Addition Polymers

There are, at heart, only two ways to build a polymer chain, and this is one of them. In addition polymerization, also called chain-growth, a monomer with a reactive double bond gets kicked into an excited state, and from then on it’s an avalanche: monomers add to the growing chain’s active tip one after another, in a blistering sequence, nothing left behind. No water, no byproduct, no leftover fragment. What goes in as monomer comes out as chain, atom for atom. The whole reaction hangs on that active tip (a radical, a cation, an anion, or a metal center), which stays hungry until something quenches it.

This is the mechanism that built the plastics age at scale. Because the chains shoot up to enormous length almost instantly, addition polymerization is fast, high-yielding, and forgiving, perfect for churning out commodity plastics by the megatonne. It’s how we make polyethylene and polypropylene (via the coordination catalysts of Ziegler and Natta), polystyrene and PVC and the acrylics (via free radicals), and a great many rubbers and specialty chains besides. The unifying thread across this whole crowd isn’t what they’re made of (the members here span wildly different chemistries) but how they’re assembled: a reactive chain end, devouring double bonds.

Set this page beside its sibling, the condensation polymers, and you have the fundamental fork in the road of polymer synthesis. Nearly every chain in this Atlas took one path or the other.