Methods of Polymerisation — Core Principles

Core Principles

Polymerisation is the process of chemically linking small molecules called monomers into large macromolecules called polymers. The two main methods are Addition Polymerisation and Condensation Polymerisation.

Addition polymerisation (also known as chain-growth) involves monomers adding to each other without the loss of any atoms, typically initiated by free radicals, cations, or anions, and is characteristic of unsaturated monomers like alkenes.

Examples include polyethylene and PVC. Condensation polymerisation (also known as step-growth) involves the reaction of monomers with two or more functional groups, leading to the formation of a polymer and the elimination of small molecules like water or alcohol.

Examples include nylon and polyesters. Understanding these methods is crucial for predicting polymer properties and identifying monomers from polymer structures, a common NEET question type. Ring-opening polymerisation is a special case for cyclic monomers.

Important Differences

vs Condensation Polymerisation

Aspect	This Topic	Condensation Polymerisation
Monomer Structure	Typically unsaturated compounds with double or triple bonds (e.g., alkenes, vinyl compounds).	Typically bifunctional or polyfunctional compounds with reactive functional groups (e.g., -OH, -COOH, -NH$_2$). Can be two different monomers.
By-product Formation	No small molecules are eliminated during the process.	Small molecules (e.g., $H_2O$, $HCl$, $CH_3OH$) are eliminated as by-products.
Empirical Formula	The empirical formula of the repeating unit is identical to that of the monomer.	The empirical formula of the repeating unit is different from that of the monomer(s) due to the loss of a small molecule.
Growth Mechanism	Chain-growth: Monomers add sequentially to an active site on a growing chain. Rapid increase in molecular weight once initiated.	Step-growth: Any two reactive species (monomer, dimer, trimer, etc.) can react. Gradual increase in molecular weight throughout the reaction.
Examples	Polyethylene, Polypropylene, PVC, Polystyrene, Teflon.	Nylon 6,6, Polyester (Dacron/Terylene), Bakelite, Urea-formaldehyde resins.

Addition and condensation polymerisation represent the two fundamental pathways for macromolecule synthesis, distinguished primarily by whether small molecules are eliminated during the process. Addition polymerisation, often called chain-growth, involves the direct linking of unsaturated monomers without any loss of atoms, resulting in a polymer repeating unit identical in empirical formula to the monomer. In contrast, condensation polymerisation, or step-growth, involves the reaction of bifunctional monomers with the expulsion of by-products like water, leading to a polymer repeating unit with a different empirical formula. This distinction is critical for understanding polymer structure and properties.