35.1 Condensation polymerisation
- Syllabus
- 9701–2028–2029
- Topic
- 35.1
- Level
- A2
Learning objectives
35.1.1Formation of polyesters• Describe the formation of polyesters:: (a) the reaction between a diol and a dicarboxylic acid or dioyl chloride; (b) the reaction of a hydroxycarboxylic acid35.1.2Formation of polyamides• Describe the formation of polyamides:: (a) the reaction between a diamine and a dicarboxylic acid or dioyl chloride; (b) the reaction of an aminocarboxylic acid; (c) the reaction between amino acids35.1.3The repeat unit of a condensation polymer• Deduce the repeat unit of a condensation polymer obtained from a given monomer or pair of monomers35.1.4Monomer(s) present• Identify the monomer(s) present in a given section of a condensation polymer molecule
Polyesters form by repeated condensation at two reactive groups
A polyester contains repeated ester links, -CO-O-. It forms when monomers each provide two chain-extending functional groups, so condensation can repeat rather than stop after one small ester forms.
| Monomer route | Link-forming groups | Small molecule eliminated |
|---|---|---|
| diol + dicarboxylic acid | -OH + -COOH | H2O |
| diol + dioyl chloride | -OH + -COCl | HCl |
| hydroxycarboxylic acid self-condensation | -OH + -COOH on each molecule | H2O |
Ethane-1,2-diol plus benzene-1,4-dicarboxylic acid gives a polyester with repeating section -O-CH2-CH2-O-CO-C6H4-CO-. Both ends of each monomer participate as the chain grows.
A monofunctional alcohol plus a monofunctional acid makes a small ester and stops. Do not claim water is always lost: using a dioyl chloride eliminates HCl.
Polyamides form from diamines or from monomers bearing both NH2 and COOH
A polyamide contains repeated amide links, -CO-NH-. Chain growth requires an amino group to condense repeatedly with a carboxylic-acid or acyl-chloride group.
| Monomer route | Link-forming groups | Small molecule eliminated |
|---|---|---|
| diamine + dicarboxylic acid | -NH2 + -COOH | H2O |
| diamine + dioyl chloride | -NH2 + -COCl | HCl |
| aminocarboxylic acid self-condensation | -NH2 + -COOH on each molecule | H2O |
| amino acids condense | amino and carboxyl groups on amino-acid residues | H2O |
Hexane-1,6-diamine and hexanedioic acid form a nylon-type polyamide containing -NH-(CH2)6-NH-CO-(CH2)4-CO- repeats. Amino-acid condensation gives peptide links, which are amide links.
The two-monomer route pairs a diamine with a dicarboxylic acid or dioyl chloride; it is not two 'amino-functional' monomers. A single aminocarboxylic acid works because each molecule contains both required groups.
Build a condensation-polymer repeat unit by linking every functional end
From the given monomer or pair: 1) mark both reactive ends; 2) remove OH from -COOH and H from -OH/-NH2, or remove Cl from -COCl and H from -OH/-NH2; 3) join the remaining atoms through -CO-O- or -CO-NH-; 4) preserve every carbon skeleton; 5) choose the smallest section whose repetition recreates the chain and put continuation bonds through the brackets.
| Functional groups joined | Link in repeat unit | Condensate |
|---|---|---|
| -COOH + -OH | -CO-O- | H2O |
| -COCl + -OH | -CO-O- | HCl |
| -COOH + -NH2 | -CO-NH- | H2O |
| -COCl + -NH2 | -CO-NH- | HCl |
With an AA + BB pair such as a diol and diacid, one repeat normally contains the carbon skeleton of each monomer. With an AB monomer such as HO-R-COOH or H2N-R-COOH, one repeat contains one monomer skeleton after loss at its two linking ends.
Check that repeating the bracketed section reconstructs every linkage and carbon in the chain. Do not draw terminal -OH, -COOH or -NH2 groups inside an ideal repeat unit unless they are part of the monomer skeleton rather than chain ends.
Recover condensation monomers by cutting links and restoring end groups
From a polymer section: 1) identify each ester -CO-O- or amide -CO-NH- link; 2) cut the acyl C-O or C-N bond that formed during condensation; 3) restore -COOH (or recognise -COCl as the alternative acyl monomer) on the carbonyl fragment and -OH or -NH2 on the other fragment; 4) extend to neighbouring repeats before deciding whether the result is two bifunctional monomers or one AB-type monomer.
| Polymer link | Restore on carbonyl side | Restore on other side | Likely monomer families |
|---|---|---|---|
| -CO-O- | -COOH or -COCl | -OH | diacid/dioyl chloride + diol, or hydroxycarboxylic acid |
| -CO-NH- | -COOH or -COCl | -NH2 | diacid/dioyl chloride + diamine, or aminocarboxylic acid/amino acid |
Cutting -O-CH2CH2-O-CO-C6H4-CO- at its ester links and restoring ends gives ethane-1,2-diol plus benzene-1,4-dicarboxylic acid (with the corresponding dioyl chloride as the acyl-chloride alternative).
Cut the bond within the ester or amide link, not a C-C bond in a monomer skeleton. Do not label every oxygen as an alcohol: carbonyl-side oxygen belongs to the acid/acyl fragment, while the single-bond O of -CO-O- reconnects to the alcohol fragment.