Forward and reverse reactions of N-methylaniline-blocked polyisocyanates: a clear step into double Arrhenius plots and equilibrium temperature of thermally reversible reactions

Abstract

Blocked polyisocyanates are important building blocks mainly used in a variety of polyurethane coatings. In this study, the synthesis, kinetics of blocking and deblocking reactions and deblocking temperatures of a series of N-methylaniline-blocked polyisocyanates were studied in detail using a hot-stage FT-IR spectrophotometer adopting neat conditions. The results were compared with an aim to resolve the complex questions on the relationship between the forward and reverse reaction parameters. As a result, double Arrhenius plots for these thermally reversible reactions were proposed which paved the way for the prediction of equilibrium temperatures, i.e., the temperature at which both the forward and reverse reactions coexist. From these plots, the equilibrium temperature and the equilibrium rate constants for the forward and reverse reactions were determined. It was found that the electronic and steric effects of the substituents were reflected uniformly in the deblocking temperature, equilibrium temperature and rate constants of the reverse reactions. Among the six N-methylanilines studied, the one having a chloro substituent at a para position was found to be equally reactive compared to the unsubstituted N-methylaniline in the forward reaction and at the same time it was easily cleaved off in the reverse reaction. On the other hand the N-methylaniline possessing a methyl ester substituent at a para position was found to be slow in the forward reaction and cleaved off with highest rate in the reverse reaction. The data such as time required for 50% conversion into product and the equilibrium temperature in combination with deblocking temperature reported in this work will increase the prospects of these blocked polyisocyanates in the polyurethane industry.