The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the properties that they exhibit. Additionally, functional groups that may be present on a polymer further define their properties. The individual units of a polymer are held together by stable covalent bonds that occur through either addition or condensation polymerization.
Addition Polymerization
Additional polymerization occurs when the monomers that participate in the reaction have double bonds. This reaction requires the presence of an external molecule that has an unpaired valence electron to initiate the reaction. This unpaired electron is highly reactive and forms a bond with one of the double bonds in a monomer, which leads to the monomer having an unpaired valence electron. This unpaired valence electron then associates with another monomer with double bonds leading to a chain reaction where the molecule keeps adding other monomers with double bonds to it. This results in the formation of a chain of monomers, with the one added most recently having an unpaired valence electron. This addition reaction continues until two chains with unpaired electrons react with each other to form a bond. This eliminates any unpaired valence electrons, resulting in the termination of the reaction.
Condensation Polymerization
Condensation polymerization, also known as step-growth polymerization, requires a catalyst, such as an acid or an enzyme, and the monomers must have at least two reactive functional groups, such as amines or carboxylic acids. Functional groups on monomers react with each other to form linkages such as ester or amide linkages and are accompanied by the loss of a small molecule, such as water. Condensation polymerization is a slower reaction compared to addition polymerization and, in many cases, requires the presence of heat.
This text is adapted from Openstax, Chemistry 2e, Chapter 20.1: Hydrocarbons.
