Thermally conductive polymer

Abstract

A polymer comprising a repeat unit of formula (I,) wherein: X1 and X2 are each independently selected from formulae (II) and (III), wherein Ar1, Ar2 and Ar3 in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2 C(R7)2 and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5 or Si(R6)2 wherein R5 is H or a substituent; each R6 is independently a substituent; and each R7 is independently H or a substituent; one of Y1 and Y2 is CR1 wherein R1 is H or a substituent; and the other of Y1 and Y2 is N; and one of Y3 and Y4 is CR1; and the other of Y3 and Y4 is N; and the polymer is substituted with a crosslinkable group.

Description

[0001] THERMALLY CONDUCTIVE POLYMER

[0002] BACKGROUND

[0003] Thermally conductive materials are used in a wide variety of applications including in underfill for flip-chips to reduce thermally induced stresses following application of a flip chip.

[0004] WO 2022 / 136584 discloses thermally conductive polymers formed by reaction of a dialdehyde and a diamine.

[0005] Jinhong Yu et al, “Polymer / boron nitride nanosheet composite with high thermal conductivity and sufficient dielectric strength”, Polymers Advanced Technologies, Volume26, Issue 5 2015 pages 514-520 discloses epoxy composites incorporating boron nitride nanosheets.

[0006] SUMMARY

[0007] The present disclosure provides a polymer comprising a repeat unit of formula (I): wherein:

[0008] X1and X2are each independently selected from formulae (II) and (III):

[0009] -(Ar^m- Formula (II)

[0010] -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1 ; p is at least 1 ; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2 C(R7)2and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5or Si(R6)2 wherein R5is H or a substituent; each R6is independently a substituent; and each R7is independently H or a substituent; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N; and the polymer is substituted with a crosslinkable group.

[0011] Optionally, the repeat unit of formula (I) is substituted with a crosslinkable group.

[0012] Optionally, at least one occurrence of at least one of Ar1, Ar2and Ar3is substituted with a crosslinkable group.

[0013] Optionally, the repeat unit of formula (I) is substituted with at least two crosslinkable groups.

[0014] Optionally, the polymer comprises crosslinkable end-groups.

[0015] Optionally, the or each crosslinkable group is a group of formula (IV):

[0016] -(Sp)y-(XL)Z

[0017] (IV) wherein Sp is a spacer group; y is 0 or 1; z is 1 if y is 0; and z is at least 1, preferably 1-4, and XL is a crosslinkable unit.

[0018] Optionally, the or each crosslinkable group comprises at least one crosslinkable unit selected from: an acyclic unit of formula -CR10=CH2 wherein R10is H or a substituent, preferably a Ci-6 alkyl group, for example vinyl, styryl acrylate or methacrylate; a cyclic alkene, preferably an optionally substituted norbomene, cyclopropene or cyclobutene; an optionally substituted epoxide; and an optionally substituted benzocyclobutene.

[0019] Optionally, Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N.

[0020] Optionally, Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a monocyclic or fused 6-12 membered arylene or an optionally fused 5- or 6-membered heteroarylene. A fused 5- or 6-membered heteroarylene preferably has 8-14 aromatic ring atoms. Optionally, Ar1, Ar2and Ar3independently in each occurrence is an unsubstituted or substituted para-phenylene. Optionally, p and q are each 1. The present disclosure provides a monomer of formula M: RG-X-RG M wherein: RG is a reactive group selected from C(=O)R1and NH2; X is selected from formulae (II) and (III): -(Ar1)m- Formula (II) -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2C(R7)2 and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5or Si(R6)2 wherein R5is H or a substituent; each R6is independently a substituent; each R7is independently H or a substituent; and X is substituted with at least one crosslinkable group. R5in each occurrence is preferably H or a C1-20hydrocarbyl group and R6in each occurrence is preferably a C1-20hydrocarbyl group. The present disclosure provides a method of forming a polymer comprising a repeat unit of formula (I), the method comprising reacting a first monomer of formula M1 and a second monomer of formula M2: RG1-X1-RG1M1 RG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and NH2 and each RG2is a second reactive group which is the other of C(=O)R1and NH2.

[0021] The present disclosure provides a film comprising a polymer comprising a repeat unit of formula (I).

[0022] The present disclosure provides a crosslinked film comprising a crosslinked polymer comprising a repeat unit of formula (I).

[0023] Optionally, the film is formed by a thermal processing method.

[0024] Optionally, the thermal processing method is selected from extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

[0025] The present disclosure provides an electronic device comprising a film as described herein disposed on a surface of a functional layer of the electronic device.

[0026] Optionally, the film is disposed in a region between the surface of the functional layer and a first surface of a first chip electrically connected to the functional layer.

[0027] Optionally, the functional layer is a printed circuit board; an interposer; or a second chip.

[0028] Optionally, the electronic device comprises a plurality of films as described herein.

[0029] Optionally, the electronic device comprises a 3D chip stack.

[0030] The present disclosure provides apparatus comprising a heat-generating device, a heat transfer device configured to transfer heat away from the heat-generating device and a film as described herein disposed between the heat-generating device and the heat transfer device.

[0031] The present disclosure provides a heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a film as described herein disposed thereon.

[0032] DESCRIPTION OF DRAWINGS Figure 1 schematically illustrates a 3D chip stack according to an embodiment of the present disclosure;

[0033] Figure 2 is a differential scanning calorimetry thermogram for a monomer according to an embodiment of the present disclosure;

[0034] Figure 3 is a differential scanning calorimetry thermogram for Polymer Example 1 according to an embodiment of the present disclosure;

[0035] Figure 4 is a differential scanning calorimetry thermogram for Polymer Example 3 according to an embodiment of the present disclosure;

[0036] Figure 5 is a differential scanning calorimetry thermogram for a model compound; and

[0037] Figure 6 is the infrared spectrum for the model compound 1.

[0038] Figure 7 schematically illustrates a substrate for measurement of thermal conductivity of a film; and

[0039] Figures 8A and 8B schematically illustrate apparatus for measurement of thermal conductivity including the substrate of Figure 4.

[0040] The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and / or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

[0041] DETAILED DESCRIPTION

[0042] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or" in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. References to a layer “over” another layer when used in this application means that the layers may be in direct contact or one or more intervening layers may be present. References to a layer “on” another layer when used in this application means that the layers are in direct contact. References to a chemical element include isotopes of that element.

[0043] The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

[0044] These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

[0045] To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

[0046] The present inventors have found that a stable, thermally conductive film can be provided by crosslinking of a polymer comprising a repeat unit of formula (I): wherein:

[0047] X1and X2are each independently selected from formulae (II) and (III):

[0048] -(Arbm- Formula (II)

[0049] -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1 ; p is at least 1 ; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2 C(R7)2and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5or Si(R6)2 wherein R5is H or a substituent; each R6is independently a substituent; and each R7is independently H or a substituent; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N; and the polymer is substituted with at least one crosslinkable group.

[0050] Polymers as described herein are preferably at least partially crystalline.

[0051] Polymers as described herein may undergo pi-pi stacking when deposited as a film. Preferably, the polymer is a linear polymer. A polymer comprising a repeat unit of formula (I) may be formed by reacting a monomer comprising at least two aldehyde groups and a monomer comprising at least two amine groups to form the imine (-Y1=Y2- and -Y3=Y4-) units of the repeat unit of formula (I). The polymerisation may be as described in WO 2022 / 136584, the contents of which are incorporated herein by reference. Preferably, the or each aldehyde monomer is a dialdehyde monomer. Preferably, the or each amine monomer is a diamine monomer. It will be understood that polymerisation of dialdehyde monomer(s) and diamine monomer(s) gives a linear polymer. Optionally, thermal conductivity of polymers as described herein is at least 0.15 Wm-1K-1, optionally at least 0.4, 0.5, 0.6 or 0.7 Wm-1K-1. The polystyrene-equivalent number-average molecular weight (Mn) measured by gel permeation chromatography of the crosslinkable polymers described herein may be in the range of about 1x103to 1x108, and preferably 1x104to 5x106. The polystyrene-equivalent weight-average molecular weight (Mw) of the polymers described herein may be 1x103to 1x108, and preferably 1x104to 1x107. L groups R5independently in each occurrence is preferably H or a C1-20hydrocarbyl group. R6independently in each occurrence is preferably a C1-20 hydrocarbyl group. R7independently in each occurrence is preferably H, F or an optionally fluorinated C1-20 hydrocarbyl group. Unless otherwise stated, a C1-20hydrocarbyl group as described anywhere herein is preferably a C1-20 alkyl or phenyl which is unsubstituted or substituted with one or more C1-14 alkyl groups, optionally one or more C1-12 alkyl groups. One or more H atoms of the methylene groups of L may be replaced with one or more substituents. Optionally, one or more H atoms are replaced with F, C1-6 alkyl or C1-6 fluoroalkyl. Preferably, L contains at least 3 or at least 4 atoms separating Ar2and Ar3. Preferably, L is a chain of methylene groups and O atoms. Exemplary groups L include, without limitation, -OCH2CH2O- and -OCH2O-.

[0052] Crosslinkable groups

[0053] In a preferred arrangement, end-groups of the polymer are substituted with at least one crosslinkable group. The polymer comprising a repeat unit of formula (I) formed by polymerisation of a dialdehyde monomer and a diamine monomer may be formed using an excess of the dialdehyde monomer or the diamine monomer so as to provide aldehyde or amine end-groups at the ends of the polymer. Crosslinkable end-groups may be formed by reacting an aldehyde end-group with a compound having the crosslinkable group substituted with an amine or by reacting an amine end-group with a compound having the crosslinkable group substituted with an aldehyde.

[0054] In another preferred arrangement, some or all of the repeat units of formula (I) are substituted with at least one crosslinkable group. More preferably, at least one of -(Ar^m-, (Ar2)p- and - (Ar3)q- is substituted with at least one crosslinkable group.

[0055] The crosslinkable groups may be provided only as polymer end-groups.

[0056] The crosslinkable groups may be provided only as substituents of the repeat unit of formula (I).

[0057] The crosslinkable groups may be provided as both polymer end-groups and substituents of the repeat unit of formula (I)

[0058] In the case where m is greater than 1, each Ar1may be substituted with at least one crosslinkable group. Preferably, only one Ar1is substituted with at least one crosslinkable group.

[0059] In the case where p is greater than 1, each Ar2may be substituted with at least one crosslinkable group. Preferably, only one Ar2is substituted with at least one crosslinkable group.

[0060] In the case where q is greater than 1, each Ar3may be substituted with at least one crosslinkable group. Preferably, only one Ar3is substituted with at least one crosslinkable group.

[0061] An Ar1, Ar2or Ar3group which is substituted with at least one crosslinkable group is preferably substituted with 1-4 crosslinkable groups, more preferably 1 or 2 crosslinkable groups. In some embodiments, every repeat unit of formula (I) is substituted with at least one, optionally 1-4, crosslmkable groups.

[0062] In some embodiments, the polymer may comprise repeat units of formula (I) which are substituted with at least one, optionally 1-4, crosslinkable groups and repeat units of formula (I) which are not substituted with a crosslinkable group. In these embodiments, 5-95 % of the repeat units of formula (I) are substituted with a crosslinkable group.

[0063] Crosslinkable groups are preferably selected from groups of formula (IV):

[0064] -(Sp)y-(XL)Z

[0065] (IV) wherein Sp is a spacer group; y is 0 or 1; z is 1 if y is 0; and z is at least 1, preferably 1-4, if y is 1; and XL is a crosslinkable unit.

[0066] Optionally, XL is selected from

[0067] (i) an acyclic unit of formula -CR10=CH2 wherein R10is H or a substituent, preferably a Ci-6 alkyl group, for example vinyl, styryl acrylate or methacrylate;

[0068] (ii) a cyclic alkene, preferably an optionally substituted norbomene, cyclopropene or cyclobutene;

[0069] (iii) an optionally substituted epoxide; and

[0070] (iv) an optionally substituted benzocyclobutene.

[0071] An optionally substituted benzocyclobutene group XL may have formula (V): wherein R8in each occurrence is H or a substituent; z is 0, 1, 2 or 3, preferably 0; R9in each occurrence is a substituent; and * is a bond to Sp or, if y is 0, a direct bond to the polymer backbone. Preferably, of two R8groups bound to the same carbon atom at least one R8is H. Optionally, each R8of formula (V) is H or only one R8of formula (V) is not H. Exemplary non-H groups R8are Ci-6 alkyl, Ci-6 alkoxy and a phenyl which is unsubstituted or substituted with one or more Ci-6 alkyl groups.

[0072] R9, if present, is preferably selected from F, Cl, NO2, CN, C1-6 alkyl and C1-6 alkoxy.

[0073] Sp is preferably selected from optionally substituted phenylene; and Ci-20 alkylene wherein one or more H atoms of the Ci-20 alkylene may be replaced with F and one or more non-adjacent C atoms may be replaced with O, S, NR5, Si(R6)2, CO, COO or CONR5wherein R5is H or a substituent as described above and each R6is independently a substituent, optionally a Ci-20 hydrocarbyl group.

[0074] Optional substituents of a phenylene group Sp are F; CN; NO2; and Ci-20 alkylene wherein one or more H atoms may be replaced with F and one or more non-adjacent C atoms may be replaced with O, S, NR5, Si(R6)2, CO, COO or CONR5.

[0075] X1and X2

[0076] Optionally, X1and X2are each independently a group of formula (II).

[0077] Preferably, at least one of X1and X2is a group of formula (III). More preferably, one of X1and X2is a group of formula (II) and the other of X1and X2is a group of formula (III).

[0078] Y' - Y4

[0079] R1is preferably H or a C 1-20 hydrocarbyl group, more preferably H or a C1-6 alkyl group, most preferably H.

[0080] Preferably, Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N. Preferably, R1of each CR1is the same.

[0081] Ar1-Ar3

[0082] Preferably, Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a 6-12 membered arylene or an optionally fused 5- or 6-membered heteroarylene, e.g., an unfused 5- or 6-membered heteroarylene or a benzene-fused 5- or 6-membered hetero arylene. Exemplary groups Ar1, Ar2and Ar3include, without limitation, para-phenylene, thiophene, furan, and benzobisoxazole, each of which may independently be unsubstituted or substituted with one or more substituents. Para-phenylene is preferred. m is preferably more than 1, preferably 2-5, more preferably 2 or 3. p and q are preferably each independently 1 or 2, more preferably 1.

[0083] Each of Ar1, Ar2and Ar3is independently unsubstituted or substituted with one or more substituents R2.

[0084] Preferably, where present R2in each occurrence is independently selected from:

[0085] F;

[0086] CN;

[0087] NO2; branched, linear or cyclic Ci-40 alkyl, preferably Ci-20 alkyl, wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO; wherein R5in each occurrence is H or a substituent, preferably H or a Ci-20 hydrocarbyl group and R6in each occurrence is independently a substituent, optionally a Ci-20 hydrocarbyl group; an aryl or heteroaryl group Ar5which is unsubstituted or substituted with one or more substituents, optionally phenyl which is unsubstituted or substituted with one or more substituents selected from F, CN, NO2 and branched, linear or cyclic Ci-20 alkyl wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO; or a crosslinkable group.

[0088] Preferably, at least one substituent R2, optionally each substituent R2, is Ci-20 alkyl, Ci-20 alkoxy, a group of formula -(Ak1)y-(OCH2CH2)Z-Ak2wherein Ak1is a C1-4 alkylene group; y is 0 or 1; z is 1-15; and Ak2is a C1-4 alkyl group; and a crosslinkable group. More preferably R2is a Ci-12 alkyl, Ci-12 alkoxy or a crosslinkable group.

[0089] Exemplary groups -(Ar^m- include, without limitation, groups of formulae (Via) and (VIb):

[0090] (Via) (VIb) wherein R2independently in each occurrence is a substituent and w in each occurrence is independently 0 or a positive integer.

[0091] A preferred group -(Ar^m- has formula (VIb-1):

[0092] (VIb-1)

[0093] Ar2and Ar3are each preferably optionally substituted 1,4-phenylene, more preferably unsubstituted 1,4-phenylene.

[0094] Polymerisation

[0095] The polymer may be formed by polymerisation of a first monomer containing X1and two reactive groups and a second monomer containing X2substituted with two reactive groups, wherein the reactive groups of the first and second monomers react to form imine (-CR^N-) linkages between X1and X2. Suitable reactive groups for reaction to form imine linkages are aldehyde or ketone (-C(=O)R1) reactive groups, preferably aldehyde reactive groups, and amine (NH2) reactive groups.

[0096] In some embodiments, polymerisation is between a first monomer of formula Ml and a second monomer of formula M2:

[0097] RG^X^RG1Ml

[0098] RG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and NH2 and each RG2 is a second reactive group which is the other of C(=O)RXand NH2. Preferably, each RG1is C(=O)R1and each RG2is NH2.

[0099] Exemplary monomers include:

[0100] The reaction between RG1and RG2may be catalysed by a Lewis acid. The Lewis acid may be a Bronsted-Lowry acid. Exemplary catalysts include, without limitation, sulfonic acids and salts thereof, for example p-toluene sulfonic acid; triflic acid; and salts thereof; and aromatic alcohols, more preferably benzene which is substituted with at least one hydroxyl group and, optionally, one or more further substituents for example one or more C1-6 alkyl groups.

[0101] An exemplary triflic acid salt is scandium triflate, Sc(Trf)3. Exemplary aromatic alcohols are phenol which is unsubstituted or substituted with one more Ci-6 alkyl groups, more preferably one or more methyl groups, for example m-cresol.

[0102] The catalyst may be provided in an amount of 0.01-2 molar equivalents, optionally 0.01-3 molar equivalents, of the total number of moles of the monomers. The catalyst is suitably soluble in the solvent or solvent mixture in which the monomer or monomers are dissolved.

[0103] Optionally, the solvent or solvents are selected from benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from Ci-12 alkyl, Ci-12 alkoxy, F and Cl; ethers; esters; halogenated alkanes; ketones; sulfoxides; and mixtures thereof. Exemplary solvents include, without limitation, toluene, xylenes, 1,2,4-trimethylbenzene, mesitylene, tetrahydrofuran, 2-methyltetrahydrofuran, 1 -methylnaphthalene, 1- chloronaphthalene, diiodomethane, anisole, N-methylpyrrolidone, 1,2-dimethoxybenzene, dimethylsulfoxide l,3-dimethyl-2-imidazolidinone and cyclopentanone.

[0104] Polymer processing

[0105] Formation of a film comprising a crosslinkable polymer as described herein may comprise formation of a liquid monomer film comprising the monomers for forming the polymer followed by polymerisation of the monomers, referred to hereinafter as in situ polymerisation.

[0106] In some embodiments, the liquid monomer film comprises or consists of molten monomers.

[0107] In some embodiments, the liquid monomer film may be a monomer formulation comprising the monomers dissolved in one or more solvents. According to these embodiments, in-situ polymerisation preferably takes place in solution.

[0108] In some embodiments, the monomers are the sole materials dissolved or dispersed in the monomer formulation. In other embodiments, the monomer formulation comprises one or more further materials, for example one or more of a polymerisation catalyst; inorganic thermally conductive particles, for example boron nitride particles; and a crosslinking agent.

[0109] Following reaction of the monomers to form a film comprising a crosslinkable polymer, the polymer may be cured. Curing may be performed by light treatment, preferably UV light treatment, and / or heat treatment. Heat treatment is preferably performed at a temperature of at least 200°C. Preferably, formation of a cured film as described herein comprises deposition of a polymer formulation comprising the crosslinkable polymer comprising a repeat unit of formula (I) dissolved or dispersed in one or more solvents. The formulation may consist of the polymer and one or more solvents or it may comprise one or more further materials, e.g., inorganic thermally conductive particles such as boron nitride particles and / or a crosslinking agent.

[0110] Solvents for a monomer or polymer formulation may be selected according to their ability to dissolve the monomers or the polymer for example, without limitation, benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from Ci-12 alkyl, Ci-12 alkoxy, F and Cl; ethers; esters; halogenated alkanes; ketones; sulfoxides; and mixtures thereof. Exemplary solvents include, without limitation, toluene, xylenes, 1,2,4- trimethylbenzene, mesitylene, 1 -methylnaphthalene, 1 -chloronaphthalene, diiodomethane, anisole, tetrahydrofuran, N-methylpyrrolidone, 1,2-dimethoxybenzene, dimethylsulfoxide 1,3- dimethyl-2-imidazolidinone, 1,2-dichlorobenzene and cyclopentanone.

[0111] The concentration of each monomer, or the polymer, dissolved in the monomer or polymer formulation is preferably in the range of about 1-50 mg / ml, more preferably about 10-40 mg / ml. The monomer or polymer formulation may be heated to achieve dissolution of the monomer or monomers or the polymer.

[0112] In a preferred embodiment, the crosslinkable polymer is heated to a curing temperature which is higher than the polymer melting point. Preferably, the polymer has a melting point of no more than 200°C. Following application, the polymer is preferably heated to a curing temperature above 150°C, preferably 200°C, preferably at least 220°C or at least 240°C.

[0113] Monomer or polymer formulations as described anywhere herein may be deposited by any suitable solution deposition technique including, without limitation, spin-coating, dip-coating, jet dispensing, drop-casting, spray coating and blade coating.

[0114] A monomer or polymer melt as described herein may be processed using a thermal processing technique, for example extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

[0115] Optionally, a cured thermal transfer film comprising or consisting of a polymer as described herein has a thickness in the range of 1-100 microns, preferably 5-100 microns or 10-100 microns. The thermal transfer film may consist of the polymer or may contain one or more further materials, optionally one or more amorphous polymers, e.g. polystyrene, polyethylene or polypropylene; and / or one or more thermally conductive materials, for example boron nitride.

[0116] Inorganic thermally conductive particles

[0117] In some embodiments, a cured thermal transfer film as described herein does not comprise any thermally conductive particles. Optionally according to these embodiments, the cured thermal transfer film consists of a polymer as described herein.

[0118] In some embodiments, the cured thermal transfer film comprises thermally conductive particles, such as boron nitride, dispersed therein.

[0119] Inorganic thermally conductive particles such as boron nitride as described herein may be substituted with an organic group, optionally an aromatic group, for example an oligo- (hetero)arylene comprising 1-10 arylene or hetero arylene groups, or a poly-(hetero)arylene. An exemplary surface group is an oligophenylene, for example biphenyl or terphenyl. Surface groups of a thermally conductive particle are disclosed in WO2022 / 207695, the contents of which are incorporated herein by reference.

[0120] Applications

[0121] A film comprising a polymer as described herein may be used in any known application of a thermally conductive film. The film as described herein may be disposed between a surface of a heat-generating device and a heat transfer device configured to transfer heat away from the heat-generating device, such as in any known thermal interface management application.

[0122] It will be understood that in this arrangement the film is configured to transfer heat from the heat-generating device to the heat transfer device. The film preferably has a first surface in direct contact with a surface of the heat-generating device and / or a second surface opposing the first surface in direct contact with a surface of the heat transfer device.

[0123] The heat-generating device may be an electronic device.

[0124] Any passive or active heat transfer device known to the skilled person may be used including, without limitation, a heat sink having a surface in contact with the film and an opposing surface comprising one or more heat-dissipating features, for example fins or a pipe or channel configured to transfer heat to a fluid flowing through the pipe or channel. The fluid may or may not undergo a phase change upon absorption of heat.

[0125] Preferably, the film is a thermally conductive layer of an electronic device.

[0126] Heat may be transferred from a surface by bringing a layer comprising a thermally conductive film as described herein adjacent to the surface. The thermally conductive film may be in direct contact with the surface or it may be spaced apart from the surface by one or more thermally conductive layers.

[0127] A film as described herein may be disposed on a surface of a heat sink opposing a surface of the heat sink having fins extending therefrom. In use, the film may be disposed between the heat sink and an electrical component.

[0128] A film as described herein may be a heat spreader layer disposed on a surface of a printed circuit board, for example a PCB for use in LED arrays.

[0129] A film as described herein may be used as an electrically non-conductive film, e.g. an underfill, between opposing surfaces of electrically connected layers, e.g. between a substrate and a semiconducting wafer or between two semiconducting wafers, such as for a flip chip including but not limited to 3D stacked multi-chips. The crosslinkable polymer may be introduced between the two layers by any method known to the skilled person, for example a capillary underfill method or pre-applied underfill method. Following formation of the film comprising the crosslinkable polymer, the film may be heated to above a melting point of the crosslinkable polymer. Following alignment and bonding between the opposing surfaces, the film may be heated to a curing temperature above the melting point to form a solid, crosslinked film.

[0130] Following crosslinking, the structure may be diced. Dicing may be performed using any method known to the skilled person.

[0131] Figure 1 illustrates a 3D stack of chips 105 according to some embodiments, wherein the chips 105 are interposed by an interposer 111 and a non-electrically conductive film 109 disposed between adjacent interposer and chip surfaces and between the substrate 101, e.g. a printed circuit board, and a first chip of the 3D stack. At least one non-electrically conductive film 109 comprises a polymer as described herein. Through-vias 115 are formed through the chips 105 and the interposers. The 3D stack may comprise a heat sink 113 disposed on a surface thereof.

[0132] In formation of the 3D stack, a first non-electrically conductive film 109 may be formed by deposition of a crosslinkable polymer as described herein followed by crosslinking of the film prior to or after formation of one or more further non-electrically conductive films 109.

[0133] Following crosslinking, the film is stable to elevated temperatures that may be used in subsequent processing steps, for example formation of the one or more further non-electrically conductive films 109. Preferably, a plurality of the non-electrically conductive films 109 comprise or consist of a crosslinked polymer as described herein. In some embodiments, a film comprising or consisting of a polymer as described herein may be disposed between an electronic device and a heat sink.

[0134] EXAMPLES

[0135] Monomer Example 1

[0136] Monomer Example 1

[0137] Monomer Example 1 – Step 1 4-Bromobenzocyclobutene (20 g, 0.11 mol), hex-5-yn-1-ol (15.9 g, 0.16 mol) and Copper Iodide (0.62 g, 3 mol%) were dissolved in DCM (100 mL) and triethylamine (209 mL, 1.53 mol). The solution was purged with nitrogen for 40 minutes before bis(triphenylphosphine)palladium (II) chloride (2.3 g, 3 mol%) was added, and the solution was heated at 75 °C overnight. After completion, the reaction mixture was brought to room temperature and filtered through a fluorosil bed. The filtrate was concentrated under vacuum and further purified by column chromatography (GRACE 230-400 mesh silica, 30 % ethyl acetate in hexane) to give Intermediate 3 as two fractions, F1: 3.5 g of 82.68 % LCMS purity; F2. 16.5 g of 93.3 % LCMS purity as a pale brown liquid. Yield = 76 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 1.68-1.79 (m, 4H), 2.47 (t, J = 6.40 Hz, 2H), 3.17 (s, 4H), 3.73 (t, J = 6.40 Hz, 2H), 6.98 (d, J = 7.60 Hz, 1H), 7.10 (s, 1H), 7.26 (d, J = 7.60 Hz, 1H). Monomer Example 1 – Step 2 Intermediate 3 (17 g, 0.08mol) was dissolved in Ethanol (350 mL) in a tiny clave stainless steel pressure reactor equipped with a nitrogen manifold. 10% Palladium on Carbon (1.8 g) was added and after flushing with N2gas, hydrogen gas was introduced (applied pressure = 8 kg / cm2). The reaction was stirred at room temperature overnight and monitored by LCMS analysis. The reaction mixture was filtered through a celite bed and the filtrate was concentrated and the crude product was purified by column chromatography (230-400 mesh silica gel, 14 % ethyl acetate in hexane) to isolate the desired product as a colourless liquid (14 g, 81 % yield). LCMS: 95.10 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 1.38-1.43 (m, 4H), 1.58-1.65 (m, 4H), 2.60 (t, J = 8.00 Hz, 2H), 3.16 (s, 4H), 3.66 (t, J = 6.40 Hz, 2H), 6.91 (s, 1H), 6.98 (d, J = 7.20 Hz, 1H), 7.02 (dd, J = 7.60, Hz, 1H). Monomer Example 1 – Step 3 Intermediate 4 (8.5 g, 0.04 mol) was dissolved in THF (130 mL) and cooled to 0 °C and a solution of sodium hydroxide (9.96 g, 6 eq.) in water (20 mL) was added to the reaction mixture. A solution of 4-toluenesulfonyl chloride (10.3 g, 0.05 mol) in THF (50 mL) was then slowly added at 0 °C. The reaction mixture was allowed to warm to room temperature and stirred for 16 hours. LCMS analysis showed incomplete reaction; additional sodium hydroxide (6.7 g, 4 eq.) in water (10 mL) was added and the reaction mixture was stirred for additional 32 hours at room temperature. The reaction mixture was then diluted with DCM and washed with water, then with saturated brine solution. The organic layer was concentrated under vacuum and the crude residue was purified by column chromatography (230-400 mesh silica gel, 2 % ethyl acetate in hexane) to give the desired product(8.8 g 59 %. LCMS: 97.49 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 1.30-1.37 (m, 4H), 1.52-1.59 (m, 2H), 1.62-1.67 (m, 2H), 2.47 (s, 3H), 2.55 (t, J = 8.00 Hz, 2H), 3.16 (t, J = 4.00 Hz, 4H), 4.04 (t, J = 6.40 Hz, 2H), 6.88 (s, 1H), 6.98 (s, 2H), 7.36 (d, J = 8.00 Hz, 2H), 7.81 (d, J = 8.40 Hz, 2H). Monomer Example 1 – Step 4 Intermediate 5 (7.7 g, 0.022 mol) was dissolved in DMF (100 mL) and 2,5-dibromobenzene- 1,4-diol (2.5 g, 0.01 mol) was added. Potassium carbonate (6.44 g, 0.05 mol), and 18-Crown- 6 (0.49 g, 0.002 mol) dissolved in DMF (5 mL) were added and the reaction mixture was heated at 100 °C for 20 hours. The reaction mixture was cooled to room temperature, diluted with ethyl acetate and washed with water twice. The combined organic layer was washed with saturated brine solution, dried over anhydrous sodium sulphate, and then concentrated under vacuum. The crude product was then purified by column chromatography (230-400 mesh silica gel, 6 % ethyl acetate in hexane) to give Intermediate 7 as a white solid (4.5 g, 33 %). LCMS: 92.95 %.1H-NMR (400 MHz, CDCl3): 1.40-1.41 (m, 4H), 1.50-1.52 (m, 4H), 1.61-1.63 (m, 4H), 1.79- 1.80 (m, 4H), 2.61 (t, J = 7.60 Hz, 4H), 3.16 (d, J = 3.60 Hz, 8H), 3.96 (t, J = 6.40 Hz, 4H), 6.91 (s, 2H), 6.97 (d, J = 7.20 Hz, 2H), 7.03 (d, J = 7.20 Hz, 2H), 7.10 (s, 2H). Monomer Example 1 – Step 5 Intermediate 7 (3 g, 4.68 mmol) and (4-formylphenyl) boronic acid (3.5 g, 0.02 mol) were dissolved in a solution of toluene (105 mL) and ethanol (45 mL). A solution of sodium carbonate (7.44 g, 0.07 mol) in water (45 mL) was added and the reaction mixture was degassed with N2 gas for 30 minutes. Tetrakis(triphenylphosphine)palladium(0) (0.27 g, 23 µmol) was added and the reaction mixture was degassed with N2 gas for 10 minutes before being heated at 85 °C for 3 hours. The reaction mixture was cooled to room temperature and extracted with ethyl acetate, the organic layers were separated, washed with water twice, saturated brine solution, dried over anhydrous sodium sulphate, and then concentrated under vacuum. The crude material was purified by column chromatography (neutral alumina, 50 % DCM in hexane) followed by recrystallization from acetonitrile (10 volume) which was heated to 60 °C, followed by introduction of toluene (2 volume) until complete dissolution. Filtration gave the desired product as a pale yellow solid (2.165 % yield). LCMS: 98.33 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 1.24-1.26 (m, 4H), 1.32-1.34 (m, 4H), 1.46-1.48 (m, 4H), 1.60-1.62 (m, 4H), 2.61-2.67 (m, 4H), 3.06 (s, 8H), 4.01 (t, J = 6.40 Hz, 4H), 7.15 (s, 2H), 7.83 (d, J = 8.00 Hz, 4H), 7.97 (d, J = 8.40 Hz, 4H). Monomer Example 2

[0138]

[0139] Monomer Example 2 – Step 1 2,5-dibromobenzene-1,4-diol (10 g, 0.03 mol) and potassium carbonate (11.3 g, 0.08 mol) were dissolved in 2-butanone (100 mL). After stirring at room temperature for 15 minutes 6- bromohexan-1-ol (13.5 g, 0.07 mol) was added and the mixture heated to 75 °C and stirred for 6 hours. The reaction mixture was then cooled to 0 °C, diluted with water (150 mL), acidified using 1.5N HCl (~50 mL) until pH reached ~ 5 to 6. It was then extracted with ethyl acetate (2 × 100 mL) and the combined organic layers were washed with saturated brine solution (100 mL), dried over anhydrous sodium sulphate (5 g) and concentrated under vacuum. The crude material was purified by column chromatography (silica, 0 to 35 % ethyl acetate in hexanes) to give two fractions: Fraction-1: (16.5 g, 92 % LCMS purity) and Fraction -2: (2 g, of 85.26% LCMS purity). Fraction-1 was triturated with n-hexane (165 mL) and filtered, dried to give the desired product (14 g, 80 %). LCMS: 94.49 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 1.47 (m, J = 6.8 Hz, 4H), 1.54 (m, J = 7.2 Hz, 4H), 1.64 (m, J = 6.8 Hz, 4H), 1.85 (m, J = 6.8 Hz, 4H), 3.69 (t, J = 6.4 Hz, 4H), 3.98 (t, J = 6.4 Hz, 4H), 7.1 (s, 2H). Monomer Example 2 – Step 2 DMF (50 mL, anhydrous) was added to sodium hydride (3.58 g, 0.09 mol, 60 % dispersion in oil) under N2. The reaction mixture was cooled to 0 °C in an ice-bath and a solution of intermediate 3 (14 g, 0.03 mol) in DMF (50 mL) was added dropwise over a period of 15 minutes. The ice bath was removed and the reaction mixture was stirred at room temperature for 30 minutes. It was then cooled to 0 °C and a solution of tert-butyl dimethylsilyl chloride(11.2 g, 0.07 mol) in DMF (50 mL) was added dropwise and the reaction mixture was allowed to warm to room temperature and stirred for about 4 hours. The reaction mixture was cooled to 0 °C and quenched with ice-cold water at 0 °C. and hexanes (150 mL) was added. The organic layer was separated and washed with saturated brine solution and dried over anhydrous sodium sulphate and reduced under vacuum. The crude product was purified by column chromatography (100-200 mesh silica, 0 to 10 % ethyl acetate in hexanes) and then again (Silica flash column, 0 to 35 % DCM in hexanes) to give the desired product (17.2 g, 83 % yield). LCMS: 93.25 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 0.07 (s, 12H), 0.92 (s, 18H), 1.45 (m, J = 7.2 Hz, 4H), 1.53 (m, J = 6.4 Hz, 4H), 1.59 (m, J = 6.8 Hz, 4H), 1.84 (m, J = 6.8 Hz, 4H), 3.64 (t, J = 6.4 Hz, 4H), 3.97 (t, J = 6.4 Hz, 4H), 7.1 (s, 2H). Monomer Example 2 – Step 3 Intermediate 4 (17.2 g, 0.02 mol) and (4-formylphenyl) boronic acid (11 g, 0.07 mol) were dissolved in tetrahydrofuran (136 mL). A solution of potassium carbonate (20.4 g, 0.15 mol) in water (34 mL) was added, the reaction was purged with nitrogen for 15 minutes and dichlorobis(triphenylphosphine)palladium (II) (1.72 g, 0.003 mol, 10 mol%) was introduced. The reaction mixture was heated to 80°C and stirred at 80 °C for 16 hours. It was then diluted with ethyl acetate, filtered through a celite bed and the filtrate washed with water (200 mL), followed by saturated brine solution (100 mL), dried over anhydrous sodium sulphate, and concentrated under vacuum. The crude material was dissolved in DCM (250 mL),passed through a florisil-plug and the plug was washed with DCM (250 mL), and concentrated. The residue was triturated with methanol (100 mL) for 30 minutes at room temperature, filtered and washed with methanol (20 mL), to give the desired product (15 g 82 %). LCMS: 97.6 %.1H-NMR (400 MHz, CDCl3): δ [ppm] 0.05 (s, 12H), 0.9 (s, 18H), 1.37 (m, J = 6.4 Hz, 8H), 1.51 (m, J = 7 Hz, 4H), 1.73 (m, J = 6.4 Hz, 4H), 3.59 (t, J = 6.4 Hz, 4H), 3.97 (t, J = 6.4 Hz, 4H), 7.03 (s, 2H), 7.8 (d, J = 8 Hz, 4H), 7.96 (d, J = 8.4 Hz, 4H), 10.1 (s, 2H). Monomer Example 2 – Step 3 Intermediate 6 (15 g, 0.20 mol) was dissolved in tetrahydrofuran (150 mL) and cooled to 0°C with an ice-bath. Tetrabutylammonium fluoride (13.1 g, 1 M in THF, 0.50 mol) was added dropwise to the reaction mixture at 0°C over a 15-minutes. The ice-bath was removed and the reaction was stirred at room temperature for 16 hours. It was then cooled to 0°C, quenched with water (250 mL) and extracted with ethyl acetate (200 mL), dried over anhydrous sodium sulfate and concentrated under vacuum. The crude material was purified by column chromatography (230-400 mesh silica, 2-10 % MeOH in DCM ) to give two fractions which were triturated with acetonitrile. The mother liquors of both fractions were combined and further purified by column chromatography (230-400 mesh silica, 2 % MeOH in DCM) and titubation with acetonitrile (15 mL) give the desired product as a green solid (1.1, g 28 %). LCMS: 96.8 %.1H-NMR (400 MHz, DMSO-D6): δ [ppm] 1.23-1.41 (m, 12H), 1.63 (m, 4H), 3.37 (m, J = 6 Hz, 2H), 4.02 (t, J = 6 Hz, 4H), 4.33 (m, J = 6 Hz, 2H), 7.16 (s, 2H), 7.84 (d, J = 8 Hz, 4H), 7.98 (d, J = 8 Hz, 4H), 10.1 (s, 2H). Monomer Example 2 – Step 4 Intermediate 7 (5.8 g, 0.11 mol), 4,4-dimethylaminopyridine (136 mg, 0.01 mol), triethyl amine (4.5 g, 0.04 mol) were combined in dichloromethane (120 mL) and the reaction mixture was cooled to 0°C. 5-methylprop-2-enoyl chloride (3.5 g, 0.03 mol) was added dropwise, then the solution was warmed to room temperature and stirred for 16 hours. It was cooled and quenched with ice-cold water (30 mL), then extracted with DCM (2 × 50 mL). The combined organic extracts were washed with water (50 mL), dried over anhydrous sodium sulphate, and concentrated under vacuum. The crude material was purified by column chromatography (silica, 0 to 5 % ethyl acetate in DCM) to give five fractions, Fraction - 1: 0.6 g 96.8 % LCMS purity. Fraction - 2: 0.9 g 90.1 % LCMS purity. Fraction - 3: 2.3 g 65.03 % LCMS purity. Fraction - 4: 1.3 g 74.5 % LCMS purity. Fraction - 5: 4.6 g 95.9 % LCMS purity. Fraction - 2, Fraction - 3 & Fraction - 4 were combined and purified by column chromatography (silica cartridge, 2 to 5 % ethyl acetate in DCM) to give 1.7 g of product of 95.4 % LCMS purity. Fraction -1 & Fraction -5 combined and purified by column chromatography (silica, 2 to 5 % ethyl acetate in DCM) to give 3.7 g of product of 97.9 % LCMS purity. These two fractions were combined and triturated twice with diethyl ether (120 mL) at room temperature for 2 hours and filtered. The solid was then dissolved in a minimum amount of DCM (18 mL) and precipitated with diethyl ether (200 mL) to give the product as a yellow solid (4.3 g, 60 % yield). LCMS: 98.3 %).1H-NMR (400 MHz, CDCl3): δ [ppm] 1.41 (m, 8H), 1.66 (t, J = 7.2 Hz, 4H), 1.74 (t, J = 7.2 Hz, 4H), 1.95 (t, J = 1.6 Hz, 6H), 3.98 (t, J = 6.4 Hz, 4H), 4.12 (t, J = 6.8 Hz, 4H), 5.56 (t, J = 6.1 (t, J = 1.8 Hz, 2H), 7.02 (s, 2H), 7.79 (d, J = 8.4 Hz, 4H), 7.97 (d, J = 8.4 Hz, , 2H). xample 3 xample 3 – Step 1 benzene-1,4-diol (15 g, 0.06 mol), 6- Bromo-1-hexene (22.8 g, 0.14 mol) and arbonate (38.7 g, 0.28 mol) were added to acetonitrile (375 mL). The reaction s heated to 90 °C for 16 hours before being cooled to room temperature, diluted (150 mL) and extracted with ethyl acetate (2 × 300 mL). The combined organic washed with water (50 mL) and saturated brine solution (50 mL), dried over odium sulphate and concentrated under vacuum. The residue was purified by repeated column chromatography ((1) silica 230-400 mesh, 6 % DCM in hexane; (2) silica 230-400 mesh, 3 % DCM in hexane and (3) antigravity, silica, 3 % DCM in hexane) to give the product as a colourless oil (9 g, 37 %). LCMS: 95.35 %.1H-NMR (400 MHz, MeOD): δ [ppm] 1.607-1.664 (m, 4H), 1.828-1.866 (m, 4H), 2.136-2.189 (m, 4H), 3.967- 3.999 (t, J = 6.40 Hz, 4H), 4.996-5.095 (m, 4)H, 5.821- 5.890 (m, 2H) , 7.109 (s, 2 H). Monomer Example 3 – Step 2 1,4-dibromo-2,5-bis(hex-5-en-1-yloxy)benzene (9.2 g, 0.02 mol) and (4- formyl phenyl)boronic acid (15.8 g, 0.12 mol) were added to a mixture of toluene (320 mL) and Ethanol (120 mL). The reaction mixture was purged with N2for 30 minutes, then a solution of sodium carbonate (33.6 g, 0.32 mol) in water (70 mL) was added and N2 purging was continued for another 10 minutes. Tetrakis(triphenylphosphine)palladium(0) (1.21 g, 0.001 mol) was then added, and N2purging was continued for further 5 minutes. The reaction mixture was then heated to 100°C for 6 hours. The reaction was cooled to room temperature, diluted with water (50 mL), extracted with ethyl acetate twice (2 × 150 mL). The combined organics were washed with water (50 mL), saturated brine solution (50 ml), dried over anhydrous sodium sulphate and concentrated under vacuum. by Purification by repeated column chromatography (silica 230-400 mesh, 10 %-12 % ethyl acetate in hexane, 3 repeats) gave the desired product as a yellow solid (4.2 g, 40 %). HPLC: 99.18%.1H-NMR (400 MHz, MeOD): δ [ppm] 1.448-1.502 (m, 4H) , 1.704-1.773 (m, 4H), 2.036-2.089 (m, 4H), 3.957-3.997 (m, 4H), 4.946- 5.021 (m, 4H), 5.741- 5.809 (m, 2H) , 7.031 (s, 2 H), 7.78- 7.801 (d, J = 8.4 Hz, 4H ), 7.955- 7.980 (m, 4H), 10.096 (s, 2H). Polymer Example 1 Polymer Example 1 was formed by reaction of dialdehyde Monomer Example 1 and the following diamine monomer 1: m-Cresol (2 equivalents) was added to a stirred solution of the diamine monomer 1 (0.23 g, 1 equivalent) and Monomer Example 1 (0.75 g, 1.1. equivalents) in a mixture of toluene and THF. The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness at 50°C, upon which a yellow oily solid formed. This was resuspended in toluene / THF (~50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was suspended in warm DCM and was precipitated into methanol. Heptane (-100 ml) was added to the fine suspension and the mixture was concentrated under reduced pressure to remove the majority of the MeOH / DCM. The product was then collected via filtration as a fine yellow powder.

[0140] Polymer Example 2

[0141] Polymer Example 2 was formed by reaction of dialdehyde Monomer Example 2 and the following diamine monomer 1 : m-Cresol (2 equivalents) was added to a stirred solution of the diamine monomer 1 (0.19 g, 1 equivalent) and Monomer Example 2 (0.60 g, 1.1 equivalents) in a mixture of toluene and THF. The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness at 50°C, upon which a yellow oily, solid formed. This was resuspended in toluene / THF (-50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was suspended in warm dichloromethane (DCM) and was precipitated into methanol, and filtered to give the product as a yellow solid.

[0142] Polymer Example 3

[0143] Polymer Example 3 was prepared according to the following reaction scheme:

[0144] Step 1: A solution of amino-benzocyclobutene -TFA salt (0.103 g, 0.147 mmol) in dichloromethane (100 mL) was washed with saturated NaHCOa aqueous solution. The aqueous solution was extracted with DCM (2 x 100 ml) and organic phases were combined, dried over anhydrous sodium sulfate and concentrate to give the free amino-BCB.

[0145] Step 2: Precursor polymer 1 was prepared as described in WO 2022 / 136584 (1.00 g) and amino- benzocyclobutene were dissolved in a mixture of dichloromethane and toluene, m- cresol (0.03 ml, 0.292 mmol) was added and the mixture was stirred at room temperature for 30 minutes. The solution was concentrated to dryness under reduced pressure at 50°C. The residue was resuspended in dichloromethane with a small amount of toluene added and was stirred again for 5 minutes, before concentrating the mixture to dryness under reduced pressure. This process was repeated a second time. The yellow solid residue was dissolved in warm dichloromethane and the solution was added dropwise to a large volume of methanol. The precipitate was collected by filtration and washed with further methanol. The solid was then dried overnight in a vacuum oven at 50 C to yield 0.934 g of yellow polymer, 91% yield. Polymer Example 4:

[0146] Polymer Example 4 was formed by reaction of dialdehyde Monomer Example 2 and the following Diamine Monomer 2 and Dialdehyde Monomer 1 :

[0147] Diamine Monomer 2

[0148] Dialdehyde Monomer 1 is disclosed in WO2022 / 136584, the contents of which are incorporated herein by reference.

[0149] Diamine Monomer 2 (0.500 g, 1.74 mmol), Dialdehyde Monomer Example 2 (0.501 g, 0.766 mmol) and dialdehyde Monomer 1(0.554 mg, 1.14 mmol) were dissolved in 2- methyltetrahydrofuran (2-MeTHF) (15 ml) at 50°C. The solution was stirred on a rotary evaporator for 30 minutes. The solution was concentrated to dryness under reduced pressure at 50°C. The residue was redissolved in 15 ml of 2-MeTHF at 50°C on the rotary evaporator for 30 minutes. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed 3 times with 30 ml of K2CO3 (10wt% aqueous) and twice with 30 ml of water. The polymer solution was added drop wise to 120 ml of methanol. The mixture was filtered and polymer was stirred with 100 ml of methanol for 30 minutes and filtered. The polymer was dried in a vacuum oven at 50°C for 20 hours. The polymer was dissolved in 10 ml of 2-MeTHF at 50°C. The solution was cooled down to room temperature and washed 3 times with 30 ml of K2CO3 (10wt% aqueous) and 3 times with 30ml of water. The polymer solution was added drop wise to 120 ml of methanol. The mixture was filtered. The polymer was stirred with 80 ml of methanol for 30 minutes and filtered. The polymer dried in a vacuum oven at 50°C for 20 hours. The polymer was sonicated with 20 ml of methanol for 30 minutes at room temperature and filtered. This was repeated once and the polymer was dried in a vacuum oven at 50°C for 20 hours to yield 1.125 g of yellow solid, 75% yield.

[0150] Polymer Example 5:

[0151] Precursor polymer 1 was prepared as follow:

[0152] Diamine Monomer 1 (20.0 g, 69.8 mmol) and Dialdehyde Monomer 1 (37.3 g, 76.7 mmol) were combined with 600 ml of 2-methyltetrahydrofuran (2-MeTHF) and m-cresol (14.5 ml, 139 mmol) and dissolved at 50°C on the rotary evaporator. The solution was stirred for 30 minutes and reduced to dryness under reduced pressure. The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed three times with 300 ml of NaOAc (10wt% aqueous) and once with 300ml of water and with 30 ml of brine. The polymer solution was added dropwise to 4 E of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer cake was triturated with 1.5 E of methanol for 30 minutes. The polymer was collected and sonicated for 1 hour with 500 ml of methanol, filtered, and dried in a vacuum oven at 50°C for 20 hours.

[0153] The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator for 1 hour. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed 2 x with 300 ml of NaOAc (10wt% aqueous) and once with 300ml of water + 30 ml of brine to help phase separation. The polymer solution was added dropwise to 4 L of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer was washed with water then methanol and dried in a vacuum oven at 50°C for 20 hours.

[0154] The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator for 1 hour. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed once with 300ml of water. The polymer solution was added dropwise to 4 L of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer was washed with methanol and dried in a vacuum oven at 50°C for 20 hours, yielding to 52.2 g of yellow solids, 95% yield.

[0155] Precursor Polymer 2 (1.0 g) was dissolved in 10 ml of 2-MeTHF at 50°C. Solution was cooled down to room temperature and 4-aminostyrene (43 mg, 0.3608 mmol) followed by m-cresol (0.05 ml, 0.4771 mmol) were added to the polymer solution. The mixture was stirred for 30 minutes at 50°C then concentrated to dryness under reduced pressure. The residue was dissolved in 2-MeTHF at 50°C and 4-aminostyrene (12 mg, 0.1006 mmol) were added to the polymer solution. The solution was concentrated to dryness under reduced pressure. The residue was dissolved in 2-MeTHF at 50 C and m-cresol (0.05 ml, 0.4771 mmol) were added to the polymer solution. The solution was concentrated to dryness under reduced. The residue was dissolved in 2-MeTHF at 50°C and added drop wise to 60 ml of methanol. The slurry was stirred for 30 minutes and filtered. The polymer cake was stirred in 60 ml methanol for 30 minutes and filtered. This step was repeated once. The polymer was suspended in 20 ml methanol, sonicated for 30 minutes at room temperature and filtered. The polymer was dried on the rotary evaporator for 15 minutes under reduced pressure then dissolved in 2-MeTHF at 50°C. The polymer solution was washed 5 times with water. The polymer solution was added drop wise to 60 ml of methanol. The slurry was stirred for 30 minutes and filtered. The polymer was washed twice with methanol and air dried. It was then dried in a vacuum oven at 50°C for 20 hours to yield 0.81 g of yellow fibrous solid, 81%yield.

[0156] Film formation

[0157] Each of Precursor Polymer 1, Precursor Polymer 2, Polymer Example 2, Polymer Example 4 and Polymer Example 5 were dissolved in 1,2-dichlorobenzene at a concentration of 20 mg / mL at 80°C. The solutions of Polymer Examples, cooled down to room temperature before adding thermal radical initiator (l,l'-Azobis(cyclohexanecarbonitrile)) in a weight concentration of 1 wt%. The solutions were then drop-casted on polyimide substrates, followed by overnight ambient drying. The example polymer films were then annealed in a nitrogen filled glovebox for 1 hour at 120°C to activate the thermal radical initiator for crosslinking. The films were measured for their thermal conductivity. The solutions of Polymer Precursor 1 and 2 were casted at 80°C and left to dry at ambient temperature. Thermal conductivities of Polymer Precursor 1 and 2 were measured after room temperature drying. Substrates with Polymer Precursor 2, Polymer Example 4 and 5 were further annealed at 170°C for 30 minutes and cooled down to room ambient temperature prior to measuring their thermal conductivity a second time.

[0158] Thermal conductivity measurement

[0159] A sensor substrate 600 (ca. 25 mmx 25 mm) illustrated in Figure 7 was used for measurement of thermal conductivity as described herein. The substrate has a polyimide (PI) film (25 microns) with a 400 nm thick heating structure consisting of a 20 micron wide heater line 610, 500 micron wide busbars 620 for application of a current and contact pads 640. A sensing structure mirrors the heating structure except that the heater line is replaced with a 200 micron wide sensor line 630.

[0160] With reference to Figures 8A and 8B, the sensor substrate 600 carrying the film to be measured is placed on a temperature-controlled aluminium block, regulated via a PID system such that the temperature may be controlled by software. The aluminium block has a long notch 720 of 1mm width and ~lmm depth cut into it. The sensor substrate 600 is placed over the notch such that the central heater line 610 is aligned with the centre of the notch 720, and the sensor line 630 is aligned with the edge of the notch. A PMMA sheet 730 (2mm thickness) with a notch cut-through matching that of the aluminium block 710 is placed over the top and an addition piece of plain PMMA sheet 740 (5mm thickness) is placed on top to enclose the device. The entire assembly is clamped using bolts and nuts at positions 750. The heater line is connected to a sourcemeter unit (Keithley 2400) using a 4- wire measurement set up. The sensor line is connected to a multimeter unit (Keithley 2001) using a 4- wire set up.

[0161] The temperature of the assembly is first stabilised at a predetermined temperature. The resistance of the heater line and the temperature sensor is then measured. To measure the resistance of the heater line without causing undue heating a low current is sourced and voltage measured in short pulses, with time allowed between pulses for heat to be dissipated. A constant DC current is then passed along the heater line to cause resistive heating. The arrangement of the substrate in the assembly causes heat to flow through the substrate and film to the aluminium block which acts as a heat sink, setting up an approximate one-dimensional steady state heat flux. The power dissipated in the heater line, and the resistance of the heater line and temperature sensor is additionally measured in this state. This process is repeated for increasing sourced current, and the complete process repeated at the next temperature setpoint.

[0162] The resistances of the heater line and sensor lines under the condition of no heat flux at different temperature setpoints are used as calibration data in a straight-line fit of resistance and temperature, allowing the temperature of the resistive elements to be determined under the condition of steady state heat flux. As such the temperature gradient, 4T, between the heater line and temperature sensor (aligned with the heatsink) can then be calculated. The power dissipated in the heater line is assumed to be completely converted to heat energy Q. A straight- line fit is then made between dT and Q with additional parameters for the length of the heater line over which power is measured (L, 14 mm), the distance between the voltage sense points) and the gap width (2w, 1mm). This provides a measure of the conductance C of the device under test and is affected by losses pertaining to conductive heat transfer in the substrate and convective and radiative heat transfer to the environment (h).

[0163] To calculate a thermal conductivity K, the same measurement process is carried out on substrates without any test film (substrate only). We assume the losses will be approximately the same when measuring a coated vs uncoated substrate. We subtract the conductance of the substrate (Cs) from the device measurement (CF+S) to adjust for these losses. The thermal conductivity ( k i ) is then calculated by dividing the resulting film only conductance by the film thickness (dp). The film thickness is determined using a digital micrometer by measuring the total thickness and subtracting the substrate thickness.

[0164] Results

[0165] Table 1

[0166] As shown in Table 1, neither the introduction of a cross -linkable group on one of the monomers of a thermally conductive polymer nor on the end-group have a negative effect on the thermal conductivity. Consequently, stable thermoset films can be formed by crosslinking without significantly reducing the thermal conductivity of the films.

[0167] Differential Scanning Calorimetry

[0168] Referring to Figure 2 (lower line, first heating cycle), thermal analysis of Monomer Example 1 by differential scanning calorimetry (DSC) showed that it melts at 132°C. On continued heating the material undergoes a broad exothermic transition peaking at 265°C. These transitions do not repeat on subsequent heating cycles, as shown in the upper line of Figure 2 for a second heating cycle, indicating the material had undergone an irreversible change which is believed to be crosslinking of the monomer.

[0169] Referring to Figure 3, thermal analysis of crosslinkable Polymer Example 1 by DSC shows that in a first heating cycle the polymer undergoes a broad exothermic transition peaking at 265°C (lower line), similar to the monomer. This transition does not repeat on subsequent heating cycles, as shown in the upper line of Figure 3 for a second heating cycle, indicating the material had undergone an irreversible change, believed to be crosslinking.

[0170] Upon heating of Polymer Example 1, it was observed that softening and melting began at about 110°C. The polymer was fully molten at about 160°C.

[0171] With reference to Figure 4, thermal analysis of Polymer Example 3 shows that the polymer undergoes a ‘melt-like’ event at 80°C, similar to its parent, Polymer precursor 1. At higher temperatures, the Polymer Example 3 undergoes a broad exothermic transition peaking at 300°C which is attributed to the thermal curing of the BCB groups on the end of the polymer chains.

[0172] Model Compound 1

[0173] To investigate the cross-linking reaction further, a small molecule with two BCB moieties was formed from a dialdehyde monomers.

[0174] Model Compound 1

[0175] Step 1: A solution of amino-benzocyclobutene -TFA salt (1.44 g, 6.17 mmol) in dichloromethane (50 ml) was washed with saturated NaHCOa aqueous solution. The aqueous phase was extracted once with further dichloromethane (1 x 50 ml) before the organic phases were combined, dried over MgSCU, filtered and the solvent removed in vacuo to give an oil that was used for the next step without any further purification.

[0176] Step 2: Amino- benzocyclobutene and a dialdehyde monomer (1.50 g, 3.08 mmol) were combined and dissolved in a mixture of toluene (50 ml) and THF (10 ml). M-cresol (0.64 ml, 6.13 mmol) was added and the mixture was stirred at room temperature for an hour. The solution was concentrated to dryness under reduced pressure at 50°C to yield a yellow solid residue. Toluene was added to it, the mixture was stirred and concentrated to dryness under reduced pressure at 50°C. This was repeated a second time. The solid was then dissolved in DCM and precipitated into methanol. A yellow, fluffy precipitate formed and was collected via filtration. The solid was washed with further methanol and dried in a vacuum oven (50°C). The reaction of Model Compound 1 is illustrated below:

[0177] With reference to Figure 5, the thermal behaviour of Model Compound 1 was analysed by thermal analysis which shows that it melts at 124°C. On continued heating the material undergoes a broad exothermic transition peaking at 324°C which is attributed to reaction of the BCB units.

[0178] These transitions do not repeat on subsequent heating cycles indicating the material had undergone an irreversible change, which is attributed to the thermal ring opening reaction of the BCB group, followed by polymerisation between BCB groups.

[0179] The change in structure for Model Compound 1 on curing can also be seen from its infrared spectrum (Figure 6). The IR spectrum of a thin film sample of Model Compound was recorded and the sample was then cured at 260°C for 1 hour (argon atmosphere) before the spectrum was measured again. Large changes in the spectrum can be seen, with peaks broadening and disappearing. This indicates a large change in structure has occurred, consistent with polymerisation. Additionally, it was no longer possible to dissolve the material after this curing, further supporting the formation of a crosslinked network.

Claims

Claims1. A polymer comprising a repeat unit of formula (I):wherein:X1and X2are each independently selected from formulae (II) and (III):-(Ar^m- Formula (II)-(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2 C(R7)2 and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5or Si(R6)2 wherein R5is H or a substituent; each R6is independently a substituent; and each R7is independently H or a substituent; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N; and the polymer is substituted with a crosslinkable group.

2. The polymer according to claim 1 wherein the repeat unit of formula (I) is substituted with a crosslinkable group.

3. The polymer according to claim 2 wherein at least one occurrence of at least one of Ar1, Ar2and Ar3is substituted with a crosslinkable group.

4. The polymer according to claim 2 or 3 wherein the repeat unit of formula (I) is substituted with at least two crosslinkable groups.

5. The polymer according to claim 1 wherein the crosslinkable group is an end-group of the polymer.

6. The polymer according to any one of the preceding claims wherein the or each crosslinkable group is a group of formula (IV):-(Sp)y-(XL)Z(IV) wherein Sp is a spacer group; y is 0 or 1; z is 1 if y is 0; z is at least 1, preferably 1-4 if y is 1; and XL is a crosslinkable unit.

7. The polymer according to any one of the preceding claims wherein the or each crosslinkable group comprises at least one crosslinkable unit selected from:(i) an acyclic unit of formula -CR10=CH2 wherein R10is H or a substituent, preferably a Ci-6 alkyl group, for example vinyl, styryl acrylate or methacrylate;(ii) a cyclic alkene, preferably an optionally substituted norbomene, cyclopropene or cyclobutene;(iii) an optionally substituted epoxide; and(iv) an optionally substituted benzocyclobutene.

8. The polymer according to any one of the preceding claims wherein Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N.

9. The polymer according to any one of the preceding claims wherein Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a 6-12 membered arylene or an optionally fused 5- or 6-membered hetero arylene.

10. The polymer according to any one of the preceding claims wherein Ar1, Ar2and Ar3independently in each occurrence is an unsubstituted or substituted para-phenylene.

11. The polymer according to any one of the preceding claims wherein p and q are each 1.

12. A monomer of formula M: RG-X-RG M wherein: RG is a reactive group selected from C(=O)R1and NH2; X is selected from formulae (II) and (III): -(Ar1)m- Formula (II) -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is selected from O, S, C=O, COO, CONR5, NR5, Si(R6)2C(R7)2and an optionally substituted chain of methylene (-CH2-) groups wherein one or more non-adjacent methylene groups may be replaced with O, S, C=O, COO, CONR5, NR5or Si(R6)2 wherein R5is H or a substituent; each R6is independently a substituent; each R7is independently H or a substituent; and X is substituted with at least one crosslinkable group.

13. A method of forming a polymer according to any one of claims 1-11 comprising reacting a first monomer of formula M1 and a second monomer of formula M2: RG1-X1-RG1M1 RG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and NH2 and each RG2 is a second reactive group which is the other of C(=O)R1and NH2.

14. A film comprising a polymer according to any one of claims 1-11.

15. A crosslinked film comprising a crosslinked polymer according to any one of claims 1- 11.

16. A method of forming a film according to claim 14 wherein the film is formed by a thermal processing method.

17. The method according to claim 16 wherein the thermal processing method is selected from extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

18. An electronic device comprising a film according to claim 16 disposed on a surface of a functional layer of the electronic device.

19. The electronic device according to claim 18 wherein the film is disposed in a region between the surface of the functional layer and a first surface of a first chip electrically connected to the functional layer.

20. The electronic device according to claim 19 wherein the functional layer is a printed circuit board; an interposer; or a second chip.

21. The electronic device according to any one of claims 18-20 wherein the electronic device comprises a plurality of films according to claim 14.

22. The electronic device according to any one of claims 18-20 wherein the electronic device comprises a 3D chip stack.

23. Apparatus comprising a heat- generating device, a heat transfer device configured to transfer heat away from the heat-generating device and a film according to claim 15 disposed between the heat-generating device and the heat transfer device.

24. A heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a film according to claim 15 disposed thereon.

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