High Pressure Organic Polymers
How I became involved in studies of intriguing organic polymers synthesized at room T and very high P [Solid State NMR but of course :) ] I love the analytical challenge and enjoy the very interesting chemistry! I look forward to more.
This is a new and interesting thing for me. High pressure technology is growing to the point that scientists are able to produce samples in the ~ mg range- which means I can do solid state NMR. Trust me if you can do SSNMR you want to do it! High pressure people come to me and I love collaborating with them- BTW doing these experiments is not trivial- I aim for perfection.
What I have developed is the ability to do NMR with really tiny samples and minimal signal background- I really think our NMR lab does this really well- as I hope to show you shortly :)
• So first I got involved in the polymerized benzene story with Tom Fitzgibbons and John Badding (PSU)- this is super cool and we are still working on it.
• then I started my collaboration with Dr. Haiyan Zheng (HPSTAR) - a brilliant chemist! first with polymerized acetonitrile - rxn at 25 GPa room T- this turned out to be extremely interesting and Hayan was a genius at getting all the data necessary to figure things out.
• I am also working with Tim Strobel (GL) and his team- they are studying some really interesting system as well.
These are challenging NMR experiments as the samples are really tiny- but we know our back ground and how to suppress it. This a really fun and fruitful collaboration.
I LOVE IT!
POLYBENZENE- a surprizing organic polymer formed at room temperature and very high pressure
Figure 1: Organic solids formed from room Temperature polymerization of benzene at 20 GPa. Tom Fitzgibbons (PSU) came to my lab with super small samples of white powder (~ 1 mg or less) that he told me were solids that he made in a Paris-Edinburough (PE) press- I recall thinking to my self- why compress benzene? What could it do- solidify? boring. I did not realize that it might polymerize and that does what it does. So Tom brings these really tiny samples to my lab and tells me that he worked with 13C enriched benzene- YES! now we are talking- so we are really great at the GL NMR facility at knowing our background and suppressing it with NMR background suppression methods- pulse games. As you can see we were able to do both CP and DP and see the same thing- ~20 % sp2 carbon and ~80 % sp3 carbon (in the form of methine: C-H). We published these data in Nature Materials (although I think only the VACP NMR data were included and only in the supplemental materials (?). So maybe this is the first the world has seen the very beautiful DP NMR data- enjoy!
Note that the surprisingly strong side bands of the prominent methine peak are a result of residual 13C-13C dipolar coupling- the benzene precursor was 13C enriched. Note that you will also see intensity at ~ 210 ppm and 175 ppm and 75 ppm - these minor peaks are likely the result of H2O addition to strained double bonds with subsequent oxydation and tautomerization to yield alchohol, keto groups, and carbonxylic groups- check out polyacetylene below.
Figure 2: The high pressure (20 GPa) polymerization of benzene yields white to pale yellow solids. 13C NMR of these (note using 13C enriched benzene) yields a solid with ~ 20 % sp2 carbon and ~ 80 % sp3 carbon- with a resonance at ~ 43 ppm- meaning most likely CH or methine carbon. One can envision a polymerization scheme like sketched (LEFT) where successive 2+4 cyclo additions yield a propagating strand of poly benzene. The theoretical limit for such a structure is 33 % sp2 and 66 % sp3 (methine) carbon. So the observed polybenzene is slightly more sp3 rich. Note that the fact that we could do both VACP MAS and DP NMR and that they are identical nails this. So does one "sow" this together tighter- there are some tantalizing points where more sigma bonds might be imagined at the expense of sp2 bonds- alas NMR can only take us this far- our imaginations will have to take us the rest of the way- I think!
There are a number of ways to change the sp2/sp3 ratio, all speculative at this point.
This is a really interesting system and more work is being done- I owe one more paper - working on it!
YES! I need some better molecular drawing software- but the white board and my cell phone camera was SOOOOO much faster.
I trust you get the picture!
Poly-Acetonitrile (CH3-CN)- another surprising polymer formed at room temperature and very high pressure
Figure 3: Working with Dr. Haiyan Zheng (HPSTAR- China) I was very happy to collaborate with quality 13C solid state NMR. (again we did both VACP and DP; only the VACP shown here). Haiyan is an incredible high pressure chemist and a true master of the Paris-Edinburgh PE press.
She cold compressed acetonitrile (CH3-CN) to 25 GPa and produced a yellowish polymer. We collected the 13C NMR spectrum of this polymer and these data were part of the story published in Angewandte Chemie (see publications).
Haiyan is a careful chemist and collected considerable information regarding this formation of the this polymer.
1) she did elemental analysis and showed that N content is reduced and stoichiometrically this can be ascribed to loss of NH3 (she tested her runs with pH paper that indicted highly basic gas/liquid after reaction supporting NH3 elimination.
2) the DP data (not shown here) reveals a little more sp2 and this is at ~ 120 ppm suggesting a bit of graphene (as suggested in the Angewandte paper- check this out).
Note that: We see no nitrile carbon (at 120 ppm) and no methyl carbon. We see amine and amidine. This is some complex chemistry- all at room T! Just need 25 GPa to get there- Haiyan knows how to do it!
Note: the obvious water addition products observed with polybenzene (above) and polyacetylene (below) - are not obvious here. Interesting- I have no explanations for this- do you? Maybe ammonia release supresses water addition(?).
Figure 4: Thoughts about the polymerization of acetonitrile. Whereas the polymerization of benzene is "simple" to understand as occurring through various 2 + 4 and 2 + 2 cycloadditions, the polymerization of polynitrile leading to the spectrum above (and in particular leading to the elimination of NH3) is not at all a simple to envision. What I mean is that whereas the polymerization of acetonitrile is easy to imagine- the actual polymer is not what you would expect!
On the left is a sketch of the what appears necessary to be the first step the formation of a methylated pol imine (sketch bottom). Above is the tautomer with secondary amine and olefin- both states are expected.
In the 13C NMR spectrum above we do see at peak at 165 ppm that is consistent with amidine functionality (corresponding to the structure below). But! We do not see any evidence of methyl groups that would be as abundant and would be expected at ~ 20-30 ppm. The prominent peak at 40 ppm has to be indicative of methine- at least I have never seen a CH3 group at such a high frequency and in any case this peak is way more intense than the amidine peak.
The tricky bit is how to you eleminate NH3 and still have a sp3 dominant polymer- actually you need to make a sp3 dominant polymer to enable NH3 elimination in the first place- but how do you do it? It is not so simple I think- This is a VERY Interesting chemical system!
Once again: White board drawing- cell phone camera- SOOO fast
Promise: will get some better software! Suggestions welcome!
Polyacetylene: Yet another really cool organic polymer made by Dr. Haiyan Zheng and her colleagues (e.g., Dr. Jiangman Sun) at room T and high Pressure-
Figure 5: Polyacetylene is an obvious target, but it takes real skill to do it- and Haiyan has the skill. Haiyan and her post Doc Jiangman Sun visited and we set out to acquire 13C solid state NMR. What we observed has recently been published (see pub.s) and is very interesting.
As seen here, the polyacetylene polymer (synthesized at 25 GPa and room temperature) is composed predominantly methine carbon (at 46 ppm) and slightly lessor amounts of aromatic/olefinic carbon (at 130 ppm).
There are also minor peaks corresponding to ketone (at ~ 200 ppm), carboxyl (~ 175 ppm) and alcohol(?) (at 75 ppm). Now, acetylenic carbon would resonate at ~ 75 ppm- but of course how would you have carbon-carbon triple bonds and be "bound" in this polymer- not possible.
The minor amount of apparently oxidized species are likely formed (my thinking!) after rxn when the polymer is exposed to water vapor in air. Given the very high P synthesis, I would not be surprised if many of the olefins are strained and more susceptible to room temperature H2O addition.
The formation of Keto groups are explained through tautomerizations, the formation of carbonxylic acids is not obvious- more than just water addition.