You can make plastic from coal, indirectly by way of synthetic methanol. China's already doing it on a large scale: http://blogs.platts.com/2015/02/11/china-coal-methanol-olefi...

(The methanol could come from renewable electricity and CO2 extracted from seawater, instead of coal, if renewables get cheap enough. Or you can make methanol from waste wood, corn stover, all kinds of biomass.)