Polymerization: Definition, Types, Uses

Polymerization simply means a chemical reaction through which monomers combine together forming a 3D chain of elements. It has large molecules made up of rings or subunits (monomers) which together forms a network. This is due to the exothermic nature of the reaction. Polymerisation is something that has huge application in the field of thermoplastic and fibre manufacturing. Let us have a detailed look on this topic, covering its types and techniques.

Keyterms: Polymerization, Chemical Reaction, Monomer, Molecules, Polyethylene, Polymer, Ethylene, Bulk polymerization, Solution polymerization

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What is Polymerization?

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A polymer is a molecule made up of a monomer, the repeating structure unit. Let us take the example of polyethylene. We all know that polyethylene is a polymer that has different applications in industries. The molecular formula of polyethylene is nCH₂==CH₂. Here, the adhering feature of the polymer is enhanced by repeating the ethylene units for about 25,000 times.

Polymerization

Thus it is clear that through polymerisation, we create a long form chain of monomers. This chain of polymers surely has their own features that make them unique. The repeating of units for the reaction of polymerization produces heat abundantly and has the chance to explode. To avoid such dangers, the unique features of polymers are understood while handling them in the industries. 

Read: High Density Polyethylene

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Techniques of Polymerisation 

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There are mainly four techniques that are used for the polymerization reaction. These techniques are again classified into two, based on the resultant features. Let us see in detail the four popular techniques of polymerization. 

Heterogeneous Techniques 

There are mainly two different techniques that fall under the head of heterogeneous techniques, namely bulk polymerization and solution polymerization. 

Heterogeneous Techniques

In the method of bulk polymerization, the monomer is taken as a liquid, letting an initiator dissolve in it. Then, the chain transferring agent is allowed to dissolve in the above said solution. This solution is later exposed to light and kept under agitation for proper mass and heat transfer. This process results in the formation of a broad molecule with heavy distribution of mass. Moreover, the mixing becomes progressively difficult with the improvement of the process. Bulk polymerization provides us with the purest form of polymer. 

Homogeneous Techniques 

The second popular method of homogeneous polymerization is solution polymerization. In this process, the monomer and chain agent are dissolved in a solvent, simultaneously. The free radical initiator dissolves in the solvent, wherever it is used. This method is recommended wherever the polymers are likely to be used as a solution. Like heterogeneous polymerization, homogeneous polymerization is also divided into two different techniques, namely, suspension polymerization and emulsion polymerization.

Homogeneous Techniques

Bulk polymerization was noticed to be producing excessive heat while dissolving with the chain agent. This was dangerous because sometimes it exploded, getting overheated. To overcome this shortcoming of bulk polymerization, suspension polymerization was developed. Here, the monomer was dissolved in water as fine drops, which was stabilized and prevented from coalescing by using suitable water-soluble protective colloids and surface agents. The droplets of monomers are formed, transferring heat in the water. But do note that this technique works only in water and thus this is applicable for water insoluble monomers.

But beyond all this, emulsion polymerization is the most popular and widely used technique of polymerization. In this technique, the monomer is dispersed in an aqueous phase as fine droplets which are then emulsified by surface agents, buffers and protective colloids. With the surface tension reduced by the agents, making the condition suitable for surfactants to form micelles. The interior of the micelles acts as a hydrocarbon phase where the monomer is emulsified. 

Read More: Micelles

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Types of Polymerisation 

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While completing the discussion on how polymerization is done (techniques of polymerization) we would have to know what the types of polymerization reactions are. Polymerization reactions are divided into two groups, namely, Addition polymerization (chain reaction) and condensation reaction (step reaction). 

• Addition Polymerisation: As the name denotes, we can see the addition of monomers to form a polymer, twice the number or even more than that. Here, we can either select monomers of the same species or of different species. When we choose the single species monomers for the polymerisation, the product thus formed is known as homopolymer (for example polythene used to make bags). When we use two different species for additional polymerization, the polymer would be known as copolymer (for example, Buna-S and Buna-N).
• Condensation Polymerisation: In the condensation polymerisation, we can find elimination of compounds like HCL, water, alcohol etc...; while the monomers condense during the reaction process. This reaction happens between tri-function and bi-function monomer units and results in the formation of polymers like nylon. 

Types of Polymerization

Read: Bipolymers

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Degree of Polymerisation and Its Calculation 

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We know that polymers have different subunits of monomers repeated like a ring of chains, forming a lengthy network. The number of such monomeric units that is present in a polymer is known as the degree of polymerisation. Then, how can we calculate the degree of a polymer? It is as simple as calculating weighted averages. For that, what you should know is just the components of the polymer and its atomic number.

Let us take an imaginary polymer AB₂ – AB₂, with a certain polymerisation degree. Here, the components are A and B. Assume that the atomic mass of A is 10 and atomic mass of B is 20. Then, we know that the molecular weight would be as follows:

Molecular weight of AB₂ – AB₂= 10 x 2 + 20 x 4

=20 + 80 =100g mol^-1

We can further assume that the molecular mass of the polymer, AB₂ – AB₂ is 1,00,000. Then how many times would the monomers A and B repeated to form this polymer? That would be the degree of polymerisation, which can be calculated by dividing the molecular mass of the polymer by the molecular mass of the monomers. In this case:

Molecular mass of polymer/molecular mass of monomers= 100000/ 100 = 100 

Therefore, the degree of polymerisation in our imaginary polymer AB₂ – AB₂ is 100.

You can try this example on any polymer of your choice.

Also read: Atoms and Molecules

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Things to Remember 

• Polymerisation refers to a chemical reaction through which monomers combine together to form a 3D chain of elements. It has large molecules made up of rings or subunits (monomers) which together form a network.
• There are mainly four techniques that are used for the polymerization, namely bulk polymerization, solution polymerization, emulsion polymerization and suspension polymerization.
• Polymerization reactions are also divided into two groups, namely, Addition polymerization (chain reaction) and condensation reaction (step reaction).
• In Addition polymerization, we can see addition of monomers to form a polymer, twice the number or even more than that. If the same species of monomer is used for addition polymerisation, then it is known as homopolymer and if two different species are used, we obtain a copolymer.
• Condensation polymerisation happens with elimination of compounds like HCL, water, alcohol etc..., while the monomers condense during the reaction process.
• The number of monomeric units that repeats in formation of polymers through polymerisation that are arranged like a ring of subunits is known as the degree of polymerisation. It is calculated by dividing the molecular mass of a polymer by the sum of the molecular mass of the monomers. 

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