The effect of a direct interchain exchange reaction (transesterification) on the microtacticity of polymer chains is studied with theory and a Monte Carlo simulation. Kinetic equations describing the evolution of the distribution over the type and length of the dyad sequences in the course of transesterification are derived. From this detailed consideration, the dependence of the average lengths of the isotactic and syndiotactic sequences on time is calculated for a melt of an initially isotactic homopolymer. It is shown that the microtacticity change proceeds at a rate similar to that of copolymer randomization during transesterification in polymer blends and much more slowly than the relaxation of the molecular mass distribution. The reacting chains for a long time consist of isotactic sequences of decreasing length and mostly isolated syndiotactic dyads. Possible effects of varying the ratio of the rate constants of the elementary reactions between dyads of different types are investigated. The agreement between the calculations and results of the Monte Carlo simulation is satisfactory. (C) 2007 Wiley Periodicals, Inc.