Chiral poly(β-hydroxyalkanoates): an adaptable helix influenced by the alkane side-chain

Abstract

Bacterial copolyesters of poly(β-hydroxyalkanoates) (PHA) with average side-chain lengths between C 5 and C 7 crystallize as a 2 1 helix in an orthorhombic lattice with two molecules per unit cell. The fibre repeat, or c dimension, is 4.55 Å on average, compared with 5.96 and 5.56 Å for poly(β-hydroxybutyrate) (PHB) and poly(β-hydroxyvalerate) (PHV), respectively. This collapse of the c dimension while maintaining the 2 1 helical symmetry may be thought of as crystallization driven by side-chain packing influences. This class of PHA with homologous hydrocarbon side-chains must form ordered sheets with the extended alkane branches in polyethylene-like domains. The melting point, glass transition temperature and fibre repeat change regularly as a function of average number of side-chain carbons for the whole family of PHA. X-ray diffraction of a melt-crystallized PHA indicated a maximum crystallinity of about 25%, achieved only after several hours at room temperature. Compared with P(HB-co-HV) polyesters, the stress-strain curves of PHA polyesters studied are typical of elastomers. 13C-n.m.r. spectra and relaxation times of the bulk phase amorphous regions of these polymers demonstrate the high degree of motional freedom of the side-chains and the significantly slower motion for the backbone carbons. 13C n.m.r. was also used to estimate the percentage crystallinity which is in agreement with the X-ray diffraction results. Modelling and conformational analysis of single chains using the virtual bond approach lead to two possible models for the crystalline helix: ‘herringbone’ versus ‘comb-like’.