Regiosequence Isomerism - Head vs Tail Addition

With 1,3-diene based polymers, greater scope for structural variation is introduced because there arc two double bonds to attack and the propagating species is a delocalized radical with several modes of addi tion possible (see 4.3.2). 

For mono- and 1,1-disubstituted monomers, steric, polar, resonance, and bond-strength terms (see Section 2.3) usually combine to favor a preponderance of tail addition Le. an almost completely isoregic structure. Fiowever, the occurrence of 

In view of the potential problems associated with discriminating between the various types of head-to-head linkages, it is perhaps curious that, while much effort has been put into finding hcad-to-hcad linkages, relatively little attention has been paid to applying spectroscopic methods to detect tail-lo-lail linkages where no such difficulty arises. 

Even allowing for the above-mentioned complication, the number of head-to-head linkages is unlikely to equate exactly with the number of tail-to-tail linkages. The radicals formed by tail addition (T ) and those formed by head addition (H ) arc likely to have different reactivities. 

Consideration of data on the reactions for small radicals (Section 2.3) suggest that the primary alkyl radical (H ) is more likely to give head addition than the normal propagaLing species (T-) for three reasons  

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