Analysis of the End Groups in Derivatized Poly(lactic acid) and the Junction Unit in Chain-extended Poly(lactic acid) by DOSY

Abstract

The hydroxyl and carboxyl end-groups in poly(lactic acid) (PLA) were derivatized with trimethylsilylation or trifluoroacetylation to be detected by ¹H DOSY or ¹⁹F DOSY with enhanced sensitivity. The diffusion coefficients (D) for the derivatized end-groups of PLA in a chloroform-d solution were determined by ¹H and ¹⁹F DOSY analysis; the values agreed well with each other. The D values for the end groups were found to be larger than those for the methyl and methine groups in repeating units of the PLA. The relative intensity of the ¹H DOSY resonances due to the trimethylsilyl end-group and the main-chain methyl group allowed us to evaluate the dependence of the degree of polymerization on D. ¹H DOSY analysis of PLA treated with diphenylmethane diisocyanate (MDI) was also carried out to determine D for the junction unit in the PLA. The result clearly indicated that the molecular weight of the PLA increased by the reaction of MDI and the hydroxyl end-groups.