Propagation backbiting

Besides propagation the radical I may also undergo backbite to yield ethyl groups. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

These two pieces of evidence show that cyclohexene can add to metal carbene complexes to some extent, but that at low temperature backbiting to give oligomers is preferred to propagation, while at room temperature the product of addition can be trapped, at least for a time, by reaction with norbornene358. 

From the analyses of NMR and electron-spray ionization mass (ESI-MS) spectroscopy, the polymers obtained from the polymerization of cyclic disulfides were found to be a cyclic structure [202], The cyclic structure consisting of poly(DT) is assumed to be formed by a backbiting reaction of propagating species [203]. Thermal and mechanical properties of the polymers, and decomposition behaviors of the polymers demonstrate that the polymers obtained from thermal polymerization of cyclic disulfides include a polycatenane structure. From polymerization of cyclic disulfides in the presence of cyclic polyethylene oxide), a polycatenane consisting of two different cyclic polymers was obtained [199]. Thus, poly(DT) contains spatial entanglements of cyclic polymers with each other (a polycatenane structure was presumed) (Fig. 61). 

In addition, short-chain branches of C5 and C7 are seen at levels of 1 per 1000 carbon atoms, compared with a maximum of 15 n-butyl branches. This also arises from a backbiting reaction of the propagating radical and the resultant intramolecular chain transfer, and the relative amounts of the branches of various lengths may vary depending on the conditions of synthesis. 

The effect of this backbiting attack of a propagating carbanion is lowering of both polymer molecular weight and polymerization rate. 

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