Polymerization:

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form 3-dimensional networks or polymer chains. There are several forms of polymerization and different systems are there to categorize them.

• Step-growth polymers are defined as polymers formed by stepwise reaction between functional groups of monomers.
• Most step-growth polymers are classified as condensation polymers also, but not all step-growth polymers release condensates.
• Step-growth polymers increase in molecular weight at a slow rate at lower conversions and reach moderately high molecular weights at high conversion.

A condensation polymer can be defined as a polymer that involves elimination of small molecules during its synthesis, or contains functional groups as part of its backbone chain, or its repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded. Chain-growth polymerization involves the connecting together of molecules incorporating double or triple chemical bonds. These unsaturated monomers have additional internal bonds which are able to break and link up with the other monomers to form the repeating chain. Chain-growth polymerization is involved in the manufacture of polymers like polyethylene, polypropylene, and polyvinyl chloride. In chemical compounds, polymerization occurs through a range of reaction mechanisms which vary in complexity because of functional groups present in reacting compounds and their inherent steric effects explained by VSEPR Theory. As alkenes can be formed in straightforward reaction mechanisms, they form useful compounds like polyethylene and polyvinyl chloride when undergoing radical reactions. Polymers such as PVC are usually referred to as homopolymers as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one molecule are referred to as co-polymers.

Other monomer units, like formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures to form trimers. The molecules consisting of 3 monomer units which can cyclize to form ring cyclic structures, or undergo reactions to form tetramers or 4 monomer-unit compounds. Further compounds being referred to as oligomers in smaller molecules. Generally, because the formaldehyde is an reactive electrophile it allows nucleophillic addition of hemiacetal intermediates, which are generally short lived and relatively unstable "mid stage" compounds which react with other molecules present to form more stable polymeric compounds. Polymerization that is not sufficiently moderated and proceeds at a fast rate can be hazardous. This phenomenon is known as Hazardous polymerization and can cause explosions and fires.