5.21 Understand that condensation polymerisation produces a small molecule, such as water, as well as the polymer.

When the two monomers come together, one or more atoms is/are lost. These join together to make a small molecule (i.e. H₂O, CH₃OH (methanol) or HCl). The monomers then join together to make the polymer.

5.20 Understand that some polymers, such as nylon, form by a different process called condensation polymerisation

Condensation polymerisation is a situation in which a polymer is formed, along with a small molecule such as H₂O, HCl or CH₃OH (methanol). 

Nylon is an example:

5.19 Explain that addition polymers are hard to dispose of as their inertness means that they do not easily biodegrade

Addition polymers are unreactive because they are saturated, which means they don't biodegrade easily. Solutions used today include:

• burning (not good, produces cancerous / harmful gases)
• landfills (takes up a lot of useful land and harmful chemicals leak into the soil)
• recycling (good for the environment but uses up energy and isn't always cheapest)

5.18 Describe some uses for polymers, including poly(ethene), poly(propene) and poly(chloroethene)

Poly(ethene)

• plastic bags
• light carrier bags
• plastic bottles

poly(propene)

• crates
• ropes
• thermal undergarments

poly(chloroethene)

• water pipes
• wire insulation

5.17 Deduce the structure of a monomer from the repeat unit of an addition polymer

In the simplest terms possible: take away the two 'floating' / empty bonds at the sides and make a carbon carbon double bond between 2 carbons instead. 

5.16 Draw the repeat unit of addition polymers, including poly(ethene), poly(propene) and poly(chloroethene)

Repeat units must be drawn:

• within large brackets
• with two bonds sticking out of the brackets
• an n at the end, representing the number of times it is repeated (i.e. n could be anything, you don't write the number, write n)

Poly(ethene) and poly(chloroethene):

Poly(propene)

5.15 Understand that an addition polymer is formed by joining up many small molecules called monomers

Exactly what the point says: addition polymers are made by joining up many small ones called monomers: "mono" meaning one and "poly" meaning many