Polymerisation

There are many methods of joining large numbers of monomers together to form long chains of repeating units. These broadly fall into two categories: addition polymerisation and condensation polymerisation.

Addition reactions to form polymers

Radical addition³ was originally found when a broken reaction vessle let in oxygen to a reaction under pressure involving ethene. The oxygen cleaved homolytically leaving very reactive lone electrons that proceeded in a chain reaction attacking the double bond in the alkene. The following diagram shows how low density poly(ethene) can be made.

The plastic made is low density because the radicals don't only attack the double bonds but will also attack other parts in the chain (called backbiting) leaving branched chains that don't pack tightly. The radical chain reaction will only stop when two lone electrons come together in the termination process.

Acid catalysed polymerisation⁴ occurs when a strong acid breaks the double bond in electrophilic addition leaving the carbocation on the monomer as the electophilic nucleophile which will attack the next monomer. Polymerisation stops when the carbocation picks up an anion.

Condensation reactions to form polymers

Nylon⁶

A condensation reaction occurs when two monomers come together and as they join emit a small molecule like H₂O or HCl. This is most commonly know as the type of polymerisation when making nylon which has an amide bond connecting the two monomers. This is made when the lone pair of electrons on the nitrogen in the amine attack the carbocation on the carboxylic acid or acyl chloride, in nucleophilic substitution resulting in the loss of the small molecule.

Polyesters⁷

Condensation reactions are also how polyesters are formed, where the lone pair of electrons on the oxygen in the alcohol attack the carbocation of the carboxylic acid To form an ester between the two monomers.

Stereoisomerism

Branching of chains causes low density plastics to be made, usually with low tensile strength because the chains can't lie close to each other, so there are weak intermolecular forces between them and so the chains can slide past each other making quite a flexible plastic. This can sometimes be a desired property but a method of controlling how branched the chains are is needed.

A research group lead by Zieglar and Natta found that by using a mixed metal compound it was possible to control where the polymerisation takes place leading to isotactic polymers (ones with side chains all in the same plain) rather than the attactic (randomly arranged side chains) that had previously been used⁸

Zieglar Natta catalysts work by making temporary bonds between the metal in the compound, quite often Titanium, and the σ bond of the monomers. When electrophilic addition occurs it first forms a complex with the catalyst and is then substituted for another monomer in exchange for the chlorine part of the catalyst which produces a product with correctly aligned side groups.

Laura White: 24/05/04