Catalytic metal-based systems for controlled statistical copolymerisation of lactide with a lactone

Polylactide (PLA) is currently considered as a major polymer which could serve as a potential substitute to the widely used petroleum-based plastics. It is typically produced by coordination-insertion polymerisation (Ring-Opening Polymerisation, ROP) of the cyclic ester, lactide (LA), a dimer of lactic acid that is extracted from biomass via biotechnological processing. However, PLA has limitations that hinder its ability to compete with conventional plastics, particularly with regard to its brittle behaviour. Different methodologies have been employed in order to improve the performances of PLA and thereby expand its range of applications. One strategy involves the statistical Ring-Opening coPolymerisation (ROcoP) of LA with another lactone, primarily e-caprolactone (e-CL), which enables the inherent properties of each homopolymer to be incorporated within the same polymer chain. Nevertheless, the difference in the reactivity of LA relative to the lactone comonomer and the occurrence of undesired transesterification creates the challenge of producing a strictly random copolymer. Herein, this review aims to present the variety of metal-based catalysts and/or initiators that target the synthesis of statistical copolymers of LA and lactone, under smooth conditions to ensure the best possible controlled polymerisation process.