Molecular weight enhancement by chain extension of poly(lactic acid) derived from polycondensation

Abstract

High molecular weight PLLAs were prepared by a two-step method. In the first step, PLLA was synthesized via direct condensation polymerization. The molecular weight ([M-bar]n) obtained was in the range of 4,000-40,000 Da. In the second step, the PLLA chains were extended by joining two PLLA chains together via the terminal hydroxyl groups using chain extender i.e. 1,1'-carbonyldiimidazole (CDI), diethyl carbonate (DEC), diphenyl carbonate (DPC), and epichlorohydrin (EC). All reactions were performed in solution using xylene as the solvent. For PLLA with ([M-bar]n)<10,000 Da, their chain lengths could be increased by as high as 107% when an excess amount of CDI (10 times the moles of PLLA) was used in the extension reaction performed at 110℃ for 3 h. The chain extension by CDI was, however, not as successful when the ([M-bar]n) of starting PLLA were higher than 10,000 Da. The reactivity of DPC and DEC was found to be lower than that of CDI. The use of EC (3 times the moles of PLLA) in combination of Na₂CO₃ as catalyst in the chain extending reaction resulted in 45% increase of ([M-bar]n) for PLLA with molecular weight of < 10,000 Da and up to 29% increase for the PLLA with ([M-bar]n) about 18,000 Da. In some cases, the decreases of molecular weight were presumably due to chain fragmentation caused by the undesired reaction with by-products. Detailed investigation on polymer structure by proton-NMR spectroscopy was also reported.