Tools of the trade: making polymers
There are a wide variety of polymers available in the world and an even greater number of monomers, which make up these polymeric chains. Understanding how monomers react to form polymeric chains and which monomers should be used to modify a polymer’s properties is crucial to developing our understanding of polymer science.
Chain growth polymerisation
The most basic polymers consist of just a single monomer. For example, polyethene (or polyethylene) is formed from ethene. Polymers made by joining together alkenes (that contain C=C bonds), is 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.
One of the most common radical sources is benzoyl peroxide. The weak O–O bond of the peroxide bond breaks via homolytic cleavage (homolysis) when heated or exposed to ultraviolet radiation. Homolytic fission gives two alkoxyl radicals (RO•).
The alkoxyl radical adds to the C=C bond in the alkene monomer to form a new carbon-centred radical, in a propagation step. Addition occurs because the new O–C bond that is formed in the product is stronger than the C=C bond that is broken in the monomer. The carbon-centred radical then adds to another C=C bond in another monomer. Subsequent addition reactions can take place, so the length of the carbon chain increases.
It is possible for two carbon-centred radicals in different chains to react with one another, thus stopping either chain from growing any further, by reacting with further monomer units; this is a termination step called combination. Alternatively, one carbon-centred radical can “steal” a hydrogen atom from another through a reaction called abstraction. This produces both saturated and unsaturated products, in equal amounts, and this type of termination reaction is called disproportionation. In chain growth polymerisation a range of polymer lengths are formed due to the variable times at which termination occurs.
This type of reaction is useful for monomers with C=C bonds or similar functional groups. However, 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.
Step growth polymerisation
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.
As shown below, the esterification step involves a nucleophilic acyl substitution reaction, in which the OH group of the alcohol (or diol) acts as a nucleophile, attacking the carbonyl carbon of the carboxylic acid (or diacid).
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.
Monomers that have three or more functional groups will form infinitely large 3D 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.
To help visualise the different ways of incorporating two or more monomers into polymers, we have added a couple of handouts (below).
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