Natural Rubber and Vulcanisation

Natural rubber is a natural, addition polymer of the monomer isoprene (2-methylbuta-1,3-diene, $\text{CH}_2{=}\text{C}(\text{CH}_3)\text{--CH=CH}_2$), obtained as a milky colloidal suspension called latex tapped from the bark of the rubber tree, Hevea brasiliensis. Biosynthesis links successive isoprene units through their 1- and 4-carbons, leaving one residual double bond per unit, and -- critically -- does so with essentially all of the double bonds in the cis configuration, giving the repeat unit $\big[-\text{CH}_2\text{--C}(\text{CH}_3){=}\text{CH--CH}_2-\big]_n$, known as cis-1,4-polyisoprene. The cis geometry keeps the chain kinked and coiled rather than extended, so natural rubber chains are naturally tangled together in a loose, disordered coil; this is what makes raw rubber elastic in principle -- stretching the material uncoils the tangled chains, and releasing the stress lets the disordered, coiled resting state re-form.

Limitations of raw rubber. In its raw, un-cross-linked form, however, natural rubber is soft and tacky (sticky), has low tensile strength, and becomes even softer and stickier when warmed and brittle when chilled, because the individual polymer chains are held to one another by nothing stronger than weak van der Waals forces and are free to slide past one another under stress -- so the material has poor elastic recovery and cannot hold a shape reliably under real mechanical or thermal use. …