Incorporation of Radicals during Ni(II)-Catalyzed Ethylene Insertion Polymerization

A possible involvement of radicals in catalytic insertion chain growth remains elusive, albeit it could account for thus far inconclusive experimental observations and enable yet unachieved synthetic reaction schemes. This work shows that radicals present, deliberately generated for this purpose, are incorporated as end groups in ethylene insertion polymerizations. Azo initiators (R-N = N-R′ with R = C­(CH3)2COOMe, Ph, 3,5-F2C6H3) added to pressure reactor Ni­(II)­salicylaldiminato-catalyzed ethylene polymerizations afforded initiator-derived aryl or alkyl end groups, respectively, as revealed by in-depth NMR studies of the polymer formed. The observation of additional solvent- derived aryl end groups, originating from radical transfer to toluene, underlines that end groups are indeed formed from radicals. Evidence for incorporation as a chain-initiating as well as -terminating end group is discussed.