Unit 18: Chemistry in the Service of Mankind

Polymers

Polymers are large molecules (macromolecules) formed by the repetitive bonding of smaller units called monomers. Depending on the manner in which monomers link, polymers are broadly classified into addition polymers, condensation polymers, elastomers, fibres, natural polymers, and synthetic polymers.

1. Addition Polymers

In addition polymerization, monomers add together without the loss of any small molecules. The process typically involves the opening of double bonds in unsaturated monomers.

• Polythene (Polyethylene): Produced from ethene (CH₂=CH₂).
  n CH₂=CH₂ → [−CH₂−CH₂−]ₙ (polyethylene). Used in packaging films, containers, and grocery bags.
• Polyvinyl Chloride (PVC): From vinyl chloride (CH₂=CHCl).
  n CH₂=CHCl → [−CH₂−CHCl−]ₙ. Employed in pipes, electrical cable insulation, and window frames.
• Polytetrafluoroethylene (Teflon): From tetrafluoroethene (CF₂=CF₂).
  n CF₂=CF₂ → [−CF₂−CF₂−]ₙ. Known for non‑stick coatings, chemical resistance, and low friction.
• Polystyrene: From styrene (C₆H₅CH=CH₂).
  n C₆H₅CH=CH₂ → [−CH(C₆H₅)−CH₂−]ₙ. Used in foam packaging, disposable cups, and insulation boards.

2. Condensation Polymers

Condensation polymerization involves the joining of monomers with the elimination of small molecules such as water, methanol, or HCl.

• Nylon (Polyamide): Formed from diamines and dicarboxylic acids (e.g., hexamethylenediamine + adipic acid).
  n H₂N-(CH₂)₆-NH₂ + n HOOC-(CH₂)₄-COOH → [−NH-(CH₂)₆-NH-CO-(CH₂)₄-CO−]ₙ + n H₂O. Utilized in fibres, ropes, gears, and automotive parts.
• Bakelite (Phenol‑Formaldehyde resin): Produced by reacting phenol with formaldehyde under acidic or basic conditions.
  n C₆H₅OH + n CH₂O → [−C₆H₄−CH₂−O−]ₙ + n H₂O. Serves as an electrical insulator, heat‑resistant handle material, and early plastic for radios.

3. Elastomers

Elastomers are polymers that exhibit rubber‑like elasticity; they can be stretched significantly and return to their original shape.

• Natural Rubber: Polymer of isoprene (C₅H₈) – poly(cis‑1,4‑isoprene). Used in tires, footwear, and seals.
• Neoprene (Polychloroprene): Synthetic elastomer from chloroprene (CH₂=CCl‑CH=CH₂). Resistant to oil, heat, and weathering; applied in wetsuits, gaskets, and adhesives.

4. Fibres

Fibres are long, thin strands of polymer with high tensile strength and modulus, suitable for textiles and reinforcement.

• Nylon (see above) – strong, abrasion‑resistant fibre for hosiery, carpets, and parachutes.
• Polyester (PET): From terephthalic acid and ethylene glycol.
  n HOOC‑C₆H₄‑COOH + n HO‑CH₂‑CH₂‑OH → [−O‑CH₂‑CH₂‑O‑OC‑C₆H₄‑CO−]ₙ + n H₂O. Used in clothing, bottles, and tire cords.
• Cotton: Natural cellulose polymer ([(C₆H₁₀O₅)ₙ]) – breathable, absorbent fibre for apparel.
• Wool: Keratin‑based protein fibre – excellent insulation and moisture‑wicking.

5. Natural Polymers

Polymers that occur in nature without human synthesis.

• Proteins: Polymers of α‑amino acids; enzymes, hormones, structural components (e.g., collagen, keratin).
• Cellulose: Linear β‑1,4‑linked glucose polymer; main constituent of plant cell walls; used in paper, textiles (as rayon), and biofuels.
• Starch: Branched polymer of glucose (amylose & amylopectin); energy storage in plants; food source and biodegradable filler.
• Natural Rubber: As described under elastomers.

6. Synthetic Polymers

Polymers manufactured via chemical processes from petro‑derived monomers.

• Polythene, PVC, Teflon, Polystyrene, Nylon, Bakelite (details above).
• Additional examples: Polypropylene (from propene), Polyacrylonitrile (PAN), Polyurethane (from diisocyanate + polyol).

Some Synthetic Polymers – Details and Applications

Dyes

Dyes are coloured substances that impart hue to substrates such as textiles, leather, paper, and plastics by forming a chemical or physical bond with the material.

Introduction

The colour of a dye arises from its chromophore (e.g., azo, nitro, carbonyl groups) and its ability to absorb specific wavelengths of visible light. Auxochromes (e.g., –OH, –NH₂) intensify the colour and improve solubility.

Types Based on Structure and Application

1. Direct Dyes
   Applied directly to the fibre from an aqueous solution; they contain sulfonic acid groups for water solubility.
   Example: Azo dyes such as Direct Red 80 used for cotton and rayon.
2. Mordant Dyes
   Require a mordant (usually a metal salt like Al³⁺, Fe³⁺, Cr³⁺) to form an insoluble complex with the fibre.
   Example: Alizarin (from madder) with Al³⁺ gives a red‑lake on wool.
3. Vat Dyes
   Insoluble in water; they are first reduced to a soluble leuco form, applied to the fibre, then oxidized back to the insoluble pigment.
   Example: Indigo (C₁₆H₁₀N₂O₂) – reduced to leuco‑indigo (yellow), applied, then oxidized to blue.
4. Reactive Dyes
   Form covalent bonds with the fibre (usually cellulose) via reactive groups such as chlorotriazine or vinyl sulfone.
   Example: Reactive Red 120 bonds to –OH groups of cotton under alkaline conditions.

Applications

• Textile industry: colouring of cotton, wool, silk, synthetic fibres.
• Leather dyeing: achieving uniform shades and fastness.
• Paper and pulp: colouring of stationery, packaging.
• Food and cosmetics (subject to strict regulations): certain azo and anthraquinone dyes.

Drugs

Drugs are chemical substances that produce a biological effect when administered to a living organism. They are used for diagnosis, prevention, treatment, or relief of symptoms of diseases.

Characteristics

• Pharmacological activity: Interaction with specific biomolecules (receptors, enzymes).
• Dosage form: Tablets, capsules, injections, topical preparations.
• Safety and efficacy: Determined through preclinical and clinical trials.

Natural Drugs

Obtained directly from plants, animals, or microorganisms.

• Morphine: Alkaloid from opium poppy (Papaver somniferum); potent analgesic.
• Quinine: Alkaloid from cinchona bark; antimalarial.
• Penicillin: β‑lactam antibiotic from Penicillium fungi; inhibits bacterial cell wall synthesis.

Synthetic Drugs

Prepared by chemical synthesis in laboratories.

• Paracetamol (Acetaminophen): Analgesic/antipyretic; inhibits cyclooxygenase in the CNS.
• Aspirin (Acetylsalicylic acid): Irreversible COX inhibitor; analgesic, antipyretic, anti‑inflammatory, antiplatelet.
• Ibuprofen: Propionic acid derivative; reversible COX inhibitor; used for pain and inflammation.

Classification by Therapeutic Action

Habit‑Forming Drugs

Substances that can lead to dependence due to their effect on the brain’s reward system.

• Narcotics (Opioids): Morphine, Heroin, Codeine – produce euphoria and analgesia; high addiction potential.
• Stimulants: Amphetamine, Methamphetamine, Cocaine – increase alertness and energy; can cause psychological dependence.
• DepressantsAlcohol, Barbiturates, Benzodiazepines – reduce anxiety, induce sedation; risk of physical dependence.