Two monomers, each with a reactive group at both ends, join head to tail and spit out a small molecule at every link. Diols and dicarboxylic acids give polyesters such as PET; diamines and dicarboxylic acids give polyamides such as nylon-6,6, Kevlar and proteins.
In condensation polymerisation monomers join with the loss of a small molecule at every link: H2O when a carboxylic acid is used, HCl when an acyl chloride is used. Each monomer needs two reactive groups. A diol plus a dicarboxylic acid (or diacyl dichloride) gives a polyester, for example PET (Terylene) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid; a single hydroxycarboxylic acid such as lactic acid can also polymerise on its own, giving poly(lactic acid). A diamine plus a dicarboxylic acid or diacyl dichloride gives a polyamide, for example nylon-6,6 from hexane-1,6-diamine and hexanedioic acid, or Kevlar from aromatic monomers. Proteins are natural polyamides made from amino acids, joined by peptide (amide) links. The repeat unit contains one of each monomer residue, so its Mr is the sum of the monomers minus the small molecules lost: 116 + 146 − 2(18) = 226 for nylon-6,6.
Watch the diacid and diol join in the simulator and note the water that leaves. A polyester forms by the same reaction, repeated at both ends of each monomer.
| Polyester | Monomer(s) | Repeat unit | Small molecule lost |
|---|---|---|---|
| PET (Terylene) | HOCH2CH2OH + HOOCC6H4COOH | –OCH2CH2OOCC6H4CO– | H2O |
| PET from the acyl chloride | HOCH2CH2OH + ClOCC6H4COCl | the same | HCl |
| poly(lactic acid), PLA | CH3CH(OH)COOH only | –OCH(CH3)CO– | H2O |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What type of compound must be reacted with a dicarboxylic acid to make a polyester?
Which small molecule is eliminated when hexane-1,6-diamine reacts with hexanedioyl dichloride to make nylon-6,6?
Which linkage joins the monomers in nylon-6,6 and in proteins?
Lactic acid, CH3CH(OH)COOH, can form a polymer on its own. Why?
The amide link forms when an NH2 group reacts with a COOH or COCl group. The same link holds nylon, Kevlar and every protein together.
| Polyamide | Monomers | Repeat unit Mr |
|---|---|---|
| nylon-6,6 | H2N(CH2)6NH2 + HOOC(CH2)4COOH (or ClOC(CH2)4COCl) | 226 |
| Kevlar | H2NC6H4NH2 + ClOCC6H4COCl (both 1,4-substituted) | 238 |
| nylon-6 | H2N(CH2)5COOH, one monomer | 113 |
| protein | many different amino acids, H2NCHRCOOH | varies |
In a condensation polymer the repeat unit contains one residue of each monomer and the links inside the chain: do not leave H on the N end and OH on the C=O end of a repeat unit. Every link releases one small molecule, so forming n repeat units of nylon-6,6 releases about 2n H2O, not n. With a diacyl dichloride the molecule lost is HCl, not water.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
What is the Mr of the repeat unit of nylon-6,6? (Mr: hexane-1,6-diamine 116, hexanedioic acid 146, water 18)
A sample of nylon-6,6 has an average Mr of 45 200. How many repeat units are there in an average chain?
Which pair of monomers forms Kevlar?
What mass of PET (repeat unit Mr 192) can be made from 16.6 g of benzene-1,4-dicarboxylic acid (Mr 166) and excess ethane-1,2-diol?
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