from abstract ideas...fundamental knowledge at Esso Research
In cationic polymerization of isobutene at low temperatures, Drs. J . P. Kennedy and R. M . Thomas, scientists at Esso Research, have found an unusual relationship between the degree of polymerization and monomer concentration. Their studies have revealed the existence of an inversion temperature at which the degree of polymerization is independent of monomer concentration. Above this temperature, molecular weight increases with increasing monomer concentration ; below this temperature, molecular weight decreases as monomer concentration increases. Isobutene polymers like that illustrated above were prepared with alum i n u m chloride catalyst solutions in a
variety of chlorinated diluents. Experiments were run in the liquid phase at as low as — 185°C. without apparent decrease of reaction rate. Lines relating the logarithm of degree of polymerization to temperature, for various monomer concentrations, crossed at — 42 °, which is the inversion temperature. T h e results indicate that the molecular weight of the polymer is determined by several competing chainbreaking processes, having dissimilar activation energies. Molecular weight increases with decreasing temperature at substantially constant conversion. In line with this hypothesis, at lower temperatures some of the important chain-breaking processes are "frozen
out." Experiments with G 1 4 methyl chloride indicate that one of m a n y possible chain-breaking processes involves chain transfer to the diluent. These studies provide new insight into the mechanism of a reaction which takes place with great violence even at ultra-low temperatures so that conventional kinetic studies cannot be applied. T h e new knowledge is of value not only to cationic low temperature polymerizations but to ionic polymerizations in general. . . . adapted from the scientists' notes at
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