Copolymerization of polyethylene terephthalate and polycaprolactone using catalytic transesterification

A random copolymer between polyethylene terephthalate (PET) and polycaprolactone (PCL) is formed using catalytic transesterification in the melt. The copolymer, poly[(ET)2-co-(CL)2], has a backbone structure similar to that of polybutylene adipate terephthalate (PBAT) and could replace it at a lower cost, while simultaneously managing waste PET. Theoretically, a biodegradable copolymer is achieved when only two consecutive ET blocks remain. This degree of transesterification requires an active catalyst, capable of selective transesterification while limiting undesired chain scission. 1H NMR spectroscopy revealed that titanium alkoxide catalysts were the most effective for rapid transesterification, leading to di-blocks of both ET and CL units. It was found that the random copolymer could only be produced by catalysts containing both highly acidic metal ions and highly basic, small linear ligands. These observations led to the proposal that catalytic transesterification is likely to occur via an insertion-coordination mechanism. Increasing catalyst loading from 0.35 to 1.1 parts per hundred when using Ti(OBu)4 significantly accelerates transesterification, but a plateau is reached at about 0.85. Pleasingly, a reaction time of 2 min was sufficient for the reaction to be complete, suggesting reactive extrusion could be feasible. DSC thermograms of Ti(OEt)4 and Ti(OBu)4 catalysed blends have no observable recrystallization and melting peaks along with a sharp glass transition temperature suggesting the copolymer was highly amorphous with no unreacted homopolymers. Gel permeation chromatography of THF soluble blends revealed that a high extent of transesterification led to a decrease in chain length such that the Mn was reduced from about 68000 (PCL) to 13000–20000 (copolymer), suggesting that there is a need to use a minimum amount of catalyst to avoid excessive chain scission. The results demonstrate a proof-of-concept that transesterification of PET with PCL is possible to a high extent and that the reaction proceeds via an insertion-coordination mechanism. The results will inspire future research towards repurposing waste PET on extrusion scales and the creation of other biodegradable polyester materials using waste polymers.