35. Polymerisation
- Syllabus
- 9701–2028–2029
- Section
- 35
- Level
- A2
Exam analysis
35.1 Condensation polymerisation
- Syllabus
- 9701–2028–2029
- Topic
- 35.1
- Level
- A2
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.
35.2 Predicting the type of polymerisation
- Syllabus
- 9701–2028–2029
- Topic
- 35.2
- Level
- A2
Predict polymerisation type from the monomer's chain-forming groups
Inspect every monomer for groups that can form a chain. A polymerisable C=C can open and join monomers by addition. A complementary pair of bifunctional groups can react repeatedly by condensation to form ester or amide links while eliminating a small molecule.
| Monomer evidence | Predicted route | Chain-forming event | Small molecule |
|---|---|---|---|
| one or more alkene monomers, using C=C | addition | C=C opens to make new C-C backbone bonds | none |
| diol + diacid/dioyl chloride, or hydroxycarboxylic acid | condensation | repeated ester-link formation | H2O or HCl |
| diamine + diacid/dioyl chloride, aminocarboxylic acid or amino acids | condensation | repeated amide-link formation | H2O or HCl |
If a monomer contains both C=C and condensation-capable groups, the structure may support more than one possible route. Identify which groups the proposed polymerisation uses rather than classifying from the mere presence of one familiar group.
The number of different monomers does not decide the type: two different alkenes can form an addition copolymer, while one hydroxycarboxylic or aminocarboxylic acid can self-condense.
Deduce polymerisation type from links in the polymer backbone
Trace the main polymer backbone through more than one repeat. A continuous C-C backbone produced from opened alkene bonds indicates addition polymerisation. Repeated ester -CO-O- or amide -CO-NH- links within the backbone indicate condensation polymerisation.
| Polymer-section evidence | Deduction | Reason |
|---|---|---|
| carbon-carbon backbone with substituents hanging from it | addition | alkene pi bonds opened; no small molecule was eliminated |
| ester links in backbone | condensation, polyester | alcohol and acid/acyl-chloride ends joined |
| amide/peptide links in backbone | condensation, polyamide | amine and acid/acyl-chloride ends joined |
Judge the backbone, not side chains. An addition polymer may carry -CN, halogen, aryl or other functional substituents without those groups lying in the chain-forming backbone.
Do not rely on the polymer name or on oxygen/nitrogen appearing anywhere. Locate the repeated backbone linkage and show how it could arise from C=C opening or small-molecule-eliminating condensation.
35.3 Degradable polymers
- Syllabus
- 9701–2028–2029
- Topic
- 35.3
- Level
- A2
Poly(alkenes) are chemically inert and often persist in the environment
In a poly(alkene), the monomer C=C bonds have become strong C-C single bonds in a saturated, largely non-polar carbon backbone. With no ester or amide link to hydrolyse, the material is relatively chemically inert.
This stability gives useful durability but also makes many poly(alkenes) difficult for organisms and ordinary environmental conditions to biodegrade.
Poly(ethene) has no hydrolysable backbone functional group. A polyester or polyamide, by contrast, contains polar backbone links that can be cleaved under suitable conditions.
Chemically inert does not mean indestructible or unable to react under extreme conditions; it means resistant under the ordinary biological and environmental conditions relevant to biodegradation.
Light can break down some polymers by photodegradation
Some polymers absorb light energy, especially ultraviolet light, and undergo bond breaking that shortens or fragments their chains. This light-driven breakdown is photodegradation.
Whether it occurs depends on the polymer structure, exposure to light and additives such as stabilisers. A polymer buried away from light may not photodegrade effectively.
Chain scission can make a material weaker or brittle and produce smaller fragments. This is evidence of degradation by light, but it does not by itself prove complete biological conversion to simple products.
Do not equate visible fragmentation with complete biodegradation: the syllabus claim is that some polymers can be degraded by light, not that all polymers disappear fully in sunlight.
Acidic and alkaline hydrolysis cleave polyester and polyamide chains
Polyesters contain ester links and polyamides contain amide links in their backbones. Aqueous acid or aqueous alkali can hydrolyse these links, cutting long chains into smaller molecules, so these polymer classes are biodegradable by acidic and alkaline hydrolysis.
| Polymer link | Acidic hydrolysis gives | Alkaline hydrolysis gives |
|---|---|---|
| polyester, -CO-O- | carboxylic acid + alcohol fragments | carboxylate salt + alcohol fragments |
| polyamide, -CO-NH- | carboxylic acid + protonated amine/ammonium fragments | carboxylate salt + amine/ammonia fragments |
Hydrolysis is possible because the chain contains susceptible ester or amide links. Poly(alkenes) lack these links, so the same acid/alkali route does not readily cut their saturated C-C backbone.
A hydrolysable link makes breakdown chemically possible, not automatically rapid in every environment. Temperature, pH, water access and polymer structure affect the rate.