Polymers  (Page 18/6)

Organic macromolecules

As its name suggests, a macromolecule is a large molecule that forms when lots of smaller molecules are joined together. In this chapter, we will be taking a closer look at the structure and properties of different macromolecules, and at how they form.

Polymers

Some macromolecules are made up of lots of repeating structural units called monomers . To put it more simply, a monomer is like a building block. When lots of similar monomers are joined together by covalent bonds, they form a polymer . In an organic polymer , the monomers are joined by the carbon atoms of the polymer 'backbone'. A polymer can also be inorganic , in which case there may be atoms such as silicon in the place of carbon atoms. The key feature that makes a polymer different from other macromolecules, is the repetition of identical or similar monomers in the polymer chain. The examples shown below will help to make these concepts clearer.

Polymer

Polymer is a term used to describe large molecules consisting of repeating structural units, or monomers, connected by covalent chemical bonds.

1. Polyethene Chapter [link] looked at the structure of a group of hydrocarbons called the alkenes . One example is the molecule ethene . The structural formula of ethene is is shown in [link] . When lots of ethene molecules bond together, a polymer called polyethene is formed. Ethene is the monomer which, when joined to other ethene molecules, forms the polymer polyethene . Polyethene is a cheap plastic that is used to make plastic bags and bottles.

   

   A polymer may be a chain of thousands of monomers, and so it is impossible to draw the entire polymer. Rather, the structure of a polymer can be condensed and represented as shown in [link] . The monomer is enclosed in brackets and the 'n' represents the number of ethene molecules in the polymer, where 'n' is any whole number. What this shows is that the ethene monomer is repeated an indefinite number of times in a molecule of polyethene.

   

2. Polypropene Another example of a polymer is polypropene (fig [link] ). Polypropene is also a plastic, but is stronger than polyethene and is used to make crates, fibres and ropes. In this polymer, the monomer is the alkene called propene .

   

How do polymers form?

Polymers are formed through a process called polymerisation , where monomer molecules react together to form a polymer chain. Two types of polymerisation reactions are addition polymerisation and condensation polymerisation .

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

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 [link] ). When molecules of ethene are joined to each other, the only thing that changes is that the double bond between the carbon atoms in each ethene monomer is replaced by a single bond so that a new carbon-carbon bond can be formed with the next monomer in the chain. In other words, the monomer is an unsaturated compound which, after an addition reaction, becomes a saturated compound.