Copolymer Randomization by End-Group Interchain Exchange Reactions

Theoretical analysis of the kinetic scheme describing formation and randomization of a binary copolymer due to end-group interchange reactions is presented. Compact analytical expressions are obtained for the transient and equilibrium values of the copolymer degree of blockiness and fractions of active end-groups of different types. The resulting copolymer is described by the first-order Markov statistics, which can be rather far from the Bernoullian one. Its structure is determined by the composition of the system and a single combination of the rate constants of four elementary reactions. The kinetics includes fast and slow stages with characteristic time periods independent of and proportional to the average polymerization degree, respectively. During the fast stage, the initial polymers disappear, whereas the copolymer degree of blockiness is still very low. The distribution of units in the copolymer is Markovian, only if the initial polymers possess the most probable molar mass distributions. At the slow stage copolymer randomization gradually progresses, whereas the distribution of end groups may change in the opposite direction compared to the fast stage. The results can be used to analyze the experimental data on the chain structure of condensation, metathesis, and dynamic covalent polymers, where interchange reactions play a significant role.