Q.Write the names of classes of polymers formed according to intermolecular forces and describe briefly their structural characteristics.
Concept understanding — Classification of Polymers
Classification of Polymers – A First Look
You already know what a polymer is: a long chain made of many small repeating units (monomers). But not all polymers are the same. Think about the difference between a rubber band, a plastic water bottle, and a cotton T-shirt. They feel different, stretch differently, and are made from different starting materials. That's why we classify polymers — to understand their behaviour and how they're made.
The classification is done along four main axes: source, structure, mode of polymerization, and intermolecular forces. Each gives you a different lens to look at a polymer.
1. Classification by Source
This is the simplest: where does the polymer come from?
- Natural polymers – found in nature. Examples: cellulose (cotton, wood), proteins (wool, silk), natural rubber, starch, DNA.
- Synthetic polymers – man-made in a lab or factory. Examples: nylon, polyester, polythene, PVC, Teflon.
- Semi-synthetic polymers – natural polymers chemically modified. Examples: cellulose acetate (rayon), vulcanized rubber.
Semi-synthetic polymers are a middle ground. Rayon is made from natural cellulose but treated with chemicals — it's not fully natural, not fully synthetic.
2. Classification by Structure
This is about how the polymer chains are arranged. Imagine a pile of spaghetti versus a ladder versus a tree branch.
- Linear polymers – long, straight chains. They pack closely, so they have high density, high melting points, and are strong. Example: high-density polythene (HDPE).
- Branched polymers – chains with side branches. They pack less tightly, so they are less dense and melt at lower temperatures. Example: low-density polythene (LDPE).
- Cross-linked polymers – chains connected by covalent bonds in a 3D network. They are rigid, hard, and do not melt on heating (thermosetting). Example: bakelite, melamine.
Don't confuse "branched" with "cross-linked". Branched chains are still separate; cross-linked chains are chemically bonded together into one giant molecule.
3. Classification by Mode of Polymerization
This is about the chemical reaction that builds the polymer. There are two fundamentally different mechanisms.
Addition polymerization – monomers add to each other without losing any small molecule. The monomer usually has a double bond that opens up. The polymer has the same empirical formula as the monomer.
Example: ethene (CH2=CH2) → polythene (−CH2−CH2−)n
Condensation polymerization – monomers join by eliminating a small molecule like water, ammonia, or HCl. The polymer has a different formula from the monomer (because something is lost). These polymers usually have functional groups like ester, amide, or ether links.
Example: hexamethylenediamine + adipic acid → nylon-6,6 + water
| Feature | Addition | Condensation |
|---------|----------|--------------|
| By-product | None | Small molecule (H₂O, NH₃, etc.) |
| Monomer | Contains C=C double bond | Two different functional groups |
| Example | Polythene, PVC, Teflon | Nylon, polyester, bakelite |
4. Classification by Intermolecular Forces
This is the most practical classification for everyday use. The forces between polymer chains determine whether the material is stretchy, stiff, or fibre-like.
The strength of intermolecular forces increases in this order:
Elastomers < Plastics (Thermoplastics < Thermosetting) < Fibres
Let's unpack that.
Elastomers – weak van der Waals forces between chains, but the chains are lightly cross-linked. They can stretch a lot and snap back. Example: natural rubber, neoprene.
Fibres – strong intermolecular forces (hydrogen bonds, dipole-dipole) plus linear, closely packed chains. They are strong, tough, and have high tensile strength. Example: nylon, polyester, silk.
Plastics – intermediate forces. They are further divided into:
- Thermoplastics – linear or branched chains with moderate forces. They soften on heating and harden on cooling (reversible). Example: polythene, PVC, polystyrene.
- Thermosetting plastics – cross-linked polymers. They become hard and infusible once set. On heating, they decompose rather than melt. Example: bakelite, melamine-formaldehyde resin.
The key exam distinction: thermoplastics can be remoulded (recyclable), thermosetting plastics cannot. This is directly linked to their structure — linear vs. cross-linked.
Putting It All Together
Here's a quick reference table that ties the four classifications together for common polymers:
| Polymer | Source | Structure | Polymerization | Intermolecular Force | Type |
|---|---|---|---|---|---|
| Polythene (LDPE) | Synthetic | Branched | Addition | Moderate | Thermoplastic |
| Polythene (HDPE) | Synthetic | Linear | Addition | Moderate | Thermoplastic |
| Nylon-6,6 | Synthetic | Linear | Condensation | Strong (H-bonds) | Fibre |
| Natural rubber | Natural | Linear (lightly cross-linked) | Addition | Weak | Elastomer |
| Bakelite | Synthetic | Cross-linked | Condensation | Very strong (covalent network) | Thermosetting plastic |
| Cellulose | Natural | Linear | Condensation (in nature) | Strong (H-bonds) | Fibre |
The final takeaway: When you see a polymer in an exam, ask yourself four questions — Where does it come from? What shape are its chains? How were they joined? How strongly do the chains stick to each other? Answer those, and you've classified it completely.
Classification of polymers opens the NCERT Class 12 Chemistry Polymers chapter and is one of the most searched topics under queries like "classification of polymers class 12 chemistry" and "polymers important questions CBSE", since board papers routinely ask students to classify a given polymer by source, structure or mode of polymerisation.
By intermolecular force, polymers are elastomers, fibres, thermoplastics or thermosetting polymers, each with characteristic chain-holding forces.
Elastomers (weak forces, stretchy), Fibres (strong forces, high tensile strength), Thermoplastics (moderate forces, resoftenable), Thermosetting (permanently cross-linked)
Step 1. Elastomers are held together by weak van der Waals forces with only a few crosslinks, so the chains can be stretched under an applied force and spring back once it is released -- e.g. vulcanized rubber, buna-S, neoprene.
Step 2. Fibres are held together by strong intermolecular forces such as hydrogen bonding and strong dipole-dipole attraction, giving high tensile strength and a crystalline structure -- e.g. nylon 6,6, terylene.
Step 3. Thermoplastics have moderately strong intermolecular forces, intermediate between elastomers and fibres, so they can be softened repeatedly by heating and hardened again by cooling -- e.g. polythene, polystyrene, polyvinyls.
Step 4. Thermosetting polymers develop extensive covalent cross-linking while being moulded hot, becoming permanently rigid, infusible and insoluble once set -- e.g. bakelite, urea-formaldehyde resin.
Elastomers (weak forces + a few crosslinks, stretchy), Fibres (strong H-bonding/dipole forces, high tensile strength, crystalline), Thermoplastics (moderate forces, resoftenable), Thermosetting polymers (extensive permanent cross-linking, rigid once set)
List the four intermolecular-force classes in increasing/permanent cross-linking order and give each class's characteristic structural feature and one example.
- Mixing up fibres (strong forces, crystalline) with thermoplastics (only moderate forces).
- Describing thermosetting hardening as reversible, when it is a one-way, permanent change.
- CBSE 2022Set HE2181 markMCQQ.Nylon is an example of -(a) Polyamide(b) Polythene(c) Polyester(d) Polysaccharide
›Reveal solutionSolution
Nylon-6,6 is made by condensation polymerisation of a diamine and a diacid, joined by repeating amide (-CONH-) bonds, which is why it is classed as a polyamide.
Nylon-6,6 is manufactured by heating hexamethylenediamine [H2N-(CH2)6-NH2] with adipic acid [HOOC-(CH2)4-COOH] under high pressure and temperature. Each condensation step eliminates a water molecule and forms an amide (-CO-NH-) linkage between the -COOH of one monomer and the -NH2 of the other:
n H2N(CH2)6NH2 + n HOOC(CH2)4COOH -> [-NH(CH2)6NH-CO(CH2)4CO-]n + 2n H2O
Because the repeating linkage is the amide group, nylon is classified as a polyamide (not a polyester, which has -COO- ester linkages, e.g. Terylene/PET; not polythene, an addition polymer with no linking functional group; and not a polysaccharide, whose repeating sugar units are joined by glycosidic linkages).
✓Final answer(A) Polyamide.
- CBSE 2020Set HE8221 markQ.Fill in the blank: Neoprene is a ______ rubber.
›Reveal solutionSolution
Neoprene is a man-made (synthetic) rubber, an elastomer formed by free-radical addition polymerisation of chloroprene (2-chloro-1,3-butadiene).
Rubbers (elastomers) are classified as natural (e.g. natural rubber, obtained from the latex of Hevea brasiliensis by polymerisation of isoprene units) or synthetic (man-made, produced industrially by polymerising suitable monomers).
Neoprene is prepared by the free-radical polymerisation of chloroprene (2-chloro-1,3-butadiene, CH2=CCl–CH=CH2), giving polychloroprene. It is valued for its resistance to oils, heat and weathering, and is used in gaskets, hoses and wetsuits.
✓Final answerNeoprene is a synthetic rubber.
- CBSE 2020Set ANNUAL1 markMCQQ.Identify synthetic polymer amongst the following:(a) Linen(b) Jute(c) Silk(d) Terylene
›Reveal solutionSolution
Terylene is the man-made polyester; the other three are natural fibres.
Linen (from flax stem), jute (from the jute plant) and silk (secreted by silkworms) are all natural fibres of plant or animal origin. Terylene, on the other hand, is polyethylene terephthalate (PET) — a synthetic polyester manufactured industrially by condensation polymerisation of ethylene glycol with terephthalic acid (or its dimethyl ester), releasing water (or methanol) at each step. Being lab/factory-made from petrochemical monomers, it is the synthetic polymer of the four.
✓Final answer(d) Terylene
- CBSE 2019Set HE1 markQ.Match the following. Column A term: backelite. Column B options to match from:(a) buna-rubber(b) thermosetting plastic(c) Isoprene(d) CaOCl2(e) lyophilic colloid(f) NaCl
›Reveal solutionSolution
Bakelite is a phenol-formaldehyde resin formed by the condensation polymerisation of phenol and formaldehyde; it sets into a hard, infusible, cross-linked (thermosetting) solid on heating/moulding, and cannot be remoulded/softened again.
Bakelite is prepared by the condensation of phenol with formaldehyde (using either an acid or a base catalyst), forming a highly cross-linked, three-dimensional network polymer once cured. Because of this rigid cross-linked network, it is classified as a thermosetting plastic (as opposed to thermoplastics, which soften on heating and can be remoulded).
Matching against Column B: (a) buna-rubber, (b) thermosetting plastic, (c) Isoprene, (d) CaOCl₂, (e) lyophilic colloid, (f) NaCl → bakelite matches (b) thermosetting plastic.
✓Final answerBakelite → (b) thermosetting plastic.
- CBSE 2019Set ANNUAL1 markMCQQ.Which of the following is a copolymer?(a) Buna-S(b) PAN(c) PVC(d) Polythene
›Reveal solutionSolution
Buna-S is a copolymer of two different monomers (1,3-butadiene and styrene); the other three options are homopolymers.
A copolymer is formed from two or more different monomer units, while a homopolymer is built from only one type of monomer.
- Buna-S: a copolymer of buta-1,3-diene and styrene ("Bu" for butadiene, "Na" for sodium catalyst, "S" for styrene) — used in making synthetic rubber/tyres.
- PAN (polyacrylonitrile): a homopolymer of a single monomer, acrylonitrile (CH2=CH−CN).
- PVC (polyvinyl chloride): a homopolymer of a single monomer, vinyl chloride (CH2=CHCl).
- Polythene: a homopolymer of a single monomer, ethylene (CH2=CH2).
Only Buna-S is built from two distinct monomers, making it the copolymer.
✓Final answer(a) Buna-S.
- CBSE 2018Set ANNUAL1 markMCQQ.In Buna-S, letter S corresponds to:(a) sodium(b) sulphur(c) styrene(d) ethene
›Reveal solutionSolution
Buna-S is a copolymer of butadiene and styrene; "Bu" = butadiene, "Na" = sodium (the polymerisation catalyst), "S" = styrene.
Buna-S is a synthetic rubber made by co-polymerising 1,3-butadiene with styrene in the presence of sodium as a catalyst. Its name is built from: Bu (1,3-Butadiene) + Na (sodium catalyst) + S (Styrene). It has good resistance to abrasion and is widely used to manufacture automobile tyres and footwear soles.
✓Final answerStyrene (option c).
- CBSE 2017Set ANNUAL1 markQ.What are thermoplastics? Give one example.
›Reveal solutionSolution
Thermoplastics have linear/slightly-branched chains held only by weak intermolecular forces, so heating lets the chains slide past each other and re-set into a new shape.
Thermoplastics are polymers made of linear or slightly branched long-chain molecules that are held together only by weak intermolecular forces (van der Waals forces, not covalent cross-links). On heating, these weak forces are overcome, the chains can move/flow past one another, and the polymer softens; on cooling it hardens again in its new shape. Because no permanent chemical bonds are broken or formed, this softening–hardening cycle can be repeated many times, allowing the plastic to be remoulded/recycled.
Example: Polythene (polyethylene) — used for buckets, bags, bottles, etc. (Other examples: PVC, polystyrene, nylon.)
✓Final answerThermoplastics soften on heating and can be repeatedly remoulded because their chains are held only by weak intermolecular forces (no cross-linking); example — polythene.
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