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Functional Spaces in Star and Single-Chain Polymers Via Living Radical Polymerization

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Functional Spaces in Star and Single-Chain Polymers Via Living Radical Polymerization Polymer Journal (2014) 46, 664–673 & 2014 The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/14 www.nature.com/pj FOCUS REVIEW Functional spaces in star and single-chain polymers via living radical polymerization Takaya Terashima This article reviews recent advances in the creation of functional spaces with core-functionalized star polymers and single-chain folding/crosslinked polymers via living radical polymerization. Various core-functionalized star polymers were efficiently prepared with functional linking agents and monomers to perform unique functions. For example, they can serve as nanoreactors for active and robust catalysis in organic reactions and polymerization and as nanocapsules for selective and stimuli-responsive molecular recognition. Single-chain folding polymers were obtained from the self-folding of amphiphilic random copolymers bearing hydrophilic poly(ethylene glycol) chains and hydrophobic alkyl pendants in water, resulting in unimer micelles with dynamic hydrophobic domains. The folded structure could be further fixed via the intramolecular crosslinking of the hydrophobic interior. In addition, cation template-assisted cyclopolymerization and concurrent tandem living radical polymerization with in situ monomer transesterification were also developed for the one-pot synthesis of cyclopolymers with large in-chain cavities and gradient and sequence-controlled copolymers. Polymer Journal (2014) 46, 664–673; doi:10.1038/pj.2014.57; published online 16 July 2014 INTRODUCTION polymerization,29–35 whereas the microgel cores of divinyl Polymers and macromolecules can provide three-dimensional com- compounds have been conventionally used as a connecting point of partments1–10 and place functional groups in a specific position11 to arm polymers. Beyond such classical understanding, we regard the serve as promising scaffolds for unique functional spaces. Proteins microgel cores of star polymers as crosslinked but solubilized and enzymes naturally possess precision nanocavities within their functional spaces8–10,36–46 that are distinct from dynamic, non- globular tertiary structure where they perform selective recognition crosslinked spaces such as micelles via the physical self-assembly of and catalysis in water. The tertiary structure consists of self-folding amphiphilic block copolymers in solvents that are poor for one of the polymer chains with perfectly controlled primary structures. Recent segments.3–5 advances in living radical polymerization and the initiating/catalytic By contrast, amphiphilic or functional linear random copolymers systems have allowed us to tailor-make functional polymers with in turn self-fold to form unimer micelles and single-chain polymeric controlled primary structure and three-dimensional and/or branched nanoparticles with intramolecular non-covalent physical (for architecture.12–22 Focusing on these features, we have recently example, hydrophobic or hydrogen bonding) interaction and/or developed functional polymer spaces (microgel-core star polymers, covalent chemical crosslinking in specific solvents.47–62 The folding single-chain folding/crosslinked polymers)8–10 and functional spaces are not only attractive as an artificial mimic of proteins and polymer chains (cyclopolymers, sequence-controlled polymers) via enzymes but they can also be more precisely designed because the ruthenium-catalyzed living radical polymerization.12,13 These primary structure (for example, molecular weight, composition of polymers perform unique functions reflecting their primary monomers and functional groups) of linear polymers is directly structure and specific environment (Figure 1). related to folding spaces. With functionalization and/or primary structure control, synthetic Here, we review recent advances in the synthesis and functions of linear polymers can be typically transformed into the following microgel-core star polymers37–46 and single-chain folding/crosslinked globular, compartmentalized polymers: (1) microgel-core star poly- polymers57–61 via living radical polymerization. In addition, smart mers;8–10,23–25 (2) micelles, vesicles and polymersomes;3–6 and (3) synthetic strategies for cyclopolymers with large in-chain cavities63 unimer micelles and single-chain polymeric nanoparticles.26–28 and sequence-controlled polymers64–66 are also briefly described. Among these, microgel-core star polymers comprise crosslinked microgels that are covered and solubilized by multiple linear arm RESULTS AND DISCUSSION polymers to be regarded as ‘solubilized gels’.29 Such star polymers are Core-functionalized star polymers generally obtained from the intermolecular crosslinking of living Design of microgels and arms. As a result of the high functionality polymers (or macroinitiators) with divinyl compounds in living tolerance and controllability of ruthenium catalysts,12,13 various Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan Correspondence: Dr T Terashima, Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: [email protected] Received 15 March 2014; revised 14 May 2014; accepted 19 May 2014; published online 16 July 2014 Functional spaces in star and single-chain polymers T Terashima 665 Functional Chains Cyclopolymers Sequence-Regulated Copolymers RX R X Core-Functionalized Ru Star Polymers Living Radical Polymerization Amphiphilic Copolymers Single-Chain Crosslinked Polymers Single-Chain Folding Polymers Functional Spaces Catalysis Molecular Recognition Substrates Oxidation Hydrogenation Living Radical Polymerization Nanoreactors Products Nanocapsules Figure 1 Design of functional polymer chains and spaces via ruthenium-catalyzed living radical polymerization. Cl Arms Functional Linker /Monomer Microgel-Core F FF Solubilized Gel F F Nano Size Isolated Space Cl + Ru Linking Multiple Functions Arm Polymers Direct Core-Functionalization F FFRuCl Core-Functionalized Functional Star Polymers Nanospaces Arm: Ru: PPh3 Cl F O Ru : O O O O n PPh Cl 3 O N O N+ I- O O N+ – O F - PPh3 Cl O I PPh2 S PMMA-Cl RuCl2 2 O 4 5 6 HN NH2 Cl O O O n O Ru O O O O O O PPh O Cl 3 O O O O OH PPh 1 3 O 9 8.5 3 8.5 PPEGMA-Cl Ru(Ind) 7 8 9 O O H H O O Cl O O O O Ru O N N n O O O (CF2)6F N N O Cl PPh3 (CF2)6 O H H O PPh3 PEG-Cl RuCp* 10 11 12 Figure 2 Core-functionalized star polymers obtained via ruthenium-catalyzed living radical polymerization of functional linkers and monomers (1–12)with chlorine-capped arm polymers (PMMA-Cl, PPEGMA-Cl and PEG-Cl). core-functionalized star polymers have been directly synthesized by hydroxyl groups (9), perfluorinated alkanes (10, 11) and urea groups the ruthenium-mediated linking reaction of chlorine-capped linear (12); (Figure 2).8–10,37–46 Efficient preparation involves the following: arm polymers (living polymers or macroinitiators) with functional (1) selection of catalytic systems (RuCl2(PPh3)3/n-Bu3N, divinyl linkers/monomers carrying phosphines (2), amines (3, 4), Ru(Ind)Cl(PPh3)2/n-Bu3N, RuCp*Cl(PPh3)2/amino alcohol (for quaternary ammonium salts (5, 6), poly(ethylene glycol) (7, 8), example, 4-dimethylaminobutanol)) and solvents (toluene, ethanol Polymer Journal Functional spaces in star and single-chain polymers T Terashima 666 1 , 2 PMMA-Cl 7 PMMA-Cl 1 , 11 PMMA-Cl Ru(Ind) RuCl2 Ru(Ind) Mn Cl Mw/Mn 11000 O 100 1.2 O 106 105 104 106 105 104 106 105 104 MW (PMMA) 5 12 PEG-Cl PEG-Cl 1 PEG-Cl O RuCp* RuCp* RuCp* M O Cl n O Mw/Mn 4800 113 1.06 106 105 104 106 105 104 106 105 104 MW (PEO) Figure 3 Synthesis of core-functionalized star polymers via Ru-catalyzed linking reactions of arm polymers (PMMA-Cl or PEG-Cl) with functional linkers and monomers ((a) 1, 2,(b) 7,(c) 1, 11,(d) 5,(e) 12,(f) 1): [arm]0/[linker]0/[monomer]0 ¼ 1/10/0 or (a) 5 or (c) 10. Catalytic system: (a)RuCl2 or (b, c) Ru(Ind) in toluene at 80 1C; (d–f) RuCp* in ethanol at 40 1C. and water) suitable for arm polymers and core-forming functional radical polymerization with ligand-bearing monomers (2, 3).8–10,37–44 linkers/monomers; and (2) optimization of the feed ratio of core- Importantly, this synthetic strategy affords a one-pot transformation forming linkers/monomers to arm chains ([R–Cl]). of ruthenium polymerization catalysts into ruthenium-bearing star Arm polymers provide the desired solubility to star polymers. polymer catalysts, which is thus more efficient than conventional Arm chains are typically obtained from one of two methods: multistep procedures: (1) synthesis of ligand-bearing (co)polymers, (1) living radical polymerization of monomers (for example, methyl followed by the post introduction of metals; and (2) synthesis of methacrylate (MMA),29,30,36–41,45 poly(ethylene glycol) methyl ether metal-bearing monomers, followed by (co)polymerization. 42–44 methacrylate (PEGMA: Mn ¼ 475; ethylene oxide units ¼ 8.5) ) Typically, MMA was first polymerized with a chloride initiator and with a chlorine-based initiator (a chlorine-capped MMA dimer: a ruthenium catalyst (RuCl2(PPh3)3/n-Bu3N) to yield chlorine-capped H-(MMA)2-Cl, ethyl 2-chloro-2-phenylacetate); and (2) esterifica- PMMA-Cl with a narrow molecular weight distribution (condition: tion of polymer chains (for example, poly(ethylene glycol) methyl [MMA]0/[initiator]0/[RuCl2(PPh3)3]0/[n-Bu3N]0 ¼ 2000/20/10/40 mM, ether (PEG-OH: Mn ¼ 5000; ethylene oxide units ¼ 113)) with an conv. ¼ B90%, 48 h, Mn ¼ 11 000, Mw/Mn ¼ B1.2). Then, PMMA- acid chloride (2-chloro-2-phenylacetyl chloride).46 The former yields Cl was directly crosslinked with ethylene glycol dimethacrylate hydrophobic poly(methyl methacrylate) arms (PMMA-Cl) and (EGDMA: 1) in the presence of a phosphine-bearing styrene amphiphilic, thermosensitive poly[poly(ethylene glycol) methyl derivative (2) in toluene ([1]0/[2]0/[PMMA–Cl]0 ¼ 10/1.25–5/1), ether methacrylate] arms (PPEGMA-Cl), whereas the latter yields leading to the production of phosphine and ruthenium-bearing amphiphilic long poly(ethylene glycol) methyl ether arms (PEG-Cl; microgel star polymers in high yield (80B90%; Figure 3a).37,38,40 Figure 2).
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