How can synthetic polyisoprene be made to resemble natural rubber?

To make synthetic polyisoprene resemble natural rubber, chemists must achieve a highly regular molecular structure that mimics the cis-1,4 configuration found in nature.12

Natural rubber consists almost exclusively of the cis-1,4 isomer, which provides its unique extensibility and "self-reinforcing" crystalline properties when stretched. Early synthetic isoprene rubber (IR) was produced using anionic and Ziegler-Natta catalysts, but these methods typically resulted in a product that was only about 98 percent cis-1,4 polyisoprene. Because this structure is less regular than that of natural rubber, the synthetic version does not crystallize as readily and lacks the same strength and tackiness in its raw state.13

The breakthrough in creating "synthetic natural rubber" occurred with the development of advanced organometallic catalysts by Karl Ziegler and Giulio Natta. These catalysts allow for precise control over the arrangement of units along the polymer chain, enabling the production of virtually 100 percent cis-polyisoprene, which is structurally identical to natural rubber.2