Two-state models for olefin polymerization using metallocene catalysts. 1.Application to fluxional metallocene catalyst systems

A kinetic model was developed to describe the propylene polymerization beha vior of fluxional, two-state metallocene catalysts. In particular, the pent ad and molecular weight distributions can be described as well as other par ameters of interest, such as the weight fraction of crystallizable sequence s and the isotactic sequence length distribution, in terms of fundamental k inetic constants and polymerization conditions that pertain to these two-st ate catalyst systems. The model was used in an attempt to describe the poly merization behavior of two, prototypical, fluxional catalyst systems, (2-Ph Ind)(2)ZrCl2/MAO (1) and (2-p-CF(3)PhInd)(2)ZrCl2/MAO (2). The model can ac curately reproduce the pentad distributions observed in PP prepared using t hese catalysts and the response of the distribution to changes in polymeriz ation conditions, specifically changes in [C3H6] at constant T. These studi es illustrate that the rate of state-to-state interconversion is slower but of comparable magnitude to the rate of monomer insertion and that the stat es have similar stability and reactivity. The broad molecular weight distri butions previously observed with this family of catalysts can be described by the model. However, the model predicts that the state-to-state interconv ersion rate has to be significantly slower than the rate of formation of de ad polymer chains, and this is inconsistent with the rate estimated from th e response of the pentad distribution to changes in the rate of propagation (i.e., [C3H6]) Recent work where propylene polymerizations using 1 were ca rried out to low conversion indicate that the broad MWD seen in earlier stu dies is partly related to variations in [C3H6] during polymerization.