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Topic: URGENT: purification of azobenzene-modified polyacrylic acid  (Read 3273 times)

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Offline gggabimandl

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Hello! I am an undergraduate student working on my Honors project, and I am in way over my head. I am extremely grateful for any suggestions. I have already spoken to grad students in my department, and now I am just super desperate and coming to the internet for help. I am posting this here because there is no way any undergraduate students are going to be able to help me!

 I am currently working on a DCC coupling of 2-(4-phenylazophenoxy)ethanol with Polyacrylic acid (MW 130,000) and I have been having trouble for months (I've done this synthesis 20+ times). I finally decided to try adding DMAP as a catalyst to aid in the esterification process, which appears to work when I do a TLC (silica TLC with toluene/HOAc), but I can't isolate my product! Previously, without using DMAP, the TLC would show the azobenzene separating from the PAA. So now that they are not separating, I am confident that the coupling appeared to work.

Reaction mixture: DCC, DMAP, 2-(4-phenylazophenoxy)ethanol and PAA, dissolved in NMP.

The protocol i am using (An Azobenzene-based Light Responsive Hydrogel by J. Fraser Stoddart et al. if you wanna look it up) is totally useless, it seems. Their workup says to add NaOH to neutralize (my solution is at 7.5 already...) and filter the precipitate and wash with MeOH and NMP and then dissolve in water and precipitate in MeOH. I actually emailed them because this totally does not work, and they said just to see if I could wash the precipitate with methanol and leave it at that.

So far, i have tried the following to isolate my product:
- Adding NaOH until pH = 9, i get a sticky orange precipitate (which is good!) and then I vacuum filter the precipitate and wash with methanol. I end up with a hard, dark brown disgusting looking thingy.
- Adding NaOH and methanol and centrifuging, again I got a dark brown ugly looking thing, but in -very- low yield.
- Pipetting the mixture into hexanes (since PAA is insoluble in hexanes) and centrifuging. I again got a dark brown thingy, which I am currently vacuum drying, rather than rotovaping because someone told me to stop rotovaping it.

Any suggestions to properly purify this stuff? I feel like im just destroying my product, which I have precious little of.

Thank you!

Offline wildfyr

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Re: URGENT: purification of azobenzene-modified polyacrylic acid
« Reply #1 on: June 26, 2016, 12:21:53 PM »
What kind of access to chemicals do you have, and are you willing to do a a little synthesis followed by a free radical polymerization? My lab does extensive work with activated ester polymers, and the PERFECT reaction for this situation is to make poly(pentafluorophenyl acrylate) polymer and do your esterification with DMAP as is seen in this theato paper (http://pubs.acs.org/doi/abs/10.1021/acs.macromol.5b02293 ). PFPA can be made pretty simply from acryloyl chloride and pentafluorophenol. It would skip all this horrible bulls&$# DCC/DCU crap associated with Steglich esterification, which is a reaction I detest. You can then gently hydrolyze the leftover PFPA with base to end up with acrylic acid.

One of the best tricks in the book is to do a Steglich in acetonitrile. The dicyclohexylurea salt evolved has very poor solubility in it, so you can just filter it off after the reaction. I see this reaction is done in NMP, you should check if PAA is soluble in MeCN (I think it will be).

I looked at that procedure, I dont get why they precipitate in MeOH when they are only 10% functionalizing the acrylic acid, and acrylic acid is soluble in MeOH. Maybe try acetone or some other solvent for precipitation. I see you tried hexanes, which is probably a little TOO nonpolar, and all the other byproducts are going to stay in your polymer. And rotovapping is not good simply because its not going to totally dry your product, which hi-vac will.

If you just need ANY azobenzene, check if someone has Disperse Red 1, its a cheap azo dye with a alkyl alcohol on it.

How dry is your PAA? If its too wet, then its going to ruin your esterification. You can probably fix this with gentle heat and hi-vac or a vacuum oven.

And uh, my last question is: How do you know that dark disgusting solid isnt your product? In that paper their product looks rather like brown goo.

One final note just for future chemistry: Don't run TLCs in Toleuene. Toluene is UV active and can mess up your observation. Use EtOAc/hexanes instead. or DCM if you need something more polar.
« Last Edit: June 26, 2016, 12:35:27 PM by wildfyr »

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