EFFECT OF THE RATIO OF REACTIVE GROUPS ON GELATION AND CYCLIZATION DURING POLYURETHANE NETWORK FORMATION

The effect of the initial molar ratio of the reactive groups, r(NCO) ( = [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)BAR, in t he pre-gel state, the critical conversion at the gel point, p(NCO)c, a nd the critical ratio, r(NCO)c, at which gelation occurs, were measure d. The decrease in 1/r(NCO)c with increasing dilution in xylene sugges ts that cyclization takes place. The experimental M(n)BAR values are a lways lower than the theoretical ones, with the latter being calculate d for the ring-free case. The fraction of bonds that are lost in the c yclization process, s, calculated from the difference between the expe rimental and theoretical values of M(n)BAR increases with conversion a nd has a value near the gel point of 0.04-006. 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 obt ained from M(n)BAR, but depend on r(NCO).