addition polymer

An addition polymer is a plastic made by snapping many small molecules together end to end, with nothing left over — every atom of the starting molecules ends up in the chain. Polyethylene (plastic bags, bottles), PVC (pipes), polystyrene (foam cups) and Teflon are all addition polymers. The starting small molecule is the monomer; the long chain is the polymer; and the secret is that the monomer carries a carbon-carbon double bond.

Here is how the chain grows, in plain steps. Take a monomer with a C=C double bond, like ethylene, CH2=CH2. An initiator opens one of the double bonds, leaving a reactive end — a free radical (or sometimes a cation or anion). That reactive end attacks the C=C of the next monomer, opening its double bond and adding it to the chain while creating a fresh reactive end. That end attacks the next monomer, and the next, thousands of times — a chain reaction, which is why this is called chain-growth polymerization. This is exactly the addition-to-an-alkene chemistry you already know (and often the radical chain mechanism), repeated over and over. Crucially, no small molecule is expelled: -CH2=CH2- units simply link into -CH2-CH2-CH2-CH2-, so the polymer's empirical formula matches the monomer's. That makes addition polymerization a high-atom-economy process.

Addition polymers matter because they are most of the everyday plastics, and they show alkene chemistry working at industrial scale. Their properties are tuned by changing the substituent on the monomer: a hydrogen gives flexible polyethylene, a chlorine gives rigid PVC, a benzene ring gives stiff polystyrene, fluorines give slippery Teflon. Same chain-growth trick, different dangling group, wildly different material — the polymer chemist's most basic lever.