Polymerization occurs through two distinct chemical pathways: addition and condensation.

Chemists group polymerization reactions into two main families. The first is addition polymerization, where monomers with double bonds simply open up and link together. No atoms are lost. It is clean and direct, like Lego bricks snapping together. This is how we get polyethylene for plastic bags. The second is condensation polymerization. Here, molecules link up by sharing a handshake that spits out a tiny byproduct—usually a molecule of water or alcohol. Nylon and polyester are made this way. For every link forged in a nylon chain, a molecule of water is cast aside.