Crude oil and polymers

Crude oil is a mixture of thousands of hydrocarbons. A refinery first separates it by fractional distillation: the oil is heated to vapor at the bottom of a tall column, and as the vapor rises and cools, each fraction condenses at its own height. Small molecules with low boiling points (refinery gas, petrol) come off the top; bigger ones (kerosene, diesel) lower down; the heaviest (fuel oil, bitumen) leave from the bottom.

There's more demand for small molecules than crude oil contains, so long chains are broken into short ones by cracking, using heat and a catalyst or steam. Cracking also makes alkenes like ethene and propene, which are the building blocks for plastics. For example, decane can crack into octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

A polymer is a long chain of repeating units called monomers. In addition polymerization, alkene monomers open their double bonds and join end to end, with nothing left over: thousands of ethenes make poly(ethene), polyethylene, the stuff of bags and bottles. Propene makes polypropylene; chloroethene makes PVC.

In condensation polymerization each link gives off a small molecule, usually water. A diacid and a dialcohol make a polyester (PET drink bottles and clothing fibres); a diacid and a diamine make a polyamide such as nylon. Proteins and starch are natural condensation polymers.

Plastics are useful because they last, which is also the problem. Addition polymers' all-single-bond carbon chains are very unreactive and break down slowly in the environment. Recycling, burning for energy and designing plastics that break down are all part of what chemists and chemical engineers work on.