In free radical addition polymerizations sometimes chain transfer agents are added. From what I understand chain transfer agents (e.g. XR' below) reduce the average molecular weight by transferring the free radical from the growing polymer chain (P•) to another molecule (R'•).

P• + XR' → PX + R'•

My question is what effect would this have on the overall "rate" of polymerization itself, i.e say the consumption of monomer.

In other words the R'• species is then free to combine with more Monomer, right? And being a smaller molecule (in general) would it be expected to have a higher rate constant for the reaction:

R'• + M--> P•


Not really.

As long as you are in solution (=unreacted monomer or solvent), it makes no difference if the reactive chain end is diffusing slow or fast. The monomer molecules come by at the same rate.

The mobile chain end would help against a local depletion of monomer, but once your mixture gets so viscous that this could happen, neither a long or a short active chain can still diffuse well (and neither can the transfer agent).

The only general difference you will see with transfer agent is that the Trommsdorff-Norish or gel effect sets in a bit later, because the overall viscosity of short chains is lower. In that sense, the transfer agent even slows down the reaction a bit. Which in that case is very welcome.

Of course the transfer agent radical can be either more reactive with the given monomer or less. (See "copolymerisation parameter".) So the chain restart can either be fast (in which case it makes practically no difference), or it can eventually be much slower. That depends on the specific transfer agent / monomer pair.

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  • $\begingroup$ Many thanks! So, if I interpret that correctly, then the effect on total rate of polymerization could be, in general, in either direction? Increase or decrease or not change? Would depend on the specific situation? $\endgroup$ – curious_cat Jun 8 at 7:14
  • $\begingroup$ I believe I said that a speedup is rather improbable?! The reaction rate of a FRP isn´t even constant to begin with, but the successive stages of the interplay of initiator dissociation, recombination, chain growth, chain transfer, gel effect, stationary state, etc. are generally well understood. Textbook stuff. ;-) Pick your favourite, read on, and don´t forget to do the exercises! $\endgroup$ – Karl Jun 8 at 7:47
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    $\begingroup$ lol Thanks! More like....back to the textbooks for me! :) Been too long! Thanks for your help! $\endgroup$ – curious_cat Jun 8 at 10:29

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