Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching

Abstract

AbstractThe full moment equations and equations using pseudo‐kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number‐ and weight‐average molecular weights (M̄n and M̄w) were compared with those found using the pseudo‐kinetic rate constant method (PKRCM). The results show that the weight‐average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ1˙ and φ2˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo‐kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo‐kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.