| Abstract: | Intramolecular reaction and gelation in reactions of a poly (propylene oxide) triol and hexamethylene di-isocyanate in benzene at 70° have been investigated. The numbers of ring structures formed in the pre-gelation region have been evaluated from cryoscopic measurements of Mn and overall extents of reaction of isocyanate groups. It is found that intramolecular reaction increases with initial dilution, and at a given dilution is a maximum for equimolar concentrations of reactive groups. Again, at a given initial dilution, more intramolecular reaction occurs than for linear polymerisations. Various ways of presenting the results are discussed, and one of them in particular, the extent of intermolecular reaction, pch, as a function of overall extent of reaction of the minority group, p, gives useful linear relationships. Also, by combining gelation and intramolecular reaction in terms of plots of pch vs. p, a distinction can be drawn between what may be termed the chemical and physical gel points. Finally, values of the Flory-Huggins interaction parameter, X, have been determined from the cryoscopic data. These show fairly normal behaviour - they are generally less than 0.5 — with some dependence on the density of hydroxyl groups and the proportion of polyoxypropylene units present. Such behaviour is in sharp contrast with that found previously for poly (ethylene-oxide)/hexamethylene di-isocyanate systems. |
