Chain Reaction in Polymerization

Chain reactions

Many gas phase and polymerization reaction are chain reactions, in which a reactive intermediate (which is often a free radical, an atom, or a fragment of stable molecule with an unpaired electron) produced in one step of the mechanism takes part in subsequent steps. Those are:

1. Initiation step
2. Propagation step
3. Branching reaction
4. Inhibition step
5. Termination step

The structure of chain reactions

The steps in a chain reaction can be classified into five types. In the initiation step the radical are formed from ordinary (spin-paired) molecules. For example, Cl atoms are formed by the dissociation of Cl₂ molecules either as a result of collisions (in a thermal reaction, one driven by heat) or as a result of absorption of light (in a photochemical reaction). The radicals produced in the initiation step attack other spin-paired molecules in the propagation step, and each attack give rise to a new radical, as in the attack of CH3 radical on ethane:


^.CH₃ + CH₃CH₃ → CH₄ + ^.CH₂CH₃
(The dot signifies the unpaired electron and marks the radical). In some cases the attack results in the production of more than one radical, and we deal with these branching reaction. In other cases the radical might attack a product molecule formed earlier in the reaction. This reduces the net rate of formation of product, and is called an inhibition step. For example, in a photochemical reaction in which HBr is formed from H₂ and Br₂, an atom H atom might attack an HBr molecule, leading to H₂ +Br. This does not break the chain, because on radical (^.H) gives rise to another (^.Br). Steps in which radicals are removed are called termination step because they end the chain of reactions. For example, when two ^.CH₂CH₃ radicals dimerize, the chain terminates. Some radicals react with the walls of the vessel and terminate the chain. Others may be scavenged by reaction with other radicals introduced into the reaction vessel. The nitrogen(II) oxide molecule, NO, has an unpaired electron and is a very efficient radical scavenger. The observation that a reaction is quenched when NO is introduced is a good indication that a chain mechanism is in operation.

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