Polymers are large molecules, or macromolecules, composed of many smaller, identical or similar units called monomers. The process of connecting these monomers into long chains is called polymerization. Chemical reactions used to form polymers are broadly categorized into two types based on how the monomers combine. This article focuses on condensation polymers, which are distinguished by a specific formation mechanism.
Defining Condensation Polymers
Condensation polymers are formed when monomers link together, creating a polymer chain and simultaneously expelling a small molecule byproduct. The monomers must possess at least two reactive functional groups, such as hydroxyl (OH), carboxyl (COOH), or amine (NH2) groups, allowing them to bond repeatedly. This multifunctional nature enables monomers to form long chains by reacting at both ends. Because a portion of the original material is lost during the reaction, the resulting macromolecule is not simply the sum of the monomers’ weights.
The Condensation Reaction Process
The defining characteristic of condensation polymerization is the elimination of a low molecular weight molecule, often water, but sometimes methanol, ammonia, or hydrogen chloride gas. This reaction is fundamentally a step-growth process, meaning the polymer chain builds up gradually. Functional groups on monomers or short chains (oligomers) react with each other. For example, a carboxylic acid group reacting with an alcohol group forms an ester linkage, releasing one molecule of water.
Chain growth requires the collision and successful bonding of functional groups, which become less concentrated as the polymer grows. To encourage the reaction to proceed and the polymer chain to lengthen, the small molecule byproduct, such as water, is typically removed from the reaction environment, often by applying heat or reduced pressure. This removal shifts the chemical equilibrium toward polymer formation, increasing its overall molecular weight. The repeated formation of new linkages continues until most functional groups have reacted, resulting in the final long-chain polymer.
Condensation vs Addition Polymerization
Condensation and addition are the two principal types of polymerization reactions. The primary chemical difference lies in the fate of the atoms that make up the monomer units. In addition polymerization, monomers typically containing double or triple bonds link together without the loss of any atoms, resulting in a polymer whose molecular formula is an exact multiple of the monomer’s formula.
In contrast, condensation polymerization results in the formation of a polymer molecule and a small byproduct molecule. Because a small molecule is lost with every bond formation, the polymer’s molecular weight is less than the combined weight of the starting monomers. Addition polymers are often chemically inert due to strong carbon-carbon bonds. Condensation polymers, however, contain heteroatom linkages (like C-O or C-N) and can be more susceptible to degradation.
Everyday Examples of Condensation Polymers
Many materials encountered daily are products of condensation polymerization, both synthetic and naturally occurring. Polyesters are common synthetic examples, formed when a dicarboxylic acid reacts with a diol. Polyethylene terephthalate (PET), a widely used polyester, is found in plastic bottles, food packaging, and clothing fibers.
Another significant class is polyamides, made from the reaction between a diamine and a dicarboxylic acid, resulting in an amide linkage. Nylon 6,6, a well-known synthetic polyamide, is used in textiles, carpets, and durable engineering plastics. Biological systems also rely heavily on condensation reactions. Proteins, for instance, are polyamides (polypeptides) formed when amino acids link together, releasing a water molecule in the formation of each peptide bond. Polysaccharides like cellulose and starch are natural condensation polymers made from glucose units.