Unit 5 | Engineering Chemistry Notes | AKTU Notes

UNIT 5: Materials Chemistry — Polymers and Organometallic Compounds

PART A: POLYMERS

5.1 Introduction to Polymers

The word polymer comes from Greek: poly = many, meros = parts/units.

A polymer is a large molecule (macromolecule) made up of a very large number of small repeating units called monomers joined together by covalent bonds.

• Monomer: Small, simple molecule that can react to form polymer. Example: ethylene (CH₂=CH₂).
• Polymer: Large molecule formed by joining many monomers. Example: polyethylene (–CH₂–CH₂–)ₙ.
• Degree of Polymerization (n): Number of repeating units in a polymer chain. Can be hundreds to thousands.
• Molecular weight of polymer: Molecular weight of monomer × degree of polymerization.

5.2 Classification of Polymers

1. Based on Source/Origin:

• Natural polymers: Found in nature. Examples: Rubber (polyisoprene), cellulose, starch, proteins (polyamides), DNA, wool, silk.
• Synthetic polymers: Made by chemical synthesis. Examples: Nylon, polyethylene, PVC, Teflon, Bakelite.
• Semi-synthetic polymers: Natural polymers chemically modified. Examples: Rayon (from cellulose), cellulose acetate.

2. Based on Structure:

• Linear polymers: Monomers joined in a straight chain. High melting point, strong. Example: PVC, Nylon.
• Branched polymers: Main chain with side branches. Lower melting point than linear. Example: Low density polyethylene (LDPE).
• Cross-linked (network) polymers: Chains connected to each other by covalent bonds — forming a 3D network. Very hard, rigid, do not melt. Example: Bakelite, vulcanized rubber.

3. Based on Thermal Behavior:

• Thermoplastics: Soften on heating, harden on cooling — repeatedly. Can be molded multiple times. Example: PVC, polyethylene, nylon.
• Thermosetting plastics: Set permanently on heating — cannot be re-melted. Irreversible cross-linking occurs. Example: Bakelite, Urea-formaldehyde resin, epoxy resin.

4. Based on Application:

• Plastics, fibers, elastomers (rubbers), adhesives, coatings.

5.3 Polymerization Processes

1. Addition Polymerization (Chain-growth polymerization):

• Monomers with double bonds or triple bonds add to each other one at a time.
• No byproduct is formed — all monomer becomes polymer.
• Requires an initiator (free radical, cation, or anion) to start the chain.
• Three steps: Initiation → Propagation → Termination.
• Example: Ethylene → Polyethylene: nCH₂=CH₂ → (–CH₂–CH₂–)ₙ
• Other examples: Propylene → Polypropylene, Vinyl chloride → PVC, Styrene → Polystyrene, Tetrafluoroethylene → Teflon.

2. Condensation Polymerization (Step-growth polymerization):

• Monomers with two or more functional groups react with each other, releasing a small molecule as byproduct (usually water, HCl, methanol).
• Requires monomers with at least two reactive functional groups (bifunctional monomers).
• Polymer grows stepwise — molecular weight builds gradually.
• Example: Nylon-6,6: Hexamethylenediamine + Adipic acid → Nylon-6,6 + H₂O
• Example: Terylene (polyester): Ethylene glycol + Terephthalic acid → Terylene + H₂O
• Example: Bakelite: Phenol + Formaldehyde → Bakelite + H₂O

Feature	Addition Polymerization	Condensation Polymerization
Monomers	Unsaturated compounds (double/triple bonds)	Bifunctional compounds
Byproduct	None	Small molecule (H₂O, HCl)
Growth	Chain grows by adding one monomer at a time	Stepwise — all intermediates present simultaneously
Examples	PE, PVC, Teflon, polystyrene	Nylon, terylene, Bakelite

5.4 Thermosetting and Thermoplastic Polymers

Thermoplastic Polymers:

• Linear or branched structure — no cross-links between chains.
• Chains are held together by weak van der Waals forces or hydrogen bonds.
• Soften and melt on heating → can be reshaped → harden on cooling.
• Recyclable — can be melted and remolded many times.
• Examples: Polyethylene, PVC, Polypropylene, Nylon, Polystyrene, Teflon.
• Applications: Bags, bottles, pipes, fibers, packaging material.

Thermosetting Polymers:

• Heavily cross-linked 3D network structure.
• On first heating, they soften and can be molded, but simultaneously cross-linking reactions occur.
• Once set (cured), they cannot be re-melted or reshaped — irreversible.
• Not recyclable — they char/burn instead of melting.
• Very hard, rigid, good heat resistance.
• Examples: Bakelite, Urea-formaldehyde resin (UF), Melamine-formaldehyde resin, Epoxy resin.
• Applications: Electrical switches, handles, adhesives, circuit boards.

5.5 Polymer Blends and Composites

Polymer Blends:

• A mixture of two or more polymers mixed together (like mixing two metals to form an alloy).
• Blending is done to improve properties that individual polymers lack.
• Example: ABS (Acrylonitrile-Butadiene-Styrene) — blend of three polymers: combines the hardness of acrylonitrile, flexibility of butadiene, and ease of processing of styrene.
• Example: PVC blended with rubber for better flexibility.
• Types: Miscible blends (compatible, single phase) and immiscible blends (two phases, need compatibilizer).

Polymer Composites:

• A material made of a polymer matrix reinforced with fillers or reinforcements (like fibers, particles) to give improved properties.
• The polymer is the matrix (binder), the filler provides strength, stiffness, or other properties.
• Examples:
• Glass Fiber Reinforced Plastic (GFRP/Fiberglass): Glass fibers in polyester or epoxy matrix. Lightweight and strong. Used in boats, car bodies, sports equipment.
• Carbon Fiber Reinforced Plastic (CFRP): Carbon fibers in epoxy matrix. Very light and extremely strong. Used in aerospace, racing cars, sports equipment (tennis rackets, bicycles).
• Rubber composites: Rubber filled with carbon black — increases strength and abrasion resistance of tyres.

5.6 Conducting Polymers

• Normally, polymers are insulators. But certain polymers with extended conjugated double bond systems can conduct electricity — these are called conducting polymers.
• They conduct electricity through delocalized π electrons along the conjugated chain.
• Conductivity is enhanced by doping — adding oxidizing or reducing agents.
• Nobel Prize in Chemistry 2000 was awarded for the discovery of conducting polymers.

Examples of Conducting Polymers:

• Polyacetylene: First conducting polymer discovered. (CH=CH)ₙ
• Polyaniline (PANI): Most widely studied. Green in oxidized (conducting) form. Stable, easy to synthesize.
• Polypyrrole: Good conductor, stable in air.
• Polythiophene: Good thermal stability.

Applications of Conducting Polymers:

• Organic solar cells and LEDs (OLEDs).
• Rechargeable batteries and supercapacitors.
• Corrosion protection coatings.
• Biosensors and chemical sensors.
• Antistatic coatings.
• Electrochromic devices (smart windows that change color).

5.7 Biodegradable Polymers

• Traditional plastics (polyethylene, PVC) are not biodegradable — they persist in the environment for hundreds of years, causing serious pollution.
• Biodegradable polymers are polymers that can be broken down by microorganisms (bacteria, fungi) into harmless natural substances (CO₂, water, biomass) within a reasonable time.

Examples:

• PLA (Polylactic acid): Made from lactic acid (from fermentation of starch). Biodegrades in compost. Used in disposable cups, food packaging, medical sutures.
• PGA (Polyglycolic acid): Used for medical sutures (dissolves in body after wound heals).
• PHB (Polyhydroxybutyrate): Produced by bacteria. Biodegrades in soil and water. Used in packaging.
• Starch-based plastics: Made from corn or potato starch. Biodegradable shopping bags.

Advantages:

• Reduce plastic pollution in soil and water bodies.
• Made from renewable resources.
• Lower carbon footprint than petroleum-based plastics.

5.8 Preparation, Properties and Industrial Applications of Important Polymers

5.8.1 Teflon (Polytetrafluoroethylene — PTFE)

• Monomer: Tetrafluoroethylene (CF₂=CF₂).
• Preparation: Addition polymerization: nCF₂=CF₂ → (–CF₂–CF₂–)ₙ
• Properties: Extremely low friction coefficient (very slippery), excellent chemical resistance (resistant to almost all chemicals — called "the most chemically resistant polymer"), high temperature stability (up to 260°C), non-stick surface, excellent electrical insulator.
• Applications: Non-stick cookware (Teflon-coated pans), gaskets, seals, and valves in chemical industries, electrical insulation, bearings and sliding surfaces, medical implants (tubing, catheters), waterproof clothing (Gore-Tex).

5.8.2 Lucite (Polymethyl Methacrylate — PMMA) / Perspex / Plexiglass

• Monomer: Methyl methacrylate (CH₂=C(CH₃)COOCH₃).
• Preparation: Addition (free radical) polymerization of methyl methacrylate.
• Properties: Optically transparent (92% light transmission — clearer than glass), lightweight (half the weight of glass), shatter-resistant (safer than glass), weather resistant, UV resistant, can be easily machined and polished.
• Applications: Aircraft windows, car headlight lenses and tail lights, aquarium tanks, display panels, safety shields, contact lenses, bone cement in orthopedic surgery, optical fibers (low-quality), advertising signs.

5.8.3 Bakelite (Phenol-Formaldehyde Resin)

• Monomers: Phenol (C₆H₅OH) and Formaldehyde (HCHO).
• Preparation: Condensation polymerization of phenol with formaldehyde under acid or alkali catalyst, forming a cross-linked 3D network. Water is released as byproduct.
• Properties: Hard and rigid, thermosetting (cannot be remolded), excellent electrical insulator, heat resistant, chemically resistant, low cost.
• Applications: Electrical switches, sockets, plugs, circuit breaker housings, telephone handsets, radio and TV cabinets (older models), billiard balls, brake pads, adhesives.

5.8.4 Kevlar (Poly-para-phenylene terephthalamide)

• Monomers: para-phenylenediamine and terephthaloyl chloride.
• Preparation: Condensation polymerization releasing HCl as byproduct.
• Properties: Extremely high tensile strength (5 times stronger than steel on weight basis), heat resistant (does not melt — decomposes above 500°C), lightweight, excellent impact resistance, rigid molecular structure due to aromatic rings and hydrogen bonding between chains.
• Applications: Bulletproof vests and body armor, helmets for military and police, cut-resistant gloves, reinforcement in tires and hoses, protective clothing for firefighters, aerospace structural components, ropes and cables.

5.8.5 Dacron (Polyethylene terephthalate — PET) / Terylene

• Monomers: Ethylene glycol (HOCH₂CH₂OH) and Terephthalic acid (para-phthalic acid).
• Preparation: Condensation polymerization releasing water as byproduct. The product is a polyester.
• Properties: High tensile strength, wrinkle resistant, good elasticity, moisture resistant, can be drawn into fine fibers or thin films, lightweight.
• Applications: Polyester clothing fibers (blended with cotton — "poly-cotton"), PET bottles for water, soft drinks (the clear plastic bottle), packaging films (Mylar), magnetic tapes, surgical sutures, sails.

5.8.6 Nylon (Polyamide)

• Nylon-6,6: Made from hexamethylenediamine + adipic acid (condensation). Two monomers each with 6 carbons — hence 6,6.
• Nylon-6: Made from caprolactam (ring-opening polymerization). Single monomer with 6 carbons.
• Properties: High tensile strength and elasticity, abrasion resistant, can be drawn into strong fibers, moisture absorption moderate, heat resistant, good chemical resistance.
• Applications: Textile fibers (stockings, sportswear, parachutes), ropes, fishing lines, toothbrush bristles, gears, bearings, and other mechanical parts, carpets.

5.8.7 Thiokol (Polysulfide Rubber)

• Monomers: Dichloro compound + sodium polysulfide (Na₂Sₓ).
• Preparation: Condensation polymerization releasing NaCl.
• Properties: Excellent resistance to oils, fuels, and organic solvents, weather resistant, ozone resistant, good flexibility at low temperatures.
• Applications: Fuel hoses, seals, and gaskets in aircraft (fuel-resistant), rocket propellant binder (solid rocket fuel contains thiokol as binder), sealants for construction and aerospace, printing rollers.

5.8.8 Buna-N (Nitrile Rubber / Acrylonitrile-Butadiene Rubber)

• Monomers: Butadiene (CH₂=CH–CH=CH₂) + Acrylonitrile (CH₂=CHCN).
• Preparation: Emulsion copolymerization (addition polymerization).
• Properties: Excellent oil and fuel resistance (due to polar nitrile groups), good heat resistance, abrasion resistant, does not swell in oils and hydrocarbons.
• Applications: Fuel and oil hoses, O-rings and gaskets in automotive industry, nitrile gloves (medical and industrial), fuel tank linings, conveyor belts, printing rollers, adhesives.

5.8.9 Buna-S (Styrene-Butadiene Rubber — SBR)

• Monomers: Butadiene (CH₂=CH–CH=CH₂) + Styrene (C₆H₅CH=CH₂) in ratio 3:1.
• Preparation: Emulsion copolymerization (addition polymerization).
• Properties: Good abrasion resistance, better aging resistance than natural rubber, moderate oil resistance, maintains flexibility at low temperatures.
• Applications: Most widely produced synthetic rubber. Automobile tyres (most important application — about 70% of SBR production), conveyor belts, shoe soles, floor tiles, wire and cable insulation, hoses.

5.8.10 Speciality Polymers

Speciality polymers are high-performance polymers designed for specific demanding applications where ordinary polymers fail.

• PEEK (Polyether ether ketone): Excellent heat resistance (up to 250°C), chemical resistance, used in aerospace and medical implants.
• Polyimides: Exceptional thermal stability (up to 400°C), used in electronics (flexible circuits), aerospace.
• Liquid Crystal Polymers (LCP): Rigid, high strength, excellent dimensional stability, used in precision electronic components.
• High-performance elastomers: Silicone rubber (used in high temperature seals, medical implants, cookware).

5.9 Environmental Impact of Polymers on Society

• Plastic pollution: Non-biodegradable plastics accumulate in soil, rivers, and oceans — harming wildlife and ecosystems. Microplastics (tiny plastic fragments) are now found everywhere — in oceans, fish, and even human blood.
• Marine pollution: Millions of tonnes of plastic enter oceans yearly — harms sea birds, turtles, whales, fish.
• Land pollution: Plastic bags and packaging clog drains, contaminate soil, affect agriculture.
• Toxic emissions: Burning of PVC releases HCl and dioxins — highly toxic. Burning of polystyrene releases styrene vapors.
• Solutions: Use biodegradable polymers, reduce single-use plastics, improve recycling systems, develop better waste management, green chemistry for polymer synthesis.

PART B: ORGANOMETALLIC COMPOUNDS

5.10 Introduction to Organometallic Compounds

Organometallic compounds are compounds that contain at least one direct metal-carbon bond (M–C bond).

• The metal can be alkali metals (Li, Na, K), alkaline earth metals (Mg), transition metals (Fe, Zn, Ti), or main group metals (Al, Sn).
• They combine properties of both organic and inorganic chemistry.
• They are extremely useful as reagents in organic synthesis and as catalysts in industrial processes.

Examples:

• Grignard Reagent: RMgX (e.g., CH₃MgBr — methylmagnesium bromide)
• Lithium Aluminium Hydride: LiAlH₄
• Ferrocene: (C₅H₅)₂Fe
• Diethyl zinc: (C₂H₅)₂Zn
• Triethylaluminium: (C₂H₅)₃Al

5.11 Grignard Reagent (RMgX)

Preparation of Grignard Reagent:

• Grignard reagents are prepared by reacting an alkyl or aryl halide (RX) with magnesium metal in dry diethyl ether as solvent.
• General reaction: RX + Mg → RMgX (in dry ether)
• Where R = alkyl or aryl group, X = Cl, Br, or I, MgX = magnesium halide part.
• Examples:
• CH₃Br + Mg → CH₃MgBr (methylmagnesium bromide)
• C₂H₅Cl + Mg → C₂H₅MgCl (ethylmagnesium chloride)
• C₆H₅Br + Mg → C₆H₅MgBr (phenylmagnesium bromide)

Conditions for preparation:

• Absolutely dry ether solvent (water destroys Grignard reagent).
• Dry glassware — no moisture.
• Inert atmosphere (N₂ or Ar) — Grignard reagents react with moisture and CO₂ in air.

Why RMgX is reactive:

• The C–Mg bond is highly polarized: Cδ⁻–Mgδ⁺ (carbon carries partial negative charge).
• The carbon acts as a carbanion (nucleophile) — it readily attacks electrophilic centers (like carbonyl carbon, C=O).

Applications of Grignard Reagent:

• Preparation of alcohols:
• RMgX + HCHO → primary alcohol (after hydrolysis)
• RMgX + RCHO → secondary alcohol
• RMgX + R₂CO (ketone) → tertiary alcohol
• RMgX + CO₂ → carboxylic acid (after hydrolysis): RMgX + CO₂ → RCOOMgX → RCOOH + Mg(OH)X
• Preparation of carboxylic acids: RMgX + CO₂ → RCOOH
• Preparation of alkanes: RMgX + H₂O → RH + Mg(OH)X
• Preparation of esters: RMgX + ester → tertiary alcohol.
• Carbon-carbon bond formation: One of the most powerful tools for making C–C bonds in organic synthesis.

5.12 Lithium Aluminium Hydride (LiAlH₄)

Preparation:

• Prepared by reacting lithium hydride (LiH) with aluminium chloride (AlCl₃) in dry diethyl ether:
• 4LiH + AlCl₃ → LiAlH₄ + 3LiCl

Properties:

• White solid, highly reactive.
• Reacts violently with water: LiAlH₄ + 4H₂O → LiOH + Al(OH)₃ + 4H₂↑ (fire hazard!).
• Must be stored and used under anhydrous (dry) conditions in ether solvent.
• Powerful reducing agent — can reduce almost all functional groups.
• It provides hydride ions (H⁻) which are nucleophilic — attack electrophilic carbonyl carbons.

Applications of LiAlH₄ as a Reducing Agent:

Starting Material	Product after LiAlH₄ Reduction
Carboxylic acid (RCOOH)	Primary alcohol (RCH₂OH)
Aldehyde (RCHO)	Primary alcohol (RCH₂OH)
Ketone (RCOR')	Secondary alcohol (RCHOHR')
Ester (RCOOR')	Two alcohols (RCH₂OH + R'OH)
Amide (RCONH₂)	Primary amine (RCH₂NH₂)
Nitrile (RCN)	Primary amine (RCH₂NH₂)
Epoxide	Alcohol

Advantages of LiAlH₄:

• Reduces functional groups that are difficult to reduce by other methods.
• Very selective under controlled conditions.
• Widely used in pharmaceutical synthesis for making drug molecules.

Limitations:

• Cannot be used in aqueous medium (reacts with water).
• Expensive compared to other reducing agents like NaBH₄.
• Flammable and requires careful handling.