Comprehensive Study Notes on Polymers

Introduction to Polymers

Polymers have revolutionized daily life and industry, serving as the backbone of plastics, elastomers, fibers, and paints. The term 'polymer' comes from the Greek words 'poly' (many) and 'mer' (unit). Polymers are macromolecules with high molecular mass ($10^3$ - $10^7$ u), formed by joining repeating structural units called monomers via covalent bonds. The process of polymer formation is called polymerization.

Objectives

After studying this Unit, you will be able to:

• Explain the terms - monomer, polymer, and polymerization and appreciate their importance.
• Distinguish between various classes of polymers and different types of polymerization processes.
• Appreciate the formation of polymers from mono- and bi- functional monomer molecules.
• Describe the preparation of some important synthetic polymers and their properties.
• Appreciate the importance of polymers in daily life.

15.1: Classification of Polymers

Polymers are classified based on their source:

1. Natural Polymers

Found in plants and animals. Examples include proteins, cellulose, starch, resins, and rubber.

2. Semi-synthetic Polymers

Cellulose derivatives like cellulose acetate (rayon) and cellulose nitrate.

3. Synthetic Polymers

Man-made polymers such as plastic (polythene), synthetic fibers (nylon 6,6), and synthetic rubbers (Buna-S).

Polymers can also be classified based on structure, molecular forces, or modes of polymerization.

15.2: Types of Polymerization Reactions

15.2.1: Addition Polymerization (Chain Growth Polymerization)

Monomers add together on a large scale to form a polymer. Monomers are unsaturated compounds like alkenes, alkadienes, and their derivatives. Chain growth occurs through free radicals or ionic species, with free radical polymerization being the most common.

15.2.1.1: Mechanism of Addition Polymerization

Free radical polymerization involves an initiator (catalyst) like benzoyl peroxide. The process includes:

• Chain Initiation: The initiator forms a free radical that adds to the ethene double bond, creating a new, larger free radical.
• Chain Propagation: The radical reacts with another monomer molecule, forming a bigger radical. This repeats.
• Chain Termination: The product radical reacts with another radical, forming the polymer.

Chain Initiation Steps

$\text{Benzoyl Peroxide} \rightarrow \text{Phenyl Free Radical}$

$\text{Phenyl Free Radical} + \text{Ethene} \rightarrow \text{New Free Radical}$

Chain Propagating Step

$\text{New Free Radical} + \text{Ethene} \rightarrow \text{Bigger Sized Radical}$

Chain Terminating Step

$\text{Radical} + \text{Radical} \rightarrow \text{Polythene}$

Homopolymers vs. Copolymers

• Homopolymers: Polymers formed from a single monomeric species (e.g., polythene).
• Copolymers: Polymers formed from two different monomers (e.g., Buna-S from buta-1,3-diene and styrene).

15.2.1.2: Some Important Addition Polymers

(a) Polythene

Linear or slightly branched long chain molecules that soften on heating and harden on cooling (thermoplastic polymers).

(i) Low Density Polythene (LDP)

Obtained by polymerizing ethene under high pressure (1000-2000 atmospheres) at 350-570 K with traces of dioxygen or peroxide initiator. It has a highly branched structure, is chemically inert, tough, flexible, and a poor conductor of electricity. Used for insulation of wires, squeeze bottles, toys, and flexible pipes.

(ii) High Density Polythene (HDP)

Formed by addition polymerization of ethene in a hydrocarbon solvent using a Ziegler-Natta catalyst (triethylaluminum and titanium tetrachloride) at 333-343 K and 6-7 atmospheres. It consists of linear molecules with high density due to close packing. It's chemically inert, tough, and hard, used for buckets, dustbins, bottles, and pipes.

(b) Polytetrafluoroethene (Teflon)

Manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures. It is chemically inert and resistant to corrosive reagents. Used in oil seals, gaskets, and non-stick surfaces.

(c) Polyacrylonitrile

Formed by addition polymerization of acrylonitrile in the presence of a peroxide catalyst. Used as a substitute for wool in commercial fibers like orlon or acrilan.

Example 15.1

Is a homopolymer or a copolymer? It is a homopolymer, and the monomer is styrene ($\text{C}<em>6\text{H}</em>5\text{CH=CH}_2$).

15.2.2: Condensation Polymerization (Step Growth Polymerization)

Involves repetitive condensation reactions between two bi-functional or trifunctional monomeric units, resulting in the loss of small molecules like water or alcohol. Each step produces a distinct functionalized species, independent of each other.

$\text{n HOCH}<em>2\text{CH}</em>2\text{OH} + \text{n HOOC-C}<em>6\text{H}</em>4\text{-COOH} \rightarrow [-\text{OCH}<em>2\text{CH}</em>2\text{OOC-C}<em>6\text{H}</em>4\text{CO-}]<em>\text{n} + \text{2n H}</em>2\text{O}$

(a) Polyamides (Nylons)

Polymers with amide linkages, prepared by condensation polymerization of diamines with dicarboxylic acids or amino acids or their lactams.

(i) Nylon 6,6

Prepared by condensation polymerization of hexamethylenediamine with adipic acid under high pressure and temperature. It is a fiber-forming solid with high tensile strength due to strong intermolecular hydrogen bonding, leading to close packing and crystalline nature. Used in sheets, bristles for brushes, and in the textile industry.

(ii) Nylon 6

Obtained by heating caprolactam with water at high temperature. Used for manufacturing tire cords, fabrics, and ropes.

(b) Polyesters

Polycondensation products of dicarboxylic acids and diols. Dacron (terylene) is a well-known example, manufactured by heating ethylene glycol and terephthalic acid at 420-460 K with zinc acetate-antimony trioxide catalyst. Dacron fiber is crease-resistant and used in blending with cotton and wool, and as reinforcing material in safety helmets.

(c) Phenol-Formaldehyde Polymers (Bakelite and related polymers)

Oldest synthetic polymers, obtained by condensation of phenol with formaldehyde using an acid or base catalyst. The reaction starts with the formation of o- and/or p-hydroxymethylphenol derivatives, which react with phenol to form compounds joined by -CH2 groups. Novolac is a linear product used in paints.

Novolac on heating with formaldehyde forms bakelite, an infusible solid mass. Bakelite is a thermosetting polymer which cannot be reused or remoulded. It is formed by cross linking of linear chains of the polymer novolac. Used for combs, phonograph records, electrical switches, and utensil handles.

(d) Melamine-Formaldehyde Polymer

Formed by condensation polymerization of melamine and formaldehyde. Used in unbreakable crockery.

15.2.3: Copolymerization

A polymerization reaction involving a mixture of more than one monomeric species to form a copolymer. The copolymer contains multiple units of each monomer in the same polymeric chain. Copolymers have properties different from homopolymers. For example, butadiene-styrene copolymer is tough and a good substitute for natural rubber, used in autotyres, floortiles, footwear components, and cable insulation.

15.2.4: Rubber

1. Natural Rubber

A natural polymer possessing elastic properties, also termed an elastomeric polymer. Chains are held together by weak intermolecular forces, allowing stretching. Crosslinks help the polymer retract to its original position after force is released. It is manufactured from rubber latex, a colloidal dispersion of rubber in water, obtained from rubber trees. Natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene), also called cis-1,4-polyisoprene. The molecule consists of chains held together by weak van der Waals interactions, has a coiled structure, and exhibits elastic properties.

Vulcanization of Rubber:

Natural rubber becomes soft at high temperatures (>335 K) and brittle at low temperatures (<283 K), with high water absorption. Vulcanization involves heating raw rubber with sulphur and additives at 373-415 K. Sulphur forms crosslinks at double bonds, stiffening the rubber. Tyre rubber uses 5% sulphur as a crosslinking agent.

2. Synthetic Rubbers

Vulcanizable rubber-like polymers capable of stretching to twice their length and returning to their original shape. They are either homopolymers of 1,3-butadiene derivatives or copolymers of 1,3-butadiene or its derivatives with another unsaturated monomer.

(1) Neoprene

Polychloroprene formed by free radical polymerization of chloroprene. It has superior resistance to vegetable and mineral oils. Used for conveyor belts, gaskets, and hoses.

(2) Buna-N

Obtained by copolymerization of 1,3-butadiene and acrylonitrile in the presence of a peroxide catalyst. It is resistant to petrol, lubricating oil, and organic solvents. Used in oil seals and tank lining.

15.3: Molecular Mass of Polymers

Polymer properties are closely related to their molecular mass, size, and structure. Polymer samples contain chains of varying lengths, so molecular mass is expressed as an average, determined by chemical and physical methods.

15.4: Biodegradable Polymers

Polymers resistant to environmental degradation cause accumulation of solid waste. Biodegradable polymers contain functional groups similar to biopolymers. Aliphatic polyesters are important biodegradable polymers.

1. Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV)

Obtained by copolymerization of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. Used in specialty packaging, orthopedic devices, and controlled drug release. Undergoes bacterial degradation in the environment.

2. Nylon 2 – Nylon 6

An alternating polyamide copolymer of glycine (H2N–CH2–COOH) and amino caproic acid [H2N(CH2)5COOH] and is biodegradable.

15.5: Polymers of Commercial Importance