Polymers are formed by two main ways called addition and condensation polymerization. In addition, polymerization, an initiator (or catalyst) reacts with a starting monomer. The result of this initiation reaction is a monomer attached to the initiator with an unsatisfied bond. The unsatisfied bond is free to react with another monomer, thus adding to the chain. The process repeats over and over again until two chains combine or another initiator binds to the end of the chain, both of which will terminate the chain. In condensation polymerization, a monomer with an exposed H (hydrogen) atom binds with a monomer with exposed OH (oxygen-hydrogen) atoms. During the reaction, water is released (compensated) as the H and OH combine to form H2O (water). The following 4-minute video discusses addition and condensation polymerization.
Unlike addition polymers where monomers react to form a single product, in a condensation polymerization reaction not only is the polymer formed but also a small molecule is eliminated or lost, normally water. Polyesters and polyamides are the two types of condensation polymer we will look at.
We will first look at polyamides. This is known as the amide link. It is formed when a carboxylic acid reacts with an amine. In the formation of nylon 6,6, we react a molecule with an amine group on each side known as hexane-1, 6-diamine and a molecule with a carboxylic acid at each end, hexanedioic acid. Since both these molecules have long chain carbons, they are only complicated the visual structure. Let us remove these and replace them with rectangular blocks. You can now see the functional groups at the ends of each molecule. A carboxylic acid and amine group positioned next to each other. These groups react to form water, which is eliminated. And a large polymer molecule is formed, held together by amide links. Hence the name Polyamide. Nylon has many uses in the textile industry but this is also structurally sound material used in engineering. Especially where low friction is required, such as in bearings or bushes.
We can now look at polyesters similarly. An ester is formed in the reaction of an alcohol with a carboxylic acid. In this example, we will show the formation of polyethylene terephthalate, more commonly known as PET. Using the carboxylic acid Benzene-1, 4-dicarboxylic acid and the alcohol ethane-1, 2-diol. Once again since those these molecules have a long carbon chain that may confuse the overall structure we will replace them with rectangular blocks. As you can see when we align the two molecules, a carboxylic acid and alcohol group can react between the molecules causing Ester links to hold the large polymer molecule together. And once again water is released. PET is commonly used as a plastic for drinks bottles and polyester is used to produce fabric for clothing.
Now at the end of this tutorial, you should understand what condensation polymers are and be able to give examples of polyesters and polyamides along with their uses.
Now that we have reviewed how polymers are formed, let’s discuss one of the possible ways to classify polymers, as thermoplastic or thermosetting.