Atom-Transfer Radical Polymerization: Method Question

[Caleb Breaux]

Hi everyone,

So I'm a chemical engineering grad student and oddly enough I have been tasked with doing chemistry (strange I know). I'm currently doing atom-transfer radical polymerization (ATRP) using a metal (CuCl or CuBr) and a ligand (dNbpy or PDMETA). My monomer is a methacrylate group with a tert-butyl dimethyl silyl tail on it. Currently, I'm having an issue with my polymerization and I think it's caused by catalyst (CuCl) poisoning, but I was hoping someone here might be able to shed some light on the situation.Current State:
I have CuCl, lightish green so it's oxidized a little bit, but I assume it's no big deal. I place in roughly a 2:1 molar ratio of ligand (dNbpy) and metal (CuCl). I then proceed to put in my monomer and initiator (the initiator is a 1:1 molar ratio with the metal compound). The dNbpy is solid and so is the CuCl, the monomer and initiator being liquids. As soon as the liquids meet with the solids, I get a reddish color that will (over the course of about ten to twenty minutes) go from red, to brown, to green. I know that Cu(II)Cl2 is a green color, so I assume that once it becomes green, the majority of the metal is in its oxidized state. For ATRP, the key (as I understand it) is for propagation to be slow and initiation to be fast. Control is possible through the activation of a radical by the initiator (or halogen capped group) and the Cu(I)Cl reacting to form a radical and Cu(II)Cl2. This is a reversible reaction so assuming that activation and deactivation occur at roughly the same speeds, I'd expect the reaction to be red with all Cu(I)Cl present, brown with a mixture of Cu(I)Cl and Cu(II)Cl2 present, and green when pure Cu(II)Cl2 is present. So I figured my solution, during polymerization, should be brown...however...the solution is green! This makes worried, especially since the polymerization is going 7 times slower than it normally should.

Today I attempted to "very quickly" add the monomer and initiator and then freeze-pump-thaw (I always do this before polymerization) three times before the solution turned green. After three freeze/pump/thaw cycles, I saw no bubbles in the solution and assumed all the oxygen was gone. The solution was still red, and I then placed it in a heated oil bath (75oC) for the polymerization. 15-20 minutes later I checked on the polymerization (it's a bulk polymerization by the way) and the solution was green. I run all my polymerizations under vacuum, rather than over nitrogen or argon.

Can anyone tell me if the color of ATRP-type polymerizations with CuCl is suppose to red, brown, or green during the polymerization? Catalyst poisoning is the only criminal I can think of for my failed reactions.

 

[Bystander]

Can you furnish any details for the actual reaction mechanism?

 

[Caleb Breaux]

https://en.wikipedia.org/wiki/Atom-transfer_radical-polymerization

The reaction mechanism is basically the picture you see at the top of the wikipedia page for ATRP, I'm even using CuCl. In the top reaction you have your initiator (R-Cl) whose bond is cleaved and radical stabilized by the Metal-ligand complex. The radical then proceeds to propagate by continuously attack the vinyl monomers. Every so often though, the metal ligand will reduce, causing a temporary "capping" of the radical. It's this capping that's suppose to allow for overall control over the molecular weight distribution.

 

[Bystander]

Ain't that interesting --- my first reaction would be to "play" with initiator:complex ratios with no monomer present --- do a little colorimetry just to verify your red-brown-green suspicion (which sounds as good a guess as any to me). From there, one or two polymerizations of monomers known to be well-behaved with this method. Then ... ?