Types of Polymerization Reactions with Mechanism and Examples
Polymerization Reaction
The word “Polymer” is a Greek word which literally means “Many parts”. A polymer is that chemical molecule which has a long set of identical building blocks, all linked with a bond, be it ionic or covalent. The building block through which any polymer molecule is built is known as a monomer. These monomers are generally very reactive molecules. The process in which monomers are all linked together to form the long-chain polymers is called a polymerization reaction or polymerization process.
Classification of Polymerization Reaction
The classification of the polymerization process is done by taking a look at the mechanism of the reaction. The mechanism here refers to how the transformation of reactants into products happens. So, there are two types of polymerization reaction listed below:
Addition Polymerization Process
Like the name “Addition” in this process, the polymers are formed when the corresponding monomers are added to each other. The structure of the polymer, i.e. if the polymer would branch or develop a long chain would depend upon three things, the catalyst used, the reaction conditions and the monomers used. There is no loss of atoms in the process. So, the law of conservation of mass and stoichiometry both are applicable in the addition polymerization process. Usually, the monomers which are used in this process are unsaturated molecules (the carbon molecules which contain double or triple covalent bonds). Additionally, there are four more sub-types of addition polymerization process:
a. Free Radical Polymerization
Free radicals are those atoms which have only one free electron in their valence shell. So, the polymerization reaction classified on the basis of free radicals in the initiation process is called free radical polymerization reaction. The formation of free radical is done in the presence of a particular catalyst known as the free radical generating initiator. Benzol peroxides or peroxides, in general, are perfect examples of the free radical generating initiator. The free radicals which are formed are highly reactive (because of just one unpaired electron instead of a proper negative or positive charge). So, when they are created, they rapidly react with other free radicals present to form longer chains of the carbon molecules known as polymers.
Cationic Polymerization
Polymerization reactions classified on the basis of using cations with the reacting monomers are known as Cationic Polymerization. In this polymerization, both the initiation and propagation steps occur with the help of a cation. So, the chain growth occurs when the cation transfers its charge to the reacting monomers to make them reactive. These reactive monomers now react with other monomers in a similar fashion to form the polymer.
Anionic Vinyl Polymerization
In this process, the anions are introduced to make the monomers reactive. These reactive anions (Usually formed when a strong Lewis base or a nucleophile transfers its charge to the reacting monomers) then react with other monomers resulting in the propagation of the chain and forming the required polymers.
Coordination Polymerization:
This is a special kind of free radical polymerization which happens in the presence of a particular catalyst known as the Ziegler-Natta catalyst (it is a metal complex). This catalyst allows us to control the free radicals and how they react, leading to the formation of polymers which are denser and more robust. The initiation occurs when this catalyst adds the monomer to itself. The propagation of the chain occurs when more monomeric molecules are added to the metal complex or the catalyst. The termination occurs when the added molecules of the monomers leave the metal complex as the required polymers.
Condensation Polymerization
In this type of reaction, the polymers are formed by elimination of simpler molecules (mainly water and small chain alcohols), thus the name condensation. The reactants are also different for the condensation polymerization reaction. Here the reactants must contain two different functional groups at the two ends of the molecule. At each step of this reaction, the molecules formed also include the same two functional groups at the two ends of the intermediate. This allows the response to further continue, and the formation of long-chain carbon molecules or polymers is completed. Also, at each step of the way, there is the elimination of smaller molecules or condensation.
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A straightforward example of the condensation reaction is displayed above. In this reaction, the carboxylic acid group and the amine group at the ends of the two reactants, react with each other to form the long chain carbon molecule or polymer. Two molecules of water are also eliminated in the process as evident from the reaction above.
FAQs on Classification of Polymerization Reactions in Chemistry
1. What is classification of polymerization reaction?
The classification of polymerization reaction is the systematic grouping of polymer-forming reactions based on their mechanism or the type of monomers involved. In chemistry, polymerization reactions are mainly classified into:
- Addition polymerization (chain-growth polymerization)
- Condensation polymerization (step-growth polymerization)
2. What are the main types of polymerization reactions?
The main types of polymerization reactions are addition polymerization and condensation polymerization.
- Addition polymerization: Involves unsaturated monomers (usually with C=C bonds) and forms polymers without eliminating small molecules. Example: nCH2=CH2 → (–CH2–CH2–)n (polyethene).
- Condensation polymerization: Involves bifunctional or polyfunctional monomers and eliminates small molecules like H2O or HCl. Example: Formation of nylon-6,6 from hexamethylenediamine and adipic acid with elimination of water.
3. What is addition polymerization with example?
Addition polymerization is a chain-growth polymerization in which unsaturated monomers add together without loss of any small molecule.
- It usually involves monomers containing a double bond (C=C).
- The reaction proceeds through initiation, propagation, and termination steps.
- Example: nCH2=CH2 → (–CH2–CH2–)n (formation of polyethene).
4. What is condensation polymerization with example?
Condensation polymerization is a step-growth polymerization in which monomers join together with the elimination of small molecules such as H2O, HCl, or NH3.
- It involves bifunctional or polyfunctional monomers.
- The polymer chain grows step by step between functional groups.
- Example: Nylon-6,6 formation from hexamethylenediamine and adipic acid with elimination of water molecules.
5. What is the difference between addition and condensation polymerization?
The main difference between addition and condensation polymerization is that addition polymerization forms polymers without by-products, while condensation polymerization eliminates small molecules.
- Monomers: Addition uses unsaturated monomers (C=C); condensation uses bifunctional monomers.
- By-products: Addition – no small molecule eliminated; Condensation – H2O, HCl, etc., are removed.
- Mechanism: Addition – chain-growth; Condensation – step-growth.
- Example: Polyethene vs Nylon-6,6.
6. What is chain-growth polymerization?
Chain-growth polymerization is a type of polymerization in which the polymer chain grows by successive addition of monomer units to an active center.
- Also called addition polymerization.
- Involves three main steps: initiation, propagation, and termination.
- Commonly initiated by free radicals, cations, or anions.
7. What is step-growth polymerization?
Step-growth polymerization is a polymerization reaction in which monomers, dimers, or oligomers react with each other step by step to form high molecular weight polymers.
- Also known as condensation polymerization in most cases.
- Requires monomers with two or more functional groups.
- Often involves elimination of small molecules like H2O.
8. How are polymerization reactions classified based on mechanism?
Polymerization reactions are classified based on mechanism into free radical, cationic, anionic, and coordination polymerization.
- Free radical polymerization: Initiated by free radicals (e.g., peroxide initiators).
- Cationic polymerization: Initiated by a positively charged species.
- Anionic polymerization: Initiated by a negatively charged species.
- Coordination polymerization: Uses transition metal catalysts like Ziegler–Natta catalysts.
9. Can you give an example of addition and condensation polymers?
An example of an addition polymer is polyethene, and an example of a condensation polymer is nylon-6,6.
- Addition polymer example: Polyethene formed by nCH2=CH2 → (–CH2–CH2–)n.
- Condensation polymer example: Nylon-6,6 formed from hexamethylenediamine and adipic acid with elimination of H2O.
10. Why is classification of polymerization reactions important?
The classification of polymerization reactions is important because it helps predict polymer structure, properties, and industrial applications.
- Determines whether by-products will form.
- Helps select suitable monomers and catalysts.
- Guides control over molecular weight and polymer architecture.
- Supports industrial production of plastics, fibers, rubbers, and resins.
