Photocleavage of the Corona Chains of Rigid-Rod Block Copolymer Micelles

A polyferrocenyldimethylsilane-block-poly(2-vinylpyridine) sample with a photocleavable o-nitrobenzyl ester (ONB) group at the junction (PFS35-hv-P2VP(400)) was synthesized by copper-catalyzed coupling of a P2VP-ONB-alkyne with a PFS-azide. Rodlike core-crystalline micelles of uniform length and uniform width were prepared in 2-propanol, a selective solvent for P2VP. Samples of these micelles were photoirradiated with UV-A light (peak emission 360 nm), which induced cleavage at the core-corona junction. Prolonged irradiation (24 h) led to aggregation and precipitation of the corona-cleaved micelles. One could see by TEM that the width of the micelles in the aggregates was significantly reduced (from 49 to 21 nm) because of the loss of the P2VP block, while the PFS core length (L) remained unchanged. For one micelle sample with L-w = 320 nm (650 polymer molecules per micelle), the time course of the irradiation was monitored by GPC, TEM, and multiangle light scattering. After 1 h irradiation, 60% of the corona chains were cleaved, but only small amounts of aggregates had formed. Most of the rodlike micelles maintained their colloidal stability even after 70% of the corona chains had been cleaved. By GPC, we detected formation of an unexpected PFS dimer that became more prominent as the irradiation continued. Dimer formation could be explained by a photoredox coupling of o-nitrosobenzaldehyde groups at the ends of adjacent PFS chains embedded in the micelle core.