Shaohua Wang, Qiongxia Shen, Mian Hasnain Nawaz and Weian Zhang

The double hydrophobic block copolymer, poly[6-(4-(phenyldiazenyl)phenoxy)hexyl acrylate]-b-polystyrene (PAzo-b-PS), was synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. This hydrophobic block copolymer can self-assemble into spherical micelles through b-cyclodextrin–

azobenzene (b-CD–Azo) host–guest inclusion. Furthermore, the spherical micelles can be dissociated and reassembled by alternating irradiation of UV/visible light. These supramolecular self-assembly and disassembly procedures were studied by dynamic light scattering (DLS), atomic force microscopy (AFM),

transmission electron microscopy (TEM) and UV-Vis spectra.

Experimental

Characterization 

1 H-NMR spectra were recorded at 400 MHz, using a BRUKER AV400 Spectrophotometer in CDCl3 with tetramethylsilane (TMS) as an internal reference. The number average weight (Mn) and polydispersity index (PDI) were determined using a gel permeation chromatograph (GPC) equipped with an LC1120 HPLC pump (polymer laboratories, UK), a differential refractive index (DRI) detector (Shodex, RI-101), a 5.0 mm bead-sized guard column (50 × 7.5 mm), and two PLgel 5.0 mm MIXED-C columns (300 × 7.5 mm) in series (Polymer laboratories, UK). Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1 mL min^-1, and toluene was used as a flow marker. The GPC system was calibrated using polystyrene as the standards. The UV-Vis spectra of the samples were measured over different irradiation time intervals by using a Thermo Scientific Evolution 220 spectrophotometer. Fluorescence spectra were measured on a Thermo Scientific Lumina Fluorescence Spectrometer. Dynamic light scattering (DLS) measurements were carried out on a BECKMAN COULTER Delasa Nano C particle analyser at a fixed angle (165°). All the measurements were carried out at room temperature. AFM observation was carried out on a Veeco's NanoScope III Multimode Atomic Force Microscope by drying a 10 mL droplet of solution onto freshly cleaved mica. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM1400 electron microscope operated at 100 kV with a carbon coated copper grid. Sample was prepared by dropping the micelle solution onto a carbon-coated copper grid and then dried at room temperature. Irradiation with UV light (365 nm) and visible light (450 nm) of the sample was respectively performed on a 500 W Xe light equipped with 365 nm and 450 nm cutoff filters (E = 2000 mW cm^-2, CEL-HXF300/CEL-HXUV300, China), and the distance between the UV-lamp and the sample is about 70 cm.

The formation of PAzo-b-PS with b-CD micelles and their reversible photo-responsive self-assembly and disassembly procedures.

Results and discussion 

Synthesis of Azo-containing diblock copolymers The Azo-containing diblock copolymer (PAzo-b-PS) was synthesized via RAFT polymerization as shown in Scheme 2. The PAzo homopolymer and PAzo-b-PS block copolymers were all successfully synthesized, which were conrmed by GPC and 1 H NMR. Fig. 1 shows the GPC traces of the PAzo homopolymer and PAzo-b-PS diblock copolymers. The polymerization results are also listed in Table 1. It can be seen that the molecular weights of block copolymers increased with the molar ratio of the styrene monomer to the macro-RAFT agent. Fig. 2 shows the 1 H NMR spectra of the PAzo homopolymer and the PAzo10-bPS130 diblock copolymer in CDCl3. The signals (a, b, c) from 6.92 to 7.85 ppm are assigned to the resonance of the aromatic protons of the Azo group. The resonance signal (d) from 3.94 to 4.06 ppm is ascribed to the methylene group of –CH2–O– in the side chains. The signal (j) from 6.42 to 6.70 ppm corresponds to the aromatic protons of the styrene unit. Therefore, GPC and 1 H NMR results indicate that the PAzo-b-PS diblock copolymer was successfully synthesized.

UV-Vis spectra of the mixtures of PAzo10-b-PS70 and b-CD at a 1 : 1 molar ratio: (A) at 365 nm UV irradiation induced trans-to-cis transition and (B) 450 nm visible light irradiation induced cis-to-trans transition. The concentration of the sample is 0.1 mg mL^-1

^Conclusions

The double hydrophobic PAzo-b-PS block copolymers were synthesized via RAFT polymerization. The hydrophobic block copolymers can self-assemble into spherical micelles by the inclusion complexation between b-CD and Azo side groups on the block copolymers. The spherical micelles can be reversibly disassembled and reassembled by alternating UV and visible light irradiation. This non-covalently connected photo-responsive supramolecular assemblies system might have potential applications in many fields.

Photocontrolled reversible supramolecular assemblies of a diblock azo-copolymer based on b-cyclodextrin– Azo host–guest inclusion complexation†.pdf