Chain propagation oriented addition

Orientation seems to hinder formation and propagation of hydroperoxides (Emanuel and Buchachenko 1987 Popov et al. 1991). Chain propagation and branching are thereby suppressed as it is achieved by the addition of chemicals as stabilisers. One therefore speaks of a structural stabilisation, which has a particularly marked effect in partially crystalline, strongly oriented polymer materials. 

The mechanism for the addition of hydrogen bromide to 1-phenylpropene in the presence of peroxides is a chain mechanism analogous to the one we discussed when we described anti-Markovnikov addition in Section 10.9. The step that determines the orientation of the reaction is the first chain-propagating step. Bromine attacks the second carbon atom of the chain because by doing so the reaction produces a more stable benzylic radical. Had the bromine atom attacked the double bond in the opposite way, a less stable secondary radical would have been formed. 

Each propagation step adds another molecule of styrene to the radical end of the growing chain. This addition always takes place with the orientation that gives another resonance-stabilized benzylic (next to a benzene ring) radical. 

For radical chain reactions to be kinetically feasible, both propagation steps must have relatively low activation barriers. Such is the case for addition of HBr, but not for HC1 or HI. Only HBr addition turns around in the presence of peroxides. HCI and HI additions remain ionic, with Markovnikov orientation, whether peroxides are present or not ... 

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