[theunraveler]

ok i assume ppl here are all much better than me in chemistry so i wont write down all the steps in this process 

I was just reading my textbook about this interesting example of using free radicals to create vinyl polymers. What I would like to know is how exactly is the process controlled so that the free radicals dun end the reaction too quickly?

[lavoisier]

I am not an expert of polymerizations, but actually free radicals are the ones that keep the reaction going, not the ones that make it 'end too quickly'.

The kinetic of chain polymerisation requires a slightly different mathematical treatment with respect to 'normal' reaction kinetics.
However, the process starts with the formation of radicals by homolytic cleavage of an initiator I (sorry for the poor formatting):

I --> A. + B.

This occurs by supplying energy (heat, radiation...), but I is designed to undergo this reaction easily, so it's not a problem.
The monomer R-H reacts with the initiator radical:

R-H + A. --> R. + AH

The monomer radical can then add to other monomers to generate a longer chain (which is the aim of the process):

R. + R-H --> R₂.
R₂. + R-H --> R₃.
etc.

Or it can abstract a hydrogen atom from another molecule (chain transfer):

R_m. + R_n-H --> R_m-H + R_n.

As you see, in all these cases a radical is present on either side of the reaction arrow, which means the reaction can proceed (as long as there's monomer available).
The cases when the radicals are quenched are:
- the addition of two growing chains:

R_n. + R_m. --> R_(n+m)

- the reaction with labile substances that yield very stable, unreactive radicals (X.):

R_n. + X-H --> R_n-H + X.

I seem to remember that as there are initiators there are also other substances one can add to moderate the reaction; please don't ask me what they are (phenols? thiols?).
But normally the result of your polymerization (mean molecular weight, distribution of molecular weights...) is determined by the initial amount of initiator and by the specific kinetic constants of the system.

[movies]

Really all you need to know is that the rate of radical addition to a vinyl group (radical chain propagation) is a lot faster than the rate of H atom abstraction (radical chain termination).

[P]

I seem to remember that as there are initiators there are also other substances one can add to moderate the reaction; please don't ask me what they are (phenols? thiols?).
But normally the result of your polymerization (mean molecular weight, distribution of molecular weights...) is determined by the initial amount of initiator and by the specific kinetic constants of the system.

2, 2, 6, 6-tetramethylpiperidin-1-oxyl    or TEMPO for short mediates a free radical polymerisation usually initiated with AIBN or BPO.     The reaction is then classed as a 'living' free radical polymerisation. As the chain grows, the free radical is capped by the TEMPO and monomer is allowed to add a a more controlled rate, so molecular weight distributions (or polydispersities) are allot tighter.   The reaction is classed as living because when all of the monomer is used up the reaction ends but the TEMPOstill caps a now 'stable' free radical on the end of each chain. If more monomer is added (and the appropriate temperature applied) the reaction will continue.   This makes them very useful for block copolymerisations as when the first reaction is over a different monomer can be applied and propagation will continue with the new monomer (presuming the compatibility is there with the initial monomer) 

ATRP or atom transfer free radical polymerisations are similar, where, you have an end capping metal complex to mediate/ controll the polymerisation.

[Random Coils]

Really all you need to know is that the rate of radical addition to a vinyl group (radical chain propagation) is a lot faster than the rate of H atom abstraction (radical chain termination).

Hydrogen abstraction does not generally terminate the chain reaction. Rather, it leads to another radical species that can, in some cases, reinitiate polymerization to start a new chain; therefore it's usually referred to as chain transfer instead of termination.

[movies]

Well, that kind of depends on what type of H is abstracted and what you consider the "end" of the process.  I was thinking in terms of ending the growth of a particular polymer chain.  I suppose to be rigorously correct, H-atom abstraction is not always chain terminating.

[P]

Really all you need to know is that the rate of radical addition to a vinyl group (radical chain propagation) is a lot faster than the rate of H atom abstraction (radical chain termination).

Hydrogen abstraction does not generally terminate the chain reaction. Rather, it leads to another radical species that can, in some cases, reinitiate polymerization to start a new chain; therefore it's usually referred to as chain transfer instead of termination.

This is why TEMPO etc.. give excellent polydispersities  -  it cuts out the chain transfer and the termination reactions - if the propergating chains meet they just ignore each other because of the end cap on the chain rather than terminating.   

Also, once the reaction has gone to compleation, you can add more monomer and heat to restart it again.  This is GREAT for doing block copolymerizations where you do the first polymerization to compleation then add monomer 2 to get a run with that. You can then go back to monomer 1 again if you want to make an AAAAAAAA-BBBBBBBBB-AAAAAAAA sort of sandwhich copolymer and so on as many times as you like.  (OK, you can make block co[polymers in other ways, but this gives precise controll over the length of each run       )