Tools of the trade: making polymers

Chain growth polymerisation
The most basic polymers consist of just a single monomer. For example, poly(ethene) (or polyethylene) is formed from ethene. Polymers made by joining together alkenes (that contain C=C bonds) are typically achieved through chain polymerisation, involving radical intermediates.The first step is called the initiation step and involves the formation of a reactive radical species with an unpaired valence electron that adds to the C=C bond in the monomer.



Step growth polymerisation
Some polymers rely on condensation reactions, where two functional groups form a covalent bond in the product, through the loss of a small molecule, such as water. The resulting polymers are commonly called step growth polymers.Polyesters
Bi-functional monomers, those which contain two functional groups, can be reacted together indefinitely in order to give long polymer chains with high molecular masses. For example, monomers containing a carboxylic acid and alcohol group can be mixed and heated with an acid catalyst in order to form ester linkages. Water is lost in each esterification step and so a series of condensation reactions forms the polyester. Condensation reactions will continue to occur until all of the free monomer units have been used up, thereby resulting in a long chain. If additional monomer units are added then condensation reactions will begin again. The polymer polyethylene terephthalate, abbreviated PET, is used to make plastic bottles for soft drinks and water, and is the most widely recycled plastic.

Polyamides
Many common polymers use a co-polymerisation method where two different monomers, with two functional groups, react to form a continuous chain. As such, these chains are classed as alternating co-polymers. One example is Kevlar – a diacyl chloride reacts with a diamine, in a series of nucleophilic acyl substitutions, to form the polyamide (notice the presence of repeating -C(=O)NH– groups).
You may have seen the ‘nylon rope trick’ where a solution of a diacyl chloride in an organic solvent floats on top of an aqueous solution of the diamine. As nylon forms at the interface, it can be pulled as fast as it is produced forming a long thread. Nylon rope is twice as strong as rope made from natural fibres such as hemp and it weighs less.
This is an additional video, hosted on YouTube.
Polymetallocarbosilanes
Monomers that have three or more functional groups will form infinitely large 3-D networks of interlinking, polymers. This method is used to create viscous polymer gels as well as to strengthen polymer chains through cross-linking. For example, silicon-titanium tetra-functional group polymers, and similarly substituted structures, are called polymetallocarbosilanes. They can be used to create heat-resistant paints and protective coatings.
See if you can identify the 9 different polymers comprising the everyday products in the collage in the downloads section below. Post your answers – we look forward to seeing your results.
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