Tuesday, July 15, 2014

An Unlikely Combination: Thiol-Ene Chemistry and Isocyanate-Free Polyurethanes

Long-time readers of this blog may recall that I spent a good deal of time with my previous employer working with thiol-ene chemistry. I really loved working with that chemistry - it was very fast and versatile, although my wife never enjoyed the experience of handling my cloths at the end of the day. Upon changing jobs a year and a half ago, I've set those malodorous chemicals aside and moved on to other monomers. These new monomers are apparently lightly scented, but I always question why someone would be sniffing them at all. You see, I'm now working with isocyanates to make polyurethanes and polyurea coatings and those aren't chemicals you shouldn't be smelling as you are probably being exposed to too high a level of them.

Isocyanates present a unique hazard to the polymer chemist - sensitization of the skins and lungs to the chemicals. Akin to an allergic reaction, once you are sensitized, you respond to increasingly smaller levels of exposure [*]. While skin sensitization is (merely) a rash, lung sensitization can induce horrible breathing problems and in extreme cases, death. Contrast that with the diamines used to make polyureas. They are corrosive and burn your skin, eyes and whatever else they contact, but that is pretty much the end of the story. You can march back into the lab the next day and carry on (albeit being more careful than before). But get sensitized to isocyanates and you are done working with them for the rest of your life. That's a risk for me and anyone working for me that always leaves me wondering whether it is justified. Fortunately, good PPE exists now that allows us to manage those risks.

Another way to manage the risk is to eliminate the use of isocyanates altogether, typically by reacting a cyclic carbonate with an diol or diamine such as is seen here:
Preparing a whole range of bifunctional cyclic carbonates can be a hassle, but a newly published patent application, discussed by the Innovations in Polyurethanes blog may change all that and best of all it does so by using my old love, thiol-ene chemistry. As is seen below, by starting with a vinyl-substituted cyclic carbonate, a dithiol (or trithiol or tetrathiol...) can react with that moiety to quickly and easily produce a difunctional cyclic carbonate, ready for polymerization.
This is only an application - the patent has not issued yet, but in a case of uncanny timing, the journal Polymer Chemistry today published online some new research showing how cyclic carbonates can be made from unsaturated fatty acids and how they can be dimerized by...thiol-ene chemistry:
Time will tell whether the application will issue and what the claims will be, but this is a world I never thought I would find myself in, combining my lovely thiol-ene reactions with isocyanate-free polyurethanes. Wait until I tell my wife!

[*] But the analogy stops there. Sensitization is not an allergic reaction. An allergic reaction produces a flood of histamines in your body, hence the application of antihistamines. Those medicines are useless for combating isocyanate sensitization.

Previous Years
July 15, 2013 - Closing the Door after the Horse has Gone Tilting at Windmills

July 15, 2011 - A New Polymer Blog

July 15, 2010 - Blow Molding on a Small Scale

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