1. Introduction

Polymers are large macromolecules composed of repeating structural units called monomers, covalently bonded in long chains. The term is derived from the Greek words poly (many) and meros (part or unit). Plastics are a broad category of synthetic or semi-synthetic polymers that can be molded or shaped, typically under heat and pressure, into useful products.

2. Classification of Polymers

A. Based on Origin

• Natural Polymers – e.g., cellulose, starch, silk, wool, natural rubber (polyisoprene).
• Synthetic Polymers – e.g., polyethylene, PVC, nylon, Bakelite.
• Semi-synthetic Polymers – e.g., cellulose acetate, vulcanized rubber.

B. Based on Polymerization Mechanism

• Addition Polymers – Formed without elimination of small molecules. Examples: polyethylene, polypropylene.
• Condensation Polymers – Formed with elimination of small molecules like H₂O, NH₃, HCl. Examples: nylon, polyester.

C. Based on Structure

• Linear Polymers – e.g., HDPE.
• Branched Polymers – e.g., LDPE.
• Cross-linked Polymers – e.g., Bakelite, melamine.

D. Based on Thermal Behavior

• Thermoplastics – Soften on heating, harden on cooling (reversible). Examples: polyethylene, PVC.
• Thermosetting Plastics – Harden irreversibly upon heating due to cross-linking. Examples: Bakelite, epoxy resins.

3. Polymerization Methods

• Free Radical Polymerization – Initiated by free radicals (e.g., benzoyl peroxide).
• Ionic Polymerization – Cationic or anionic initiators.
• Coordination Polymerization – Ziegler–Natta catalysts.
• Condensation Polymerization – Step-growth reaction with elimination of small molecules.

4. Properties of Polymers

• High strength-to-weight ratio, flexibility, and elasticity.
• Low thermal conductivity, variable thermal stability.
• Resistance to corrosion and chemicals.
• Good electrical insulation properties.

5. Plastics — Overview

Plastics are a subclass of polymers that can be shaped by heat and pressure. They may be:

• Commodity Plastics – e.g., polyethylene, polypropylene, PVC, polystyrene.
• Engineering Plastics – e.g., polycarbonate, nylon, PET.
• Specialty Plastics – e.g., Teflon (PTFE), PEEK.

6. Advantages of Plastics

• Lightweight and strong.
• Corrosion-resistant.
• Good insulators.
• Moldable into complex shapes.
• Cost-effective in mass production.

7. Disadvantages of Plastics

• Non-biodegradable (most types).
• Environmental pollution from waste.
• Limited heat resistance.
• Can release toxic gases upon burning.

8. Environmental Impact & Recycling

• Marine plastic pollution, microplastics, landfill accumulation.
• Plastics are labeled (1 to 7) for recycling identification.
• Biodegradable alternatives include polylactic acid (PLA), starch-based plastics.
• Waste management strategies: Reduce, Reuse, Recycle, Recover.

9. Applications

• Packaging: films, containers, bottles.
• Construction: pipes, insulation, panels.
• Automotive: dashboards, bumpers, interior fittings.
• Electronics: casings, insulation, connectors.
• Healthcare: syringes, catheters, prosthetics.