Diisocyanate-based chain extension via Mg(II) catalyzed amide formation to high-molecular-weight poly(lactic acid)

Abstract

A simple chain-extension reaction using diisocyanate as a chain extender was adopted to increase the molecular weight of low-molecular-weight (LMW) poly(lactic acid) (PLA) synthesized via direct polycondensation and to recover the molecular weight of commercial PLA after service life. The slow reaction between diisocyanate and the carboxylic acid terminus of PLA was successfully accelerated using a Mg(II) catalyst, affording a linear chain-extended PLA (cePLA) connected through amide bonds. To increase the number of amide bonds, which exhibit higher thermal stability than the urethane bonds that are formed in the more common chain-extension reactions between isocyanates and the hydroxy terminus of PLA, a telechelic LMW-PLA having carboxylic acid groups at both termini was prepared. Subsequent reaction of this LMW-PLA with diisocyanate in the presence of a Mg(II) catalyst afforded a cePLA with high molecular weight (M_w > 180 × 10³ g/mol) and enhanced stability against thermal degradation, while showing identical mechanical properties and biodegradability as commercial PLA.