Towards lignin-based polyurethanes: a computational study of the influence of the reactants structure on the formation of urethanes in the presence of tin(II) dicarboxylate as a catalyst

Abstract

An in-depth computational investigation of the tin-catalysed reaction of an alcohol with an isocyanate forming a urethane has been performed to elucidate the influence of the reactants, most importantly the type of alcohol or phenol, on its kinetics. The model alcohols were chosen based on those alcohol moieties present in lignin and comprised aliphatic, aromatic and benzylic alcohols, and reactions with both aliphatic and aromatic isocyanates were studied by modelling the rate-determining step of the urethane formation based on the mechanism proposed by Devendra et al. The results show that N-coordination is both kinetically and thermodynamically favoured for all reactions under consideration. Aliphatic alcohols react more readily than phenols with an aliphatic isocyanate, while benzylic alcohols react similarly to a bulky aliphatic alcohol. In the case of phenols, the kinetics are not influenced by the presence and/or number of ortho‑methoxy groups, but these will slightly alter the thermodynamics of the reaction. Aliphatic alcohols show slightly improved kinetics when reacting with an aromatic isocyanate, whereas phenols show no such preference. The reactions with the aliphatic isocyanate were also modelled in DMSO, but no significant changes in reaction kinetics were observed.