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One of my duties at the company where I work is to perform intrinsic viscosity (IV) experiments on PAEK (poly(arly ether ketone)) polymers. These polymers don't dissolve well in most solvents, so the solvent for these experiments is concentrated sulfuric acid. I've found that if I leave the mixture to dissolve overnight and/or at too high of a temperature, the resulting IV is much lower (approximately half) than if I run the experiment as soon as all the material has dissolved at lower temperatures. The PAEK compounds I'm referencing are ones like PEK, PEEK, and PEKK (the results I've detailed above were seen primarily, maybe exclusively, with PEKK). The structures for these polymers is below.


PEK:
PEK

PEEK:
enter image description here

PEKK:
enter image description here

My thought on this was that with extra time and heat, the water in the sulfuric acid could attack the carbon alpha to the ether oxygen, hydrolyzing the polymer and creating shorter chain lengths. One of my colleagues disagrees, but to be honest, I haven't asked her to explain why.

Aromatic sulfonation can also occur when PAEK polymers are dissolved in concentrated sulfuric acid, as documented here, but I wouldn't expect that to have such a large impact on the viscosity (and I would also expect the effect of sulfonic acids to increase viscosity, not decrease it, though I'm less confident about that). My background is organic chemistry, not polymer chemistry/materials science, so I'm not super familiar with how different aspects affect things like viscosity. I do know shorter chains leads to a lower viscosity, though.

So here's my question: Is it likely that a significant amount of hydrolysis is occuring, or is it more likely some other factor causing this effect?

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    $\begingroup$ Well, what is PAEK? I know the other abbreviations, but not this one. $\endgroup$ – Karl Aug 16 '18 at 18:09
  • $\begingroup$ @Karl it's a generic name for the polymer family. It stands for poly(aryl ether ketone). $\endgroup$ – SendersReagent Aug 16 '18 at 18:14
  • $\begingroup$ Well, to hydrolyse something, you need, well, water... and there's hardly any in conc. H2SO4 $\endgroup$ – Mithoron Aug 16 '18 at 18:28
  • $\begingroup$ @Mithoron I realize it's low. We're using 95-98% sulfuric acid. Of course, most (approximately all) of that is bound up as hydronium, which is I realize is probably much less electrophilic, but with enough time and energy, I thought the reaction might happen. $\endgroup$ – SendersReagent Aug 16 '18 at 18:43
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    $\begingroup$ Not really, it dehydrates not adds water. You think about cleavage of chains, which might happen in other ways. $\endgroup$ – Mithoron Aug 16 '18 at 19:11
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I resist concluding this but nothing else I can think of makes sense. The only way the polymer solution to become less viscous is through a cleavage of bonds reducing the $\mathrm{M_w}$ of the polymer or dilution. Cleavage at the ether linkages would require reduction in an oxidizing environment or nucleophilic aromatic substitution which this polymer lacks the electron-withdrawing groups to effect, so those mechanisms should be eliminated (I concur with your colleague). Thus we can deduce that the cleavage must be occurring at the carbonyl groups.

To cleave the polymer at the carbonyl groups, the PAEK is likely undergoing an electrophilic aromatic substitution at the carbonyl groups. Though not commonly utilized, it is possible to substitute any electrophile on an aromatic ring in including acyl groups (especially with sulfuric acid which provides a high concentration of $\ce{H+}$ electrophiles) as shown in Figure 1—essentially Friedel Craft's Acylation in reverse.

enter image description here
Figure 1. Electrophilic Aromatic Substitution Removing an Acyl Group.

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  • $\begingroup$ How is cleavage at the ether bond a reduction? If H2SO4 protonates the ether oxygen and its conjugate base attacks the alpha-carbon, this SNAr reaction shouldn't result in an oxidation or reduction. Am I missing something in there? I like your reaction, though. Certainly a possibility. $\endgroup$ – SendersReagent Aug 17 '18 at 4:01
  • $\begingroup$ @SendersReagent, nucleophilic aromatic substitution is possible, but requires a strong electron-withdrawing group(s), Sandmeier Reaction, or breaking aromaticity see: WikiPedia Artice. $\endgroup$ – A.K. Aug 17 '18 at 4:12
  • $\begingroup$ The ketone para to the ether is a decent EWG, but I admit I wasn't/am not sure it is strong enough to cause the SNAr reaction I've described. I have seen it done primarily with nitro groups in the past. $\endgroup$ – SendersReagent Aug 17 '18 at 4:41
  • $\begingroup$ Actually, now that I think about it, PEEK is synthesized through an SNAr reaction (see this image). Of course, this is done under basic conditions, not acidic conditions. Protonation of the ketone will definitely affect its electron-withdrawing capabilities, but protonation of the ether oxygen will also make it a better leaving group for an SNAr reaction. Though the reactions are in very different conditions, I'm not convinced that an SNAr reaction is impossible for chain cleavage here. $\endgroup$ – SendersReagent Aug 17 '18 at 5:04
  • $\begingroup$ That's a very good point. The only problem is that sulfate and bisulfate are poor nucleophiles. $\endgroup$ – A.K. Aug 17 '18 at 5:28

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