6.2.12 Addition polymers from alkenes

What is a Monomer?

A small molecule that can join with other monomers to form a polymer.

What is a polymer?

• A giant molecule made from many repeating monomer units.

• A polymer may contain several thousand monomer units.

What is addition polymerisation?

• An addition reaction in which many small molecules (monomers) join together to form one giant molecule (a polymer).

• The monomers are usually alkenes.

• No small molecule is eliminated as a by-product.

How does addition polymerisation work?

• The C=C double bond contains one sigma (\(\sigma\)) bond and one pi (\(\pi\)) bond.

• During addition polymerisation, the pi bond opens.

• Each carbon forms a new single bond with a neighbouring monomer.

• This joins the monomers together into a long carbon chain.

• The polymer backbone consists of carbon–carbon single bonds.

What are the 3 stages of free radical polymerisation (ethene example)

1. Initiation

2. Propagation

3. Termination

What occurs during the initiation stage of the free radical polymerisation of ethene?

• An initiator, often a peroxide, is used.

• Peroxides contain a weak O–O single bond.

• This bond breaks homolytically.

• Each oxygen atom takes one electron from the bond.

• Two free radicals are produced.

What is homolytic fission?

Breaking a covalent bond so that each atom receives one electron.

What occurs during the propagation stage of the free radical polymerisation of ethene?

• A free radical reacts with an ethene molecule.

• The radical adds across the C=C double bond.

• A new C–C single bond forms.

• A new radical is produced at the other end of the growing chain.

• This radical reacts with another ethene molecule.

• The process repeats, causing the polymer chain to grow.

What occurs during the termination stage of the free radical polymerisation of ethene?

• Two growing polymer-chain radicals combine.

• Their unpaired electrons pair to form a covalent bond.

• The radicals are removed, so that chain growth stops.

What are the conditions needed for manufacturing low density poly(ethene)?

• High pressure.

• High temperature.

• Initiator, often a peroxide.

• Free-radical polymerisation.

What are the structure and properties of low density poly(ethene)?

• Very long polymer chains.

• Many branches.

• Branches prevent the chains from packing closely together.

• Weaker intermolecular attractions between neighbouring chains overall.

• Lower density.

What are the conditions needed for the manufacturing of high density poly(ethene)?

• Relatively low pressure.

• Relatively low temperature.

• Special catalyst.

What is the structure and properties of high density poly(ethene)?

• Extra-long chains.

• Very little branching.

• Chains pack closely together.

• Stronger intermolecular attractions between neighbouring chains overall.

• Higher density.