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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:

enter image description here

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!

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    $\begingroup$ Not sure but steric hindrance might play a role. Because if we attack on CH to form tertiary radical it will be having two phenyl group that might be unstable. $\endgroup$ Oct 5 at 6:55
  • $\begingroup$ So bascially, if it comes to hindrance, that would indicate a kintetically controlled process, because it is more likely to attack on the bridge, and form sterically stable bridges, if I am not mistaken. $\endgroup$
    – 冰淇淋
    Oct 5 at 11:18
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    $\begingroup$ See if we talk about intermediate that intermediate will be more stable in ur case like removing H from tertiary carbon containing phenyl group but as final product it won't be much stable to have two phenyl group on single carbon. So intermediate is stable but product is not so I am not sure whether it will come under TCP or KCP. $\endgroup$ Oct 5 at 13:28
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The image shown in the question can be found with some context at https://www.open.edu/openlearn/science-maths-technology/science/chemistry/introduction-polymers/content-section-4.3.3

In that document, they say of chain transfer:

A similar mechanism accounts for the side branches in LDPE where it is a more important mode of termination than in polystyrene

This indicates that chain transfer is a rare event in a typical synthesis of polystyrene.

[...] 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?

As this is a rare event, you don't expect any of the intermediates to be the most favorable ones.

Or is it a kinetically controlled process [...]

If the chain transfer is rare, you would expect faster processes to react with the radical intermediate faster than it forms (because these steps are less rare). This means it is kinetically controlled.

Moreover, all the steps with two radicals combining to form a bond would be expected to be kinetically controlled because once formed, the covalent bond would not fall apart into radicals (that is why you need initiator molecules in the first place).

References

  1. Synthesis of Polystyrene and Molecular Weight Determination by 1H NMR End-Group Analysis, Jay Wm. Wackerly and James F. Dunn, J. Chem. Educ. 2017, 94, 11, 1790–1793
  2. Preparation and Properties of Branched Polystyrene through Radical Suspension Polymerization, Wenyan Huang et al., Polymers (Basel). 2017 Jan; 9(1): 14
  3. Self-Branching in the Polymerization of Styrene J. C. Bevington, G. M. Guzman and H. W. Melville Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences Vol. 221, No. 1147 (Feb. 9, 1954),
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