M. Nele et al., Two-state models for olefin polymerization using metallocene catalysts. 1.Application to fluxional metallocene catalyst systems, MACROMOLEC, 33(20), 2000, pp. 7249-7260
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.