Polymerization

Polymerization is the process of creating polymers. Several monomers combine by process of polymerization to form various polymers. These polymers are then processed to make various products. Polymers are of great importance in our daily life. In this article, we will learn about What is Polymerization, Mechanism of Polymerization, the Degree of Polymerization and others in detail.

👁 Polymerization

What is Polymerization?

The process through which large chains of monomer molecules combine together to form the polymer is called polymerization. The polymers formed by this method are of great importance. Polymerization can be achieved through different categories such as step-growth polymerization, chain-growth polymerization, condensation polymerization and others.

Polymers are long-chained compounds and their chain can even contain several thousand repeating units called monomers. These monomer molecules are usually linked by covalent bonds to form such a huge polymer. Polythene, Buna-S, and Nylon-66 are some examples of polymers.

Polymerization Mechanism

Polymerization consists of three steps that include,

• Initiation
• Propagation
• Termination

The process of completing the polymerization reaction involves two different methods which include the step-growth mechanism and the chain-growth mechanism.

Step Growth Polymerization

When the pairs of reactants of any length, fuse or combine at each step in order to form a longer polymer molecule, then this particular way of forming the polymerization is regarded as step-growth polymerization. In this process of polymerization, the average molar mass increases gradually but slowly, while the long chains take time to be formed.

Condensation Polymerization

Condensation polymers are formed by a reaction between two different bifunctional or tri-functional monomer units. These monomeric units undergo a condensation reaction to form polymers and small molecules of simple compounds like HCl (Hydrogen chloride), H₂O (Water), alcohol, etc. are eliminated. Silicon, Bakelite, Nylon, etc. are some examples of condensation polymers. 

Chain Growth Polymerization

When the step of chain extension adds to the monomer in a growing chain, with an active centre such as a cation, or free radical, then the such formation of the polymerization, is regarded as a Chain-growth polymerization because here a chain reaction takes place. The unsaturated monomers, especially those which contain the double bonds of carbon-carbon, are all linked together in the chain-growth polymerization. While due to the formation of a new sigma bond the pi bond is lost.

Addition or Chain Growth Polymerization

When the same or different monomer molecules (unsaturated compounds) are successively added together on a large scale to form a polymer, this type of polymerization is known as addition or chain-growth polymerization.

1. Polyaddition: It is a polymerization reaction that forms polymers through respective independent addition reactions. Polyaddition occurs as a reaction between functional groups on molecules to crystallize a category of altitudinous molar mass, with low degrees of polymerization, resemblant to dimers, trimers, and oligomers.
2. Polycondensation: It is a chemical abridgement for delivering a polymer by associating single or multiple varieties of monomers to crystallize long chains releasing water or a correspondent simple substance.

This involves free radical or ionic mechanisms of which free radical one is the most common mode.

Polymerization Chemical Reaction

Chemical reactions of organic monomers are referred to as polymerization chemical reactions. Polymers are formed by combining these monomers in infinitely long chains. This reaction includes either monomer adsorption polymerization or emulsion polymerization.

Preparation of Polymers

Preparation of polymers is achieved through various methods. Let's learn about the preparation of some important polymers in detail.

Polyethene

There are two types of polyethene which are,

• Low-Density Polythene (LDP)
• High-Density Polythene (HDP

Low-Density Polythene (LDP)

Low-Density Polythene is obtained by the polymerization of ethene under high pressure of 1000 to 2000 atmospheres and high temperature of 350 to 520 k in the presence of oxygen gas or peroxide initiator as the catalyst.

LDP is formed by free radical addition and H-atom. They have a highly branched structure. They are chemically inert in nature, tough but flexible and are poor conductors of electricity. LDP is widely used for the manufacturing of toys, bottles and flexible pipes.

High-Density Polythene (HDP)

High-Density Polythene is prepared by the polymerization addition of ethene under low pressure of 3 to 4 atmospheres and 343 k temperature, in presence of like triethyl aluminium and titanium tetrachloride which works as a catalyst.

HDPs are chemically inert similar to LDPs but are comparatively tougher and harder. They are used for manufacturing buckets, dustbins, pipes, etc.

Polytetrafluoroethylene

Polytetrafluoroethylene is also known as Teflon. It is manufactured by heating tetrafluoroethylene under high pressure with a free radical. Teflon is highly inert and doesn't react to other chemicals very easily. This property of Teflon makes it ideal for surfaces which are frequently exposed to other chemicals. It is used to make the nonstick coating, gas caskets, etc

Polyacrylonitrile

Polyacrylonitrile is formed by the additional polymerization of acrylonitrile in the presence of a peroxide which acts as a catalyst. It is used as a substitute for wool for making commercial fibres such as Acrilan and others.

Condensation or Step Growth Polymerization

It is associated with a repetitive condensation reaction between two bi-functional monomers. In these polycondensation reactions, some unpretentious molecules like water, alcohol, etc. are lost and high molecular mass condensation polymers are formed. Such as terylene, nylon-6,6, nylon-6, phenol-formaldehyde polymer, melamine-formaldehyde polymer, etc.

1. Radical Polymerization: A variety of unsaturated compounds such as alkanes or dienes or their derivatives are polymerized by free radical addition polymerization in the presence of a free radical generating initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. This type of polymerization is performed by heating the monomer with a small amount of the initiator (usually peroxide) or by exposing the monomer to light.
2. Coordinative Polymerization: Coordination polymerization is a form of polymerization catalyzed by transition metal salts and complexes.
3. Cationic Polymerization: When the initiator is a cation, it binds to the double bond to form an ionic intermediate to which p is added in a process called cationic addition polymerization. It starts with acid. A commonly used acid for the catamite polymer sulfuric acid HSO, HF, BF is in the presence of a small amount of water.
4. Anionic Polymerization: This type of polymerization is initiated by ions, which can be bases or other nucleophiles. The main chain carries the nucleophile addition of ions to an unsaturated monomer. This proceeds through the formation of the carbanion. Butyl lithium or potassium amide K⁺NH₂^-.

Polymerisation Techniques

Each type of polymerisation has a set of techniques using which chemical reactions take place for the formation of polymers. Here, look at the various polymerization techniques which are used.

• Condensation Polymerisation
• Solution polycondensation
• Melt polycondensation
• Addition Polymerization

Solution Polymerization: In this industrial polymerization method, a monomer is to be dissolved in the non-reactive solvent having an initiator or catalyst, which increases the speed of the reaction. The polymer formed in the process is also soluble in the same non-reactive solvent.

Bulk Polymerization: It is also known as mass polymerization and is formed by adding a radical initiator to the pure monomer. However, the monomer must be in a liquid state, and the radical initiator should be soluble. The reaction is exothermic.  

Emulsion Polymerization: It is a process of radical polymerization which involves a monomer, water that incorporates emulsion, and surfactant.

Suspension Polymerization: In this process of heterogeneous radical polymerization, mechanical agitation is used for mixing monomers in their liquid state.

Degree of Polymerization

The frequency of repeating units present that form a polymer is called the degree of polymerization. For example, for polymer A which is made up of 25 monomers called (M), then the degree of polymerization of A will be 25.

Polymerization of PVC

PVC polymerization is a chemical process where smaller Vinyl Chloride units join together to create long chains, forming Polyvinyl Chloride, or PVC. In this process we produces a versatile plastic material used in numerous applications like pipes, vinyl flooring, and cable insulation due to its durability and versatility.

Classification of Polymerization

Classification of Polymerization is achieved based on several factors which include source, structures, mode of polymerization, molecular forces and others. Further in this article, we will learn about these in detail.

Based on Source of Polymers

Polymers are further classified into three subcategories based on the source of their monomers which includes,

• Natural Polymerization: These polymers are directly extracted from natural materials such as plants and animals. Some examples of natural polymers include resins, starch and rubber.
• Semi-Synthetic Polymerization: Natural polymers are modified to form semi-synthetic polymers. Natural rubber is treated with chemicals to form semi-synthetic polymers. Some other examples of semi-synthetic polymers include cellulose acetate and cellulose nitrate.
• Synthetic Polymerization: Synthetic Polymers are completely man-made. Some examples of synthetic polymers include polyethene, Nylon 66, and others.

Based on Structure of Polymers

Polymers are further classified into three subcategories based on their structure which include,

• Linear Polymerization: Linear Polymers as the name suggest consists of a long and straight-chain of monomers. Example of Linear Polymers is PVC.
• Branched Polymerization: Branched Polymers are sub categieories of linear polymers that contain branches. Example of Branched Polymers include Low-density Polythene.
• Network or Cross-linked Polymerization: Cross-linked Polymers are polymers having cross-linked bonds with each other. They are generally formed by bi-functional or tri-functional monomers. Examples of cross linked polymers include bakelite and melamine.

Based on Mode of Polymerization

Polymers are further classified into two subcategories based on their mode of polymerization which includes,

• Addition Polymerization: Addition Polymers formed by the repeated addition of monomers. They possess double or triple bonds. When same species of monomers are added to form polymers they are called homopolymers and when different monomers are added they are called copolymers. Examples of addition polymers include polythene and Buna-s respectively.
• Condensation Polymerization: Condensation polymers are formed by repeated condensation of tri or bifunctional monomeric units. These reaction eliminate some small molecules like water, hydrogen chloride, etc. Examples of condensation polymers include Terylene and Nylon 6,6.

Based on Forces Between Molecules

Polymers are further classified into four subcategories based on the force between their molecules which include,

• Elastomers: Polymers that are rubber-like solids and have elastic properties are called elastomers. Their polymer bonds are held together by weak intermolecular forces and that allows these polymers to stretch. Examples of elastormes include Buna-s and Buna-n
• Fibres: Polymers having strong intermolecular forces like hydrogen bonding are called fibers. Due to their strong intermolecular bond they closely packed. They are crystalline in nature. Examples are Polyamide and polyesters.
• Thermoplastic Polymers: Plastic polymers that softened on heating and hardened on cooling are called thermoplastic. Examples of thermoplastic are Polyvinyls, polystyrene etc.
• Thermosetting Polymers: Thermosetting Polymers are polymer which are heavily branched or cross-linked, they mould on heating and can’t regain the original shape. Example of thermosetting polymer is Bakelite.

Also, Check

• Polymers
• Polymers of Commercial Importance