Polymers & Chemistry in Everyday Life

1. Classification of Polymers

By source: Natural (proteins, cellulose, starch, natural rubber), Semi-synthetic (cellulose acetate, vulcanized rubber), Synthetic (polyethylene, nylon, Bakelite).

By structure: Linear (HDPE, nylon — chains packed closely, high density, high tensile strength), Branched (LDPE — irregular packing, low density, flexible), Cross-linked/Network (Bakelite, melamine — extensive covalent cross-links, hard, infusible, insoluble).

By polymerization type: Addition (chain-growth) polymers form by addition of monomers with C=C bonds — no by-product lost (PE, PP, PVC, PS, Teflon, polyacrylonitrile). Condensation (step-growth) polymers form by reaction of bifunctional monomers with loss of small molecule (H₂O, HCl) — nylon-6,6, polyester (Terylene/Dacron), Bakelite, melamine.

By monomer type: Homopolymer (one type of monomer: PE, PVC, PS), Copolymer (two or more monomers: Buna-S from butadiene + styrene, Buna-N from butadiene + acrylonitrile, nylon-6,6 from hexamethylenediamine + adipic acid).

2. Addition (Chain-Growth) Polymerization

Mechanism proceeds in three stages: Initiation (free radical, cationic, or anionic), Propagation (chain grows by adding monomers), Termination (chain growth stops by combination/disproportionation).

Free radical polymerization: Initiator (benzoyl peroxide) generates radicals → radical attacks C=C → chain propagates → termination by coupling or disproportionation. Used for PE, PVC, PS, PMMA, Teflon.

Key addition polymer monomers:

Styrene (monomer for polystyrene)

Vinyl chloride (monomer for PVC)

Acrylonitrile (monomer for PAN/Orlon)

Methyl methacrylate (monomer for PMMA/Plexiglass)

Key addition polymers:

• Polyethylene (PE): Monomer = CH₂=CH₂. LDPE (high P, high T, peroxide initiator — branched, flexible, packaging). HDPE (Ziegler-Natta catalyst TiCl₄/Al(C₂H₅)₃ — linear, rigid, pipes/containers).
• Polypropylene (PP): CH₂=CHCH₃. Ziegler-Natta gives isotactic (all CH₃ on same side) — high m.p., strong fibers.
• PVC: Monomer C=CCl. Rigid (pipes) or plasticized with phthalates (flexible — cables, flooring).
• Polystyrene (PS): Monomer . Transparent, brittle. Expanded PS = Thermocol (foam insulation).
• Teflon (PTFE): CF₂=CF₂. Chemically inert (C-F bond very strong), non-stick, high m.p. Used in cookware, gaskets.
• Polyacrylonitrile (PAN/Orlon): Monomer C=CC#N. Synthetic wool substitute.
• PMMA (Plexiglass): Monomer . Transparent, shatter-resistant, optical fibers.

3. Condensation (Step-Growth) Polymerization

Bifunctional or trifunctional monomers react with elimination of H₂O, HCl, etc.

Key condensation polymers:

• Nylon-6,6: Hexamethylenediamine + adipic acid → polyamide. Numbers indicate carbons in each monomer (6+6). Strong, elastic, used in textiles, ropes, gears.
• Nylon-6: Ring-opening polymerization of caprolactam (single monomer, 6 carbons). Similar properties to nylon-6,6.

Caprolactam (monomer for Nylon-6):

• Terylene (Dacron/PET): Ethylene glycol + terephthalic acid → polyester. Used in fabric blends, PET bottles.
• Bakelite: Phenol + formaldehyde. Novolac (linear, acid catalyst, excess phenol) → cross-linked Bakelite (heated with more HCHO + base catalyst). Thermosetting — hard, used in electrical fittings.

Bisphenol A (monomer for polycarbonates and epoxy resins):

• Melamine-formaldehyde: Melamine (triamine) + HCHO → highly cross-linked. Heat-resistant dinnerware, laminates.

4. Thermoplastics vs Thermosetting

Thermoplastics: Soften on heating, can be remolded repeatedly. Linear/branched chains held by intermolecular forces. Examples: PE, PVC, PS, nylon, polyester.

Thermosetting: Harden permanently on heating (cross-linking). Cannot be remelted. Examples: Bakelite, melamine, epoxy resins, vulcanized rubber.

5. Rubber

Natural rubber: cis-1,4-polyisoprene (from isoprene = 2-methyl-1,3-butadiene). Soft, sticky, not elastic enough for practical use.

Vulcanization: Heating natural rubber with sulfur (2-5%) introduces S-S cross-links between chains → improves elasticity, tensile strength, and resistance to heat/solvents. Discovered by Charles Goodyear.

Synthetic rubbers: Neoprene (polychloroprene — oil/solvent resistant), Buna-S (butadiene + styrene — automobile tires), Buna-N (butadiene + acrylonitrile — oil-resistant seals/gaskets).

6. Biodegradable Polymers

PHBV (poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate): Biodegraded by soil bacteria. Used for packaging, orthopedic devices.

Nylon-2-nylon-6: Polyamide from glycine (2C) and aminocaproic acid (6C). Biodegradable, structurally similar to proteins.

7. Chemistry in Everyday Life

Drugs: Analgesics — aspirin (non-narcotic, anti-inflammatory), morphine (narcotic). Antibiotics — penicillin (bactericidal, inhibits cell wall synthesis), tetracycline (broad-spectrum). Antiseptics — Dettol (chloroxylenol), boric acid. Disinfectants — phenol (1%), SO₂ gas. Antifertility drugs — norethindrone, ethynylestradiol (synthetic progesterone/estrogen derivatives).

Soaps vs Detergents: Soaps = sodium/potassium salts of long-chain fatty acids (RCOONa). Fail in hard water (Ca₂+/Mg₂+ form insoluble scum). Detergents = sodium alkyl sulfates (R-OSO₃Na) or alkylbenzene sulfonates (R-C₆H₄-SO₃Na). Work in hard water (calcium salts are soluble). Non-biodegradable if branched (hard detergents); biodegradable if linear (soft detergents).

Food chemistry: Artificial sweeteners — aspartame (200x sweeter), saccharin (550x), sucralose. Preservatives — sodium benzoate (E₂₁₁), potassium metabisulfite. Antioxidants — BHA (butylated hydroxyanisole), BHT.

The key problem-solving concept is matching monomers to polymers and distinguishing addition from condensation polymerization based on whether a by-product is lost and whether monomers have C=C bonds or bifunctional groups.