SYNTHESIS AND CHAIN EXTENSION OF BRANCHED POLY(L-LACTIDE)

High molecular weight branched poly(L-lactide) (PLLA) was prepared by a two-step method. In the first step, linear and branched PLLAs were synthesized via ring opening polymerization (ROP). The obtained molecular weights were in the range of 10,000 – 38,000 g/mol. In the second step, the branched PLLA chain was extended by joining linear PLLA chains together via the terminal hydroxyl groups using hexamethylene diisocyanate (HDI) as chain extender. All reactions were performed in solution using xylene as the solvent, at 110 °C. Starting from PLLA prepolymer with molecular weight of 20,000 g/mole, the final extended 3-arm branched PLLA with molecular weight as high as 55,000 g/mol was obtained when the mole ratio of branched PLLA/linear PLLA/chain extender as 1/3/6 and the reaction time of 3 hr. Using excess HDI, however, yielded polymer gel that was not dissolved in the solvent. High molecular weight branched PLLA formed higher crystallinity and withstood higher thermal decomposition temperature than the pre-polymers. Adding branched PLLA into linear PLLA could also improve its melt strength.