MODELING FREE-RADICAL COPOLYMERIZATION KINETICS - EVALUATION OF THE PSEUDO-KINETIC RATE-CONSTANT METHOD .1. MOLECULAR-WEIGHT CALCULATIONS FOR LINEAR COPOLYMERS

The moment equations for binary copolymerization in the context of the terminal model have been solved numerically for a batch reactor opera ting over a wide range of conditions. Calculated number- and weight-av erage molecular weights were compared with those found using pseudo-ki netic rate constants with the method of moments and with the instantan eous property method for homopolymerization. With the pseudo-kinetic r ate constant method under polymerization conditions where number-avera ge molecular weights (M(n)BAR) are below about 10(3) the error in calc ulating M(n)BAR exceeds 5%. The error increases rapidly with decrease in molecular weight for M(n)BAR < 10(3). M(n)BAR measured experimental ly for polymer chains (homo- and copolymers) have error limits of grea ter than +/- 5% at the 95% confidence level. Therefore, for all practi cal purposes, the pseudo-kinetic rate constant method is valid for M(n )BAR greater than 10(3). Errors in calculating weight-average molecula r weights (M(w)BAR) or higher averages are always smaller than those f or M(n)BAR when applying the pseudo-kinetic rate constant method. The assumptions involved in molecular weight modelling using the pseudo-ki netic rate constant approach are thus proven to be valid, and therefor e it is recommended that the pseudo-kinetic rate constant method be em ployed with the instantaneous property method to calculate the full mo lecular weight distribution and averages for linear chains synthesized by multicomponent chain growth polymerization.