Introduction

Polymers are very large molecules (macromolecules) with high molecular masses ($\textsf{10}^3 - \textsf{10}^7$ u). The word 'polymer' comes from Greek: 'poly' (many) and 'mer' (unit or part). Polymers are formed by the extensive joining of smaller, simple, reactive molecules called **monomers**. The monomers link together via covalent bonds to form repeating structural units within the polymer chain.

The process by which monomers combine to form polymers is called **polymerisation**.

Polymers have significantly changed daily life and industry, being essential components in plastics, elastomers (rubbers), fibres, paints, and varnishes. They are found in countless products, from common household items like buckets and toys to synthetic clothing, tyres, and electrical insulation.

Classification Of Polymers

Polymers can be classified in various ways. One common method is based on their origin or source.

• 1. Natural polymers: These polymers are found in nature, in plants and animals. Examples include proteins (polymers of amino acids), cellulose (polymers of glucose, found in plant cell walls), starch (polymers of glucose, plant energy storage), natural rubber (polymer of isoprene), and some natural resins.
• 2. Semi-synthetic polymers: These are derived from naturally occurring polymers, usually through chemical modification. Common examples are cellulose derivatives like cellulose acetate (rayon, a regenerated fibre) and cellulose nitrate.
• 3. Synthetic polymers: These are man-made polymers, synthesised in laboratories and industries. They are widely used in modern life. Examples include plastics like polythene, synthetic fibres like nylon 6,6, and synthetic rubbers like Buna-S.

Other classifications can be based on structure (linear, branched, cross-linked), molecular forces (elastomers, fibres, thermoplastics, thermosetting polymers), or mode of polymerisation (addition, condensation).

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Types Of Polymerisation Reactions

There are two main types of polymerisation reactions:

1. Addition Polymerisation (or Chain Growth Polymerisation) 2. Condensation Polymerisation (or Step Growth Polymerisation)

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Addition Polymerisation Or Chain Growth Polymerisation

In this type of polymerisation, monomers add to one another sequentially to form a growing polymer chain. This typically involves unsaturated monomers containing double or triple bonds (e.g., alkenes, alkadienes, and their derivatives). The polymer chain grows through a chain reaction mechanism, which can be initiated by free radicals or ionic species (cations or anions). Free radical addition polymerisation is the most common mode.

Mechanism of Addition Polymerisation (Free Radical Mechanism): This process occurs in the presence of a free radical initiator (catalyst), such as organic peroxides (e.g., benzoyl peroxide, acetyl peroxide, tert-butyl peroxide).

The mechanism involves three main steps:

• Chain Initiation: The initiator molecule decomposes to form free radicals. A radical from the initiator adds to the double bond of a monomer molecule, creating a new, larger free radical.
• Chain Propagation: The new free radical reacts with another monomer molecule, adding it to the chain and generating an even larger radical. This process repeats, causing the polymer chain to grow rapidly.
• Chain Termination: The growing polymer chains eventually terminate the chain reaction. This occurs when two free radicals combine to form a neutral polymer molecule, consuming the radicals. Termination can happen by combination (coupling) or disproportionation of radical chains.

Addition polymers formed from a single type of monomer are called **homopolymers** (e.g., polythene from ethene). Polymers formed by addition polymerisation of two or more different monomers are called **copolymers** (e.g., Buna-S from buta-1,3-diene and styrene, discussed under Copolymerisation).

Some Important Addition Polymers:

• (a) Polythene: A very common plastic. Two main types:
  • (i) Low Density Polythene (LDP): Produced by polymerisation of ethene under high pressure (1000-2000 atm) and temperature (350-570 K) with dioxygen or peroxide initiator. Forms a highly branched structure. It is flexible, tough, chemically inert, and a poor electrical conductor. Used for insulation, squeeze bottles, toys, flexible pipes.
  • (ii) High Density Polythene (HDP): Produced by polymerisation of ethene at lower pressure (6-7 atm) and temperature (333-343 K) in a hydrocarbon solvent using a Ziegler-Natta catalyst (triethylaluminium and titanium tetrachloride). Forms a linear, closely packed structure. It is chemically inert, tougher, and harder than LDP. Used for buckets, dustbins, bottles, pipes.
• (b) Polytetrafluoroethene (Teflon): Formed by heating tetrafluoroethene with a free radical or persulphate catalyst under high pressure.
  It is highly chemically inert and resistant to corrosive reagents. Used for non-stick coatings on utensils, oil seals, gaskets.
• (c) Polyacrylonitrile (PAN): Formed by addition polymerisation of acrylonitrile in the presence of a peroxide catalyst.
  Used as a substitute for wool in commercial fibres like Orlon or Acrilan.

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Condensation Polymerisation Or Step Growth Polymerisation

This type of polymerisation involves repetitive condensation reactions between two different bi-functional or tri-functional monomer units. These reactions typically result in the loss of small molecules like water, alcohol, $\textsf{HCl}$, etc. Each step in the polymer chain formation is a separate condensation reaction between functional groups. Since the polymer grows step-by-step by the reaction between functionalised species, it's also called **step growth polymerisation**.

Example: Formation of Terylene (Dacron) from ethylene glycol and terephthalic acid involves the condensation of hydroxyl and carboxyl groups, with the elimination of water.

Some Important Condensation Polymers:

• (a) Polyamides (Nylons): Polymers containing amide linkages (–CO–NH–). Important synthetic fibres. Prepared by condensation polymerisation of diamines with dicarboxylic acids, or amino acids or their lactams.
  • (i) Nylon 6,6: Prepared from hexamethylenediamine ($\textsf{H}_2\text{N–(CH}_2)_6\text{–NH}_2$) and adipic acid ($\textsf{HOOC–(CH}_2)_4\text{–COOH}$) by condensation polymerisation under high pressure and temperature.
    It is a strong, fibre-forming solid with high tensile strength due to strong intermolecular hydrogen bonding, leading to close packing and crystalline nature. Used in sheets, brushes (bristles), textile industry.
  • (ii) Nylon 6: Obtained by heating caprolactam with water at high temperature. Caprolactam is a cyclic amide that undergoes ring-opening polymerisation.
    Used for tyre cords, fabrics, ropes.
• (b) Polyesters: Polycondensation products of dicarboxylic acids and diols. **Dacron** (Terylene) is the best-known example, formed from ethylene glycol and terephthalic acid (shown above). Dacron fibre is crease-resistant, used in blending with cotton and wool, and as a glass reinforcing material.
• (c) Phenol–Formaldehyde Polymer (Bakelite and related polymers): Oldest synthetic polymers, obtained by condensation of phenol and formaldehyde catalyzed by acid or base. Initial products are o- and p-hydroxymethylphenol derivatives which condense to form linear chains (Novolac) or cross-linked structures (Bakelite). Novolac is used in paints. Heating Novolac with formaldehyde leads to cross-linking, forming a rigid, infusible, thermosetting polymer called Bakelite. Bakelite cannot be remoulded once hardened.
  Used for combs, electrical switches, handles of utensils, phonograph records.
• (d) Melamine—Formaldehyde Polymer: Formed by condensation polymerisation of melamine and formaldehyde. Used in the manufacture of unbreakable crockery.

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Copolymerisation

Copolymerisation is a polymerisation reaction involving a mixture of **more than one type of monomeric species**. This process forms a **copolymer**, which contains repeating units derived from each of the different monomers present in the reaction mixture. Copolymerisation can occur via both addition and condensation polymerisation mechanisms.

Example: Polymerisation of a mixture of buta-1,3-diene and styrene forms a copolymer called Buna-S (Styrene-Butadiene Rubber, SBR).

Copolymers often have properties that are significantly different from, and sometimes superior to, those of the homopolymers made from the individual monomers. For instance, Buna-S is a tough rubber substitute used in tyres, footwear, and cable insulation.

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Rubber

Rubber is a natural polymer with elastic properties, classified as an elastomer. Elastomers are polymers where chains are held by weak intermolecular forces. They can be stretched but return to their original shape due to a few crosslinks between chains.

• 1. Natural rubber: A linear polymer of **isoprene** (2-methyl-1,3-butadiene). It is also known as **cis-1,4-polyisoprene**.
  The chains are coiled and held by weak van der Waals forces, giving it elasticity. Obtained from rubber latex (colloidal dispersion) from rubber trees.
• Vulcanisation of rubber: Natural rubber is soft at high temperatures, brittle at low temperatures, absorbs water, is soluble in non-polar solvents, and is reactive towards oxidising agents. To improve these properties, it is vulcanised by heating with sulfur (and additives) at 373-415 K. Sulfur forms crosslinks between polymer chains, usually at the double bonds, stiffening the rubber and improving its strength, elasticity, and resistance to chemicals and temperature changes. About 5% sulfur is used for tyre rubber.
• 2. Synthetic rubbers: Vulcanisable, rubber-like polymers capable of stretching and returning to shape. They are typically homopolymers of 1,3-butadiene derivatives or copolymers of 1,3-butadiene/derivatives with other unsaturated monomers.
  • 1. Neoprene (Polychloroprene): Formed by free radical polymerisation of chloroprene (2-chloro-1,3-butadiene).
    Resistant to oils, used for conveyor belts, gaskets, hoses.
  • 2. Buna–N: Copolymer of 1,3-butadiene and acrylonitrile, formed in the presence of a peroxide catalyst.
    Resistant to petrol, oils, organic solvents. Used in oil seals, tank linings.

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Molecular Mass Of Polymers

Polymer properties (like tensile strength, viscosity, melting point) depend heavily on their molecular mass, size, and structure. During synthesis, polymer chains don't all grow to exactly the same length, resulting in a distribution of chain lengths in a polymer sample. Therefore, the molecular mass of a polymer is always expressed as an **average** value.

Various chemical and physical methods are used to determine the average molecular mass of polymers, such as osmometry, viscometry, light scattering, and gel permeation chromatography (GPC).

Biodegradable Polymers

Many synthetic polymers are very resistant to degradation by environmental processes (like microbial action, sunlight, moisture). This leads to the accumulation of large amounts of persistent polymeric solid waste, causing significant environmental problems.

To address this, new **biodegradable synthetic polymers** are being developed. These polymers are designed to contain functional groups that are susceptible to degradation by microorganisms or natural processes in the environment. They often mimic the types of linkages found in natural biopolymers (like proteins and polysaccharides).

An important class of biodegradable polymers is **aliphatic polyesters**.

Examples of biodegradable polymers:

• 1. Poly $\beta$-hydroxybutyrate – co-$\beta$-hydroxy valerate (PHBV): A copolymer formed by the copolymerisation of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. It contains ester linkages.
  PHBV undergoes bacterial degradation in the environment. Used in speciality packaging, orthopaedic devices, and for controlled drug release.
• 2. Nylon 2–Nylon 6: An alternating polyamide copolymer synthesised from two different monomers: Glycine ($\textsf{H}_2\text{N–CH}_2\text{–COOH}$, a two-carbon amino acid) and amino caproic acid ($\textsf{H}_2\text{N–(CH}_2)_5\text{–COOH}$, a six-carbon amino acid). This polymer contains amide linkages similar to natural proteins and is biodegradable.

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Polymers Of Commercial Importance

Beyond the polymers discussed previously, several other polymers are commercially important. Their structure is derived from specific monomers, leading to unique properties and applications.

Some examples:

Name of Polymer	Monomer	Structure	Uses
Polypropene	Propene ($\textsf{CH}_3\text{CH=CH}_2$)	-[CH(CH$_3$)-CH$_2$]-$_n$	Manufacture of ropes, toys, pipes, fibres, etc.
Polystyrene	Styrene ($\textsf{C}_6\text{H}_5\text{CH=CH}_2$)	-[CH(C$_6$H$_5$)-CH$_2$]-$_n$	As insulator, wrapping material (Styrofoam), manufacture of toys, radio and television cabinets.
Polyvinyl chloride (PVC)	Vinyl chloride ($\textsf{CH}_2\text{=CHCl}$)	-[CHCl-CH$_2$]-$_n$	Manufacture of rain coats, hand bags, vinyl flooring, water pipes, electrical insulation.
Urea-formaldehyde Resin	(a) Urea ($\textsf{H}_2\text{NCONH}_2$) (b) Formaldehyde ($\textsf{HCHO}$)		For making unbreakable cups and laminated sheets.
Glyptal	(a) Ethylene glycol ($\textsf{HOCH}_2\text{CH}_2\text{OH}$) (b) Phthalic acid (Benzene-1,2-dicarboxylic acid)		Manufacture of paints and lacquers.
Bakelite	(a) Phenol ($\textsf{C}_6\text{H}_5\text{OH}$) (b) Formaldehyde ($\textsf{HCHO}$)		For making combs, electrical switches, handles of utensils and computer discs. (Thermosetting)

Polymers are essential to modern life, providing materials with a wide range of properties that can be tailored for specific applications, from flexible films and fibres to rigid structural components and chemically resistant coatings.

Summary

Polymers are high molecular mass macromolecules formed by the repetition of structural units derived from monomers. They can be natural, semi-synthetic, or synthetic.

• Addition polymerisation (chain growth) involves the direct addition of unsaturated monomers, often via a free radical mechanism (e.g., polythene, Teflon, Orlon). Forms homopolymers or copolymers.
• Condensation polymerisation (step growth) involves repetitive condensation between bi/polyfunctional monomers with the elimination of small molecules (e.g., Nylon, Bakelite, Dacron, melamine-formaldehyde polymer).
• Copolymerisation involves polymerising a mixture of different monomers to form a copolymer with units from each monomer in the chain (e.g., Buna-S, Buna-N).
• Rubber is a natural elastomer (cis-1,4-polyisoprene). Its properties are improved by vulcanisation with sulfur (forming crosslinks). Synthetic rubbers (Neoprene, Buna-N) are also important elastomers.
• Polymer properties depend on their molecular mass (expressed as an average) and structure.
• Due to environmental concerns with non-biodegradable polymer waste, **biodegradable polymers** (e.g., PHBV, Nylon 2-Nylon 6), often aliphatic polyesters or polyamides, are being developed.
• Many polymers have crucial commercial applications, forming the backbone of the plastics, elastomers, fibres, paints, and varnishes industries.

Exercises

Questions covering the definition, classification, types of polymerisation, mechanisms, structure, properties, preparation, uses, and environmental aspects of polymers.

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