Abstract
The important observation made by Katz in 1925, using x-ray diffraction technique, showed that the mere stretching of crude or soft vulcanized Hevea rubber caused at least a portion of the polymer to undergo crystallization. This and the subsequent experiments of Hock emphasized the point that successful attempts to synthesize a polymer resembling Hevea rubber would have to take regularity into account. The situation is complicated in isoprene polymers by the variety of ways the monomer can enter the growing polymer chain. Four isomeric forms of polyisoprene are shown in Figure 1. Since head-to-tail orientations must be considered because of the asymmetry of the isoprene molecule, there are eight possible arrangements of the units that can occur in polymerized isoprene. Sodium, potassium, emulsion, and alfin polyisoprenes have been shown to contain all four forms of the repeating units. All these uncured polymers have poor tack, and their pure-gum vulcanizates show poor physical properties. They do not crystallize when stretched or cooled. It has long been recognized that polymers of conjugated hydrocarbon dienes would have to be very regular in structure before their properties would approach those of Hevea rubber. Looking back over the last fifteen to twenty years, there has been a slow development, almost imperceptible at first, of catalyst systems which produce polymers that in some degree show an oriented structure. This orientation could result if the monomer, during the propagation phase of the polymerization, is held in a definite position with respect to the growing chain, as by cybotactic forces where polymerization is initiated at the melting point of the monomer, or by adsorption of the monomer onto a solid surface—perhaps to the surface of the catalyst itself. The work reported here deals primarily with the properties of a rubber made by directed polymerization.
Effect of (100) and (001) Hexagonal WO3 Faceting on Isoprene and Acetone Gas Selectivity
