Organic Chemistry - Polymers: Addition Polymers

Objective

Deduce the structure or repeat unit of an addition polymer from a given alkene and vice versa.

Addition Polymerisation

Addition polymerisation is a chemical process where small monomer molecules (monomers) combine to form a large macromolecule (polymer) with little or no loss of atoms.

This process typically involves unsaturated monomers, meaning monomers containing carbon-carbon double (alkene) or triple (alkyne) bonds.

The double or triple bond in the monomer breaks, and the atoms rearrange to form a single bond in the polymer chain.

Deducing the Structure of a Polymer from an Alkene

To deduce the structure of an addition polymer, consider the following:

1. Identify the monomer(s) involved in the addition polymerisation.
2. Determine the type of bond that is broken during the addition reaction (double or triple bond).
3. The repeat unit of the polymer will consist of the atoms from the monomer(s) that are present in the polymer chain.
4. The number of monomers that join together to form the polymer will be indicated by the repetition of the repeat unit.

Example:

Ethylene ($C_2H_4$) polymerises to form polyethylene ($(-CH_2-CH_2-)^n$). The monomer is ethylene, which has a double bond. During polymerisation, the double bond breaks, and the carbon atoms form single bonds. The repeat unit is $-CH_2-CH_2-$, and ethylene units join together to form the polymer.

Deducing the Alkene from a Polymer Structure

To deduce the alkene that could have formed a given addition polymer, consider the following:

1. Identify the repeat unit of the polymer.
2. Determine the type of bond present in the repeat unit (single or double).
3. If the repeat unit contains a double bond, the monomer is an alkene.
4. The structure of the alkene monomer will be half the size of the repeat unit (excluding the atoms that are shared in the repeat unit).

Example:

If the repeat unit of a polymer is $-CH_2-CH_2-$, the polymer is polyethylene. The corresponding alkene monomer is ethylene ($C_2H_4$).

Example 2:

If the repeat unit of a polymer is $-CH_2-CH=CH_2$, the polymer is polypropylene. The corresponding alkene monomer is propene ($C_3H_6$).

Common Addition Polymers

Polymer	Monomer	Chemical Formula	Uses
Polyethylene	Ethylene	$(-CH_2-CH_2-)^n$	Plastic bags, bottles, films
Propylene	Propene	$(-CH_2-CH=CH_2)^n$	Plastic films, containers
Polyvinyl Chloride (PVC)	Vinyl Chloride	$(-CH_2-CHCl-)^n$	Pipes, flooring, window frames
Polytetrafluoroethylene (PTFE) - Teflon	Tetrafluoroethylene	$(-CF_2-CF_2-)^n$	Non-stick coatings, seals

Practice Questions

1. The repeat unit of a polymer is $-CH_2-CH=CH_2$. What is the name of the polymer and what is the corresponding alkene monomer?
2. What is the structure of the repeat unit in polypropylene? What is the structure of the alkene monomer?
3. An alkene with the formula $C_4H_8$ polymerises to form a polymer used in the manufacture of plastic bottles. Deduce the structure of the polymer and the monomer.