What is the primary difference between addition and condensation polymerisation?

The primary difference lies in the fate of atoms during the polymerisation process. In addition polymerisation, monomer units add to each other without the loss of any atoms, meaning the empirical formula of the monomer is identical to that of the repeating unit in the polymer. For example, ethene polymerises to polyethylene. In contrast, condensation polymerisation involves the joining of monomers with the simultaneous elimination of small molecules, such as water, alcohol, or hydrogen chloride. This means the empirical formula of the repeating unit in the polymer is different from that of the monomer(s).

Can a single monomer undergo both addition and condensation polymerisation?

Generally, no. A monomer's structure dictates its polymerisation method. Monomers for addition polymerisation typically contain carbon-carbon double or triple bonds (e.g., alkenes, alkynes, vinyl compounds). Monomers for condensation polymerisation typically possess at least two reactive functional groups (e.g., -OH, -COOH, -NH$_2$) that can react with each other or with another type of monomer. There are exceptions, like cyclic monomers undergoing ring-opening polymerisation, which can be considered a type of addition polymerisation but involves specific cyclic structures.

What are 'initiators' in polymerisation, and why are they important?

Initiators are compounds that start the polymerisation process, particularly in chain-growth (addition) polymerisation. They generate reactive species (like free radicals, carbocations, or carbanions) that attack the monomer, forming an active center from which the polymer chain grows. Without an initiator, many monomers would not polymerise under practical conditions. Common initiators include peroxides and azo compounds for free radical polymerisation, Lewis acids for cationic polymerisation, and organometallic compounds for anionic polymerisation.

What is 'step-growth' polymerisation, and how does it relate to condensation polymerisation?

Step-growth polymerisation is another name for condensation polymerisation. It describes the mechanism where polymer chains grow in a stepwise fashion. Unlike chain-growth (addition) polymerisation where monomers add one by one to a growing chain, in step-growth, any two reactive molecules (monomer, dimer, trimer, etc.) can react with each other to form a larger molecule. This means that monomers are consumed early in the reaction, forming small oligomers, which then react further to build up high molecular weight polymers over time.

Give an example of a polymer formed by free radical addition polymerisation and its monomer.

A classic example is polyethylene, which is formed from the monomer ethene ($CH_2=CH_2$). In free radical addition polymerisation, an initiator (like benzoyl peroxide) generates free radicals. These radicals attack the ethene double bond, opening it up and forming a new radical. This new radical then reacts with another ethene molecule, and the process repeats, leading to the formation of long polyethylene chains. Polyethylene is widely used in packaging, films, and various plastic products due to its versatility.

What is the significance of the 'living polymerisation' concept in anionic polymerisation?

Living polymerisation is a special characteristic often observed in anionic polymerisation, where the active chain ends retain their reactivity even after all the monomer has been consumed. This means that if more monomer is added, the polymerisation can restart. This 'living' nature allows for precise control over polymer molecular weight, narrow molecular weight distribution, and the synthesis of block copolymers by sequentially adding different monomers. It's a powerful tool for creating highly tailored polymer architectures.

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Version 1Updated 22 Mar 2026

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Polymerisation is a fundamental chemical process in which small, repeating molecular units, known as monomers, chemically link together to form a large, long-chain molecule called a polymer. This transformation involves the formation of new covalent bonds between monomer units, leading to a macromolecular structure with significantly different physical and chemical properties compared to its const…

Quick Summary

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.

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Key Concepts

Addition Polymerisation (Chain-Growth)

This method is characterized by the direct addition of monomer units to a growing polymer chain, without the…

Condensation Polymerisation (Step-Growth)

In contrast to addition polymerisation, condensation polymerisation involves the reaction between monomers…

Free Radical Polymerisation Mechanism

This is a specific type of addition polymerisation that proceeds through a free radical intermediate. It's a…

Polymerisation: — Monomers

\rightarrow

Polymer

Addition Polymerisation (Chain-Growth):

- Monomers: Unsaturated (C=C, C $\equiv$ C) - No by-product eliminated. - Polymer empirical formula = Monomer empirical formula. - Mechanisms: Free Radical, Cationic, Anionic. - Examples: Polyethylene, PVC, Teflon, Polystyrene.

Free Radical Mechanism: — Initiation (radical formation)

\rightarrow

Propagation (chain growth)

\rightarrow

Termination (radical deactivation).

Condensation Polymerisation (Step-Growth):

- Monomers: Bifunctional/Polyfunctional (e.g., -OH, -COOH, -NH $_2$ ) - Small by-product eliminated (e.g., $H_2O$ , $HCl$ ). - Polymer empirical formula $\neq$ Monomer empirical formula. - Examples: Nylon 6,6, Dacron (Polyester), Bakelite.

Ring-Opening Polymerisation: — For cyclic monomers (e.g., Caprolactam

\rightarrow

Nylon 6). No by-product.

To remember the two main types of polymerisation and their key features:

All Always Add And No By-products (Addition Polymerisation: Always adds, No By-products)

Condensation Cuts Constantly, Creating Water (Condensation Polymerisation: Cuts small molecules, often Water)

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