Every year, millions of tonnes of plastics are manufactured. Understanding the chemical properties that govern their stability: and their ability to decompose: is crucial for developing sustainable waste management solutions.
🔑 Key Principle
The differences in biodegradability between addition and condensation polymers arise directly from their chemical structures. Addition polymers consist of strong, non-polar carbon carbon single bonds, whereas condensation polymers contain polar ester or amide links that can be targeted by chemical reagents and biological enzymes.
Biodegradability Comparison
We can classify and contrast the stability of polymer chains based on their chemical composition:
A polymer that can be broken down into small, non-toxic molecules by the action of living organisms: typically bacteria or fungi: over a reasonable timeframe.
1. Addition Polymers (Non-Biodegradable)
Addition polymers like poly(ethene), poly(propene), and PVC are made from alkenes. Their main backbone consists entirely of carbon carbon single bonds (\\( \text{C-C} \\)).
- No polar bonds: The carbon carbon bonds in the backbone are completely non-polar. This means they are not susceptible to nucleophilic attack by water or enzymes.
- Chemically inert: Addition polymers are highly unreactive. Since they lack any functional groups, decomposer organisms cannot break them down. Consequently, they remain in landfills for hundreds of years.
2. Condensation Polymers (Biodegradable)
Condensation polymers like polyesters (Terylene, PLA) and polyamides (Nylon, Kevlar) contain polar carbonyl groups (\\( \text{C=O} \\)) adjacent to oxygen or nitrogen atoms.
- Polar bonds: The carbonyl carbon carries a partial positive charge (\\( \delta^+ \\)) because oxygen is more electronegative. This makes it vulnerable to attack by nucleophiles: such as water (\\( \text{H}_2\text{O} \\)) or hydroxide ions (\\( \text{OH}^- \\)).
- Hydrolysis: The ester and amide linkages can be broken down using water in a process called hydrolysis. In the environment, enzymes produced by microorganisms act as catalysts to speed up this process, making condensation polymers biodegradable.
Hydrolysis Reactions
Hydrolysis can be catalysed by acids or bases. You must be able to write the structural products formed under both conditions:
The cleavage of an organic bond using water in the presence of an aqueous acid catalyst (e.g. dilute \\( \text{HCl} \\)).
The cleavage of an organic bond using water in the presence of an aqueous base (e.g. dilute \\( \text{NaOH} \\)). In this reaction, carboxylic acids are converted to their carboxylate salt forms.
- Acid Hydrolysis of Polyamides: Produces the dicarboxylic acid and the protonated diamine salt (e.g. \\( \text{H}_3\text{N}^+\text{-R-NH}_3^+ \\)). \[ \text{[-CO-R-CO-NH-R'-NH-]} + 2\text{H}_2\text{O} + 2\text{H}^+ \rightarrow \text{HOOC-R-COOH} + \text{H}_3\text{N}^+\text{-R'-NH}_3^+ \]
- Base Hydrolysis of Polyamides: Produces the dicarboxylic acid salt (carboxylate salt, e.g. \\( \text{Na}^+ \text{ } ^-\text{OOC-R-COO}^- \text{Na}^+ \\)) and the free diamine. \[ \text{[-CO-R-CO-NH-R'-NH-]} + 2\text{OH}^- \rightarrow \text{^-OOC-R-COO}^- + \text{H}_2\text{N-R'-NH}_2 \]
- Base Hydrolysis of Polyesters: Produces the dicarboxylic acid salt and the free diol. \[ \text{[-CO-R-CO-O-R'-O-]} + 2\text{OH}^- \rightarrow \text{^-OOC-R-COO}^- + \text{HO-R'-OH} \]
Step 1: Identify the link type. The polymer is a polyester, containing ester links (\\( \text{-CO-O-} \\)).
Step 2: Cleave the ester bonds. Breaking the ester bonds yields the starting monomers: benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
Step 3: Adjust for alkaline (NaOH) conditions.
- Ethane-1,2-diol is an alcohol and does not react with aqueous NaOH. It remains as \\( \text{HO-CH}_2\text{CH}_2\text{-OH} \\).
- Benzene-1,4-dicarboxylic acid contains two acidic carboxylic acid groups (\\( \text{-COOH} \\)). These react with NaOH to form the sodium carboxylate salt: sodium benzene-1,4-dicarboxylate (\\( \text{Na}^+ \text{ } ^-\text{OOC-C}_6\text{H}_4\text{-COO}^- \text{Na}^+ \\)).
Answer: The products are sodium benzene-1,4-dicarboxylate and ethane-1,2-diol.
Environmental Considerations and Disposal Methods
Because plastics do not break down easily, choosing the appropriate disposal method is a major environmental challenge. You must be able to evaluate the advantages and disadvantages of each option:
| Disposal Method | Advantages | Disadvantages |
|---|---|---|
| Landfill | Cheap, easy to manage, requires no sorting of plastics. | Wastes valuable land, visually polluting, addition polymers do not decay, risk of local environment contamination. |
| Incineration | Reduces volume of waste dramatically, allows heat/energy recovery to generate electricity. | Releases carbon dioxide (a greenhouse gas) and toxic gases (e.g. \\( \text{HCl} \\) from burning PVC, or toxic heavy metals from additives). |
| Recycling | Saves precious crude oil resources, reduces land waste and carbon footprint. | Sorting plastic types is difficult and expensive, requires washing and melting, down-cycling degrades polymer quality over time. |
Photodegradable Polymers
To reduce plastic litter, scientists have developed photodegradable polymers. These polymers contain carbonyl functional groups incorporated into their addition backbones. The carbonyl groups absorb ultraviolet (UV) light, causing bonds in the main chain to break. This causes the plastic to break down into smaller pieces when exposed to sunlight.
Students often mistakenly state that because condensation polymers are biodegradable, they decompose rapidly in standard landfill sites. In reality, landfills are highly compacted and lack oxygen, moisture, and light. Under these anaerobic conditions, even biodegradable polymers take a very long time to decompose. Always clarify this distinction in your written answers.
Get flashcards and quizzes in ChemEasy, or plan your revision with ChemPlan IB.