Green polymers are biodegradable or bio-based alternatives to conventional plastics. CBSE Class 12 covers them in the polymers chapter; NEET asks direct questions on biodegradability and examples.
Core Concepts
Why conventional plastics are a problem
Non-biodegradable. Persist in environment for centuries. Enter food chains as microplastics. Clog drains and oceans. Mumbai and Delhi both face massive plastic waste challenges.
The scale is staggering — India generates about 26,000 tonnes of plastic waste daily. Most of it is single-use packaging made from polyethylene and polypropylene that no microorganism can break down in any reasonable timeframe.
Biodegradable polymers
Break down naturally by microbial action into water, CO2 and biomass. Examples — PHBV (poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate), polylactic acid (PLA), nylon-2-nylon-6.
The key difference from conventional plastics is the presence of ester or amide linkages in the backbone that microorganisms can hydrolyse. Polyethylene has only C-C bonds in the backbone, which are extremely resistant to biological attack.
PHBV
Produced by bacteria like Alcaligenes eutrophus. Can be used in packaging and medical implants. Fully biodegradable.
PHBV is a copolymer of 3-hydroxybutyric acid and 3-hydroxyvaleric acid. The ratio of the two monomers controls the flexibility — more hydroxyvalerate makes it softer and more flexible, while more hydroxybutyrate makes it stiffer. This tunability is what makes PHBV attractive for medical devices.
Polylactic acid (PLA)
Made from lactic acid, which is made from corn starch. Used in compostable cutlery, cups and 3D printing filament. Breaks down in industrial composting.
The synthesis pathway: corn starch → glucose (by hydrolysis) → lactic acid (by fermentation) → lactide (cyclic dimer) → PLA (by ring-opening polymerisation). It is a condensation polymer with ester linkages.
Nylon-2-nylon-6
A biodegradable copolymer of glycine (2-carbon amino acid) and aminocaproic acid (6-carbon amino acid). Alternative to traditional nylon for packaging. The amide bonds in the backbone can be cleaved by proteolytic enzymes in soil.
Bio-based vs biodegradable
Bio-based means made from biological sources (like corn starch). Biodegradable means it breaks down naturally. Not all bio-based polymers are biodegradable, and vice versa.
For example, bio-polyethylene made from sugarcane ethanol is bio-based but not biodegradable — it has the same C-C backbone as petroleum-based polyethylene. On the other hand, some petroleum-derived polyesters are biodegradable because their ester linkages can be hydrolysed.
Key Formulas and Structures
Lactic acid undergoes self-condensation. The -OH of one molecule reacts with the -COOH of another, releasing water and forming ester linkages.
3-Hydroxybutyric acid:
3-Hydroxyvaleric acid:
Both monomers polymerise through ester bond formation between -OH and -COOH groups.
The “2” in nylon-2 comes from glycine having 2 carbons; the “6” comes from aminocaproic acid having 6 carbons. The amide bonds formed are similar to those in proteins, which is why soil microbes can digest this polymer.
Worked Examples
PLA needs high temperature (50-60°C) and specific microbial communities to break down efficiently. Regular backyard composts are too cool and take years.
Because it breaks down safely in the body, PHBV is used for drug delivery, sutures and scaffolds for tissue engineering. No second surgery needed to remove.
The C-C backbone of polyethylene has no functional group that enzymes can attack. Microbial enzymes work by hydrolysing specific linkages (ester, amide, glycosidic). With nothing to hydrolyse, polyethylene just sits there.
If a PLA cup (mass 10 g) is fully biodegraded, the products are CO2 and H2O. The carbon returns to the atmosphere and can be captured again by corn — making PLA carbon-neutral in principle. However, the energy used in manufacturing is not zero, so the full life-cycle is not perfectly neutral.
Comparison Table
| Property | Conventional (PE, PP) | PHBV | PLA | Nylon-2-Nylon-6 |
|---|---|---|---|---|
| Source | Petroleum | Bacterial fermentation | Corn starch | Chemical synthesis |
| Backbone bond | C-C | Ester | Ester | Amide |
| Biodegradable | No | Yes | Yes (industrial) | Yes |
| Strength | High | Moderate | Moderate | Moderate |
| Cost | Low | High | Medium | Medium |
| Common use | Bags, bottles | Medical implants | Cups, 3D printing | Packaging |
Common Mistakes
Saying all bioplastics are fully biodegradable. Some are only partially.
Confusing bio-based and biodegradable. They are different concepts.
Writing that PLA is the same as polythene. They are chemically different and have different fates.
Saying PHBV is a synthetic polymer. It is produced by bacteria — it is a natural polyester that bacteria use for energy storage, similar to how we store glycogen.
Assuming biodegradable means it breaks down anywhere. PLA needs industrial composting conditions. In a landfill without oxygen, even PLA can persist for decades.
Exam Weightage and Revision
Green polymers appear in NEET about once every two years as a direct question. CBSE boards ask it more regularly — typically a 2-3 mark question in Class 12 chemistry. The question pattern is predictable: “Give an example of a biodegradable polymer and state its use” or “Distinguish between biodegradable and non-biodegradable polymers.”
| Exam | Frequency | Typical Question |
|---|---|---|
| NEET | Every 2 years | Name a biodegradable polymer with its use |
| CBSE Class 12 | Almost yearly | Difference between biodegradable and non-biodegradable with examples |
| JEE Main | Rare | Structure-based question on PHBV or PLA |
The three names you must know are PHBV, PLA, and nylon-2-nylon-6. For each, know the source, the type of linkage (ester or amide), and one specific application. That covers every PYQ on this topic from the last decade.
Practice Questions
Q1. PHBV is a copolymer of which two monomers? Why is it useful in medicine?
PHBV is a copolymer of 3-hydroxybutyric acid and 3-hydroxyvaleric acid. It is useful in medicine because it biodegrades safely inside the body, making it ideal for drug delivery systems, biodegradable sutures, and tissue engineering scaffolds. No removal surgery is needed.
Q2. Why is nylon-2-nylon-6 biodegradable but nylon-6,6 is not easily biodegradable?
Both are polyamides, but nylon-2-nylon-6 has shorter segments between amide bonds (especially the glycine-derived 2-carbon unit), making it structurally closer to natural peptides. Soil proteases can attack these bonds. Nylon-6,6 has longer hydrocarbon segments between amide bonds and a tightly packed crystal structure, making it harder for enzymes to access.
Q3. A student says “PLA is carbon-neutral, so we can use as much as we want.” What is wrong with this claim?
While PLA is theoretically carbon-neutral (the CO2 released equals what the corn absorbed), growing corn requires fertilisers (made from fossil fuels), irrigation, and transportation. The manufacturing also uses energy. The full life-cycle carbon footprint is lower than conventional plastics but not zero. Also, if PLA ends up in a landfill instead of an industrial composter, it may not biodegrade at all.
Q4. Classify as biodegradable or non-biodegradable: (a) PHBV (b) Teflon (c) PLA (d) Polypropylene (e) Cellulose
(a) PHBV — biodegradable (ester linkages, bacterial origin). (b) Teflon — non-biodegradable (C-F bonds are among the strongest in organic chemistry). (c) PLA — biodegradable (ester linkages). (d) Polypropylene — non-biodegradable (C-C backbone). (e) Cellulose — biodegradable (glycosidic linkages, many organisms produce cellulase).
FAQs
Why are biodegradable polymers more expensive?
Production scale is smaller, raw material costs are higher (bacterial fermentation for PHBV is expensive), and the technology is newer. As scale increases, costs are expected to drop — PLA prices have already fallen significantly in the last decade.
Can we make polyethylene biodegradable by adding starch?
Adding starch to polyethylene makes the material fragment faster (the starch is eaten by microbes, leaving small PE particles), but the polyethylene itself does not biodegrade. This creates microplastics, which may actually be worse. True biodegradability requires breaking down the polymer backbone.
What is the future of green polymers in India?
India banned many single-use plastics in 2022. The government is pushing alternatives including jute, paper, and biodegradable polymers. PHBV and PLA production facilities are being set up. The market is growing fast but cost and composting infrastructure remain challenges.
How is the polymer backbone structure related to biodegradability?
The backbone determines everything. Polymers with hydrolysable bonds (ester, amide, glycosidic) in the backbone can be attacked by microbial enzymes — these are biodegradable. Polymers with only C-C bonds in the backbone (polyethylene, polypropylene, polystyrene, PVC, teflon) have no point of enzymatic attack — these are non-biodegradable. When choosing or designing a green polymer, the first question is always: what bonds are in the backbone?
Condensation vs addition polymerisation and biodegradability
Most biodegradable polymers are condensation polymers — they form by losing small molecules (water) and contain ester or amide linkages. Most non-biodegradable polymers are addition polymers — they form by opening double bonds and contain only C-C bonds. This is not an absolute rule (some addition polymers can be designed to be biodegradable with special side groups), but it holds for the polymers covered in CBSE and NEET.
| Polymerisation Type | Backbone Bonds | Biodegradable? | Examples |
|---|---|---|---|
| Condensation | Ester (-COO-) | Usually yes | PLA, PHBV, polyester |
| Condensation | Amide (-CONH-) | Sometimes | Nylon-2-nylon-6 (yes), nylon-6,6 (poorly) |
| Addition | C-C only | Usually no | Polyethylene, PVC, polystyrene |
Environmental impact of improper disposal
Even biodegradable polymers can cause problems if disposed of improperly. PLA in a landfill (anaerobic conditions) produces methane — a greenhouse gas 25 times more potent than CO2. The best outcome requires industrial composting with controlled temperature, aeration, and microbial communities. Without proper infrastructure, switching to biodegradable plastics alone does not solve the waste problem.
The ideal lifecycle of a green polymer:
- Made from renewable feedstock (corn, sugarcane)
- Used for its intended purpose
- Collected separately from regular waste
- Processed in an industrial composting facility
- Breaks down to CO2 and water
- CO2 is captured by the next crop — closing the carbon loop
This closed loop works in theory but requires collection and composting infrastructure that most Indian cities do not yet have.
Memorise three biodegradable polymers — PHBV, PLA, nylon-2-nylon-6 — with one use each.
Green polymers are where chemistry meets environmental responsibility. The field is growing fast, and questions on it are getting more common.