Polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.There are many forms of polymerization and different systems exist to categorize them.

Mechanism of Addition Polymerization

The formation of a polymer by addition polymerization is an example of a chain reaction. Once a chain reaction gets started, it is able to keep itself going. The three steps of this reaction to focus on are
how the reaction gets started (INITIATION)
how the reaction keeps going (PROPAGATION)
how the reaction stops (TERMINATION)

Example: polymerization will combine ethylene (ethene) monomers (CH2=CH2), so product will be polyethylene. (Polyethylene is used to make food wrap, milk jugs, garbage bags, and many other plastic products.)
I. INITIATION (How the reaction gets started)

If you looked at Part 3 of this tutorial, you have already seen the first part of the initiation step of addition polimarization chain reaction. A peroxide molecule breaks up into two reactive free radicals. Light or heat can provide the energy needed for this process.

We can write an equation for this process:

The second part of initiation occurs when the free radical initiator attacks and attaches to a monomer molecule. This forms a new free radical, which is called the activated monomer.

We can write an equation for this process, too:

II. PROPAGATION (How the reaction keeps going)

During a chain reaction, most of the time is spent in the propagation phase as the polymer chain grows. In the propagation phase, the newly-formed activated monomer attacks and attaches to the double bond of another monomer molecule. This addition occurs again and again to make the long polymer chain.

Once again, we can write an equation for this reaction:

The “n” stands for any number of monomer molecules, typically in the thousands.

III. TERMINATION (-How reaction ends-)
This chain reaction cannot go on forever. The reaction must terminate, but how? A growing polymer chain joins with another free radical. We watched a peroxide break up to form two radicals. It makes sense that two free radicals could join to make a stable bond.

The equation representing this step of the chain reaction can be written simply as:

Remember: The R and R’ groups here can be the original free radicals, the growing polymer chains, or even one of each. Termination reactions can, however, be more complicated looking.

An Important Note:
Chemists can control the way a polymer does each of these steps by varying the reactants, the reaction times, and the reaction conditions.

The physical properties of a polymer chain depend on the polymer’s average length, the amount of branching, and the constituent monomers.