Ch 3 · Degradable polymers · AS & A Level Chemistry 9701

Topic 3 of 3 in Polymerisation — AS & A Level Chemistry 9701 — 48 lessons.

Introduction

Once discarded, different polymers behave very differently in the environment: some persist for centuries while others break down under sunlight or by hydrolysis, and choosing between them involves weighing durability in use against the environmental cost of disposal.

Key ideas — Poly(alkenes) resist biodegradation

Poly(alkenes) such as poly(ethene) and poly(propene) have backbones made entirely of strong, non-polar mathrmC-C and mathrmC-H single bonds. No micro-organism produces an enzyme able to attack these unreactive bonds, so poly(alkenes) are not broken down biologically and persist in landfill and the oceans for decades or longer, accumulating as environmental waste.

🧑‍🏫 Teacher's explanation

Imagine a long, unbreakable chain made of smooth, non-sticky plastic beads. If you throw this chain into a forest, the bacteria and fungi (the 'recyclers' of nature) won't be able to grab onto it or break it apart because there are no 'handles' or chemical 'hooks' for their enzymes to grab.

📖 Definitions

🔍 Worked example — Easy

Explain why poly(ethene) is considered non-biodegradable.

🔍 Worked example — Standard

A student claims that poly(propene) can be degraded by hydrolysis. Evaluate this statement.

🔍 Worked example — Exam-style

Compare the environmental persistence of poly(ethene) and poly(lactic acid) (a polyester).

⚠️ Common errors

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🎯 Exam question

Key ideas — Photodegradable polymers

Some polymers contain bonds, or additives such as light-sensitive carbonyl groups built into the backbone, that absorb ultraviolet radiation in sunlight; absorbing this energy breaks the weakened bond homolytically, fragmenting the long polymer chain into much shorter pieces. This process, photodegradation, does not require micro-organisms and can be deliberately built into a poly(alkene) by copolymerising it with a small proportion of a carbonyl-containing monomer, making an otherwise inert plastic break down more readily in sunlight.

🧑‍🏫 Teacher's explanation

Imagine a long, heavy steel chain holding a gate shut. If you keep the chain intact, it stays strong for years. But if you use a chain made of links that become brittle and snap when exposed to sunlight, the gate will eventually fall. This is how photodegradation works. Most common plastics, like poly(ethene), are made of long, incredibly stable carbon-carbon chains that act like a continuous, unbreakable rope. Because they lack 'weak points,' bacteria cannot easily eat them, leading to plastic pollution.

📖 Definitions

🔍 Worked example — Easy

Explain why poly(ethene) is considered environmentally persistent compared to a polyester.

🔍 Worked example — Standard

A polymer is synthesized by copolymerising ethene with a small amount of a monomer containing a carbonyl group (C=O). Describe the mechanism of degradation when exposed to sunlight.

🔍 Worked example — Exam-style

Calculate the percentage mass increase of a sample of poly(lactic acid) (PLA) if it undergoes hydrolysis where 10% of the ester bonds are broken, assuming the fragments released are small organic acids. (Note: This is a conceptual qualitative check for stoichiometry in degradation). If a polymer chain of M_r = 1000 is cleaved into two fragments of M_r = 500 by the addition of H_2O, what is the total mass of the products?

⚠️ Common errors

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🎯 Exam question

(a) Explain why Plastic A is more likely to persist in the environment than Plastic B. [2] (b) Suggest how the addition of a small amount of a carbonyl-containing monomer to Plastic A would change its environmental impact. [2] [4 marks]

Key ideas — Hydrolytic degradation of polyesters and polyamides

Unlike poly(alkenes), polyesters and polyamides contain a polar, hydrolysable linkage at every repeat unit: the ester linkage, -mathrmCO-O-, and the amide linkage, -mathrmCO-NH-, are both susceptible to hydrolysis by aqueous acid or aqueous alkali, which attacks the linkage and splits the chain back into shorter fragments and eventually the original monomers. Acidic hydrolysis reforms the carboxylic acid and the alcohol (or amine); alkaline hydrolysis instead forms the carboxylate salt. Because this hydrolysis can be carried out by naturally occurring moisture, acids and, in some cases, microbial enzymes, polyesters and polyamides are generally biodegradable, though the rate depends on the polymer's structure and the conditions of disposal.

🧑‍🏫 Teacher's explanation

Imagine a long chain of paperclips linked together. If you want to break that chain, you need to find a way to snap the individual loops. In chemistry, poly(alkenes) like poly(ethene) are like a chain of smooth, non-stick plastic beads; there is no 'weak point' for water or chemicals to grab onto, making them very stable and hard to break down.

📖 Definitions

🔍 Worked example — Easy

Identify the functional group in a polymer that makes it susceptible to hydrolysis and state the products of the hydrolysis of a polyester using water.

🔍 Worked example — Standard

A polyamide is hydrolyzed using an aqueous acid. Write the balanced chemical equation for the hydrolysis of a generic amide linkage (-CONH-) to show the formation of a carboxylic acid and an amine.

🔍 Worked example — Exam-style

Compare the environmental persistence of poly(ethene) and poly(lactic acid) (a polyester). Explain why one is more degradable than the other.

⚠️ Common errors

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🎯 Exam question

(a) Explain why Polymer Y is more susceptible to degradation by water than Polymer X. [2] (b) Predict the products formed when Polymer Y undergoes hydrolysis in the presence of aqueous sodium hydroxide (NaOH). [2] [4 marks]

Key ideas — Evaluating environmental impact

A non-degradable poly(alkene) is chemically stable and durable in use, which is valuable for packaging and construction, but this same stability means it accumulates as long-lived waste, harms wildlife through ingestion or entanglement, and — if it does break down physically — fragments into persistent microplastics rather than disappearing chemically. A degradable polyester or polyamide breaks down into smaller, non-toxic fragments over a realistic timescale, reducing long-term accumulation, but degradable plastics can be less durable in use, may require specific industrial composting conditions (controlled temperature and moisture) to degrade within a useful time, and photodegradable poly(alkenes) still fragment into microplastics rather than being removed from the environment as molecules.

🧑‍🏫 Teacher's explanation

Think of polymers like construction materials. A poly(alkene) like poly(ethene) is like a solid block of granite; it is incredibly tough and doesn't react with water or acid, making it great for a water bottle, but terrible for the environment because it will sit in a landfill for centuries. It doesn't 'go away'; it just breaks into smaller and smaller pieces of plastic (microplastics).

📖 Definitions

🔍 Worked example — Easy

Compare the chemical stability of poly(ethene) and poly(lactic acid) (a polyester) when exposed to water.

🔍 Worked example — Standard

A student argues that photodegradable plastics are a perfect solution to plastic pollution. Evaluate this statement based on the mechanism of degradation.

🔍 Worked example — Exam-style

A polymer contains the repeating unit -[NH-CH_2-CO]-. Identify the type of polymer and the specific reaction that would degrade it.

⚠️ Common errors

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🎯 Exam question

Worked example

Question. A drinks bottle can be made from poly(ethene) or from poly(lactic acid), a biodegradable polyester. (a) Explain why poly(ethene) does not readily biodegrade in landfill. [2] (b) Explain how poly(lactic acid) can be broken down by hydrolysis. [2] (c) Suggest one advantage and one disadvantage of replacing poly(ethene) bottles with poly(lactic acid) bottles. [2]

Ch 3 · Degradable polymers · AS & A Level Chemistry 9701

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