Photoinduced Reversible Worm-to-Vesicle Transformation of Azo- Containing Block Copolymer Assemblies Prepared by Polymerization-Induced Self-Assembly Qiquan Ye, † Meng Huo, † Min Zeng, † Lei Liu, † Liao Peng, † Xiaosong Wang, ‡ and Jinying Yuan* ,† † Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China ‡ Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue, Waterloo, ON N2L 3G1, Canada * S Supporting Information ABSTRACT: A series of azo-containing copolymeric assemblies based on poly(N,N- dimethylaminoethyl methacrylate)- b-poly[(benzyl methacrylate)- co-(4-phenyl- azophenyl methacrylate)] [PDMA-b-P(BzMA-co-AzoMA)] were prepared by reversible addition-fragmentation chain transfer polymerization-induced self-assembly at high solid contents. Depending on the chain length of P(BzMA-co-AzoMA), spheres, worms, and vesicles were readily prepared. These azo-containing wormlike micelles underwent reversible worm-to-vesicle transformation upon alternative UV/vis light irradiation. By investigating the morphology evolution, a series of intermediates were observed, including coalesced worms as well as “octopus”- like and “jellyfish”-like structures. The morphology transformation was rationalized by the volume change of the P(BzMA-co- AzoMA) block caused by the trans-cis isomerization of the azobenzene groups. It is the first demonstration of light-stimulated reversible worm-to-vesicle transition and would benefit for the understanding of morphology evolution of polymer assemblies under external stimuli. ■ INTRODUCTION Stimuli-responsive polymeric assemblies, which undergo changes in structures and properties in response to external stimuli, have been extensively studied for potential applications in the fields of artificial organelles, 1,2 smart nanoreactors, 3-6 drug and gene delivery, 7-11 etc. Various stimuli, including temperature, 12,13 pH, 14 redox, 15-17 salt, 18,19 gas, 20,21 and light, 22,23 have been utilized to trigger the shape transformation of polymeric assemblies. Among these triggers, light stimulus enables precise regulation in intensity, duration time, and irradiation site, so that controlled stimuli response can be readily achieved without introducing additional reagents. 24-31 For example, Zhao et al. 32 prepared photoresponsive polymeric assemblies based on the azo-containing amphiphilic diblock copolymers poly(acrylic acid)- b -poly{6-[4-(4-methoxy- phenylazo)phenoxy]hexyl methacroylate}. Because of the photoisomerization of the azobenzene mesogens, the micelles could be disrupted by ultraviolet (UV) irradiation and re- formed by visible light. To study the photoinduced reversible fusion and fission of polymersomes, Zhou et al. 33,34 prepared two kinds of micrometer-scale vesicles based on β-cyclodextrin- and azobenzene-terminated poly(3-ethyl-3-oxetanemethanol)- star-poly(ethylene glycol), respectively. Because of the host- guest molecular recognition between β-cyclodextrin and azobenzene units, these vesicles, after mixing up, aggregated and fused into vesicle aggregates, which underwent reversible fission and fusion upon UV/vis light trigger. Besides, reversible photocontrolled swelling-shrinking behaviors of spherical micelles 35 and vesicles 36 were also reported. However, all of these assemblies are prepared using traditional postpolymeriza- tion self-assembly strategies, which were laborious and inefficient to target desired morphologies. Polymerization-induced self-assembly (PISA) is an emerging technique to prepare polymeric assemblies with controllable morphology and high concentration (10-50 wt %). 23,37-41 Taking advantage of “living”/controlled dispersion or emulsion polymerization, PISA enables the preparation of amphiphilic block copolymer assemblies in situ via chain extension of the solvophilic block with solvophobic block. 42-53 Compared with the traditional postpolymerization self-assembly strategies, PISA offers convenient size and morphology control, so a variety of morphologies, including spherical micelles, wormlike micelles (WLMs), nanosheets, and vesicles, can be easily produced. 54-64 Taking advantage of PISA, Pan et al. 65 fabricated photoresponsive polymeric vesicles via incorporation of spiropyran groups onto the solvophilic block. They prepared macro-chain-transfer agent (macro-CTA) by copolymerization of the spiropyran-derived methacrylate with 4-vinylpyridine and used this macro-CTA to mediate the PISA of styrene in methanol. This vesicle dispersion changed from colorless to red without morphology alteration upon UV irradiation, which corresponds to the spiropyran-to-merocyanine transformation. Received: February 13, 2018 Revised: April 6, 2018 Article Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b00340 Macromolecules XXXX, XXX, XXX-XXX