Ch 15 covers polymers — high molecular weight molecules made from repeating monomer units. Topics include classification, polymerisation types, important polymers, rubber, and biodegradable polymers.
By source: natural (rubber, cellulose), semi-synthetic (cellulose acetate), synthetic (nylon, PE). By structure: linear (HDP), branched (LDP), cross-linked (Bakelite). Polymerisation: addition (chain growth — free radical, e.g. polyethylene) and condensation (step growth — e.g. nylon-6,6 from adipic acid + hexamethylenediamine). Copolymers: two different monomers (e.g. Buna-S).
Nylon-6,6: condensation, used in fibres. Bakelite: phenol + formaldehyde, thermosetting. Teflon (PTFE): non-stick, chemically inert. Natural rubber: cis-polyisoprene. Vulcanisation: cross-linking with S → stronger, less sticky. Biodegradable: PHBV, nylon-2-nylon-6.
Download: https://ncert.nic.in/textbook/pdf/lech206.pdf | Part II: https://ncert.nic.in/textbook/pdf/lech2ps.zip
Thermoplastics (e.g. polyethylene) have linear/slightly branched chains held by weak intermolecular forces — heating softens them for remoulding. Thermosetting plastics (e.g. Bakelite) have extensive cross-links (strong covalent bonds) between chains. Once set, these cross-links cannot be broken by heating — further heating decomposes the polymer instead of softening it.
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