Will PAEK polymers hydrolyze in concentrated sulfuric acid?

Question

One of my duties at the company where I work is to perform intrinsic viscosity (IV) experiments on PAEK (poly(aryl 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:

PEEK:

PEKK:

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?

Answer 1

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 MW 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–Crafts acylation in reverse.

Figure 1. Electrophilic Aromatic Substitution Removing an Acyl Group.