Effect of Catalyst and Polymerization Conditions on the Preparation of Low Molecular Weight Lactic Acid Polymers

The synthesis of low molecular weight (M̄n(NMR) < 27 000 g/mol) lactic acid polymers by condensation polymerization of l-lactic acid was investigated. All polymerizations were carried out in the melt, using different catalysts and polymerization temperatures. The products were characterized by DSC, GPC, titrimetric methods, and 13C-NMR. According to NMR, the resulting polymers contained less than 1 mol % of lactic acid monomer and less than 6.6 mol % of lactide. In 13C-NMR studies, the molecular weights were calculated by using the previously identified end group peaks in the methine area. The calculated molecular weights were systematically smaller than the weight-average molecular weights determined by GPC and on the same order as the molecular weights determined by titrimetric methods. The weight-average molecular weights of prepared prepolymers determined by GPC varied from 3600 to 32 600 g/mol, depending on the catalyst and polycondensation conditions. In DSC studies the glass transition temperatures of the resulting polymers varied from 24 to 51 °C, and crystallinity varied from 0% to 52%. The annealing of the polymer samples had only a small effect on glass transition temperatures and crystallinity. According to our results, the best polycondensation catalyst was sulfuric acid, which produced the highest molecular weights and over 50% crystallinity. Sn(II) octoate produced quite a high molecular weight polymer which was totally amorphous (the proportion of d-lactic acid structures was about 48 mol %).