Comparison of Small Molecule and Polymeric Urethanes, Thiourethanes, and Dithiourethanes: Hydrogen Bonding and Thermal, Physical, and Mechanical Properties

The hydrogen bonding behavior of a homologous series of small molecule and polymeric urethanes, thiourethanes, and dithiourethanes was investigated in solution, melt, and solid states. The relative hydrogen bonding strengths in both small molecule and polymer systems were evaluated, and the results were compared to theoretical calculations of hydrogen bonding strength. The results for NMR and FTIR analysis of the small molecule models indicated that the NH protons on the carbamate and thiocarbamates have greater hydrogen bonding strengths than the NH protons of the dithiocarbamate. The polyurethane and polythiourethanes were found to have approximately equivalent physical and mechanical properties as a result of a similar extent of hydrogen bonding, whereas the polydithiourethane, due to a lower degree of hydrogen bonding, has reduced thermal and mechanical transition temperatures as well as lower hardness values. The polythiourethane and polydithiourethane networks exhibit narrower glass transitions compared to polyurethane networks, apparently the result of an efficient isocyanate/isothiocyanate−thiol reaction with little or no side products. Because of weakness of the C−S bond compared to the C−O bond, thiourethanes and dithiourethanes have lower thermal stability than the corresponding urethanes. Finally, the polythiourethanes and polydithiourethane have higher refractive index values than their polyurethane counterpart.