What is chain polymerization reaction?

Hint: Polymers are compounds of very high molecular mass consisting of a large number of repeating structural units. Monomers are the simple compounds from which polymers are made. Thus, monomers are the repeating structural units. The process of forming polymers from suitable monomers is called polymerisation.

Complete answer:
The word polymer springs from two Greek words: poly' means many and 'mer' means unit or part. The term polymer is defined as very large molecules which generally have high molecular mass . Some polymers are formed by joining of repeating structural units on an outsized scale, these also are termed as macromolecules. The repeating structural units are derived from some simple and reactive molecules called monomers and are linked to every other by covalent bonds. This process of formation of polymers from respective monomers is termed polymerisation. The interaction of hexamethylene diamine and carboxylic acid as well as the transformation of ethene to polyethene which give formation of Nylon 6, 6 are samples of addition and condensation polymerization.
 Addition Polymerisation or Chain Growth Polymerisation: In addition polymerisation, the molecules of the identical monomer or different monomers add together to make polymer. This mode of polymerisation results in an increase in chain length through the formation of either free reactions or ionic species. Eg: Polymerisation of ethene to polyethene consists of heating a combination of ethene with whitener initiator.

Note:
Polymerization reaction are of 2 types :
1. Addition polymers or chain growth polymers.
2. Condensation polymers or step growth polymers.