process

Abstract

There is provided herein a method of preparing cyclic polybutylene terephthalate (c-PBT) oligomers, the method comprising the steps of: (a) providing a composition comprising a solvent, polybutylene terephthalate (PBT), and a catalyst of formula (I), wherein R1 is selected from the list consisting of a C1-20 alkyl, optionally substituted; aryl, optionally substituted; cycloalkyl, optionally substituted; (b) exposing the composition to a temperature of no greater than about 185°C and at a pressure of no less than about atmospheric pressure to provide a product stream; and (c) isolating c-PBT oligomers from the product stream of step (b).

Description

[0001] PROCESS

[0002] Field of the invention

[0003] The present invention relates to a method for preparing cyclic polybutylene terephthalate (c-PBT) oligomers from polybutylene terephthalate (PBT).

[0004] Background of the invention

[0005] Polybutylene terephthalate (PBT) is a widely used thermoplastic polyester known for its excellent mechanical properties, thermal stability, and versatility in various applications, particularly in the manufacture of connectors and in the automotive industries.

[0006] Macrocyclic polyester oligomers, including the cyclic oligomers of polybutylene terephthalate (c-PBT), can be formed by the depolymerisation reaction of PBT. c-PBT oligomers, which include dimers, trimers, and higher-order oligomers, have been identified as valuable intermediates that can enhance the properties of PBT-based materials. These cyclic structures can improve processing characteristics, reduce viscosity, and enhance compatibility with other polymers, making them suitable for a range of applications, including in connectors, polymer composites, and advanced engineering materials.

[0007] Despite the potential benefits, existing methods for producing c-PBT oligomers often involve complex and inefficient processes that can lead to low yields and high production costs. Therefore, there is a need for improved methods that facilitate the efficient synthesis of c-PBT oligomers while ensuring high purity. c-PBT oligomers may be formed by the depolymerisation reaction of PBT. However, the depolymerisation of PBT into c-PBT typically progresses relatively slowly and produces undesired byproducts, including the hydroxybutylester linear oligomers, which are difficult to remove from the c-PBT product stream.

[0008] The current approach taken in the art is to use high processing temperatures (approximately 245 °C) to convert any hydroxybutylester linear oligomers to their acid- terminated linear oligomers which can then be readily separated from the c-PBT product stream. This process is shown in reaction scheme 1 below. In addition, due to the high temperatures required this process is energy intensive.

[0009]

[0010] Reaction 1 - Thermal conversion of hydroxy butyl terminated linear oligomer to acid- terminated oligomers

[0011] Accordingly, there exists a need for a faster, more efficient, less energy intensive and cheaper method for depolymerizing PBT into its c-PBT form.

[0012] This invention addresses these challenges by providing a novel low temperature approach to the production of c-PBT oligomers from PBT. The resulting c-PBT oligomers are produced in good yield and purity and without the undesirable hydroxybutyl ester linear oligomer side product.

[0013] Summary of the invention

[0014] The present invention seeks to address the problems highlighted above with regards to the production of c-PBT. The method of the present invention provides a method for producing c-PBT from PBT via tin catalysis. The inventors have surprisingly found that the method of the invention, which utilises a tin catalyst, allows for the production of c-PBT at lower temperatures than what has previously been reported but still results in good yield and purity. Furthermore, the method of the invention advantageously does not form the undesirable hydroxybutyl linear oligomer byproduct, allowing for a simpler workup to yield a c-PBT product stream.

[0015] In a first aspect of the invention, there is provided a method of preparing c-PBT oligomers, the method comprising the steps of:

[0016] (a) providing a composition comprising a solvent, polybutylene terephthalate (PBT), and a catalyst of formula (I) Formula (I) wherein R1is selected from the list consisting of a C1-20 alkyl, optionally substituted; aryl, optionally substituted; cycloalkyl, optionally substituted;

[0017] (b) exposing the composition to a temperature of no greater than about 185 °C and at a pressure of no less than about atmospheric pressure to provide a product stream; and

[0018] (c) isolating c-PBT oligomers from the product stream of step (b).

[0019] In a second aspect of the invention, there is provided a c-PBT obtainable by a method according to the first aspect.

[0020] Description of the figures

[0021] Examples of the present invention will now be described in detail with reference to the accompanying figures, in which:

[0022] FIG. 1 is a process flow diagram of an embodiment of the invention and depicts unit operations of a process for obtaining: i) c-PBT; and ii) acidic-terminated oligomeric PBT; from an input stream of PBT.

[0023] Detailed description of the invention

[0024] Unless indicated otherwise, all technical and scientific terms used herein will have their common meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0025] The term "comprising", or variants thereof, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term "consisting", or variants thereof, is to be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps.

[0026] When used herein in relation to a specific value (such as an amount), the term "about" (or similar terms, such as "approximately") will be understood as indicating that such values may vary by up to 10% (particularly, up to 5%, such as up to 1%) of the value defined. It is contemplated that, at each instance, such terms may be replaced with the notation "±10%", or the like (or by indicating a variance of a specific amount calculated based on the relevant value). It is also contemplated that, at each instance, such terms may be deleted.

[0027] The term "atmospheric boiling point" is understood to mean the boiling point of a liquid at atmospheric pressure. Atmospheric pressure is 1 atm (101,325 Pa).

[0028] Unless otherwise specified, Ci-z alkyl groups (where z is the upper limit of the range) defined herein may be straight-chain or, when there is a sufficient number (i.e. a minimum of two or three, as appropriate) of carbon atoms, be branched-chain, and / or cyclic (so forming a Cs-z cycloalkyl group). When there is a sufficient number (i.e. a minimum of four) of carbon atoms, such groups may also be part cyclic (so forming a C4-z partial cycloalkyl group). For example, cycloalkyl groups that may be mentioned include cyclopropyl, cyclopentyl and cyclohexyl. Similarly, part cyclic alkyl groups (which may also be referred to as "part cycloalkyl" groups) that may be mentioned include cyclopropylmethyl. When there is a sufficient number of carbon atoms, such groups may also be multicyclic (e.g. bicyclic or tricyclic) and / or spirocyclic. For the avoidance of doubt, particular alkyl groups that may be mentioned include straight chain (i.e. not branched and / or cyclic) alkyl groups.

[0029] As may be used herein, the term aryl may refer to Ce-i4 (e.g. Ce-io) aromatic groups. Such groups may be monocyclic or bicyclic and, when bicyclic, be either wholly or partly aromatic. Ce-io aryl groups that may be mentioned include phenyl, naphthyl, 1,2, 3, 4- tetra hydronaphthyl, indanyl, and the like (e.g. phenyl, naphthyl, and the like). For the avoidance of doubt, the point of attachment of substituents on aryl groups may be via any suitable carbon atom of the ring system.

[0030] For the avoidance of doubt, the skilled person will understand that aryl groups that may form part of compounds of the invention are those that are chemically obtainable, as known to those skilled in the art. Particular aryl groups that may be mentioned include phenyl and naphthyl, such as phenyl.

[0031] For the avoidance of doubt, where groups are referred to herein as being optionally substituted it is specifically contemplated that such optional substituents may be not present (i.e. references to such optional substituents may be removed), in which case the optionally substituted group may be referred to as being unsubstituted.

[0032] Further, for the avoidance of doubt, when it is specified that a substituent is itself optionally substituted by one or more substituents, the substituents are substituted by one or more selected from the list consisting of the group consisting of halo, Ci-6 alkyl, aryl, =0, -0Re, -(0)Re, wherein Rerepresents H or a Ci-io alkyl. These substituents where possible may be positioned on the same or different atoms. Such optional substituents may be present in any suitable number thereof (e.g. the relevant group may be substituted with one or more such substituents, such as one such substituent).

[0033] For the avoidance of doubt, it will be understood that references herein to particular aspects of the invention (such as the first aspect of the invention) will include references to all embodiments and particular features thereof, which embodiments and particular features may be taken in combination to form further embodiments and features of the invention.

[0034] In a first aspect of the invention, there is provided a method of preparing cyclic polybutylene terephthalate (c-PBT) oligomers, the method comprising the steps of:

[0035] (a) providing a composition comprising a solvent, polybutylene terephthalate (PBT), and a catalyst of formula (I) Formula (I) wherein R1is selected from the list consisting of a C1-20 alkyl, optionally substituted; aryl, optionally substituted; cycloalkyl, optionally substituted;

[0036] (b) exposing the composition to a temperature of no greater than about 185 °C and at a pressure of no less than about atmospheric pressure to provide a product stream; and

[0037] (c) isolating c-PBT oligomers from the product stream of step (b). In some embodiments of the first aspect, R1represents a C1-20 alkyl group, optionally substituted. For example, in certain embodiments, R1represents a C1-10 alkyl group, optionally substituted, for example a Ci-s alkyl group, optionally substituted, such as a Ci- 6 alkyl group, optionally substituted, or a Ci-4 alkyl group, optionally substituted.

[0038] In preferred embodiments of the first aspect, R1is a C4 alkyl group, optionally substituted. In even more preferred embodiments, R1is a linear, unsubstituted C4 alkyl group, such that the catalyst of Formula (I) is a catalyst of Formula (la) Formula (la).

[0039] In some embodiments of the first aspect the product stream of step (b) comprises acidcapped PBT oligomers, and c-PBT. In preferred embodiments, the product stream of step (b) is substantially free of hydroxybutyl ester linear PBT oligomers.

[0040] As used herein, the term "acid-capped" or "acid terminated" refers to an oligomer with a carboxylic acid end group (-COOH).

[0041] As used herein, the term "hydroxybutyl ester capped" or "hydroxybutyl ester terminated" refers to a polymer or oligomer with the following end group -COO(CH2)4OH.

[0042] As used herein, the term "substantially free" means that the product stream comprises less than 5 wt.%, preferably less than 3 wt.%, such as less than 2 wt.%, such as less than 1 wt.%, more preferably less than 0.5 wt.% of hydroxybutyl ester linear PBT oligomers.

[0043] In some embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration greater than about 0.1 meq / kg, greater than about 0.5 meq / kg, greater than about 0.9 meq / kg, greater than about 1.0 meq / kg, greater than about 1.5 meq / kg, greater than about 5.0 meq / kg, greater than about 6.0 meq / kg, greater than about 7.0 meq / kg, greater than about 10 meq / kg, greater than about 15 meq / kg, greater than about 18 meq / kg, greater than about 20 meq / kg, greater than about 30 meq / kg, or greater than about 38 meq / kg. Preferably, the PBT used in step (a) has an acid concentration of greater than about 0.9 meq / kg, greater than about 1.5 meq / kg, greater than about 7.0 meq / kg, greater than about 18 meq / kg, or greater than about 38 meq / kg. For the avoidance of doubt the units "meq / kg" are to be understood as milliequivalents per kilogram. It is a commonly used unit which expresses the concentration of ions or reactive species in a solution or material, such as a polymer.

[0044] In other embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration of from about 0.1 to less than about 0.9 meq / kg.

[0045] In other embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration of from greater than about 0.9 to less than about 7 meq / kg, such as from about 1.0 to about 6.9 meq / kg. For example, it may be that the PBT has an acid concentration of from greater than about 0.9 to less than about 1.4 meq / kg, such as from about 1.0 to about 1.3 meq / kg.

[0046] In other embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration of from greater than about 1.5 to less than about 7.0 meq / kg, such as from about 1.6 to about 6.9 meq / kg.

[0047] In other embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration of from greater than about 7.0 to less than about 18 meq / kg, such as from about 7.1 to about 17.9 meq / kg.

[0048] In other embodiments of the first aspect, the PBT used in step (a) of the method of the invention has an acid concentration of from greater than about 18 to less than about 38 meq / kg, such as from about 18.1 to about 37.9 meq / kg.

[0049] In step (b) in the method of the invention, the reaction is carried out at a temperature no greater than about 185 °C. In some embodiments of the first aspect, step (b) is carried out at a temperature of no greater than about 182 °C, no greater than about 180 °C, no greater than about 178 °C, no greater than about 176 °C, no greater than about 174 °C, no greater than about 172 °C, or no greater than about 170 °C.

[0050] In other embodiments of the invention, step (b) is carried out at a temperature of from about 120 to about 185 °C, from about 130 to about 185 °C, from about 140 to about 185 °C, from about 150 to about 185 °C, from about 160 to about 185 °C, or from about 170 to about 185 °C. Preferably, step (b) is carried out at a temperature of from about 150 to about 185 °C, such as from about 150 to about 183 °C. More preferably step (b) is carried out at a temperature of from about 160 to about 183 °C, such as from about 170 to about 183 °C.

[0051] In some embodiments of the first aspect, the solvent comprises an organic solvent, optionally wherein the organic solvent is one or more organic solvents selected from the list consisting of tetra decane, hexadecane, octadecane, toluene, xylene, trimethylbenzene, tetramethylbenzene, ethylbenzene, propylbenzene, naphthalene, methylnaphthalene, biphenyl, triphenyl, diphyenyl ether (or a halogenated derivative thereof), anisol, methylene chloride, dimethyoxybenzene, chlorobenzene, dichlorobenzene, trichlorobenzene, chloronaphthalene, dichloronaphthalene, a perfluorocarbon, or mixtures thereof. In preferred embodiments, the solvent comprises an ortho-xylene. In more preferred embodiments, the organic solvent comprises ortho-dichlorobenzene (oDCB).

[0052] In some embodiments of the first aspect, the solvent comprises oDCB in a concentration of greater than about 50 weight % of the solvent solution, such as greater than about 60, about 70, about 80, about 90, about 95, about 98 or about 99 weight %. Preferably, the solvent consists essentially of oDCB.

[0053] In some embodiments of the first aspect, the organic solvent may further comprise an alkane, such as tetradecane and hexadecane. In other embodiments, the organic solvent may further comprise a perfluorocompound.

[0054] In preferred embodiments of the first aspect, the solvent used preferably has a boiling point of no less than about 110 °C.

[0055] In some embodiments of the first aspect, the molar concentration of PBT in the composition of step (a) is from about 0.01 to about 1.0 M, from about 0.01 to about 0.9 M, from about 0.01 to about 0.8 M, from about 0.01 to about 0.7 M, from about 0.01 to about 0.6 M, from about 0.01 to about 0.5 M, from about 0.01 to about 0.4 M, from about 0.01 to about 0.3 M, from about 0.01 to about 0.2 M, from about 0.01 to about 0.1 M, from about 0.01 to about 0.08 M, from about 0.01 to about 0.06 M, from about 0.01 to about 0.05 M, from about 0.01 to about 0.04 M, from about 0.01 to about 0.03 M, or from about 0.01 to about 0.02 M.

[0056] In certain embodiments of the first aspect, the molar concentration of PBT in the composition of step (a) is from about 0.02 to about 0.2 M, from about 0.03 to about 0.1 M, from about 0.04 to about 0.08 M, or from about 0.05 to about 0.06 M. In preferred embodiments of the first aspect, the molar concentration of PBT in the composition of step (a) is from about 0.01 to about 0.2 M, for example from about 0.01 to about 0.1 M, such as from 0.01 to about 0.05 M.

[0057] In some embodiments of the first aspect, the molar concentration of the catalyst in the composition of step (a) is from about 0.10 to about 3.00 mM, from about 0.10 to about 2.75 mM, from about 0.10 to about 2.50 mM, from about 0.10 to about 2.25 mM, from about 0.10 to about 2.00 mM, from about 0.10 to about 1.75 mM, from about 0.10 to about 1.50 mM, from about 0.10 to about 1.25 mM, or from about 0.10 to about 1.00 mM. Preferably, the molar concentration of the catalyst in the composition of step (a) is from about 0.10 to about 0.90 mM, from about 0.20 to about 0.80 mM, or from about 0.30 to about 0.80 mM. Advantageously, the molar concentration of the catalyst in the composition of step (a) is from about 0.40 to about 0.75 mM, preferably from about 0.5 to about 0.75 mM.

[0058] In some embodiments of the first aspect, step (c) comprises:

[0059] (i) cooling the product stream of step (b) to less than 150 °C such that a precipitate comprising acid-capped PBT oligomers is formed;

[0060] (ii) removing the precipitate of step (i) by filtration; and

[0061] (iii) removing any remaining solvent from the filtrate to provide a solid comprising c-PBT oligomers.

[0062] In some embodiments of the first aspect wherein step (c) comprises sub-steps (i) to (iii) above, sub-step (ii) is performed at about 50 °C, this ensures compete precipitation of the acid-capped PBT oligomers. Alternatively, or additionally, in sub-step (iii), basic alumina (AI2O3) is added to the liquid filtrate and removed by filtration before the remaining solvent from the filtrate of step (ii) is removed.

[0063] In some embodiments of the first aspect, the method of the invention is carried out in a batchwise manner. In alternative embodiments, the method of the invention is carried out in a continuous operation. Preferably, the method of the invention is carried out in a continuous operation.

[0064] In some embodiments of the first aspect, the acid-capped PBT oligomers are recycled, preferably at least a portion is recycled for use in a method for preparing PBT.

[0065] In some embodiments of the first aspect, at least a portion of the PBT in step (a) is prepared using recycled acid-capped PBT oligomers. According to a second aspect of the invention, there is provided a c-PBT obtainable by a method according to the first aspect of the invention.

[0066] Detailed Description of the Figures

[0067] In the flow diagram depicted in FIG. 1, one or more input streams 101 comprising high molecular weight PBT is refluxed in a solvent of ortho dichlorobenzene (oDCB) at a temperature of approximately 180 °C in one or more unit operations 102. The PBT dissolves to provide an output solvate stream 103. Optionally, any insoluble material, such as any composite material (e.g., fibres, glass fibres etc), is removed.

[0068] The output solvate stream comprising dissolved PBT 103 is then optionally dried before being introduced into one or more input streams into one or more depolymerisation (cyclisation) reactors 105. The one or more input streams also comprises a depolymerisation tin catalyst 104.

[0069] The output solvate stream 103 comprising dissolved PBT may alternatively and / or additionally comprise PBT derived from one or more polycondensation reactors, such as 114 described below. It is envisaged that in some embodiments unit operator 102 can be omitted such that the solvate stream 103 comprising dissolved PBT, which is introduced into one or more depolymerisation (cyclisation) reactors 105, is derived entirely from output stream 115.

[0070] The one or more depolymerisation reactors 105 is refluxed at atmospheric pressure and at a temperature of approximately 180 °C for approximately 1.5 hrs. In the process of FIG. 1, an output stream 106 of the depolymerization reaction may comprise c-PBT product in oDCB solvent, as well as byproducts including, for example, residual acid-terminated oligomeric PBT, catalyst residue, and other compounds.

[0071] In such an embodiment, it may be that the depolymerisation reactor 105 is the same vessel as unit operation 102 which was used to dissolve the initial input stream of PBT in oDCB, or alternatively it may be a separate vessel.

[0072] The output stream 106 from the depolymerisation step then provides the one or more input streams for one or more unit operations 107. The one or more unit operations 107 may comprise cooling the solution to facilitate the precipitation of the acid-terminated PBT oligomers followed by filtration to separate the c-PBT filtrate 108 from the precipitated acid-terminated PBT oligomers 112. In such an embodiment, in practice unit operation 107 may be the same vessel as the depolymerisation reactor 105, or alternatively it may be a separate vessel.

[0073] The output stream from the unit operator 107 comprising the c-PBT filtrate 108 is then purified and worked-up in one or more unit operations 110. In the one or more purification and work-up unit operations 110, the input stream 108 is: i) treated with basic alumina (AI2O3); and ii) filtered and concentrated to provide a c-PBT product stream 111.

[0074] The output stream from unit operation 107 comprising the acid-terminated oligomeric PBT 112 can be recycled into one or more input streams for one or more polycondensation unit operations 114 used in the production of PBT (e.g., high Mw PBT) 115. The polycondensation unit operations may also have additional input streams comprising dimethyl terephthalate (DMT) and butanediol (BDO) 113. As described earlier, output stream 115 can optionally be recycled into the one or more depolymerisation (cyclisation) reactors 105.

[0075] Example

[0076] Example 1 - Preparation of Cyclic Polv(Butylene Terephthalate) (c-PBT) via Tin Catalysis

[0077] To 350 mL of ortho Dichlorobenzene (oDCB) heated to reflux was added 5.28 g (24 mM) of Polybutylene Terephthalate (PBT). All of the PBT quickly dissolved resulting in a clear homogeneous solution. After complete dissolution, approximately 10 mL of oDCB was removed by distillation to azeotropically remove traces of water. To this dried solution was added 174.9 mg (0.716 mM) of n-butyltin(IV) chloride dihydroxide and the resulting solution was heated at reflux for 1.5 hours.

[0078] Subsequently, the reaction was allowed to cool to 50 °C with no agitation. It was observed that at 140 °C the acid terminated PBT oligomer began to precipitate as a white precipitate. The white precipitate was removed by filtration at 50 °C.

[0079] The filtrate was treated with 1 g of basic alumina (AI2O3) and after stirring for 10 minutes the alumina was removed by filtration.

[0080] The resulting filtrate was dried to remove remaining solvent to yield 4.7 g of c-PBT consisting of only the oligomeric array of c-PBT. Examination of the reaction mixture and the isolated product by HPLC analysis showed a c-PBT product yield of 89% yield and the cyclic oligomers were all in the usual distribution. The HPLC trace was also void of the hydroxyl terminated oligomers observed in titanium catalysed examples.

[0081] Comparative Example 1 - Preparation of Cyclic PolvfButvIene Terephthalate) (c-PBT) via Titanium Catalysis

[0082] To 130 mL of oDichlorobenzene (oDCB) was added 1.54g (7.0 mmol), of Celanex 2001, PBT. This mixture was brought to reflux under an atmosphere of nitrogen, where the PBT went into solution. After complete dissolution, approximately 20 mL of oDCB was removed by distillation to azeotropically remove traces of water. To this dried solution was added

[0083] 8.7 mg, (0.035 mM) of tetra isopropyl titanate and the resulting reaction mixture was heated at reflux for 45 minutes.

[0084] HPLC analysis revealed a 56.9 % yield of c-PBT was produced. The HPLC trace however had a number of additional peaks between the cyclic dimer and the cyclic tetramer that are attributed to hydroxyl terminated PBT oligomers. These contaminants are absent when the reaction is performed at elevated temperatures.

Claims

Claims1. A method of preparing cyclic polybutylene terephthalate (c-PBT) oligomers, the method comprising the steps of:(a) providing a composition comprising a solvent, polybutylene terephthalate (PBT), and a catalyst of formula (I)Formula (I) wherein R1is selected from the list consisting of a C1-20 alkyl, optionally substituted; aryl, optionally substituted; cycloalkyl, optionally substituted;(b) exposing the composition to a temperature of no greater than about 185°C and at a pressure of no less than about atmospheric pressure to provide a product stream; and(c) isolating c-PBT oligomers from the product stream of step (b).

2. The method according to claim 1, wherein the product stream of step (b) comprises acid-capped PBT oligomers, and c-PBT.

3. The method according to claim 1 or claim 2, wherein the product stream of step (b) is substantially free of hydroxybutyl ester linear PBT oligomers.

4. The method according to any one of the preceding claims, wherein the PBT has an acid concentration greater than about 0.9 meq / kg.

5. The method according to any one of the preceding claims, wherein step (b) is carried out at a temperature of from about 160 °C to about 183°C.

6. The method according to any one of the preceding claims, wherein R1is a C1-10 alkyl.

7. The method according to any one of the preceding claims, wherein the catalyst of formula (I) is a catalyst of formula (la),Formula (la).

8. The method according to any one of the preceding claims, wherein the solvent comprises an organic solvent, preferably ortho dichlorobenzene (ODCB).

9. The method according to any one of the preceding claims, wherein the molar concentration of PBT in the composition of step (a) is from about 0.01 M to about 0.2 M, for example from about 0.01 M to about 0.1 M, such as from 0.01 M to about 0.05 M.

10. The method according to any one of the preceding claims, wherein the molar concentration of the catalyst in composition of step (a) is from about 0.5 mM to about 1 mM, such as from about 0.5 mM to about 0.75 mM.

11. The method according to any one of the preceding claims, wherein step (c) comprises:(i) cooling the product stream of step (b) to less than about 150°C such that a precipitate comprising acid-capped PBT oligomers is formed;(ii) removing the precipitate by filtration;(iii) removing any remaining solvent from the filtrate to provide a solid comprising c-PBT oligomers.

12. The method according to any of the preceding claims wherein the method is carried out in a batchwise or continuous operation, preferably wherein the method is carried out in a continuous operation.

13. The method according to any one of the preceding claims, wherein the acid-capped PBT oligomers are recycled, preferably for use in a method for preparing PBT.

14. The method according to claim 13, wherein at least a portion of the PBT in step (a) is prepared using recycled acid-capped PBT oligomers.

15. A c-PBT prepared according to the method of any one of claims 1 to 14.

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