Dihydroxy-terminated poly(L-lactide) obtained by controlled ring-opening polymerization: Investigation of the polymerization mechanism

Hydroxy telechelic poly(L-lactide)s of different molecular weight have been synthesized by controlled ring-opening polymerization using cyclic tin alk oxides. NMR analysis showed that the propagation proceeded through an inser tion mechanism. No free hydroxyl or carboxyl end groups were detected in th e polymerization mixture. Complete reaction of the initiator was observed o ver the entire range of compositions studied. Both tin-oxygen bonds were re active and participated in the propagation reaction. Peak assignments were obtained by H-1, C-13, distortionless enhancement polarization transfer (DE PT), and heteronuclear multiple quantum coherence-gradient selected (hmqc-g s) nuclear magnetic resonance spectroscopy. The kinetics of the solution po lymerization of L-lactide has been investigated and showed a first order in monomer. The external order in initiator has been determined to be 3/4 for initiator concentrations above 5 mmol/L and to 2 below 2 mmol/L. The molec ular weight could easily be adjusted by the monomer-to-initiator ratio, and the molecular weight distribution remained narrow even at high molecular w eight(MWD < 1.15). The polymerization products were characterized by size e xclusion chomatography (SEC) as well as H-1 and C-13 NMR.