[gus_massa]

More details from https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment the reaction is

2 CO2 + heat --> 2 CO + O2

CO is a gas, so they let it escape. (It's toxic for mammals (in particular humans), but there is no one around to worry about. It will get transformed back to CO2 in the atmosphere when we react with something else that has an oxygen to steal. And in extremely low concentrations it's not a problem. So it's safe to let it escape for now.)

It's energy limited. You need heat to make the transformation.,

[marcosdumay]

Hum... There are plenty of ways to turn that CO into plastics, like the GP wants (but all will require even more energy).