Polymerisation

Polymerisation is the process in which simple chemical compounds, known as monomers, are joined together by forming true homopolar bonds to form larger, chain-shaped molecules known as polymers or macromolecules. A classic example of this is the conversion of ethylene into the plastic polyethylene.

This process of polymerisation typically proceeds in four phases:

1. Initiation: This phase comprises the reaction that starts the chain reaction and thus initiates the formation of polymers. This step is also referred to as chain start or primary reaction. In this process, a reactive centre is created that triggers the polymerisation.
2. Growth reaction: In this phase, the length of the molecular chains increases. This phase is also referred to as build-up reaction, propagation reaction or propagation. Monomers are successively added to the reactive centre, causing the polymer chains to grow.
3. Chain transfer: During this phase, the growing molecular chains can form branches. This occurs through the transfer of reactivity from one chain to another, which leads to the formation of branched polymer structures.
4. Chain termination: This phase ends the chain reaction. This reaction, also referred to as termination, stops the growth of the polymer chains by neutralising the reactive centres.

A special type of polymerisation is radical polymerisation. This occurs when radicals, i.e. particles with unpaired electrons, which are generally very reactive, are involved. A well-known example is the formation of the macromolecule polystyrene (PS) through a radical reaction of styrene. In this process, a radical initiates the reaction, sets the chain growth reaction in motion and ultimately leads to the formation of the polystyrene.