Question

How many methods of chain termination are there in the free radical mechanism?

Answer 1

A chain reaction mechanism step in which one or more than one chain carrying species reacts without forming another chain carrying molecule is called chain termination. A reaction which involves free radicals is called a free-radical reaction. Many radical reactions are chain reactions which have a chain initiation step, a chain propagation step and a chain termination step.

Complete answer:
Basically, there are three ways in which a chain terminates. The first method is known as coupling and it occurs when two free radicals join together. This type of mechanism increases the molecular mass if the result is joining two polymer chains. This mechanism is the main mechanism which terminates the polymerization of styrene. Disproportionation is an alternative mechanism which occurs when two radicals interact with each other. In this mechanism, one molecule abstracts hydrogen atoms from the other molecule and the other molecule will form a double bond. This method will not affect the molecular mass. The third method is the chain transfer mechanism. In this method, a radical abstracts a hydrogen atom from its neighboring molecule.
Therefore, the three methods of chain termination in a free radical mechanism are: Coupling, disproportionation and chain transfer.

Note:
Free radical polymerization involves three important steps: Initiation, propagation and termination. Initiation involves the formation of radicals and the formed radical reacts with a vinyl monomer. Propagation is the rapid addition of monomers to the growing polymer chain without changing the active center. Termination involves the destruction of the growth active center by combination or disproportionation or coupling of the radicals of two growing polymer chains.