Condensation Polymerisation

Polyamides are formed through the reaction of a diamine and a dicarboxylic acid, resulting in the creation of an amide link ($-CONH-$). This specific linkage is identical to the peptide bond found in proteins, which are natural polyamides formed from amino acid monomers.

Synthetic polyamides like Nylon 6,6 are produced by reacting a six-carbon diamine with a six-carbon dicarboxylic acid. The resulting polymer is highly valued for its strength and elasticity, which arise from the extensive hydrogen bonding between the polar amide groups on adjacent chains.

Kevlar is an aromatic polyamide synthesized from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. Its exceptional heat resistance and tensile strength are due to the rigid planar structure of the benzene rings and the alignment of hydrogen bonds between the polymer strands.

Polyesters are synthesized by the condensation of a diol and a dicarboxylic acid, forming an ester link ($-COO-$) between the repeating units. This reaction involves the loss of a water molecule for every ester bond created in the chain.

A common industrial polyester is poly(ethylene terephthalate), or PET, which is used extensively in plastic bottles and textile fibers. It is formed from the reaction between ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Alternatively, polyesters can be formed from a single type of monomer known as a hydroxycarboxylic acid. These monomers contain both a hydroxyl group and a carboxylic acid group, allowing them to react with themselves to form a continuous chain.

While dicarboxylic acids are commonly used, dioyl dichlorides (acyl chlorides) are often preferred in laboratory settings because they are significantly more reactive. When an acyl chloride reacts with a diamine or diol, the byproduct is hydrogen chloride ($HCl$) gas rather than water.

The use of acyl chlorides allows the reaction to proceed rapidly at room temperature without the need for an acid catalyst. This is particularly useful for demonstrating polymerisation at an interface, such as the 'nylon rope trick' where the polymer forms at the boundary of two immiscible liquids.

In biological systems, enzymes facilitate the condensation of amino acids to form polypeptides. Each amino acid has an amine group and a carboxylic acid group, and the sequence of these monomers determines the specific structure and function of the resulting protein.

Identifying Monomers: To find the monomers from a polymer structure, locate the amide or ester link and 'break' it by adding $H$ back to the nitrogen/oxygen and $OH$ back to the carbonyl carbon. This effectively reverses the condensation process.

Drawing Repeating Units: When drawing the repeating unit from monomers, ensure you remove the correct atoms ($OH$ from the acid and $H$ from the amine/alcohol). Always include the trailing bonds at the ends of the brackets to indicate the chain continues.

Common Mistake: Students often forget that for every $n$ monomers, $2n-1$ small molecules are lost in a long chain, or simply $n$ molecules per repeating unit in a simplified model. Always check if the question asks for the byproduct.