Group 12 and 13 metal-alkenyl promoted generation of long-chain branching in metallocene-based polyethylene

In situ synthesis of topologically modified linear polyethylenes (PEs) using single-site polymerization catalysis is a challenging task that enables the production of advanced materials with tailored properties. We describe here our investigations aimed at an efficient generation of long-chain branches (LCB) in linear PEs using discrete B-, Al- and Zn-alkenyl co-reagents in combination with homogeneous rac-{EBTHI}ZrCl2 (1)/ or (nBuCp)ZrCl2 (2)/MAO catalytic systems. As corroborated by extensive rheological studies and 13C NMR spectroscopy, Al-and Zn-based reagents promote LCB formation via a two-step mechanism involving both vinylic group insertion and M → Zr transmetallation steps. In striking contrast, the B-alkenyl species was found to form hydrolytically stable B-centred cross-linked PE structures. The Mg-based reagent appeared to be reluctant towards Mg → Zr transmetallation reaction, providing only the products of vinylic group insertion, which after hydrolysis afforded short-chain branched (SCB) PEs. The experimental observations were rationalized by DFT computations.