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 carboxy l-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 ca talyst. The products were characterized by differential scanning calor imetry, gel permeation chromatography (GPC), ER, H-1-NMR, and C-13-NMR . According to NMR, the resulting prepolymers contained less than 1 mo l % of lactic acid monomer and less than 4.1 mol % of lactide. End gro up analysis of the polymers was carried out by comparing the NMR spect ra of different polymers. According to NMR, the lactic acid can be cop olymerized so that the resulting prepolymer chains have only one kind of end group, hydroxyl or carbonyl. The integrated area of the identif ied end group peak (hydroxyl or acid) was then used in molecular weigh t calculations. In C-13-NMR studies, the molecular weights were calcul ated by using the peaks in the methine area. The molecular weights wer e also calculated by using the peak integrals of H-1-NMR spectra of di fferent polymers. The calculated molecular weights were systematically smaller than the molecular weights determined by GPC, and on about th e same order as the molecular weights determined by titrimetric method s. The number-average molecular weights of prepared prepolymers determ ined by GPC Varied from 2800 to 18 000 g/mol, depending on the amount of difunctional substance added. The glass transition temperatures var ied from 16.7 to 46 degrees C.