The relationship between chain structure and thermal stability has been investigated with three classes of polymers: aromatic copolyesters, polyketanils and poly(azomethinesters). The influence of the microstructure of the polymer chain, i.e. the sequence distribution of comonomer units, on the glass transition temperature and on the dielectric Ê3-relaxation was investigated, using the copolyterephthalate of 4,4'-bi(1-naphthol) and 2, 2-bis(4-hydroxyphenyl)-propane as model substance.