Exploration of tertiary aminosquaramide bifunctional organocatalyst in controlled/living ring-opening polymerization of l-lactide

Abstract

A series of tertiary aminosquaramides as bifunctional organocatalysts in the ring-opening polymerization (ROP) of l-lactide (l-LA) were developed, allowing the activation of both the l-LA monomer and the alcohol group of the initiator/propagating species. Further, the impact of tertiary nitrogen substituents on catalytic activity in ROP of l-LA was explored. The tertiary aminosquaramide— an air-stable and moisture-stable catalyst—exhibited superior activity in contest with thiourea counterpart when both were equipped with a similar tertiary amine group. Kinetic and chain-extension experiments indicated that the formed poly(l-LA) is featured with narrow polydispersity and high end-group fidelity, hallmarks of a living polymerization process. The initiator efficiency was further executed at ease by preparation of an ABA triblock copolymer poly (l-LA)-b-poly (ethylene glycol)-b-poly (l-LA) in the presence of a dual-headed PEG macroinitiator. 1H NMR titration experiments suggested a bifunctional catalytic mechanism, wherein both the l-LA monomer and the propagating hydroxyl group were activated en route to polymerization. The 1H NMR, SEC, and MALDI-TOF MS measurements validated the quantitative incorporation of the initiator in the polymeric chains and enchainment over competitive trans-esterification reaction. Overall, the structure-activity relationships were surveyed to uncover aminosquaramide as a new bifunctional dual hydrogen-bond donor catalyst for living ROP of l-LA. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017