Board exam focus — Polymers (CBSE & HBSE)

Classification of polymers, addition and condensation polymerization, natural and synthetic rubbers, biodegradable and non-biodegradable polymers.

Classification and Addition Polymerization

Classification of Polymers

Based on Source

• Natural polymers: Occur in nature. Examples: starch, cellulose, natural rubber (cis-1,4-polyisoprene), proteins, nucleic acids.
• Semi-synthetic polymers: Chemically modified natural polymers. Examples: cellulose acetate (rayon), cellulose nitrate (gun cotton).
• Synthetic polymers: Made in laboratory/industry. Examples: nylon, PVC, Teflon, Bakelite, Dacron, polyethylene.

Based on Structure

Based on Molecular Forces

Based on Synthesis

• Addition polymers: Formed by repeated addition of monomers with double bonds without loss of atoms. No by-product.
• Condensation polymers: Formed by repeated condensation with loss of small molecules (H2O, HCl, etc.).

Addition Polymerization

Free Radical Mechanism:

1. Initiation: Initiator (benzoyl peroxide) generates free radical R•

R• + CH2=CH2 → R-CH2-CH2•

2. Propagation: Monomer adds to growing chain

R-CH2-CH2• + CH2=CH2 → R-CH2-CH2-CH2-CH2• (chain grows by successive addition)

3. Termination: Two radicals combine or disproportionate

R-(CH2-CH2)n• + •(CH2-CH2)m-R → terminated polymer

Important Addition Polymers

Polyethylene:

• LDPE (Low Density Polyethylene): Made at 200°C, 1000-2000 atm by free radical. Branched chains, low density, flexible. Used for bags, insulation, squeeze bottles.
• HDPE (High Density Polyethylene): Made at low pressure with Ziegler-Natta catalyst (TiCl4 + Al(C2H5)3). Linear chains, high density, stiff. Used for pipes, bottles, toys.

PVC (Polyvinyl Chloride): Monomer = vinyl chloride (CH2=CHCl). Hard, rigid, PVC pipes, insulation, raincoats, records.

Polystyrene: Monomer = styrene (C6H5-CH=CH2). Transparent, brittle. Expanded polystyrene = Thermocol (foam cups, insulation).

Teflon (PTFE - Polytetrafluoroethylene): Monomer = tetrafluoroethylene (CF2=CF2). Extremely chemically inert (C-F bond very strong), non-stick surface (Teflon coated cookware), low friction coefficient, high temperature resistant.

Orlon/Acrilan (Polyacrylonitrile, PAN): Monomer = acrylonitrile (CH2=CH-CN). Acrylic fibre. Used as substitute for wool in warm clothing.

Natural Rubber: cis-1,4-polyisoprene. Monomer = isoprene (CH2=C(CH3)-CH=CH2). Sticky, poor elasticity, softens at high temp, brittle at low temp.

Condensation Polymerization

Condensation Polymerization

In condensation polymerization (also called step-growth polymerization), monomers with two functional groups react repeatedly with elimination of small molecules (water, HCl, methanol, etc.).

General feature: n A-A + n B-B → [-A-A-B-B-]n + (2n-1) small molecules

Nylon-6,6

Monomers: Hexamethylenediamine H2N-(CH2)6-NH2 + Adipic acid HOOC-(CH2)4-COOH Reaction: -NH2 + HOOC- → -NH-CO- + H2O (amide bond) Structure: [-NH-(CH2)6-NH-CO-(CH2)4-CO-]n Properties:

• Fibre with very high tensile strength (H-bonds between C=O and N-H of adjacent chains)
• High melting point (~264°C)
• Resistant to oils and solvents

Uses: Stockings, ropes, parachutes, toothbrush bristles, seat belts.

Nylon-6

Monomer: Caprolactam (cyclic amide, ring-opening polymerization) Process: Ring of caprolactam opens and polymerizes in presence of trace water Structure: [-NH-(CH2)5-CO-]n Uses: Tyre cords, ropes, fabrics.

Dacron/Terylene (Polyethylene Terephthalate, PET)

Monomers: Ethylene glycol HO-CH2-CH2-OH + Terephthalic acid HOOC-C6H4-COOH Reaction: -OH + HOOC- → -O-CO- + H2O (ester bond) Structure: [-O-CH2-CH2-O-CO-C6H4-CO-]n Properties: Fibre, good tensile strength, wrinkle resistant Uses: Fabric (Terylene), PET bottles, Mylar film, magnetic recording tapes.

Bakelite

Monomers: Phenol + Formaldehyde (HCHO) Catalyst: Acid or base Mechanism:

1. First step: Phenol + HCHO → ortho- and para-hydroxymethylphenol (addition of -CH2OH groups)
2. Second step: Condensation of these with more phenol units → linear Novolac resin (thermoplastic)
3. Heating with HCHO (cross-linking): Forms 3D cross-linked Bakelite network (thermosetting)

Properties: Hard, rigid, thermosetting, excellent electrical insulator, chemical resistant. Uses: Electrical switches, plugs, automobile parts, consumer electronics bodies.

Melamine-Formaldehyde Resin

Monomers: Melamine + Formaldehyde Melamine has 3 -NH2 groups → can form highly cross-linked polymer Uses: Dinnerware (Melamine plates/cups), decorative laminates (Formica), fire-retardant materials.

Glyptal

Monomers: Ethylene glycol + Phthalic acid Bond: Ester bonds Uses: Alkyd paints, surface coatings.

BUNA Rubbers

Buna-S (SBR - Styrene Butadiene Rubber): Copolymer of butadiene (CH2=CH-CH=CH2) + styrene (C6H5-CH=CH2) Most widely used synthetic rubber (used in car tyres)

Buna-N (NBR - Nitrile Rubber): Copolymer of butadiene + acrylonitrile (CH2=CH-CN) Oil and solvent resistant; used for hoses, gaskets, oil tanks.

Rubber and Biodegradable Polymers

Natural Rubber and Vulcanization

Natural Rubber

Source: Latex from Hevea brasiliensis tree Chemical name: cis-1,4-polyisoprene Monomer: Isoprene (2-methylbuta-1,3-diene): CH2=C(CH3)-CH=CH2 Structure: cis configuration at C1-C4 double bond Properties:

• Soft, sticky at room temperature
• Becomes hard and brittle below 10°C
• Softens and melts above 65°C
• Low tensile strength
• Affected by organic solvents (swells/dissolves)
• Poor elasticity and resilience

Vulcanization of Rubber

Process: Heating natural rubber with sulphur (S8) at 150-180°C What happens: Sulphur cross-links form between polymer chains through C=C double bonds Structure: chains connected by -S-S- or -S-S-S- bridges (polysulphide links) Typical sulphur content: 1-3% for soft vulcanized rubber; up to 30% for ebonite (hard rubber)

Properties IMPROVED by vulcanization:

Synthetic Rubbers

1. Neoprene (Polychloroprene): Monomer: Chloroprene (CH2=CCl-CH=CH2) Addition polymerization Properties: Resistant to oils, solvents, ozone, heat Uses: Wet suits, hoses, belts, wires, gaskets

2. Buna-S / SBR (Styrene Butadiene Rubber): Monomers: 1,3-butadiene + styrene (random copolymer, 75:25 ratio) World's most used synthetic rubber (car tyres) Properties: Good abrasion resistance, cheaper than natural rubber

3. Buna-N / NBR (Nitrile Rubber): Monomers: 1,3-butadiene + acrylonitrile Excellent oil/fuel/solvent resistance (polar nitrile groups) Uses: O-rings, hoses, petrol tanks, gloves

4. Thiokol: Monomer: Ethylene dichloride + sodium polysulphide Extremely resistant to oxidation, heat, solvents Uses: Rocket fuel binders, caulking

Biodegradable Polymers

Polymers that are broken down by microorganisms (bacteria, fungi) in reasonable timeframes.

1. PHBV (Poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate):

• Copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid
• Produced by bacteria (Alcaligenes eutrophus)
• Biodegrades in soil by microbial action
• Used for packaging, disposable cups/bottles

2. PLA (Polylactic Acid / Polylactide):

• Monomer: Lactic acid (from fermentation of corn starch)
• Biodegrades in weeks to months
• Biocompatible (medical sutures, drug delivery)
• Used in packaging, bottles, textiles

3. Nylon-2-nylon-6 (PEPAX):

• Copolymer of glycine (2-amino acid) and aminocaproic acid (6-amino acid)
• Biodegradable polyamide

Non-Biodegradable Polymers and Environment

Problem: PVC, polyethylene, polystyrene persist in environment for 500-1000 years. Microplastics: Fragmented plastic particles (<5mm) found in oceans, soil, even human blood. Solutions: Reduce/Reuse/Recycle, bioplastics, incineration (energy recovery), photodegradable plastics.

Frequently asked questions

Are these Polymers notes free?

Yes — the Polymers notes for chemistry (Class 12) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Polymers notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Polymers chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Polymers.