Reactivity of urethanes at high temperature: Transurethanization and side reactions

ABSTRACT The reactivity of urethanes based on 1,6‐hexamethylene diisocyanate (HDI) and 4,4′‐methylene diphenyl diisocyanate (MDI) was investigated at temperatures between 190 °C and 235 °C. Diurethane model compounds end‐capped with either 1‐dodecanol (D‐core‐D) or 1‐hexadecanol (H‐core‐H) were mixed and annealed at high temperature. The core was either MDI or HDI. The transurethanization reaction was followed based on the formation of the compounds (H‐core‐D). The amount of H‐core‐D and of side products, which had formed after variable annealing times, were identified with 1H NMR, FTIR, SEC, and MALDI‐TOF. Transurethanization was considerably faster for MDI‐based urethanes than for HDI‐based urethanes. Only traces of side products were formed during annealing of MDI‐based urethanes, whereas a significant amount of allophanates was formed from HDI‐based urethanes under the same conditions. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 621–629 Both polyurethane vitrimers and the chemical recycling of polyurethane materials take advantage of urethane group exchange reactions at high temperature. Using small‐molecule model compounds, it is shown that urethane exchange reactions are much faster for aromatic urethanes than for their aliphatic counterparts between 190 °C and 220 °C. Aromatic urethanes generate traces of uretdiones and ureas as side products, whereas aliphatic urethanes yield allophanates and isocyanurates.