Natural and Synthetic like Polythene, Nylon, Polyesters, Bakelite, Rubber — Core Principles

Core Principles

Polymers are large molecules made of repeating smaller units called monomers. They are broadly classified as natural or synthetic. Natural polymers, like natural rubber (monomer: isoprene), are found in nature and often require vulcanization (with sulfur) to improve properties like elasticity and strength. Synthetic polymers are man-made and include a wide range of materials.

Polythene (monomer: ethene) exists as Low-Density (LDPE, branched, flexible) and High-Density (HDPE, linear, rigid) types, used in packaging and containers. Nylon is a polyamide, with Nylon-6,6 (monomers: hexamethylenediamine and adipic acid) and Nylon-6 (monomer: caprolactam) being common examples, known for high tensile strength in fibers and ropes.

Polyesters, such as Terylene/Dacron/PET (monomers: ethylene glycol and terephthalic acid), contain ester linkages and are used in fabrics and bottles. Bakelite (monomers: phenol and formaldehyde) is a thermosetting plastic, forming a rigid, cross-linked structure that cannot be remolded, making it ideal for electrical switches and utensil handles.

These synthetic polymers are crucial to modern industry and daily life, each with specific monomers, polymerization types (addition or condensation), and characteristic properties.

Important Differences

vs Thermoplastic Polymers

Aspect	This Topic	Thermoplastic Polymers
Behavior on Heating	Soften on heating, harden on cooling. Can be repeatedly melted and remolded.	Undergo irreversible chemical change on heating. Become hard and rigid, cannot be softened or remolded.
Structure	Linear or branched polymer chains, held by weak intermolecular forces.	Highly cross-linked, three-dimensional network structure, formed by strong covalent bonds.
Intermolecular Forces	Weak van der Waals forces, dipole-dipole interactions, hydrogen bonding (in some cases).	Strong covalent bonds forming a rigid network.
Recyclability	Generally recyclable, as they can be melted and reshaped.	Generally not recyclable, as they decompose upon reheating rather than melting.
Examples	Polythene, Nylon, Polyesters, PVC, Polypropylene.	Bakelite, Urea-formaldehyde resins, Melamine-formaldehyde resins.

The distinction between thermosetting and thermoplastic polymers is crucial in material science and for NEET. Thermoplastics, like polythene, can be repeatedly softened by heating and then hardened by cooling, making them recyclable. This is due to their linear or branched structures and weaker intermolecular forces. In contrast, thermosetting polymers, such as Bakelite, undergo a permanent chemical change upon initial heating, forming a rigid, cross-linked network. Once set, they cannot be remelted or reshaped without degradation, making them non-recyclable but highly durable and heat-resistant for specific applications.