Revisiting the long-chain branch formation mechanism in metallocene catalyzed polyethylenes

The effect of polymerization parameters, such as pressure (ethylene concentration), temperature and comonomer type and concentration, on several long-chain branched (LCB) ethylene homopolymers and ethylene/α-olefin copolymers was investigated with respect to their degree of long-chain branching. Common long-chain branched metallocene polyethylenes (LCB-mPEs) and comb-like LCB-PEs, which have high viscosity and significantly differ from conventional LCB-mPEs, have been used to evaluate different proposed mechanisms for LCB-formation based on available characterization methods. The correlations between polymerization parameters and degree of long-chain branching were compared with the correlations predicted by different formation mechanisms (intermolecular auto-copolymerization by re-incorporation of β-hydride terminated chains, intramolecular insertion of stored vinyl terminated chains, chain walking and σ-bond metathesis reactions) to elucidate which of these mechanisms is the correct one. The intermolecular auto-copolymerization mechanism was found to be the predominant mechanism behind LCB-formation in slurry phase polymerization. However, σ-bond metathesis reactions cannot be entirely ignored.