Toward Controlled Hierarchical Heterogeneities in Giant Molecules

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Research Article http://pubs.acs.org/journal/acscii Toward Controlled Hierarchical Heterogeneities in Giant Molecules with Precisely Arranged Nano Building Blocks † † † † † † ‡ † Wei Zhang, Mingjun Huang, Hao Su, Siyu Zhang, Kan Yue, Xue-Hui Dong, Xiaopeng Li, Hao Liu, † † § ∥ ⊥ † Shuo Zhang, Chrys Wesdemiotis, , Bernard Lotz, Wen-Bin Zhang,*, Yiwen Li,*, † and Stephen Z. D. Cheng*, † Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325-3909, United States ‡ Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas 78666, United States § Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601, United States ∥ Institut Charles Sadron, CNRS−Universitéde Strasbourg, 23, Rue du Lœss, Strasbourg, 67034, France ⊥ Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Center for Soft Matter Science and Engineering, Peking University, Beijing 100871, China *S Supporting Information ABSTRACT: Herein we introduce a unique synthetic methodology to prepare a library of giant molecules with multiple, precisely arranged nano building blocks, and illustrate the influence of minute structural differences on their self- assembly behaviors. The T8 polyhedral oligomeric silsesquiox- ane (POSS) nanoparticles are orthogonally functionalized and sequentially attached onto the end of a hydrophobic polymer chain in either linear or branched configuration. The heterogeneity of primary chemical structure in terms of composition, surface functionality, sequence, and topology can be precisely controlled and is reflected in the self-assembled supramolecular structures of these giant molecules in the condensed state. This strategy offers promising opportunities to manipulate the hierarchical heterogeneities of giant molecules via precise and modular assemblies of various nano building blocks. ■ INTRODUCTION molecule;14 and employing DNA templates to assist efficient, sequence-specific polymerization of peptide nucleic acid In biological systems, the activity and function of biomacro- 15,16 molecules are dictated not only by their primary chemical aldehydes. The classic solid state synthesis developed by Merrifield remains the gold standard for precisely defined structures but also by their secondary, tertiary, and quaternary 17 1−3 macromolecules, despite the limit at low molecular weight hierarchical structures. This is best illustrated with nuclei 18 acids and proteins where self-assembly, molecular recognition, oligomers using small-molecule motifs. See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Downloaded via SOUTH CHINA UNIV OF TECHNOLOGY on October 9, 2018 at 14:33:31 (UTC). “ and replication/translation are closely associated with their In 1960, Feynman raised a fundamental question: What 3−5 would the properties of materials be if we could really arrange the functions. In the past half-century, nature has inspired the ” 19 ’ pursuit of precision synthetic macromolecules in the field of atoms the way we want them? . In chemist s language, it is polymer chemistry. The state-of-the-art synthetic polymers necessary not only to design and synthesize precise primary chemical structures but also to accurately control the usually consist of covalently linked repeating units in various ffi chain topologies (linear, cyclic, branched, etc.). Their properties supramolecular structures for e ciently transferring and amplifying the microscopic functions to macroscopic proper- are recognized more as a function of molecular weights, 1,2,7,20 ’ polydispersity, and chain topology, rather than the arrangement ties. Echoing Feynman s question, we propose a modular approach to construct precise macromolecules using function- of repeating units along the chain and the collective interaction “ ” among them.1 The precise control of sequence in polymers has alized molecular nanoparticles (MNPs, or nanoatoms ) as the − been the “Holy Grail” of polymer science.3,6 8 Many efforts fundamental building blocks. It has previously been shown that “ ” 21−24 have been devoted to this aspect with varying degrees of T8-POSS is an important class of model nanoatoms . It can be used to build up a library of POSS-based giant success. Recent examples include controlling the monomer 25 sequence distribution using radical chain polymerization9 or molecules, including giant Janus particles, giant surfac- ring opening metathesis polymerization;10 synthesizing se- − quence-defined oligomers using “click” reactions;11 13 design- Received: December 3, 2015 ing digitally encoded and information-containing macro- Published: January 27, 2016 © 2016 American Chemical Society 48 DOI: 10.1021/acscentsci.5b00385 ACS Cent. Sci. 2016, 2, 48−54 ACS Central Science Research Article Figure 1. Synthetic routes and representative macromolecular design of giant molecules. (a) General synthetic strategy toward giant molecules. (b) “ ” The chemical structure of the POSS MNPs, PS-N3, and the click adaptors . (c) A library of giant molecules with controlled heterogeneities: (i) a giant molecule with distinct POSSes in a designed sequence; (ii) giant molecules with tunable number of POSSes; (iii) giant molecules with tunable functionalized POSSes; (iv) giant molecules with POSS cages arranged in dendritic topology. tants26,27 and giant polyhedra.28 Other examples include giant In addition, two kinds of small-molecule “click adaptors” molecules based on [60]fullerene and polyoxometalates.29,30 (Figure 1b) are used to convert one click functionality to Their self-assembly exhibits an unusual and remarkable another without the need for protection and deprotection.33,34 sensitivity to their primary chemical structures. Hierarchically This strategy allows us to sequentially arrange the “nanoatoms” ordered supramolecular structures can be modularly built up in any desired way as in Feynman’s question. Notably, the and finely tuned in the bulk, solution, and thin films. We architectures of resulting giant molecules can be tuned by envision that, by attaching multiple MNPs into one giant simply using click adaptors with different geometry. In each molecule in a precisely defined sequence and geometry, their step, purification was conveniently achieved by repeated self-assembly would show controlled heterogeneity at higher precipitation into methanol owing to the presence of a long length scales. A systematic study on these precise macro- polystyrene tail as the purification tag. The tag can be removed molecules would be critical in understanding their self-assembly if a cleavable linker is present between PS and the first POSS and functions. In this article, we focus on the design and building block. In the present paper, we are interested in the synthesis of giant molecules with tandem interconnected POSS interplay between PS chain and POSSes in self-assembly. cages in desirable sequences and topologies, and present a Therefore, the linkage is kept intact. The process promises thorough evaluation of their phase structures with controlled convenient and modular construction of giant molecules, which heterogeneity. are illustrated in the following proof-of-concept examples. For a trial to test the newly deliberated chemical synthesis ■ RESULTS AND DISCUSSION route, we first designed a giant molecule with five different The general synthetic methodology is depicted in Figure 1a. POSS “nanoatoms” attached along the polymeric chain in a The strategy is inspired by the solid state peptide synthesis predesigned sequence (Figure 1c.i). The entire process starts using “click” chemistry and other highly efficient chemical from azido-terminated polymer (i.e., PS135-N3, Figure 1b). The transformations. Simple precipitation is used instead of the first SPAAC reaction installs the first XPOSS onto the polymer beads to improve the reaction efficiency. The use of tail, affording the macromolecular precursor with an aldehyde “nanoatoms” as the fundamental building blocks greatly chain end. The complete reaction is confirmed by the accelerates the modular construction of giant molecules. The disappearance of the characteristic vibrational band of the basic nano building block in this case is DIBO-(XPOSS)− azido group in Fourier transform infrared spectroscopy (FT- − CHO (Figure 1b), where X represents a variety of periphery IR) at 2100 cm 1 (Figure S6) and appearance of an aldehyde functional groups including vinyl (V), isobutyl (B), fluoroalkyl peak at δ 10 ppm in the 1H NMR (Figure S2). A “click” (F), and chloroalkyl (Cl), etc. The dibenzocyclooctyne (DIBO) adaptor is used to reinstall the azido functionality, as shown by group is ready for copper free strain-promoted azide−alkyne the disappearance of the chemical shift signal of aldehyde in 1H cycloaddition (SPAAC) and the aldehyde can undergo oxime NMR (Figure S3).33,35 The cycle is then repeated. Only a slight ligation.31,32 Those two reactions are orthogonal to each other. excess of reactants relative to that of the polymer (typically 49 DOI: 10.1021/acscentsci.5b00385 ACS Cent. Sci. 2016, 2, 48−54 ACS Central Science Research Article 1.05−1.2 equiv) is used to ensure a complete reaction in each the following samples were designed and prepared (see Figure step instead of large excess. Near quantitative

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