The mechanisms of nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium 2-mercaptothiobenzoate (NaMBT), as studied in recent works, have been outlined. Like substitution with other nucleophiles formerly studied, the reaction proceeds only by the tetrad mmr and the pentad nmr, the termini of the isotactic and syndiotactic sequences, respectively. In contrast, unlike in those nucleophiles, mmr is found to react exclusively by the mm triad, because the competing reaction by the mr triad proves to be nil with NaMBT. This is demonstrated to be the reason why the evolution of T-g with substitution, as studied herein, agrees thoroughly with the ratio of mmr to rrmr, contrary to what happened with the nucleophiles that exhibit a minority reaction by the mr triad of mmr. Such a novel quantitative correlation, compared with the more tentative ones obtained previously, allows one to furnish a valuable insight into the prevailing role of stereochemical microstructure in the process of glass transition in PVC. (C) 2000 Elsevier Science Ltd. All rights reserved.