Thermal degradation of polyurethanes by hexamethyldisilazane: Syntheses of polyhexamethyleneurea with organosilicon reagents

Abstract

AbstractRepresentative aliphatic and aromatic polyurethanes undergo degradation upon treatment with hexamethyldisilazane (HMDS) at elevated temperatures. The course of the reaction is dependent on the nature of the polyurethane. Thus heating poly‐[ethylene methylene bis(4‐phenylcarbamate)] with HMDS in a sealed tube at 197°C gives high yields of 4,4′‐diaminodiphenylmethane, trimethylisocyanatosilane, and 1,2‐bis(trimethylsiloxyethane) along with lesser amounts of hexamethyldisiloxane, bis‐(trimethylsilyl)carbodiimide, and ammonia. Under the same conditions, poly(ethylene N,N′‐hexamethylenedicarboxylate) gives no diamine, but good yields of polyhexamethyleneurea and 1,2‐bis(trimethylsiloxy)ethane together with smaller quantities of the other named products are obtained. In the course of this study, two novel routes to polyalkyleneureas were developed. For example, polyhexamethyleneurea is obtained in good yield by treatment of 1,6‐hexanediamine with trimethyklisocyanatosilane at elevated temperatures in a sealed tube. The reaction of 1,1′‐hexamethylenediurea with HMDS under these conditions results in formation of the same product. A mechanism rationalizing the foregoing results is proposed which involves initial nucleophilic attack by HMDS on the polyurethane to give an intermediate disilylated urea. Thermal decomposition of this intermediate by alternative routes would give the observed products.