Chain Reaction

Run the night shift at a polymer works. You are the growing end of a polymer chain: steer it round the reactor and swallow monomers. Each one's C=C double bond opens and it joins the chain, end to end. Dodge the alkanes (no C=C, so they can't join), fill each order, then draw the repeating unit with its brackets and n, and work back from a polymer to its monomer.