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
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
Preparation and Properties of Branched Polystyrene through Radical Suspension Polymerization, Wenyan Huang et al., Polymers (Basel). 2017 Jan; 9(1): 14
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),
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
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
Preparation and Properties of Branched Polystyrene through Radical Suspension Polymerization, Wenyan Huang et al., Polymers (Basel). 2017 Jan; 9(1): 14
Self-Branching in the Polymerization of StyreneJ. C. Bevington, G. M. Guzman and H. W. MelvilleProceedings of the Royal Society of London. Series A, Mathematical and Physical SciencesVol. 221, No. 1147 (Feb. 9, 1954),
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:
This indicates that chain transfer is a rare event in a typical synthesis of polystyrene.
As this is a rare event, you don't expect any of the intermediates to be the most favorable ones.
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
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:
This indicates that chain transfer is a rare event in a typical synthesis of polystyrene.
As this is a rare event, you don't expect any of the intermediates to be the most favorable ones.
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
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