VKC Notes · Polymers · Civil Services P-II · IFoS P-II · Geo-Scientist P-III

9.1 Polymers: Polymerisation, Ziegler–Natta and Molar Mass

★★★ 2021–2025: 36 parts, 240 marks (Civil Services 14 parts / 100 m · IFoS 18 / 115 m · Geo-Scientist 4 / 25 m), plus Prelims. Civil Services 2025 gave a whole question (Q4) to polymers.

Syllabus lines. Civil Services: “Preparation and properties of polymers: organic polymers: polyethylene, polystyrene, polyvinyl chloride, Teflon, nylon, terylene, synthetic and natural rubber”. IFoS: the same, plus “Inorganic polymers: phosphonitrilic halides, borazines, silicones and silicates” and “Physical chemistry of polymers: number and weight average molecular weights; determination by sedimentation, light scattering, osmotic pressure, viscosity, end group analysis”. Geo-Scientist: “polymerization reactions of alkenes and substituted alkenes, Ziegler-Natta catalyst for polymerization, polyurethane, and conducting polymers” (under addition reactions).

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

9.1.1 Chain growth and step growth ★★

	Chain growth (addition)	Step growth (condensation)
Who reacts	monomer adds only to an active chain end (radical, ion, metal–C)	any two molecules with the right end groups
High polymer	at once, even at low conversion	only near the end (p > 0.99)
By-product	none	H₂O, HCl, MeOH (none for polyurethanes)
Examples	PE, PP, PS, PVC, Teflon, rubbers	nylons, PET, polyurethanes, epoxies, Bakelite

Carothers equation: if p is the fraction of end groups that have reacted, the number-average degree of polymerisation is X̄ₙ = 1/(1 − p). 0.2 % unreacted –COOH → p = 0.998 → X̄ₙ = 500.SN
p-Hydroxybenzoic acid (an AB monomer) gives the aromatic polyester [–O–C₆H₄–CO–]ₙ, repeat unit 120 g mol⁻¹: M̄ₙ ≈ 500 × 120 = 6.0 × 10⁴ g mol⁻¹, a linear step-growth (condensation) polyester; the rigid para chain makes it a liquid-crystalline, high-melting thermoplastic CSE 2025 Q4(a)(i) 7m CSE 2025 Q4(a)(ii) 3m.SN
For a copolymer, the average repeat-unit mass is the mole-weighted mean: A₃.₅₅B₂.₂₀C₁.₀₀ with 104, 184, 128 → (369.2 + 404.8 + 128)/6.75 = 133.6 IFoS 2023 Q8(d) 5m.

(9.1.1)

9.1.2 Free-radical polymerisation ★★★

Fig. 9.1.1 Radical chain polymerisation of ethylene, with fishhook arrows.

Draw it in 60 s: O–O of benzoyl peroxide: two fishhooks, one to each O; R• + CH₂=CH₂: one fishhook from R•, one from the π bond to the same C, one to the far C; back-biting: radical end reaches the H on C-5.

Initiation: the weak O–O bond of benzoyl peroxide (≈ 130 kJ mol⁻¹) breaks on heating; PhCO₂• may lose CO₂ to give Ph•; the radical adds to the CH₂ end of the monomer. AIBN works the same way with loss of N₂.SN
Propagation: each step turns a C=C π bond into a C–C σ bond (about −90 kJ mol⁻¹), so the chain runs downhill; styrene adds head-to-tail because the new radical is benzylic.
Termination: two chain radicals combine (one long chain, both ends carry initiator fragments) or disproportionate (one H moves: a saturated and an unsaturated chain end).SN
Chain transfer and branching: the chain-end radical takes an H from its own chain five carbons back through a six-membered transition state (back-biting, 1,5-H shift): butyl branches. Transfer to another chain gives long branches. Branched chains pack badly: low-density polyethylene (made at 1000–3000 atm, 200 °C), soft and flexible CSE 2024 Q6(a)(i) 10m Geo 2023 Q8(c) 5m.SN

(9.1.2)

Inhibitors are radicals or radical traps that end chains: DPPH (a stable radical) couples with or takes an H from the polystyryl radical, giving a “dead” chain; hydroquinone, benzoquinone and O₂ do the same. A radical polymer is not living IFoS 2022 Q5(b) 5m.SN
Monomers: ethylene, styrene, vinyl chloride (PVC), CF₂=CF₂ (Teflon), methyl methacrylate, acrylonitrile. Propene fails: the chain radical takes an allylic H and makes a stable allyl radical that cannot propagate.

9.1.3 Ionic and living polymerisation ★★★

Anionic: initiator n-BuLi, Na/naphthalene, NaNH₂; needs a monomer whose carbanion is stabilised by an electron-withdrawing group (styrene, butadiene, acrylonitrile, methyl methacrylate). For para-styrenes the chain end is a benzylic carbanion: p-NO₂ > p-CH₃ > p-OCH₃ (–M stabilises; +M destabilises most) CSE 2021 Q5(b)(i) 5m IFoS 2023 Q5(a) 5m.SN
Cationic: initiator BF₃·H₂O, AlCl₃, H₂SO₄; needs an electron-donating group that stabilises the carbocation (isobutylene → butyl rubber, vinyl ethers, p-methoxystyrene). Prelims rule: donor-substituted styrenes go cationic, acceptor-substituted go anionic Geo Pre 2024 Q107.SN
Living polymer (Szwarc, 1956): in dry, O₂-free THF an anionic chain end has no way to terminate; it stays active until a proton source is added. Consequences: chains grow at the same rate, so Đ ≈ 1 (narrow distribution); X̄ₙ = [M]₀/[I]₀; adding a second monomer grows a block copolymer IFoS 2024 Q6(b)(i) 10m.SN

Solved PYQ CSE 2021 Q7(a) 20m IFoS 2023 Q7(b) 5m

I. Styrene + n-BuLi → A; A + excess 1,3-butadiene → B; B + H₂O → C. II. Cyclohexanone + NH₂OH → A; A + 55 % H₂SO₄ → B; B, heat → C.

I. A = Bu–[CH₂CH(Ph)]ₙ–CH₂CH(Ph)⁻ Li⁺, living polystyryllithium (a deep red solution).
B = the same chain end now adding butadiene (mostly 1,4 in hydrocarbon, more 1,2 in THF): Bu–(styrene)ₙ–(butadiene)ₘ⁻ Li⁺, a living block.
C = Bu–(styrene)ₙ–(butadiene)ₘ–H + LiOH: a polystyrene-block-polybutadiene copolymer (the SBS thermoplastic rubber idea).SN
II. A = cyclohexanone oxime; B = ε-caprolactam (Beckmann rearrangement); C = nylon-6 (Perlon) by ring-opening polymerisation.

9.1.4 Ziegler–Natta catalysis and tacticity ★★★

Fig. 9.1.2 Isotactic, syndiotactic and atactic chains.

Draw it in 60 s: draw a zigzag of 12 bonds three times; on every second carbon put R on a wedge (all), wedge-hash alternately, or at random.

Tacticity is the relative configuration of successive stereocentres along a chain: isotactic all R on one side, syndiotactic alternate, atactic random CSE 2023 Q6(b)(ii) 5m.SN
Regular chains crystallise. Polystyrene: atactic is an amorphous, clear, brittle glass that softens near 100 °C (T_g); isotactic is semicrystalline, T_m ≈ 240 °C; syndiotactic crystallises fast, T_m ≈ 270 °C, stiff and solvent resistant CSE 2024 Q6(a)(ii) 5m.SN
Radical polymerisation gives atactic chains. Syndiotactic polystyrene: a half-sandwich titanium catalyst, CpTiCl₃ (or Cp*Ti(OMe)₃) with methylaluminoxane (MAO), in toluene at 50–70 °C CSE 2025 Q4(b)(ii) 5m.SN

Fig. 9.1.3 Cossee–Arlman mechanism at titanium.

Draw it in 60 s: Ti with the chain P on CH₂ and propene beside it; arrow from C=C to Ti, arrow from the Ti–C bond to the CH of propene; product Ti–CH₂–CH(CH₃)–CH₂–P.

Catalyst: TiCl₄ (or TiCl₃) + Al(C₂H₅)₃ in dry heptane; propene at 50–70 °C, 1–10 atm. AlEt₃ alkylates surface titanium: a Ti–C bond beside a vacant site.SN
Propene binds at the vacant site as a π-complex; the chain migrates onto it through a four-centre transition state (migratory insertion); a new vacant site opens and the cycle repeats.SN
The chiral crystal surface lets propene bind one way only: every CH₃ ends up on the same side, isotactic polypropylene CSE 2022 Q6(b)(i) 10m.
Advantages: low pressure and temperature; linear chains (HDPE, no back-biting); stereocontrol (isotactic PP, cis-1,4-polyisoprene); propene polymerises at all, which radicals cannot do. Ziegler and Natta, Nobel 1963.SN

The same insertion with acetylene gives polyacetylene (9.1.8) Geo 2022 Q7(a) 10m.

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