Ethylene homo- and copolymerization chain-transfers: A perspective from supported ( n BuCp) 2 ZrCl 2 catalyst active centre distribution

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Polymerization chain termination reactions and unsaturation of the polymer backbone end are related. Therefore, in this study, the parameters resulting from the modelling of the active centre distribution of the supported catalyst—silica/MAO/(nBuCp)2ZrCl2—were applied to evaluate the active-centre-dependent ethylene homo- and copolymerization rates, as well as the corresponding chain termination rates. This approach, from a microkinetic mechanistic viewpoint, elucidates better the 1-hexene-induced positive comonomer effect and chain transfer phenomenon. The kinetic expressions, developed on the basis of the proposed polymerization mechanisms, illustrate how the active site type-dependent chain transfer phenomenon is influenced by the different apparent termination rate constants and momoner concentrations. The active centre-specific molecular weight M ni (for the above homo- and copolymer), as a function of chain transfer probability, \(p_{CT_{i}}\), varied as follows: \(log\left ({p_{CT_{i}} } \right )=log\left ({mw_{ru}} \right )-log\left ({M_{ni}} \right )\), where mw ru is the molecular weight of the repeat unit. The physical significance of this finding has been explained. The homo- and copolymer backbones showed all the three chain end unsaturations (vinyl, vinylidene, and trans-vinylene). The postulated polymerization mechanisms reveal the underlying polymer chemistry. The results of the present study will contribute to develop in future supported metallocene catalysts that will be useful to synthesize polyethylene precursors having varying chain end unsaturations, which can be eventually used to prepare functional polyethylenes. The active-centre-dependent ethylene homo- and copolymerization rates and the corresponding chain termination rates support the 1-hexene-induced positive comonomer effect and chain transfer phenomenon from a microkinetic mechanistic viewpoint. The chain transfer probability \(p_{CT_{i}}\) decreased with increasing active centre-specific molecular weight M ni