Polymers — Classification, Polymerization, Natural and Synthetic

Giant Molecules from Simple Units

Polymers are large molecules made by joining many small repeating units (monomers). From the plastic in your phone case to the proteins in your muscles, polymers are everywhere. This chapter classifies them, explains how they are made, and covers important examples that examiners test.

CBSE Class 12 boards give 3-5 marks. NEET tests 1-2 questions, usually on monomer identification or classification. The questions are largely factual — know the tables and you score.

Key Concepts

Classification by source

Type	Description	Examples
Natural	Found in nature	Proteins, cellulose, natural rubber, starch, nucleic acids
Semi-synthetic	Natural polymers modified chemically	Cellulose acetate (rayon), vulcanised rubber, gun cotton
Synthetic	Man-made in laboratories	Polythene, nylon, PVC, Teflon, Bakelite

Classification by structure

Type	Chain arrangement	Properties	Examples
Linear	Long, straight chains	High density, high tensile strength	HDPE, nylon
Branched	Chains with side branches	Low density, flexible	LDPE
Cross-linked	Chains connected by covalent bonds	Hard, rigid, infusible	Bakelite, vulcanised rubber, melamine

Classification by molecular forces (physical properties)

Type	Intermolecular forces	Properties	Examples
Elastomers	Weak van der Waals, some cross-links	Highly elastic, return to original shape	Rubber, buna-S
Fibres	Strong H-bonds or dipole-dipole	High tensile strength, low elasticity	Nylon, polyester, silk
Thermoplastics	Moderate intermolecular forces	Soften on heating, can be remoulded	Polythene, PVC, polystyrene
Thermosetting	Extensive cross-links	Hard, do not soften on heating, cannot be remoulded	Bakelite, melamine, epoxy

The distinction between thermoplastics and thermosetting polymers is tested frequently. Thermoplastics = linear/branched, can be reshaped. Thermosetting = cross-linked, permanent once set. Bakelite is the go-to example for thermosetting.

Types of polymerisation

Addition (chain-growth) polymerisation: Monomers with C=C double bonds add together. The double bond opens and links to the next monomer. No by-product is formed. The polymer has the same empirical formula as the monomer.

n(\text{CH}_2=\text{CH}_2) \to (-\text{CH}_2-\text{CH}_2-)_n

Initiated by free radicals, cations or anions. Three stages: initiation, propagation, termination.

Condensation (step-growth) polymerisation: Monomers with two functional groups react, losing a small molecule (H $_2$ O, HCl, NH $_3$ ) at each step. The polymer has a different empirical formula from the monomer.

Example: Nylon-6,6 from hexamethylenediamine + adipic acid → polyamide + water.

Copolymerisation: Two or more different monomers polymerise together. Example: Buna-S (1,3-butadiene + styrene), SBR rubber.

Important polymers — the master table

Polymer	Monomer(s)	Type	Structure	Use
LDPE	Ethene	Addition	Branched	Bags, squeeze bottles
HDPE	Ethene	Addition	Linear	Pipes, bottles
PVC	Vinyl chloride ( $\text{CH}_2=\text{CHCl}$ )	Addition	Linear	Pipes, flooring, cables
Polystyrene	Styrene ( $\text{C}_6\text{H}_5\text{CH}=\text{CH}_2$ )	Addition	Linear	Packaging, insulation
Teflon (PTFE)	Tetrafluoroethylene ( $\text{CF}_2=\text{CF}_2$ )	Addition	Linear	Non-stick cookware, seals
Polypropylene	Propene	Addition	Linear	Ropes, carpets
Polyacrylonitrile (PAN/Orlon)	Acrylonitrile	Addition	Linear	Woollen substitute
Nylon-6,6	Hexamethylenediamine + Adipic acid	Condensation	Linear	Textiles, ropes, gears
Nylon-6	Caprolactam	Condensation	Linear	Fibres, tyre cords
Terylene (PET/Dacron)	Ethylene glycol + Terephthalic acid	Condensation	Linear	Bottles, fabric
Bakelite	Phenol + Formaldehyde	Condensation	Cross-linked	Electrical switches, handles
Melamine-formaldehyde	Melamine + Formaldehyde	Condensation	Cross-linked	Crockery, laminates
Natural rubber	Isoprene (2-methyl-1,3-butadiene)	Addition	Linear (cis-1,4)	Tyres, tubes
Buna-N	1,3-butadiene + Acrylonitrile	Addition (copolymer)	—	Oil-resistant rubber
Buna-S (SBR)	1,3-butadiene + Styrene	Addition (copolymer)	—	Tyre manufacture

For NEET/boards, the most common question is: “Identify the monomer of…” Know the monomer-polymer pairs in the table above. Also know that nylon-6,6 has two monomers (6 carbons each), while nylon-6 has one monomer (6 carbons). The name tells you the number of carbons in each monomer.

Natural rubber and vulcanisation

Natural rubber is cis-1,4-polyisoprene — a linear addition polymer of isoprene. It is soft, sticky when warm and brittle when cold due to weak intermolecular forces and no cross-links.

Vulcanisation (Charles Goodyear, 1839): Heating rubber with sulphur introduces sulphur cross-links between chains. This:

Increases hardness and elasticity
Improves heat and solvent resistance
Makes rubber suitable for tyres and industrial applications

The extent of sulphur addition determines hardness: light vulcanisation → soft rubber (tyres), heavy sulphur → ebonite (hard, used in battery cases).

Biodegradable polymers

Traditional synthetic polymers persist for centuries. Biodegradable alternatives:

Polymer	Composition	Use
PHBV	Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)	Packaging, drug delivery
Nylon-2-nylon-6	Glycine + aminocaproic acid polyamide	Specialty fibres
PLA	Polylactic acid	Packaging, medical implants

These are decomposed by microorganisms, reducing environmental pollution.

Molecular weight of polymers

Polymers have a distribution of chain lengths, not a single molecular weight. Two averages are used:

\bar{M}_n = \frac{\sum N_i M_i}{\sum N_i}

where $N_i$ is the number of molecules with molecular weight $M_i$ .

\text{DP} = \frac{\text{Molecular weight of polymer}}{\text{Molecular weight of monomer}}

If polythene has MW = 28,000 and ethene has MW = 28, then DP = 1000 (1000 monomer units per chain).

Solved Examples

Example 1 (Easy — CBSE)

What type of polymer is Bakelite? Name its monomers.

Bakelite is a condensation polymer with a cross-linked structure. Monomers: phenol and formaldehyde. It is a thermosetting polymer — once set, it cannot be reshaped by heating. The cross-links between phenol rings make it rigid.

Example 2 (Medium — NEET)

Distinguish between thermoplastics and thermosetting polymers.

Thermoplastics (e.g., polythene, PVC) have linear or branched chains with moderate intermolecular forces — they soften on heating and can be remoulded repeatedly. Thermosetting polymers (e.g., Bakelite, melamine) are extensively cross-linked — once hardened, they cannot be softened because breaking cross-links would require breaking covalent bonds.

Example 3 (Medium — CBSE)

Monomer: vinyl chloride ( $\text{CH}_2=\text{CHCl}$ ). The C=C double bond opens during addition polymerisation:

n(\text{CH}_2=\text{CHCl}) \to (-\text{CH}_2-\text{CHCl}-)_n

No by-product is formed. PVC is a thermoplastic used in pipes and electrical insulation.

Example 4 (Hard — JEE/NEET)

A polymer has the repeating unit: $(-\text{NH}-(CH_2)_6-\text{NH}-\text{CO}-(CH_2)_4-\text{CO}-)_n$

This is Nylon-6,6. The amide linkages ( $-\text{CO}-\text{NH}-$ ) identify it as a polyamide. The first monomer has 6 carbons between the two NH groups (hexamethylenediamine), and the second has 6 carbons including the two CO groups (adipic acid). This is why it is called 6,6.

Example 5 (Application)

Teflon (PTFE) is made from tetrafluoroethylene ( $\text{CF}_2=\text{CF}_2$ ). The C-F bond is one of the strongest single bonds in organic chemistry (about 485 kJ/mol). The fluorine atoms completely shield the carbon backbone, making Teflon chemically inert — resistant to acids, bases and almost all solvents. Its low surface energy means nothing sticks to it, making it ideal for cookware and chemical equipment.

Common Mistakes to Avoid

Confusing addition and condensation. Addition: no by-product, requires C=C double bond in monomer. Condensation: small molecule lost (usually water), requires two functional groups per monomer. If the question says “no by-product”, it is addition.

Mixing up nylon-6 and nylon-6,6. Nylon-6: one monomer (caprolactam, 6 carbons, ring-opening). Nylon-6,6: two monomers (both with 6 carbons — hexamethylenediamine and adipic acid). The number of comma-separated values tells you the number of monomers.

Calling natural rubber a condensation polymer. Natural rubber (polyisoprene) is an addition polymer of isoprene. No small molecule is lost during its formation.

Saying LDPE and HDPE have different monomers. Both are made from ethene. LDPE is branched (made at high pressure, free radical initiation). HDPE is linear (made at low pressure, Ziegler-Natta catalyst). Same monomer, different conditions, different structures.

Thinking vulcanisation adds sulphur atoms to the chain. Sulphur forms cross-links between different polyisoprene chains, not along a single chain. These bridges restrict chain movement and increase rigidity.

Exam Weightage and Strategy

Polymers carries 3-5 marks in CBSE Class 12 boards and 1-2 NEET questions per year. The questions are heavily table-based — know the monomer-polymer pairs, the type of polymerisation, and the classification by structure and properties. This is one of the easiest scoring chapters if you memorise the master table.

Make a single table with columns: polymer name, monomer(s), polymerisation type (addition/condensation), structure (linear/branched/cross-linked), property class (thermoplastic/thermosetting/elastomer/fibre), and one use. About 15 rows cover all NEET-relevant polymers. That table is your complete revision.

Practice Questions

Q1. Name the monomer of PVC.

Vinyl chloride ( $\text{CH}_2=\text{CHCl}$ ). PVC is poly(vinyl chloride), an addition polymer. It is a thermoplastic used in pipes, cables and flooring.

Q2. What is vulcanization? Why is it done?

Heating natural rubber with sulphur to form cross-links between polyisoprene chains. It improves hardness, elasticity, and resistance to heat and organic solvents. Unvulcanised rubber is sticky when warm and brittle when cold — vulcanisation fixes both problems. Discovered by Charles Goodyear in 1839.

Q3. Classify the following as addition or condensation: (a) Polythene (b) Nylon-6,6 (c) Teflon (d) Bakelite.

(a) Addition — ethene has C=C, no by-product. (b) Condensation — two monomers join with loss of water. (c) Addition — tetrafluoroethylene has C=C. (d) Condensation — phenol + formaldehyde with loss of water.

Q4. What is a copolymer? Give an example.

A polymer made from two or more different monomers. Example: Buna-S rubber (1,3-butadiene + styrene). Other examples: Buna-N (1,3-butadiene + acrylonitrile), SBR rubber. Copolymers combine properties of both monomers.

Q5. What is the difference between LDPE and HDPE?

Both are made from ethene. LDPE (low density polyethene): made at high pressure (~1000-2000 atm) with free radical initiation, branched chains, lower density and melting point, used for bags and squeeze bottles. HDPE (high density polyethene): made at low pressure with Ziegler-Natta catalyst, linear chains, higher density and melting point, used for pipes and bottles.

Q6. Why is Bakelite used for electrical switches?

Bakelite is a cross-linked (thermosetting) polymer that is rigid, heat-resistant, and a good electrical insulator. It does not soften when heated (unlike thermoplastics), making it safe for electrical applications where heat is generated. Its cross-linked structure makes it mechanically strong and resistant to solvents.

Q7. What are biodegradable polymers? Why are they important?

Polymers that can be broken down by microorganisms into harmless products. Examples: PHBV, Nylon-2-nylon-6, PLA. They are important because conventional plastics (polythene, PVC) persist in the environment for hundreds of years, causing soil and water pollution, harming wildlife, and creating microplastics. Biodegradable alternatives reduce long-term environmental damage.

FAQs

What are biodegradable polymers?

Polymers that can be broken down by microorganisms. Examples: PHBV (polyhydroxybutyrate-co-valerate) and Nylon-2-nylon-6. Important for reducing plastic pollution. They decompose in months to years rather than centuries.

Why is Teflon chemically inert?

The C-F bond is extremely strong (485 kJ/mol). Fluorine atoms shield the carbon backbone completely, making Teflon resistant to almost all chemicals, acids, and solvents. Only molten alkali metals and elemental fluorine at high temperature can attack it.

What is the difference between elastomers and fibres?

Elastomers (rubber) have weak intermolecular forces with some cross-links — they stretch significantly and return to original shape. Fibres (nylon, polyester) have strong hydrogen bonds or dipole interactions — they have high tensile strength and resist stretching. The difference is in intermolecular force strength.

What is the Ziegler-Natta catalyst?

A catalyst system (typically $\text{TiCl}_4$ + $\text{Al(C}_2\text{H}_5)_3$ ) used to produce stereoregular polymers. It allows polymerisation at low pressure and temperature, producing linear, high-density polymers like HDPE. Karl Ziegler and Giulio Natta won the Nobel Prize in 1963 for this work.