Polymers (JEE Chemistry)

Cheatsheet Content

1. Introduction to Polymers Polymers: Large molecules (macromolecules) formed by the repetitive bonding of small molecules (monomers). Monomers: The repeating structural units from which polymers are formed. Polymerization: The process of forming polymers from monomers. Degree of Polymerization (n): Number of monomer units in a polymer chain. 2. Classification of Polymers 2.1. Based on Source Natural Polymers: Found in nature (plants, animals). Examples: Starch, Cellulose, Proteins, Natural rubber. Synthetic Polymers: Man-made in laboratories. Examples: Polyethylene, PVC, Nylon, Bakelite. Semi-synthetic Polymers: Chemically modified natural polymers. Examples: Cellulose acetate (Rayon), Cellulose nitrate. 2.2. Based on Structure Linear Polymers: Monomer units linked to form long straight chains. High density, high tensile strength. Examples: HDPE, PVC, Nylon, Polyesters. Branched-chain Polymers: Monomer units form chains with side branches. Lower density, lower tensile strength. Examples: LDPE, Amylopectin. Cross-linked (Network) Polymers: Monomer units are linked together to form a 3D network. Hard, rigid, brittle. Examples: Bakelite, Melamine-formaldehyde resin, Vulcanized rubber. 2.3. Based on Mode of Polymerization Addition Polymers: Formed by the direct addition of monomers without the elimination of any by-product molecules. Monomers are usually unsaturated compounds (alkenes, alkadienes). The empirical formula of the polymer is the same as the monomer. Examples: Polyethylene, Polypropylene, PVC, Teflon, PAN, Natural rubber. Condensation Polymers: Formed by the repetitive condensation reaction between two bi-functional or tri-functional monomers with the elimination of small molecules like $H_2O$, $CH_3OH$, $HCl$, etc. Examples: Nylon-6,6, Nylon-6, Polyesters (Dacron), Bakelite, Melamine-formaldehyde resin. 2.4. Based on Molecular Forces Elastomers: Rubber-like solids with elastic properties. Weak intermolecular forces, coiled structure. Examples: Natural rubber, Buna-S, Buna-N, Neoprene. Fibres: Thread-forming solids with high tensile strength and high modulus. Strong intermolecular forces (H-bonding, dipole-dipole). Crystalline nature. Examples: Polyamides (Nylon-6,6), Polyesters (Dacron), Silk, Wool. Thermoplastics: Linear or slightly branched polymers that soften on heating and harden on cooling. Can be remoulded. Intermolecular forces intermediate between elastomers and fibres. Examples: Polyethylene, PVC, Polystyrene, Nylon, Teflon. Thermosetting Polymers: Cross-linked or heavily branched polymers that become hard and infusible on heating. Cannot be remoulded. Strong covalent bonds. Examples: Bakelite, Melamine-formaldehyde resin, Urea-formaldehyde resin. 3. Types of Polymerization 3.1. Addition Polymerization (Chain Growth Polymerization) Mechanism: Usually involves free radical, cationic, or anionic mechanisms. Free Radical Mechanism: Initiated by peroxides, persulphates, etc. Initiation: Initiator forms free radical ($R \cdot$). $R \cdot + CH_2=CH_2 \rightarrow R-CH_2-\dot{C}H_2$ Propagation: Radical adds to more monomer units. $R-(CH_2-CH_2)_n-\dot{C}H_2 + CH_2=CH_2 \rightarrow R-(CH_2-CH_2)_{n+1}-\dot{C}H_2$ Termination: Two radicals combine or disproportionate. Examples: Polyethylene: LDPE (Low Density Polyethylene): Branched, formed under high pressure (1000-2000 atm), 373-573 K, $O_2$ or peroxide initiator. Used in squeeze bottles, toys, flexible pipes. HDPE (High Density Polyethylene): Linear, formed under low pressure (6-7 atm), 333-343 K, Ziegler-Natta catalyst ($TiCl_4 + Al(C_2H_5)_3$). Used in buckets, dustbins, pipes. Polypropene (Polypropylene): Monomer: Propene. Catalyst: Ziegler-Natta. Used in ropes, carpets, pipes. Polytetrafluoroethene (Teflon): Monomer: Tetrafluoroethene ($CF_2=CF_2$). Catalyst: Persulphate initiator. Non-stick coatings, oil seals. Polyacrylonitrile (PAN): Monomer: Acrylonitrile ($CH_2=CH-CN$). Peroxide initiator. Orlon, Acrilan (substitute for wool). Polyvinyl Chloride (PVC): Monomer: Vinyl chloride ($CH_2=CH-Cl$). Used in pipes, floorings. Polystyrene: Monomer: Styrene ($C_6H_5-CH=CH_2$). Used in packaging, insulation. 3.2. Condensation Polymerization (Step Growth Polymerization) Involves reactions between monomers with functional groups like $-OH, -COOH, -NH_2$. Proceeds in a stepwise manner, eliminating small molecules. Examples: Polyamides: Polymers with amide linkages ($-CONH-$). Nylon-6,6: Monomers: Hexamethylenediamine ($H_2N-(CH_2)_6-NH_2$) + Adipic acid ($HOOC-(CH_2)_4-COOH$). Strong, used in sheets, brushes, textiles. Nylon-6: Monomer: Caprolactam. Polymerized by heating with water. Used in tire cords, fabrics, ropes. Polyesters: Polymers with ester linkages ($-COOR-$). Terylene (Dacron): Monomers: Ethylene glycol ($HO-CH_2-CH_2-OH$) + Terephthalic acid ($HOOC-C_6H_4-COOH$). Used in fabric blends, safety belts, recording tapes. Phenol-Formaldehyde Polymer (Bakelite): Monomers: Phenol + Formaldehyde. Intermediate products: Novolac (linear, thermoplastic) and Resole (branched). Novolac on heating with formaldehyde forms cross-linked Bakelite (thermosetting). Used in electrical switches, handles of utensils, phonograph records. Melamine-Formaldehyde Resin: Monomers: Melamine + Formaldehyde. Used in unbreakable crockery. Urea-Formaldehyde Resin: Monomers: Urea + Formaldehyde. Used in adhesives, laminates. 4. Copolymerization Process of polymerization in which two or more different monomers are allowed to polymerize together. The product is called a copolymer . Examples: Buna-S (Styrene-butadiene rubber): Monomers: 1,3-Butadiene + Styrene. Used in tire manufacturing, footwear components. Buna-N (Acrylonitrile-butadiene rubber): Monomers: 1,3-Butadiene + Acrylonitrile. Resistant to oils and organic solvents. Used in oil seals, tank linings. 5. Natural Rubber Monomer: Isoprene (2-methyl-1,3-butadiene). It is a cis -polyisoprene. Has coiled structure, weak intermolecular forces, elastic properties. Vulcanization of Rubber: Process of heating natural rubber with sulfur (3-5%) to improve its physical properties (toughness, elasticity, resistance to abrasion). Sulfur forms cross-links between polymer chains. Higher sulfur content leads to ebonite (hard rubber). 6. Biodegradable Polymers Polymers that can be degraded by microorganisms. Essential for environmental protection. Examples: PHBV (Poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate): Monomers: 3-Hydroxybutanoic acid + 3-Hydroxypentanoic acid. Used in specialty packaging, orthopedic devices, drug release. Nylon-2-Nylon-6: Monomers: Glycine ($H_2N-CH_2-COOH$) + Aminocaproic acid ($H_2N-(CH_2)_5-COOH$). PGA (Polyglycolic Acid) PLA (Polylactic Acid) 7. Important Monomers and Their Polymers (Summary Table) Monomer Polymer Type Uses Ethene Polyethylene (PE) Addition Bags, bottles, pipes Propene Polypropylene (PP) Addition Ropes, carpets Vinyl chloride Polyvinyl chloride (PVC) Addition Pipes, flooring Tetrafluoroethene Teflon Addition Non-stick surfaces Acrylonitrile Polyacrylonitrile (PAN) Addition Orlon, Acrilan (synthetic wool) 1,3-Butadiene + Styrene Buna-S Copolymer Tires, footwear 1,3-Butadiene + Acrylonitrile Buna-N Copolymer Oil seals, tank linings Isoprene Natural Rubber Addition Tires, elastic goods Chloroprene Neoprene (Synthetic Rubber) Addition Gaskets, hoses Hexamethylenediamine + Adipic acid Nylon-6,6 Condensation Textiles, brushes Caprolactam Nylon-6 Condensation Tire cords, fabrics Ethylene glycol + Terephthalic acid Terylene (Dacron) Condensation Fabrics, magnetic tapes Phenol + Formaldehyde Bakelite Condensation Switches, handles Melamine + Formaldehyde Melamine-Formaldehyde Resin Condensation Unbreakable crockery 3-Hydroxybutanoic acid + 3-Hydroxypentanoic acid PHBV Condensation (Biodegradable) Packaging, medical implants