S. Lin et al., Kinetics of propylene polymerization using bis(2-phenylindenyl)zirconium dichloride/methylaluminoxane, J AM CHEM S, 122(46), 2000, pp. 11275-11285
The kinetics of propylene polymerization in toluene solution by bis(2-pheny
lindenyl)zirconium dichloride/methylaluminoxane at 20 degreesC were investi
gated. As the structure and properties of elastomeric polypropylenes produc
ed by these catalysts depend sensitively on the reaction conditions, a deta
iled study of the kinetics was carried out to evaluate the influence of the
se parameters on the polymerization behavior. Studies of the solubilities a
nd mass-transfer rates reveal that dissolved atactic polypropylene has litt
le effect on propylene solubility but influences the mass-transfer rate of
propylene into solution. The rates of propylene polymerization reach a maxi
mum after 10-20 min and then decrease. The decrease in rate over time is fa
ster at higher monomer concentrations. Catalyst activity was negligible at
[Al]/[Zr] = 1000 but constant from [Al]/[Zr] = 2500 to [Al]/[Zr] = 10 000.
Analysis of molecular weights as a function of monomer concentration reveal
P-hydride elimination to be the primary chain-transfer mechanism. Narrow m
olecular weight distributions (M-w/M-n = 2.0-2.6) were obtained. The increa
se of the isotactic dyads and pentads ([m] and [mmmm]) with increasing mono
mer concentration reveals an additional kinetic event which competes with t
he stereodifferentiating olefin insertion step. Modeling studies are more c
onsistent with a mechanism involving interconversion of the catalyst betwee
n isospecific and aspecific states than a mechanism involving epimerization
of the stereogenic centers of the growing polymer chain.