The substitution reaction of poly(vinyl chloride) (PVC) with sodium azide was performed in cyclohexanone. The evolution of unreacted iso-, hetero- and syndiotactic triad contents with the degree of substitution has been followed by C-13 NMR spectroscopy. By quantitatively comparing the microstructure changes with degree of substitution, taking into account that the reaction is of S(N)2 type, only the mechanisms of substitution through the mm triad of mmr tetrad and the rm of rrmr pentads are shown to react. This conclusion was confirmed by FT-IR. From this stereospecific chemical modification of PVC, the thermal decomposition of azide-modified PVC and the consequent reaction with styrene offer a method for the preparation of stereoselective graft copolymers. After grafting, no variation of the microstructure of the chain has been demonstrated. These results have been used to study the effect of the aforementioned structures on the evolution of density as a function of free-volume of the graft copolymers, and thus provide new approaches to a better understanding of the structure-properties relationships at the molecular level. (c) 2006 Wiley Periodicals, Inc.