Abstract:
Polyurethane is made by reacting a polyisocyanate with a polyol. Organotin dicarboxylates are used as
catalysts for this reaction. However, a deeper understanding of the mechanism in catalysis is essential
in the polyurethane product manufacture. Although catalysis of aromatic and aliphatic isocyanates
using organotin dicarboxylate in urethane formation was claimed to have the same mechanism for
both, our work showed that they were different. In this work, we investigated urethane formation for
both aliphatic and aromatic isocyanates in the presence of different organotin carboxylate compounds
as catalysts using experimental and computational methods. In experimental methods, kinetic studies
and FTIR analysis were used. As the computational method, the DFT/B3LYP-D3 functional with the
LANL2DZ basis set for tin and the 6-31G* basis set for light elements was used. The results from
the experimental and theoretical studies showed that an alkoxide complex was formed initially by
the interaction between organotin di-carboxylate catalyst and alcohol. Then the interaction between
alkoxide complex and isocyanate molecule was modelled using a computational method. This
interaction resulted in the formation of organotin carbamate and carboxylic acid. Then the newly
formed organotin carbamate was computationally modelled to interact with alcohol or carboxylic
acid to form the urethane and to regenerate the active catalyst. By using thermochemical data from
the computational investigation, it was possible to show that aliphatic isocyanates can be more
sensitive to the carboxylic ligand content of the organotin carboxylate catalyst compared to aromatic
isocyanates in urethane formation. Based on these findings, different mechanisms for aromatic and
aliphatic isocyanates in urethane formation can be proposed when an organotin dicarboxylate is used
as a catalyst. The new knowledge from our studies was essential to minimize the viscosity increase
in storage of urethane pre-polymer when synthesised using organotin dicarboxylate as the catalyst.