Polyurethanes are commonly synthesized by leveraging the high reactivity of the isocyanate group towards hydroxyl compounds. However, the sensitivity of isocyanates to water makes industrial storage of these compounds difficult. Therefore, blocking agents are used to cap the functional group of isocyanate and stable condition against moisture is established. The synthesis of phenol-blocked polyisocyanate has already been carried out, where firstly two equivalents of toluene-2,4-diisocyanate (TDI) react with one equivalent of poly(tetrahydrofuran)diol to form the isocyanate terminated polyurethane prepolymer. Then, phenol-derived blocking agents react with the prepolymer to obtain phenol-blocked polyisocyanates. Amine based catalysts are used in blocking reactions, where the acidity of blocking agents directly correlates with the rate constants of amine-catalyzed blocking reactions. In this research, computational tools were used to explore the blocking reaction mechanisms of isocyanate derivatives taking solvents and the catalyst effects into account. The phenols with electron withdrawing substituents showed relatively lower activation barriers than the ones with electron donating substituents. Moreover, the activation barriers of phenols with substituents at <i>para-</i> positions had slightly lower barriers than the ones with <i>-ortho </i>derivatives. Two different mechanisms have previously been suggested for the tertiary amine catalyzed reactions: concerted and stepwise. The stepwise catalyzed mechanism was found to be preferred over the concerted mechanism. Furthermore, the computed rate constants of blocking reactions increased as the polarity of the solvent increased.

Role of blocking agents in the synthesis of polyurethanes: A computational study

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.