Poly(alkenes) are chemically inert, which is why a plastic bag can outlive the person who used it. Polyesters and polyamides have a weak spot, the ester or amide link, that water, acids, alkalis and enzymes can attack, and C=O groups let some polymers fall apart in sunlight.
Poly(alkenes) such as poly(ethene) contain only strong, non-polar C–C and C–H bonds, so they are chemically inert and non-biodegradable: they persist in landfill and the oceans. Polyesters and polyamides contain polar C=O groups in their ester and amide links, so they can be hydrolysed by hot aqueous acid, alkali or by enzymes in microorganisms; they are biodegradable. Acid hydrolysis gives the free acid and the alcohol or the ammonium salt of the amine; alkaline hydrolysis gives the carboxylate salt and the alcohol or free amine. Photodegradable polymers contain C=O groups that absorb ultraviolet radiation; the energy breaks nearby bonds in the chain so the polymer crumbles. Burning plastics releases CO2 and can release toxic gases such as HCl from PVC.
Hydrolyse the polyester chain in the simulator with water, acid and then alkali. The chain is broken in exactly the same way at every ester link.
| Polymer | Hot dilute acid, H+(aq) | Hot aqueous alkali, OH−(aq) |
|---|---|---|
| PET (polyester) | HOOCC6H4COOH + HOCH2CH2OH | −OOCC6H4COO− + HOCH2CH2OH |
| nylon-6,6 (polyamide) | HOOC(CH2)4COOH + +H3N(CH2)6NH3+ | −OOC(CH2)4COO− + H2N(CH2)6NH2 |
| poly(lactic acid) | CH3CH(OH)COOH | CH3CH(OH)COO− |
| poly(ethene) | no reaction: no polar link to attack | |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why is poly(ethene) not biodegradable?
Which type of reaction breaks down polyesters and polyamides?
Which functional group makes a polymer photodegradable?
Which gas is a particular problem when PVC is incinerated?
Not every polymer can be made hydrolysable. The alternatives each solve one problem and create another.
| Option | Advantage | Drawback |
|---|---|---|
| landfill | cheap and simple | poly(alkenes) persist for centuries; land is used up |
| incineration | energy recovered; volume removed | CO2 emitted; PVC gives toxic HCl, and dioxins if burnt badly |
| recycling | saves crude oil and energy | plastics must be sorted; quality falls each cycle |
| photodegradable polymers | C=O groups absorb UV and break the chain | need light: buried waste does not break down; fragments can become microplastics |
| biodegradable polyesters | broken down by microorganisms | need the right conditions; can contaminate recycling streams |
Poly(alkenes) are not biodegradable because their C–C and C–H bonds are non-polar and strong, not because they are "too big". Biodegradable and photodegradable are different: one needs hydrolysis (water, acid, alkali or enzymes) at ester or amide links, the other needs UV light absorbed by C=O groups. In alkaline hydrolysis the acid monomer appears as its carboxylate salt, and in acid hydrolysis of a polyamide the amine appears as its ammonium salt.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Nylon-6,6 is heated under reflux with dilute hydrochloric acid. What are the organic products?
PET is heated with excess aqueous sodium hydroxide. Which products form?
Which polymer is most likely to be biodegradable?
A photodegradable plastic bag is buried deep in a landfill site. Why might it not break down?
This skill is now recorded on your profile. Keep the chain going.