Chemistry

Sub-topics in Classification of Polymers

Methods of Polymerisation

Updated 22 Mar 2026

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.

Full explanation

Polymerisation is the cornerstone of polymer chemistry, a process that transforms simple, low-molecular-weight compounds (monomers) into complex, high-molecular-weight macromolecules (polymers). The choice of polymerisation method profoundly influences the polymer's structure, properties, and ultimately its utility. Broadly, polymerisation methods are classified into two main categories: Addition Polymerisation and Condensation Polymerisation, with a few other specialized types.

Conceptual Foundation

At its heart, polymerisation is about forming new covalent bonds. Monomers must possess specific functional groups or structural features that allow them to react and link repeatedly. For instance, unsaturated compounds with double or triple bonds are prime candidates for addition polymerisation, while compounds with two or more reactive functional groups (like -OH, -COOH, -NH2_2) are suitable for condensation polymerisation.

The driving force for polymerisation is often the reduction in free energy, leading to a more stable, larger molecular structure.

Key Principles and Laws

1. Addition Polymerisation (Chain-Growth Polymerisation)

Addition polymerisation involves the successive addition of monomer units to a growing polymer chain without the elimination of any small molecules. The empirical formula of the monomer and the repeating unit of the polymer are identical. This process typically occurs rapidly once initiated, leading to high molecular weight polymers. It's often referred to as chain-growth polymerisation because the polymer chain grows by adding one monomer at a time to an active site.

Mechanism: Addition polymerisation usually proceeds via three main types of active species:

  • Free Radical Polymerisation: — This is the most common method for polymerising unsaturated monomers (alkenes, dienes, vinyl compounds). It involves three distinct steps:

1. Initiation: A free radical initiator (e.g., peroxides, azo compounds) decomposes to form highly reactive free radicals. These radicals then attack the monomer's double bond, creating a new monomer radical.

R⋅+CH2=CHX→R−CH2−C˙HXR \cdot + CH_2=CHX \rightarrow R-CH_2-\dot{C}HX
2. Propagation: The newly formed monomer radical reacts with another monomer molecule, extending the polymer chain and regenerating a new radical at the chain end.

This step repeats thousands of times.

R−(CH2−CHX)n−C˙HX+CH2=CHX→R−(CH2−CHX)n+1−C˙HXR-(CH_2-CHX)_n-\dot{C}HX + CH_2=CHX \rightarrow R-(CH_2-CHX)_{n+1}-\dot{C}HX
3. Termination: The growing polymer chains combine or disproportionate, leading to the deactivation of the radicals and the cessation of chain growth.

This can happen by coupling of two radicals or by hydrogen abstraction.

R−(CH2−CHX)n−C˙HX+R′−(CH2−CHX)m−C˙HX→R−(CH2−CHX)n+m+1−R′ (Coupling)R-(CH_2-CHX)_n-\dot{C}HX + R'-(CH_2-CHX)_m-\dot{C}HX \rightarrow R-(CH_2-CHX)_{n+m+1}-R' \text{ (Coupling)}
2R−(CH2−CHX)n−C˙HX→R−(CH2−CHX)n−CH=CHX+R−(CH2−CHX)n−CH2−CH2X (Disproportionation)2 R-(CH_2-CHX)_n-\dot{C}HX \rightarrow R-(CH_2-CHX)_n-CH=CHX + R-(CH_2-CHX)_n-CH_2-CH_2X \text{ (Disproportionation)}
* Examples: Polyethylene (from ethene), Polypropylene (from propene), Polyvinyl chloride (PVC, from vinyl chloride), Polystyrene (from styrene), Polyacrylonitrile (PAN, from acrylonitrile), Teflon (from tetrafluoroethene).

  • Cationic Polymerisation: — This method is used for monomers with electron-donating groups (e.g., isobutylene, vinyl ethers) that can stabilize a carbocation. It requires a Lewis acid catalyst (e.g., BF3BF_3, AlCl3AlCl_3) and a co-initiator (e.g., water, alcohol).

1. Initiation: The Lewis acid-co-initiator complex generates a proton, which adds to the monomer's double bond, forming a carbocation.

H++CH2=C(CH3)2→CH3−C+(CH3)2H^+ + CH_2=C(CH_3)_2 \rightarrow CH_3-\overset{+}{C}(CH_3)_2
2.

Propagation: The carbocation attacks another monomer, extending the chain and regenerating a new carbocation at the chain end.

CH3−C+(CH3)2+CH2=C(CH3)2→CH3−C(CH3)2−CH2−C+(CH3)2CH_3-\overset{+}{C}(CH_3)_2 + CH_2=C(CH_3)_2 \rightarrow CH_3-C(CH_3)_2-CH_2-\overset{+}{C}(CH_3)_2
3.

Termination: Occurs via proton transfer to the counterion or to a monomer, or by combination with the counterion. * Examples: Polyisobutylene (butyl rubber).

  • Anionic Polymerisation: — This method is effective for monomers with electron-withdrawing groups (e.g., acrylonitrile, methyl methacrylate, styrene) that can stabilize a carbanion. It uses strong nucleophiles like organometallic compounds (e.g., n-butyllithium) or alkali metal amides as initiators.

1. Initiation: The initiator adds to the monomer's double bond, forming a carbanion.

R−+CH2=CHX→R−CH2−C−HXR^- + CH_2=CHX \rightarrow R-CH_2-\overset{-}{C}HX
2. Propagation: The carbanion attacks another monomer, extending the chain and regenerating a new carbanion.

R−(CH2−CHX)n−C−HX+CH2=CHX→R−(CH2−CHX)n+1−C−HXR-(CH_2-CHX)_n-\overset{-}{C}HX + CH_2=CHX \rightarrow R-(CH_2-CHX)_{n+1}-\overset{-}{C}HX
3. Termination: Often, anionic polymerisation can be 'living' (i.e., propagation continues until all monomer is consumed or a terminating agent is added), leading to very narrow molecular weight distributions.

Termination can be induced by adding protic solvents (e.g., water, alcohol). * Examples: Polystyrene, Poly(methyl methacrylate) (PMMA).

2. Condensation Polymerisation (Step-Growth Polymerisation)

Condensation polymerisation involves the reaction between monomers with two or more reactive functional groups, leading to the formation of a polymer and the simultaneous elimination of small molecules such as water, alcohol, or hydrogen chloride.

The repeating unit of the polymer does not have the same empirical formula as the monomer(s). This process is often called step-growth polymerisation because the polymer grows in a stepwise fashion, with any two reactive molecules (monomer, dimer, trimer, etc.

) being able to react with each other.

Mechanism: This typically involves a series of independent reactions between functional groups. For example, in polyester formation, a dicarboxylic acid reacts with a diol:

  • Step 1: — Esterification between a carboxyl group and a hydroxyl group, forming an ester linkage and releasing water.

HOOC−R−COOH+HO−R′−OH→HOOC−R−COO−R′−OH+H2OHOOC-R-COOH + HO-R'-OH \rightarrow HOOC-R-COO-R'-OH + H_2O

  • Step 2: — The resulting dimer (which still has reactive -COOH and -OH groups) can react with another monomer, or another dimer, or a trimer, and so on.

HOOC−R−COO−R′−OH+HOOC−R−COOH→HOOC−R−COO−R′−OOC−R−COOH+H2OHOOC-R-COO-R'-OH + HOOC-R-COOH \rightarrow HOOC-R-COO-R'-OOC-R-COOH + H_2O
This continues until high molecular weight polymers are formed.

  • Examples:

* Polyesters: From dicarboxylic acids and diols (e.g., Dacron/Terylene from terephthalic acid and ethylene glycol). * Polyamides: From diamines and dicarboxylic acids (e.g., Nylon 6,6 from hexamethylenediamine and adipic acid), or from amino acids/lactams (e.g., Nylon 6 from caprolactam). * Phenol-Formaldehyde Resins (Bakelite): From phenol and formaldehyde. * Urea-Formaldehyde Resins: From urea and formaldehyde.

3. Ring-Opening Polymerisation (ROP)

This is a special type of polymerisation where cyclic monomers (e.g., lactams, lactones, cyclic ethers) are opened up and linked together to form linear polymers. It can proceed via anionic, cationic, or coordination mechanisms. While it shares characteristics with addition polymerisation (no small molecule elimination), the mechanism involves ring strain relief.

  • Examples: — Nylon 6 (from caprolactam), Poly(lactic acid) (PLA, from lactide).

Real-World Applications

  • Polyethylene (Addition): — Packaging films, bottles, pipes, toys.
  • PVC (Addition): — Pipes, window frames, electrical insulation, flooring.
  • Teflon (Addition): — Non-stick coatings, chemical-resistant linings.
  • Nylon (Condensation): — Fibers for clothing, carpets, engineering plastics.
  • Polyester (Condensation): — Fabrics, bottles (PET), films.
  • Bakelite (Condensation): — Electrical switches, handles, early plastics.

Common Misconceptions

    1
  1. Confusing Addition and Condensation: — The most common mistake is not distinguishing between the two. Remember, addition polymerisation involves no loss of small molecules, and the polymer's repeating unit has the same empirical formula as the monomer. Condensation polymerisation always involves the elimination of a small molecule (like H2OH_2O, HClHCl, CH3OHCH_3OH), and the polymer's repeating unit has a different empirical formula than the monomer(s).
    2. 2
  2. Monomer Structure for Polymerisation Type: — Students often struggle to identify which type of monomer leads to which type of polymerisation. Monomers for addition polymerisation typically have double or triple bonds. Monomers for condensation polymerisation typically have at least two distinct functional groups (e.g., -OH, -COOH, -NH2_2) that can react with each other.
    3. 3
  3. Mechanism Details: — While NEET might not delve into intricate mechanistic steps for all types, understanding the basic initiation, propagation, and termination for free radical polymerisation is important. For condensation, knowing that it's a step-growth process involving functional group reactions is key.

NEET-Specific Angle

For NEET, the focus is primarily on:

  • Identifying the type of polymerisation — (addition vs. condensation) given the monomer(s) or polymer structure.
  • Identifying the monomer(s) — given a polymer structure.
  • Recognizing common examples — of polymers and their respective polymerisation methods (e.g., polyethylene is addition, nylon 6,6 is condensation).
  • Understanding the by-product — in condensation polymerisation (usually water).
  • Basic understanding of free radical mechanism — (initiation, propagation, termination).
  • Distinguishing between homopolymers and copolymers — (though copolymerisation is a variation, not a distinct method).

Mastering these distinctions and examples will equip you to tackle most NEET questions on methods of polymerisation.

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…

Often confused with

Side-by-side differences the NEET paper likes to test.

Methods of Polymerisation vs Condensation Polymerisation
AspectMethods of PolymerisationCondensation Polymerisation
Monomer StructureTypically 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 FormationNo small molecules are eliminated during the process.Small molecules (e.g., $H_2O$, $HCl$, $CH_3OH$) are eliminated as by-products.
Empirical FormulaThe 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 MechanismChain-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.
ExamplesPolyethylene, 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.

Why it is tested: For NEET, understanding the core differences between addition and condensation polymerisation is paramount. Questions frequently test the ability to identify the type of polymerisation based on monomer structure, polymer structure, or the presence/absence of a by-product. Knowing key examples for each category is also highly relevant, as is a basic grasp of the free radical mechanism for addition polymerisation.

Questions students ask

6 answered on this topic.

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, -NH2_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 (CH2=CH2CH_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.

Revise in 30 seconds

  • Polymerisation: — Monomers →\rightarrow Polymer
  • Addition Polymerisation (Chain-Growth):

- Monomers: Unsaturated (C=C, C≡\equivC) - 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, -NH2_2) - Small by-product eliminated (e.g., H2OH_2O, HClHCl). - 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)