Introduction and types of polymerisation

Definition

Polymerisation is the chemical process by which a large number of small molecules called monomers chemically join together to form a high molecular weight substance called a polymer.

A monomer is a simple, low molecular mass molecule capable of linking with other similar or different molecules, while a polymer is the resulting giant molecule made up of many repeating structural units. Depending on the mechanism, polymerisation can occur by chain growth, step growth, or by the action of initiators, catalysts, heat, light, or pressure.

Main Content

1. Types of polymerisation based on mechanism: Addition polymerisation and condensation polymerisation

Addition polymerisation

occurs when monomers containing double or triple bonds combine without the elimination of any small molecule. The monomers add directly to form a polymer chain. This usually involves unsaturated monomers such as ethene, propene, styrene, and vinyl chloride. Examples include polyethylene, polypropylene, polystyrene, and PVC. In this process, the molecular formula of the monomer and polymer often differs only by the opening of the double bond, not by loss of atoms.

Condensation polymerisation

occurs when monomers with two or more reactive functional groups react step by step, usually with the elimination of small molecules such as water, methanol, hydrochloric acid, or ammonia. Examples include nylon-6,6, polyester (PET), Bakelite, and urea-formaldehyde resin. This type of polymerisation is common in the formation of fibres, resins, and engineering plastics.

2. Types of polymerisation based on growth pattern: Chain-growth polymerisation and step-growth polymerisation

Chain-growth polymerisation

takes place through a rapid chain reaction involving initiation, propagation, and termination. Once a reactive species such as a free radical, cation, or anion is formed, monomers are added one by one to the active chain end. The polymer chain grows quickly after initiation. Free-radical polymerisation of ethene to polyethylene is a common example.

Step-growth polymerisation

occurs when any two molecules present in the reaction mixture, whether monomers, dimers, or oligomers, can react with each other step by step. High molecular weight polymer is formed only after many stages of reaction. Condensation polymerisation usually follows step-growth mechanism, as seen in the preparation of nylon and polyesters. This process generally requires high purity of monomers and careful control of stoichiometry.

3. Types of polymerisation based on reaction conditions and mechanism of initiation

Free-radical polymerisation

is initiated by compounds that produce free radicals, such as benzoyl peroxide or azobisisobutyronitrile (AIBN). It is widely used because it is simple, versatile, and suitable for many vinyl monomers. It is commonly employed in the manufacture of polystyrene, poly(methyl methacrylate), and polyethylene.

Ionic polymerisation

includes cationic and anionic polymerisation. In cationic polymerisation, the active center is a positively charged species, while in anionic polymerisation, it is a negatively charged species. These methods are useful for monomers that are sensitive to radical conditions and are used in the preparation of special-purpose polymers, elastomers, and high-purity materials.

Coordination polymerisation

uses transition metal catalysts, especially Ziegler–Natta and metallocene catalysts, to control the arrangement of monomer units. This method produces polymers with specific stereochemistry and improved properties. It is important in the industrial production of isotactic polypropylene and high-density polyethylene.

Working / Process

1. Selection of monomer and polymerisation type

The process begins by choosing suitable monomers based on the desired polymer structure and properties. For example, ethene is used for polyethylene, while ethylene glycol and terephthalic acid are used for PET. The type of polymerisation is then selected according to whether the reaction involves addition, condensation, chain growth, or step growth.

2. Initiation and chain formation

In chain-growth processes, an initiator, catalyst, heat, or light generates an active species that starts the reaction. The monomer molecules then begin linking together. In step-growth processes, functional groups of monomers start reacting gradually, often releasing small molecules as by-products.

3. Growth, termination, and polymer recovery

The polymer chain continues to grow until the reaction is stopped by termination, chain transfer, or complete exhaustion of monomers. The polymer is then separated, purified, and processed into usable forms such as pellets, fibres, sheets, films, or moulded products.

Advantages / Applications

Polymerisation allows the production of materials with a wide range of properties such as strength, elasticity, chemical resistance, heat resistance, and light weight.
It is used in the manufacture of plastics, synthetic fibres, rubber, coatings, adhesives, biomedical materials, packaging materials, and electrical insulation.
Different polymerisation methods make it possible to design polymers for specific uses, such as durable engineering plastics, flexible elastomers, strong fibres, and biocompatible medical polymers.

Summary

Polymerisation is the process of forming polymers from monomers.
It may occur by addition, condensation, chain-growth, step-growth, free-radical, ionic, or coordination methods.
It is the basis of many important industrial and daily-use materials.
Important terms to remember: monomer, polymer, addition polymerisation, condensation polymerisation, chain-growth, step-growth, initiator, catalyst, free radical, polymer chain.