[Mahmoud Refaat]

Hello everyone.

I have been working on a project about polymerization of DADMAC.

My procedure is according to US Patent 4151202. However the method doesn't explain how to control the molecular weight of the produced chain.

I know that reaction temperature and mixing speed are very important, So is there a way to predict the M.Wt. using pre-calculated temp. and speed ?

I don't have much experience in this field so any guides or manuals are always appreciated even if not very related to the subject.

Thanks.

[wildfyr]

Oof this is a hard one. It is theoretically possible to control free radical polymerization MW, however many factors will affect molecular weight. Solution temperature, viscosity, monomer concentration, purity, and the presence of inhibitors, plus initiator identity, purity, concentration, and your ability to exclude oxygen, and I'm sure I'm missing some.

Also, DADMAC is a weird polymerization because your form a 5 membered ring from a diallyl compound.

Its likely the best way to do this is to do a series of experiments and empirically determine how to get your MW. To give you at least a target to hit, I suggest looking at Principles of Polymerization by G. Odian. Its available cheaply in hardcopy or in pdfs floating around the internet. Good luck.

[Mahmoud Refaat]

Oof this is a hard one. It is theoretically possible to control free radical polymerization MW, however many factors will affect molecular weight. Solution temperature, viscosity, monomer concentration, purity, and the presence of inhibitors, plus initiator identity, purity, concentration, and your ability to exclude oxygen, and I'm sure I'm missing some.

Also, DADMAC is a weird polymerization because your form a 5 membered ring from a diallyl compound.

Its likely the best way to do this is to do a series of experiments and empirically determine how to get your MW. To give you at least a target to hit, I suggest looking at Principles of Polymerization by G. Odian. Its available cheaply in hardcopy or in pdfs floating around the internet. Good luck.

I found the book it was quite helpful , thanks .

[P]

I thought the MW of a free radical polymerization was determined by the initiator concentration, no?

You have a finite amount of monomer molecules and a finite number of chains. 2 chains for each initiator molecule. So, crudely, you can work out the MW from the yield of the reaction as you know how much monomer has been used up by how many chains....  thus you can work out the average chain length..   Of course this ignores termination reactions, unpredictable scissions and side chain reactions etc...  but you should get a reasonable average by just working it out from the monomer MW and some maths involving the concentrations of initiator and monomer (if you keep everything else constant).

Am I missing something? (probably the something to do with the rings? - I have never polymerized DADMAC)
 

[wildfyr]

You are right, but, just for instance, the termination rates and pathways are different for allyl vs acrylate vs methacrylate vs styrenic. If I took acrylic acid and methacrylic acid at the same concentration and same initiator concentration, I would see different MWs.
This paper discusses it. Even just skimming, look at Figure 2 (http://pubs.acs.org/doi/pdf/10.1021/ma960367h)


Such confounding factors make it really tough to apply the theoretical calculations to real life conditions. Heres another confounding factor: initiator concentration is not constant. Towards the end of a reaction you get higher MW material since the radical concentration in solution is lower.

Another fascinating effect is the Trommsdorf effect, in which the MW of a rxn increases as viscosity increases during the polymerization. This is because monomers can diffuse much faster than chains, so monomers will diffuse to the growing chain end more efficiently, making termination due to polymer-polymer termination events less likely. Its often a rapid onset effect.

[phth]

Like others said concentration. 
My two cents:
If the reaction happens too fast, then cool it down to slow the rate constant to below the rate of mixing.  Typical Rxns with mixing problems are complete within 5 s -1 min.  I would just to test the mixing hypothesis by splitting a matrix in 2 and using 2 stir plates.

[P]

You are right, but, just for instance, the termination rates and pathways are different for allyl vs acrylate vs methacrylate vs styrenic. If I took acrylic acid and methacrylic acid at the same concentration and same initiator concentration, I would see different MWs.
This paper discusses it. Even just skimming, look at Figure 2

I'll check it out. First thought though would be that the equations contain a constant or a fudge factor for each different reaction. You can run a few reactions and calculate your fudge factor for the set of reactants you have.

Such confounding factors make it really tough to apply the theoretical calculations to real life conditions. Heres another confounding factor: initiator concentration is not constant. Towards the end of a reaction you get higher MW material since the radical concentration in solution is lower.

Another fascinating effect is the Trommsdorf effect, in which the MW of a rxn increases as viscosity increases during the polymerization. This is because monomers can diffuse much faster than chains, so monomers will diffuse to the growing chain end more efficiently, making termination due to polymer-polymer termination events less likely. Its often a rapid onset effect.

I think this is why the reaction in solvent yields much better PolyDispersity control over the polymer. Keep constant temperature, high solvent conc and don't let the reaction run to completion (set the time) and you can get pretty good control. (For vinyl and acrylate type polymers anyway - I don't have a lot of experience with others, but I think the principles are the same).

For better/tighter controls over polydispersities I started to try 'Living' Free radical reactions using TEMPO as a chain end cap that slows the reaction down and allows more controlled growth. 

[wildfyr]

Ha, there hasn't been much literature using nitroxide mediated polymerization in a while, I tried it too (from the surface) as a first year grad student and got good results. Unfortunately I was too much of a rookie to reliably synthesize the initiator. For me the gold standard CRP right now is RAFT. ATRP (and its easier buddies ARGET and AGET) requires mucking around with copper species and is still kind of a pain.

For OP though, these aren't really considered commerically viable. Are you using it in industry, P? Because I'm sure Craig Hawker would love to hear that!

I agree that somewhat diluted polymerizations will afford him decent control. I think DADMAC is done in water. This isn't a terribly difficult project, it just may require him to run a bunch of polymerizations to figure out what knobs need to be dialed to get a given MW. Temp and initiator concentration/identity are what I would focus on, and don't let it go above 90% conversion or become highly viscous because you can lose control.

[P]

Ha, there hasn't been much literature using nitroxide mediated polymerization in a while,...
For OP though, these aren't really considered commerically viable. Are you using it in industry, P? Because I'm sure Craig Hawker would love to hear that!

I haven't done a polymerization for years now. I used TEMPO to get better polydispersities in a system where better polydispersities gave better product performance. It was part of my Ph.D. This was some time ago though. Others in our group were using ATRP and RAFT. I must confess that I do not know what ARGET or AGET are even. lol - it has been a long time. Although I use polymers at work now I do not do polymerisations any more.

[wildfyr]

ARGET and AGET are more oxygen tolerant versions of ATRP. Its often written as ARGET-ATRP or AGET-ATRP. They include a reducing agent (ascorbic acid I think) to fend off oxidation due to triplet oxygen and keep your Cu(I) in play. With AGET you can do ATRP in a scint vial with the cap on, no nitrogen flow.

[P]

OK Thanks  -  that's good - I remember it being very oxygen sensitive. We had to de gas everything and get everything very pure by distillation before even thinking about starting a reaction.