Synthesis of Tri-arm PLLA-PDLA Block Copolymers and Its Stereocomplex Crystallization Behavior

The tri-arm poly (L-lactide) was prepared by ring-opening polymerization (ROP) of L-lactide (LLA) with 1,1,1-tris(hydroxymethyl)ethane(TME) as an initiator. The terminal hydroxyls of the pre-polymer were activated by stannous octanoate[Sn(Oct)(2)]. Then a series of novel tri-arm poly(L-lactide) -poly(D-lactide) (PLLA-PDLA) block copolymers were synthesized by ROP of D-lactide(DLA) with functioned PLLA-SnOct pre-polymer as a macroinitiator. Nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography(GPC) results confirmed the configuration of the tri-arm PLLA-PDLA block copolymers with high isotacticity and high molecular weight. The block structure of copolymer could be readily adjusted in terms of the feed ratio of monomer with the PLLA pre-polymer. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) demonstrated that the stereocomplex PLA (Sc-PLA) were formed between intramolecular and intermolecular PLLA and PDLA blocks. The melting temperature (T-m) of Sc-PLA crystals was above 200 degrees C, which was much higher than that of pure PLLA. These results show that a novel tri-arm PLLA-PDLA block copolymer would be promising as a heat-resistant PLA engineering material.