Copolymerization between mono- and di-isocyanates by organometallic catalysts

The copolymerization between aliphatic and aromatic mono- and di-isocyanate couples by organometallic catalysts, expecially tributyltin oxide (TBTO), was studied. Unlike the reactions between 4,4′-diphenyl methane diisocyanate (MDI) and BuNCO, hexamethylene diisocyanate (HDI) and BuNCO, and MDI and PhNCO, where low yields of copolymer were obtained, the reaction between HDI and PhNCO catalysed by TBTO gave high yields of copolymer. Detailed study showed that, in the early stages, there is considerable formation of homotrimer of PhNCO but this product disappears in the later stages of the reaction with formation of coopolymer with HDI. The extent of formation of (PhNCO)₃ depended on the reaction conditions; its extent decreased sharply on working with (HDI), instead of HDI, due to lowering of the dielectric constant of the system. Study of the rate of trimerization of PhNCO alone or in the presence of HDI or (HDI)₃ showed that the rate increased on passing from pure PhNCO to (HDI)₃ to HDI, the rate in (HDI)₃ being comparable to that of the aliphatic isocyanate groups thus permitting efficient copolymerization. Zr(OBu)₄ and Pb-naphthenate gave a faster reaction but a smaller degree of copolymerization between PhNCO and HDI than when TBTO was used.