reasonable assumption

Free radicals formed in number will decide the chain length of the polymer. More free radicals will form many polymeric chains with shorter chain length but it may narrow down the distribution profile of molecular weights of polymer or polydispersity.

Dear all, whatever the source for free radicals generation (thermal or radiation) it is a matter of free radicals concentration, in the case of acrylamide polymerization the propagating radical species are protected against premature termination, which means propagation is much more prefered. In fact this is why polyacrylamide has a MW in the order of 20 millions. Years ago and in a similar question I attached some documents that support an answer. Best Regards

Abdelkader BOUAZIZ I looked up the information you gave on this in 2014.You say, " There is a simple mechnism behind such a phenomenon, in which the propagating species are efficiently protected against any premature termination." but you don't really explain what you mean by protected against premature termination. I'm pretty new to polymer chemistry, and I don't have a polymer expert here at my university, so I'm not really sure what you mean by this? Could I get some more detail about the mechanism that causes this protection?

Also, in the same question, you mention increasing the initiator to control MW somewhat. This would increase the number of radicals in the solution and make shorter chains. Our EB initiation takes less than 1 min, so that means many radicals are being produced at once and reacting very quickly, right? Our reaction is done with a sparged solution open to air.

Since there is a limited supply of monomer in the solution, the radicals will eventually deplete the monomer, almost completely, or "starve". At that point you end up with a lot of smaller chains, from the way I understand it. These radicals are then quenched from the O2 in the air, or in some other way I'm not aware of. We don't add anything to quench them. I think this would give me small chains.

Am I on the right track, or is this completely wrong? Thanks to everyone for their help on this!

Dear Lena Allen, concerning the first paragraph I attached the two scanned pages from my master thesis in which it is explained how propagation is more favored by protecting the propagating chains by protonation (in water) or by hydrogen bonds (in organic solvent).

The second paragraph I didn't said what you mentioned and your reasoning is correct : increasing initiator concentration reduces MW.

the referenced you need (18,86) are :

18- moradi-araghi, a., Iqbal, a., water-soluble polymers (oil recovery applications), in : Concise Polymeric Materials Encyclopedia, by Joseph C. Salamone (1998).

86- Allen, S. G., FRS, Bevington, J. C., 'Comprehensive Polymer Science ' 3, Perganon Press, 1989.

It is the dissolved O2 that is harmful for free radicals. The ambient O2 needs time to dissolve to reach the free radicals. This is why bubbling with an inert gas is done at the beginning of the reaction.

Best Regards

This is the link I meant : https://www.researchgate.net/post/How_to_control_the_molecular_weight_in_polymerizing_acrylamide_with_free_radical_polymerization/amp

Abdelkader BOUAZIZ Thank you so much for this information!

No thanks, good luck.

Solid acrylamide is one of the easiest monomers to polymerize into solid polyacrylamide. One of my distinguished English supervisors told me once jokingly , during my postgraduate research, : "Nizar, acrylamide will get polymerized if you just look at it !".

The tip , for our research team at that time, from a U.K giant chemical industry , was to either reduce the amount of radical initiator or to decrease the time of exposing the monomer to irradiation in order to get polymeric chains with reasonable lengths. This has worked well.