[curiouscat]

During the synthesis of a high boiling diol (BP ~200 C) we seem to end up with a fair bit of dissolved residual polymerization by-products. Vacuum distillation (~50 torr) gives ~10% (w/w) of solid gummy residue at the end. Distillation as an analytical procedure is slow and cumbersome.

What other ways could we use to quantify the residue? We normally use a GC to quantify the liquid phase; but that is of no use here; the solid residues don't show up at all. Would an HPLC be a good option?

[Arkcon]

What don't these large polymers show up?  Are they never visible, regardless of GC conditions?  If they're very large polymers, they might show up on HPLC.  If they're huger, on the order of biopolymer sized, they may show up on a size-exclusion column, otherwise, a standard reverse phase column would be good for a large polymer.  I worry about what wavelength you'll use, do you see a possible UV absorbing ability in your molecule.   

[curiouscat]

What don't these large polymers show up?  Are they never visible, regardless of GC conditions?  If they're very large polymers, they might show up on HPLC.  If they're huger, on the order of biopolymer sized, they may show up on a size-exclusion column, otherwise, a standard reverse phase column would be good for a large polymer.  I worry about what wavelength you'll use, do you see a possible UV absorbing ability in your molecule.

Thanks Arcon. As far as I can guess they aren't really biopolymer sized.

"Regardless of GC conditions" is something I'll look into. I'll post the temp. program we are using. I was under the impression, maybe misconception, that anything boiling at higher than the GC temp. (or essentially non-volatiles) wouldn't show up.

One thing I can try though is to take a sample of the post-distillation residue and heat it in our furnace to see if it will decompose or vaporize at a certain high temp.

What determines if or not an HPLC will show a compound?

[Arkcon]

You need at least a few conjugated double bonds to absorb some UV wavelengths.  A more specific answer is beyond my knowledge.  If the wavelength is too low, you begin to have problems with the eluent UV cutoff.

[discodermolide]

Why not add a high boiling carbowax to the stuff you want to distill. This should stop any gummy residues forming. Use a molecular distillation set-up if you can.

[curiouscat]

Why not add a high boiling carbowax to the stuff you want to distill. This should stop any gummy residues forming. Use a molecular distillation set-up if you can.

Thanks! I never knew that trick. What does the Carbowax (PEG, right?) do that it kills the residue? Will it help if the residue had been formed already in the preceding reaction step?

[discodermolide]

It should stop the residue forming and acts as a heat sink to stop decomposition and generally diluted the stuff being distilled so that it remains stirrable. I used carbowax 600. If the residue has been formed it may dissolve it but I doubt that it will. Try it anyway.
Use a molecular distillation as you will get less decomposition that way.