It is a general rule that for a polymer to crystallize, it must have highly regular polymer chains. Highly irregular polymers are almost inevitably amorphous. Polymer chains can have isomeric forms that decrease the regularity of the chains. There are three important forms of isomerism in polymers.
Double bonds in the polymer chain can show cis- or trans-isomerism. When a monomer with two conjugated double bonds, such as isoprene, undergoes chain polymerization one double bond can remain in the chain.
This segment of the polyisoprene chain can have four different isomers.
The difference between the properties of the cis- and trans-isomers is apparent for naturally-occurring polyisoprenes13. Gutta percha is predominantly trans-1,4-polyisoprene, which has a regular structure that allows crystallization. As a result, gutta percha is hard and rigid. Natural rubber is cis-1,4-polyisoprene, which has a less symmetrical structure that does not allow easy crystallization.
Gutta Percha (trans-1,4-polyisoprene)
Natural rubber is an amorphous rubbery material.
Natural Rubber (cis-1,4-polyisoprene)
Sequence Isomerism (Head-to-Tail or Head-to-Head)
In chain polymerization monomers with pendant groups can attach in two ways.
The usual arrangement is head-to-tail with the pendant groups on every other carbon atom in the chain
When a chiral center is present in a polymer molecule, different configurations or optical isomers are possible. Three of them are shown below for a monosubstituted vinyl polymer.
Stereochemistry can have an important effect on chain packing. Isotactic polypropylene (PP), for instance, is highly crystalline because the regular chains can pack closely together. Isotactic PP has a melting point of 160oC. Atactic PP, on the other hand is a soft noncrystalline polymer with a melting point of only 75oC.
David Whisnant (Wofford College). Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grants DUE #9950809 and DUE #9950296. Additional support was provided by the Camille and Henry Dreyfus Foundation.