Experimental Evidence Shedding Light on the Origin of the Reduction of Branching of Acrylates in ATRP

Intramolecular chain transfer to polymer and subsequent propagation of tertiary radicals cause extensive branching in radical polymerization of acrylic monomers. Studies in the literature have shown that under controlled radical polymerization conditions the extent of branching is significantly reduced. There are two competing theories to explain this effect. In one, the cause of reduced branching is attributed to a reduction in the number of backbiting events, and in the other that has been specifically applied to atom-transfer radical polymerization (ATRP), the cause is due to trapping of the tertiary radical by a fast deactivation step. In this article we show that trapping of the tertiary radical is not the cause for the reduction in branching fraction. This is shown by the absence of the corresponding patched midchain bromide structure as revealed by quantitative C-13 NMR and by the ability to chain extend from a poly(butyl acrylate-co-butyl 2-bromoacrylate) copolymer by ATRP. These results are complemented by quantum mechanical computations.