New polymer syntheses, 92. Biodegradable, thermotropic copolyesters derived from β‐(4‐hydroxyphenyl)propionic acid

Binary copolyesters were prepared by polycondensation of the trimethylsilylester of acetylated β‐(4‐hydroxyphenyl)propionic acid (HPPA) with acetylated 4‐hydroxybenzoic acid (HBA), vanillic acid (VA), 6‐hydroxy‐2‐naphthoic acid (HNA), 4‐hydroxycinnamic acid (HCA) or 4′‐hydroxybiphenyl‐4‐carboxylic acid (HBCA). Only the binary copolyester derived from 4‐hydroxybenzoic acid proved to be enantiotropic nematic, whereas all other binary copolyesters were more or less crystalline and infusible. Furthermore eight ternary copolyesters were prepared, which all proved to be soluble and thermotropic. 13C NMR spectroscopy proved a random nature of the sequences of the copolyesters, which were only obtained when transesterification catalysts such as Ti(OiPr)4 or MgO were used for the polycondensations. Optical microscopy revealed that five of the eight copolyesters form a homogeneous nematic phase above 220 or 250°C. However, in three cases the nematic melt contained crystalline particles which did not melt below 400°C. According to the WAXS powder patterns and DSC measurements most binary and ternary copolyesters are semicrystalline and form an imperfect orthorhombic crystal lattice. In three cases, the solid phase is best described as a hexagonal columnar mesophase.