Is plastic e.g. polyproylene, polystyrene etc mostly inert to most substances at room temperature? I only know that plastic does not like heat.

They aren’t fans of acetone either 

Nor UV light 

15 minutes ago, kenny1999 said:

Is plastic e.g. polyproylene, polystyrene etc mostly inert to most substances at room temperature? I only know that plastic does not like heat.

Many are dissolved by organic solvents. There's a simple compatibility chart here that gives you an idea of what various polymers resist and what they are attacked by:

https://www.calpaclab.com/chemical-compatibility-charts/

It doesn't usually rain organic solvents.

To add to what exchemist said and what you may be thinking about.

Plastics do not take part in many acid - base or redox reactions in normal environmental conditions.

This is unlike many other materials, from wood to metals to soils etc.

This is why the are considered less reactive but as has been pointed out in the right environment they are every bit as reactive.

I didn't see diethyl phthalate on the list but can say from experience it eats up many plastics.