You can make pretty much any hydrocarbon, including those used to make plastics, starting from carbon dioxide and hydrogen. You can get carbon dioxide out of se

You can make pretty much any hydrocarbon, including those used to make plastics, starting from carbon dioxide and hydrogen. You can get carbon dioxide out of seawater via electrochemical pH shift and hydrogen via electrolysis. Or, probably better due to starting with more concentrated feedstocks, you can get synthesis gas from high temperature gasification of biomass and top up any hydrogen deficits with electrolytic hydrogen. It's less selective, also less finicky, than low temperature enzymatic/fermentation processes. Alkanes, aromatics, alkenes... it's all accessible starting from methanol. China's already making plastic feedstocks from methanol on an industrial scale though the methanol comes from coal rather than CO2 and clean electricity.