CBSE Class 12 Chemistry Chapter 15 Polymers Revision Notes

Glossary of Essential Polymer Terms

Polymers

High molecular mass macromolecules (10^3 - 10^7 u) formed by the linkage of a large number of repeating structural units.

Monomers

The simple, reactive starting molecules that combine chemically with each other to form a polymer.

Homopolymer

A polymer derived from only one single species of monomer (e.g., Polythene from ethene).

Copolymer

A polymer formed by the polymerization of two or more different monomer species (e.g., Buna-S from butadiene and styrene).

Elastomers

Elastic polymers with weak intermolecular forces (Van der Waals forces) that can stretch and return to their original shape (e.g., Buna-S, Neoprene).

Thermoplastics

Linear or slightly branched long-chain polymers that soften on heating and harden on cooling, permitting remolding (e.g., Polythene, Polystyrene).

Thermosetting Polymers

Cross-linked or heavily branched polymers that undergo permanent chemical change (cross-linking) on heating, becoming infusible and insoluble (e.g., Bakelite, Melamine).

Biodegradable Polymers

Polymers that can be broken down by environmental microorganisms over time, preventing ecological accumulation (e.g., PHBV, Nylon-2-nylon-6).

Classification of Polymers Based on Molecular Forces

Polymers are widely classified according to the strength of their internal intermolecular forces, which directly dictate their mechanical properties such as tensile strength, elasticity, and thermal stability:

1. Elastomers: These possess the weakest intermolecular forces of attraction, typically weak Van der Waals forces. These weak forces allow the polymer to stretch under stress. Elasticity is further enhanced by introducing a few cross-links between the chains (such as during the vulcanization of rubber), which help the polymer return to its original position when the stress is released.
2. Fibers: These exhibit the strongest intermolecular forces of attraction, specifically strong hydrogen bonding or dipole-dipole interactions. This close packing gives them high tensile strength, low elasticity, and sharp melting points, making them ideal for threads (e.g., Nylon 6,6, Terylene).
3. Thermoplastics: These are linear polymers having intermolecular forces intermediate between elastomers and fibers. They possess no cross-linking, which allows the polymer chains to slide past each other upon heating. Consequently, they can be repeatedly recycled and remolded.
4. Thermosetting Polymers: These form three-dimensional network structures with heavy cross-linking. When heated, they undergo extensive chemical cross-linking in the mold and set into a hard, infusible mass. They cannot be remolded or reused, and heating them further only leads to decomposition.

Addition Polymerization vs. Condensation Polymerization

Mechanism of Free Radical Addition Polymerization

1. Chain Initiation Step — An initiator molecule (such as benzoyl peroxide or acetyl peroxide) undergoes homolytic cleavage upon heating or exposure to light, generating active free radicals. These radicals then attack the double bond of the monomer (e.g., ethene) to generate a new, larger monomer-free radical.
2. Chain Propagation Step — The newly formed monomer radical attacks another monomer molecule. This process repeats continuously in a cascade, rapidly propagating the active radical center and lengthening the carbon chain.
3. Chain Termination Step — The reaction is brought to a halt when two active growing chain radicals collide and combine (coupling), or undergo disproportionation, forming a stable, long-chain polymer molecule.

Key Monomers and Polymer Structures (Must Remember)

• Nylon 6,6: Prepared from Hexamethylenediamine [$H_2N-(CH_2)_6-NH_2$] and Adipic acid [$HOOC-(CH_2)_4-COOH$]. Used in making bristles for brushes and textile fibers.
• Nylon 6: Prepared by heating Caprolactam with water at high temperatures. It contains a single monomer unit with 6 carbon atoms.
• Terylene (Dacron): Formed by condensation of Ethylene glycol [$HO-CH_2-CH_2-OH$] and Terephthalic acid [para-$HOOC-C_6H_4-COOH$].
• Bakelite: A phenol-formaldehyde polymer. Formed by cross-linking Novolac (a linear polymer of phenol and formaldehyde) with additional formaldehyde upon heating.
• Buna-S: An addition copolymer of 1,3-Butadiene [$CH_2=CH-CH=CH_2$] and Styrene [$C_6H_5-CH=CH_2$] in a 3:1 ratio.
• Buna-N: An addition copolymer of 1,3-Butadiene and Acrylonitrile [$CH_2=CH-CN$]. Highly resistant to the action of petrol and oils.
• Neoprene: Synthetic rubber produced by the free radical polymerization of Chloroprene (2-chloro-1,3-butadiene).
• PHBV (Poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate): A biodegradable copolymer made from 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. Used in specialty packaging and orthopedic devices.

Worked Chemical Equations & Monomer Identifications

• {"title":"Example 1: Synthesis of Nylon 6,6","description":"Identify the monomers and write the polymerization equation for Nylon 6,6.\n\nReaction:\nn H2N-(CH2)6-NH2 (Hexamethylenediamine) + n HOOC-(CH2)4-COOH (Adipic Acid) --(553K, High Pressure)--> [-NH-(CH2)6-NH-CO-(CH2)4-CO-]n + 2n H2O\nMonomers: Hexamethylenediamine and Adipic Acid."}
• {"title":"Example 2: Polymerization of Teflon","description":"Write the monomer and reaction condition for preparing Teflon (Polytetrafluoroethylene).\n\nReaction:\nn CF2=CF2 (Tetrafluoroethene) --(Catalyst, High Pressure)--> [-CF2-CF2-]n (Teflon)\nProperties: Chemically inert and highly resistant to heat, used as a non-stick coating on cookware."}
• {"title":"Example 3: Biodegradable Polymer PHBV Formation","description":"Identify the monomers of the biodegradable polymer PHBV.\n\nMonomers:\n1. 3-hydroxybutanoic acid [CH3-CH(OH)-CH2-COOH]\n2. 3-hydroxypentanoic acid [CH3-CH2-CH(OH)-CH2-COOH]\nLinkage type: Ester linkage (Polyester)."}

CBSE Board Exam Traps & High-Yield Strategy

• The 'Nylon' Trap: Students often confuse Nylon 6 and Nylon 6,6. Remember that Nylon 6 has only one monomer (Caprolactam, which has 6 carbons), whereas Nylon 6,6 has two monomers, each containing 6 carbons (Hexamethylenediamine and Adipic acid).
• Representing Polymeric Units: When writing equations in the exam, always include the subscript 'n' outside the square brackets and balance the output by showing the eliminated simple molecules (like $2n H_2O$ or $(2n-1) H_2O$). Skipping this will lead to a deduction of half a mark.
• Vulcanization of Rubber: Be prepared for a conceptual 2-mark question on why natural rubber is vulcanized. Natural rubber becomes soft at high temperatures and brittle at low temperatures. Heating it with 3-5% Sulfur creates cross-linking disulfide bridges, making it tough, elastic, and temperature-resistant.

Quick Revision Check

• Q1: What are the monomeric units of Bakelite? A: The monomers are Phenol ($C_6H_5OH$) and Formaldehyde ($HCHO$).
• Q2: Distinguish between homopolymers and copolymers with one example each. A: Homopolymers are made from a single monomer type (e.g., PVC from vinyl chloride). Copolymers are made from two or more different monomers (e.g., Buna-S from butadiene and styrene).
• Q3: Why is vulcanized rubber preferred over natural rubber? A: Vulcanization introduces sulfur cross-links between polymer chains. This makes the rubber stronger, more elastic, less water-absorbent, and highly resistant to temperature fluctuations and oxidation.
• Q4: Write the monomer structure of Neoprene. A: The monomer is Chloroprene (2-chloro-1,3-butadiene), which has the structure: $CH_2=C(Cl)-CH=CH_2$.

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