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Biodegradable Polymers
World D — Polymers & Biomolecules
42.A — THE CRISIS & THE SOLUTION
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R42.01 The Non-Biodegradable Crisis
- Most synthetic polymers (Polythene, PVC) have unreactive C-C backbones.
- They persist in the environment for centuries.
- Lead to severe ecological damage and microplastic pollution.
R42.02 Hydrolysis (The Mechanism)
- Biodegradable polymers contain functional groups like Esters (-COO-) and Amides (-CONH-).
- These bonds are susceptible to hydrolysis (breaking by water).
- Splits the giant chain back into small, manageable monomers.
R42.03 The Role of Microbes
- Bacteria and fungi secrete specific enzymes.
- Enzymes catalyze the cleavage of the polymer bonds.
- The resulting small molecules are consumed by the microbes, producing $CO_2$, $H_2O$, and biomass.
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R42.04 Natural Biodegradables
- Nature's polymers are inherently biodegradable.
- Polysaccharides: Cellulose, Starch.
- Proteins: Wool, Silk, Collagen.
- They have built-in hydrolyzable ester/amide bonds.
42.B — SYNTHETIC BIODEGRADABLES
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R42.05 PHBV
- Poly $\beta$-hydroxybutyrate - co - $\beta$-hydroxyvalerate.
- Copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
- An aliphatic polyester. Degrades completely in soil.
- Used for specialty packaging and orthopedic devices.
R42.06 Nylon-2-Nylon-6
- An alternating polyamide copolymer.
- Monomers: Glycine (2 carbons) and Aminocaproic acid (6 carbons).
- Unlike regular Nylon, its specific structure allows enzymatic breakdown.
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R42.07 PLA & PGA
- PLA (Polylactic Acid): Made from fermented plant starch (corn). Widely used in 3D printing and bio-plastics.
- PGA (Polyglycolic Acid): Simpler structure, degrades faster.
- Both are aliphatic polyesters.
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R42.08 Aliphatic Polyesters
- The most common class of synthetic biodegradable polymers.
- The ester link (-COO-) is the key structural vulnerability.
- Easily attacked by water and bacterial enzymes.
42.C — APPLICATIONS
R42.09 Medical Implants & Sutures
- PGA and PHBV are used for post-operative stitches.
- They dissolve safely in the body over weeks via natural hydrolysis.
- Eliminates the need for follow-up surgery to remove stitches.
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R42.10 Industrial Composting
- PLA and other bio-plastics replace single-use packaging.
- However, they often require industrial composting (high heat, high humidity) to degrade quickly.
- Will not easily degrade in a cold, dry landfill or the ocean.