Polymerisation is the chemical process in which small monomer molecules bond together through covalent bonds to form large polymer chains. There are two primary types of polymerisation: addition polymerisation (chain-growth, no byproducts) and condensation polymerisation (step-growth, releases small molecules like water). Subtypes include free radical, cationic, anionic, ring-opening, and coordination polymerisation.
Table of Contents
What Is Polymerisation? (Definition & Basic Concept)
If you have ever held a plastic bottle, worn a nylon jacket, or stretched a rubber band, you have already experienced the end result of polymerisation; you just did not know it yet.
Polymerisation is the chemical process by which large numbers of small molecules, called monomers, join together through covalent bonds to form long, repeating chains or three-dimensional networks called polymers. The word itself comes from the Greek poly (many) and meros (parts); essentially, a polymer is a molecule made of many parts.
Think of monomers as individual LEGO bricks. On their own, each brick is small and not particularly useful. But when you connect thousands of them together in a specific pattern, you can build something much larger, more structured, and far more functional. That is exactly what polymerisation does at the molecular level.
What Are Monomers and Polymers?
A monomer is a small, reactive molecule that serves as the building block of a polymer. Common monomers include ethene (used to make polyethene), styrene (used to make polystyrene), and amino acids (used to build proteins naturally). Monomers must have at least one reactive site, typically a double bond or a specific functional group, that allows them to link with neighbouring molecules.
A polymer, on the other hand, is the resulting macromolecule formed from the repeated bonding of these monomers. Polymers can consist of thousands to millions of monomer units, which is why they tend to have unusually high molecular weights and distinctive physical properties, from flexibility and toughness to heat resistance and electrical insulation.
Why Does the Type of Polymerisation Matter?
Not all polymerisation reactions work the same way, and the type of reaction used determines almost everything about the final polymer, its molecular weight, chemical stability, mechanical strength, recyclability, and suitability for industrial applications.
Choosing the wrong type of polymerisation for a given monomer can result in incomplete reactions, unwanted byproducts, structural defects in the polymer chain, or material properties that simply do not match what is needed. Understanding the different types is therefore essential, whether you are a chemistry student preparing for exams, a materials scientist developing new products, or simply someone curious about the materials that shape modern life.
Why Polymerisation Is Important in Chemistry
Polymerisation sits at the heart of modern materials science and chemical engineering. Without it, there would be no plastics, no synthetic fibres, no rubber tyres, no medical implants, and no adhesives. In biological systems, polymerisation also produces DNA, proteins, and cellulose molecules that sustain all life on Earth.
From an industrial perspective, polymerisation underpins a multi-trillion-dollar global industry. The products it creates touch nearly every sector: packaging, automotive, aerospace, textiles, electronics, healthcare, and construction. Understanding how and why polymerisation works is not just academic; it drives real-world innovation and problem-solving.
In chemistry education, polymerisation also serves as a unifying concept that brings together organic chemistry, thermodynamics, kinetics, and structural chemistry into a single coherent framework.
“The era of polymer chemistry has only just begun. The substances which will be made from giant molecules will transform every domain of human life.”
— Hermann Staudinger, Nobel Prize in Chemistry, 1953
Classification of Polymerisation Overview Table
Before diving into each type in detail, here is a clear overview of how polymerisation is classified. There are two primary classification systems: one based on the reaction mechanism (how bonds form and grow) and one based on the functional group chemistry involved.
| Classification Basis | Type | Key Feature | Examples |
|---|---|---|---|
| Reaction mechanism | Addition (Chain-Growth) | Monomers add to active chain end; no byproduct | Polyethylene, PVC, Polystyrene |
| Reaction mechanism | Condensation (Step-Growth) | Any two molecules react; small byproduct released | Nylon, PET, Polycarbonate |
| Initiation method | Free Radical | Initiated by free radical species | Poly(methyl methacrylate), PVC |
| Initiation method | Cationic | Initiated by a carbocation | Polyisobutylene |
| Initiation method | Anionic | Initiated by a carbanion | Polystyrene (living polymers) |
| Initiation method | Coordination | Ziegler-Natta catalysts used | High-density polyethylene (HDPE) |
| Special category | Ring-Opening | Cyclic monomer ring opens to form chain | The cyclic monomer ring opens to form a chain |
This table shows that polymerisation types are not mutually exclusive. For example, free radical polymerisation is a subtype of addition polymerisation. The primary division is always between addition and condensation, and the subtypes describe how the addition process is initiated and controlled.
Addition Polymerisation Explained (With Example)
Addition polymerisation, also called chain-growth polymerisation, is the process by which monomers that contain carbon-carbon double bonds (C=C) repeatedly add to a growing polymer chain without losing any atoms in the process. The defining feature is that no byproducts are formed. Every atom present in the original monomers ends up in the final polymer.
How It Works
The process occurs in three distinct stages:
1. Initiation — A reactive species (called an initiator) generates an active site, typically a free radical, cation, or anion. This active site attacks the double bond of the first monomer, opening it up and creating a new bond between the initiator and the monomer while transferring the reactive site to the other end of the monomer.
2. Propagation — The newly activated monomer end attacks the double bond of the next monomer molecule. This process repeats thousands of times in rapid succession, with the chain growing by one monomer unit at a time and the reactive site migrating to the end of the chain.
3. Termination — The chain stops growing when the reactive site is neutralised. This can happen through combination (two growing chain ends join together), disproportionation (a hydrogen atom transfers between chains), or addition of a terminating agent.
Example: Polyethene from Ethene
The simplest and most widely produced addition polymer is polyethene, made from the monomer ethene (CH₂=CH₂).
During initiation, a free radical (R•) attacks one of the electrons in ethene’s double bond, forming: R–CH₂–CH₂•
During propagation, this radical attacks another ethene molecule: R–CH₂–CH₂–CH₂–CH₂•
This continues thousands of times, producing: –[CH₂–CH₂]ₙ–
The result is a long, saturated carbon chain with no byproducts. Depending on the conditions used (temperature, pressure, catalyst), you can produce low-density polyethene (LDPE), which is soft and flexible, or high-density polyethene (HDPE), which is rigid and strong.
Key Characteristics of Addition Polymerisation
- No atoms are lost, and the empirical formula of the polymer is identical to that of the monomer
- Chain growth is rapid, producing high molecular weight polymers quickly
- Requires monomers with C=C double bonds (or sometimes C≡C triple bonds)
- Produces homochain polymers (backbone contains only carbon atoms)
- Generally results in chemically inert polymers due to strong C–C bonds
- Products include polyethene, polypropylene, PVC, polystyrene, and PTFE (Teflon)
Condensation Polymerisation Explained (With Example)
Condensation polymerisation, also called step-growth polymerisation, is a fundamentally different process. Here, monomers with two or more functional groups react with each other in a stepwise fashion, and each bond formed between monomers releases a small molecule as a byproduct. That byproduct is most commonly water, but can also be methanol, hydrogen chloride, or ammonia, depending on the chemistry involved.
Unlike addition polymerisation, there is no distinct chain end that must be activated. In condensation polymerisation, any two molecules can react with any other at any point in the process; a monomer can react with another monomer, a dimer, a trimer, or a long chain. This means the molecular weight builds up slowly and gradually across the entire reaction mixture rather than rapidly at specific chain ends.
How It Works
Step 1 — Two bifunctional monomers come together, and their reactive functional groups react (e.g., a carboxylic acid –COOH group reacts with a hydroxyl –OH group).
Step 2 — A covalent bond forms between the two monomers, and a small molecule (water, in this case) is expelled.
Step 3 — The resulting dimer still has reactive functional groups at both ends. It can react with another monomer or with another dimer or oligomer.
Step 4 — This stepwise process continues until long polymer chains are formed. Because there is no real termination step, the chain ends remain reactive throughout.
Example: Nylon 6,6 from Diamine and Diacid
Nylon 6,6 is one of the most commercially important condensation polymers. It is made by reacting two bifunctional monomers:
- Hexamethylene diamine: H₂N–(CH₂)₆–NH₂ (an amine with two –NH₂ groups)
- Adipic acid: HOOC–(CH₂)₄–COOH (a dicarboxylic acid with two –COOH groups)
When these react, the –COOH group of one monomer reacts with the –NH₂ group of the other, forming an amide linkage (–CO–NH–) and releasing one molecule of water per bond formed. The reaction repeats along both ends of each molecule, ultimately producing long polyamide chains, which is what nylon is.
Example: PET Polyester from Diol and Diacid
PET (polyethene terephthalate) used in plastic bottles and polyester clothing is formed by reacting ethylene glycol (a diol with two –OH groups) with terephthalic acid (a dicarboxylic acid). Each ester linkage formed releases one molecule of water, and the repeating ester bond (–CO–O–) gives PET its name as a polyester.
Key Characteristics of Condensation Polymerisation
- A small molecule (water, HCl, methanol) is released with every bond formed
- Monomers must have two or more functional groups (bifunctional or polyfunctional)
- Molecular weight increases gradually throughout the reaction
- Any molecule can react with any other, not just at chain ends
- Produces heterochain polymers (backbone contains atoms other than carbon, such as oxygen or nitrogen)
- Condensation polymers are often more biodegradable than addition polymers due to weaker backbone bonds
- Products include nylon, PET, polycarbonate, polyurethane, and natural proteins
Mechanism-Based Polymerisation: Free Radical, Cationic, and Anionic
Within the broad category of addition polymerisation, the mechanism by which the chain is initiated and propagated leads to three important subtypes. Understanding these matters is important because each produces polymers with different molecular weights, tacticities (spatial arrangements of side groups), and end-use properties.
Free Radical Polymerisation
Free radical polymerisation is the most common and industrially important mechanism for chain-growth polymers. It is initiated by free radicals, highly reactive species with an unpaired electron in their outer shell.
Common initiators include organic peroxides and azo compounds, which decompose under heat or UV light to generate free radicals. The radical attacks the double bond of a monomer, opening it and transferring the radical character to the monomer end. This process propagates rapidly until two radicals meet and neutralise each other (termination).
Free radical polymerisation is relatively straightforward and tolerant of impurities, making it easy to scale industrially. However, it offers limited control over molecular weight distribution and polymer architecture.
Key products: PVC, poly(methyl methacrylate) (acrylic glass), polystyrene, polytetrafluoroethylene (PTFE/Teflon)
Typical initiators: Benzoyl peroxide, azobisisobutyronitrile (AIBN)
Cationic Polymerisation
In cationic polymerisation, the active chain end is a carbocation (a carbon atom with a positive charge). This is initiated by a strong Lewis or Brønsted acid such as boron trifluoride (BF₃) or sulfuric acid, which donates a proton or cation to the monomer, generating the reactive carbocation.
Cationic polymerisation works best with electron-rich monomers, such as vinyl ethers and isobutylene. The reaction must typically be carried out at low temperatures to reduce side reactions and increase chain length.
Key products: Polyisobutylene (used in inner tubes and sealants), poly(vinyl ethers)
Typical initiators: BF₃, AlCl₃, H₂SO₄
Anionic Polymerisation
Anionic polymerisation uses a carbanion (a negatively charged carbon species) as the reactive chain end. It is initiated by strong bases or organometallic compounds such as n-butyllithium.
One of the most important features of anionic polymerisation is that it can proceed as living polymerisation, that is, the chain continues to grow for as long as monomer is available, and there is no spontaneous termination step. This allows chemists to precisely control molecular weight and produce block copolymers with very narrow molecular weight distributions.
Key products: High-purity polystyrene, polybutadiene, block copolymers used in thermoplastic elastomers (e.g., Styrene-Butadiene-Styrene, SBS)
Typical initiators: n-Butyllithium, sodium naphthalene
Coordination Polymerisation (Ziegler-Natta)
Coordination polymerisation uses transition metal catalysts, most famously the Ziegler-Natta catalysts (developed by Karl Ziegler and Giulio Natta, who were awarded the Nobel Prize in Chemistry in 1963), to insert monomers into a growing metal-carbon bond in a highly controlled, stereospecific manner.
This method allows precise control over the stereochemistry of the polymer backbone, producing isotactic or syndiotactic polymers (where side groups are arranged in a regular pattern) rather than the randomly arranged atactic polymers that free radical polymerisation often produces. This control over spatial arrangement has a dramatic effect on crystallinity, melting point, and mechanical strength.
Key products: High-density polyethene (HDPE), isotactic polypropylene (used in food packaging, textiles, and automotive parts)
Comparison Table: Addition vs Condensation Polymerisation
This is the most important comparison table in polymer chemistry. Study it carefully; nearly every exam question on this topic can be answered with a solid understanding of these differences.
| Property | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Alternative name | Chain-growth polymerisation | Step-growth polymerisation |
| Monomer requirement | Must contain C=C double bond (or C≡C) | Must have two or more functional groups (–OH, –COOH, –NH₂, etc.) |
| Byproduct formed | None no atoms are lost | Small molecule released (water, HCl, methanol, etc.) |
| Chain growth | Rapid at reactive chain ends only | Slow and gradual; any two molecules can react |
| Stages of reaction | Three distinct stages: initiation, propagation, termination | No atoms is lost |
| Molecular weight build-up | Molecular weight jumps quickly; lots of unreacted monomer remains | Molecular weight increases slowly across entire mixture |
| Polymer backbone | Homochain contains only carbon (C–C bonds) | Heterochain contains C, O, N, S in the backbone |
| Chemical stability | High C–C backbone is inert to most chemicals | Polyethene, PVC, Polystyrene, PTFE, Polypropylene |
| Biodegradability | Generally non-biodegradable | Often more biodegradable |
| Recyclability | Difficult to recycle chemically | No atoms are lost |
| Examples of polymers | Polyethylene, PVC, Polystyrene, PTFE, Polypropylene | Nylon, PET, Polycarbonate, Polyurethane, Epoxy resins |
| Empirical formula of polymer | Same as monomer | Different from either monomer (small atoms lost) |
Comparison Table: Mechanism-Based Types of Addition Polymerisation
| Feature | Free Radical | Cationic | Anionic |
|---|---|---|---|
| Active chain species | Free radical (unpaired electron) | Carbocation (positive charge) | Carbanion (negative charge) |
| Initiator type | Organic peroxides, azo compounds | Lewis/Brønsted acids | Strong bases, organolithium compounds |
| Monomer type | Broad, vinyl monomers, acrylates | Electron-rich monomers (vinyl ethers, isobutylene) | Electron-deficient monomers (styrene, acrylonitrile) |
| Temperature | Room temperature to moderate heat | Often requires low temperatures | Variable, often low temperature |
| Living polymerisation? | No, spontaneous termination occurs | Generally no | Yes, enables precise molecular weight control |
| Molecular weight control | Limited | Moderate | Excellent |
| Tacticity control | Poor (atactic polymers typical) | Moderate | Good (can produce isotactic/syndiotactic) |
| Key industrial products | PVC, Polystyrene, PMMA | Polyisobutylene | Block copolymers, narrow-distribution PS |
Stages of Polymerisation, Brief Overview
Whether you are looking at free radical, cationic, or anionic addition polymerisation, the process always passes through three recognisable stages:
Initiation is the birth of the reactive species. An initiator decomposes or reacts to generate the active entity, a radical, cation, or anion, which then reacts with the first monomer unit to start the chain. This step is typically slow and rate-determining.
Propagation is the growth phase, where the activated monomer end repeatedly attacks new monomer molecules, extending the chain by one unit with each reaction. Propagation is rapid and accounts for the vast majority of monomer consumption. The reactive end migrates to the new chain terminus with every addition.
Termination ends the growth. In free radical systems, two radical chain ends may combine (coupling) or exchange a hydrogen atom (disproportionation), both of which destroy the radicals. In cationic systems, the chain is often quenched by nucleophiles in the reaction medium. In anionic living systems, true termination requires deliberate addition of a terminating agent.
For condensation polymerisation, these three stages do not apply in the same way. There is no distinct termination; the reaction can theoretically continue until all functional groups are consumed or until the system reaches equilibrium.
Industrial Applications of Polymerisation
The scale at which polymerisation operates industrially is extraordinary. Global polymer production exceeds 400 million tonnes per year, and virtually every manufactured product we encounter contains at least one polymer.
Packaging is dominated by addition polymers. LDPE and HDPE make plastic films and bottles; polypropylene forms containers and caps; polystyrene is used for foam packaging; PET (a condensation polymer) is used for drinks bottles and food trays.
Textiles rely heavily on condensation polymers. Nylon fibres are used in sportswear, hosiery, and ropes. Polyester (PET) is the world’s most-produced synthetic fibre, found in clothing, carpeting, and geotextiles. Polyurethanes are used in elastic fibres like Spandex/Lycra.
Automotive and construction industries use both types extensively. Polypropylene is found in bumpers, dashboards, and interior panels. Epoxy resins (condensation polymers) are used as structural adhesives and composite matrix materials in high-performance components.
Healthcare and medical devices depend on carefully engineered polymers. Polycarbonates (condensation) are used in medical equipment and eyeglass lenses. Poly(methyl methacrylate), an addition polymer, is used in bone cements and contact lenses. Biodegradable polyesters such as polylactic acid (PLA) are used in resorbable sutures and drug delivery systems.
Electronics rely on polymers for insulation, encapsulation, and flexible substrates. PTFE (addition) insulates high-frequency cables; epoxy resins encapsulate circuit components; conducting polymers are an emerging class with applications in displays and batteries.
Common Examples: PVC, Nylon, and Polyethene
Understanding a few key examples in depth is the most effective way to consolidate your knowledge of polymerisation types.
Polyethene: The World’s Most Produced Plastic
Polyethene is the product of addition polymerisation of ethene monomers. Depending on the conditions and catalyst used, the reaction produces different grades. Low-density polyethene (LDPE) is made using free radical polymerisation at high pressure, producing a branched chain structure that gives it low density and flexibility ideal for cling film and plastic bags. High-density polyethene (HDPE) is produced using Ziegler-Natta coordination catalysts, yielding straighter, more crystalline chains with higher stiffness used in milk bottles, pipes, and outdoor furniture.
PVC: Poly(vinyl chloride)
PVC is produced by free radical addition polymerisation of vinyl chloride (CH₂=CHCl). It is one of the most versatile polymers known, able to be formulated in both rigid and flexible forms through the addition of plasticisers. Rigid PVC is used in window frames, pipes, and credit cards. Flexible PVC is found in electrical cable insulation, flooring, and raincoats. Globally, PVC is the third most widely produced synthetic polymer by volume.
Nylon: A Condensation Polymer
Nylon is a family of polyamides produced by condensation polymerisation. Nylon 6,6 (as described earlier) is made from hexamethylene diamine and adipic acid, with water released at each linkage. Nylon 6 is produced slightly differently by ring-opening polymerisation of caprolactam, though it resembles a condensation polymer in structure. Nylon’s combination of high tensile strength, low friction, and resistance to abrasion makes it ideal for gears, bearings, toothbrush bristles, and fabrics.
Industrial Polymerisation Techniques — A Quick Map
The type of polymerisation (addition or condensation) tells you the chemistry. The industrial technique tells you how that chemistry is physically carried out at scale. These are not the same thing, and conflating them is one of the most common sources of confusion in polymer science.
Bulk Polymerisation
Bulk polymerisation is the simplest industrial technique — the monomer itself acts as the reaction medium, with initiator added directly. No solvent is required, which means the final polymer is pure and no solvent recovery costs are incurred. The challenge is heat management: polymerisation reactions are exothermic, and in a large volume of undiluted monomer, removing heat quickly enough to prevent runaway reactions and localised overheating is technically demanding. Bulk polymerisation is used to produce clear polystyrene (crystal PS), PET, and poly(methyl methacrylate) for optical applications where purity is paramount.
Solution Polymerisation
In solution polymerisation, both the monomer and the initiator are dissolved in a solvent. The solvent acts as a heat sink, moderating the exothermic reaction and allowing much better temperature control than bulk methods. The trade-off is that the final polymer must be separated from the solvent — either by precipitation, evaporation, or drying — which adds cost and environmental burden. Solution polymerisation is widely used for producing polyacrylonitrile (PAN) for carbon fibre precursors and for manufacturing certain grades of neoprene rubber.
Suspension Polymerisation
In suspension polymerisation, the monomer is dispersed as fine droplets in water using mechanical agitation and stabilising agents. Each droplet acts like a tiny bulk polymerisation reactor. The water phase carries away heat efficiently, giving much better thermal control than true bulk methods. The polymer is produced as small beads or pearls that are easy to filter, wash, and dry. This technique is used for PVC, polystyrene beads, and ion-exchange resins.
Emulsion Polymerisation
Emulsion polymerisation is superficially similar to suspension but mechanistically very different. Here, the monomer is emulsified in water using surfactants, and polymerisation occurs inside micelles — nanoscale surfactant assemblies — rather than in large monomer droplets. This allows very high molecular weight polymers to be produced at relatively high rates, because the polymerisation sites (micelles) are separated from each other, reducing termination frequency. Emulsion polymerisation produces latex products — the polymer remains dispersed in water as fine particles. This technique is used to make synthetic rubber (SBR), water-based paints, and adhesives.
How to Choose the Right Type of Polymerisation
This section answers a question that textbooks rarely address directly: given a monomer and a desired product, how do you decide which polymerisation type and method to use?
Step 1 — Look at Your Monomer’s Functional Groups
This is the first and most decisive factor. If your monomer contains a carbon-carbon double bond (C=C) and no other reactive functional groups, addition polymerisation is your route — the double bond is the reactive site. If your monomer contains two functional groups (such as –OH, –COOH, –NH₂, or –NCO), condensation polymerisation is indicated, because the reaction proceeds by pairing up complementary groups between monomers. If your monomer is a cyclic compound (such as caprolactam or ethylene oxide), ring-opening polymerisation is a strong candidate.
Step 2 — Decide What Byproduct Behaviour You Need
If your application cannot tolerate water or other small-molecule byproducts being trapped in the final material — for example, in optically clear films or precision moulded parts — addition polymerisation is strongly preferred. If you can manage byproduct removal (through drying, vacuum, or reactive distillation), condensation polymerisation opens up a richer range of backbone chemistries.
Step 3 — Consider Your Molecular Weight Target
For very high molecular weight polymers produced rapidly (in packaging films, fibres, and structural plastics), addition polymerisation — especially free radical or coordination — is best. For moderate molecular weights with precise control over end groups and chain architecture (in speciality polymers, biomedical materials, and thermosets), condensation or living anionic polymerisation offers greater precision.
Quick Decision Table — Monomer Type → Polymerisation Method
| Monomer Type | Reactive Feature | Recommended Polymerisation | Typical Product |
|---|---|---|---|
| Alkene (e.g., ethene, propene) | C=C double bond | Free radical or coordination (addition) | Polyethylene, Polypropylene |
| Vinyl compound (e.g., vinyl chloride) | C=C with substituent | Free radical addition | PVC |
| Electron-rich alkene (e.g., isobutylene) | Electron-donating group | Cationic addition | Polyisobutylene |
| Styrene or acrylonitrile | Electron-withdrawing group | Anionic addition | Polystyrene (narrow MW) |
| Diol + Diacid | –OH and –COOH groups | Condensation (polyester) | PET, Dacron |
| Diamine + Diacid | –NH₂ and –COOH groups | Condensation (polyamide) | Nylon 6,6 |
| Cyclic lactam | Ring strain | Ring-opening | Nylon 6 |
| Diisocyanate + Diol | –NCO and –OH groups | Condensation (step-growth) | Polyurethane |
Polymer Architecture — How Chain Structure Affects Properties
Two polymers can be made from identical monomers using the same polymerisation type, yet behave completely differently as materials. The reason is often chain architecture — the three-dimensional arrangement of polymer chains.
Linear Polymers — Properties and Examples
Linear polymers consist of a single, unbranched main chain. All the monomer units are connected end-to-end in an uninterrupted sequence. Linear polymers tend to pack together efficiently, resulting in high crystallinity, elevated melting points, and good tensile strength. They are generally thermoplastic — they soften on heating and can be reshaped, which makes them recyclable.
- Examples: High-density polyethylene (HDPE), PET, Nylon 6,6
- Properties: High density, stiff, strong, recyclable
- Applications: Pipes, bottles, structural fibres, packaging films
Branched Polymers — How Branching Changes Behaviour
Branched polymers have side chains extending off the main backbone at irregular intervals. These side chains disrupt efficient packing, reducing crystallinity and density while increasing flexibility. The most familiar example is low-density polyethylene (LDPE), produced by free radical polymerisation at high pressure, where chain-transfer reactions create the branching. Branching lowers melting point, increases flexibility, and reduces tensile strength relative to the linear equivalent.
- Examples: Low-density polyethylene (LDPE), glycogen (biological branched polymer)
- Properties: Lower density, flexible, lower melting point
- Applications: Plastic bags, cling film, squeeze bottles
Cross-Linked Polymers — When Chains Connect to Each Other
Cross-linked polymers have covalent bonds formed between adjacent chains, creating a three-dimensional network structure. Light cross-linking produces an elastomer — a rubber-like material that stretches and recovers elastically. Heavy cross-linking produces a thermoset — a rigid, infusible material that cannot be remelted once formed. Vulcanised natural rubber (cross-linked with sulfur) and epoxy resin (cross-linked with a curing agent) are classic examples.
- Examples: Vulcanised rubber, epoxy resin, Bakelite, polyurethane foams
- Properties: Cannot be remelted (thermosets), elastic recovery (lightly cross-linked elastomers), solvent resistant
- Applications: Tyres, adhesives, electronic encapsulants, structural composites
Summary and Key Takeaways
Polymerisation is one of the most fundamental and consequential processes in all of chemistry. Here is what you need to carry away from this guide:
The two primary types of polymerisation are addition (chain-growth) and condensation (step-growth). Addition polymerisation joins monomers with double bonds without releasing any byproducts; condensation polymerisation joins bifunctional monomers in a stepwise process that releases small molecules like water.
Within addition polymerisation, the four major subtypes based on mechanism are free radical, cationic, anionic, and coordination (Ziegler-Natta). Each differs in the nature of the active chain species, the type of monomer it can polymerise, and the degree of control it offers over the final polymer’s properties.
The type of polymerisation determines everything about the resulting polymer, its molecular weight, architecture, chemical resistance, thermal stability, mechanical strength, and recyclability. Choosing the right mechanism is not just a theoretical exercise; it is an engineering decision with enormous practical consequences.
In everyday life, addition polymers dominate packaging, construction, and electronics (polyethene, PVC, polystyrene), while condensation polymers dominate textiles, engineering plastics, and adhesives (nylon, PET, polycarbonate, epoxy).
Frequently Asked Questions
What are the main types of polymerisation?
The two main types of polymerisation are addition polymerisation (also called chain-growth polymerisation) and condensation polymerisation (also called step-growth polymerisation). Within addition polymerisation, there are four mechanistic subtypes: free radical, cationic, anionic, and coordination polymerisation. Ring-opening polymerisation is sometimes classified as a third primary type, as it shares features of both addition and condensation processes.
What is the difference between addition and condensation polymerisation?
The key differences are: addition polymerisation produces no byproducts (every atom in the monomer ends up in the polymer), while condensation polymerisation releases a small molecule, usually water, with every bond formed. Addition polymerisation requires monomers with double bonds and proceeds through chain reactions at active chain ends; condensation polymerisation requires monomers with functional groups (like –OH or –COOH) and proceeds stepwise, with any two molecules capable of reacting at any time.
What are the stages of polymerisation?
In addition to chain-growth polymerisation, there are three stages: initiation (creation of the reactive species), propagation (rapid chain growth), and termination (deactivation of the chain end). In condensation (step-growth) polymerisation, these distinct stages do not apply; the reaction proceeds stepwise without a defined termination event, as chain ends remain reactive throughout.
What is free radical polymerisation?
Free radical polymerisation is a type of addition polymerisation where the reactive chain end is a free radical, a species with an unpaired electron. It is initiated by compounds such as organic peroxides or azo initiators that decompose to generate radicals. The radical attacks a monomer’s double bond, starting the chain, and the process continues by rapidly adding more monomers. It is the most industrially common form of addition polymerisation and produces polymers such as PVC, polystyrene, and PMMA.
Where is polymerisation used in industry?
Polymerisation is used across virtually every major industry. It produces the plastics used in packaging, pipes, and consumer goods; the synthetic fibres in clothing and carpets; the rubbers in tyres and seals; the adhesives and coatings in construction and electronics; the biomedical polymers in sutures and implants; and the engineering plastics in automotive and aerospace components. Globally, polymer manufacturing is one of the largest sectors of the chemical industry.
What are examples of addition polymers?
Major examples of addition polymers include polyethylene (PE) used in bottles, bags, and pipes; poly(vinyl chloride) (PVC) used in window frames, cables, and flooring; polypropylene (PP) used in food containers and car parts; polystyrene (PS) used in foam packaging and disposable cups; polytetrafluoroethylene (PTFE/Teflon) used in non-stick coatings; and poly(methyl methacrylate) (PMMA/acrylic glass) used in displays and optical products.
Is nylon an addition or condensation polymer?
Nylon is a condensation polymer (specifically a polyamide). It is formed by the stepwise reaction between a dicarboxylic acid and a diamine (in the case of Nylon 6,6) or by ring-opening polymerisation of caprolactam (in the case of Nylon 6). In both cases, the resulting polymer contains amide linkages (–CO–NH–) in its backbone, and the synthesis releases water as a byproduct, the defining feature of condensation polymerisation.
Ready to go deeper? Explore our full mechanism guides on Addition Polymerisation and Condensation Polymerisation, or dive into the Stages of Polymerisation — from Initiation to Termination — for a complete mechanistic breakdown.