Natural and Synthetic like Polythene, Nylon, Polyesters, Bakelite, Rubber — Explained

Detailed Explanation

Polymers are fascinating macromolecules that form the backbone of much of our material world, from the clothes we wear to the plastics that encapsulate our technology. At their core, polymers are large molecules (macromolecules) composed of repeating structural units called monomers, linked together by covalent bonds. The process by which monomers combine to form polymers is termed polymerization.

Conceptual Foundation

1
Monomers and Polymers: — A monomer is a single, simple molecule capable of reacting with other monomer molecules to form a polymer. For example, ethene is the monomer for polythene.
2
Degree of Polymerization: — This refers to the number of monomer units in a polymer chain. It influences the polymer's properties.
3
Classification of Polymers: — Polymers can be classified based on several criteria:

* Origin: Natural, Synthetic, Semi-synthetic. * Structure: Linear, Branched, Cross-linked. * Mode of Polymerization: Addition, Condensation. * Molecular Forces: Elastomers, Fibers, Thermoplastics, Thermosetting plastics.

Key Principles of Polymerization

There are two primary modes of polymerization:

1
Addition Polymerization: — Monomers add to one another in a chain reaction without the elimination of any small molecules (like water, alcohol, etc.). This typically occurs with unsaturated monomers (containing double or triple bonds). The empirical formula of the monomer and the repeating unit of the polymer are the same. Examples include polythene from ethene, PVC from vinyl chloride.
2
Condensation Polymerization: — Monomers react to form a polymer with the simultaneous elimination of small molecules such as water, alcohol, or ammonia. This usually involves monomers with two or more functional groups. Examples include nylon-6,6 from hexamethylenediamine and adipic acid, polyesters from diols and dicarboxylic acids, and Bakelite from phenol and formaldehyde.

Natural Polymers: Natural Rubber

Natural Rubber: This is a classic example of a natural polymer, primarily obtained from the latex of the Hevea brasiliensis tree. Chemically, natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene).

Monomer: — Isoprene (
$$ext{CH}_2=\text{C}(\text{CH}_3)-\text{CH}=\text{CH}_2$$
)
Structure: — Natural rubber is predominantly the *cis*-polyisoprene. The *cis* configuration allows for a coiled structure, giving it elasticity. The polymer chains are held together by weak van der Waals forces.
Properties: — It is soft, sticky at high temperatures, brittle at low temperatures, and has high water absorption capacity. It is soluble in non-polar solvents and is non-resistant to oxidation.
Vulcanization: — To improve its properties, natural rubber undergoes vulcanization, a process developed by Charles Goodyear. This involves heating natural rubber with sulfur (typically 3-5%) at 100-140°C. Sulfur forms cross-links between the polymer chains, making the rubber harder, stronger, more elastic, less sticky, and more resistant to abrasion and chemical attack. The degree of cross-linking determines the hardness of the rubber.
Uses: — Tires, elastic bands, footwear, waterproof materials (after vulcanization).

Synthetic Polymers

1. Polythene (Polyethylene): This is one of the most widely used synthetic polymers, formed by the addition polymerization of ethene (ethylene).

Monomer: — Ethene (
$$ext{CH}_2=\text{CH}_2$$
)
Types:

* Low-Density Polythene (LDPE): Produced under high pressure (1000-2000 atm) and temperature (350-570 K) in the presence of a peroxide initiator or oxygen. It has a highly branched structure, leading to loose packing, low density, and low tensile strength.

It is transparent. Uses: Squeeze bottles, toys, flexible pipes, plastic bags, electrical insulation. * High-Density Polythene (HDPE): Produced under low pressure (6-7 atm) and temperature (333-343 K) in the presence of a Ziegler-Natta catalyst (e.

g., triethylaluminium and titanium tetrachloride). It has a linear structure with minimal branching, allowing for close packing, high density, and greater toughness and hardness. Uses: Buckets, dustbins, bottles, pipes, containers.

Properties: — Chemically inert, tough, flexible, good electrical insulator.

2. Nylon: A generic name for synthetic polyamides. The term 'NY' for New York and 'LON' for London is a popular but unconfirmed origin story. Nylons are characterized by the presence of amide (

$$-\text{CONH}-$$

) linkages.

Nylon-6,6: — Formed by the condensation polymerization of hexamethylenediamine (
$$ext{H}_2\text{N}-(\text{CH}_2)_6-\text{NH}_2$$
) and adipic acid (
$$ext{HOOC}-(\text{CH}_2)_4-\text{COOH}$$
). Water molecules are eliminated during polymerization.

* Properties: High tensile strength, elasticity, abrasion resistance, lustrous, easily washable. It is a fiber-forming polymer. * Uses: Fabrics, carpets, ropes, bristles for brushes, fishing nets, tire cords.

Nylon-6: — Formed by the condensation polymerization of caprolactam. Caprolactam is heated with water at high temperatures to form Nylon-6.

* Properties: Similar to Nylon-6,6 but often used where slightly lower melting point and easier processing are desired. * Uses: Tire cords, fabrics, ropes.

3. Polyesters: These are polymers containing ester (

$$-\text{COO}-$$

) linkages. They are typically formed by the condensation polymerization of a dicarboxylic acid and a diol.

Terylene (Dacron/PET): — One of the most common polyesters. Formed by the condensation polymerization of terephthalic acid (
$$ext{HOOC}-\text{C}_6\text{H}_4-\text{COOH}$$
) and ethylene glycol (
$$ext{HOCH}_2\text{CH}_2\text{OH}$$
). Water is eliminated.

* Properties: High tensile strength, excellent resistance to creasing, chemical resistance, low moisture absorption, good electrical insulator. * Uses: Fabric blends (with cotton or wool), safety belts, tire cords, sails, magnetic recording tapes, bottles (PET).

4. Bakelite: This is a classic example of a thermosetting plastic, a phenol-formaldehyde resin.

Monomers: — Phenol (
$$ext{C}_6\text{H}_5\text{OH}$$
) and formaldehyde (
$$ext{HCHO}$$
).
Polymerization: — Involves a series of condensation reactions. Initially, phenol reacts with formaldehyde in the presence of an acid or base catalyst to form ortho and para hydroxymethylphenols. These intermediates then react further to form linear Novolac (if acid catalyst and excess phenol) or cross-linked Resols (if base catalyst and excess formaldehyde). Bakelite is formed by heating Novolac with formaldehyde, leading to extensive cross-linking.
Structure: — Highly cross-linked, three-dimensional network structure. This cross-linking is responsible for its thermosetting nature.
Properties: — Hard, rigid, scratch-resistant, excellent electrical insulator, heat resistant, insoluble in organic solvents, non-fusible (cannot be remolded once set).
Uses: — Electrical switches, handles of utensils, telephone casings, computer discs, varnishes, lacquers.

Real-World Applications and NEET-Specific Angle

Understanding the specific monomers, the type of polymerization (addition or condensation), and the key properties and uses of each polymer is paramount for NEET. Questions often revolve around identifying the monomer from a given polymer structure, classifying polymers (e.

g., natural vs. synthetic, thermoplastic vs. thermosetting), or matching polymers with their applications. For instance, knowing that Bakelite is a thermosetting polymer explains why it's used for electrical switches (it won't melt when heated by current).

Common Misconceptions

Thermoplastic vs. Thermosetting: — Students often confuse these. Thermoplastics (like polythene, nylon, polyesters) can be softened on heating and remolded, as their chains are held by weaker intermolecular forces. Thermosetting plastics (like Bakelite) undergo irreversible chemical changes upon heating, forming a rigid, cross-linked structure that cannot be softened or remolded. They 'set' permanently.
Addition vs. Condensation: — Remember, addition polymerization involves no loss of small molecules, while condensation polymerization always involves the elimination of a small molecule (usually water).
Natural vs. Synthetic: — While natural polymers are biologically derived, synthetic polymers are man-made. Semi-synthetic polymers are derived from natural polymers but chemically modified (e.g., cellulose acetate).

Mastering these distinctions and the specific details of each polymer will equip you well for polymer-related questions in NEET.