MICROSTRUCTURE OF POLY[(R,S)-BETA-HYDROXYBUTYRATE] BY C-13 NMR

High-resolution C-13 NMR spectra were obtained for a wide tacticity range of racemic poly[(R,S)-beta-hydroxybutyrate] samples and interpreted with regard to several statistical models. None of the samples followed Bernoullian statistics, but the syndiotactic and atactic fractions essentially obeyed first-order Markov statistics, as derived for racemic monomer. Fitting to a second-order Markovian model confirmed previously-held theories regarding the blocky nature of the polymer. An enantiomorphic model described the more isotactic samples well but did not adequately describe the atactic and syndiotactic ones; a two-site model involving enantiomorphic and first-order Markovian sites was therefore proposed, and the parameters necessary to describe each sample were calculated. Average isotactic and syndiotactic block lengths were also calculated and used in the interpretation of degradation and single-crystal results.