Structure 2.4.6 — Condensation Polymers: Polyamides and Polyesters

Condensation polymers are formed when monomers with two functional groups react to form a long-chain polymer, with the elimination of a small molecule such as water (\( \text{H}_2\text{O} \)) or hydrogen chloride (\( \text{HCl} \)).

Monomer Requirements:

• Each monomer must have two reactive functional groups.
• Common functional groups: carboxylic acids (\( -\text{COOH} \)), alcohols (\( -\text{OH} \)), amines (\( -\text{NH}_2 \)).

Types of Condensation Polymers:

• Polyesters: Formed from diols and dicarboxylic acids (or esters).
• Polyamides: Formed from diamines and dicarboxylic acids (or amino acids).

1. Polyesters:

Formed by ester linkages (\( -\text{COO}- \)) between a dicarboxylic acid and a diol, with the loss of water.

General reaction:

\( n \text{HO-R-OH} + n \text{HOOC-R’-COOH} \rightarrow [-\text{O-R-OCO-R’-CO}-]_n + 2n\text{H}_2\text{O} \)

Example: Terylene (PET) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

2. Polyamides:

Formed by amide linkages (\( -\text{CONH}- \)) between a dicarboxylic acid and a diamine, with the loss of water.

General reaction:

\( n \text{H}_2\text{N-R-NH}_2 + n \text{HOOC-R’-COOH} \rightarrow [-\text{NH-R-NHCO-R’-CO}-]_n + 2n\text{H}_2\text{O} \)

Example: Nylon-6,6 from hexamethylenediamine and adipic acid.

Properties of Condensation Polymers:

• Stronger intermolecular forces (often hydrogen bonding).
• Higher melting points than addition polymers.
• Often biodegradable (especially polyesters).

Biological Macromolecules: Condensation and Hydrolysis

All biological macromolecules (carbohydrates, proteins, and nucleic acids) are formed by condensation reactions and broken down by hydrolysis reactions.

Condensation Reaction

A condensation reaction is a chemical process in which two molecules combine to form a larger molecule, releasing a small molecule such as water (\( \text{H}_2\text{O} \)) as a byproduct.

Hydrolysis Reaction:

Hydrolysis is the reverse of condensation. It involves breaking a covalent bond in a molecule using water, producing smaller subunits.

Biological Examples:

Carbohydrates:

Monosaccharides (e.g., glucose) combine to form disaccharides and polysaccharides (e.g., starch, cellulose, glycogen) via glycosidic bonds.

Proteins:

Amino acids join to form dipeptides and polypeptides via peptide bonds.

Nucleic Acids:

Nucleotides link via phosphodiester bonds to form DNA and RNA.

 Important: All of these biological polymers are hydrolyzed back into their monomers during digestion or cellular breakdown processes.