Polymers
Easy Overview
Polymers are giant molecules made by linking many small repeating units (monomers) together. Think of them as molecular chains β like beads on a string, where each bead is a monomer. Polymers are everywhere: the plastic bottle you drink from (PET), the bag you carry groceries in (polyethylene), the rubber tires on cars (styrene-butadiene rubber), the synthetic fiber in your clothes (nylon, polyester), the Teflon coating on non-stick pans, and even the DNA in your cells (a natural polymer). The word polymer comes from Greek β poly = many, meros = parts. Polymers are classified in several ways: by source (natural β cellulose, proteins, rubber; synthetic β nylon, polyethylene; semi-synthetic β cellulose acetate), by structure (linear, branched, cross-linked), by polymerization mechanism (addition β chain growth; condensation β step growth), and by thermal behavior (thermoplastics β melt on heating, can be reshaped; thermosets β set irreversibly on heating, cannot be reshaped). The properties of a polymer depend on the monomer(s) used, the molecular weight, the degree of polymerization (number of monomer units), and the arrangement of chains (crystalline vs amorphous regions). The discovery and development of synthetic polymers is one of the great stories of industrial chemistry. Wallace Carothers at DuPont invented nylon in 1935 β the first synthetic fiber. Roy Plunkett accidentally discovered Teflon in 1938 while trying to make a new refrigerant. Karl Ziegler and Giulio Natta developed catalysts that control polymer stereochemistry (Nobel Prize 1963). Paul Flory won the Nobel Prize (1974) for his theoretical work on polymers. And Stephanie Kwolek invented Kevlar (aramid fiber, five times stronger than steel by weight) in 1965. Today, global plastic production exceeds 400 million tons per year β and the environmental problem of plastic waste is one of our greatest challenges. Biodegradable polymers (PLA, PHBV) and recycling are areas of active research. In the Maharashtra Board syllabus, you need to know the classification, methods of polymerization, and the preparation and uses of important polymers.
Classification of polymers
On the basis of source: (1) Natural polymers β obtained from nature: cellulose (cotton), starch, proteins (wool, silk, leather), natural rubber (latex from Hevea brasiliensis), nucleic acids. (2) Semi-synthetic polymers β chemically modified natural polymers: cellulose acetate (rayon), cellulose nitrate (celluloid), vulcanized rubber. (3) Synthetic polymers β man-made from monomers: polyethylene, nylon, polyester (Terylene/Dacron), polystyrene, PVC, Teflon, Bakelite. On the basis of structure: Linear polymers β chains linked end-to-end (thermoplastics like polyethylene, PVC). Branched polymers β side chains attached to the main chain (low-density polyethylene, LDPE). Cross-linked polymers β chains connected by covalent bonds into a network (Bakelite, melamine-formaldehyde). On the basis of polymerization mechanism: Addition polymers β formed by repeated addition of monomers with double bonds (free radical or ionic mechanism). Condensation polymers β formed by condensation reactions between monomers with loss of small molecules (HβO, NHβ, HCl). On the basis of thermal behavior: Thermoplastics β soften on heating, harden on cooling, can be reprocessed (linear/branched polymers). Thermosetting polymers β become permanently hard on heating (cross-linked), cannot be remelted (Bakelite, urea-formaldehyde).
Addition polymerization β free radical mechanism
Addition polymerization (chain growth) involves monomers with C=C double bonds that open up to form long chains. The mechanism has three steps: Initiation, Propagation, and Termination. Let's take ethene β polyethene: Initiation: a free radical initiator (like benzoyl peroxide or AIBN) decomposes on heating to form free radicals. The radical attacks the C=C bond, forming a new C-C bond and creating a new radical at the chain end. Propagation: the radical at the chain end attacks another monomer molecule, adding to the chain and generating a new radical. This process repeats thousands of times, growing the chain rapidly (each addition takes fractions of a second). The chain adds about 1000-2000 monomers per second! Termination: two growing chains combine (combination β two radicals meet and form a single bond) or disproportionate (one radical transfers a hydrogen to another, giving one saturated and one unsaturated end). Other types of addition polymerization: Cationic (requires a proton or Lewis acid initiator β for monomers with electron-donating groups like isobutylene β butyl rubber). Anionic (requires a strong base/alkali metal initiator β for monomers with electron-withdrawing groups like styrene). Coordination (Ziegler-Natta catalysts β TiClβ/Al(CβHβ )β) gives stereoregular polymers (isotactic, syndiotactic) with controlled tacticity. Ziegler-Natta polymerization produces high-density polyethylene (HDPE) and isotactic polypropylene.
Condensation polymerization
Condensation polymerization (step growth) involves the reaction between two different functional groups, with the elimination of a small molecule (HβO, NHβ, CHβOH). The monomers have at least two functional groups each. The polymerization is stepwise β dimers form, then trimers, tetramers, and so on. The process is slower than addition polymerization (takes hours), and the molecular weight builds up only at very high conversion (greater than 99% conversion needed for high molecular weight). Examples: (1) Nylons β polyamides. Nylon-6,6: hexamethylenediamine + adipic acid (heat) β nylon-6,6 + HβO. The numbers indicate number of carbons in diamine and diacid. Nylon-6: caprolactam (ring-opening polymerization, not strictly condensation). (2) Polyesters β Terylene (Dacron): ethylene glycol + terephthalic acid β polyester + HβO. (3) Polycarbonates (Lexan): bisphenol A + phosgene β polycarbonate (used in DVD discs, bulletproof glass). (4) Phenol-formaldehyde (Bakelite): phenol + formaldehyde (HβΊ or OHβ» catalyst) β cross-linked polymer. The first synthetic polymer (Leo Baekeland, 1907). Bakelite is thermosetting β used for electrical switches, handles. (5) Urea-formaldehyde β used for adhesives and laminates.
Polyethylene β types and properties
Polyethylene (polythene, -(CHβ-CHβ)-β) is the most widely produced plastic in the world. Low-density polyethylene (LDPE): prepared by free radical polymerization at high pressure (1000-3000 atm) and temperature (150-300Β°C). The mechanism produces branched chains (backbiting during propagation creates short branches). Branches prevent close packing β lower density (0.91-0.93 g/cmΒ³), lower crystallinity (50-60%), lower melting point (110Β°C). LDPE is flexible, transparent, and soft. Uses: plastic bags, squeeze bottles, film wrap, wire insulation. High-density polyethylene (HDPE): prepared using Ziegler-Natta or Phillips catalysts at low pressure (1-50 atm) and temperature (70-150Β°C). No branching β linear chains pack tightly β higher density (0.95-0.97 g/cmΒ³), higher crystallinity (80-90%), higher melting point (135Β°C). HDPE is rigid, opaque, and stronger. Uses: milk jugs, detergent bottles, pipes, hard hats, cutting boards. Cross-linked polyethylene (PEX): chemical or radiation cross-linking gives a thermoset with heat resistance and shape memory. Used for hot water pipes. Ultra-high molecular weight polyethylene (UHMWPE): extremely long chains (MW > 3 million) β very high impact resistance, wear resistant. Used for artificial joints.
Teflon, PVC, polystyrene, and other addition polymers
Polytetrafluoroethylene (PTFE, Teflon): -(CFβ-CFβ)-β. Prepared by free radical polymerization of tetrafluoroethylene. Extremely inert due to strong C-F bonds (F is the most electronegative element β shields carbon backbone completely). Teflon is non-stick, heat-resistant (up to 260Β°C), chemically inert (resists aqua regia!), has very low coefficient of friction (ice on ice is more frictional), and is hydrophobic. Uses: non-stick cookware, wire insulation, gaskets, seals, lab equipment. Polyvinyl chloride (PVC): -(CHβ-CHCl)-β. Prepared from vinyl chloride monomer (CHβ=CHCl). PVC is hard and brittle alone β plasticizers (like phthalates) are added to make it flexible. Properties depend on additives: rigid PVC (unplasticized) for pipes, window frames; flexible PVC (plasticized) for hoses, flooring, shower curtains, artificial leather. PVC has good flame retardance (chlorine content), but burning produces HCl gas and dioxins (toxic). Polystyrene (PS): -(CHβ-CH(CβHβ ))-β. Prepared from styrene monomer (CβHβ CH=CHβ). Polystyrene is clear, rigid, and brittle. Expanded polystyrene (EPS, Styrofoam) is foamed with pentane β 95% air, 5% polystyrene β excellent thermal insulator. Uses: disposable cups, packaging material (peanuts), insulation boards. Polyacrylonitrile (PAN): -(CHβ-CH(CN))-β. Used for acrylic fibers (Orlon, Acrilan β synthetic wool). Also precursor for carbon fibers (carbonized at high temperature, used in aerospace composites). Poly(methyl methacrylate) (PMMA, Plexiglas, Lucite): clear glass substitute, shatter-resistant, lighter than glass. Uses: windows, skylights, aquariums, acrylic paints.
Natural rubber and vulcanization
Natural rubber (NR) is obtained from the latex of Hevea brasiliensis (rubber tree). Latex is a colloidal suspension of rubber particles. The monomer is isoprene (2-methyl-1,3-butadiene, Cβ Hβ). Natural rubber is cis-1,4-polyisoprene β the isoprene units are joined in a cis configuration at the 1 and 4 positions. The cis configuration gives a coiled, elastic structure because the chain kinks back on itself. Gutta-percha (trans-1,4-polyisoprene) is a hard, inelastic, crystalline material β found in some trees, used for golf ball covers and electrical insulation. Drawback of natural rubber: it becomes soft and sticky in summer, hard and brittle in winter. Charles Goodyear (1839) discovered vulcanization by accident β he dropped a mixture of rubber and sulfur on a hot stove. Vulcanization: heating natural rubber with sulfur (2-5% for soft rubber, 10-30% for hard rubber = ebonite). Sulfur atoms form cross-links (disulfide bridges, -S-S-) between polymer chains. This restricts chain movement β rubber becomes stronger, more elastic, and less temperature-sensitive. The cross-links prevent the chains from flowing past each other (permanent deformation) but still allow stretching and recovery. Vulcanized rubber is used for tires, shoe soles, hoses, and countless other products. Synthetic rubbers: Styrene-butadiene rubber (SBR) β the most widely used synthetic rubber (for car tires), prepared by emulsion polymerization. Butyl rubber (isobutylene + isoprene) β excellent air impermeability (used for inner tubes). Neoprene (2-chloro-1,3-butadiene) β oil-resistant, used for wetsuits and gaskets.
Biodegradable polymers
Synthetic polymers are generally non-biodegradable β they persist in the environment for hundreds of years. Biodegradable polymers can be broken down by microorganisms into COβ, HβO, and biomass under appropriate conditions. Natural biodegradable polymers: starch, cellulose, proteins, polyhydroxyalkanoates (PHAs). Synthetic biodegradable polymers: Poly(lactic acid) (PLA) β made from corn starch, used for compostable cups, 3D printing filament, surgical sutures. Poly(Ξ΅-caprolactone) (PCL) β used in drug delivery systems. Poly(hydroxybutyrate-hydroxyvalerate) (PHBV) β made by bacteria from glucose or starch, used for packaging. Poly(vinyl alcohol) (PVA) β water-soluble, biodegradable under certain conditions. The importance of biodegradable polymers has grown enormously due to the plastic pollution crisis. The Great Pacific Garbage Patch (a floating accumulation of plastic waste in the ocean) contains an estimated 1.8 trillion pieces of plastic. Microplastics (particles <5 mm) from the fragmentation of larger plastics are now found everywhere β in drinking water, fish, and even human blood. Biodegradable polymers are part of the solution, along with reducing plastic use, improving recycling, and developing better waste management systems. In the exam, you may be asked about the structure and applications of specific biodegradable polymers.
Key Points
- β’Polymers: large molecules made of repeating monomer units
- β’Classification: natural (cellulose, rubber), semi-synthetic (rayon), synthetic (nylon)
- β’Addition (chain-growth): C=C monomers, free radical/ionic mechanism
- β’Condensation (step-growth): difunctional monomers, elimination of small molecule
- β’Free radical polymerization steps: initiation β propagation β termination
- β’Ziegler-Natta catalysts: stereoregular polymers (isotactic, syndiotactic)
- β’Thermoplastics (linear/branched, meltable) vs thermosets (cross-linked, infusible)
- β’LDPE: branched, flexible (bags); HDPE: linear, rigid (bottles, pipes)
- β’PVC: requires plasticizers for flexibility; Teflon: inert, non-stick
- β’Natural rubber: cis-1,4-polyisoprene; vulcanization: S cross-links β strength
- β’Synthetic rubbers: SBR (tires), Neoprene (oil-resistant), Butyl (airtight)
- β’Biodegradable polymers: PLA (corn starch), PHBV (bacterial), PCL
- β’Plastic pollution: 400M tons/year; microplastics are a global concern
Practice Questions
- Classify polymers based on (a) source (b) structure (c) polymerization mechanism with examples.
- Explain the free radical mechanism of addition polymerization using ethene as an example.
- Distinguish between addition and condensation polymerization with examples.
- Write the preparation, properties, and uses of (a) Nylon-6,6 (b) Terylene (c) Teflon.
- What is vulcanization? Why is natural rubber vulcanized? Explain the role of sulfur.
- Explain Ziegler-Natta catalysis. What is its importance in polymer chemistry?
- Define the following: (a) Thermoplastics (b) Thermosetting polymers (c) Biodegradable polymers. Give two examples of each.
- Compare LDPE and HDPE in terms of preparation, structure, properties, and uses.