Lichao Liu, Leilei Rui, Yun Gao and Weian Zhang* Self-assembly of stimuli-responsive block copolymers in aqueous solution has attracted considerable attention in the past few decades. Herein, we report a supramolecular modular synthetic approach for the fabrication of photo-responsive block-controllable supramolecular polymers (BSPs) based on the assembly of two homopolymers in water. The homopolymers, namely, the polystyrene modifified with adamantane and azobenzene as the end groups (Ad–PS–Azo) and poly(ethylene glycol) modifified with b-CD (PEG–b-CD), were successfully synthesized via atom transfer radical polymerization (ATRP) and click chemistry, respectively. The supramolecular triblock copolymer (STP) was constructed through host–guest interactions between b-CD and Ad/Azo moieties, and further self-assembled into micelles in aqueous solution, which were investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS). This well-defined supramolecular triblock copolymer can reversibly disassemble into a supramolecular diblock copolymer (SDP) by alternating irradiation of UV/visible light, which was revealed by TEM, UV-vis and 1 H NMR.

Introduction 

Supramolecular self-assembly is ubiquitous in nature and daily life. Over the past quarter century, the development of supramolecular chemistry based on intermolecular non-covalent interactions has paved the way toward apprehending and exploring the storage of information at the molecular level, selforganization at the supramolecular level into well-dened architectures, and the principles of biology, such as DNA and proteins in biological systems at the interface with biology and chemistry.1–3 Supramolecular polymer, fabricated via straight forwardly connecting micromolecular or polymeric segments through reversible non-covalent interactions, has recently become a unique class of novel smart materials with stimuliresponsive characteristics and sophisticated structures expanded from supramolecular chemistry.4–7 The most recent advances in stimuli-responsive supramolecular polymers constructed via macrocyclic hosts, such as crown ether, cyclodextrin, calixarene, cucurbituril, pillararene, and their derivatives, have been highlighted.8–13 Nowadays, extensive attention has been focused on supramolecular polymers formed by the incorporation of host–guest inclusion complexation, especially through end functionalization of a polymer with supramolecular moieties, namely cyclodextrins as the linker moieties.14–16 Harada et al.17–20 designed various cyclodextrinbased supramolecular polymers, including main-chain, side chain polypseudorotaxanes and polyrotaxanes, supramolecular oligomers and polymers. Li et al.21–23 prepared a novel system comprising a CD-core star-shaped polymer which formed by supramolecular self-assembly with the polymer bearing an adamantyl end through host–guest complexation. Ritter et al.24 presented the formation of a novel supramolecular ABA triblock macromolecular architecture based on cyclodextrin complexation. More interestingly, most of the supramolecular polymers are responsive to external stimuli such as pH, light, temperature, redox, electricity and so on. Recently, Yuan et al.25–29 fabricated a series of two end-decorated homopolymers which can orthogonally self-assemble into supramolecular diblock copolymers in aqueous solution based on the terminal host– guest interactions between CDs and responsive guest molecules, and these supramolecular polymers further reversibly self-assembled into different morphologies in water, which can be applied as a stimuli-responsive drug delivery system. To our best knowledge, almost all reported cyclodextrin-based supramolecular block copolymers have been constructed by host– guest interactions only with one kind of guest molecules. There is no report on supramolecular approaches for the construction of block-controllable supramolecular polymers with two kinds of guest molecules as end moieties in one polymer chain.

Herein, we report for the rst time a convenient, exible and modular synthetic approach to prepare photo-responsive block controllable supramolecular polymers (BSPs). The recognition between CDs and Ad/Azo was employed to achieve the self assembly of BSPs from two different polymeric building blocks and the reversible transition between supramolecular triblock and diblock copolymers during the self-assembly and disas sembly in water by alternating irradiation of UV/visible light (Scheme 1). In this paper, polystyrene (PS) was selected to study the self-assembly behavior of classical amphiphiles with poly ethylene glycol (PEG) constructed via host–guest inclusion complexation between b-CD and Ad/Azo moieties. The inner building block polystyrene homopolymer modied with adamantane and azobenzene (Ad–PS–Azo) was synthesized via atom transfer radical polymerization (ATRP) using brominated adamantane to initiate the polymerization of styrene and subsequently “click chemistry” with 4-propargyl azobenzene. The outer building block poly(ethylene glycol) homopolymer modied with b-CD (PEG–b-CD) was synthesized also via “click chemistry” between b-CD–N3 and alkynyl-PEG. The supramolecular triblock polymer (STP) was constructed through host– guest interactions between b-CD and Ad/Azo moieties, and further self-assembled into vesicles in aqueous solution to prove the formation of supramolecular amphiphiles. This well dened supramolecular triblock polymer can reversibly disas semble into a supramolecular diblock copolymer (SDP) by alternating irradiation of UV/visible light in water.

Experimental 

Materials

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 absorption spectra of all products were recorded on an AVATAR 360 ESP FT-IR spectrometer and the results were collected at 30 scans with a spectral resolution of 1 cm 1 . The number average weight (Mn) and polydispersity index (PDI) were determined using a Waters 1515 gel permeation chromatograph (GPC) equipped with a refractive index detector and ultrastyragel columns of 100–10 000 ˚ A porosities. The GPC system was calibrated with polystyrene as the standards and tetrahydrofuran (THF) as the eluent at a ow rate of 1 mL min- 1 . The UV-vis spectra of the samples were measured over different irradiation time intervals by using a Thermo Scientic Evolution 220 spectrophotometer. Dynamic light scattering (DLS) measurements were carried out with a BECKMAN COULTER Delasa Nano C particle analyzer. All the measurements were carried out at room temperature. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM1400 electron microscope operated at 100 kV. Samples for TEM were 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 (CEL-HXF300/CEL-HXUV300, China).