Synthesis and Characterization of Stereoblock Poly(lactic acid)s with Nonequivalent D/L Sequence Ratios

Stereoblock poly(lactic acid)s (sb-PLA) consisting of different ratios of d - and l -sequences were synthesized by a process involving solid-state polycondensation (SSP) of polymer blends of poly( l -lactic acid) (PLLA) and poly( d -lactic acid) (PDLA) of medium molecular weight. Both PLLA and PDLA prepolymers were first prepared by direct melt-polycondensation of l - and d -lactic acids, respectively. These prepolymers were melt-blended in different ratios where their respective partial stereocomplexation could aid in the crystallization of the rich polymer. The melt-blends were then subjected to SSP at a temperature of 140−160 °C, which was lower than the melting temperature of the homochiral polymers. The resultant polymers consisted of longer block chains of the rich components and had molecular weights of higher than 100 kDa when either the PLLA or PDLA ratio in the feed was larger.