= 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.
\b[The principal surviving <polymer> is a <polystyrene> chain bearing the junction $\mathrm{HOCH_2CH(Ph)-}$ 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.
Back to article page
