[rec_cris]

Hi,

I'm a non-chem grad student, but working on polyethylene glycol and l-lactide synthesis. I have trouble to characterize the copolymer.

I read from a paper using trifluoroacetic anhydride to react with the OH end-group for NMR. Can anyone provide more information of how TFAA is used in NMR normally? And how to predict the shift?

Also, by chance, if anyone knows PEG-LA or NMR well enough, hope you can tell me at what ppm can I confirm the reaction (i.e. I can calculate the yield).

Thanks.
Cris

[movies]

My guess is that you would use this to confirm the assignment of your hydroxyl peak in the NMR.  You could do this pretty easily: first take an NMR of your polymer (save, print, all that), then take the NMR tube out of the instrument and add a few drops of TFAA and shake the tube around a bit.  The hydroxyl will react pretty much instantly with the TFAA to make the trifluoroacetate ester.  Take an NMR again and you should see that the hydroxyl peak is gone.  You might see a peak for trifluoroacetic acid at about 11 ppm (way out of the range of anything else in your polymer) but otherwise the TFA will be invisible by 1H NMR.  The chemical shift of the protons adjacent to the oxygen of your hydroxyl group might shift a little, but not much.

I hope that helps.