Start with one molecule of terephthalic acid and one of ethylene glycol. The acid has a COOH group at each end, and the glycol has an OH group at each end. An esterification joins one COOH to one OH and releases a molecule of water. The product still has a free COOH at one end and a free OH at the other, so it can react again at both ends. Repeat that thousands of times and you have PET, the plastic of drinks bottles.
PET is a condensation polymer: a chain built from monomers that each carry two reactive groups, with a small molecule released at every join. The name condensation refers to that released molecule. Each join is an acyl substitution, the reaction that makes esters and amides (Nucleophilic acyl substitution). What a polymer is covers monomers, repeat units and how step-growth chains get long.
Polyesters
A diacid, with two COOH groups, plus a diol, with two OH groups, gives a polyester: a chain whose units are joined by ester groups. The one to know is PET, short for poly(ethylene terephthalate). The figure builds it from its two monomers.
PET is the plastic of drinks bottles and the fiber of polyester fabric. It is also one of the most widely recycled plastics. Most of that recycling is mechanical: used bottles are collected, washed, shredded and melted into new products. The ester bonds add a second route that an all-carbon chain such as polyethylene lacks. Under forcing conditions, water or an alcohol can cut every ester bond and take the chain back to its monomers, which can then make new PET.
Polyamides
Swap the diol for a diamine, a molecule with two NH2 groups, and each join becomes an amide. The chain is then a polyamide. Nylon 6,6 is made this way from hexamethylenediamine and adipic acid, and each of those monomers has six carbons.
Mixed cold, the two monomers only form a salt, as an alkylamine and a carboxylic acid do. Strong heating drives water out of the salt and turns each ammonium carboxylate pair into an amide bond.
The two sixes give the name. In nylon X,Y the first number counts the carbons in the diamine and the second counts the carbons in the diacid, carboxyl carbons included. Nylon 6,6 hides the order, but nylon 6,10 shows it: hexamethylenediamine, with six carbons, joined to sebacic acid, the ten-carbon diacid.
Step 1 — read the name. The first number belongs to the diamine, so 6 means hexamethylenediamine, H2N(CH2)6NH2. The second belongs to the diacid, so 10 means sebacic acid, HOOC(CH2)8COOH. The ten carbons include the two carboxyl carbons, so the run of CH2 between them is only eight. Missing that is the usual slip.
Step 2 — make one amide. An amine nitrogen bonds to an acid carbonyl carbon, and the OH leaves with an H from the nitrogen as water. That gives –NH(CH2)6NH–CO(CH2)8CO–.
Step 3 — bracket it. –[NH(CH2)6NH–CO(CH2)8CO]–n. The repeat unit has sixteen carbons and two amide bonds.
Step 4 — check by mass. The diamine, C6H16N2, is 116, and the diacid, C10H18O4, is 202, which makes 318 together. The repeat unit, C16H30N2O2, is 282. The difference of 36 is two waters, one per amide, so it balances.
The trap. Writing nylon 6,10 from decanediamine and adipic acid puts the same two numbers in the wrong slots, and that is a different polymer. The diamine always comes first.
Esters and amides under attack
Hydrolysis treats the two linkages differently. An amide is the least reactive acyl derivative, because the nitrogen lone pair donates into the C=O more strongly than an ester oxygen’s lone pair does (Esters and amides). So hot aqueous base cuts ester bonds much faster than amide bonds, and polyester fabric weakens in hot alkali sooner than nylon does.
That does not make nylon the tougher polymer overall. The two fail under different conditions. Strong acid, chlorine bleach and sunlight all damage nylon. PET stands up to acid and bleach in everyday use and gives way to hot base.
Nylon 6, and why one number is not two
Nylon 6 has a single number because it comes from a single monomer, 6-aminohexanoic acid, H2N(CH2)5COOH. That is a six-carbon chain with an amine at one end and a carboxylic acid at the other. One molecule carries both partners, so it can join to copies of itself, losing a water each time. Lactic acid does the same with its OH and COOH groups and gives the polyester PLA, poly(lactic acid).
Industrial nylon 6 comes from caprolactam, the lactam of 6-aminohexanoic acid. As What a polymer is shows, opening its ring loses nothing, yet the chain is a polyamide. Started by a strong base, it grows by chain-growth; the water-started industrial route is step-growth.
That order gives a nylon 6 chain a direction. Every unit is joined head to tail, so every amide in the chain reads the same way along it: N–H end first, then C=O. In nylon 6,6 the amides alternate, one reading NH–CO and the next CO–NH, so the chain reads the same from either end.
Direction decides how neighboring chains hydrogen-bond. In a flat sheet, two nylon 6 chains can pair every facing N–H with a C=O only when they run in opposite directions. Two nylon 6,6 chains pair fully whichever way round they lie. The figure shows the best fit in each case.
So nylon 6,6 chains line up more easily. That is the usual explanation for its higher melting point, about 265 °C against about 220 °C for nylon 6, though it is not the only factor.
Kevlar, and what aromatic rings buy
Kevlar is also a polyamide. Its diacid is terephthalic acid, the same one PET uses, and its diamine is para-phenylenediamine, a benzene ring with NH2 groups on opposite corners. Both monomers are aromatic and para-substituted.
Kevlar is not made from the free acid. An aromatic amine is a weak nucleophile, because its lone pair is spread into the ring (Amine structure and basicity), and a carboxylic acid is a weak electrophile. Together they cannot reach the near-complete conversion a strong fiber needs. So the acid goes in as its acid chloride, terephthaloyl chloride, and each join releases HCl instead of water.
The shapes of the two monomers decide the fiber’s properties:
- The rings are flat and rigid, so the chain cannot coil.
- Para substitution makes the chain straight, so neighboring chains lie against each other along their full length.
- Every amide makes a hydrogen bond to the next chain, and straight chains line those bonds up into sheets.
Compare nylon 6,6. The amide groups and the hydrogen bonds are the same, and only what sits between the amides differs. Nylon has a flexible run of CH2 groups that can twist out of line, so its chains lie straight only in patches. Kevlar has a para ring that cannot twist the chain out of line, so each chain stays straight from end to end and its hydrogen bonds add up all along it.
The result is a fiber stronger than steel for its weight.
Polycarbonates
Take bisphenol A, two phenol rings joined through one carbon that also carries two methyl groups. It is a diol, with both OH groups on rings. Its partner is phosgene, Cl2C=O, the diacid chloride of carbonic acid. The reaction runs in aqueous sodium hydroxide, where each OH loses its proton to give a phenoxide (Phenols). Each join is then an acyl substitution: the phenoxide oxygen attacks the carbonyl carbon and chloride leaves. Overall each join loses HCl, which the base neutralizes.
The product is a polycarbonate. Its linkage, O–C(=O)–O, is a carbonate: one carbonyl carbon with an oxygen on each side, so it is a diester of carbonic acid.
Newer plants avoid phosgene. They heat bisphenol A with diphenyl carbonate, and at each join a bisphenol oxygen replaces a phenoxy group on the carbonate and phenol is released. Swapping one alcohol part of an ester for another like this is a transesterification. The linkage is the same; only the leaving group differs.
The carbonate is also the weak point. Hydroxide or an amine attacks its carbonyl carbon, so polycarbonate slowly fails in hot alkali and can be broken down on purpose.
Polycarbonate is stiff because every repeat unit carries two rigid rings. It is also glass-clear. The central carbon is sp3, so the two rings leave it at an angle and its two methyl groups stick out. The chains therefore cannot pack into the ordered, crystal-like regions that make many plastics cloudy. A polymer whose chains stay disordered like this is called amorphous.
It stays rigid at room temperature because that is far below its glass transition, near 150 °C, the temperature above which its chain segments start to move and it softens. Structure and properties takes up chain packing and the temperatures at which polymers soften and melt. Stiff and clear together is exactly what safety glasses and bullet-resistant windows need.
Polyurethanes: nothing lost
An isocyanate is R–N=C=O, one carbon double-bonded to both a nitrogen and an oxygen. You met it as the intermediate of the Hofmann rearrangement (Amine synthesis). A diisocyanate carries two of them. Mix one with a diol, and each O–H adds across the C=N bond of an N=C=O group, so the chain grows. The link that forms, R–NH–C(=O)–O–R′, is a carbamate. Its older name, urethane, gives the polymer its name: polyurethane.
The isocyanate carbon sits between two electronegative atoms, so it is strongly electrophilic. The alcohol oxygen attacks it, and the electrons of the C=N π bond move onto nitrogen. That leaves the nitrogen negative and the oxygen positive. The nitrogen then takes the proton from the oxygen. This is nucleophilic addition to a C=X double bond, just like addition to a ketone, and nothing has to leave.
So nothing is released, and the repeat unit weighs exactly what one diol and one diisocyanate weigh together. A polyurethane still grows the step-growth way, with any two pieces able to join, but it is not a condensation. That is why the whole class is better called step-growth polymers.
Which one loses what
| Polymer | Monomers | New linkage | Lost |
|---|---|---|---|
| Polyester (PET) | Terephthalic acid + ethylene glycol | –CO–O– | H2O |
| Polyamide (nylon 6,6) | Adipic acid + hexamethylenediamine | –CO–NH– | H2O |
| Polyamide (nylon 6) | Caprolactam, ring-opening | –CO–NH– | nothing |
| Kevlar | Terephthaloyl chloride + p-phenylenediamine | –CO–NH– | HCl |
| Polycarbonate | Bisphenol A + phosgene | –O–CO–O– | HCl |
| Polyurethane | Diol + diisocyanate | –O–CO–NH– | nothing |
Read the last column before answering any question about this class. Two of the six lose nothing and two lose HCl rather than water, so “condensation loses water” describes only the first two rows. The small molecule depends on the monomers, not on the linkage. PET made from dimethyl terephthalate, the methyl diester of the acid, releases methanol instead of water. So a drawn chain cannot tell you what was lost; for that you have to go back to the monomers. What the six share is a C=O joined to O or N in the backbone, not the molecule lost.
Pushing a step-growth reaction to the end
What a polymer is showed that at conversion p, with equal amounts of the two groups, the average chain holds 1/(1−p) monomer units. The same page gave the three requirements that follow: exact stoichiometry, pure monomers, and continuous removal of the by-product. That removal matters wherever the join is an equilibrium: esterification, amide formation from the acid, and transesterification. The acid-chloride joins on this page (Kevlar, phosgene) do not reverse, so there is no equilibrium to drive. Their HCl still has to be neutralized, or it would protonate the amine or phenoxide that does the attacking.
The setup. The formula 1/(1−p) assumes the two kinds of group are present in exactly equal numbers. When they are not, let r be the ratio of the minority group to the majority one, and let p be the conversion of the minority group. The average chain then holds (1+r)/(1+r−2rp) monomer units.
Step 1 — take the best case. Even at full conversion, p = 1, the expression simplifies to (1+r)/(1−r).
Step 2 — put in the error. A 2% excess of the diol makes r = 0.98. Then (1+r)/(1−r) = 1.98/0.02 = 99.
The point. A 2% weighing error caps the chains near a hundred units however long the reaction runs. That is the arithmetic behind the words “exact stoichiometry”.
What carries forward
A diacid with a diol gives a polyester, and a diacid with a diamine gives a polyamide; the small molecule lost depends on the monomers. Amides resist hydrolysis better than esters, so polyester gives way first in hot base. Acid, chlorine and sunlight attack nylon instead: each polymer survives different conditions. Rigid para-linked aromatic monomers give the straight, fully hydrogen-bonded chains of Kevlar. And 1/(1−p) explains why these reactions are pushed almost to completion, far further than anyone would push a single esterification.