[ET]

Hi... this is a rather dumb question, but I can't seem to find the answer that will ease my mind...
I will state the thoughts that lead to my question:

1. In radical polymerization the initiator forms a radical species which attacks a monomer binds to it and then the monomer (plus the initiator already bounded to it) has a radical on a different atoms and it goes on and so forth until there's a termination by either two radicals eliminating each other (makes you wonder about what would happen if people were radicals) or by disproportionation.

2. Radical polymerization has (during the reaction) only unreacted monomers, unreacted initiators and high MW chains with no intermediate size molecules.

My question is, why are there no intermediate size molecules? If radical rxns eliminate themselves why can't it occur in the initiation step, or during the propagation step? Are there really no tetramers formed (with the addition of bits from the benzoyl peroxide on the ends) in the end product when doing PVC (for example)?
Is it because the affinity radicals have for stable structures over other radicals (if so, any numbers for rxn constants)?

I'll be happy for answers, references, whatever you think fit.

[Borek]

It is all about probability of termination. As long as the probability of termination is substantially lower than the probability of the growth, the latter is favored. It doesn't mean there are no intermediate size chains present, more like their number is very low.