Free radical polymerization/Chain reaction/Addition polymerization:
Addition polymerization is characterized by a self addition of the monomer molecules to each other very rapidly through a series of stage are:
1. Generation of free radical
2. Initiation
3. Propagation
4. Termination
1. Generation of free radical:
The initiation of polymer chain growth is brought about by free radicals are produced by the decomposition of compounds called initiators. initiators are thermally unstable compounds & decomposed by catalyst like heat, light, etc to form free radical. The initiators can be written as-
2. Initiation:
Pair a free radical contain a lone electron. A lone electron is always looking for another lone electron to couple with & get established. This is the process of initiation.
The free radical R° attacks the double bond in the monomer molecule.
3. Propagation:
After initiation comes propagation. In this step the radical site at the first monomer unit attacks the double bond of a fresh monomer molecule to the growing chain & all these reactions are knows as propagation.
The chain still contains a free radical & can attacks another monomer molecule.
Here, n denotes the number of monomer units added with radical site in chain growth. The mode of addition of the incoming monomer to the growing chain can be as follow-
4. Termination:
After propagation comes termination. There several types of termination. These are given as below-
(A) Coupling termination
(B) Disproportion termination
(C) Chain transfer termination.
(A) Coupling termination:
In this type of termination the two growing chains are united by the coupling of the lone electron present in each to form an electron pair & thus nullify their reactiveness. Science this process involves in coupling the two lone pair electrons, this type of termination is known as coupling termination.
(B) Disproportionation termination:
In this case one H atom from one growing chain is abstracted by the other growing chain & utilised by the lone electron for getting stabilised by the formation of a double bond.
Here two polymer molecules of short chain length instead of a single molecule of a longer chain. This type of termination is called disproportionation termination.
(C) Chain transfer termination:
In this case the growth of polymer chain is stopped or arrested by forming a dead polymer. There is simultaneous generation of a free radical capable of a fresh polymer chain growth.
This reaction takes place by the abstraction of a hydrogen atom or some other atom from the initiator, monomer, polymer or any other species present in the system.
