condensation polymerization

Now imagine linking train cars, where every time you couple two cars together a small bolt pops out and drops on the track. You still build one long train, but each connection sheds a little piece. Condensation polymerization works like that: two monomers with reactive ends join to form a bond, and each join throws off a small molecule — most often water. The chain grows steadily, one link at a time, leaving a trail of little byproduct molecules behind.

The mechanism is step growth. You start with monomers that carry reactive groups on their ends — say a diamine with -NH2 groups and a diacid with -COOH groups. An -NH2 reacts with a -COOH to make an amide bond and release a molecule of water; do this over and over and you build nylon. Because a small molecule leaves at every join, the mer weighs less than the monomers that made it. Polyesters like PET, nylons, and epoxies are all condensation polymers, and the same chemistry builds proteins in your body from amino acids.

It matters because the process is fundamentally different from addition: it is slow, chains lengthen gradually, and you only reach high molecular weight when the reaction is driven almost to completion — which usually means continuously removing the water byproduct so the reaction keeps going forward. One honest caveat: because average chain length climbs so steeply near the very end of the reaction, small losses of conversion or an imbalance of the two monomers can leave you stuck with short, weak chains. Contrast addition polymerization, which is a fast chain reaction with no byproduct at all.