AbstractAlthough the necessary conditions are known for formation of tough films from polyurethane intermediates, the chemistry is too complex to allow certain predictions of how variances in the polymerization process effect molecular structure and polymer properties. This research examined the hope that a correlated study of solubility and mechanical behaviors might show the effect of different catalysts and environments on crosslinking or other structural features which can modify film quality. Films (0.01 in. thick) were cast on mercury of intermediates derived from reaction in solvent of 2 moles of diisocyanate per mole of hydroxyl terminated polyesters, catalysts likeN‐methylmorpholine, methyldiethanolamine, and cobalt and/or lead ions being used to promote film‐forming reactions. The films, when dry, were aged at different levels of temperature and humidity. The solubility behavior was rated by measuring swelling ratios, rates of extraction, and intrinsic viscosity of extracts. Mechanical properties were rated by measuring tensile and stress relaxation behavior. The stress‐induced microstructure and fracture process were observed microscopically in a few typical specimens. The data support the theory that hydrogen bonding provides a strong crosslinking mechanism which dominates even the effects of moderate covalent crosslinking. Polymers derived from the same intermediates show very similar mechanical properties, even though significant differences in solubility properties suggest variances in other details of molecular stru