4:47 explain problems in the disposal of addition polymers, including: their inertness and inability to biodegrade, the production of toxic gases when they are burned

Polymers are inert (unreactive) as they have strong C-C bonds.

This makes them non-biodegradeable.

Biodegradable: the breakdown of a substance by microorganisms.

if burnt the addition polymers could produce toxic gases such as carbon monoxide and hydrogen chloride.