Polymerisation Techniques Questions and Answers

Answer: C

Heat dissipation is least efficient in Bulk polymerization.

In bulk polymerisation there is only monomer and initiator. Addition polymerisation is strongly exothermic, and as conversion rises the viscosity soars, so the heat cannot be stirred or conducted away. Local hot spots accelerate the reaction further (the gel or Trommsdorff effect) and can lead to runaway.

Solution polymerisation dilutes the system with a solvent that absorbs heat and keeps it fluid. Suspension and emulsion polymerisation disperse the monomer as droplets or particles in water, whose high heat capacity and low viscosity remove heat very efficiently.

Answer: D

The match is P-2; Q-3; R-4; S-1.

• Butyl rubber is made by cationic copolymerisation of isobutylene with a little isoprene, using a Lewis acid initiator at very low temperature.
• Cold SBR is emulsion-polymerised at about 5 °C, which needs a redox initiator system to generate radicals at that temperature.
• PET is a polyester made by condensation polymerisation of terephthalic acid (or its dimethyl ester) with ethylene glycol.
• Polypropylene is made by coordination polymerisation, including with metallocene catalysts that control its tacticity.

Options assigning metallocene catalysts to butyl rubber or SBR, or condensation to polypropylene, mismatch the chemistry of at least two polymers.

Answer: B

Interfacial polymerization can be used to prepare Nylon 66.

Interfacial polymerization is a step-growth condensation at the boundary of two immiscible liquids: a diamine in water and a diacid chloride in an organic solvent. Hexamethylenediamine and adipoyl (or sebacoyl) chloride react at the interface to give nylon 66 (or 610); this is the classic 'nylon rope trick'.

Nylon 6 is made by ring-opening polymerization of caprolactam. Polyacrylonitrile and poly(butyl acrylate) are vinyl polymers made by free-radical chain-growth polymerization, not by condensation of two reactive monomers.

Answer: A

The match is P-2; Q-1; R-4; S-3.

• LLDPE is made by copolymerizing ethylene with an alpha-olefin on Ziegler-Natta (or metallocene) catalysts.
• Nylon 6 is made by ring-opening polymerization of caprolactam.
• Styrene-butadiene rubber is made mainly by emulsion polymerization.
• Aromatic polyamides (aramids) are made by condensation of an aromatic diamine with an aromatic diacid chloride.

The other options agree on LLDPE but give nylon 6 a condensation route, or put SBR and the aramid in each other's places.

Answer: B

The correct correlation is P - chain growth; Q - living polymerization; R - step growth.

Chain growth (P): each chain starts, grows to full length in a fraction of a second and terminates. High molecular weight chains appear from the very start, so \(\overline{M}_{n}\) jumps up at low conversion and then stays roughly constant while more monomer is converted into more chains.

Living polymerization (Q): all chains start together and there is no termination, so every chain keeps growing as monomer is consumed. \(\overline{M}_{n}\) rises linearly with conversion.

Step growth (R): any two species can react, so monomers first form dimers, trimers and short oligomers. By the Carothers equation \(\overline{X}_{n} = 1/(1 - p)\), high molecular weight appears only at very high conversion, near 99%, hence the sharp rise at the end.

Options assigning the linear line to chain growth, or the late rise to chain growth, contradict these mechanisms.

Answer: D

The correct matching is P-II ; Q-I ; R-IV ; S-III.

• BF₃/H₂O: a Lewis acid with a co-initiator that supplies a proton, the classic initiator for cationic polymerization (II).
• n-Butyllithium: a carbanion source for anionic (living) polymerization (I).
• FeCl₂/K₂S₂O₈: a redox pair in which Fe²⁺ reduces persulfate to sulfate radicals, starting free radical polymerization (IV).
• Metallocene/methylaluminoxane: a single-site coordination catalyst for olefin polymerization (III).

Options A and C call butyllithium a coordination or cationic initiator, and option B makes BF₃/H₂O a radical initiator and the redox pair cationic.