Polyethylene Chain Growth on Zinc Catalyzed by Olefin Polymerization Catalysts:  A Comparative Investigation of Highly Active Catalyst Systems across the Transition Series

Abstract

Highly active transition metal ethylene polymerization catalysts across the transition series have been investigated for their ability to catalyze chain growth on zinc. In reactions of various catalysts with ZnEt(2), product distributions range from Schulz-Flory to Poisson, with several catalysts showing intermediate behavior. A statistical modeling program is introduced to correlate product distributions with the relative rates of propagation, chain transfer to zinc, and beta-H transfer. Six regimes have been identified, ranging from Schulz-Flory to pure Poisson where chain transfer to metal is the only termination process, through to combined alkane/alkene distributions where beta-H transfer is competitive with chain transfer to metal. It is concluded that, while catalyzed chain growth (CCG) is favored by a reasonable match between the bond dissociation energies of both the main group and transition metal alkyl species, the M-C bond energies of the bridging alkyl species, and hence the stabilities of any hetero-bimetallic intermediates or transition states, are key. The latter are strongly influenced by the steric environment around the participating metal centers, more bulky ligands leading to a weakening of the bonds to the bridging alkyl groups; CCG is thus usually more favored for sterically hindered catalysts.

How to Polymerize Ethylene in a Highly Controlled Fashion?

“…[21] Systems identified by the authors as very fast, reversible, PE chain-transfer catalysts are only taken for discussion. We do not review on chain transfer in general within this contribution.…”

“…Pre-catalysts summarised in Scheme 3 were activated with MAO and used as CGS. [21] Nine out of nineteen transition-metal complexes listed in Scheme 3 show catalytic polymerisation characteristics indicative of chain transfer to Zn. Polymerisation details of these complexes are listed in Table 7.…”

Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

“…The amount of chain transfer can be quantified in two ways: first, as the amount (mol) of main group metal alkyl groups extended (equation 1), 7 and second, by examining the number of chains initiated per total molar amount of polymerization catalyst (equation 2). Figure 4A shows a plot of the number of chains initiated versus the concentration of ZnEt 2 .…”

Propylene Polymerization Catalyzed by rac-Et(Ind)2ZrCl2/Cp2ZrCl2 in the Presence of ZnEt2

“…The polymerization was terminated at a fixed time by the addition of acidified ethanol. The product was filtered, washed and dried in vacuum at 60 • C [21]. The oligomer produced was isolated by evaporation of toluene and the remaining solution containing polymer was added into hexane, then the resultant solution was washed with a large amount of acidified ethanol to remove aluminum salts, and finally the oligomer could be obtained after evaporating the solvent.…”