Ultrahighly Branched Main‐Chain‐Functionalized Polyethylenes via Inverted Insertion Selectivity

: Branched polyolefin microstructures resulting from 'chain walking' are a fascinating feature of late transition metal catalysts, but so far it has not been demonstrated how any desirable branched polyolefin microstructures can be generated hereby. We show how ultrahighly-branched polyethylenes with exclusively methyl-branches (220 Me/1000 C) and very high molecular weights (~10 6 g mol -1 ), that reach the branch density and microstructure of commercial ethylene-propylene elastomers, can be generated from ethylene alone. At the same time polar groups on the main chain can be generated by in-chain incorporation of methyl acrylate. Key is a novel rigid environment in an α-diimine Pd(II) catalyst with a steric constraint that allows for excessive chain walking and branching, but restricts branch formation to methyl branches, hinders chain transfer to afford a living polymerization, and inverts the regioselectivity of acrylate insertion to a 1,2-mode.