while I was studying the many side reactions of radical polymerization, I stumbled across this source which illustrates the chain transfer side reaction in polystyrene synthesis:
Of course, there is a typo in this picture, it shouldn’t be -CH2-H- but -CH-H-, but you get the idea. A monomer radical can abstract a hydrogen atom from an already existing chain in the first step, so that it’s radical function is forwarded to the chain.
The chain then, in a second step attacks another styrene molecule from the back side of the double bond so that the more stable, secondary radical forms, so the molecule can also form branches.
Everything alright with me, but I don’t get the regiochemistry here. In the first step,a secondary carbon atom is attacked, which also forms a secondary radical intermediate; but wouldn’t an attack on the tertiary -CH group be more favorable thermodynamically as it is a tertiary radical intermediate which is also stabilized by the phenyl group? Or is it a kinetically controlled process, where more hydrogens can be subtracted on the bridging carbon atoms, so that after the chain transfer, the bridging carbons are tertiary as a result?
Any tips would really be appreciated!