Biodegradable Polymers
Solving the plastic problem: Master PHBV and Nylon-2-nylon-6.
Most synthetic polymers (like Polythene, PVC, and Teflon) are incredibly durable and chemically inert. While this is great for manufacturing, it creates a massive environmental problem: they do not break down in nature, leading to severe solid waste accumulation. To combat this, chemists developed Biodegradable Polymers.
1. What makes a polymer biodegradable?
Natural polymers (biopolymers) like proteins, cellulose, and starch degrade naturally because microorganisms and enzymes in the environment can break their chemical bonds.
To make synthetic polymers biodegradable, chemists design them to contain functional groups similar to those found in biopolymers (such as ester or amide linkages). Environmental bacteria and enzymes can recognize these linkages, attack them, and break the massive chain into harmless, small molecules like $CO_2$ and $H_2O$.
Figure 1: Environmental enzymes recognize and cleave the specific functional groups (like esters or amides) in the polymer backbone.
There are two highly important biodegradable synthetic polymers you must memorize for your exams:
2. PHBV
Full Name: Poly $\beta$-hydroxybutyrate - co - $\beta$-hydroxyvalerate
PHBV is a biodegradable polyester. It is a copolymer formed by the condensation polymerization of two different monomers.
3-Hydroxybutanoic acid
$CH_3-CH(OH)-CH_2-COOH$
3-Hydroxypentanoic acid
$CH_3-CH_2-CH(OH)-CH_2-COOH$
$\textbf{[-O-CH(CH}_3\textbf{)-CH}_2\textbf{-CO-O-CH(CH}_2\text{CH}_3\textbf{)-CH}_2\textbf{-CO-]}\mathbf{_n} + 2n\text{H}_2\text{O}$
- Linkage: Ester linkage ($-CO-O-$).
- Properties: The 3-hydroxybutanoic acid provides stiffness, while the 3-hydroxypentanoic acid imparts flexibility.
- Uses: Specialty packaging, orthopedic devices, and extensively in controlled drug release capsules (the capsule slowly degrades in the body, releasing the drug steadily).
- Degradation: It undergoes bacterial degradation in the environment.
3. Nylon-2-nylon-6
Unlike standard Nylon 6 or Nylon 6,6 (which persist in the environment for decades), Nylon-2-nylon-6 is an alternating polyamide copolymer that is completely biodegradable.
Glycine
$H_2N-CH_2-COOH$
Aminocaproic acid
$H_2N-(CH_2)_5-COOH$
$\textbf{[-NH-CH}_2\textbf{-CO-NH-(CH}_2\textbf{)}_5\textbf{-CO-]}\mathbf{_n} + 2n\text{H}_2\text{O}$
- Linkage: Amide linkage ($-CO-NH-$), exactly the same peptide bond found in natural proteins.
- Biodegradability: Because its backbone perfectly mimics natural proteins, environmental enzymes easily break it down.
Other important biodegradable polymers occasionally tested include PGA (Polyglycolic acid) and PLA (Polylactic acid). They are highly biocompatible polyesters famously used for post-operative internal sutures (stitches) because they dissolve naturally in the body after the wound heals, eliminating the need to have the stitches removed.
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