Formation and Depolymerization of Oligomers during Thermal Cracking of Silicon‐Containing Carbamates

Abstract

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.

Degradation of silicone‐based materials as a driving force for recyclability

“…Then they heated the mixture of products in the presence of urea and alcohols at 190 °C to get the recycled product of silicone‐containing carbamate. They were able to recover 96% by mass by this cycling procedure 127 …”

“…They were able to recover 96% by mass by this cycling procedure. 127 Polysiloxane depolymerization can also occur using acid chlorides, acetic acid or methanol in the presence of cheap iron salts at 190 °C. Enthaler explored the recyclability of siloxanes using these methods and found that, when he used acid chlorides with Fe catalyst, he got dichlorodimethylsilane as the primary product.…”

“…Depolymerization can be catalytically or non‐catalytically driven depolymerization from a strong electrophile or a nucleophile (25–110 °C), catalytic depolymerization (110–350 °C), thermal depolymerization (over 350–500 °C) and radical scission and calcination (over 500 °C) 125 . The catalysts or catalytic activity involved in depolymerizations of siloxanes are mainly Lewis or Brønsted acids and bases, 49 organo‐metallic catalysts, 126 alcohols 127,128 and radiation 129,130 . Overall depolymerization mechanisms in siloxane systems are based on the degradation process (temperature, oxidative or reductive atmospheres, sample mass and geometry) and the network architecture (structure and chemical makeup) 131,132 .…”

Green non&lt;italic&gt;-&lt;/italic&gt;phosgene process for manufacturing isocyanates

“…Possible dissociation mechnism of polyurea[28] (color online).图 8 (a) 光气法生产异氰酸酯过程; (b) 非光气制异氰酸酯三步过程(网络版彩图) (a) Phosgene process for isocyanate; (b) three-step non-phosgene process for isocyanate production (color online).…”