The reactivity of urethanes based on 1,6-hexamethylene diisocyanate (HDI) and 4,4 '-methylene diphenyl diisocyanate (MDI) was investigated at temperatures between 190 degrees C and 235 degrees 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 H-1 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. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 621-629