Everything Science Grade 12

2.2 CHAPTER 2. ORGANICMACROMOLECULES

DEFINITION: Polymerisation

In chemistry, polymerisation is a process of bonding monomers, or

single units together through a variety of reaction mechanisms to form

longer chains called polymers.

Addition polymerisation ESCAE

In this type of reaction,monomer molecules are added to a growing polymer chain
one at a time. No small molecules are eliminated in the process. An example of this
type of reaction is the formation of polyethene from ethene (fig 2.1). When molecules
of ethene are joined to each other, the only thingthat changes is that thedouble bond
between the carbon atoms in each ethene monomer is replaced by a single bond so
that a new carbon-carbon bond can be formedwith the next monomerin the chain.
In other words, the monomer is an unsaturated compound which, after an addition
reaction, becomes a saturated compound.

Extension: Initiation, propagation and termination

There are three stages inthe process of additionpolymerisation. Initiation

refers to a chemical reaction that triggers off another reaction. In other words,

initiation is the starting point of the polymerisation reaction. Chain propagation

is the part where monomers are continually added to form a longer and longer

polymer chain. Duringchain propagation, it isthe reactive end groupsof the

polymer chain that react in each propagation step, to add a new monomer to

the chain. Once a monomer has been added, thereactive part of the polymer is

now in this last monomer unit so that propagation will continue. Termination

refers to a chemical reaction that destroys the reactive part of the polymerchain

so that propagation stops.

Example 1: Polymerisation reactions

QUESTION

A polymerisation reaction takes place and the following polymer is formed: