Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section A / 19

Let $R=\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Ph})-\mathrm{O}-\mathrm{TEMPO}$, so B is $\mathrm{HO}-\mathrm{R}$. Its hydroxyl group initiates ring-opening polymerization of lactide, givingEach lactide contributes two lactyl repeat units. Dry monomer, exclusion of water and an alcohol-compatible catalyst such as tin(II) 2-ethylhexanoate under heating provide a practical route. Alcohol/metal-alkoxide exchange followed by lactide coordination and acyl–oxygen cleavage extends the ester chain while retaining a terminal hydroxyl group.

Heating C under oxygen-free conditions homolytically cleaves its benzylic C–O bond to the nitroxide, releasing a benzylic radical and TEMPO. The radical adds styrene repeatedly; reversible trapping by TEMPO controls the active radical concentration. D is therefore a poly(lactide)–polystyrene block copolymer with junction $\mathrm{PLA}-\mathrm{OC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Ph})-[\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Ph}){]}_{\mathrm{m}}-\mathrm{O}-\mathrm{TEMPO}$ and the hydroxyl end of PLA retained. This is nitroxide-mediated polymerization; a temperature around 120–130 °C is a representative condition for a TEMPO/styrene system, rather than an exact universal optimum.

Styrene, benzoyl peroxide and TEMPO provide the required radical addition–trapping sequence. Heating the peroxide breaks its O–O bond into benzoyloxy radicals. Before decarboxylation, $\mathrm{PhC}{\mathrm{O}}_2^{\bullet }$ can add to styrene's terminal carbon, generating the resonance-stabilized radical $\mathrm{PhC}{\mathrm{O}}_2\mathrm{C}{\mathrm{H}}_2\dot{\mathrm{C}}\mathrm{HPh}$. Coupling with the oxygen-centred TEMPO radical then gives $\mathrm{PhC}{\mathrm{O}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Ph})-\mathrm{O}-\mathrm{TEMPO}$, the benzoate-protected alkoxyamine A.

The connectivity distinguishes this route from initiators that leave a carbon-based radical fragment at the chain end. Competition with benzoyloxy-radical decarboxylation affects yield, so this is a plausible preparation rather than a claim of exclusive product formation. Subsequent benzoate cleavage reveals B's hydroxyl group without requiring rupture of its alkoxyamine linkage.

The ideal number-average molar mass is approximately 25.11 kg mol${}^{-1}$. Quantitative uptake of 100 lactide and 100 styrene molecules per bifunctional initiator givesB has formula ${\mathrm{C}}_{17}{\mathrm{H}}_{27}\mathrm{N}{\mathrm{O}}_2$. Lactide's entire cyclic diester mass is incorporated; multiplying its molar mass by two again would double-count its two lactyl units. The estimate assumes quantitative initiation, conversion, nitroxide recapping and no chain transfer or loss of chain-end fragments.

Hydroxide attacks a PLA ester carbonyl, moving the $\pi$ electrons onto oxygen. Collapse of the tetrahedral intermediate expels an alkoxide; proton transfer produces an alcohol and a carboxylate. The carboxylate stabilizes the hydrolysis products and makes this saponification effectively irreversible under the basic conditions. Repetition cleaves the PLA block, including its terminal ester linkage to the junction alcohol, while the carbon–carbon polystyrene backbone remains intact.

The principal surviving polymer is a polystyrene chain bearing the junction $\mathrm{HOC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Ph})-$ end and its nitroxide-derived distal end. In gel permeation chromatography, the intact block copolymer normally elutes earlier than this smaller surviving polystyrene coil. Lactate and short hydrolysis fragments elute later still, provided the column and detector resolve such small solutes. The polymer peak therefore shifts to larger elution volume; its height need not remain constant under a mass-sensitive detector. Elution reflects hydrodynamic volume, so calibration against polystyrene standards does not supply an exact absolute mass for the original mixed block.

Replacing lactide by ethylene oxide givesThe first chain grows by alkoxide attack on ethylene oxide, followed by protonation at work-up; the alkoxyamine subsequently controls styrene polymerization. The final material is an amphiphilic block copolymer. In water, polystyrene avoids solvent exposure while the poly(ethylene oxide) block is hydrated. Above a concentration-dependent assembly threshold, micelles with a polystyrene core and hydrated poly(ethylene oxide) corona are a reasonable morphology. Unimers remain in equilibrium with aggregates. Core–corona assembly lowers unfavourable water contact but costs chain-stretching and translational entropy. Block volume fractions, solvent history and concentration can instead favour cylinders or vesicles, so the chemical structure alone does not uniquely establish spherical micelles.