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

Most addition polymers are inert. All this means is that they do not react easily (due to the fact that the carbon bonds are very strong and hard to break). This means that it takes a really long time for addition polymers to biodegrade. Furthermore, burning plastics releases toxic chemicals so that's not a good idea either. Because of this, it is hard to dispose of polymers (that's why we recycle them).