Petrochemicals are used to build production modules on Arctic resource buildings, increasing their output. As one of the Rare Resources, it can be found in Combat Missions, rewards from optional Assignments, and as an item purchasable from Ville Jorgensen, the Arctic trader, or Dr. Esther Neyland, the Tundra trader. They can also be gathered from the Ikkuma Glacier sector after completing any phase of its Sector Project. With the Orbit DLC, you can also automatically generate Petrochemicals continuously at a pretty high rate with a tier 3 technology in the Biotech tree in the Technology Nexus.