Cis and Trans isomers

Beyond the basics of being able to recognize cis isomers and trans isomers, it is interesting to note the physical implications of the isomers. The best and most basic example of naturally occuring cis and trans isomers are the polymers of isoprene. Isoprene polymers occur naturally in rubber plants and gutta-percha plants. However they differ in the ways the isoprene units are arranged. Notice that the two double bonds of isoprene are connected by a single bond which allows for rotation. Polymers of isoprene can be connceted in either linkages or trans linkages. The highest priority chains will be the ones that link with the rest of the polymer.
The cis isomer looks like thisand the trans isomer repeats like this

The two isomers have vastly differing properties because of their geometries. The cis isomer cannot be packed tightly with another cis isomer, whereas chains of the trans isomers fit nicely together. The extent of cross-linking between two polymer chains is greatly affected by how compact the polymer chains can become. The cis isomer has fewer cross-links and the separate polymer chains can slip more-making a the stretchy, rubbery polymer we know from the rubber tree. The trans isomer is much harder. It is sometimes used in golf ball coverings.

Somewhere inbetween the two naturally occuring isomer polymers we have the rubber that we come in contact with most often. Industry has created a system of introducing sulfur cross-links between the cis polymer. Very soft rubber has a smaller percentage of sulfur cross-linkages, while harder rubber will contain more.