Physicist asking dumb questions alert:
I want to synthesise polyacrylamide with a molecular weight ~ millions of Da. I want to do this so that I can add a small quantity of other monomers into the backbone. I've made a PAM gel before using TEMED as an initiator and bis-acrylamide as the crosslinker. 1. Do I simply do the same thing without the crosslinker? 2. How do I control the Mw (is it just a question of time?) 3. I've read that bisacrylamide is a common impurity in acrylamide monomer samples. Is there something I can do to prevent this forming crosslinks? 4. Would it be better to buy a low Mw PAM and perform the same chemistry?
Dear Michael Smith;
there is absolutely no problem to get ultra high molecular weight (MW) in the case of acrylamide polymerization. To avoid to a great extent the aforementioned problems you have to carry out the polymerization in water at low temperature using preferebly a redox couple initiating system (e.g., Potassium- or ammonium- persulfate and sodium metabisulfate). You may reach even 20 millions (g/mol) MW. There is a simple mechnism behind such a phenomenon, in which the propagating species are efficiently protected against any premature termination (if more details are needed, please tell me). On the other hand the ultra high MW polyacrylamides are difficult to solubilize in most common solvants including water, though it is a water-soluble polymer. Here the number of entanglements are so high that the chains are physically crosslinked.
Acrylamide is usually purified by recrystallization before use, chloroform and other common solvants (non-solvants) is used for this purpose.
There are other forms of intra- and inter-chains groups that may be formed during the polymerization which may cause chemical crosslinks, e.g., imide group, but this usually has a limited and minor effects.
To control the MW there are many posibilities: - increasing the concentration of the initiator to a certain limit, - use of transfer agents, - to stop the polymerization at low conversion, - mechnical degradation of the high MW polymer.
I hope I have given as much as you can get an insight view to how broad is this question. Regards
If you want to end up with a material that you can manipulate in any way, rather than an intractable crosslinked mass, it is best to work in an inverse emulsion system. Here you would dissolve your monomer and half the redox couple in water, disperse this phase in an oil using an appropriate surfactant, and add the other half of the redox couple in aqueous solution as a feed over time. Because it is a free radical chain polymerisation, the MW will be approximately the same at every stage during the reaction, there will just be a lower yield earlier on.
Here is a recipe we used to make high MW cationic polyacrylamide a while ago which is adapted from an industrail recipe. (Journal of Applied Polymer Science, Vol. 90, 316–325 (2003))
"Aqueous solutions of AAm and cationic monomer containing a small amount of potassium bromate were dispersed in hydrocarbon oil (LT20; Conoco, Victoria, Australia), with sorbitan monoleate as emulsifier and ethylenediaminetetraacetic acid (EDTA; Aldrich) as a chelating agent to remove the inhibitor. The aqueous phase was dispersed in the oil phase at 1000 rpm using an Ultra-Turrax, after which the system was purged with nitrogen overnight (at 25°C and at 200 rpm) to remove residual oxygen, which can inhibit the radical reactions. An initiator solution containing sodium metabisulfite was then injected in aliquots to start the polymerization and the progress of the exothermic reaction monitored. All polymerizations were isothermal at 40+/-0.5°C and were carried out under nitrogen with equal volumes of water and oil, corresponding to 570 g of aqueous phase and 430 g of organic phase. Samples for characterization were obtained by precipitation in acetonitrile."
Thanks for your reply Chris. My aim in all this is ultimately to have very dilute aqueous solns of high Mw fluorescent polymer. Since I can buy the polyacrylamide, might it be easier to react a fluorescent dye with the amide side groups? I found things like 5(6)-Carboxytetramethylrhodamine. Would this undergo a condensation reaction with the amide group? I'm just struggling to work out the chemistry of what group I need and then some sensible conditions. Maybe I am being unrealistic but I want to keep this as simple as possible since I am not a chemist.
Dear all,
the amide group is open to a wide and easy chemical modification, so just enough skill is needed to choose the appropriate flurorescent species with a suitable reactive group with the amide of the polymer matrix. Would you please take a look at the following documents. Please feel free to ask about any thing more specific, it would be of a great pleasure for me. Regards
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