THERMOLYSIS AND METHANOLYSIS OF POLY(BETA-HYDROXYBUTYRATE) - RANDOM SCISSION ASSESSED BY STATISTICAL-ANALYSIS OF MOLECULAR-WEIGHT DISTRIBUTIONS

Samples of a bacterial polyester, poly(beta-hydroxybutyrate), have-been degraded thermally as a thin film on a thermocouple-controlled filament at 350 degrees C. In one series of these thermal experiments a short pyrolysis time (5 s) was used in order to ensure only partial pyrolysis, while in another series of thermal experiments the pyrolysis was allowed to proceed until all the sample had decomposed to volatiles. Samples from the same batch were also degraded in solution by methanolysis under acid conditions, while under reflux at 84 degrees C. In all cases, the yields and molecular weights of the products were assessed by gas chromatography. The number and weight distributions of the oligomeric products observed have been compared with those predicted statistically on the basis of random scissions. The results show that total thermolysis can be interpreted exclusively in terms of a random chain scission mechanism. However, the partial pyrolysis results are not consistent with random scission statistics; these results imply that there are some kinetically favored scissions occurring near the ends of the molecules. The product distributions from methanolysis are also somewhat inconsistent with those expected on the basis of random scission and may suggest that dimers hydrolyze more readily than higher molecular weight species.