Homopolymerization and copolymerization of ε-caprolactone and l-lactide organocatalyzed by carboxylic acids with mono-, di-, and tri-functionality

Abstract

A challenge in the design of biocompatible polymers involves the use of non-cytotoxic catalysts in the ring-opening polymerization (ROP) of lactones. In this study, a family of seven mono-, di-, and tricarboxylic acids was used as organocatalysts for the ROP of ε-caprolactone (CL) and L-lactide (L-LA). The use of a long-chain aliphatic alcohol such as 1-docosanol (C₂₂OH) has been demonstrated to accelerate the polymerization rate. For the ROP of CL, carboxylic acids with pk_a values less than 4 and an increase in functional groups exhibited the highest catalytic efficiency. In contrast, the ROP of L-LA exhibited conversion that was independent of the number of carboxyl groups or pk_a. Random copolymers of PCL-co-PLLA were successfully synthesized using citric acid as an organocatalyst and C₂₂OH as an initiator. Increasing the L-LA content was observed to reduce the melting enthalpy without compromising semi-crystallinity, which is a behavior that is attributed to the docosyl terminal group.