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Question
Answer the following.
Explain in detail free radical mechanism involved during preparation of addition polymer.
Solution
The free radical mechanism is most common in addition to polymerization. It is also called a chain reaction which involves three distinct steps. These are as follows:
i) Step 1: Chain initiation:
a. The chain reaction is initiated by a free radical which is formed from an initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc.
b. For example, acetyl peroxide generates methyl radical as shown below:
c. The free radical (say `"R"^•`) so formed attaches itself to the olefin (vinyl monomer) and produces a new radical, made up of two parts, namely, the attached radical and the monomer unit.
ii. Step 2: Chain propagation:
a. The new radical formed in the initiation step reacts with another molecule of vinyl monomer, forming another still bigger sized radical, which in turn reacts with another monomer molecule.
b. The repetition of this sequence takes place very rapidly. It is called chain propagation.
c. This step is very rapid and leads to high molecular mass radical.
iii. Step 3: Chain termination:
a. At some stage, termination of the growing chain takes place. It may occur by several processes.
b. One mode of termination is by combination of two growing chain radicals.
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