Plastic can be synthesized via syngas (a mix of hydrogen and carbon monoxide), which itself can be produced through sustainable mechanisms like biofuels or hydr

Plastic can be synthesized via syngas (a mix of hydrogen and carbon monoxide), which itself can be produced through sustainable mechanisms like biofuels or hydrogen electrolysis and CO2 capture. That process starts from a lower energy level than starting from methane or oil, and so overall its more expensive. We would have to be willing to commit to generating a "surplus" of carbon-free power (e.g. nuclear, wind, solar) to support that sort of manufacturing, but it would be possible. Also, a great thing about running electrochemical processes off of a periodic energy source like solar or wind is that the economics are easy to load-shift -- you care about the aggregate work output, not the consistency of work output.