Synthesis and properties of crystalline dicarboxylated poly( L ‐lactic acid) prepolymers

Crystalline dicarboxylated poly( L ‐lactic acid)s (dcPLLAs) with number‐average molecular weights ( M n 's) of 10 3 to 10 4 g/mol were synthesized via the melt polycondensation of L ‐lactic acid (LLA) in the presence of succinic anhydride (SAD), with tin(II) chloride and toluene‐4‐sulfonic acid as binary catalysts. They were characterized by end‐group titration, 1 H‐NMR, differential scanning calorimetry, and wide‐angle X‐ray diffraction. The terminal COOH percentage reached over 98%, and the molecular weight could be controlled by the molar ratio of LLA to SAD. The thermal behaviors depended on the molecular weight. The poly( L ‐lactic acid)s (PLLAs) crystallized slowly for M n ≤ 2000 but quickly for M n ≥ 4000. The crystallinity increased from 27 to 40% when M n grew from 4000 to 10,000. With comparison to ordinary PLLA, the dcPLLA had the same crystallization structure but a slightly lower crystallizability. The glass‐transition temperature was clearly higher than that of amorphous dcPLLAs. With a controllable molecular weight, high COOH percentage, and crystallinity, the dcPLLA with M n ≥ 4000 appeared to be a suitable prepolymer for the preparation of high‐molecular‐weight crystalline PLLA via chain extension. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011