The maximum extent of reaction in gelled systems

Just how far reactions can go after gelation during the cure of telechelic prepolymers has been a debatable point for some time. Utilizing a recently devised method,⁷ the curing reactions of some telechelic prepolymers were followed after the gel point. Extents of reaction above 90% occurred only in systems of average functionality close to two, functionality being the average number of reactive groups per molecule. Among systems of higher functionality, maximum extents of reaction of about 70% were most common. The final extent of reaction was only a few per cent above the extent of reaction at the gel point. The maximum extent of reaction varied with the concentration of reactive species and the relationship was a linear one at each functionality of the system. The data were consistent with P_A²/r = {0.88/(h ? 1)(j ? 1)]} + 0.10 where P_A is the fraction of prepolymer reactive groups initially present which have reacted, r is the ratio of the initial number of crosslinking groups to prepolymer reactive groups, and h and j are weighted average functionalities of the two reactants. It is suggested that the limiting factor in defining the final extent of reaction in these systems is the accessibility of reactive groups as determined by solid geometry rather than thermodynamics or reaction kinetics. The final extent of cure is regularly dependent on functionality and one cannot regard functionality and maximum extent of reaction as independent variables.