The beta transition of a poly(vinyl chloride) after either nucleophilic substitution to various extents or plasticization with distinct amounts of dioctyl phthalate has been measured by Dynamical Mechanical Thermal Analysis. Both sets of experiments are shown to result in similar loss of beta transition. The results are argued in the light of prior work on the substitution mechanism determining role of the mmr chain segments at the end of isotactic sequences. They strongly suggest the mmr structures to interact specifically with plasticizer thereby becoming stiffer (antiplasticization).