Synthesis of Poly(Urethane‐Imide): Effect of Solvents with and without Basic Nitrogen Atom and Other Parameters on the Imide Formation Reaction Between Blocked‐Isocyanate Prepolymers and Pyromellitic Dianhydride

Abstract

N‐Methylaniline‐ and 2‐Butanone oxime‐blocked isocyanate prepolymers were prepared and their reaction with pyromellitic dianhydride (PMDA) was studied with a view to understand the effect of different parameters like solvents, temperature, blocking agents and deblocking catalysts [diazabicyclo 2.2.2 octane, (DABCO) and dibutyltin dilaurate, (DBTDL)] on the poly(urethane‐imide) formation reaction. The reactions were followed by the carbon dioxide evolved during the imidization reaction. The effect of each individual parameter was studied with the reaction time and GPC data. It was found that the reactions proceeded at a faster rate in hexamethyl phosphoric acid triamide (HMPA), a solvent having basic nitrogens, than in mesitylene. Also, in HMPA, it was found that the molecular weights (Mw) of the polymers increased with an increase in temperature up to 125°C and then decreased, whereas in mesitylene, it was found that there was no appreciable change in molecular weight up to 150°C. Of the two blocked isocyanate prepolymers studied, 2‐butanone oxime‐blocked prepolymer undergoes completion of reaction with anhydride rapidly. Among the two distinct types of deblocking catalysts tried, in the solvent HMPA, the tertiary amine showed no catalytic activity, while the organotin compound showed catalytic activity. In mesitylene, however, the effect of the catalysts on the title reaction was just the opposite. The polydispersity of the polymers was found to be relatively narrow in all cases.