Polymers: Properties and Disposal
A concise revision guide to the properties and disposal of polymers: why polyalkenes are chemically inert and non-biodegradable, why polyesters and polyamides can be hydrolysed, and polymers degraded by light.
Before You Start
What happens to a polymer after use depends on the bonds in its chain. This page compares polyalkenes with polyesters and polyamides.
- Condensation polymers, their ester and amide links and the forces between their chains: Condensation Polymers: Polyamides and Polyesters.
- Addition polymers from alkenes: Polymerisation Reactions.
- The earlier treatment of waste polymers: Polymer Waste.
- Hydrolysis of esters with acids and alkalis: Esters, Hydrolysis and Polyesters.
GCSE Recap: Monomers and Plastics
Three quick questions on the GCSE ideas this page builds on.
Polyalkenes Are Chemically Inert
Polyalkenes such as poly(ethene), poly(propene) and PVC are addition polymers. Their chains contain only C–C and C–H bonds, and that makes them chemically inert.
- C–C and C–H bonds are strong and non-polar: no carbon atom is δ+, so there is no site for a nucleophile such as water or OH⁻ to attack.
- Acids, alkalis, water and the enzymes of micro-organisms leave the chain untouched.
- Polyalkenes are therefore non-biodegradable: they persist in the environment for hundreds of years.
- This inertness is useful in service (food packaging, water pipes) but a problem after disposal.
Remember: Inert because the chain has only strong, non-polar C–C and C–H bonds: nothing for a nucleophile to attack, so no hydrolysis and no biodegradation.
Polyesters and Polyamides Can Be Hydrolysed
Condensation polymers carry polar links in the backbone: the ester link –COO– and the amide link –CONH–. These can be hydrolysed, so the chain breaks back into its monomers.
- The C=O, C–O and C–N bonds are polar. The carbonyl carbon is δ+ and is attacked by water or by hydroxide ions.
- Hydrolysis is slow in water alone but fast with hot aqueous acid or alkali.
- Enzymes from bacteria and fungi catalyse the same hydrolysis in soil or compost, so polyesters and polyamides are biodegradable.
- The same chemistry breaks down proteins and the ester links of fats in digestion.
| Polyalkenes | Polyesters and polyamides | |
|---|---|---|
| bonds in the chain | C–C and C–H only | ester or amide links as well |
| polarity | non-polar | polar C=O, C–O and C–N |
| attacked by water or OH⁻? | no | yes, at the δ+ carbonyl carbon |
| hydrolysed by acid or alkali? | no | yes |
| biodegradable? | no | yes |
Non-polar C–C and C–H bonds give an inert chain; polar ester and amide links give a chain that water and hydroxide ions can attack.
Exam focus: The explanation is the same both ways: polar links with a δ+ carbon can be attacked and hydrolysed; a non-polar carbon chain cannot.
Check: Inert or Hydrolysable?
Use the polarity of the bonds in a polymer chain to decide whether it resists attack or can be broken down.
Quick Check: Which Chains Can Be Hydrolysed?
Each repeat unit is heated with aqueous sodium hydroxide. Decide what happens to the backbone of the polymer.
Hydrolysis Equations
Hydrolysis reverses the condensation. With acid the monomers are recovered as the acid and the alcohol or the amine salt; with alkali the carboxylic acid ends up as its salt.
Polyesters
- Acid hydrolysis: heat the polyester with dilute H₂SO₄ or HCl under reflux. Each ester link breaks to give the dicarboxylic acid and the diol.
- Terylene → benzene-1,4-dicarboxylic acid, HOOC–C₆H₄–COOH, + ethane-1,2-diol, HOCH₂CH₂OH.
- Base hydrolysis: heat with NaOH(aq) under reflux. The acid forms as its sodium salt and the diol is released.
- Terylene + 2n NaOH → sodium benzene-1,4-dicarboxylate, Na⁺⁻OOC–C₆H₄–COO⁻Na⁺, + ethane-1,2-diol.
Polyamides
- Acid hydrolysis: heat with dilute HCl under reflux. The dicarboxylic acid is released and the diamine is protonated to its salt.
- Nylon-6,6 → hexanedioic acid, HOOC(CH₂)₄COOH, + hexane-1,6-diammonium chloride, ⁺H₃N(CH₂)₆NH₃⁺ 2Cl⁻.
- Base hydrolysis: heat with NaOH(aq) under reflux. The acid forms as its sodium salt and the diamine is released as the free amine.
- Nylon-6,6 + 2n NaOH → sodium hexanedioate, Na⁺⁻OOC(CH₂)₄COO⁻Na⁺, + hexane-1,6-diamine, H₂N(CH₂)₆NH₂.
Worked example
Question. A polyester has the repeat unit –O–CH₂CH₂–O–CO–(CH₂)₂–CO–. Give the products of its hydrolysis with hot sodium hydroxide solution.
Answer. Ethane-1,2-diol, HOCH₂CH₂OH, and sodium butanedioate, Na⁺⁻OOC(CH₂)₂COO⁻Na⁺.
Acid and base hydrolysis of a Terylene section and of a nylon-6,6 section: the acid gives the dicarboxylic acid, the base gives its salt.
Conditions and products of acid and alkaline hydrolysis for a polyester and a polyamide, and why polyalkenes are untouched.
Remember: Acid: dicarboxylic acid + diol, or dicarboxylic acid + diammonium salt. Alkali: dicarboxylate salt + diol, or dicarboxylate salt + diamine.
Quick Check: Hydrolysing a New Polyamide
Work out the products for a polyamide that is new to this page.
Polymers Degraded by Light
Some polymers are photodegradable: they break down when exposed to light, usually the ultraviolet part of sunlight.
- C=O groups built into the chain absorb UV radiation. The energy breaks bonds near the carbonyl group and the chain splits.
- Shorter chains are brittle, so the plastic crumbles into fragments that bacteria can attack more easily.
- Photodegradable plastics are made by adding carbonyl-containing monomers or light-sensitive additives to the polymer.
- Uses: agricultural mulch films and some carrier bags. Drawback: they do not degrade when buried away from light.
Ultraviolet light is absorbed by C=O groups built into the chain, the chain splits and the brittle film crumbles.
Remember: Photodegradable: C=O groups absorb UV light and the chain breaks. Biodegradable: polar ester or amide links are hydrolysed by acids, alkalis or enzymes.
Check: Breaking Polymers Down
Decide which polymers can be broken back down to small molecules and which persist, and give the reasons.
Common Exam Points
- Polyalkenes are chemically inert because their C–C and C–H bonds are strong and non-polar: no δ+ carbon for a nucleophile to attack.
- Polyesters and polyamides have polar C=O, C–O and C–N bonds; water or OH⁻ attacks the δ+ carbonyl carbon, so they can be hydrolysed.
- Hydrolysable polymers are biodegradable; polyalkenes are not.
- Acid hydrolysis gives the dicarboxylic acid with the diol or the diammonium salt; alkaline hydrolysis gives the carboxylate salt with the diol or the diamine.
- Conditions: heat under reflux with dilute acid or with NaOH(aq).
- Photodegradable polymers contain C=O groups that absorb UV light, breaking the chain.
- Explain every answer through the polarity of the bonds in the chain, not through the name of the polymer alone.
FAQs
Use these quick answers to check why polyalkenes are chemically inert, why polyesters and polyamides can be hydrolysed and the products of their hydrolysis, and photodegradable polymers.
Why are polyalkenes not biodegradable?
Their backbone is made of strong, non-polar C–C and C–H bonds.
There is no δ+ carbon for a nucleophile such as water or a hydroxide ion to attack, so acids, alkalis and bacteria cannot break the chain. Poly(ethene) and PVC therefore persist for many years.
Why can polyesters and polyamides be hydrolysed?
The ester and amide links contain a polar C=O: the carbonyl carbon is δ+ and open to attack by water, hydroxide ions or acid. Hydrolysis breaks the link and reverses the condensation reaction, so these polymers are biodegradable and bacteria in soil break them down.
What are the products of hydrolysing a polyester?
With hot dilute acid the ester links break to give the dicarboxylic acid and the diol: Terylene gives benzene-1,4-dicarboxylic acid and ethane-1,2-diol. With hot sodium hydroxide the dicarboxylate salt, sodium benzene-1,4-dicarboxylate, forms with the diol.
How is hydrolysing a polyamide different from hydrolysing a polyester?
A polyamide link gives a carboxylic acid group and an amine group. In acid the amine is protonated, so the products are the dicarboxylic acid and the diammonium salt; in alkali they are the dicarboxylate salt and the diamine. A polyester gives a diol instead.
What is a photodegradable polymer?
One that is broken down by light. C=O groups in the chain absorb ultraviolet radiation and the energy breaks nearby bonds, so the chains shorten and the plastic crumbles. Some polymers are made with these groups or additives deliberately, so litter degrades in sunlight.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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