Utility of propenyl groups in free radical polymerization: Effects of steric hindrance on formation and reaction behavior as versatile intermediates

2-Substituted-2-propenyl groups can be introduced as polymer ω-end-groups by a variety of means in free radical polymerization. Addition-fragmentation chain transfer (AFCT) and catalytic chain transfer have been employed to introduce the 2-substituted-2-propenyl groups almost quantitatively under appropriate conditions when bimolecular termination cannot compete effectively with these processes as an end forming event. Unsaturated oligomers (dimers, trimers, tetramers and oligomers of higher degrees of polymerization) of methyl methacrylate and methyl acrylate function as carbon-carbon bond cleavage type AFCT agents and can be used to introduce the propenyl end-group. However, these compounds are not as reactive chain transfer agents as carbon-heteroatom bond cleavage type AFCT agents such as α-(bromomethyl)-, α-(alkylthiomethyl)- and α-(arenesulfonylmethyl)acrylates, which effectively reduce the molecular weight of the polymer formed and introduce the propenyl end-group. Mid-chain radicals formed during polymerization of acrylic esters are structurally similar to the radicals generated by radical addition to the 2-carbalkoxy-2-propenyl end-group. β-Fragmentation of the mid-chain radical can be regarded as the reverse reaction of radical addition to the propenyl end-group. Steric congestion accelerates formation of the propenyl end-group via β-fragmentation of the adduct radical, and suppresses intermolecular reactions of the adduct radicals to a larger extent than addition of the propagating radical to the propenyl end-group. Low molecular weight polymers bearing 2-carbalkoxy-2-propenyl and 2-phenyl-2-propenyl ω-end-groups undergo copolymerization as macromonomers with common vinyl monomers, with the exception of poly(methyl methacrylate)s bearing the propenyl end-groups, which primarily act as AFCT agents. Reactivities and reaction modes of α-(substituted methyl)acrylates and related monomers are also described in relation to the behavior of propenyl groups bound to polymer chains.