Effect of the ratio of reactive groups on gelation and cyclization during polyurethane network formation

Abstract

The effect of the initial molar ratio of the reactive groups, rNCO (= [NCO]0 [OH]0=1.0 or 0.8), on the gelation of polyurethane networks based on tris(4-isocyanatophenyl)thiophosphate and poly(oxypropylene) diol, was studied. The number-average molecular weight, M ̄n, in the pre-gel state, the critical conversion at the gel point, pcNCO, and the critical ratio, rcNCO, at which gelation occurs, were measured. The decrease in 1 rcNCO with increasing dilution in xylene suggests that cyclization takes place. The experimental M ̄n values are always lower than the theoretical ones, with the latter being calculated for the ring-free case. The fraction of bonds that are lost in the cyclization process, s, calculated from the difference between the experimental and theoretical values of M ̄n, increases with conversion and has a value near the gel point of 0.04-0.06. It is demonstrated both theoretically and experimentally that the values of s calculated from the shift of the gel point conversion are not equivalent to those obtained from M ̄n, but depend on rNCO. © 1993.