In 1963 Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry for the development, in the 1950’s, of their eponymous catalysts for the production of stereoregular polymers from propylene. Their catalyst, an organoaluminum compound coupled with a transition metal, led to the development of synthetic rubbers with a structure closely resembling natural rubber. At about the same time, researchers elsewhere developed alkyllithium catalysts which resulted in similar “synthetic natural” rubber structures. With those developments, the structure of natural rubber, thought only to be the provenance of nature’s enzymatic control, could be copied by man. But this did not lead to the displacement of natural rubber in industry. More than 50 years after the development of synthetic high cis polyisoprene and high cis polybutadiene, natural rubber still occupies an irreplaceable position in the rubber industry. Why has synthetic rubber not replaced natural rubber? It is because of the unique structure of natural rubber and the properties this structure confers to industrial products. This structure/property relationship will be investigated in this paper.

Natural Rubber Structure
The characterization of polymer materials is more complex than the characterization of simple organic molecules. It has become the norm to separate macrostructure from microstructure when discussing polymer structures. Macrostructure includes the average molecular weight and molecular weight distribution of the individual polymer molecules. Microstructure refers to the way individual monomer units are distributed along the chain and the geometry in which they are distributed.

Macro Considerations
Natural rubber is a polymer, a long, chain like molecule that contains repeating subunits. The term polymer comes from the Greek “poly” meaning many and “mer” meaning parts. The chemical name for natural rubber is polyisoprene. The monomer (meaning “one-part”) from which it is built is isoprene. It is worth mentioning here that, although natural rubber is built of repeating isoprene units, isoprene is not the starting monomer for the natural product. Natural rubber is the result of a series of biochemical reactions which start with isopentenyl pyrophosphate within the tree. As mentioned earlier, in the mid 1950’s, two catalytic methods were developed which were then used to attempt production of “synthetic natural” rubber, i.e. stereospecific polyisoprene and polybutadiene. Both of these processes were developed in the U.S.A. The search for these processes may have been motivated by fears that further world conflict after World War II would again cut off supplies of natural rubber from the Far East and that the U.S. should not again be in a positon of dependence on foreign sources of natural rubber. The alkyllithium polymerization process yielded polyisoprene with a cis-1,4 content of approximately 94%. The Ziegler-Natta process yielded polyisoprene with a cis-1,4 content of approximately 96%.10 Although these two polymers, along with some high cis polybutadiene polymers with up to 99% cis content, exhibit strain induced crystallization, they still did not have the required rate and degree of crystallization to provide all of the properties of natural rubber in tire applications.