Question
Classify polymers based on four different criteria: source of origin, structure, mode of polymerization, and molecular forces. Give examples for each category. What is the difference between addition and condensation polymerization?
(NEET + CBSE Board — classification)
Solution — Step by Step
| Category | Description | Examples |
|---|---|---|
| Natural | Found in nature | Starch, cellulose, proteins, natural rubber, nucleic acids |
| Semi-synthetic | Chemically modified natural polymers | Cellulose acetate (rayon), cellulose nitrate, vulcanised rubber |
| Synthetic | Man-made in laboratories | Nylon, polyester, PVC, polythene, Bakelite, Teflon |
| Structure | Description | Examples |
|---|---|---|
| Linear | Long straight chains | HDPE (high-density polyethylene), nylon, polyester |
| Branched | Side chains branching off main chain | LDPE (low-density polyethylene), amylopectin, glycogen |
| Cross-linked | Chains connected by covalent bonds (3D network) | Bakelite, melamine, vulcanised rubber |
| Type | Mechanism | By-product? | Examples |
|---|---|---|---|
| Addition (chain-growth) | Monomers add one by one to a growing chain via double bond opening | No by-product | Polythene, PVC, polystyrene, Teflon |
| Condensation (step-growth) | Monomers react with loss of small molecules (H₂O, HCl, NH₃) | Yes — small molecule lost | Nylon 6,6 (loses H₂O), polyester (loses H₂O), Bakelite |
| Category | Molecular Forces | Properties | Examples |
|---|---|---|---|
| Elastomers | Weak van der Waals | Elastic, can stretch and return | Natural rubber, neoprene, buna-S |
| Fibres | Strong H-bonds | High tensile strength, used in textiles | Nylon 6,6, polyester (terylene) |
| Thermoplastics | Moderate intermolecular | Soften on heating, can be remoulded | Polythene, PVC, polystyrene |
| Thermosetting | Extensive cross-links | Do NOT soften on heating, permanent shape | Bakelite, melamine, urea-formaldehyde |
graph TD
A[Polymer Classification] --> B[By Source]
A --> C[By Structure]
A --> D[By Polymerization]
A --> E[By Molecular Forces]
B --> B1["Natural / Semi-synthetic / Synthetic"]
C --> C1["Linear / Branched / Cross-linked"]
D --> D1["Addition / Condensation"]
E --> E1["Elastomers / Fibres / Thermoplastic / Thermosetting"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style D fill:#86efac,stroke:#000
style E fill:#93c5fd,stroke:#000Why This Works
Polymers are classified from four different angles because each classification tells us something different. Source tells us origin. Structure tells us physical arrangement. Polymerization type tells us how it was made. Molecular forces tell us mechanical behaviour and applications. A single polymer can be classified in all four systems — for example, nylon 6,6 is synthetic, linear, condensation, and a fibre.
Common Mistake
Students often confuse thermoplastic with thermosetting polymers. Thermoplastics can be reheated and reshaped (like polythene bags). Thermosetting polymers form permanent cross-links during moulding and CANNOT be reshaped — heating them causes decomposition, not melting. Bakelite (used in electrical switches) is thermosetting; PVC (used in pipes) is thermoplastic.
For NEET, the most common trick question: “Is natural rubber a thermoplastic or elastomer?” It is an elastomer. After vulcanisation (cross-linking with sulphur), it becomes a thermosetting polymer. Knowing these transitions helps answer tricky MCQs.