Synthesis and Self-Assembly of Coil−Rod Double Hydrophilic Diblock Copolymer with Dually Responsive Asymmetric Centipede-Shaped Polymer Brush as the Rod Segment

We report on the synthesis and self-assembly of well-defined coil−rod double hydrophilic diblock copolymer with pH- and thermo-responsive asymmetric centipede-shaped polymer brush as the rod segment via a combination of atom transfer radical polymerization (ATRP) and click chemistry (Schemes 1 and 2). At first, poly(ethylene oxide)-b-poly(glycidyl methacrylate), PEO-b-PGMA, was prepared by ATRP using PEO-based macroinitiator. The ring-opening of pendent epoxide moieties in PEO-b-PGMA with NaN3 followed by esterification with 2-bromoisobutyryl bromide afforded multifunctional PEO-b-[PGMA-(N3)(Br)] bearing one azide and one bromine moieties on each monomer repeating unit of PGMA. The subsequent ATRP of 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) using PEO-b-[PGMA-(N3)(Br)] as the macroinitiator yielded PEO-b-[PGMA-g-(N3)(PMEO2MA)] coil−brush diblock copolymer possessing one residual azide moiety at each grafting site. Finally, the target coil−rod diblock copolymer with asymmetric centipede-shaped polymer brush as the rod segment, PEO-b-[PGMA-g-(PDEA)(PMEO2MA)], was obtained via the click reaction of PEO-b-[PGMA-g-(N3)(PMEO2MA)] with an excess of alkynyl-terminated poly(2-(diethylamino)ethyl methacrylate) (alkynyl-PDEA). All the intermediate and final products were characterized by 1H NMR, Fourier transform infrared spectroscopy (FT-IR), and gel permeation chromatography (GPC). Atomic force microscopy (AFM) analysis revealed that PEO-b-[PGMA-g-(PDEA)(PMEO2MA)] coil−rod diblock unimer chains adopt a wormlike conformation in aqueous solution at pH 4 and room temperature. Possessing pH-responsive PDEA and thermo-responsive PMEO2MA grafts arranged in an asymmetric centipede manner within the rod segment, PEO-b-[PGMA-g-(PDEA)(PMEO2MA)] self-assembles into two types spherical aggregates in aqueous solution, depending on solution pH and temperatures. The multiresponsive switching between wormlike unimers and two types of micellar aggregates were characterized by temperature-dependent optical transmittance, dynamic laser light scattering (LLS), AFM, and transmission electron microscopy (TEM).