Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 1. Molecular weight calculations for linear copolymers

Abstract

The moment equations for binary copolymerization in the context of the terminal model have been solved numerically for a batch reactor operating over a wide range of conditions. Calculated number- and weight-average molecular weights were compared with those found using pseudo-kinetic rate constants with the method of moments and with the instantaneous property method for homopolymerization. With the pseudo-kinetic rate constant method under polymerization conditions where number-average molecular weights (Mn) are below about 103 the error in calculating Mn exceeds 5%. The error increases rapidly with decrease in molecular weight for Mn < 103. Mn measured experimentally for polymer chains (homo- and copolymers) have error limits of greater than ±5% at the 95% confidence level. Therefore, for all practical purposes, the pseudo-kinetic rate constant method is valid for Mn greater than 103. Errors in calculating weight-average molecular weights (Mw) or higher averages are always smaller than those for Mn when applying the pseudo-kinetic rate constant method. The assumptions involved in molecular weight modelling using the pseudo-kinetic rate constant approach are thus proven to be valid, and therefore it is recommended that the pseudo-kinetic rate constant method be employed with the instantaneous property method to calculate the full molecular weight distribution and averages for linear chains synthesized by multicomponent chain growth polymerization.