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 / d / Solution

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.