... can be a mixture of cis and trans isomers. (2) 191 Natural rubbers configuration across the double bond is 98 % cis (both CH2 groups are on the same side of the double bond) and this allows the chain to increase in strength when they are stretched (they form crystalline regions). These are regions where the long chains of monomers line up very close together in straight lines. An example of cis and trans isomers (2) The chain of poly(isoprene) in a crystalline regions (1) We know that rubber is very stretchy (high resilience). This has to do with entropy(1). We also know that any molecules want to get as high entropy as possible. The same rule applies for rubber. It has high entropy and when stretched the molecules line up in one direction so entropy decreases. rubber in normal state rubber stretched Natural rubber has a property called Thermoplasticity, which tends to make ...

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