The polymerization reaction in many chemical compounds may occur through a variety of reaction mechanisms that differ in complexity because of the presence of the functional groups in the reactants as well as due to their inherent steric effects.
How is the Polymerization Reaction Classified?
Polymers are formed from repeating units of a monomer, which are linked to each other by covalent bonds. The reaction which leads to the formation of polymers from their respective monomers is called Polymerization Reaction. We are talking about polymers but have you thought, ‘what is a monomer?’, a monomer is a reactive molecule which has a simple structure and when these molecules combine and form chains with the help of covalent bonds, a polymer is formed. We can broadly classify the polymerization reaction under the following category:
1. Addition polymerization:
This is the type of polymerization in which the molecules of a similar monomer or different monomer add up together on a large scale to form a polymer. When we keep adding monomers to obtain a large chain, such a process is also called as the chain growth polymerization. The monomers that are used here are unsaturated compounds (the carbon compounds which are connected by double or triple covalent bonds).
- For example, alkenes, alkynes and alkadienes.
The increase in the chain length occurs through the formation of either free radicals or ion species. The most common mode of the polymerization reaction, in this case, is the reaction governed by the free radicals. Let us discuss the free radical mechanism here.
Free radical mechanism:
The compound of alkenes and their derivatives are polymerized in the presence of a catalyst which is also known as the free radical generating initiator.
- For example, benzoyl peroxides.
When we polymerize ethene to polythene, we first heat the mixture of ethane with a small amount of benzoyl peroxide initiator. There is an addition of phenyl free radical that is formed by peroxide to the double bond of ethane, which results in generating a new and large free radical. This step is known as the chain initiating step, as the chain formation is initiated from this very step. This free radical when reacts with another molecule of ethane leads to the formation of bigger sized radicals. The reaction is carried forward with the repetition of this sequence and formation of new and bigger radicals and is therefore known as the chain propagating step. At a later stage, we will find that the product radical is formed and this radical reacts with another radical to form the final polymerized product, and hence this is called the chain terminating step.
This type of reaction involves the repetitive condensation between two bi-functional monomers. There is a loss of some simple molecules such as water and alcohol due to the condensation mechanism. Condensation mechanism refers to combining of smaller molecules to obtain larger molecules. In these types of the polymerization reaction, the product that is formed after every step is again a bi-functional species, and so these species undergo a sequence of the condensation process. At each step, we have distinct functional species and every species is independent of the other species, and so we call this process as the step growth polymerization.
- Examples, in this case, are ethylene glycol and terephthalic acid.
So far we have seen that there are two types of the polymerization reaction, how the polymerization of monomers take place and the addition and condensation mechanism. The concepts here are explained in brief, for further details and clarification on the topic of polymerization reaction download BYJU’S the learning app.