Production of high molecular weight polyester (CO)polymer

Abstract

Production of a polyester (co)polymer, comprising the steps of a) adding a di(alkyl oxalate) ethylene glycol diester to a starting polyester (co)polymer at least comprising carboxylic acid derived monomer units and 1,2-diol derived alcohol end groups, wherein the alkyl group in the di(alkyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon; and b) in case the temperature is not already higher than 220 °C, elevating the temperature of the mixture resulting from step (a) to at least 220 °C, and reducing the pressure, for a period of time suicient to obtain a polyester (co)polymer product having a higher molecular weight than that of the starting polyester (co)polymer, wherein the amount of oxalate units that remain in the (co)polymer is less than the amount added in the form of the di(alkyl oxalate) ethylene glycol diester.

Description

PRODUCTION OF HIGH MOLECULAR WEIGHT POLYESTER (CO)POLYMERField of the invention

[0001] The present invention relates to a (catalyst free) process for the production of a high molecular weight polyester (co)polymer. The invention further relates to monomeric di(alkyl oxalate) ethylene glycol diester.Description of the background art

[0002] An important goal of polymerization processes is to obtain polymers with a molecular weight high enough for the desired application(s). This is important as the molecular weight of the polymer relates to polymer performance such as strength, toughness and durability. The use of polymers with insufficient molecular weight may lead to application failures. Therefore, many studies concerning polymerization processes and the conditions used in those processes relate to realizingthe target (high) molecularweight.

[0003] Polyesterification is a reversible reaction with a relatively low equilibrium constant. As a consequence, removal of condensation product has an impact on the molecularweight that can be achieved. Melt polycondensation at reduced pressure is commonly used in polyesterification processes for removal of condensation product. However, the increase of molecular weight of the polymers during that process also increases the viscosity of the melt material, which complicates the removal of condensation product. This may eventually become a limiting factor. Removal of condensation product can be improved, for example by using higher temperatures, longer reaction times, catalysts and improved reactor designs. However, under melt conditions, limited mass transfer due to high viscosity of the melt material, in combination with longer residence times and (potential) chemical degradation, may limit the possibilities to obtain higher molecular weights. For example, to obtain high molecular weights of polyethylene terephthalate (PET) necessary for bottles (intrinsic viscosity (IV) of 0.73 - 0.85 dL / g) and industrial yarns (IV >1 .2 dL / g), an additional solid-state polymerization (SSP) step may be required. In SSP, polymer pellets are heated below the melting point while being under a nitrogen flow or vacuum, optionally being rotated. A drawback of SSP is that due to the low mobility of the end groups and condensate in the solid state, this is time and energy consuming, and therefore an expensive process.

[0004] When less reactive diols such as isosorbide are introduced, it becomes even more difficult to obtain sufficiently high molecular weights. Incorporation of isosorbide is interesting due to its additional benefits on thermomechanical stability and mechanical performance, which opens new possibilities for applications. Isosorbide is less reactive due to its secondary alcohol groups, and melt polycondensation becomes considerably more difficult with increasing isosorbide content. Furthermore, the crystallinity of the polymer is strongly reduced if isosorbide contents are above around 15%. This makes it not possible to use SSP, as an amorphous polymerwould clump together.

[0005] An alternative route for producing high molecular weight polymers could be by utilizing a so-called chain extender after melt polycondensation (see e.g. P. Raffa et al., Reactive & Functional Polymers 72 (2012) 50-60). Chain extenders are very reactive molecules which react with the remaining alcohol and / or acid chain ends to increase the molecularweight. Only little chain extender is needed, as already a considerable chain length is usually obtained after melt polycondensation. Due to the high reactivity of the chainextender considerably less time and less harsh conditions are required to obtain high molecular weight. This can reduce costs by cutting down on polymerization conditions (temperature, time, catalyst, reactor), or by eliminating the need for SSP. Various chain extenders have been used for the production of high molecular weight polyesters, for example ethylene carbonate, bis-oxazolines, pyromellitic dianhydride, organic phosphites, di-isocyanates, di-epoxides, carbonyl biscaprolactam, diphenyl carbonate, diphenyl terephthalate, bisketenimines, and bislactams.

[0006] However, the use of chain extenders also comes with drawbacks. Chain extenders become incorporated in the polymer and thus become part of the molecular structure of the polyester. The groups that are incorporated into the polymer backbone inevitably influence the properties of the material. Further, often side reactions occur, such as crosslinking or chain scission, which also change the physical properties of the polymer. Moreover, some chain extenders are considered toxic, on their own, or as residue in polymer, thereby excluding the use thereof for food-grade applications. These drawbacks are amongst the reasons why today chain extenders are rarely used in commercial polyester production. In addition, as a consequence of the use of the currently known chain extenders, when the produced polymer is to be recycled, the chemical structure of the resulting polymer is further altered after each recycling session. This complicates the end life of the polymer and limits the amount of possible recycling sessions.

[0007] WO 2024 / 094621 and WO 2024 / 094633 describe processes for the production of a molecular weight polyester (co)polymer comprising adding a diphenyl oxalate ester to a starting polyester (co)polymer comprising alcohol end groups and units derived from 1 ,2- diols, wherein the phenyl group in the diphenyl oxalate ester is optionally substituted, the substituent being selected from one or more of o-, m- and p- C1 -C6 alkoxy, and o-, m- and p- C1 -C6 alkyl. This step is followed by in case the temperature is not already higher than 220 °C, elevating the temperature of the mixture resulting from step (a) to at least 220 °C, and reducing the pressure, for a period of time sufficient to obtain a polyester (co)polymer product wherein the amount of oxalate units that remain in the (co)polymer is less than added in the form of the diphenyl oxalate ester; and no or less than 1 mole % oxalate units are present, the percentage relative to the total amount of monomer units, the product having a higher molecular weight than that of the starting polyester (co)polymer.

[0008] There is a desire for a process for the production of a high molecular weight polyester (co)polymer in which condensation products have at most low toxicity.

[0009] Furthermore, there is a desire for processes for the production of high molecular weight polyesters which do not require the presence of a metal catalyst. For example, the majority of PET is currently produced using antimony (Sb) catalysts. In medical applications, metal catalysts are often undesired because of possible toxicity.

[0010] Therefore, there is a need for alternative, improved processes for the production of high molecular weight polyesters, which do not require high energy consumption and preferably do not have one or more of the drawbacks of the use of the commonly known chain extenders.Brief summary of the invention

[0011] According to the present invention, such an improved process is provided. The present invention relates to a process forthe production of, preferably high molecularweight,polyester (co)polymer, comprising a) adding a di(alkyl oxalate) ethylene glycol diester to a starting polyester (co)polymer at least comprising carboxylic acid derived monomer units and 1 ,2-diol derived alcohol end groups, wherein the alkyl group in the di(alkyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon; b) in case the temperature is not already higher than 220 °C, elevating the temperature of the mixture resulting from step (a) to at least 220 °C, and reducingthe pressure, for a period of time sufficientto obtain a polyester (co)polymer product having a higher molecular weight than that of the starting polyester (co)polymer, wherein the amount of oxalate units that remain in the (co)polymer is less than the amount added in the form of the di(alkyl oxalate) ethylene glycol diester. Preferably, wherein the amount of oxalate units that remain in the (co)polymer is less than 50% of the amount added in the form of the di(alkyl oxalate) ethylene glycol diester, more preferably wherein in the (co)polymer product no or less than 1 mol% oxalate units are present, the percentage relative to the total amount of monomer units. The expression (co)polymer polyester indicates that the polyester can be either a polymer polyester wherein two types of monomers are joined in the same polymer main chain or a copolymer polyester in which case three or more types of monomer units are joined in the same polymer main chain.

[0012] In this process, the di(alkyl oxalate) ethylene glycol diester is used as a “molecular weight booster” under relatively mild conditions instead of a so-called “chain extender” according to prior art processes. As discussed above, commonly used chain extenders cope with drawbacks, as they are incorporated into the polymer chain and alter the physical properties of the polymer and may leave toxic residues. Advantageously, the di(alkyl oxalate) ethylene glycol diester used according to the process of the invention acts as a molecular weight booster rather than a chain extender. The molecular weight booster tends not to be incorporated into the polymer chain of the final product. As a consequence, it does not influence the polymer properties apart from the molecular weight and properties related to the molecular weight only and does not leave toxic residue in the polymer end product.

[0013] Furthermore, by using the molecular weight booster according to the invention the use of catalyst, especially metal catalyst, in the process may be avoided altogether. Polyester chains of low molecular weight can be formed via autocatalysis in the absence of metal catalyst and subsequently connected by using the highly reactive di(alkyl oxalate) ethylene glycol diester which is subsequently removed from the polymer chain upon heating.

[0014] The high reactivity of the di(alkyl oxalate) ethylene glycol diester provides flexibility in its use in polymerization processes. It can be used as alternative to SSP in the case of crystalline polymers, or it may open up routes to produce polymers starting from less reactive or unstable monomers at less severe reaction conditions while still obtaining acceptable molecular weights. For example, for polymers containing isosorbide-derived units it is important to obtain high molecular weight directly after melt polycondensation as they are amorphous if the isosorbide content is over about 10-15 mole % and SSP is not possible. The process of the present invention may further allow for more effective recycling of certain polymers, as the process allows to increase the number of recycle sessions without altering the physical properties of the polymers that are produced in each of the recycle sessions.

[0015] In addition to the high reactivity, the di(alkyl oxalate) ethylene glycol diester molecular weight boosters allow for facile evaporation of alkanol byproducts such as methanol andethanol and tend to have low toxicity. Di(alkyl oxalate) ethylene glycol diester may be produced in efficient and effective processes known to the skilled worker.

[0016] The present invention provides an advantageous process for the preparation of polyester (co)polymers while obtaining high number average molecular weights of the polyester end product. For example, advantageously, according to the presently claimed di(alkyl oxalate) ethylene glycol diester (PEF) or polyethylene terephthalate (PET) without the addition of a metal catalyst. The absence of a metal catalyst can be advantageous from multiple perspectives. For example, the majority of PET currently is produced using antimony (Sb) catalysts, however, there are concerns about depletion of Sb reserves. Furthermore, in medical applications metal catalysts are often undesired because of toxicity.

[0017] The high molecular weight polyester (co)polymers produced according to the invention can advantageously be used in a broad range of commercial applications, such as in fibers, injection molded parts and bottles, 3D printing and packaging materials.

[0018] The present invention further relates to monomeric di(alkyl oxalate) ethylene glycol diester for producing polyester (co)polymer which ethylene glycol diester is free of oligomers and wherein the alkyl group in the d i(a Ikyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon. Preferably, the di(alkyl oxalate) ethylene glycol diester is di(methyl oxalate) ethylene glycol diester or di(ethyl oxalate) ethylene glycol diester.

[0019] A further embodiment is the use of di(alkyl oxalate) ethylene glycol diester in producing polyester (co)polymer wherein the alkyl group in the di(alkyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon. Preferably, the di(alkyl oxalate) ethylene glycol diester is di(methyl oxalate) ethylene glycol diester or di(ethyl oxalate) ethylene glycol diester.Detailed description

[0020] The following is a description of certain embodiments of the invention, given byway of example only.

[0021] The present invention relates to a process for the production of a polyester (co)polymer, in particular for the production of a high molecular weight polyester (co)polymer.

[0022] By a “polyester” herein is understood a polymer comprising a plurality of monomer units linked via ester functional groups in its main chain. An ester functional group can be formed by reacting a hydroxyl group (-OH) with a carboxyl / carboxylic acid group (-C(=O)OH). Typically, a polyester is a synthetic polymer formed by the reaction of one or more bifunctional carboxylic acids with one or more bifunctional hydroxyl compounds. Polyesters may also comprise units derived from monomers carrying both a hydroxyl group and a carboxylic acid group, such as hydroxycarboxylic acids, like lactic acid (LA) and glycolic acid (GA), and hydroxyalkanoates (HA), and the like. By a “polyester copolymer” is herein understood a polyester wherein three or more types of monomer units are joined in the same polymer main chain.

[0023] By a “monomer unit” is herein understood a unit as included in a polyester (co)polymer or oligomer, which unit can be obtained after polymerization of a monomer, that is, a “monomer unit” is a constitutional unit contributed by a single monomer or monomercompound to the structure of the polymer or oligomer, herein in particular the smallest diol or di-acid repeating unit.

[0024] By a “monomer” or “monomer compound” is herein understood the smallest building block used as the starting compound to be polymerized, such as a diol or di-acid compound, but also a hydroxycarboxylic acid.

[0025] By an “oligomer” or “oligomer compound” is herein understood a molecular structure comprising an average number of monomer units of in the range from equal to or more than 2 to equal to or less than 50 monomer units, and preferably at least 25 monomer units. Next to diol and di-acid derived monomer units, also other monomer units may be part of the oligomer, such as hydroxycarboxylic acid derived monomer units, in particular derived from a-hydroxycarboxylic acids, such as glycolic acid, lactic acid, mandelic acid and 3-alkoxy carbonic acid.

[0026] The present invention relates to a process for the production of a, preferably high molecular weight, polyester (co)polymer, comprising the use of a di(alkyl oxalate) ethylene glycol diester as molecular weight booster. The term “molecular weight booster” herein relates to a chemical that is used to increase the molecular weight of a polymer by assisting chain extension of the polymer, thereby forming a polymer product with higher molecular weight without becoming part of the polymer product itself, i.e. the chemical is not incorporated as a unit into the polymer chain.

[0027] The present process relates to the production of a polyester (co)polymer, comprising adding a d i(a Ikyl oxalate) ethylene glycol diester as molecular weight booster to a starting polyester (co)polymer at least comprising alcohol end groups and units derived from 1 ,2- diols. Particularly advantageous are starting polyester (co)polymers comprising glycol derived end groups. The term “glycol derived end groups” herein means: end groups derived from monomers wherein two hydroxyl (-OH) groups are attached to different carbon atoms, such as is the case when the end group is derived from for example mono ethylene glycol, 1 ,2-propanediol, 2,3-butanediol, and the like A preferred glycol derived end group is the group derived from mono ethylene glycol namely the 2-hydroxyethyl end group.

[0028] In the current process only a small amount of the di(alkyl oxalate) ethylene glycol diester is needed to boost the molecular weight. Amounts as small as 0.2 mol % of the di(alkyl oxalate) ethylene glycol diester may be suitable, more preferably 0.4 mol %; in particular, an amount of up to 25 mol % is used, preferably up to 15 mol %, more preferably up to 10 mol %, even more preferably up to 5 mol%, the percentages being relative to the total amount of carboxylic acid-derived monomer units in the starting polyester (co)polymer. Ideally, the amount of d i(a Ikyl oxalate) ethylene glycol diester used is about half the amount of the total amount of alcohol end groups present in the starting polyester (co)polymer. With a theoretical stoichiometric ratio of the d i(a Ikyl oxalate) ethylene glycol diester to the reactive alcohol end groups it is likely that very high molecular weight can be obtained within a very short time. Thus, ideally, and if appropriate, the amount of moles of the di(alkyl oxalate) ethylene glycol diester used in the process is about equal to half the amount of moles of glycol derived end groups present in the starting polyester (co)polymer. An advantage of using a di(alkyl oxalate) ethylene glycol diester is that due to a low volatility of this molecular weight booster, there is no evaporation of the booster before reacting. Typically, all added booster is allowed to react in the process according to the invention. The d i(a Ikyl oxalate) ethyleneglycol diester may be added either in portions or all at once, depending on the circumstances and the desired product.

[0029] The molecular weight booster of the present invention is based on alkyl esters of oxalate. Each oxalate forms an ester bond with the ethylene glycol and an ester bond with the alkyl group, wherein the alkyl group is an aliphatic alkyl group and wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon. Herein, a primary carbon means that the carbon is attached to one carbon atom only or to zero carbon atoms. In other words, the primary carbon in the dialkyl oxalate ester is a CH2group that is attached to an oxygen atom in the ester bond and to either a third hydrogen or to a carbon atom. In one embodiment, the alkyl chain is substantially linear, which herein means that counting from the ester group, at least the first three, preferably at least the first four, more preferably at least the first five carbon atoms form a linear alkyl chain without alkyl side chains. In one embodiment, the alkyl group is an aliphatic linear alkyl group without any branching. Examples of such alkyl groups include methyl, ethyl, propyl, butyl and pentyl groups. Especially good results were obtained with di(methyl oxalate) ethylene glycol diester (DMO- EG) and di(ethyl oxalate) ethylene glycol diester (DEO-EG).

[0030] Di(alkyl oxalate) ethylene glycol diesters possess a high reactivity towards alcohol groups, even to such an extent that in the process an additional catalyst may not be needed. The reactivity comes from the good leaving group ability of the alkyl groups and the volatility of the alkanol condensation products (in case of DMO and DEO: methanol and ethanol) in combination with the structure of oxalate where the carboxyl groups are directly connected to each other.

[0031] Without being bound to any theory, it is hypothesized that in the course of the process of the invention an oxalate ring product is formed from the first oxalate species and the 1 ,2- diol derived end groups, for example glycol-derived, or other suitable diol derived units that were originally present in the starting polyester (co)polymer. A second oxalate ring product is formed from the second oxalate species and the ethylene glycol in the di(alkyl oxalate) ethylene glycol diester. Both oxalate ring products (1 ,4-dioxane-2, 3-dione) may evaporate from the reaction mixture. It is believed that chain ends couple, leading to an increase of the molecular weight of the polyester, and the oxalate ring product is removed at elevated temperature and low pressures, preferably vacuum, while further removing the related alkanol for example methanol or ethanol, that may be released during the process. Furthermore, in the course of the reaction, it is possible that also other suitable diol derived units within an intermediate polymer product react with an intermediate oxalate species to form an oxalate ring product that can be removed at elevated temperature and vacuum thereby boosting the molecular weight. In such case, reactions between chains take place. While the oxalate ring product and alkanol are removed from the process, high molecular weight polymers are formed without incorporating a substantial amount of oxalate derived units in the polymer chain. Only minor, hardly detectable or indetectable, amounts of oxalate derived units were found to have remained in the polymer chain of the product.

[0032] Advantageously, no or essentially no residue of units of the molecularweight booster remains in the polymer chain. Specifically, the polyester (co)polymer contains less than 1 mol% oxalate units relative to the total amount of monomer units in the polyester). Such amount does not alter the physical and chemical properties of the polymer produced at the end of the process. This is a significant advantage, especially when a polymer product issubjected to repeated recycling. Consequently, the polymer product properties are not changed by the current process, even after recycling. It is further advantageous for recycling processes to have no catalyst present in the polymer to avoid build-up of metals. Favorably, according to the process of the invention the use of a catalyst may be avoided.

[0033] A further advantage of the current process is that the cyclic oxalate species and the alkanol may suitably be recycled. Advantageously, building blocks like oxalate, ethylene glycol and ethanol or methanol may be re-introduced into the process for re-use.

[0034] The current process relates to production of a high molecular weight polyester (co)polymer, particularly wherein at least one of the units derived from a 1 ,2-diol is derived from an aliphatic diol selected from mono ethylene glycol, 1 ,2 propanediol, 1 ,2-butanediol, 2,3-butanediol and 1 ,2-cyclohexanediol.

[0035] Additional diols that may be used in the current process, other than the linear diols as defined herein above, may be any suitable diol, primary and secondary, and may preferably be selected from cis- and / or trans- 2,2,4,4-tetramethyl-1 ,3-cyclobutanediol, and 1 ,4:3,6-dianhydrohexitols, in particular isosorbide. Thus, the starting polyester (co)polymer may comprise additional diol derived units, preferably selected from cis- and / or trans- 2, 2, 4, 4-tetramethyl-1 ,3-cyclobutanediol, and 1 ,4:3,6-dianhydrohexitols, in particular isosorbide.

[0036] Suitably, the starting polyester may be any (co)polymer comprising alcohol end groups and units derived from 1 ,2-diols. Preferably, the starting polyester (co)polymer comprises dicarboxylic acid derived units, the dicarboxylic acids or any esters thereof selected from aromatic dicarboxylic acids which can contain one or more heteroatoms or any esters thereof, and C2-C18 aliphatic dicarboxylic acids or any esters thereof, which may be linear, cyclic or branched dicarboxylic acids, such as, but not limited to, 1 ,4- cyclohexanedicarboxylic acid, diglycolic acid, and especially linear dicarboxylic acids of the formula HOOC(CH2)nCOOH wherein n is an integer of 1 to 20. The aromatic dicarboxylic acid preferably is selected from terephthalic acid, a terephthalic acid monoester, a terephthalic acid diester, a furandicarboxylic acid, a furandicarboxylic acid monoester, and a furandicarboxylic acid diester.

[0037] In a preferred embodiment, the starting polyester (co)polymer contains dicarboxylic acid units derived from terephthalic acid and / or a furandicarboxylic acid, 2,5- furandicarboxylic acid, and mono ethylene glycol derived units, and optionally isosorbide derived units. More preferably, the starting polyester polyester (co)polymer consists of dicarboxylic acid units derived from terephthalic acid and / or a furandicarboxylic acid, 2,5- furandicarboxylic acid, and mono ethylene glycol derived units, and optionally isosorbide derived units.

[0038] The process of the invention is preferably performed as follows, comprising (i) providing or producing the starting polyester (co)polymer, wherein the process for producing the polyester (co)polymer comprises esterification or transesterification and polycondensation; (ii) adding the di(alkyl oxalate) ethylene glycol diester to the starting polyester (co)polymer; (iii) elevating the temperature of the mixture resulting from step (ii) to at least 220 °C, or, in case the temperature is already higher than 220 °C, maintaining the temperature for a period of time; (iv) reducing pressure, preferably to at most 5 mbar, more preferably to equal to or below 1 mbar, thereby removingthe alkanol (comingfrom the di(alkyl oxalate) ethylene glycol diester) and producing a (co)polymer product; and (v) optionallyrepeating steps (ii) to (iv) one or more times with as starting material the (co)polymerwith the higher molecular weight of the previous step (iv) until a final polyester (co)polymer product is produced wherein no or less than 1 mole % oxalate units are present, the percentage being relative to the total amount of monomer units, and with the desired high molecular weight properties.

[0039] A preferred step (i) comprises producing the starting polyester (co)polymer by reacting a dicarboxylic acid or ester thereof with a diol and / or polyol and / or oligomer with diol derived end groups [such as bis(2-hydroxyethyl) terephthalate {BHET}, and other terephthalate oligomers such as PET glycolysis products, see e.g. T. Spychaj in "Handbook of thermoplastic polymers", 2002 Wiley, Chapter 27, p 1259-61 ], optionally in the presence of a catalyst, wherein at least one of the diols and / or polyols and / or oligomers with diol derived end groups comprises a vicinal diol group. Preferably, the diol and / or polyol and / or oligomer with diol derived end groups is selected from a linear diol selected from saturated C2-C12 aliphatic diol compounds, and BHET, and preferably from mono ethylene glycol, 2,3- butanedioland 1 ,2-cyclohexanediol, and at least comprises adjacent hydroxy groups in other words vicinal diols.

[0040] In a preferred embodiment of step (i), the dicarboxylic acid or ester thereof is reacted with an excess of the diol and / or polyol and / or oligomer with diol derived end groups. The use of an excess increases the relative amount of alcohol end groups in the starting polyester (co)polymer and decreases the relative amount of carboxylic end groups The excess may be in the range between 1.005 and 1.50 equivalents of diol relative to 1 equivalent of the dicarboxylic acid or ester groups, for example in the range between 1 .01 and 1 .10 or between 1 .05 and 1 .40, for example around 1 .25 equivalents. A too low amount of diol results in an increased number of carboxylic acid or its ester end groups, whereas a too high amount of diols may result in a starting polyester (co)polymer with a very low molecular weight.

[0041] As indicated, the starting polyester (co)polymer in step (a) of the process may be readily prepared for this purpose in step (i), as described herein above, but may also advantageously be a recycled polymeric material or derived therefrom. Preferably, such polyester (co)polymerfrom recycling sources contains an aliphatic 1 ,2-diol derived unit, the diol selected from mono ethylene glycol, 1 ,2-propanediol, 1 ,2-butanediol, 2,3-butanediol and 1 ,2-cyclohexanediol.

[0042] The temperature in step (iii) depends on the type of the polyester (co)polymer that is produced. Suitably, the temperature is high enough when after addition of the di(alkyl oxalate) ethylene glycol diester the material melts or is still in a molten state and the mixture can properly be stirred. For more flexible polyester (co)polymers with low melting points, the temperature may for example be at least 220 °C, for more rigid polyester (co)polymers, such as PET or PEF polymers, the temperature preferably needs to be at least 230 °C,

[0043] In a scaled up process (kilogram scale), the molecular weight booster may advantageously be added at the end of the polycondensation (see step (ii) above) or in an extruder, together with the (co)polymer that is to be extruded.

[0044] The molecular weight boosting strategy of the present invention was shown to be relatively easy to scale up. As an example, the d i(a Ikyl oxalate) ethylene glycol diesters, for example DMO-EG and DEO-EG can be effectively synthesized as described below. Advantageously, only a relatively small amount of DEO-EG or DMO-EG is required to have the desired molecular weight boosting effect. This makes this strategy economically attractive,but also easy to apply in existing reactor setups. The d i(a Ikyl oxalate) ethylene glycol diester can for example be fed to the reaction vessel via a catalyst addition funnel. Further, by increasing the polycondensation time or adding a catalyst, the required amount of the di(alkyl oxalate) ethylene glycol diester can be reduced.

[0045] In a ton scale polymer production, the result may still be the production of kilograms of, for example, ethanol (in case DEO-EG is used) and ethylene oxalate ring as condensates. To make the process more economic and sustainable, and commercially attractive, the condensates should have a designated use. Ethanol could be recycled and reused in the DEO synthesis process. Ethylene oxalate may also be reused for DEO synthesis as well, however it is probably more efficient to use it directly as a monomer. Ethylene oxalate may for example be used to produce polyethylene oxalate by ring opening polymerization, or it may be combined with other ring opening monomers, such as lactide and glycolide to produce PLA and PGA polymers comprising oxalate units.

[0046] In a further aspect, the invention relates to (co)polyesters obtainable by, or obtained by, the currently claimed process. The process allows the preparation of a range of existing and novel polyester (co)polymers with high molecular weights that conventionally would not be obtainable. In an embodiment of the invention, the process comprises a step (i) wherein the starting polyester (co)polymerwas produced using a catalyst, followed by steps (ii) to (v). Such a process combines the favourable features of the present invention and features of existing polyester production methods. In another preferred embodiment no catalyst is used in the entire process, allowing the production of metal catalyst free polyester (co)polymers, that may be advantageous for certain uses requiring the absence of any catalyst, such as medical uses of polyesters.

[0047] Metal catalyst free polyester is considered to be polyester comprising less than 100 ppm, preferably less than 10 ppm, preferably less than 1 ppm by weight of metals based on total amount of polyester.

[0048] Thus, in an embodiment, preferably the (novel) polyester that is produced is a metal catalyst free polyester (co)polymer which was produced without addition of a metal catalyst, and preferably with Mn of 20kDa or more, the polyester preferably being selected from:- polyethylene furan-2,5-dicarboxylate);- poly(ethylene co-isosorbide furan-2,5-dicarboxylate);- poly(ethylene co-isosorbide co-cyclohexanedimethylene furan-2,5-dicarboxylate);- poly(ethylene co-isosorbide terephthalate) with isosorbide content of 15% or higher;- poly(ethylene co-isosorbide co-cyclohexanedimethylene terephthalate);- poly(ethylene furan-2,5-dicarboxylate co-terephthalate);- poly(ethylene co-isosorbide furan-2,5-dicarboxylate co-terephthalate); wherein the Mn is measured using gel permeation chromatography with poly(methyl methacrylate) standards as reference material.

[0049] In a preferred embodiment, the polyester (co)polymer product that is produced with the process according to the invention has a molecular weight (Mn) of 20 kDa or more, preferably 30 kDa or more, even more preferably 40 kDa or more.

[0050] The weight average molecular weight (Mw) and the number average molecular weight (Mn) can be determined by means of gel permeation chromatography (GPC) at 35 °C, using for the calculation poly(methyl methacrylate) standards as reference material, and using hexafluoro-2-propanol as eluent.

[0051] The glass transition temperature (Tg) of the polyester copolymer can be measured by conventional methods, in particular by using differential scanning calorimetry (DSC) with a heating rate of 10 °C / minute in a nitrogen atmosphere. All glass transition temperatures herein are determined as described under the analytical methods section of the examples. In a preferred embodiment, the polyester (co)polymer that is obtainable with the process according to the invention has a Tg of at least 80 °C, preferably at least 90 °C, more preferably at least 100 °C, even more preferably at least 110 °C.

[0052] The polyester (co) polymer obtainable by or obtained by the process of the invention can suitably be combined with additives and / or other (co)polymers and therefore the invention further provides a composition comprising said polyester copolymer and in addition one or more additives and / or one or more additional other (co)polymers. Such composition can for example comprise, as additive, nucleating agents. These nucleating agents can be organic or inorganic in nature. Examples of nucleating agents are talc, calcium silicate, sodium benzoate, calcium titanate, boron nitride, zinc salts, porphyrins, chlorin and fluorine.

[0053] The composition according to the invention can also comprise, as additive, nanometric (i.e. having particles of a nanometric size) or non-nanometric and functionalized or non-functionalized fillers or fibres of organic or inorganic nature. They can be silicas, zeolites, glass fibres or beads, clays, mica, titanates, silicates, graphite, calcium carbonate, carbon nanotubes, wood fibres, carbon fibres, polymer fibres, proteins, cellulose fibres, lignocellulose fibres and nondestructured granular starch. These fillers or fibres can make it possible to improve the hardness, the stiffness or the permeability to water or to gases. The composition can comprise from 0.1 % to 75% by weight, for example from 0.5% to 50% by weight, of fillers and / or fibres, with respect to the total weight of the composition. The composition can also be of composite type, that is to say can comprise large amounts of these fillers and / or fibres.

[0054] The composition can also comprise, as additive, opacifying agents, dyes and pigments. They can be chosen from cobalt acetate and the following compounds: HS-325 Sandoplast Red BB, which is a compound carrying an azo functional group also known under the name Solvent Red 195, HS-510 Sandoplast Blue 2B, which is an anthraquinone, Polysynthren Blue R and Clariant RSB Violet. Sandoplast and Clariant are trademarks of Clariant. Polysynthren is a trademark of the Heubach group.

[0055] The composition can also comprise, as additive, a processing aid for reducing the pressure in the processing device. A mould-release agent, which makes it possible to reduce the adhesion to the equipment for shaping the polyester, such as the moulds or the rollers of calendering devices, can also be used. These agents can be selected from fatty acid esters and amides, metal salts, soaps, paraffins or hydrocarbon waxes. Specific examples of these agents are zinc stearate, calcium stearate, aluminium stearate, stearamide, erucamide, behenamide, beeswax or Candelilla wax.

[0056] The composition can also comprise other additives, such as stabilizers, etc. as mentioned herein above.

[0057] In addition, the composition can comprise one or more additional polymers other than the one or more polyester (co)polymers according to the invention. Such additional polymer(s) can suitably be chosen from the group consisting of polyamides, polystyrene, styrene copolymers, styrene / acrylonitrile copolymers, styrene / acrylonitrile / butadienecopolymers, polymethyl methacrylates, acrylic copolymers, poly(ether / imide)s, polyphenylene oxides, such as poly(2,6-dimethylphenylene oxide), polyphenylene sulfide, poly(ester / carbonate)s, polycarbonates, polysulphones, polysulphone ethers, polyetherketones and blends of these polymers.

[0058] The composition can also comprise, as additional polymer, a polymer which makes it possible to improve the impact properties of the polymer, in particular functional polyolefins, such as functionalized polymers and copolymers of ethylene or propylene, core / shell copolymers or block copolymers.

[0059] The compositions according to the invention can also comprise, as additional polymer(s), polymers of natural origin, such as starch, cellulose, chitosan, alginates, proteins, such as gluten, pea proteins, casein, collagen, gelatine or lignin, it being possible or not for these polymers of natural origin to be physically or chemically modified. The starch can be used in the destructured or plasticized form. In the latter case, the plasticizer can be water ora polyol, in particular glycerol, polyglycerol, isosorbide, sorbitans, sorbitol, mannitol or also urea. Use may in particular be made, in order to prepare the composition, of the process described in the document WO 2010 / 010282A1 .

[0060] These compositions can suitably be manufactured by conventional methods for the conversion of thermoplastics. These conventional methods may comprise at least one stage of melt or softened blending of the polymers and one stage of recovery of the composition. Such blending can for example be carried out in internal blade or rotor mixers, an external mixer, or single-screw or co-rotating or counter-rotating twin-screw extruders. However, it is preferred to carry out this blending by extrusion, in particular by using a co-rotating extruder. The blending of the constituents of the composition can suitably be carried out at a temperature ranging from 220 to 300°C, preferably under an inert atmosphere. In the case of an extruder, the various constituents of the composition can suitably be introduced using introduction hoppers located along the extruder.

[0061] The invention also relates to an article comprising a polyester (co)polymer according to the invention or a composition comprising a polyester (co)polymer according to the invention and one or more additives and / or additional polymers. The polyester (co)polymer may conveniently be used in the manufacturing of films, fibers, injection molded parts and packaging materials, such as for example receptacles. The use of the polyester (co)polymer is especially advantageous where such films, fibers, injection molded parts or packaging materials need to be heat-resistant or cold-resistant.

[0062] The article can also be a fibre for use in for example the textile industry. These fibres can be woven, in order to form fabrics, or also nonwoven. The article can also be a film or a sheet. These films or sheets can be manufactured by calendering, cast film extrusion or film blowing extrusion techniques. These films can be used for the manufacture of labels or insulators.

[0063] This article can be a receptacle especially for use for hot filling and reuse applications. This article can be manufactured from the polyester (co)polymer or a composition comprising a polyester (co)polymer a nd one or more additives and / or additional polymers using conventional conversion techniques. The article can also be a receptacle for transporting gases, liquids and / or solids. The receptacles concerned may be baby’s bottles, flasks, bottles, for example sparkling or still water bottles, juice bottles, soda bottles, carboys, alcoholic drink bottles, medicine bottles or bottles for cosmetic products, dishes,for example for ready-made meals or microwave dishes, or also lids. These receptacles can be of any size. The article may for example be suitably manufactured by extrusion-blow moulding, thermoforming or injection-blow moulding.

[0064] The present invention therefore also conveniently provides a method for manufacturing an article, comprising the use of one or more polyester (co)polymers according to the invention and preferably comprising the following steps: 1 ) the provision of a polyester (co)polymer obtainable by or obtained by the process of this invention; 2) melting said polyester (co)polymer, and optionally one or more additives and / or one or more additional polymers, to thereby produce a polymer melt; and 3) extrusion-blow moulding, thermoforming and / or injection-blow moulding the polymer melt into the article.

[0065] The article can also be manufactured according to a process comprising a stage of application of a layer of polyester in the molten state to a layer based on organic polymer, on metal or on adhesive composition in the solid state. This stage can be carried out by pressing, overmoulding, lamination, extrusion-lamination, coating or extrusion-coating.

[0066] Advantageously, high molecular weight polyester (co)polymers produced according to the process of the invention can be used in 3D printing. In case very high molecular weights are desired, the use of alternative types of reactors could potentially be a solution, such as extruders and compounders that are known to be used with polymers produced with several types of chain extenders. For example, a spinning disk reactor is highly suitable for processing highly viscous polymers.

[0067] In a further aspect, the invention relates to a monomeric di(alkyl oxalate) ethylene glycol diesterthat is free of oligomers, wherein the alkyl group in the d i(a Ikyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon. Preferably, the monomeric di(alkyl oxalate) ethylene glycol diester is di(methyl oxalate) ethylene glycol diester (DMO-EG) or di(ethyl oxalate) ethylene glycol diester (DEO-EG).

[0068] Herein, with a monomeric di(alkyl oxalate) ethylene glycol diester that is free of oligomers is meant that the molecule consists of one ethylene glycol (EG) derived unit that forms ester bonds with two oxalate units, which each form an ester bond with an alkyl group, respectively. Oligomers in this respect refer to molecules having a structure corresponding to alkyl-(oxalate - EG)n-oxalate-alkyl, wherein n is larger than 1. The monomeric di(alkyl oxalate) ethylene glycol diester herein refers to such a structure wherein n is 1 .

[0069] The monomeric di(alkyl oxalate) ethylene glycol diester according to the invention may be used as a molecular weight booster, for example in a process according to the invention. Advantageously, the monomeric di(alkyl oxalate) ethylene glycol diester has a lower volatility compared to, for example, dialkyl oxalate. This means that when the molecular weight booster is added to a starting polyester at elevated temperatures (above room temperature), it will not immediately evaporate and is therefore allowed to react with the end groups of the starting polyester. As a result, less molecular weight booster needs to be added to the starting polyester to achieve a boost in molecular weight of the polyester.

[0070] The invention further relates to a method for producing monomeric di(alkyl oxalate) ethylene glycol diester, wherein the method comprises the steps of reacting ethylene glycol with an excess of dialkyl oxalate as described in example 1 . Preferably, the dialkyl oxalate isreacted with ethylene glycol in an excess molar amount of 1 :2 ratio (stoichiometric) to 1 :8, preferably 1 :3 to 1 :5, such as, for example, 1 :4 ratio.

[0071] The method according to the invention allows for an easy and effective method to produce di(alkyl oxalate) ethylene glycol diester with high purity ( / .e. more than 99% purity) and with high yield

[0072] The invention is further illustrated by the following non-limiting examples.

[0073] List of abbreviationsMonomers EG ethylene glycol FDCA 2,5-furandicarboxylic acid ISO isosorbideBoosters DMO-EG di(methyl oxalate) ethylene glycol diester DEO-EG di(ethyl oxalate) ethylene glycol diester DMO dimethyl oxalate DEO diethyl oxalate OA oxalic acid Chemicals TEA hydroxide tetra ethyl ammonium hydroxide TCE-d2 deuterated tetrachloroethane TFA-d1 deuterated trifluoroacetic acid Polymers PEF polyethylene furanoate) PEIF poly(ethylene-co-isosorbide furanoate) PET polyethylene terephthalate) PEIT polyethylene-co-isosorbide terephthalate)Analysis methods NMR Nuclear Magnetic Resonance DSC Differential Scanning Chromatography GPC Gel Permeation ChromatographyGeneral terms eq. equivalent h hour RH relative humidity RT room temperature TE / PC transesterification / polycondensationMaterials and reagents

[0074] Triethylamine (99.5%), TFA-d1 (99.5%), diethyl oxalate (>99%), di-tert- butyl oxalate (99%), oxalic acid (98%), ethylene glycol (99.5%), Ti(IV)isopropoxide (97%), and tetraethylammonium hydroxide 35% w / w aq. soln, were supplied by Sigma Aldrich. Isosorbide (>99.5%) was supplied by Roquette. Dimethyl oxalate (>99%) was bought from TCI chemicals. TCE-d2 (99.5%) was ordered from Thermo Scientific. 2,5-Furandicarboxylic acid (99%, polymer grade) was supplied by Avantium, RNP. All chemicals were used as received.CharacterizationNMR

[0075] 1H-NMR spectra were recorded at appropriate frequencies on a Bruker AV300-II (1 H, 300.10 MHz) spectrometer with a 5 mm BBOF ATMA probe, and a Bruker AV400 (1 H, 400.13 MHz) spectrometer with a 5 mm BBO ATMA probe. Chemicals shift are referenced to the residual proton in the specified solvent.1H NMR sample preparation: ~7 mg of polymer was dissolved in 0.6 mLTCE-d2 orTFA-d1 .DSC

[0076] Differential scanning calorimetry thermograms were acquired using a Mettler Toledo DSC 3 STAR® system. Approximately 5 mg of the sample was measured in a conventional aluminum crucible (40 pl). The analysis was conducted in three stages, employing a N2flow of 50 ml*min-1. Initially, the sample was stabilized at 20 °C for 5 minutes and then subjected to analysis, with a temperature increase from 20 to 250 °C at a rate of 10 °C*min-1. Subsequently, the sample was cooled down to the initial temperature of 20 °C at a cooling rate of 50 °C*min-1. Finally, the initial step was repeated, and the Tgvalue from the second cycle is used for reporting.GPC

[0077] GPC analysis was conducted using a Hitachi Chromaster 5450 equipped with an Agilent HPLC system featuring two PFG 7 micrometer (pm) Linear M (300><7.5 mm) columns. PMMA standards were used for calibration. Each sample, comprising 10 mg, was dissolved in 10 mL HFIP. The mobile phase, a combination of HFIP and potassium trifluoroacetate (0.02 mol) to prevent bacterial growth in the HFIP, maintained a consistent flow rate of 1 mL / min at a temperature of 35 °C. Analysis was facilitated with the refractive index detector (Chromaster 5450), and subsequent data processing was accomplished using ASTRA 6.1 software (Wyatt Technology).Example 1 : Synthesis of the DMO-EG-diester

[0078] A 100 mL three neck round bottom flask was charged with 9.38 g (0.15 mol; 1 eq) ethylene glycol and 71 .25 g (0.6 mol; 4 eq) dimethyl oxalate. The round bottom flask was put in an oil bath and equipped with a mechanical stirrer, nitrogen inlet and distillation path connected with thermometer and Schlenk flask, all suited for high vacuum. Nitrogen flow was set to 50 mL / min and the temperature of the oil bath was set to 100 °C. After two hours, the oil temperature was set to 120 °C. After 2.5 hours of total reaction time, vacuum was applied (300 mbar) and temperature was increased to 140 °C. After 4 hours of reaction time, the temperature was set to 170 °C and vacuum was further reduced. At 150 mbar the distillation temperature increased to 105-110 °C. When, 25 mbar was reached the Schlenk flaskwas replaced with a clean one. Subsequently, full vacuum was applied and at a vacuum of 0.5 mbar the distillation temperature increased to 135-140 °C. 3.3 g of the collected product was recrystallized in 15 mL hot methanol. The crystals were filtered and washed with 3x 10 mL cold methanol. A total of 2.6 g product was collected and subjected to DSC and1H NMR analysis.

[0079] DSC analysis showed that the product had a melting point of 77.8 °C and 99.4 % purity.1H NMR(CDCl3) 53.92 (s, 6H, CH3), 64.58 (s, 4H, CH2).

[0080] A 500 mL three neck round bottom flask was charged with 99.9 g 2,5- furandicarboxylic acid (640 mmol; 1 eq), 36.6 g ethylene glycol (590 mmol; 0.92 eq), isosorbide (206 mmol; 0.32 eq) and TEA hydroxide (75 pL35wt% water solution, 0.174 mmol; 0.27 meq). The round bottom flask was put in an oil bath and equipped with a mechanical stirrer, nitrogen inlet and short path connected Schlenk flask, all suited for high vacuum. The polymerization was carried out in three steps, esterification, pre-polycondensation and molecular weight boosting.

[0081] In the esterification step, nitrogen flow was set to 50 mL / min and the temperature of the oil bath was set to 230 °C. After 13 minutes the temperature of the oil bath reached 150 °C and stirring was started (100 RPM). The set temperature was reached in about 30 minutes and the reaction was continued for another 4 hours. The reaction was left to cool overnight under a nitrogen atmosphere.

[0082] In the pre-polycondensation step, the temperature was first set to 225 °C (00:00 hours). At 00:19 hours, the temperature passed 200 °C and stirring was started (30 RPM). At 00:31 hours, the stir speed was set to 100 RPM. 00:50 hours, vacuum was started (< 2mbar) and the oil temperature was set to 255 °C. At 03:00 hours the set temperature was increased to 265 °C. After a total reaction time of 4:11 hours, the pre-polycondensation polymer was taken out of the reaction flask under a nitrogen atmosphere.

[0083] The pre-polycondensation polymer was taken for further GPC analysis and1H NMR analysis.1H NMR analysis concludes that the polymer contains 27% isosorbide. GPC analysis determines a molecular weight of 4.5 / 11.3 (kg / mol) Mn / Mw.DMO-EG diester

[0084] A 100 mL three neck round bottom flask was charged with 10.221 g (50 mmol; 1 eq) of the PEIF pre-polycondensation polymer of Example 2 and 0.360 g (1.5 mmol; 3.1 mol%, relative to the total amount of FDCA in the starting polyester) of the DMO-EG diester. The round bottom flask was put in an oil bath and equipped with a mechanical stirrer, nitrogen inlet and short path connected Schlenk flask, all suited for high vacuum.

[0085] The oil temperature was first set to 275 °C (00:00 hours). At 00:22 hours, the temperature reached 250 °C and stirring was started (30 RPM). At the same time, 0.5 mL of 10 mg / mL Titanium(IV)isopropoxide in toluene was added to the reaction flask. At 00:23 hours, vacuum was started (<1 mbar). At 1 :36 hours vacuum was replaced by nitrogen and the polymer was taken out of the reaction flask. During the reaction the torque increased from 14 Ncm to 28 Ncm, as measured with an IKA Lab stirrer MINISTAR 80 control. The skilled person knows that the measured torque is the power that is required to rotate / stir at a given RPM (rounds per minute) for a given stirrer apparatus.

[0086] The final polymer was taken for further GPC analysis. GPC analysis determines a molecular weight of 28.5 / 76.2 (kg / mol) Mn / Mw.examole without a booster

[0087] A 100 mL three neck round bottom flask was charged with 10.533 g of the PEIF prepolycondensation polymerfrom Example 2. The round bottom flaskwas put in an oil bath andequipped with a mechanical stirrer, nitrogen inlet and short path connected Schlenk flask, all suited for high vacuum.

[0088] The oil temperature was first set to 275 °C (00:00 hours). At 00:28 hours, the temperature reached 260 °C and stirring was started (30 RPM). At the same time, 0.5 mL of 10 mg / mL Titanium(IV)isopropoxide in toluene was added to the reaction flask. At 00:30 hours, vacuum was started (<1 mbar). At 1 :40 hours vacuum was replaced by nitrogen and the polymer was taken out of the reaction flask. During the reaction the torque increased from 14 Ncm to 18 Ncm, as measured with an IKA Lab stirrer MINISTAR 80 control.

[0089] The final polymer was taken for further GPC analysis. GPC analysis determines a molecular weight of 11 .3 / 30.6 (kg / mol) Mn / Mw.

[0090] Table 1 shows an overview of the molecular weight results as obtained with GPC analysis for examples 2-4Table 1: Overview GPC(a)and DSC(b)data including the Mn, Mw, PDI and Tgvalues (average of 2 spectra) of the PEIF polyester copolymers obtained in Examples 2-4.

Claims

CLAIMS1 . A process for the production of a polyester (co)polymer, comprising the steps of a. adding a di(alkyl oxalate) ethylene glycol diester to a starting polyester (co)polymer at least comprising carboxylic acid derived monomer units and 1 ,2- diol derived alcohol end groups, wherein the alkyl group in the di(alkyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon; b. in case the temperature is not already higher than 220 °C, elevating the temperature of the mixture resulting from step (a) to at least 220 °C, and reducing the pressure, for a period of time sufficient to obtain a polyester (co)polymer product having a higher molecular weight than that of the starting polyester (co)polymer, wherein the amount of oxalate units that remain in the (co)polymer is less than the amount added in the form of the di(alkyl oxalate) ethylene glycol diester.

2. A process according to claim 1 , wherein in step b) the period of time is sufficient to obtain a polyester (co)polymer product wherein the amount of oxalate units that remain in the (co)polymer is less than 50% of the amount added in the form of the di(alkyl oxalate) ethylene glycol diester.

3. A process according to claim 1 or claim 2, wherein in step b)the period of time is sufficient to obtain a polyester (co)polymer product wherein no or less than 1 mol% oxalate units are present, the percentage relative to the total amount of monomer units.

4. A process according to any of the preceding claims, wherein in step a) an amount of 0.2 to 25 mole% of the di(alkyl oxalate) ethylene glycol diester is added to the starting polyester (co)polymer, the percentages being relative to the total amount of carboxylic acid derived monomer units in the starting polyester (co)polymer.

5. The process according to any of the preceding claims, wherein the di(alkyl oxalate) ethylene glycol diester is di(methyl oxalate) ethylene glycol diester or di(ethyl oxalate) ethylene glycol diester.

6. The process according to any of the preceding claims, wherein at least one of the units derived from a 1 ,2-diol is derived from an aliphatic diol selected from mono-ethylene glycol, 1 ,2 propanediol, 1 ,2-butanediol, 2,3-butanediol and 1 ,2-cyclohexanediol.

7. The process according to any of the preceding claims, wherein the starting polyester (co)polymercomprises additional diol-derived units, preferably selected from cis- and / or trans- 2,2,4,4-tetramethyl-1 ,3-cyclobutanediol, and 1 ,4:3,6-dianhydroxohexitols, in particular isosorbide.

8. The process of any of the preceding claims, wherein the starting polyester (co)polymer comprises dicarboxylic acid derived units, the dicarboxylic acids or any esters thereofselected from (hetero)aromatic dicarboxylic acids or any esters thereof and C2-C18 aliphatic dicarboxylic acids or any esters thereof, which may be linear, cyclic or branched.

9. The process of any of the preceding claims, comprising the steps of(i) providingor producingthe starting polyester (co)polymer, wherein the processfor producing the polyester (co)polymer comprises esterification or transesterification and polycondensation;(ii) adding the di(alkyl oxalate) ethylene glycol diester to the starting polyester (co)polymer;(iii) elevating the temperature of the mixture resulting from step (ii) to at least 220 °C, or, in case the temperature is already higher than 220 °C, maintaining the temperature for a period of time;(iv) reducing pressure, preferably to at most 5 mbar, more preferably to equal to or below 1 mbar, thereby producing a (co)polymer product; and(v) optionally repeating steps (ii) to (iv) one or more times with as starting polyester (co)polymer the (co)polymer product with the higher molecular weight of the previous step (iv) until a final polyester (co)polymer product is produced wherein no or less than 1 mole % oxalate units are present, the percentage relative to the total amount of monomer units, and with the desired high molecular weight properties.

10. The process of claim 8, wherein step (i) comprises producing the starting polyester (co)polymer by reacting dicarboxylic acids or esters thereof with diols and / or polyols and / or oligomers with diol derived end groups, optionally in the presence of a catalyst, wherein at least one of the diols and / or polyols and / or oligomers with diol derived end groups comprises a vicinal diol group.

11. The process of claim 8 or 9, wherein the starting polyester (co)polymer is a recycled polyester material or derived from a recycled polyester material.

12. A process according to any of the preceding claims, wherein step a) of the process is caried out in the absence of a metal catalyst.

13. A process according to any of the preceding claims, wherein the process is carried out in absence of a metal catalyst.

14. Monomeric di(alkyl oxalate) ethylene glycol diester for producing polyester (co)polymer which ethylene glycol diester is free of oligomers and wherein the alkyl group in the d i(a Ikyl oxalate) ethylene glycol diester is an aliphatic alkyl group wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon.

15. Use of di(alkyl oxalate) ethylene glycol diester in producing polyester (co)polymer wherein the alkyl group in the d i(a Ikyl oxalate) ethylene glycol diester is an aliphatic alkylgroup wherein the carbon in the alkyl group that is attached to the oxalate is a primary carbon.

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