Polymerization reactions in the thermal cracking of N-substituted
carbamates to produce the corresponding isocyanates
significantly reduce the yield of isocyanates and thus vastly
hinder the industrial application of such non-phosgene routes.
Herein, we tried to recycle the oligomers generated during the
thermal cracking of 3-ethylcarbamatopropyltriethoxysilane
(CPTS) to produce 3-isocyanatopropyltriethoxysilane (IPTS).
Firstly, the polymerized substrates of the pyrolysis reaction
were analyzed by NMR, IR, MALDI-TOF-MS and TG, indicating
the pyrolysis substrates were mixtures of CPTS (25 wt%),
polyureas (74 wt%), imines, and other compounds (<1 wt%).
The polyureas were generated by reaction of CPTS and IPTS.
Then, the depolymerization of these polyureas was realized via
alcoholysis in the presence of urea. It was found that the urea
not only provides a carbonyl group source, but also forms
hydrogen bonds with polyureas. In addition, NH3 co-produced
can also modify the reaction system microenvironment, which
might be favorable for the dissociation of polyureas. With
optimized conditions, more than 96% of polymerized substrates
could be reverted to CPTS for secondary cracking.