Abstract A set of PVC samples with various stereochemical microstructures, especially the mmmr and the mmmmrx ( x = m or r placement) repeating stereosequences, termini of isotactic sequences, has been prepared whether by changing the temperature of polymerization or by S N2 substitution reaction which was demonstrated to occur through a highly stereospecific mechanism. The stereochemical microstructures were accurately analysed by 13C NMR spectroscopy. For purposes of stating the microstructure/β relaxation relationship, the mechanical behaviour of all samples was studied by a dynamic mechanical thermal analyser (DMTA). The results are discussed on the ground of earlier fundamental contributions within the framework of the property/stereochemical microstructure relationships as abundantly studied in our laboratory. Basically it is shown that β relaxation decreases in both temperature and intensity with the content of the aforementioned microstructures, thereby suggesting that relaxation to stem from the enhanced free volume and favoured rotational motion facilities of the mmmr pentad under GTGTTG − TT conformation.