Effect of polar aprotic solvents on the trimerization of aliphatic isocyanates by organometallic catalysts

The polymerization of aliphatic isocyanates by organometallic catalysts such as tributyltin oxide, Ti(OBu)₄ or Zr(OBu)₄ was slow and no enhancement in rate was found on addition of DMF or DMSO, contrary to what was found with PhNCO. However, similar to the latter, fast reactions were observed with catalysts such as Pb- or Co-naphthenate in the presence of DMSO. The results were explained, on the basis of spectral evidence, as due to the weak interaction of DMSO or DMF with the aliphatic isocyanate and formation of a charge transfer complex between them in very low concentration.