Dual wavelength photopolymerization of amphiphilic block copolymers via a chalcone-2959-based initiator

Abstract

The synthesis of high-purity amphiphilic block copolymers (BCPs) via photopolymerization remains challenging due to homopolymer impurities and oligomer byproducts generated by conventional photoinitiators, in which a single initiator fragment is unfavorable for generating two or more free radicals. To address this, we report the rational design of a novel unimolecular dual-color photoinitiator, IAB-2959, synthesized by bridging Irgacure 2959 to an indole-chalcone chromophore. Under sequential blue (425 nm) and UV (275 nm) irradiation, this initiator leverages distinct mechanisms: hydrogen abstraction polymerizes the first monomer, while α-cleavage of the resultant α-hydroxy ketone initiates polymerization of the second monomer, yielding well-defined A–B BCPs. Although minor oligomers were produced during the initiation stage, they were readily removed during the second-stage purification by leveraging solubility differences. Comprehensive characterization including DFT calculations, UV-Vis spectra, ESR and photolysis tests confirms the initiation mechanism. Using the initiator IAB-2959, PMMA_m-b-PHEA_n and PHEA_m-b-PMMA_n diblock copolymers were successfully synthesized with high purity and low dispersity (Đ < 1.75) and verified by GPC analysis and ¹H NMR spectroscopy. This work pioneers a dedicated photoinitiator strategy, enabling precise and efficient BCP synthesis with minimal impurities.