Click chemistry synthesis, stereocomplex formation, and enhanced thermal properties of well-defined poly(l-lactic acid)-b-poly(d-lactic acid) stereo diblock copolymers

Abstract

Stereoblock copolymerization of lactide enantiomers has been a feasible method to prepare stereocomplexed poly(lactic acid) (PLA) with highly improved thermal resistance. However, synthesis of high-molecular-weight (HMW) poly(L-lactic acid)-b-poly(D-lactic acid) (PLLA-b-PDLA) stereoblock copolymers with controlled stereoblock length and composition is still challenging. Herein we synthesized well-defined PLLA-b-PDLA stereo diblock copolymers with different molecular weights (MWs, 14–110 kDa) and PLLA and PDLA block lengths by a combination of ring-opening polymerization and azide/alkyne click chemistry. The crystallization kinetics, polymorphic crystalline structure, lamellar morphology, and thermomechanical properties of the PLLA-b-PDLAs were systematically investigated. All the PLLA-b-PDLAs exhibit fast crystallization and predominantly form stereocomplexes (SCs) during the cooling and heating processes. Symmetric PLLA-b-PDLAs with similar PLLA and PDLA block lengths exclusively crystallize in the SCs at all the investigated crystallization temperatures (T_cs) in melt crystallization; but asymmetric PLLA-b-PDLAs with very different PLLA and PDLA block lengths crystallize in both SCs and homocrystallites (HCs) at a low T_c (<160 °C). Because of the formation of high-melting-point SCs, HMW PLLA-b-PDLAs exhibit better thermal resistance and higher storage moduli at a high temperature range (170–200 °C) than the homocrystalline PLLA.