Class 12 Polymers — Types, Preparation and Degree of Polymerisation
Polymers is a short chapter that rewards organised memory more than clever reasoning, and that is exactly why it is worth doing properly: the marks are reliable. Almost every question reduces to one of four things — name the monomer, name the type, state the preparation conditions, or calculate a degree of polymerisation. This guide covers all four, with the arithmetic worked out in full.
The vocabulary, stated precisely
Note the wording: repeating unit, not monomer. For an addition polymer the two are the same thing. For a condensation polymer they are not, because a small molecule is lost when the monomers join — and forgetting that is the single most common numerical error in this chapter.
Classification 1 — by source
| Type | Meaning | Examples |
|---|---|---|
| Natural | Occur in nature | Cellulose, starch, natural rubber, proteins |
| Semi-synthetic | A natural polymer chemically modified | Cellulose acetate (rayon), cellulose nitrate |
| Synthetic | Made entirely in the laboratory or plant | Polythene, nylon, Teflon, PVC, buna-S |
Classification 2 — by structure
| Structure | Chains | Consequence | Example |
|---|---|---|---|
| Linear | Long straight chains that pack closely | High density, high tensile strength, higher melting point | High density polythene (HDPE), PVC |
| Branched | Chains with side branches that stop close packing | Lower density, more flexible, lower melting point | Low density polythene (LDPE) |
| Cross-linked | Chains joined to each other by covalent links | Hard, rigid, brittle; cannot be remoulded | Bakelite, melamine, vulcanised rubber |
Classification 3 — by mode of polymerisation
This is the classification questions ask about most.
| Addition (chain growth) | Condensation (step growth) | |
|---|---|---|
| Monomer requirement | Must contain a double or triple bond | Must contain two functional groups |
| Small molecule lost? | No — the polymer has the same empirical formula as the monomer | Yes — usually H₂O, sometimes HCl, CH₃OH or NH₃ |
| Growth pattern | Chains grow rapidly to full length; monomer is consumed steadily | Monomers form dimers, trimers and so on; long chains appear only late |
| Examples | Polythene, PVC, polypropene, Teflon, polyacrylonitrile | Nylon 6,6, nylon 6, terylene, bakelite, melamine–formaldehyde |
Classification 4 — by molecular forces
| Class | Force between chains | Behaviour | Example |
|---|---|---|---|
| Elastomers | Weakest — chains held by a few cross-links only | Stretch a long way and snap back | Buna-S, buna-N, neoprene, natural rubber |
| Fibres | Strong — hydrogen bonding between chains | High tensile strength, thread-like, high melting point | Nylon 6,6, terylene |
| Thermoplastics | Intermediate — no cross-links | Soften on heating, harden on cooling, can be remoulded repeatedly | Polythene, polystyrene, PVC |
| Thermosetting | Heavily cross-linked into a three-dimensional network | Set permanently on moulding; cannot be softened and reused | Bakelite, melamine–formaldehyde |
Addition polymers and their conditions
Polythene is the standard example, and the two grades are a favourite comparison question.
- Low density polythene (LDPE) — ethene heated at about 350–570 K under roughly 1000–2000 atm, with a trace of dioxygen or a peroxide as initiator. The mechanism is free radical, and chain transfer produces branching, which is why the product is branched, flexible and less dense. It is a poor conductor, used for squeeze bottles, toys and insulation.
- High density polythene (HDPE) — ethene polymerised with a Ziegler–Natta catalyst (triethylaluminium with titanium tetrachloride) at about 333–343 K and 6–7 atm. The chains come out linear, pack closely, and give a harder, denser plastic used for buckets, pipes and containers.
The free radical mechanism has three stages worth being able to name: initiation (the peroxide breaks homolytically to give radicals), propagation (a radical adds across the C=C of a monomer, regenerating a radical at the new chain end, over and over), and termination (two radicals combine and growth stops).
| Polymer | Monomer | Notes / use |
|---|---|---|
| Polythene | Ethene, CH₂=CH₂ | LDPE and HDPE as above |
| Polypropene | Propene, CH₂=CH–CH₃ | Ropes, pipes, moulded articles |
| Polyvinyl chloride (PVC) | Vinyl chloride, CH₂=CH–Cl | Pipes, raincoats, cable insulation |
| Teflon | Tetrafluoroethene, CF₂=CF₂ | Made with a persulphate or free radical catalyst under pressure; chemically inert, non-stick |
| Polyacrylonitrile (acrilan) | Acrylonitrile, CH₂=CH–CN | Substitute for wool |
| Polystyrene | Styrene, C₆H₅–CH=CH₂ | Packaging, insulation |
Condensation polymers and their monomer pairs
| Polymer | Monomers | Conditions (as given in NCERT) | Small molecule lost |
|---|---|---|---|
| Nylon 6,6 | Hexamethylenediamine + adipic acid | Heated under high temperature and pressure (about 553 K) | Water |
| Nylon 6 | Caprolactam (one monomer only) | Heated with water at about 533–543 K | None — ring opening, then condensation |
| Terylene (dacron) | Ethylene glycol + terephthalic acid | About 420–460 K with a zinc acetate–antimony trioxide catalyst | Water |
| Bakelite | Phenol + formaldehyde | Acid or base catalyst; the linear novolac is then cross-linked on heating | Water |
| Melamine–formaldehyde | Melamine + formaldehyde | Condensation; used for unbreakable crockery | Water |
Nylon 6 is worth a second look, because it is the one condensation polymer made from a single monomer. The name refers to the six carbon atoms in caprolactam, not to two different monomers — which is exactly the distinction the "6,6" in nylon 6,6 is making.
Copolymers and rubber
A copolymer is made from more than one kind of monomer. Two are named in the syllabus:
- Buna-S — 1,3-butadiene with styrene, using sodium as catalyst. Used for tyres and floor tiles. The "S" stands for styrene, and the "Bu" and "Na" come from butadiene and the sodium catalyst.
- Buna-N — 1,3-butadiene with acrylonitrile. Resistant to oils and petrol, so it is used for oil seals and hoses.
Natural rubber is cis-1,4-polyisoprene. The cis geometry stops the chains packing neatly, which is what makes it elastic. (The trans isomer, gutta-percha, is hard and non-elastic — same monomer, different geometry, opposite properties.) Raw rubber is sticky when hot and brittle when cold, so it is vulcanised: heated with sulphur at about 373–415 K, which introduces sulphur cross-links between the chains and makes the rubber harder, stronger and far more useful.
Biodegradable polymers
Two are named in the syllabus. PHBV is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid, used in packaging and in controlled drug release. Nylon-2-nylon-6 is an alternating polyamide of glycine and aminocaproic acid. Both are broken down by micro-organisms, unlike the ordinary plastics above.
Worked example 1 — degree of polymerisation of polythene
A sample of polythene has an average molar mass of 42,000 g/mol. Find its degree of polymerisation.
Step 1 — molar mass of the repeating unit. For an addition polymer this is the monomer
itself, ethene C₂H₄:
C: 2 × 12.011 = 24.022
H: 4 × 1.008 = 4.032
M(C₂H₄) = 24.022 + 4.032 = 28.054 g/mol
Step 2 — divide:
n = 42,000 ÷ 28.054
28.054 × 1000 = 28,054
28.054 × 400 = 11,221.6 → running total 39,275.6
28.054 × 90 = 2,524.86 → running total 41,800.46
28.054 × 7 = 196.378 → running total 41,996.84
Remainder: 42,000 − 41,996.84 = 3.16, and 3.16 ÷ 28.054 ≈ 0.11
n ≈ 1497, i.e. about 1.50 × 10³ repeating units per chain.
Check: 1497 × 28.054 = 41,996.8 g/mol, which rounds to 42,000. ✓
Worked example 2 — working backwards for PVC
A PVC chain contains 800 repeating units. What is its molar mass?
Repeating unit = vinyl chloride, C₂H₃Cl:
C: 2 × 12.011 = 24.022
H: 3 × 1.008 = 3.024
Cl: 1 × 35.45 = 35.450
M = 24.022 + 3.024 = 27.046; 27.046 + 35.450 = 62.496 g/mol
Molar mass of the chain = 800 × 62.496 = 49,996.8 ≈ 5.00 × 10⁴ g/mol
Worked example 3 — a condensation polymer, where the water matters
Find the molar mass of one repeating unit of nylon 6,6.
Hexamethylenediamine, H₂N(CH₂)₆NH₂ = C₆H₁₆N₂:
C: 6 × 12.011 = 72.066; H: 16 × 1.008 = 16.128; N: 2 × 14.007 = 28.014
M = 72.066 + 16.128 = 88.194; + 28.014 = 116.208 g/mol
Adipic acid, HOOC(CH₂)₄COOH = C₆H₁₀O₄:
C: 6 × 12.011 = 72.066; H: 10 × 1.008 = 10.080; O: 4 × 15.999 = 63.996
M = 72.066 + 10.080 = 82.146; + 63.996 = 146.142 g/mol
Route 1 — subtract the water lost. Joining the two monomers at both ends
releases two molecules of water:
116.208 + 146.142 = 262.350
262.350 − (2 × 18.015) = 262.350 − 36.030 = 226.320 g/mol
Route 2 — build the repeating unit's formula. It is C₁₂H₂₂N₂O₂:
C: 12 × 12.011 = 144.132; H: 22 × 1.008 = 22.176; N: 2 × 14.007 = 28.014;
O: 2 × 15.999 = 31.998
144.132 + 22.176 = 166.308; + 28.014 = 194.322; + 31.998 = 226.320 g/mol ✓
Both routes agree. If you had forgotten the two water molecules you would have written 262.35 instead of 226.32 — an error of about 16%, which changes every answer that depends on it.
Mistakes that lose marks
- Calling nylon an addition polymer. Nylon 6,6 is a condensation polymer — two functional groups, water lost.
- Assuming condensation always releases water. Water is the usual small molecule, but HCl, ammonia or an alcohol can be released instead, depending on the monomers.
- Using the monomer mass for a condensation polymer's repeating unit. Subtract the small molecule lost, as in worked example 3.
- Mixing up thermoplastic and thermosetting. Thermoplastics soften on heating and can be remoulded; thermosetting polymers are cross-linked and set permanently.
- Swapping the LDPE and HDPE conditions. LDPE is the high-pressure, free radical, branched one; HDPE is the Ziegler–Natta, low-pressure, linear one.
- Thinking the "S" in buna-S means sulphur. It means styrene. Sulphur belongs to vulcanisation, which is a different process entirely.
- Writing natural rubber as trans-polyisoprene. It is the cis isomer; the trans form is gutta-percha and is not elastic.
Where polymers is examined
| Exam | Typical question |
|---|---|
| CBSE Class 12 | Name the monomers of a given polymer; classify by mode of polymerisation and by molecular force; distinguish LDPE and HDPE; explain vulcanisation |
| ISC Class 12 | Preparation and uses of nylon, terylene and bakelite; addition vs condensation with examples |
| NEET | Direct monomer-to-polymer matching questions |
| IIT-JAM / GATE | Degree of polymerisation, number-average versus weight-average molar mass, mechanisms |
Check the current syllabus on your board's official website before deciding how much time to give this chapter, as its coverage is revised from time to time.
Do the degree-of-polymerisation arithmetic without slips. Enter the monomer formula — C2H4, C2H3Cl, C6H16N2 or C6H10O4 — into the Molar Mass & Composition calculator to get the repeating unit mass, then divide the polymer's molar mass by it. Remember to subtract the small molecule lost for a condensation polymer.
Open the Molar Mass & Composition Calculator →Working through the Class 12 chemistry syllabus before boards? ABC Chemistry runs Class 11–12 chemistry coaching at its Gurugram centre and online classes across India — details at abcchemistry.in.