Until recently high-solids synthetic latices were made only by expensive and time-consuming concentration of dilute latex. It is now possible to produce stable, fluid latices of 55-60% total solids content directly in the reactor.

In the ten-year period preceding the outbreak of hostilities with Japan, imports of natural latex into the United States assumed a sharply rising trend. (Figure 1 shows that the 1940 figure is somewhat high owing to the inception of stockpiling.) Roughly 80 per cent of this latex was shipped in as a concentrate of 60–65 per cent solids, since this eliminated shipping excess water originally present in the latex, and constituted a material suitable for various applications in which the normal latex could not be used. Although small quantities of special-purpose synthetic latices had been produced domestically before 1941, it was not until 1944 that substantial production of a general-purpose synthetic latex got under way (Figure 1). With this production, there arose the problem of producing a concentrate for use in applications where a high-solids latex is essential. Natural rubber latex can be concentrated readily, either by centrifuge or a creaming process, since it has a large particle size. Synthetic rubber latex, on the contrary, has a very small particle size, which makes concentration a more difficult and costly operation. In practical creaming operations a significant amount of polymer is usually lost in the serum. This article presents a description of two new synthetic latices made to 55–60 per cent solids directly in the reactor and a short description of the methods of preparation. Since the production of these high-solids latices is only slightly more expensive than the production of low-solids types, they can be sold at only a small increase in cost over the latter.

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3-Methyl-1,3-pentadiene can be prepared pure without difficulty. However, unlike butadiene, isoprene, piperylene, and 2,3-dimethyl-1,3-butadiene, it apparently does not polymerize to an elastic, resilient solid product, and therefore the polymerization of 1,3-dienes to rubber-like products does not depend solely on the presence of the vinyl group.