Synthesis and characterization of lactic acid based telechelic prepolymers

The synthesis of low molecular weight (M(n) (NMR) < 7000 g/mol) lactic acid prepolymers by condensation polymerization of L-lactic acid was investigated. Besides the L-lactic acid polymer, hydroxyl- and carboxyl-terminated telechelic prepolymers were also prepared by the addition of small amounts of 1,4-butanediol and adipic acid, respectively. All polymerizations were carried out in a melt with tin octoate as the catalyst. The products were characterized by differential scanning calorimetry, gel permeation chromatography (GPC), ER, H-1-NMR, and C-13-NMR. According to NMR, the resulting prepolymers contained less than 1 mol % of lactic acid monomer and less than 4.1 mol % of lactide. End group analysis of the polymers was carried out by comparing the NMR spectra of different polymers. According to NMR, the lactic acid can be copolymerized so that the resulting prepolymer chains have only one kind of end group, hydroxyl or carbonyl. The integrated area of the identified end group peak (hydroxyl or acid) was then used in molecular weight calculations. In C-13-NMR studies, the molecular weights were calculated by using the peaks in the methine area. The molecular weights were also calculated by using the peak integrals of H-1-NMR spectra of different polymers. The calculated molecular weights were systematically smaller than the molecular weights determined by GPC, and on about the same order as the molecular weights determined by titrimetric methods. The number-average molecular weights of prepared prepolymers determined by GPC Varied from 2800 to 18 000 g/mol, depending on the amount of difunctional substance added. The glass transition temperatures varied from 16.7 to 46 degrees C.