CBSE Class 12 Chemistry: Polymers — Complete Notes 2026

Introduction to Polymers

• Polymer: large molecule from repeating monomer units; prefix 'poly' means many
• Monomer: small molecule that links together; ethylene → polyethylene (LDPE, HDPE)
• Degree of polymerisation: number of monomers in a polymer chain; higher DP → higher molecular weight

Addition Polymers

• Chain growth: monomer adds to growing chain; no by-product; alkene monomers (C=C)
• Polyethylene: ethene → LDPE (branched, flexible, bags) or HDPE (linear, rigid, pipes)
• PVC (vinyl chloride → poly vinyl chloride): pipes, flooring; Teflon (tetrafluoroethylene): non-stick surfaces

Condensation Polymers

• Step growth: two different monomers with reactive groups (−OH, −COOH, −NH₂); by-product H₂O or HCl released
• Nylon-6,6: hexamethylenediamine + adipic acid; −NH− + −CO− forms amide linkage; strong, used in fibres
• Polyesters (Dacron/PET): ethylene glycol + terephthalic acid; −O− ester linkage; bottles, fibres

Natural and Synthetic Rubber

• Natural rubber: cis-polyisoprene from Hevea brasiliensis; elastic; low tensile strength; not water-resistant
• Vulcanisation: cross-linking rubber with sulphur at 150°C; S bridges between chains; increases strength, elasticity
• Synthetic rubber: neoprene (oil-resistant), Buna-S (car tyres), Buna-N (oil-resistant seals)

Thermoplastics and Thermosetting Polymers

• Thermoplastic: soften on heating; can be remoulded; linear/branched chains; PVC, LDPE, nylon
• Thermosetting: cross-linked on heating; permanent shape; cannot be remelted; Bakelite, urea-formaldehyde resin
• Bakelite: phenol + formaldehyde; 3D cross-linked network; first synthetic polymer (1907); used in electrical fittings

Biodegradable Polymers

• PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate): bacterial polymer; biodegradable; surgical implants
• PLA (polylactic acid): from glucose fermentation; compostable; packaging, medical sutures
• Environmental issues: conventional plastics persist 500+ years; microplastics in ocean; push for bioplastics

CBSE Board Focus

• Polymers: 4–5 marks; addition vs condensation mechanism; draw repeating unit from monomer
• Nylon-6,6, PET, PHBV: know monomer, linkage type, use; frequently asked
• Thermoplastic vs thermosetting: define and give 2 examples of each