[wildfyr]

Hi everyone,
I got a real headscratcher here. I think it belongs here and not the organic forums. I have a hyperbranched polyester that is OH terminated. Its this basic structure

 

with all the red balls =-OH. I dissolved it in dry DMF, and at 0°C added 1.2 equivalents TEA vs the hydroxyl groups along with 1.1 eq freshly distilled acryloyl chloride. It was stirred overnight at room temp then worked up by precipitating into cold isopropanol. The solid was collected and IR and GPC were taken. I'm here because the IR is extremely unusual. The bottom green trace is the starting hyperbranched polyester, the bottom is the acrylated (?) product. Zoomed in on each half of the spectra for easier time reading the wavenumbers.







Here is ethyl acrylate for some comparison



I see some of the alkene at 1634. My issue is, what the heck is a peak doing at 2738 (aldehyde?!). And the peaks from 2495 to 2600 I am completely unable to explain. Aside from CO₂ (normally a doublet at 2350) this is usually no mans land for a spectrum. Azides, diazos, CN, and alkynes will show up between 2000-2500, but none of these moieties were present, nor do they typically go above 2250. This was done on a brand new IR using an ATR setup on a diamond. Backgrounded to air, and the spectrum was repeated a day later with similar results.

Any thoughts? I couldn't come up with this spectrum on purpose if you paid me. This has been sent for NMR.

[Irlanur]

Any thoughts? I couldn't come up with this spectrum on purpose if you paid me. This has been sent for NMR.

I can't really help you with the IR (or, don't have the time to find out if I can...), but I am eager to see the NMR. It is a solid you said, so did you send it for ssNMR? or are you just hoping to see something in the liquid NMR anyway?

[wildfyr]

It should be a pretty ordinary polymer so I'm just getting a normal liquid proton NMR to see how high the degree of a acrylation is

[Corribus]

Your peak labels are really unclear so it's hard to address too many of the specifics (i.e., the 2738 peak you mention), and in any case you may be over thinking it. Clearly the spectra supports an ester transformation - your carbonyl resonance is still there but almost all OH is gone.

The curious ones I agree are the ones located around 2500 wavenumber. Not a typical region to get peaks for standard organic molecules. I do note you used a lot of triethylamine, and amine salts can have strong combination bands in this region. Are you sure it's not a contamination due to a trialkylamine salt?

[wildfyr]

The reaction was chock full of TEA-HCl salt, it is certainly a possible contaminant. I checked on SDBS, and TEA-HCl does have strong peaks at 2599 and 2490 (even one at 2739! my likely mislabeled aldehyde hydrogen), certainly similar to this. I will explore this, it should be very visible on the NMR. It seems very likely. And it should be easy to remove with a few repetitive precipitations or a water rinse.

Thanks Corribus, you insight is, as always, invaluable.

[wildfyr]

As an update: yep it was TEA HCl.