addition polymerization

Imagine a line of people who each want to hold the next person's hand, but each person's hand is 'locked' until someone touches it. Touch the first person and they unlock, grab the next, which unlocks that one, and so on down the line in a fast chain reaction until you run out of people. Addition polymerization works just like that: monomers with a locked-up double bond are opened one after another and clicked directly onto the growing chain, with nothing dropped off along the way.

The mechanism runs in three steps. In initiation, an initiator molecule breaks into a highly reactive free radical (an unpaired electron looking for a partner) that opens the double bond of one monomer. In propagation, that opened monomer now has its own reactive end, so it grabs the next monomer, then the next, adding thousands per second and growing the chain explosively. In termination, two growing chains meet and cap each other off. Because monomers simply add on with no byproduct, the mer has exactly the same atoms as the monomer: ethylene (CH2=CH2) becomes the -CH2-CH2- mer of polyethylene, every atom kept.

This is how the big-volume commodity plastics are made: polyethylene, polypropylene, PVC, and polystyrene all come from addition polymerization of a double-bonded monomer. It matters because it is fast, needs only a trace of initiator, and wastes almost no atoms. One honest point: because growth depends on a reactive radical, tiny amounts of oxygen or impurities can kill a chain early, and controlling the average chain length is part of the art. Contrast it with condensation polymerization, which builds chains slowly, step by step, and spits out a small molecule at every join.