Mechanisms of branch formation in metal-catalyzed ethene polymerization

Branches are an important aspect of the structure of real polyethylene. Branches can be short (Me, Et) or longer; long-chain branches (LCB, >100 carbons), in particular, are important because they can have a dramatic effect on polymer properties. In this review, we summarize mechanistic information from organometallic and computational chemistry and use this to examine the most probable sources of each type of branch. Short branches can be introduced deliberately by copolymerization with an α-olefin (possibly formed in situ from ethene). Me branches may be formed by one-carbon chain walking and propagation, and/or from insertion of an oligomer/macromer in an M Me bond formed via chain transfer to the cocatalyst [Me3Al or methylaluminoxane (MAO)]. Et branches are most likely formed through β-hydrogen transfer to ethene, followed immediately by reinsertion of the newly formed macromer. LCBs have usually been ascribed to reinsertion of macromers. However, certain catalysts exhibit LCB formation patterns that are hard to reconcile with this model, and a ‘two-monomer’ model was recently proposed to explain the observations for these systems. In this review, we present an alternative explanation (chain walking) that would fit the same facts for these catalysts.