Solid state polymerization of poly(L‐lactide): Multiple‐fold increase in molecular weight via an efficient catalyst system

Abstract

AbstractWe report the synthesis of high molecular weight (MW), high crystallinity (Xc) poly(L‐lactic acid) (PLLA), with improved thermal stability, by solid state polymerization (SSP) of low MW PLLA prepolymer, pre‐PLLA. SSP enabled pre‐PLLA is synthesized by ring opening polymerization (ROP) of L‐lactide using a ternary catalyst complex of L‐lactide, stannous octoate, and L‐lactic acid oligomer (OLLA). We demonstrate that this ROP, although a chain‐growth reaction, yields pre‐PLLA with end‐groups, which enable step growth SSP. MW increases ∼9 fold, with Xc reaching beyond 90%, during the SSP process. Such high values of crystallinity could be due to the synchronization of the crystallization rate with that of the slower and longer lasting step‐growth SSP. Nuclear magnetic resonance spectroscopy confirms that SSP is indeed a step‐growth reaction. In comparison, Zn(OLLA)2 only couples two pre‐PLLA chains during SSP. In addition, the radial spherulite growth rate has been quantified with annealing time of SSP synthesized PLLA. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers