A condensation polymer is built by acyl substitution — esterification or amide formation — run repeatedly on monomers that each carry two reactive groups. A small molecule is expelled at every join, which is what the word condensation means and what distinguishes these from addition polymers.
Polyesters
A diacid plus a diol gives a polyester, losing water at each ester bond. The one to know is PET — polyethylene terephthalate — from terephthalic acid and ethylene glycol. It is the plastic of drinks bottles and of polyester fabric, and it is the most recycled plastic there is, partly because the ester bonds can be hydrolyzed back to the monomers.
The ester group is what makes that possible and is also its weakness: a polyester is attacked by strong acid or base, which is why it can be depolymerized deliberately and why it degrades in the wrong conditions.
Polyamides
A diacid plus a diamine gives a polyamide, losing water at each amide bond. Nylon 6,6 comes from hexamethylenediamine (six carbons) and adipic acid (six carbons), which is where the name comes from. Read the numbers in that order: in nylon X,Y the first number counts the carbons in the diamine and the second counts the carbons in the diacid. Nylon 6,6 hides the distinction, but nylon 6,10 does not — that is hexamethylenediamine with the ten-carbon sebacic acid, not the other way around.
And because amides are the least reactive acyl derivative, a polyamide is far more resistant to hydrolysis than a polyester — which is why polyester fabric weakens in hot alkali faster than nylon does. Be careful not to turn that into a blanket ranking: the two fail under different conditions. Nylon is the one attacked by acid, by chlorine and by sunlight, while PET shrugs off acid and bleach and gives way to base.
Kevlar, and what aromatic rings buy
Kevlar is a polyamide from terephthalic acid and para-phenylenediamine — both aromatic, both para-substituted. Three things follow, and they compound:
- The rings are flat and rigid, so the chain cannot coil.
- Para substitution makes the chain straight, so chains lie against each other along their full length.
- Every amide contributes a hydrogen bond between neighboring chains, and a straight chain lines those up in sheets.
The result is a material stronger than steel by weight. Nothing exotic is happening — it is amide hydrogen bonding, the same interaction that holds a beta sheet together, multiplied by perfect alignment.
Polycarbonates and polyurethanes
A polycarbonate has a carbonate linkage, O–CO–O, joining two oxygens to one carbonyl. It is transparent and extremely impact-resistant, which is why it makes safety glasses and bulletproof windows.
A polyurethane comes from a diol plus a diisocyanate, and the carbamate link it forms is the one genuine exception in this section: nothing is lost. The alcohol simply adds across the N=C=O group. So a polyurethane is a step-growth polymer that is technically not a condensation, which is why the broader name for this class is step-growth rather than condensation.
Why the kinetics feel different
In an addition polymerization, a growing chain is either active or dead, and active chains reach full length almost immediately. In a step-growth, any two pieces can join, so the mixture goes through a long stage of dimers and trimers before anything long exists.
What carries forward
Diacid plus diol gives a polyester; diacid plus diamine gives a polyamide; both lose water. Amides resist hydrolysis better than esters, which is why nylon outlasts polyester. Aromatic para-substituted monomers give rigid, perfectly aligned chains and therefore Kevlar. And the 1/(1−p) relationship explains why these reactions are run to a completeness that would be pointless anywhere else.