| Abstract
| - A kinetic model was developed to describe the propylene polymerization behavior of fluxional,two-state metallocene catalysts. In particular, the pentad and molecular weight distributions can bedescribed as well as other parameters of interest, such as the weight fraction of crystallizable sequencesand the isotactic sequence length distribution, in terms of fundamental kinetic constants and polymerization conditions that pertain to these two-state catalyst systems. The model was used in an attempt todescribe the polymerization behavior of two, prototypical, fluxional catalyst systems, (2-PhInd)2ZrCl2/MAO (1) and (2-p-CF3PhInd)2ZrCl2/MAO (2). The model can accurately reproduce the pentad distributionsobserved in PP prepared using these catalysts and the response of the distribution to changes inpolymerization conditions, specifically changes in [C3H6] at constant T. These studies illustrate that therate of state-to-state interconversion is slower but of comparable magnitude to the rate of monomerinsertion and that the states have similar stability and reactivity. The broad molecular weight distributionspreviously observed with this family of catalysts can be described by the model. However, the modelpredicts that the state-to-state interconversion rate has to be significantly slower than the rate of formationof dead polymer chains, and this is inconsistent with the rate estimated from the response of the pentaddistribution to changes in the rate of propagation (i.e., [C3H6]). Recent work where propylene polymerizations using 1 were carried out to low conversion indicate that the broad MWD seen in earlier studies ispartly related to variations in [C3H6] during polymerization.
|
