Effect of polymerization temperature and pressure on the microstructure ofNi-diimine-catalyzed polyethylene: parameter identification for Monte-Carlo simulation

The degree of short chain branch (SCB) formation during ethylene polymeriza tion catalyzed with nickel-diimine/trimethylaluminum. systems is a function of the relative rates of polymerization and catalyst chain walking. The pr oper choice of polymerization temperature (30-50 degreesC) and ethylene pre ssure (1.0-18.4 kgf cm(-2)) allows one to tailor make the SCB content of th e polyethylene produced with these catalysts. Within such range, polyethyle ne can be prepared with 10-100 branches per 1000 carbon atoms in the backbo ne. The unique branch distribution, which mimics the one obtained in the co polymerization of ethylene and alpha -olefins, is described in detail using Monte-Carlo simulation of polymerization and chain walking. The four proba bility parameters used in the model were correlated to polymerization tempe rature, T, and ethylene concentration, [E]. With this model, it is possible to predict polyethylene microstructure as a function of T and [E]. The dep endence of each parameter on these two polymerization conditions is helpful to clarify the mechanism of branch formation with these intriguing catalys t systems. (C) 2001 Elsevier Science Ltd. All rights reserved.