Significance of Stereoregularity of Rubber

Student: Please explain the significance of "stereoregularity" of rubber molecules.

John Woon (Senior Latex Consultant): Stereoregularity indicates the ability of the long-chain molecules to crystallize. This is one reason why natural rubber inherently has high tensile strength compared to some synthetic elastomers since the the stereoregular cis-1,4-polyisoprene rubber molecules tend to undergo crystallization when the rubber is being stretched (strain-induced crystallization)

However stereoregularity has a drawback i.e. the rubber molecules also undergoes crystallization on storage albeit at a very much lower rate. This results in undesirable stiffening of the rubber when exposed to low temperatures. Of course careful design of vulcanization recipes to deliberately introducing cyclic sulphidic cross-linkings, could minimize this problem.