That is a terrible idea, for many reasons, and it would not reduce CO2 emissions... probably the opposite.

A much better idea would be to reserve petroleum and natural gas for production of high value products such as plastics, instead of using it as an energy source now that renewable energy is competitive on cost.

Is that so? I recently bought some fake-plastic bags made out of plant matter (not fossilized mind you) in the hope that they would be more environmentally sustainable. As I remember it, googling did not turn up any conclusive evidence to the contrary. Why do you think banning plastics would be a bad idea?

Because:

- there are very many plastics with different properties, and - AFAIK - not all of them can be replaced with plant matter

- replacing all plastics with plants will likely compete with agriculture for land use

- if you just went and banned plastics, we'd have to use things like glass, metals and woods instead; not sure how the manufacturing footprint compares between metals and plastics, but just consider that if every plastic item you have around you was made from steel instead, how much heavier everything would be, and how it would impact energy requirements to move those things around

Plastics are pretty awesome materials.