Polymeric photoinitiating compositions

Abstract

Branched polymers prepared by free radical vinyl polymerisation, comprising residues of a multivinyl monomer, residues of a chain transfer agent, and optionally residues of a monovinyl monomer, which carry photoinitiator and / or photosynergist moieties, and photocurable compositions comprising such polymers, are particularly though not exclusively of use in photocurable applications in the fields of 3D printing, printing inks, coatings, adhesives, electronics, packaging (e.g. food packaging), dentistry and biomedical products. TBRT polymers are suitable polymers in this context. Such polymers may be prepared by free radical techniques.

Description

[0001] POLYMERIC PHOTOINITIATING COMPOSITIONS

[0002] The present invention relates to the field of photoinitiators and photosynergists, and in particular to polymers carrying photoinitiator and / or photosynergist moieties, and photocurable compositions comprising such polymers. In particular, though not exclusively, said polymers are of use in photocurable applications in the fields of 3D printing, printing inks, coatings, adhesives, electronics, packaging (e.g. food packaging), dentistry and biomedical products. The present invention also relates to the preparation of such polymers by free radical techniques.

[0003] Photocurable technologies play an important role in both well-established and modern industrial applications, in fields such as printing inks, coatings, adhesives, electronics, dentistry and 3D printing. Such technologies generally rely on the free radical or cationic photopolymerisation of multifunctional monomers / oligomers to prepare cured polymeric materials (thermosets). The process is initiated under light irradiation, whereby a photoinitiator and / or associated photosynergist is responsible for converting photolytic energy into a reactive radical or cationic species to initiate the photopolymerisation process.

[0004] Many commercial photoinitiators and associated photosynergists on the market suffer from common issues detrimental to their application. Notably, common issues involve migration of toxic and / or coloured (e.g. yellow) and / or odorous species derived from the photoinitiator and / or photosynergist in the cured product, poor compatibility across a wide range of photocurable formulations, and handling issues such as toxicity and high VOC. Strict and increasing regulatory demands in the areas of food / medicine packaging, coatings, dentistry, 3D printing and various other applications have magnified several of these issues. For example, regulatory bodies such as the European Chemicals Agency (ECHA) have classified several photoinitiators, including the commonly used diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (commercially known as TPO), as substances of very high concern (SVHC).

[0005] Consequently, in recent years, the design and development of polymeric photoinitiating systems for photocurable technologies has been attracting increasing attention due to the potential to address some of these application issues. However, commercial product examples are limited in number, due to the challenge of balancing reactivity, performance and cost. Those available often suffer from either slower reactivity, increased cost and / or similar issues associated with their small molecule counterparts. Examples include free radical Type I photoinitiators (e.g., Omnipol TP from IGM Resins), free radical Type II photoinitiators (e.g., Genopol TX-2 by RAHN, Omnipol BP / TX from IGM Resins) and multifunctional amine photosynergists (e.g., Genopol AB-2 by RAHN).

[0006] EP2451847B1 (IGM Group BV) describes polymer-bound bisacylphosphine oxides (BAPO) linked via the phosphorus atom to the oligomer or polymer backbone. The inventors describe various methodologies to obtain such materials fundamentally either via preparation of a BAPO-functional monomer or attachment of this moiety to a pre-formed polymer, both synthetic and natural in origin. EP2960303 B1 (Fujifilm Corporation) concerns photo-curable ink-jet formulations containing high molecular weight photoinitiators which are hyperbranched or dendritic in nature. The photoinitiator groups are acyl phosphine oxide units and have at least a thioether bond present and a molecular weight of greater than 1,000 g / mol containing 3 to 10 acylphosphine oxide units; these are covalently attached to the hyperbranched or dentritic backbone through the aryl unit. US 10941307B2 (Fujifilm Corporation) describes a polymeric photoinitiator containing acyl phosphine oxide units whereby the units are attached to the polymer scaffold via the acyl unit and where the weight average molecular weight is between 300 and 10,000 g / mol. W02023 / 214010 (IGM Group BV) concerns photoinitiator packages comprising at least one or more functional bisacyphosphine oxide photoinitiators where the phosphorus atom has been covalently attached to an ether or poly / oligoether unit and describes their use in coatings, encapsulants and 3-D printing applications. US2023 / 0265231 A1 (Arkema) discloses a polymer-bound photoinitiator containing an amide linking unit where the initiating component is chosen from benzophenone or thioxanthone units. EP2060589B1 (Henkel AG) relates to a polymeric photoinitiator for use in UV curable adhesives or coatings where the photoinitiator is linked to the polymer backbone by an amide linkage and the initiator groups are chosen from benzophenone units and the polymers are elastomeric in nature. EP3149013B1 (IGM Resins Italia S.r.l.) describes photoinitiators based on mono and bis-acylphosphine oxides attached to a linking unit via an amino, ether or thioether group to form a branched molecule which may consist of branched polyethers such as ethoxylated trimethylolpropane or ethoxylated pentaerthritol. WO2023 / 227525A1(Covestro) relates to oligomeric photoinitiators useful in free-radical coatings or ink compositions composed of branched oligomers or polymers containing a pyruvate ester functionality.

[0007] Considerable challenges remain in providing polymeric photoinitiating systems which are synthetically manageable, suitable for industrial scaleup and tailorable to different applications, and which enable access to appropriate viscosities, solubilities, reactivities and molecular weights. We have now found that a certain class of polymers is particularly well suited to these objectives and facilitates excellent photocuring.

[0008] From one aspect the present invention provides a branched vinyl TBRT polymer comprising multiple units of photoinitiator moiety.

[0009] The term “TBRT polymer” has a well understood meaning to a polymer chemist. In further detail, the present invention provides a polymer which is a branched vinyl polymer prepared by transfer-dominated branching radical telomerisation (TBRT) of multivinyl monomer(s) and optionally monovinyl monomer(s) in the presence of chain transfer agent(s), and wherein said polymer comprises photoinitiator(s).

[0010] In an alternative aspect, the present invention provides a polymer which is a branched vinyl polymer prepared by free radical polymerisation of multivinyl monomer(s) and optionally monovinyl monomer(s) in the presence of chain transfer agent(s), and wherein said polymer comprises photoinitiator(s).

[0011] The photoinitiator units are carried by the polymer. In this context, it will be understood that “carried by” denotes “covalently bound to”. In other words, they are moieties present on the polymer macromolecule, rather than being separate small molecules.

[0012] In further aspects, the present invention provides uses of the above-defined photoinitiatorcarrying branched vinyl polymers for photocuring, including in applications such as 3D printing, photocurable compositions and kits of parts comprising said polymers and curable materials, and methods of preparing said polymers by free radical polymerisation and incorporation of photoinitiator moieties.

[0013] The present invention also provides the use of a branched polymer for photocuring wherein said branched polymer is a branched vinyl TBRT polymer comprising multiple units of photosynergist moiety.

[0014] The photosynergist units are carried by the polymer. In this context, it will be understood that “carried by” denotes “covalently bound to”. In other words, they are moieties present on the polymer macromolecule, rather than being separate small molecules.

[0015] The present invention further provides the use of a branched polymer for photocuring, wherein said branched polymer is prepared by free radical vinyl polymerisation, and comprises residues of multivinyl monomer(s), residues of chain transfer agent(s), and optionally residues of monovinyl monomer(s), wherein said branched polymer comprises multiple units of a covalently-bound photosynergist.

[0016] In further aspects, the present invention provides photocurable compositions and kits of parts comprising said photosynergist-containing polymers and photoinitiator(s) and / or curable material(s).

[0017] The afore-mentioned polymeric products comprising photoinitiators and / or photosynergists provide key application benefits vs. many current commercial photoinitiator and photosynergist products, notably low product migration and cytotoxicity, high efficiency, low melt and solution viscosity, and compatibility across a wide range of photocurable formulations.

[0018] The polymer-bound photoinitiator moiety is capable of generating a reactive initiating species upon irradiation with light of a suitable wavelength, usually in the UV or visible region. This may be with or without the presence of a suitable photosynergist, depending on the type of photoinitiator used. The photoinitiating moiety can be derived from various photoinitiator types, for example free radical Norrish type I (e.g., acylphosphine oxides, a-hydroxyphenones, etc.), free radical Norrish type II (e.g., thioxanthone, benzophenone, etc.), or cationic / photoacid generators (e.g., triaryl sulfonium or diaryl iodonium salts).

[0019] The polymer-bound photosynergist moiety is an electron or hydrogen donor group that is capable of accelerating the photopolymerisation process. This is achieved by donation of an electron or hydrogen atom to a relatively unreactive radical species formed during the photopolymerisation, forming a more reactive radical species on the photosynergist moiety that can readily initiate further photopolymerisation. In this context, the relatively unreactive radical species is often derived from a type II photoinitiator (also referred to as a sensitiser) that forms upon light irradiation or a peroxy radical that forms through reaction of oxygen with a growing radical chain. Examples of photosynergist moieties are tertiary amines, esters, ethers and thiols.

[0020] The products are branched and highly functional and generally possess a combination of highly desirable physicochemical properties for application as photoinitiators and / or photosynergists. This can include one or more of high chemical, photoinitiator and / or photosynergist functionalities, high molecular weight, low melt and solution viscosity, high solubility in a range of photocurable formulations and low colour. The products can exhibit other significant application benefits, including low product migration, low cytotoxicity, high reactivity, improved handling, and the ability to interact with other common components in photocurable formulations.

[0021] In addition to these advantages, such polymeric photoinitiators and photosynergists maintain comparable reactivity to commercial offerings of the same photoinitiator type, particularly under LED wavelengths.

[0022] The products provide solutions to market needs for the development of safer commercial photocuring technologies, particularly in the areas of food packaging inks, dental and biomedical applications.

[0023] Photocuring allows the formation of a cross-linked, insoluble network. Cured products may be solid (e.g. hard). The branched polymeric photoinitiating products may be used in, or with, photocurable compositions.

[0024] The photoinitiator and / or other components are distributed in numerous locations throughout the polymeric structure (e.g. TBRT structure) and thereby facilitate effective and efficient curing. The number of photoinitiator sites, and their location relative to other functional groups and within particular monomer residues, can be controlled and tailored by choice of monomer feedstock type and relative quantities. It is possible to simultaneously maintain various properties for example selected from molecular weight, viscosity, solubility and colour targets. The tunability allowed by the free radical polymerisation system (e.g. the TBRT system) also facilitates control regarding miscibility and solubility in curable formulations.

[0025] The use of the present polymerisation technology (e.g. TBRT technology) addresses problems of leaching which can be present in other systems. Initiating moieties are present within the branched, high molecular weight polymers and therefore their diffusion through / out of cured polymer products is extremely limited. Effectively, the molecular weight (size) of the polymers causes the photoinitiator species and associated by-products to become physically entrapped in the product. Further to this, the opportunity to easily incorporate multiple photoinitiating moieties and other functionalities per polymer also increases the probability of covalent binding into the cured polymer network.

[0026] In contrast to some other polymeric photoinitiator systems, the systems of the present invention have an inherently straightforward synthetic methodology. The photoinitiator or photosynergist can be present on one of the components in the feedstock used for the polymerisation (e.g. TBRT polymerisation) and thereby statistically incorporated into the polymer (e.g. TBRT polymer). Alternatively another functional group can be present on one of the components in the feedstock used for the polymerisation and thereby statistically incorporated into the polymer, which functional group may then (after the polymer has been prepared) be reacted (“post-functionalized”) to add on photoinitiator or other moiety.

[0027] The photinitiator or photosynergist may be incorporated (either into one of the components used in the polymerisation, or onto the polymer after it has been formed) via any suitable functional group. A suitable functional group may for example be an epoxide, hydroxyl, acid or unsaturated group. Accordingly, a suitable reaction to incorporate photoinitiator and / or synergist may for example be an epoxide ring-opening reaction, esterification reaction, transesterification reaction, or reaction involving an unsaturated group. The skilled person will understand that other functional groups, and other types of conjugating or linking reaction, are possible, so long as they are compatible with the system.

[0028] The use of the polymerisation technology (e.g. TBRT technology) of the present invention yields industrially-feasible polymers with key application benefits that address issues commonly associated with small molecule photoinitiators and photoinitiator synergists. The present invention facilitates the preparation of polymeric products with photoinitiator equivalent weights greater than their small molecule counterparts.

[0029] In contrast to some other polymeric photoinitiator systems, the systems of the present invention (e.g. the TBRT systems of the present invention) enable manageable viscosities combined with high molecular weights. This enables significantly reduced product migration and toxicity (due to high molecular weight) whilst also aiding handling and application processes (due to low viscosity).

[0030] Some prior art polymeric initiators or photosynergists may ameliorate leaching but at the price of high viscosities. The hyperbranched nature of the systems of the present invention, combined with dense architectures with highly distributed reactive sites, result in effective, photocurable properties. At the same time, the present invention facilitates the generation of high molecular weight products.

[0031] The systems of the present invention overcome issues in some prior art systems relating to compatibility and handling. Compatibility across a wide range of photocurable formulations is possible due to the ability to easily incorporate a range of chemical functionalities into branched vinyl polymer (e.g. TBRT polymers) through careful selection of monomer or telogen / chain transfer agent feedstocks. For example, it is possible to introduce chemical functional groups of contrasting polarities. It is also possible to tune the interactions of the photoinitiators and synergists with other common components in photocurable formulations. Another benefit of the present invention is that single-component photocuring systems can be easily achieved (this is often challenging with other synthetic methodologies). Not only photoinitiators but also other moieties can be incorporated via being present on polymerizable monomers or by being added in post-functionalization steps. Notably, it is possible to achieve TBRT structures (e.g of high molecular weight) whereby both photoinitiator moieties and synergist moieties are bound to the same branched polymer. For example, this is particularly appropriate and beneficial for Type II systems where a synergist (coinitiator) is required alongside the photoinitiator (sensitiser) component. This significantly contributes to their ease of handling.

[0032] It will also be recognised that it is possible to use various combinations of components. As discussed above, polymeric photoinitiators may be used, and / or polymeric photosynergists may be used, or polymer systems may be used which carry both photoinitiators and photosynergists. Alternatively, a polymeric photoinitiator of the present invention may be used with a different type of photosynergist (if required), e.g. a conventional small molecule photosynergist. Or a polymeric photosynergist of the present invention may be used with a different type of photoinitiator (if required), e.g. a conventional small molecule photoinitiator.

[0033] The polymeric photoinitiator and / or polymeric photosynergist of the present invention may be used to cure a curable material of any suitable kind. The curable material may, for example, be selected from an acrylate, methacrylate, vinyl ether, epoxide or oxirane monomer. Typically, free-radically curable materials include for example acrylate, methacrylate and vinyl ether monomers, and cationically curable materials include for example vinyl ether, epoxide and oxirane monomers. Nevertheless, other materials may also be cured.

[0034] Some aspects of TBRT methodology will now be discussed in further detail.

[0035] Branched vinyl polymers produced by TBRT, which may be termed “branched vinyl TBRT polymers”, are a recognised group of polymers, as described in S. R. Cassin, P. Chambon and S. P. Rannard, Polym. Chem., 2020, 11, 7637-7649 and patent publications WO 2018 / 197885, WO 2018 / 197884 and WO 2020 / 089649.

[0036] By way of background, the polymerisation of vinyl monomers using free radical chain-growth chemistry is well known. Where a vinyl monomer has only one polymerisable vinyl group (i.e. is a monovinyl monomer), polymerisation of said monomer typically results in a linear polymer. Where a vinyl monomer has more than one polymerisable vinyl group (i.e. is a multivinyl monomer), e.g. has two polymerisable vinyl groups (i.e. is a divinyl monomer), polymerisation of said monomer typically results in a branched polymer, because each of, or some of, the multivinyl monomers can become part of more than one vinyl polymer chain. Depending on the reaction conditions, said branched polymer may be highly cross-linked and gelled. Several methods have been used to control this branching and avoid gelation.

[0037] TBRT is a method for controlling branching during the free radical polymerisation of multivinyl monomers. It entails controlling the extent of propagation relative to the extent of chain transfer, and results in a hyperbranched polymer containing a large number of interconnected linear vinyl polymer chains, wherein the average length of each linear vinyl polymer chain is short. Limited propagation relative to chain transfer can be achieved by using suitable polymerisation conditions, and in particular by using a relatively large amount of chain transfer agent.

[0038] TBRT is a type of telomerisation. Telomerisation is a method of polymerisation of a polymerisable monomer (a “taxogen”) having an unsaturated group, resulting in a chain of taxogen residues (“taxomons”) with fragments of a further molecule (a “telogen”) attached terminally to the chain of taxomons. This produces a product termed a “telomer” of formula Y(A)nZ in which A is a taxomon, a residue of a taxogen, and Y and Z are fragments of a telogen, YZ.

[0039] In TBRT in the field of vinyl polymerisation, the taxogen is a multivinyl monomer (often a divinyl monomer) and the telogen is a chain transfer agent (often a thiol RSH in which, according to the above terminology, Y is RS and Z is H).

[0040] Polymers produced by TBRT of multivinyl monomers exhibit interesting and unusual properties. They are formed by chain-growth polymerisation, yet may exhibit characteristics of step-growth polymers. Retrosynthetic analysis of some TBRT vinyl polymers can cause them to be viewed as comprising a mixture of polyfunctional step-growth monomer residues. Although they are vinyl polymers, many of them do not resemble vinyl polymers because their chemistry may be dominated by functional groups present in the parts of the multivinyl monomer which link the vinyl groups, and / or by functional groups in the chain transfer agent. TBRT methodology enables new architectures containing step-growth motifs which would be difficult or impossible to achieve using step-growth polymerisation.

[0041] The methodology may also be understood when defined using different terminology to achieve the same or similar outcomes. Therefore, from further aspects, the present invention can be defined as follows. From a further aspect, the present invention provides a polymer which is a branched polymer prepared by free radical vinyl polymerisation comprising residues of multivinyl monomer(s), residues of chain transfer agent(s), and optionally residues of monovinyl monomer(s), and wherein said polymer comprises multiple units of covalently-bound photoinitiator and / or covalently-bound photosynergist.

[0042] From a further aspect, the present invention provides a polymer which is a branched polymer comprising vinyl polymer chains wherein the vinyl polymer chains comprise residues of vinyl groups of multivinyl monomers and optionally monovinyl monomers, wherein the longest chains in the polymer are not the vinyl polymer chains but rather extend through the linkages between double bonds of the multivinyl monomers, and wherein said polymer comprises multiple units of covalently-bound photoinitiator and / or covalently-bound photosynergist.

[0043] From a further aspect, the present invention provides a polymer which is a step-growth polymer comprising a mixture of polyfunctional step-growth monomer residues formed by vinyl polymerisation, and wherein said polymer comprises multiple units of covalently-bound photoinitiator and / or covalently-bound photosynergist.

[0044] Regardless of whether the invention is understood according to the first aspect above or according to the further aspects above, the principles are the same and the description of the invention, including the description herein clarifying the features of the invention and specifying further, optional or preferred features, applies to all aspects.

[0045] From yet further aspects, the present invention provides the use of the polymer defined in each of the aspects above to form a photocured product.

[0046] The photoinitiator and / or photosynergist may be incorporated via co-polymerised monovinyl monomer(s), via the multivinyl monomer(s), or via chain transfer agent(s). Alternatively, or additionally, they may be incorporated by post-functionalisation, i.e. by reaction(s) after polymerisation. For example, an epoxide may be present on the polymer; such groups (and others) can be functionalised using other reagents to add on further functional groups.

[0047] The multivinyl monomer residue(s) may be divinyl monomer residue(s) and optionally the polymer may comprise on average between 0.9 and 1.1 chain transfer agent residues per divinyl monomer residue. In some cases the polymer may comprise on average between a lesser amount (e.g. 0.8) and a greater amount (e.g. 2.5) of chain transfer agent residues per divinyl monomer residue. The multivinyl monomer may have more than two vinyl groups, i.e. within the scope of the invention are polymers which may be made from not only divinyl monomers but also, for example, trivinyl and / or tetravinyl monomers. In such scenarios, the polymer may comprise on average between 0.9 and 3.3 chain transfer agent residues per multivinyl monomer residue. In some cases the polymer may comprise on average between a lesser amount (e.g.

[0048] 0.5) and a greater amount (e.g. 5) chain transfer agent residues per multivinyl monomer residue.

[0049] The polymer may comprise a multiplicity of vinyl polymer chain segments having an average length of between 1 and 3 multivinyl monomer residues.

[0050] One of the major advantages of the present invention is that extremely branched polymers can be achieved via radical polymerisation that would be impossible, or challenging, or extremely dangerous, to make using step-growth polymerisation. It is advantageous to be able to control the polymerisation to avoid gelled polymers. Conventional step-growth polymerisation methods have been known to result in gelled branched polymers within industrial-scale reactors, which have taken weeks to recover; this can clearly be very costly and time-consuming.

[0051] The invention involves the synthetic process, composition and application of hyperbranched polymeric photoreactive materials whereby at least one photoinitiating moiety and / or photosynergist moiety is covalently bound to a larger macromolecule derived from transfer-dominated branching radical telomerisation (TBRT). Such polymer-bound moieties are capable of initiating or facilitating a free radical or cationic polymerisation upon irradiation with UV or visible light.

[0052] The photoinitiating moiety can be derived from various photoinitiator types, notably free radical Norrish type I (e.g., acylphosphine oxides, a-hydroxyphenones, etc.), free radical Norrish type II (e.g., thioxanthone, benzophenone, etc.), or cationic / photoacid generators (e.g., triaryl sulfonium or diaryl iodonium salts). The polymeric photoinitiator is prepared by addition of photoinitiator bearing polymerisable group(s) to a TBRT process or by grafting photoinitiator onto a TBRT-derived polymer.

[0053] One particularly effective way of incorporating photoinitiator and / or photosynergist is by postfunctionalisation of a TBRT polymer. Another is via such group being present on a feedstock (e.g. monovinyl monomer) which is polymerized in the TBRT process. The polymeric product may optionally have a number average molecular weight greater than 1000 g mol-1, or 1,000 to 100,000, or 1,000 to 50,000, or 1,000 to 10,000, or 1,000 to 5,000, or 1,000 to 3,000, or 2,000 to 100,000, or 2,000 to 50,000, or 2,000 to 10,000, or 2,000 to 5,000, or 2,000 to 3,000, or 3,000 to 100,000, or 3,000 to 50,000, or 3,000 to 10,000, or 3,000 to 5,000, or 5,000 to 100,000, or 5,000 to 50,000, or 5,000 to 10,000, or 10,000 to 100,000, or 50,000 to 100,000 g mol-1.

[0054] The photoinitiator equivalent weight is typically greater than in its small molecule counterpart, and may optionally be within the range of 100 to 10,000 g / mol, or 100 to 5,000 g / mol, or 100 to 3,000 g / mol, or 100 to 2,000 g / mol, or 100 to 1,500 g / mol, or 100 to 1,000 g / mol, or 100 to 900 g / mol, or 100 to 850 g / mol, or 250 to 5,000 g / mol, or 250 to 3,000 g / mol, or 250 to 2,000 g / mol, or 250 to 1 ,500 g / mol, or 250 to 1 ,000 g / mol, or 250 to 900 g / mol, or 250 to 850 g / mol, or 300 to 5,000 g / mol, or 300 to 3,000 g / mol, or 300 to 2,000 g / mol, or 300 to 1,500 g / mol, or 300 to 1,000 g / mol, or 300 to 900 g / mol, or 300 to 850 g / mol, or 500 to 10,000 g / mol, or 500 to 5,000 g / mol, or 500 to 3,000 g / mol, or 500 to 2,000 g / mol, or 500 to 1 ,500 g / mol, or 500 to 1 ,000 g / mol, or 500 to 900 g / mol, or 500 to 850 g / mol, or 1 ,000 to 10,000 g / mol, or 1,000 to 5,000 g / mol, or 1,000 to 3,000 g / mol, or 1,000 to 2,000 g / mol, or 2,000 to 10,000 g / mol, or 2,000 to 5,000 g / mol, or 2,000 to 3,000 g / mol, or 2,000 to 3,000 g / mol, or 2,000 to 3,000 g / mol, or 2,000 to 3,000 g / mol, or 3,000 to 10,000 g / mol, or 3,000 to 5,000 g / mol, or 4,000 to 10,000 g / mol, or 4,000 to 5,000 g / mol.

[0055] In this invention, TBRT technology is utilised to prepare photocurable products with a combination of desirable physicochemical properties - e.g., high chemical and / or photoinitiator functionality, high molecular weight, low melt and solution viscosity, high solubility in UV formulations, etc. Without TBRT technology, the structures and properties would be extremely challenging to obtain, particularly industrially. Further, the polymeric products are highly tuneable and can be designed to contain other moieties that support the performance of the photoinitiator product - e.g., synergistic moieties such as tertiary amines, solubilising moieties, photocurable moieties, etc. Consequently, this range of materials offer various advantages over current commercial photoinitiator offerings, notably: lower product migration and toxicity; lower volatility / odour; lower colour development; improved handling and compatibility in UV formulations; and opportunity for single-component photoinitiating systems (particularly beneficial for type II systems where a synergist is required). In addition to these advantages, such polymeric photoinitiators maintain good photoreactivity.

[0056] TBRT technology can be used to successfully prepare soluble, high molecular weight, hyperbranched copolymers with pendant photoinitiating moieties or photosynergist moieties. The polymers have low odour and volatility. Likewise, the materials of the present invention are often low colour in contrast to other polymeric photoinitiators.

[0057] Furthermore, the hyperbranched polymeric photoinitiators can have improved solubility (compatibility) relative to their small molecule counterparts in various formulations - e.g., improving the solubility of certain hydrophobic small molecule photoinitiators (e.g TPO-L, benzophenone etc) in hydrophilic media by using hydrophilic polymer backbone and / or pendant groups.

[0058] As demonstrated in the examples, hyperbranched polymeric photoinitiators prepared by TBRT technology can effectively initiate acrylic resins under LED wavelengths - i.e. , A = 385 nm, a common wavelength used by commercial 3D printers.

[0059] The UV curing parameters of hyperbranched polymeric photoinitiators of the present invention are comparable to small molecule equivalents, e.g. TPO-L. These are combined with significant benefits (e.g., lower migration).

[0060] Furthermore, hyperbranched polymeric photoinitiators prepared by TBRT technology can be used as the sole initiator in photocurable technologies such as vat photopolymerisable 3D printing - e.g. using digital light processing technology with a A = 385 nm light source.

[0061] One of the main benefits of hyperbranched polymeric photoinitiators and / or synergists prepared by TBRT technology is low product migration. When hyperbranched polymeric photoinitiators and / or synergists of the present invention are used in place of their small molecule photoinitiating counterparts, considerably lower leaching of species derived from the photoinitiator and / or synergist components can be achieved in the cured product. In addition, when hyperbranched polymeric photoinitiators and / or synergists of the present invention are used in place of their small molecule photoinitiating counterparts, higher degree of cures can be achieved, which can lead to reduced leaching of all components in cured product. Photoinitiating materials that offer lower product migration and / or lower toxicity products are desirable in many commercial photocuring technologies, notably in the areas of food and medicine packaging, coatings, dentistry, biomedical and wearable devices, and 3D printing.

[0062] Photoinitiators

[0063] In relation to the present invention, a photoinitiator moiety is a polymer-bound unit (e.g. a TBRT polymer-bound unit) that is capable of generating a reactive initiating species upon irradiation with light of a suitable wavelength. The wavelength may be (and typically is) in the UV or visible region. This may or may not require the presence of a suitable photosynergist (coinitiator), depending on the type of photoinitiator used. The photoinitiator moiety can be of certain generic types, three of which are as follows:

[0064] Free radical type I photoinitiators

[0065] Such photoinitiators operate via a photocleavage mechanism. Upon excitation with radiation of a specific wavelength, these photoinitiators undergo homolytic bond cleavage providing two reactive initiating radicals. Cleavage often occurs at the a-position to a carbonyl (or equivalent) functional group. In respect of this invention, at least one of the reactive initiating radicals will remain bound to the TBRT polymer. The reactive initiating radicals will predominantly initiate the photopolymerisation in the presence of radically-polymerisable monomer(s) and become covalently bound to the cured polymer network. Key examples of free radical type I photoinitiator moieties include, but are not limited to, acylphosphine oxides (including bisacylphosphine oxides), a-hydroxyketones (including a-hydroxyacetophenones and benzoins), a-alkoxyketones (including a-alkoxyacetophenones, benzoin ethers and benzoin ketals), a-aminoketones (including a-aminoacetophenones, a-morpholinoketones and a-morpholinoacetophenones), a-ketoesters, oxime esters (including O-acyl-a-oximino ketones and O-acyl-a-oximino acetophenones), acylgermanes (including bisacylgermanes and tetraacylgermanes), acylstannates (including bisacylstannates and tetraacylstannates), and derivatives thereof.

[0066] Free radical type II photoinitiators

[0067] Such photoinitiators are also referred to as sensitisers and rely on a photoreduction mechanism. The presence of an electron or hydrogen donor, commonly referred to as a photosynergist or coinitiator, is required. Upon excitation with radiation of a specific wavelength, these photoinitiators abstract an electron or hydrogen atom from the donor source, forming a non-initiating radical derived from the photoinitiator and a reactive initiating radical derived from the donor. In respect of this invention, the non-initiating radical derived from the photoinitiator remains bound to the TBRT polymer. The reactive initiating radical derived from the donor will predominantly initiate the photopolymerisation in the presence of radically-polymerisable monomer(s) and become covalently bound to the cured polymer network. Key examples of free radical type 11 photoinitiator moieties include, but are not limited to, benzophenones, thioxanthones, anthraquinones, benzoquinones, 1,2-diketones (including benzils and camphorquinones), coumarins (including ketocoumarins), flavonoids, naphthalimides, titanocenes, and derivatives thereof.

[0068] Cationic photoinitiators Such photoinitiators are also referred to as photoacid generators (PAGs) and often composed of an ionic pair. Upon excitation with radiation of a specific wavelength, these photoinitiators fragment via homolytic and / or heterolytic scission and, in turn, form a strong acid (H+source). In respect of this invention, at least one of the fragmented photoinitiator fragments remains bound to the TBRT polymer. The strong acid will initiate (catalyse) the cationic photopolymerisation in the presence of cationically-polymerisable monomer(s). Key examples of cationic photoinitiator moieties include, but are not limited to, aryl sulfonium salts (notably, triaryl sulfonium salts) and aryl iodonium salts (notably, diaryl iodonium salts). Optionally, such cationic photoinitiators can be sensitised under LED wavelengths with other LED-absorbing species such as thioxanthones, anthracenes, and similar compounds. In respect of this invention, such species may also be bound to a TBRT polymer for this purpose.

[0069] Photosynergists

[0070] In relation to the present invention, a photosynergist moiety is a polymer-bound electron or hydrogen donor group (e.g. a TBRT polymer-bound electron or hydrogen donor group) that is capable of accelerating the photopolymerisation process. This is achieved by donation of an electron or hydrogen atom to a relatively unreactive radical species formed during the photopolymerisation, forming a more reactive radical species on the photosynergist moiety that can readily initiate further photopolymerisation. In this context, the relatively unreactive radical species is often derived from a type II photoinitiator (also referred to as a sensitiser) that forms upon light irradiation or a peroxy radical that forms through reaction of oxygen with a growing radical chain. Usually, the photosynergist moiety does not strongly absorb at the wavelength of irradiating light used to initiate the photopolymerisation. Key examples of photosynergist moieties include, but are not limited to, aliphatic tertiary amines, aromatic tertiary amines (including aniline, p-toluidine and aminobenzoate derivatives), thiols, silanes, boranes, and transition metals such as cobalt, manganese or iron complexes.

[0071] Types of branched polymers

[0072] The branched vinyl polymers may be considered to be scaffolds to which the photoinitiator, or photosynergist are attached, and may comprise any suitable chemistry.

[0073] Suitable types of branched vinyl polymers include branched polyesters, which may be made from the polymerisation of monomers which comprise vinyl groups as well as ester-containing or ester-forming functionality. Suitable monomers include methacrylates, acrylates and vinyl esters. Suitable divinyl and multivinyl monomers include dimethacrylates, diacrylates, divinyl esters, multimethacrylates, multiacrylates, and multivinyl esters. Said branched polyesters may be aliphatic polyesters, or may be aromatic polyesters if aromatic groups are also present in the monomer(s), or may be mixed aromatic / aliphatic polyesters.

[0074] Said branched polyesters may also contain other functional groups, for example by inclusion of certain chemical moieties within the monomers used in the feedstock.

[0075] Therefore, further suitable types of branched vinyl polymers include branched poly(urethane-ester)s. These may be made from the polymerisation of monomers which comprise: (i) vinyl groups; (ii) ester-containing or ester-forming functionality; and (iii) urethane-containing or urethane-forming functionality. Suitable monomers include urethane dimethacrylate.

[0076] Types of polymer which have been found to be particularly effective in accordance with the present invention include

[0077] Suitable types of polymer include those containing functionality selected from the following: carbonates; carbonate esters; amides; amide esters; urethanes; urethane esters; urethane amides; urethane carbonates; ureas.

[0078] The branched polymer may be prepared by the free radical polymerisation of a multivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the multivinyl monomer, the monovinyl monomer and / or the chain transfer agent; or by post-functionalisation. We have found that it is convenient and effective to incorporate to photoinitiator or photosynergist via it being present on a monovinyl monomer which is polymerized. The extent of propagation may be controlled relative to the extent of chain transfer to prevent gelation of the polymer.

[0079] The term multivinyl monomer denotes monomers which have more than one free radical polymerisable vinyl group. One particular class of such monomers are those which have two such vinyl groups, i.e. divinyl monomers.

[0080] Therefore, the branched polymer may be prepared by the free radical polymerisation of a divinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the divinyl monomer, the monovinyl monomer and / or the chain transfer agent; or by post-functionalisation. We have found that it is convenient and effective to incorporate to photoinitiator or photosynergist via it being present on a monovinyl monomer which is polymerized. The extent of propagation may be controlled relative to the extent of chain transfer to prevent gelation of the polymer.

[0081] Thus, in contrast to some prior art methods, cross-linking and insolubility are avoided not by using a combination of a predominant amount of monovinyl monomer and a lesser amount of divinyl monomer, but instead by controlling the way in which a divinyl monomer, or other multivinyl monomer, reacts.

[0082] The branched polymer may be prepared by the free radical polymerisation of a divinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of divinyl monomer residues per vinyl polymer chain is between 1 and 3. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the divinyl monomer, the monovinyl monomer and / or the chain transfer agent.

[0083] The branched polymer may be prepared by the free radical polymerisation of a multivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of multivinyl monomer residues per vinyl polymer chain is between 1 and 3. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the multivinyl monomer, the monovinyl monomer and / or the chain transfer agent.

[0084] The branched polymer may be prepared by the free radical polymerisation of a trivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of trivinyl monomer residues per vinyl polymer chain is between 1 and 2. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the trivinyl monomer, the monovinyl monomer and / or the chain transfer agent.

[0085] The branched polymer may be prepared by the free radical polymerisation of a tetravinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of tetravinyl monomer residues per vinyl polymer chain is between 1 and 1.7. A photoinitiator or photosynergist is present on the polymer and may be incorporated by being present on the tetravinyl monomer, the monovinyl monomer and / or the chain transfer agent.

[0086] Combinations of different types of multivinyl monomer may be incorporated, regardless of whether or not a monovinyl monomer is incorporated. For example, it may be that any combination of two or more of a divinyl monomer, a trivinyl monomer, a tetravinyl monomer, or other multivinyl monomer, are incorporated, and that optionally a monovinyl monomer may also be incorporated. Additionally or alternatively it may be that the polymer contains more than one of each type of monomer and / or more than one type of chain transfer agent. For example, where the polymer contains divinyl residues and monovinyl residues and chain transfer agent residues, the divinyl residues may be all the same or different, the monovinyl residues may be all the same or different, and the chain transfer agent residues may be all the same or different.

[0087] The polymer is prepared by free radical polymerisation and any suitable source of radicals can be used. For example, this could be an initiator such as AIBN. Other materials can be used, for example organic peroxides. A thermal or photochemical or other process can be used to provide free radicals. In contrast to some prior art methods, a large amount of initiator is not required; only a small amount of a source of radicals is required in order to initiate the reaction.

[0088] To prepare the polymer, the skilled person is able to control the chain transfer reaction relative to the propagation reaction by known techniques. This may be done by using a sufficiently large amount of a chain transfer agent (CTA). The chain transfer agent caps the vinyl polymer chains and thereby limits their length. It also controls the chain end chemistry. Various chain transfer agents are suitable and of low cost, and impart versatility to the method and resultant product.

[0089] The primary chains may be kept very short so that gel formation is avoided, whilst at the same time a high level of branching is achieved.

[0090] An important advantage of the present invention is that the polymer may be prepared by industrial free radical polymerisation. This is completely scalable, very straightforward and extremely cost effective. In contrast, some prior art polymers are more complex and / or more costly and / or require the use of initiator systems or more complex purification procedures.

[0091] Optionally the only reagents to prepare the branched polymer are one or more multivinyl monomer (for example a divinyl monomer), a chain transfer agent, a source of radicals, and optionally a solvent. Thus, in contrast to some prior art methods, the present invention relates to polymers which can be prepared by the homopolymerisation of multivinyl monomers.

[0092] Monovinyl monomers are not required to prepare the polymer. The photoinitiator or photosynergist may be present on a monovinyl monomer, but may alternatively be present on multivinyl monomer or chain transfer agent.

[0093] Nevertheless, monovinyl monomers may be incorporated, i.e. optionally a copolymerisation may be carried out to produce the polymer. Monovinyl monomers are convenient means of introducing photoinitiator or photosynergist.

[0094] For example, the polymer may incorporate not only a divinyl monomer but also an amount, optionally a lesser amount, of monovinyl monomer. The molar amount of divinyl monomer relative to monovinyl monomer may be greater than 50%, greater than 75%, greater than 90% or greater than 95%, for example. Optionally, the ratio of divinyl monomer residues to monovinyl monomer residues may be greater than or equal to 1 : 1 , or greater than or equal to 3:1, greater than or equal to 10: 1 or greater than or equal to 20: 1.

[0095] Alternatively, in some scenarios, more monovinyl monomer may be used. Optionally, the polymer may incorporate not only one or more divinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers used are divinyl monomers. Optionally, the polymer may incorporate not only one or more divinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers residues in the product are divinyl monomer residues.

[0096] The possible incorporation of monovinyl monomers is applicable not just with divinyl monomers but also with other types of multivinyl monomers. Accordingly, the polymer may incorporate not only one or more multivinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers used are multivinyl monomers. Optionally, the polymer may incorporate not only one or more multivinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers residues in the product are multivinyl monomer residues. The polymer may comprise on average between 0.25 and 5 monovinyl monomer residues per multivinyl monomer (e.g. divinyl) residue, for example 0.25 to 4, or 0.25 to 3, or 0.25 to 2, or 0.25 to 1 , or 0.25 to 0.5, or 0.5 to 5, or 0.5 to 4, or 0.5 to 3, or 0.5 to 2, or 0.1 to 1 , or 1 to 5, or 1 to 4, or 1 to 3, or 1 to 2, or 2 to 5, or 2 to 4, or 2 to 3, or 3 to 5, or 3 to 4, or 4 to 5. In some cases the polymer may comprise on greater amounts of monovinyl monomer residues per multivinyl monomer (e.g. divinyl) residue, for example up to 6 or up to 7 or up to 8 or up to 9 or up to 10.

[0097] Divinyl Monomer

[0098] One type of multivinyl monomer residue which may be present in the polymer is a divinyl monomer.

[0099] The divinyl monomer contains two double bonds each of which is suitable for free radical polymerisation. It may contain one or more other group which for example may be selected from, but not limited to: aliphatic chains; esters; amides; esters; urethanes; silicones; amines; aromatic groups; oligomers or polymers; or a combination of one or more of these; and / or which may optionally be substituted. For example there may be PEG groups or PDMS groups between the double bonds, or a benzene ring (e.g. as in the monomer divinyl benzene) or other aromatic groups.

[0100] Each vinyl group in the divinyl monomer may for example be an acrylate, methacrylate, acrylamide, methacrylamide, vinyl ester, vinyl aliphatic, or vinyl aromatic (e.g. styrene) group.

[0101] Due to the large amount of chain transfer agent in the reaction, the vinyl polymer chains in the final product are generally quite short and the chemistry of the longest chains in the polymer may be governed by the other chemical species in the monomer. Thus, for example, monomers which contain, in addition to two vinyl groups, ester linkages (e.g. dimethacrylates, such as EGDMA) polymerise to form polyester structures, wherein the longest repeating units comprise esters. Similarly, monomers which contain, in addition to two vinyl groups, amide linkages (e.g. bisacrylamides) polymerise to form polyamide structures, wherein the longest repeating units comprise amides.

[0102] Thus the polymers may be polyesters, polyamides or other polymers.

[0103] The monomer residues may comprise other linkages or moieties, (e.g. urethane units), thereby resulting in other types of polymer (e.g. polyurethanes). The monomer residues may contain more than one type of moiety, thereby resulting in hybrid polymers. For example, monomers which contain, in addition to two vinyl groups, ester linkages and urethane linkages (e.g. dimethacrylates containing urethane linkages, such as urethane di methacrylate (LIDMA)) polymerise to form polymers which may be termed poly(urethane-ester)s or polyurethane esters.

[0104] The divinyl monomer may be stimuli-responsive, e.g. may be pH, thermally, or biologically responsive. The response may be degradation. The linkage between the two double bonds may for example be acid- or base-cleavable, for example may contain an acetal group. This allows the preparation of a commercial product which is a stimuli-responsive branched polymer. Alternatively, a further step of cleaving divinyl monomer may be carried to remove bridges in the polymer, to produce product in which the linkages between vinyl polymer chains have been removed or reduced.

[0105] Optionally the polymer may be prepared using a mixture of divinyl monomers. Thus two or more different divinyl monomers may be copolymerised.

[0106] Other types of multivinyl monomer

[0107] Multivinyl monomers other than divinyl monomers may be used, for example, trivinyl monomers, tetravinyl monomers and / or monomers with more vinyl groups. Trivinyl monomers, in particular, are useful, as they can be sourced or prepared without significant difficulty, and allow further options for producing different types of branched polymers. The discussion, disclosures and teachings herein in relation to divinyl monomers also apply where appropriate, mutatis mutandis, to other multivinyl monomers.

[0108] Chain transfer agent (CTA)

[0109] Any suitable chain transfer agent may be used.

[0110] These include thiols, including optionally substituted aliphatic thiols, such as dodecane thiol (DDT). Another suitable chain transfer agent is alpha-methylstyrene dimer. Another is 2-isopropoxyethanol. Other compounds having functionality which is known to allow the transfer of radical chains may be used. These can be bespoke to bring about desired functionality to the polymers. The chain-end chemistry can be tailored by the choice of CTA. Thus, hydrophobic / hydrophilic behaviour and other properties can be influenced. Alkyl thiols can have quite different properties to alcohol-containing groups, acid-containing groups, or amine-containing groups, for example.

[0111] Optionally, a mixture of CTAs may be used. Thus, two or more different CTAs may be incorporated into the product.

[0112] Relative amounts of chain transfer agent and divinyl monomer

[0113] The relative amounts of chain transfer agent and divinyl monomer can be modified easily and optimised by routine procedures to obtain non-gelled polymers without undue burden to the skilled person. The analysis of the products can be carried out by routine procedures, for example the relative amounts of chain transfer agent and divinyl monomer can be determined by NMR analysis.

[0114] Regarding the reagents used, optionally at least 1 equivalent, or between 1 and 10 equivalents, or between 1.2 and 10 equivalents, or between 1.3 and 10 equivalents, or between 1.3 and 5 equivalents, or between 1 and 5 equivalents, or between 1 and 3 equivalents, or between 1 and 2 equivalents, or between 1.2 and 3 equivalents, or between 1.2 and 2 equivalents, of chain transfer agent may be used relative to divinyl monomer. The presence of a large amount of chain transfer agent means that on average the primary vinyl polymer chains react, and are capped by, chain transfer agent, whilst they are short. This procedure amounts to telomerisation, i.e. the formation of short chains with small numbers of repeat units. Optionally, at least 0.7 equivalents, or at least 0.8 equivalents, or at least 0.9 equivalents, of chain transfer agent may be used relative to divinyl monomer.

[0115] In the final product, there may be n+1 chain transfer agent moieties per n divinyl monomer moieties (thus tending to a 1:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (n+1):n are possible. Optionally, on average between 0.5 and 2 chain transfer agent moieties are present per divinyl monomer moiety, optionally between 0.7 and 1.5, optionally between 0.75 and 1.3, or between 0.8 and 1.2, or between 0.9 and 1.1, or between 1 and 1.05, or approximately 1.

[0116] Relative amounts of chain transfer agent and trivinyl monomer Where the multivinyl monomer used is a trivinyl monomer, the following may optionally apply.

[0117] Regarding the reagents used, optionally at least 2 equivalents, or between 2 and 20 equivalents, or between 2.4 and 20 equivalents, or between 2.6 and 20 equivalents, or between 2.6 and 10 equivalents, or between 2 and 10 equivalents, or between 2 and 6 equivalents, or between 2 and 4 equivalents, or between 2.4 and 6 equivalents, or between 2.4 and 4 equivalents, of chain transfer agent may be used relative to trivinyl monomer. Optionally, at least 1 equivalent, or at least 1.5 equivalents, or at least 1.75 equivalents, of chain transfer agent may be used relative to divinyl monomer.

[0118] In the final product, there may be 2n+1 chain transfer agent moieties per n trivinyl monomer moieties (thus tending to a 2:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (2n+1):n are possible. Optionally, on average between 1 and 4 chain transfer agent moieties are present per trivinyl monomer moiety, optionally between 1.4 and 3, optionally between 1.5 and 2.6, or between 1.6 and 2.4, or between 1.8 and 2.2, or between 2 and 2.1, or approximately 2.

[0119] Relative amounts of chain transfer agent and tetravinyl monomer

[0120] Where the multivinyl monomer used is a tetravinyl monomer, the following may optionally apply.

[0121] Regarding the reagents used, optionally at least 3 equivalents, or between 3 and 30 equivalents, or between 3.6 and 30 equivalents, or between 3.9 and 30 equivalents, or between 3.9 and 15 equivalents, or between 3 and 15 equivalents, or between 3 and 9 equivalents, or between 3 and 6 equivalents, or between 3.6 and 9 equivalents, or between 3.6 and 6 equivalents, of chain transfer agent may be used relative to tetravinyl monomer. Optionally, at least 2 equivalents, or at least 2.5 equivalents, or at least 2.75 equivalents, of chain transfer agent may be used relative to divinyl monomer.

[0122] In the final product, there may be 3n+1 chain transfer agent moieties per n tetravinyl monomer moieties (thus tending to a 3:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (3n+1):n are possible. Optionally, on average between 1.5 and 6 chain transfer agent moieties are present per tetravinyl monomer moiety, optionally between 2.1 and 4.5, optionally between 2.25 and 3.9, or between 2.4 and 3.6, or between 2.7 and 3.3, or between 3 and 3.15, or approximately 3.

[0123] Relative amounts of chain transfer agent and multivinyl monomer

[0124] In summary, without wishing to be bound by theory, in certain idealised scenarios the number of CTA residues per n MVM residues in the final product may be as follows:

[0125]

[0126] Thus it can be seen that, as the valency of the monomer increases, more and more CTA is required to be present in the final product to cap the chains, unless some other mechanism (e.g. intramolecular reaction) does that.

[0127] In general the following may optionally apply across the various types of multivinyl monomers discussed herein. Regarding the reagents used, optionally at least 1 equivalent, or between 1 and 30 equivalents, or between 1.2 and 30 equivalents, or between 1.3 and 30 equivalents, or between 1.3 and 15 equivalents, or between 1 and 15 equivalents, or between 1 and 9 equivalents, or between 1 and 6 equivalents, or between 1.2 and 9 equivalents, or between 1.2 and 6 equivalents, of chain transfer agent may be used relative to multivinyl monomer. In the final product, optionally, on average between 0.5 and 6 chain transfer agent moieties are present per multivinyl monomer moiety, optionally between 0.7 and 4.5, optionally between 0.75 and 3.9, or between 0.8 and 3.6, or between 0.9 and 3.3, or between 1 and 3.15, or between approximately 1 and approximately 3. Optionally, at least 0.7 equivalents, or at least 0.8 equivalents, or at least 0.9 equivalents, of chain transfer agent may be used relative to divinyl monomer.

[0128] The polymer may comprise on average between 0.9(x-1) and 1.1 (x-1) chain transfer agent residues per multivinyl monomer residue, where “x” is the number of polymerisable vinyl groups on the multivinyl monomer.

[0129] Where the multivinyl monomer is a divinyl monomer, x=2, and accordingly the polymer may comprise on average between 0.9 and 1.1 chain transfer agent residues per divinyl monomer residue. Extent of vinyl polymerisation

[0130] We believe that one important feature of the present invention is that the average length of the vinyl polymer chains within the overall polymer is short. A typical polymeric molecule prepared as described herein will contain many vinyl polymer chains (each of which is on average quite short) linked together by the moiety which in the multivinyl monomer is between the double bonds.

[0131] This is achieved by adjusting the conditions, including the amount of chain transfer agent, so that the rate of chain transfer competes with the rate of vinyl polymerisation to the desired extent. The identities of the multivinyl monomer and the chain transfer agent, as well as other factors, affect this balance, but the progress of the reaction can be easily monitored and the properties of the resultant polymer easily determined, by known, routine, techniques. Therefore there is no undue burden to the skilled person. The resulting chain length in this context is the kinetic chain length.

[0132] Extent of vinyl polymerisation when using divinyl monomers

[0133] The number of propagation steps (i.e. how many divinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.3 and 2, between 1.5 and 2, between 1.7 and 2, between 1.8 and 2, between 1.9 and 2, or between 1.95 and 2, or of approximately 2, divinyl monomer residues, is suitable.

[0134] Whilst the average may optionally be between 1 and 3, a small number of vinyl polymer chains may contain significantly more divinyl monomer residues, for example as many as 10, 15, 18, 20 or more.

[0135] Optionally 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer. Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are n+1 chain transfer agent moieties per n divinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 2n double bonds per n divinyl monomers, the number of double bond residues per chain will on average be 2n / (n+1) which will tend towards 2 as the molecular weight increases.

[0136] The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one DVM, i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher.

[0137] Optionally, the product may contain a large amount of divinyl monomer residues wherein one of the double bond residues is capped with a chain transfer agent (as opposed to being part of a chain), i.e. has a nominal chain length of 1. The other double bond residues of those divinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one divinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one divinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 4 or 5, e.g. 5, divinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one divinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 4 or 5, e.g. 5, divinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 4 or 5, e.g. 5.

[0138] Extent of vinyl polymerisation when using trivinyl monomers The number of propagation steps (i.e. how many trivinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 2, between 1 and 1.8, between 1 and 1.7, between 1 and 1.5, between 1.1 and 1.5, between 1.2 and 1.5, between 1.25 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, or between 1.45 and 1.5, or of approximately 1.5, trivinyl monomer residues, is suitable.

[0139] Whilst the average may optionally be between 1 and 2, a small number of vinyl polymer chains may contain significantly more trivinyl monomer (TVM) residues, for example as many as 5, 10, 15, 18, 20 or more.

[0140] Optionally 90 % of the vinyl polymer chains contain fewer than 8 TVM residues, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 5 or fewer, or 75% have a length of 8 or fewer, or 75% have a length of 6 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer.

[0141] Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are 2n+1 chain transfer agent moieties per n trivinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 3n double bonds per n trivinyl monomers, the number of double bond residues per chain will on average be 3n / (2n+1) which will tend towards 1.5 as the molecular weight increases.

[0142] The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one TVM, i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher.

[0143] Optionally, the product may contain a large amount of trivinyl monomer residues wherein two of the double bond residues are capped with a chain transfer agent (as opposed to being part of a chain), i.e. have a nominal chain length of 1. The other double bond residues of those trivinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one trivinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one trivinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g.

[0144] 3 or e.g. 4, trivinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one trivinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g. 3 or e.g. 4, trivinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g. 3 or e.g. 4.

[0145] Extent of vinyl polymerisation when using tetravinyl monomers

[0146] The number of propagation steps (i.e. how many tetravinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 1.7, between 1 and 1.5, between 1 and 1.4, between 1 and 1.33, between 1.1 and 1.33, between 1.2 and 1.33, between 1.25 and 1.33, or between 1.3 and 1.33, or of approximately 1.33, tetravinyl monomer residues, is suitable.

[0147] Whilst the average may optionally be between 1 and 1.7, a small number of vinyl polymer chains may contain significantly more tetravinyl monomer residues, for example as many as 3, 5, 10, 15, 18, 20 or more.

[0148] Optionally 90 % of the vinyl polymer chains contain fewer than 6 tetravinyl monomer residues, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 6 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer.

[0149] Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are 3n+1 chain transfer agent moieties per n tetravinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 4n double bonds per n tetravinyl monomers, the number of double bond residues per chain will on average be 4n / (3n+1) which will tend towards 1.33 as the molecular weight increases.

[0150] The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one tetravinyl monomer residue i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher.

[0151] Optionally, the product may contain a large amount of tetravinyl monomer residues wherein three of the double bond residues are capped with a chain transfer agent (as opposed to being part of a chain), i.e. have a nominal chain length of 1. The other double bond residues of those tetravinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one tetravinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one tetravinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 6, e.g. between 2 and 5, e.g. between 2 and 4, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g. 3 or e.g. 4, tetravinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one tetravinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 6, e.g. between 2 and 5, e.g. between 2 and 4, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g. 3 or e.g. 4, tetravinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 6, e.g. between 3 and 5, e.g. 3 or 4, e.g. 3 or e.g. 4.

[0152] Extent of vinyl polymerisation when using multivinyl monomers in general

[0153] Numerical relationships and theoretical assessments have been presented above for each of divinyl monomers, trivinyl monomers and tetravinyl monomers. In summary, without wishing to be bound by theory, in certain idealised scenarios the average number of multivinyl monomer residues per vinyl polymer chain may be as follows, where the product contains n multivinyl monomer residues:

[0154]

[0155] Thus it can be seen that, as the valency of the monomers increases, the average vinyl chain length is required to decrease.

[0156] In general the following may optionally apply across the various types of multivinyl monomers discussed herein.

[0157] The average vinyl polymer chain length may contain the following number of multivinyl monomer residues: between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.1 and 2, between 1.2 and 2, between 1.3 and 2, between 1.33 and 2, between 1.5 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, between 1.2 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, between 1.45 and 1.5, between 1.1 and 1.4, between 1.2 and 1.4, between 1.2 and 1.33, or between 1.3 and 1.33.

[0158] Whilst the average may optionally be between 1 and 3, a small number of vinyl polymer chains may contain significantly more multivinyl monomer residues, for example as many as 3, 5, 8, 10, 15, 18, 20 or more.

[0159] Optionally 90 % of the vinyl polymer chains contain fewer than 10 multivinyl monomer residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 95% have a length of 5 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer. Optionally, the product may contain a large amount of multivinyl monomer residues wherein all but one of the double bond residues in the multivinyl monomer residue is capped with a chain transfer agent (as opposed to being part of a chain), i.e. has a nominal chain length of 1. The remaining double bond residue of the multivinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one multivinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one multivinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.

[0160] 3, e.g. 4 or e.g. 5 multivinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one multivinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3, e.g. 4 or e.g. 5, multivinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g. 3, 4 or 5.

[0161] Source of radicals

[0162] The source of radicals may be an initiator such as azoisobutyronitrile (Al BN). Optionally the amount used relative to divinyl monomer may be 0.001 to 1, 0.01 to 0.1, 0.01 to 0.05, 0.02 to 0.04 or approximately 0.03 equivalents. In view of the presence of two double bonds per monomer this equates to 0.0005 to 0.5, 0.005 to 0.05, 0.005 to 0.025, 0.01 to 0.02 or approximately 0.015 equivalents relative to double bond.

[0163] It has been found that the reactions proceed effectively when only small amounts of initiator are used. Reducing the amount of initiator means that the reactions may proceed more slowly but still at speeds which are industrially acceptable. Lower amounts of initiator are beneficial in terms of cost, residual effect in the product, and controlling the exotherm to enhance safety and facilitate manageable reactions even when scaled up.

[0164] Other possible sources of radicals include peroxides, organo-boranes, persulfates or UV-initiated systems.

[0165] Reaction conditions The polymers may be prepared under conventional industrial free radical polymerisation conditions. Optionally a solvent such as for example toluene may be used.

[0166] As the reaction conditions become more dilute, the amount of CTA in the product can decrease. Without wishing to be bound by theory, this may be because at greater dilution intramolecular reaction is more likely, meaning that, effectively, reaction of the molecule with itself takes the place of reaction of the molecule with a CTA molecule. Accordingly, this can alter the numerical relationships discussed above, because these assume a theoretical situation in which there is no intramolecular reaction.

[0167] This provides a further way of controlling the chemistry and tailoring the type of product and its properties. For example, whereas in some scenarios it may be desirable to have a large amount of CTA residue in the product, in other scenarios it is desirable not to, for example to reduce the amount of thiol residues. Furthermore, carrying out the same reaction at different dilutions can lead to different physical properties such that for example some products are solids and others are liquids. Ways of manipulating the glass-transition temperature and / or melting temperature can be useful for various applications.

[0168] Residual vinyl content in the polymer

[0169] The polymer is formed by vinyl polymerisation. Said polymerisation may optionally have proceeded to the extent that the polymer product contains very little, substantially no, or no, residual vinyl functionality. Optionally, no more than 20mol%, no more than 10mol%, no more than 5mol%, no more than 2mol%, or no more than 1mol%, of the radically polymerisable double bonds of the multivinyl monomer, e.g. of the divinyl monomer, remain in the polymer.

[0170] Nevertheless, where it is desired to contain unsaturated groups, the reaction may be controlled accordingly, and / or unsaturated groups or other groups may be done by postfunctionalisation.

[0171] Further examples of suitable branched polymers

[0172] The branched polymer may comprise divinyl monomer residues and chain transfer residues, wherein the molar ratio of chain transfer residues to divinyl monomer residues is between 0.5 and 2. The ratio is optionally between 0.7 and 1.5, optionally between 0.75 and 1.3, optionally between 0.8 and 1.2, optionally between 0.9 and 1.1, optionally between 1 and 1.05, optionally approximately 1, or optionally approximately 0.9, or optionally approximately 0.8. Some of the vinyl polymer chains may contain as many as 18, or 15, divinyl monomer residues. Only a small proportion are this long, however: the average, for high molecular weight materials, may be around 2.

[0173] Optionally 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer).

[0174] Thus the branched polymer product may comprise divinyl monomer residues and chain transfer residues, wherein 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer).

[0175] During preparation of the polymer, it is possible that neither of the two carbon atoms of a vinyl group forms a bond to another vinyl group (instead they could form a bond to a CTA residue or hydrogen, or, in some cases, other moiety such as initiator residue or solvent residue), or it is possible that one of the two carbon atoms of a vinyl group forms a bond to another vinyl group, or it is possible that both carbon atoms of a vinyl group form bonds to other vinyl groups. Therefore, in the product, each vinyl residue may be directly linked to 0, 1 or 2 other vinyl residues as closest neighbours. Optionally the branched polymer comprises divinyl monomer residues and chain transfer residues, wherein each vinyl residue is directly vinyl polymerised to on average 0.5 to 1.5 other divinyl monomer residue. Optionally this may be 0.8 to 1.2, 0.8 to 1.1, 0.9 to 1, or approximately 1, on average.

[0176] Thus the polymers are characterised by having a large amount of chain transfer agent incorporation, and also by having short distinct vinyl polymer chains. Whereas, conventionally, a vinyl polymer chain will normally comprise a long saturated backbone, during the preparation of the present polymers - even though they are built up using vinyl polymerisation - most of the double bonds only react with one other double bond, or react with no other double bonds, rather than react with two other double bonds. This means that the linkages between the two double bonds in the monomer, which linkages conventionally bring about branching between polymer chains in the prior art, instead form the backbone of the longest polymer chains in the present branched polymers. The branched polymer may comprise divinyl monomer residues and chain transfer residues, in which there is a multiplicity of vinyl polymer chain segments having an average length of between 1 and 3 divinyl monomer residues.

[0177] The average length may be between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.3 and 2, between 1.5 and 2, between 1.7 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, or approximately 2.

[0178] The skilled person will understand how the number of double bond residues affects the carbon chain length of the resultant vinyl polymer segment. For example, where a polymer chain segment comprises 2 double bond residues, this equates to a saturated carbon chain segment of 4 carbon atoms.

[0179] The incorporation of monovinyl monomers as well as divinyl monomers may affect the average vinyl chain length but does not affect the average number of divinyl monomer residues per chain. It can be a way of increasing the vinyl chains without increasing branching.

[0180] The branched polymer may comprise divinyl monomer residues and chain transfer residues wherein the divinyl monomer residues comprise less than 20mol% double bond functionality.

[0181] In other words, in such polymer products, at least 80% of the double bonds of the divinyl monomers have reacted to form saturated carbon-carbon chains.

[0182] The residues may comprise less than 10mol%, or less than 5mol%, or less than 2mol%, or less than 1mol%, or substantially no, double bond functionality.

[0183] Another way of defining the polymer is in terms of its Mark Houwink alpha value. Optionally, this may be below 0.5.

[0184] The above description of polymer products relates in particular to those containing divinyl monomer residues. Analogously, polymer products containing other multivinyl monomer residues may include for example trivinyl monomer residues and / or tetravinyl monomer residues. Definitions and disclosures herein apply mutatis mutandis.

[0185] The molar ratio, on average, of chain transfer residues to multivinyl monomer residues may optionally be: - for multivinyl monomers generally:

[0186] between 0.5 and 6, between 0.7 and 4.5, between 0.75 and 3.9, between 0.8 and 3.6, between 0.9 and 3.3, between 1 and 3.15, or between approximately 1 and approximately 3;

[0187] - for trivinyl monomers:

[0188] between 1 and 4, between 1.4 and 3, between 1.5 and 2.6, between 1.6 and 2.4, between 1.8 and 2.2, between 2 and 2.1, or approximately 2;

[0189] - for tetravinyl monomers:

[0190] between 1.5 and 6, between 2.1 and 4.5, between 2.25 and 3.9, between 2.4 and 3.6, between 2.7 and 3.3, between 3 and 3.15, or approximately 3.

[0191] Optionally:

[0192] - for multivinyl monomers generally:

[0193] 90 % of the vinyl polymer chains contain fewer than 10 multivinyl monomer residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 95% have a length of 5 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer;

[0194] - for trivinyl monomers:

[0195] 90 % of the vinyl polymer chains contain fewer than 8 TVM residues, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 5 or fewer, or 75% have a length of 8 or fewer, or 75% have a length of 6 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer;

[0196] - for tetravinyl monomers:

[0197] 90 % of the vinyl polymer chains contain fewer than 6 tetravinyl monomer residues, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 6 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer Optionally each vinyl bond is directly vinyl polymerised to on average:

[0198] - for multivinyl monomers generally:

[0199] 0.1 to 1.5, 0.2 to 1.2, 0.825 to 1.1, or approximately 0.3 to 1, other multivinyl monomer residue;

[0200] - for trivinyl monomers:

[0201] 0.2 to 1.3, 0.25 to 1.2, 0.3 to 1 , 0.4 to 0.7, or approximately 0.5, other trivinyl monomer residue;

[0202] - for tetravinyl monomers:

[0203] 0.1 to 1, 0.2 to 0.8, 0.25 to 0.5, or approximately 0.3, other tetravinyl monomer residue.

[0204] Optionally the branched polymer product comprises a multiplicity of vinyl polymer chain segments having an average length of:

[0205] - for multivinyl monomers generally:

[0206] between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.1 and 2, between 1.2 and 2, between 1.3 and 2, between 1.33 and 2, between 1.5 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, between 1.2 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, between 1.45 and 1.5, between 1.1 and 1.4, between 1.2 and 1.4, between 1.2 and 1.33, or between 1.3 and 1.33 multivinyl monomer residues;

[0207] - for trivinyl monomers:

[0208] between 1 and 2, between 1 and 1.8, between 1 and 1.7, between 1 and 1.5, between 1.1 and 1.5, between 1.2 and 1.5, between 1.25 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, or between 1.45 and 1.5, or of approximately 1.5, trivinyl monomer residues;

[0209] - for tetravinyl monomers:

[0210] between 1 and 1.7, between 1 and 1.5, between 1 and 1.4, between 1 and 1.33, between 1.1 and 1.33, between 1.2 and 1.33, between 1.25 and 1.33, or between 1.3 and 1.33, or of approximately 1.33, tetravinyl monomer residues.

[0211] The incorporation of monovinyl monomers as well as multivinyl monomers may affect the average vinyl chain length but does not affect the average number of multivinyl monomer residues per chain. It can be a way of increasing the length of the vinyl chains without increasing branching. Optionally a branched polymer product comprises multivinyl monomer residues and chain transfer residues wherein the multivinyl monomer residues comprise less than 20mol% double bond functionality. The residues may comprise less than 10mol%, or less than 5mol%, or less than 2mol%, or less than 1mol%, or substantially no, double bond functionality.

[0212] Figures

[0213] Non-limiting examples in accordance with the present invention will now be described in further detail with reference to the following Figures in which:

[0214] Figure 1 shows two possible methods for incorporation of multiple units of a photoinitiator into a branched polymer in accordance with embodiments of the present invention;

[0215] Figures 2a and 2b show greater structural detail in respect of an embodiment according to the first (upper) method of Figure 1;

[0216] Figure 3 shows an example of a working cure plot in relation to a polymeric photoinitiator of the present invention;

[0217] Figures 4 and 5 show photocuring parameters of polymeric photocuring products of the present invention in comparison with small molecule photocuring products;

[0218] Figures 6 and 7 show DSC curves in respect of photocurable polymeric products of the present invention in comparison with corresponding small molecules;

[0219] Figure 8 shows relative leaching of a polymeric photoinitiator product in accordance with the present invention in comparison with a small molecule equivalent; and

[0220] Figure 9 is a photograph showing an insoluble small molecule photoinitiator in isobornyl acrylate (left) compared to a soluble polymeric photoinitiator in accordance with the present invention in isobornyl acrylate (right).

[0221] Examples

[0222] The examples which follow illustrate the invention in more detail, without limiting the invention scope to such chosen examples. A range of examples is provided, spanning synthetic procedures, measurements and application data relevant to the invention. Synthesis of photoinitiator and photosynergist small molecule precursors

[0223] Small molecules containing three different photoinitiator functionalities are the subject of examples 1 to 5 below. A small molecule containing an example of a photosynergist functionality is the subject of example 6. Each compound of examples 3 to 6 contains a vinyl functionality as well as a photoinitiator (or photosynergist) functionality: this provides reactivity by which they may polymerised into a vinyl polymer, as shown in subsequent examples.

[0224] It will be seen that one of the series of examples relates to the incorporation into TBRT polymers of photoinitiator units based on acylphosphine oxide photoinitiator TPO-L.

[0225] Example 1 : Preparation of sodium phenyl(2,4,6-trimethylbenzoyl)phosphinate (PI-1)

[0226]

[0227] Sodium iodide (1.00 equiv.) and ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (TPO-L, 1.00 equiv.) were dissolved in butanone (70% w / w) under stirring at ambient temperature in a suitably-sized reaction flask. The flask was heated to 70 °C and left for 24 h. A precipitate formed after ~ 10 min. After cooling, the precipitate was isolated by vacuum filtration, washed with butanone and dried in vacuo at 40 °C for 24 h. A white powder product was obtained in 90% yield.1H,13C and31P NMR were used to confirm successful preparation.

[0228] Example 2: Preparation of phenyl(2,4,6-trimethylbenzoyl)phosphinic acid (PI-2)

[0229]

[0230] Sulfuric acid (1 M) was slowly added to a solution of PI-1 in deionised water (~ 80% w / w) under vigorous stirring until pH ~ 1 was achieved. The product gradually precipitated during the addition. The precipitate was isolated by vacuum filtration, washed with deionised water and dried in vacuo at 60 °C for 48 h. A pale-yellow powder product was obtained in 96% yield.

[0231] 1H,13C and31P NMR were used to confirm successful preparation.

[0232] Example 3: Preparation of 2-hydroxy-3-{[phenyl(2,4,6-trimethylbenzoyl)phosphoryl]oxy}propyl 2-methylprop-2-enoate (PI-3)

[0233]

[0234] Glycidyl methacrylate (1.25 equiv.), phenyl(2,4,6-trimethylbenzoyl)phosphinic acid (1.00 equiv.), inhibitor mixture (1000 ppm) and butanone (67% w / w) were mixed under stirring at ambient temperature in a suitably-sized reaction flask. The flask was heated to 60 °C to obtain a clear solution and left for 16 h to yield a crude pale-yellow solution of the product.1H NMR spectroscopy was used to determine reaction extent of the phosphinic acid (98%), epoxide (96%) and methacrylate (< 1%) functionalities.

[0235] Example 4: Preparation of 2-[4-(2-hydroxy-2-methylpropanoyl)phenoxy]ethyl 2-methylprop-2-enoate (PI- 4)

[0236]

[0237] 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methylpropan-1-one (Irgacure 2959, 1.00 equiv.), DMAP (0.40 equiv.) and DCM (90% w / w) were charged to a suitably-sized reaction flask. The reaction was cooled to 0°C with stirring. Neat methacrylic anhydride (1.10 equiv.) was charged to the stirred, cooled reaction mix at such a rate as to maintain the temperature below 5°C. The resultant clear solution was stirred out for 1 h at 0°C then allowed to warm to room temperature overnight with stirring. The reaction was sampled for thin-layer chromatography. The reaction mixture was quenched with deionised water and left to stir for 1 h. The reaction mix was washed with aqueous sodium carbonate (1 M), saturated brine and deionised water, respectively. The DCM solution was subsequently dried using magnesium sulfate, filtered and 4-methoxyphenol (500 ppm) was added. The solvent was removed under vacuum prior to purification by column chromatography (silica, 9:1 petroleum ether / ethyl acetate) to yield a yellow viscous liquid in 58% yield.1H and13C NMR spectroscopy, elemental analysis and mass spectroscopy were used to confirm successful preparation. Example 5: 3-(3-benzoylbenzoyloxy)-2-hydroxypropyl 2-methylprop-2-enoate (PI- 5)

[0238]

[0239] 3-benzoylbenzoic acid (1.00 equiv.), CTAB (0.05 equiv.), butylated hydroxytoluene (0.05 equiv.) and ethyl acetate (50% w / w) were charged to a suitably-sized reaction flask. The reaction was heated to 70 °C (reflux) with stirring. Glycidyl methacrylate (1.20 equiv.) was charged to the stirred, refluxing reaction mix dropwise. The resultant cloudy solution was stirred for 24 h. Following stirring, the solution exhibited a cloudy to clear yellow colour change. The reaction was sampled for thin-layer chromatography. The solvent was removed under vacuum prior to purification by column chromatography (silica, 9:1 petroleum ether / ethyl acetate) to yield a yellow viscous liquid in 90% yield.1H and13C NMR spectroscopy and mass spectroscopy were used to confirm successful preparation.

[0240] Example 6: Preparation of 2-hydroxy-3-({2-hydroxy-3-[(2-methylprop-2-enoyl)oxy]propyl} (phenyl)amino)propyl 2-methylprop-2-enoate (PS-1)

[0241]

[0242] A solution of glycidyl methacrylate (2.20 equiv.) in xylene (40% w / w) was added dropwise to a solution of p-toluidine (1.00 equiv.), triphenyl phosphine (1000 ppm), inhibitor mixture (400 ppm) and xylene (40% w / w) under stirring at 120 °C in a suitably-sized reaction flask. The dropwise addition was performed over 2 h. After addition, the reaction mixture was left to stir at 120 °C for 16 h to yield a crude orange solution of the product. Gas chromatography with flame ionisation detection (GC-FID) and1H NMR spectroscopy were used to determine reaction extent of the p-toluidine (> 99%), epoxide (90%) and methacrylate (<1%) functionalities.

[0243] Synthesis of branched polymeric photoinitiators and photosynergists using transfer-dominated branching radical telomerisation

[0244] Figure 1 exemplifies two possible synthetic routes to branched polymeric photoinitiators and / or photosynergists prepared by transfer-dominated branching radical telomerisation: (a) addition of polymerisable photoinitiator or photosynergist to TBRT reaction (top), and (b) grafting photoinitiator or photosynergist onto pre-formed TBRT polymer (bottom). Figure 1 shows the two routes for a system based on certain components including a photoinitiator derived from TPO-L, but the same methodology also applies to systems based on other components including with other photoinitiator types or photosynergists.

[0245] Figure 2a shows an example of the former route in greater detail, namely a branched polymeric photoinitiator prepared by addition of a photoinitiator monovinyl monomer [PI-3 (white spheres)] to a transfer-dominated branching radical telomerisation of a divinyl monomer [ethylene glycol dimethacrylate (black spheres)] in the presence of a chain transfer agent [butyl 3-mercaptopropionate (black cubes)]. The full detail of the chemical structure of a fragment of the polymer (within the ellipse of Figure 2a) is shown in Figure 2b.

[0246] As noted above, each compound of examples 3 to 6 contains a vinyl functionality as well as a photoinitiator (or photosynergist) functionality. This makes each of these compounds suitable for incorporation into a polymer, by using the compound as monovinyl monomer, together with a multivinyl monomer and a chain transfer agent, in a free radical vinyl polymerisation procedure. Alternatively it is possible to post-functionalise a polymer which has a functionality (e.g. epoxide functionality) reactive with certain functionality in a small molecule which also comprises a photoinitiator or photosynergist, thereby grafting the photoinitiator / photosynergist functionality onto a pre-formed polymer. This is exemplified here using PI-2. In both cases the photoinitiator or photosynergist functionality is covalently bound in or to the polymer scaffold.

[0247] Branched polymeric photoinitiators and / or photosynergists can be prepared by addition of photoinitiator and / or photosynergist bearing polymerisable group(s) to a transfer-dominated branching radical telomerisation (e.g., Examples 7 to 10, 12, 13 and 15) or by grafting photoinitiator and / or photosynergist groups onto a pre-formed branched polymer (e.g., Examples 11 and 14).

[0248] Example 7: Preparation of a branched polymeric photoinitiator containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties by addition of PI-3 to a transfer-dominated branching radical telomerisation (PPI-6)

[0249] PI-3 (1.00 equiv.) was added to a solution of ethylene glycol dimethacrylate (1.00 equiv.) and butyl 3-mercaptopropionate (1.43 equiv.) in butanone (50% w / w) at ambient temperature in a suitably-sized reaction flask. The solution was heated to 80 °C under stirring and a low N2 flow. Once the temperature had stabilised, Al BN (2.0 mol% equiv. w.r.t. methacrylate groups) was added (t = 0) and the reaction solution was left to stir at 80 °C, under a low N2 flow, for 6 h.1H NMR spectroscopy and GC-FID were used to determine depletion of methacrylate (> 99%), acylphosphine oxide (<1%) and thiol (65%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into hexane under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. A pale-yellow, solid product was obtained with Mn= 5,128 g rnof1and MVJ= 55,7Q7 g mol'1(TD-SEC).

[0250] Example 8: Preparation of an amphiphilic branched polymeric photoinitiator containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties by addition of PI-3 to a transfer-dominated branching radical telomerisation (PPI-7)

[0251] PI-3 (2.00 equiv.) was added to a solution of tetraethylene glycol dimethacrylate (1.00 equiv.) and 1-dodecanthiol (2.00 equiv.) in butanone (50% w / w) at ambient temperature in a suitably-sized reaction flask. The solution was heated to 80 °C under stirring and a low N2 flow. Once the temperature had stabilised, Al BN (2.0 mol% equiv. w.r.t. methacrylate groups) was added (t = 0) and the reaction solution was left to stir at 80 °C, under a low N2 flow, for 6 h.1H NMR spectroscopy and GC-FID were used to determine depletion of methacrylate (> 99%), acylphosphine oxide (<1%) and thiol (68%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into hexane under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. A colourless, viscous liquid product was obtained with Mn= 4,180 g mol-1and MVJ= 23,034 g mol-1(TD-SEC).

[0252] Example 9: Preparation of a branched polymeric photoinitiator containing pendant Irgacure 2959 moieties by addition of PI-4 to a transfer-dominated branching radical telomerisation (PPI-8a-c)

[0253] PI-4 (x equiv., where x = 0.10 [a], 0.50 [b] or 1.0 [c]) was added to a solution of ethylene glycol dimethacrylate (1.00 equiv.), 1-dodecanthiol (1.25 equiv.) and AIBN (1.5 mol% equiv. w.r.t. methacrylate groups) in ethyl acetate (50% w / w) at ambient temperature in a suitably-sized reaction flask. The solution was deoxygenated using a nitrogen purge under stirring for 30 minutes before being heated to 70 °C and left to stir for 24 h.1H NMR spectroscopy was used to determine depletion of methacrylate groups (> 99%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was diluted in THF and precipitated in methanol at ambient temperature, before drying in vacuo at 40 °C for 24 h. A white to off-white solid product was obtained. Molecular weights varied with PI-4 equiv.: Mn= 5,600 g mol’1and Mw= 360,000 g mol’1for x = 0.10 (a); Mn= 7,500 g mol’1and / Ww= 138,000 g mol’1for x = 0.50 (b); and Mn= 9,600 g mol’1and MVJ= 111,000 g mol’1for x = 1.00 (c) (TD-SEC). Example 10: Preparation of a branched polymeric photoinitiator containing pendant 3-benzoylbenzoate moieties by addition of PI-5 to a transfer-dominated branching radical telomerisation (PPI-9a-c)

[0254] PI-5 (x equiv., where x = 0.10 [a], 0.50 [b] or 1.0 [c]), was added to a solution of ethylene glycol dimethacrylate (1.00 equiv.), 1-dodecanthiol (1.25 equiv.) and AIBN (1.5 mol% equiv. w.r.t. methacrylate groups) in ethyl acetate (50% w / w) at ambient temperature in a suitably-sized flask. The solution was deoxygenated using a nitrogen purge under stirring for 30 minutes before being heated to 70 °C and left to stir for 24 h.1H NMR spectroscopy was used to determine depletion of methacrylate groups (> 99%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was diluted in THF and precipitated in methanol at ambient temperature, before drying in vacuo at 40 °C for 24 h. A white to off-white, solid product was obtained. Molecular weights varied with PI-5 equiv.: Mn= 3,700 g mol-1and Mw= 332,000 g mol'1for x = 0.10 (a); Mn= 4,300 g mol'1and / Ww= 133,000 g mol'1for x = 0.50 (b); and Mn= 3,700 g mol'1and MVJ= 98,000 g mol'1for x = 1.00 (c) (TD-SEC).

[0255] Example 11: Preparation of a branched polymeric photoinitiator containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties by grafting PI-2 onto an epoxy branched polymer prepared by transfer-dominated branching radical telomerisation (PPI-10)

[0256] A dried epoxy-functional branched polymer was prepared by the transfer-dominated branching radical telomerisation of ethylene glycol dimethacrylate (1.00 equiv.), glycidyl methacrylate (1.00 equiv.) and butyl 3-mercaptopropionate (1.43 equiv.) in xylene (50% w / w) at 150 °C using di(t-butyl) peroxide (2.0 mol% equiv. w.r.t. methacrylate groups) as a radical source. Said TBRT epoxy polymer (1.00 equiv.), phenyl(2,4,6-trimethylbenzoyl)phosphinic acid (1.00 equiv.) and butanone (50% w / w) were mixed under stirring at ambient temperature in a suitably-sized reaction flask. The flask was heated to 60 °C to obtain a clear solution and left for 16 h to yield a pale-yellow, low viscosity solution.1H NMR spectroscopy was used to determine reaction extent of the phosphinic acid (76%) and epoxide (> 99%) functionalities. To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into hexane under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. A pale-yellow, solid product was obtained with Mn= 29,337 g mol'1and MVJ= 39,810 g mol'1(TD-SEC).

[0257] Example 12: Preparation of a branched polymeric photosynergist containing backbone aromatic tertiary amine moieties by transfer-dominated branching radical telomerisation of PS-1 (PPS-2) PS-1 (1.00 equiv.) and 1 -dodecanethiol (4.00 equiv.) were dissolved in xylene (50% w / w) by stirring at ambient temperature in a suitably-sized reaction flask. The solution was heated to 80 °C under stirring and a low N2 flow. Once the temperature had stabilised, AIBN (2.0 mol% equiv. w.r.t. methacrylate groups) was added (t = 0) and the reaction solution was left to stir at 80 °C, under a low N2 flow, for 3 h.1H NMR spectroscopy and GC-FID were used to determine depletion of methacrylate (> 99%) and thiol (19%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into MeOH under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. A pale-yellow, powder product was obtained.

[0258] Example 13: Preparation of a branched polymeric photosynergist containing pendant aliphatic tertiary amine moieties by addition of 2-(diethylamino)ethyl methacrylate to a transfer-dominated branching radical telomerisation (PPS-3)

[0259] 2-(diethylamino)ethyl methacrylate (2.00 equiv.) was added to a solution of tetraethylene glycol dimethacrylate (1.00 equiv.) and 1 -dodecanethiol (2.00 equiv.) in butanone (50% w / w) at ambient temperature in a suitably-sized reaction flask. The solution was heated to 80 °C under stirring and a low N2 flow. Once the temperature had stabilised, AIBN (3.0 mol% equiv. w.r.t. methacrylate groups) was added (t = 0) and the reaction solution was left to stir at 80 °C, under a low N2 flow, for 6 h.1H NMR spectroscopy and GC-FID were used to determine depletion of methacrylate (98%) and thiol (75%). To obtain a sample of dried polymer for analysis and testing, the solvent was removed under vacuum at 40 °C. An orange, liquid product was obtained with Mn= 2,857 g mol'1and MVJ= 5,533 g mol'1(TD-SEC).

[0260] Example 14: Preparation of a branched polymeric photosynergist containing pendant aromatic tertiary amine moieties by grafting / V-methyl-p-toluidine onto an epoxy branched polymer prepared by transfer-dominated branching radical telomerisation (PPS-4)

[0261] A dried epoxy-functional branched polymer was prepared by the transfer-dominated branching radical telomerisation of ethylene glycol dimethacrylate (1.00 equiv.), glycidyl methacrylate (1.00 equiv.) and 1 -dodecanethiol (1.25 equiv.) in xylene (50% w / w) at 150 °C using di(t-butyl) peroxide (1.0 mol% equiv. w.r.t. methacrylate groups) as a radical source. Said TBRT epoxy polymer (1.00 equiv.) and / V-methyl-p-toluidine (1.00 equiv.) were dissolved in xylene (50% w / w) under stirring at ambient temperature in a suitably-sized reaction flask. The flask was heated to 150 °C and left for 24 h to yield an orange, low viscosity solution.1H NMR spectroscopy was used to determine reaction extent of the amine (82%) and epoxide (> 99%) functionalities. To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into methanol under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. A yellow, viscous liquid product was obtained with Mn= 11,190 g mol-1and / Ww= 31,870 g mol-1(TD-SEC).

[0262] Example 15: Preparation of a “one-component” branched polymeric photoinitiator containing pendant type II photoinitiator and photosynergist moieties by addition of 2-(diethylamino)ethyl methacrylate to a transfer-dominated branching radical telomerisation of a type II photoinitiator multivinyl monomer (PPI-PPS-1)

[0263] 2-(diethylamino)ethyl methacrylate (2.00 equiv.) was added to a solution of photoinitiator multivinyl monomer (1.00 equiv.) and butyl 3-mercaptopropionate (3.00 equiv.) in ethyl acetate (50% w / w) at ambient temperature in a suitably-sized reaction flask. The solution was heated to 80 °C under stirring and a low N2 flow. Once the temperature had stabilised, Al BN (3.0 mol% equiv. w.r.t. methacrylate groups) was added (t= 0) and the reaction solution was left to stir at 80 °C, under a low N2 flow, for 6 h.1H NMR spectroscopy and GC-FID were used to determine depletion of methacrylate (83%) and thiol (87%). To obtain a sample of purified polymer for analysis and testing, the crude reaction solution was precipitated into hexane under vigorous stirring at ambient temperature, before drying in vacuo at 40 °C for 48 h. An orange, viscous liquid product was obtained with Mn= 4,080 g mol-1and MVJ= 28,125 g mol-1(TD-SEC).

[0264] Assessing photoreactivity of branched polymeric photoinitiators and photosynergists under LED wavelengths

[0265] Example 16: Photoreactivity evaluation of an amphiphilic branched polymeric photoinitiators containing pendant phenyl(2, 4, 6-trimethylbenzoyl)phosphinate moieties (PPI-7 from Example 8) using 3D printing working curves

[0266] Key photocuring parameters of PPI-7, a TBRT-derived polymeric photoinitiator containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties, were obtained using 3D printing working curves, as described below. 1,6-hexanediol diacrylate (HDDA) and polyethylene glycol diacrylate (PEGDA, Mn= 575 g mol-1) were chosen as the resin systems for assessment. PPI-7 showed good solubility in both HDDA (hydrophobic) and PEGDA (hydrophilic). The tests were conducted at constant photoinitiator molar equivalent, relative to TPO-L small molecule added at 1.0% w / w w.r.t. HDDA / PEGDA.

[0267] 3D printing working curves Working curves were used to obtain the critical energy (Ec) and penetration depth (Dp), which are key characteristics used to determine printability and printer settings for successful 3D printing. Ecrepresents the minimal energy required to initiate polymerisation at the resin surface (i.e., inversely proportional to resin reactivity). Dprepresents the average distance the light source travels through the resin (i.e., proportional to resin sensitivity). All working curves were obtained using the material spot test on an Asiga UV Max digital light processing printer (A = 385 nm).

[0268] Material spot test procedure

[0269] The build platform and vat was removed from the printer and the light intensity was set to I = 5.0 mW cm-2. A fluorinated ethylene propylene (FEP) vat 3D printing film, with known thickness, was placed on the printer base and a small circle of resin (d~ 2 cm) was added to the centre. The printer’s material spot test was selected (d= 3 mm) for chosen exposure time (e.g., 10 s). Once completed the film was removed and uncured resin was carefully removed using isopropyl alcohol and a tissue. The thickness (cure depth, Cd) of the cured spot was subsequently measured using a thickness gauge. This process was repeated 5 more times, at varying exposure times (energies), to obtain a working curve from which Ecand Dpcan be calculated.

[0270] Calculating Ecand Do

[0271] Ecand Dpwere obtained using principles of Jacobs’ basic working curve equation (P. F. Jacobs, J. Manuf. Syst., 1993, 12,

[0272]

[0273] where Cd is the depth of cure (thickness) and Eo is the light energy dosage at the surface (Eo is equal to light intensity multiplied by exposure time).

[0274] For each system, Cd was plotted against In(Eo). The gradient of the linear pot was taken as Dpwhilst the x-intercept (Cd = 0) was taken as Eo.

[0275] Figure 3 provides an example of a working cure plot, specifically that of PPI-7 (TBRT-derived polymeric photoinitiator) added at 2.7% w / w in HDDA.

[0276] Table 1: Key details of TPO-L small molecule photoinitiator and PPI-7 (a TBRT-derived toinitiator). Quoted viscosity at T= 20 °C and y = 10 s~1.

[0277]

[0278]

[0279] Figure 4 shows that the photocuring parameters of PPI-7 (TBRT-derived polymeric photoinitiator containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties) are comparable to TPO-L small molecule in HDDA (black) and PEGDA (white) at constant photoinitiator molar equivalent.

[0280] Example 17: Photoreactivity evaluation of a “one-component” branched polymeric photoinitiator containing pendant type II photoinitiator and photosynergist moieties (PPI-PPS-1 from Example 15) using 3D printing working curves

[0281] Key photocuring parameters of PPI-PPS-1, a TBRT-derived polymeric photoinitiator containing pendant type II photoinitiator and photosynergist moieties, were obtained using 3D printing working curves, as described previously. 1,6-hexanediol diacrylate (HDDA) and polyethylene glycol diacrylate (PEGDA, Mn= 575 g mol-1) were chosen as the resin systems for assessment. PPI-PPS-1 showed good solubility in both HDDA (hydrophobic) and PEGDA (hydrophilic). The tests were conducted at constant photoinitiator molar equivalent, relative to a type II photoinitiator small molecule reference added at 0.3% w / w w.r.t. HDDA / PEGDA. Importantly, for the systems containing the reference type II photoinitiator small molecule, an amine photosynergist small molecule (1.0% w / w w.r.t. HDDA / PEGDA) was also required. This was not added to the systems containing PPI-PPS-1 as the polymer contains amine photosynergist moieties.

[0282] Table 2: Key details of type II photoinitiator small molecule reference and PPI-PPS-1 (a TBRT-derived polymeric type II photoinitiator).

[0283]

[0284] Figure 5 compares the photocuring parameters of PPI-PPS-1 (TBRT-derived polymeric photoinitiator containing pendant type II photoinitiator and photosynergist moieties) to a small molecule type II photoinitiator reference in HDDA (black) and PEGDA (white) at constant photoinitiator molar equivalent. It is important to note that no additional amine photosynergist was added to the systems containing PPI-PPS-1. This demonstrates the opportunity for “one-component” type II photoinitiating systems using TBRT technology.

[0285] Example 18: Photoreactivity evaluation of a branched polymeric photoinitiator containing pendant Irgacure 2959 moieties (PPI-8b from Example 9) using photo-differential scanning calorimetry

[0286] Photoreactivity of PPI-8b, a TBRT-derived polymeric photoinitiator containing pendant Irgacure 2959 moieties, was investigated using photo-differential scanning calorimetry (photo-DSC). 1,6-hexanediol diacrylate (HDDA) was chosen as the resin system for assessment.

[0287] PPI-8b was added to the resin at 0.8 photoinitiator molar equivalent, relative to Irgacure 2959 small molecule added at 1.0% w / w w.r.t. HDDA. Heat flow was used to indicate degree of polymerisation (exothermic).

[0288] Photo-DSC analysis

[0289] Photo-DSC analysis was conducted using a TA Instruments Discovery DSC 25 instrument equipped with a Photocalorimeter Accessory (PCA). The experiments were conducted in an isothermal mode (30 °C) with a UV intensity of 20 mW / cm2under N2 atmosphere. The samples were accurately weighed into an uncovered aluminium pan (5 ± 1 mg) in darkness and analysed immediately. An empty, uncovered aluminium pan was employed as a reference. After equilibration, samples were irradiated with unfiltered UV light (320-500 nm) for 10.0 sec. The heat flow of the photopolymerization was recorded as a function of time.

[0290] Table 3: Key details of Irgacure 2959 small molecule photoinitiator and PPI-8b (a TBRT-

[0291]

[0292] Figure 6 shows that the heat of polymerisation from the photocuring of HDDA with PPI-8b (TBRT-derived polymeric photoinitiator containing pendant Irgacure 2959 moieties) (solid line) was comparable to that with Irgacure 2959 small molecule (dashed line), i.e. that similar degrees of polymerisation were achieved. This is despite the addition of PPI-8b at lower photoinitiator molar equivalent (0.8) relative to Irgacure 2959 small molecule. Negligible heat evolution was observed when no photoinitiator was present in HDDA.

[0293] Example 19: Photoreactivity evaluation of a branched polymeric photoinitiator containing pendant 3-benzoylbenzoate moieties (PPI-9b from Example 10) using photo-differential scanning calorimetry

[0294] Photoreactivity of PPI-9b, a TBRT-derived polymeric photoinitiator containing pendant 3-benzoyl benzoate moieties, was investigated using photo-differential scanning calorimetry (photo-DSC). 1,6-hexanediol diacrylate (HDDA) was chosen as the resin system for assessment. The tests were conducted at constant photoinitiator molar equivalent, relative to 3-benzoylbenzoic acid small molecule added at 0.5% w / w w.r.t. HDDA. Heat flow was used to indicate degree of polymerisation (exothermic).

[0295] Table 4: Key details of benzophenone small molecule photoinitiator and PPI-9b (a TBRT-

[0296] "

[0297]

[0298] Figure 7 shows that the heat of polymerisation from the photocuring of HDDA with PPI-9b (TBRT-derived polymeric photoinitiator containing pendant 3-benzoylbenzoate moieties) (solid line) was greater than that with benzophenone small molecule (dashed line) at constant photoinitiator molar equivalent - i.e., higher degree of polymerisation was achieved with PPI-9b. Negligible heat evolution was observed when no photoinitiator was present in HDDA.

[0299] Assessing 3D printability and photoinitiator migration from 3D printed specimen

[0300] Example 19: Vat photopolymerisation 3D printing of test specimens using TBRT-derived polymeric photoinitiators containing pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties 3D printing was conducted on Asiga UV Max digital light processing printer (A = 385 nm, I = 5.0 mW cm-2, z increment = 50 pm). Photocuring parameters, obtained using working curves, were used to determine the optimal print parameters for each photoinitiating system. Dynamic mechanical thermal analysis (DMTA) test specimens (56 x 13 x 3 mm3cuboids) were successfully printed from HDDA and PEGDA resins photoinitiated by TBRT-derived polymeric photoinitiators. All photoinitiators were added at constant photoinitiator molar equivalent, relative to TPO-L small molecule added at 1.0% w / w w.r.t. HDDA / PEGDA. Each specimen was removed from the build plate, cleaned in a fresh isopropyl alcohol bath for 2 min and UV post-cured in post-processing station at 60 °C for 1 h (A = 405 nm).

[0301] Example 3D prints were produced using PEGDA, containing either: TPO-L small molecule (1), TBRT-derived polymeric photoinitiators (2-4), or commercial non-TBRT polymeric photoinitiator (5). This demonstrated 3D printability using TBRT-derived polymeric photoinitiators. Further, those printed using a TBRT-derived polymeric photoinitiator (2-4) were less coloured (less yellow) than those printed using either TPO-L small molecule (1) or a commercial non-TBRT polymeric photoinitiator (5).

[0302] Example 20: Migration studies on 3D printed HDDA specimen containing TBRT-derived polymeric photoinitiator with pendant phenyl(2,4,6-trimethylbenzoyl)phosphinate moieties (PPI-7 from Example 8)

[0303] DMTA specimens were 3D printed from HDDA resin containing either PPI-7 or TPO-L reference, in line with the procedure provided in Example 20. This ensured that representative and reproducible test specimens were obtained for migration studies. The specimen were added to 14 mL vials (2 specimen per vial) with chloroform (12.5 mL). The vials were continuously shaken (500 rpm) at 50 °C for 96 h on a VWR Incubating Microplate Shaker. Once cooled, the chloroform was allowed to evaporate under ambient conditions, and the residue was weighed to determine total leached mass and subsequently analysed for phosphorus content by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0304] Inductively coupled plasma optical emission spectroscopy (ICP-OES)

[0305] Phosphorus contents were determined using an Agilent 5110 ICP-OES instrument on radial view (A = 186, 214 & 215 nm). Samples for ICP-OES were digested in a mixture of concentrated nitric acid (9 mL) and hydrogen peroxide (1 mL) using an Ethos Easy Milestone Connect microwave digestion system (T = 210 °C, t= 2 x 15 min hold, P = 1800 W). Figure 8 displays the relative phosphorus residue by ICP-OES analysis (white bars) and total residue by gravimetric analysis (black diamonds) from migration studies on 3D printed HDDA specimens in chloroform. The graph has been normalised to TPO-L phosphorus leaching (2.34% w.r.t. added phosphorus) and total leaching from TPO-L specimen (0.60% w.r.t. specimen mass). This data shows the considerably lower product migration that can be achieved, particularly from the photoinitiator component, when a TBRT-derived polymeric photoinitiator is used in place of TPO-L small molecule.

[0306] Assessing compatibility of branched polymeric photoinitiators in acrylic monomers

[0307] Example 21: Comparison of the solubility of a TBRT-derived polymeric photoinitiator containing pendant Irgacure 2959 moieties (PPI-8c from Example 9) and Irgacure 2959 small molecule in isobornyl acrylate

[0308] PPI-8c and Irgacure2959 were both charged to a suitably-sized sample vial at constant photoinitiator molar equivalent, relative to Irgacure 2959 small molecule added at 1.0% w / w w.r.t. isobornyl acrylate (IBOA). IBOA is a common reactive diluent used in photocurable technologies. The vials were rolled at ambient temperature for 2 h before assessing their solubility. Irgacure 2959 demonstrated limited solubility in IBOA, whereas PPI-8c was fully soluble in IBOA.

[0309] Figure 9 is a photograph showing poorly soluble Irgacure 2959 in isobornyl acrylate (left) compared to fully soluble PPI-8c in isobornyl acrylate (right).

Claims

CLAIMS1. A branched polymer prepared by free radical vinyl polymerisation comprising residues of a multivinyl monomer, residues of a chain transfer agent, and optionally residues of a monovinyl monomer, and wherein said branched polymer comprises multiple units of a covalently-bound photoinitiator.

2. A branched polymer as claimed in claim 1 which is a TBRT polymer.

3. A branched polymer as claimed in claim 1 or claim 2, wherein said photoinitiator is a free radical Norrish type I photoinitiator, a free radical Norrish type II photoinitiator, or a cationic photoinitiator.

4. A branched polymer as claimed in any preceding claim, wherein said photoinitiator is covalently bound via a residue of a reactive group, for example via a residue of an epoxide, hydroxyl, acid or unsaturated group.

5. A branched polymer as claimed in any preceding claim, wherein said branched polymer additionally comprises multiple units of covalently-bound photosynergist.

6. A branched polymer as claimed in claim 5, wherein said photosynergist is covalently bound via a residue of a reactive group, for example via an epoxide, hydroxyl, acid or unsaturated group.

7. Use of a branched polymer as claimed in any preceding claim for photocuring.

8. A photocurable composition comprising a branched polymer as claimed in any of claims 1 to 6 and a curable material.

9. A kit of parts comprising a branched polymer as claimed in any of claims 1 to 6 and one or more of a photosynergist and a curable material.

10. A photocurable composition as claimed in claim 8 or a kit of parts as claimed in claim 9, wherein the curable material is an acrylate, methacrylate, vinyl ether, epoxide or oxirane.

11. A method of preparing a branched polymer as claimed in any of claims 1 to 6, comprising the free radical polymerisation of a multivinyl monomer and optionally a monovinyl monomer in the presence of a chain transfer agent, using a source of radicals, wherein thephotoinitiator is present on the monovinyl monomer, multivinyl monomer or chain transfer agent.

12. A method as claimed in claim 11, wherein the photoinitiator is present on the monovinyl monomer.

13. A method as claimed in claim 11 or claim 12, comprising a preceding step of conjugating the photoinitiator to the monovinyl monomer, multivinyl moiety or chain transfer agent, optionally via an epoxide ring-opening reaction, esterification reaction, transesterification reaction, or reaction involving an unsaturated group.

14. A method of preparing a branched polymer as claimed in any of claims 1 to 6, comprising the free radical polymerisation of a multivinyl monomer and optionally a monovinyl monomer in the presence of a chain transfer agent, using a source of radicals, and subsequently grafting the photoinitiator onto said branched polymer.

15. A method as claimed in claim 14, wherein the photointiator is grafted onto the branched polymer via suitable functionality, optionally via an epoxide ring-opening reaction, esterification reaction, transesterification reaction, or reaction involving an unsaturated group.

16. A method as claimed in any of claims 11 to 15, wherein the free radical polymerisation is TBRT.

17. Use of a branched polymer for photocuring, wherein said branched polymer is prepared by free radical vinyl polymerisation, and comprises residues of a multivinyl monomer, residues of a chain transfer agent, and optionally residues of a monovinyl monomer, wherein said branched polymer comprises multiple units of a covalently-bound photosynergist.

18. Use as claimed in claim 17, wherein said branched polymer is a TBRT polymer.

19. Use as claimed in claim 17 or claim 18, wherein said photosynergist is covalently bound via a residue of a reactive group, for example via a residue of an epoxide, hydroxyl, acid or unsaturated group.

20. A photocurable composition comprising a branched polymer as defined in any of claims 17 to 19 and one or more of a photoinitiator and a curable material.

21. A kit of parts comprising a branched polymer as defined in any of claims 17 to 19 and one or more of a photoinitiator and a curable material.

22. A photocurable composition as claimed in claim 20 or a kit of parts as claimed in claim 21, wherein the curable material is an acrylate, methacrylate, vinyl ether, epoxide or oxirane.

23. Use, composition or kit of parts as claimed in any of claims 17 to 22, wherein the photosynergist is present on the residue of monovinyl monomer, the residue of the multivinyl monomer, or the residue of the chain transfer agent, optionally wherein said photosynergist is covalently bound via a residue of a reactive group, for example via a residue of an epoxide, hydroxyl, acid or unsaturated group.

24. Use of a branched polymer as claimed in any of claims 1 to 6 or as defined in any of claims 17 to 19, a photocurable composition as claimed in claim 8, 20 or 22, or a kit of parts as claimed in claim 9, 21 or 22, for printing, for example 3D printing or inkjet printing, or for coating, adhesive, electronic, dentistry or biomedicine applications.

25. A composition suitable for printing, for example 3D printing or inkjet printing, or for coating, adhesive, electronic, dentistry or biomedicine applications, comprising a branched polymer as claimed in any of claims 1 to 6 or as defined in any of claims 17 to 19, a photocurable composition as claimed in claim 8, 20 or 22, or a kit of parts as claimed in claim 9, 21 or 22.

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