🎯 Key Points
- Addition polymerisation: monomers with C=C add directly, no byproduct (PE, PVC, Teflon); Condensation: small molecule (H₂O/HCl) eliminated each step (Nylon, Dacron, Bakelite)
- Thermoplastics soften and re-mould on heating (PVC, PE); Thermosets harden permanently after curing (Bakelite, melamine)
- Nylon = amide bonds (-CO-NH-); Dacron/polyester = ester bonds (-CO-O-)
- Homopolymer = single monomer type (polyethylene); Copolymer = 2+ monomer types (Buna-S from butadiene+styrene)
- Vulcanisation = adding sulphur cross-links to rubber, increasing strength/elasticity, reducing tackiness
- Ziegler-Natta catalyst gives more linear HDPE; free-radical mechanism gives more branched LDPE
Addition polymerisation simply links monomers containing a double bond with nothing left over, while condensation polymerisation joins two different monomers through a reaction (often forming an ester or amide bond) that releases a small molecule like water at each step.
What is a Polymer?
A polymer is a large molecule made of many repeating structural units called monomers. The process of forming polymers is called polymerisation.
Types of Polymerisation
- Addition polymerisation: Monomers with double bonds link by opening the bond; no byproduct
- Polyethylene (PE): nCH₂=CH₂ → -(CH₂-CH₂)ₙ-
- PVC: nCH₂=CHCl → -(CH₂-CHCl)ₙ- (pipes, flooring)
- Teflon (PTFE): nCF₂=CF₂ → -(CF₂-CF₂)ₙ- (non-stick coating)
- Polystyrene: used in packaging and insulation
- Condensation polymerisation: Monomers have two functional groups; small molecule (H₂O or HCl) is eliminated in each step
- Nylon-6,6: hexamethylene diamine + adipic acid → polyamide (strong fibre, rope)
- Dacron (polyester): ethylene glycol + terephthalic acid (clothing, bottles)
- Bakelite: phenol + formaldehyde → thermosetting plastic (electrical fittings)
Addition polymerisation: unsaturated monomers such as vinyl chloride join by opening their C=C double bonds, so the polymer (PVC) has the same repeating formula as the monomer — nothing is eliminated. Image: Jü, Public Domain, via Wikimedia Commons.
Condensation polymerisation: two difunctional monomers (here hexamethylenediamine and adipic acid) link through amide bonds to give nylon-6,6, expelling a small molecule (H2O) at every step — the key contrast with addition polymerisation. Image: NadirSH, CC BY 4.0, via Wikimedia Commons.
Classification by Properties
- Thermoplastic: Soften on heating, re-mouldable; PVC, PE, polystyrene
- Thermosetting: Permanently hard after curing; Bakelite, melamine
- Elastomers: Highly elastic; natural rubber, Buna-S, Neoprene
Natural Polymers
- Natural rubber: polyisoprene; vulcanisation (with sulphur) increases strength and elasticity
- Cellulose: polymer of beta-glucose; structural role in plants; basis of cotton and paper
- Proteins and nucleic acids (DNA, RNA) are also naturally occurring polymers
Quick Tips
- Nylon has amide bonds (-CO-NH-); Dacron has ester bonds (-CO-O-)
- Vulcanisation adds cross-links between rubber chains via sulphur bridges
- Biodegradable polymers: PHBV, polylactic acid (PLA)
Classification by Source and Structure
- Natural polymers: Occur in nature; proteins, cellulose, natural rubber, starch, nucleic acids
- Synthetic polymers: Man-made; polyethylene, nylon, PVC, Bakelite
- Semi-synthetic polymers: Chemically modified natural polymers, e.g., cellulose acetate (rayon), cellulose nitrate
- Linear, branched, and cross-linked polymers: classified by the arrangement of monomer chains; cross-linked polymers (like Bakelite) are rigid and infusible due to covalent bonds joining chains
- Homopolymer vs copolymer: Homopolymers are made from a single repeating monomer (polyethylene); copolymers are made from two or more different monomers (Buna-S from butadiene and styrene)
Mechanism of Addition Polymerisation
- Free-radical mechanism: Initiator (like benzoyl peroxide) generates a free radical, which adds to the monomer, propagates the chain, and is finally terminated by combination or disproportionation
- Used to make polyethylene (low-density, LDPE) and polystyrene
- Ziegler-Natta catalyst (TiCl₄ + Al(C₂H₅)₃) is used to make high-density polyethylene (HDPE) with a more linear, less branched structure
More Important Polymers and Their Monomers
- Nylon-6: Made by self-condensation of caprolactam; used in fibres, ropes, and fabrics
- Buna-N: Copolymer of 1,3-butadiene and acrylonitrile; oil-resistant synthetic rubber
- Neoprene: Polymer of chloroprene; resistant to oils and used in conveyor belts, gaskets
- Melamine-formaldehyde resin: Condensation polymer used to make unbreakable crockery and laminates
- Glyptal: Made from ethylene glycol and phthalic acid; used in paints and lacquers
Biodegradable Polymers in Detail
- PHBV (poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate): Copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid; used in packaging and as a biodegradable substitute for plastics, broken down by soil bacteria
- PLA (polylactic acid): Made from lactic acid units; biodegrades into harmless products, used in medical sutures and packaging
- Biodegradable polymers help reduce plastic pollution since they can be broken down by microorganisms into simpler, non-toxic products
Classification Based on Molecular Forces
- Elastomers: polymer chains held by the weakest intermolecular forces, allowing them to be stretched and to spring back; the weak forces permit elasticity, e.g. natural rubber, Buna-S, Buna-N
- Fibres: strong intermolecular forces (hydrogen bonds, dipole-dipole) give close packing, high tensile strength and crystallinity, e.g. nylon-6,6, terylene, silk
- Thermoplastics: intermolecular forces intermediate between elastomers and fibres; soften on heating and re-harden on cooling, e.g. polythene, polystyrene, PVC
- Thermosetting polymers: heavily cross-linked, three-dimensional network polymers that set permanently and cannot be re-moulded, e.g. bakelite, melamine-formaldehyde resin
Molecular Mass of Polymers
- A polymer sample is a mixture of chains of different lengths, so its molecular mass is an average value
- Number-average molecular mass is obtained by counting the number of molecules of each size; determined by colligative-property methods (e.g. osmotic pressure)
- Weight-average molecular mass weights each chain by its mass; determined by light-scattering and ultracentrifugation, and is always greater than or equal to the number-average value
- Polydispersity index (PDI) = weight-average / number-average mass; PDI = 1 for a perfectly uniform (monodisperse) polymer, and is greater than 1 for real synthetic polymers
Rubber: Natural and Synthetic
- Natural rubber is cis-1,4-polyisoprene (an addition polymer of the diene isoprene, 2-methyl-1,3-butadiene); the cis double bonds give it a coiled, elastic structure
- Vulcanisation: heating raw rubber with sulphur (about 3-5%) introduces sulphur cross-links (-S-S- bridges) between chains, greatly increasing strength, elasticity and resistance to heat and abrasion, while reducing tackiness
- Buna-S (SBR): copolymer of 1,3-butadiene and styrene; used in tyres and footwear
- Buna-N: copolymer of 1,3-butadiene and acrylonitrile; oil- and petrol-resistant
- Neoprene: polymer of chloroprene (2-chloro-1,3-butadiene); resistant to oils, used in gaskets and hoses
Polyacrylonitrile and Other Addition Polymers
- Polyacrylonitrile (Orlon/Acrilan): addition polymer of acrylonitrile (CH₂=CH-CN); used as a substitute for wool in sweaters and blankets
- Polytetrafluoroethylene (Teflon/PTFE): polymer of tetrafluoroethene; chemically inert, heat-resistant non-stick coating for cookware
- Poly(methyl methacrylate) (PMMA, Plexiglas/Lucite): transparent, shatter-resistant substitute for glass
- Nylon-2-nylon-6: an alternating polyamide copolymer of glycine and aminocaproic acid — an important biodegradable synthetic polymer
🚀 JEE Advanced Edge
LDPE vs HDPE structure-property relationship: LDPE's branched chains pack loosely, giving lower density, lower crystallinity, and flexibility (used for bags, films). HDPE's linear chains (from Ziegler-Natta catalysis) pack tightly, giving higher density, higher crystallinity, and rigidity (used for pipes, bottles, containers) — branching is the single structural feature explaining both the name and the property difference.
Why vulcanised rubber is stronger than raw rubber: Raw natural rubber (cis-polyisoprene) has weak chains that can slip past each other under stress. Vulcanisation introduces sulphur cross-links between different polymer chains, restricting this slippage — more cross-links (more sulphur, longer heating) gives a harder, more rigid product, which is why vulcanisation degree is tuned for the application (soft rubber bands vs hard tyres).
Worked problem: Identify the type of polymerisation and the repeat unit for the formation of Nylon-6,6 from hexamethylenediamine (H₂N-(CH₂)₆-NH₂) and adipic acid (HOOC-(CH₂)₄-COOH). Approach: This is condensation polymerisation — each amine reacts with each carboxylic acid, eliminating H₂O and forming an amide (-CO-NH-) linkage. The repeat unit is [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]ₙ, with water eliminated at every linkage formed.