Universal Access to Water-Compatible and Nanostructured Materials via the Self-Assembly of Cationic Alternating Copolymers.

Herein, we report the water-assisted self-assembly of alternating copolymers bearing imidazolium cations and hydrophobic groups to create water-compatible and nanostructured materials. The copolymers efficiently absorbed water into the cationic segments from the outer environments, depending on the relative humidity. The absorbed water serves as hydrophilic molecules to modulate the weight fraction of hydrophilic/hydrophobic units in the samples. Thus, the morphologies and domain spacing of the nanostructures can be controlled by not only the side chains, but also the amount of absorbed water. The self-assembly of the cationic copolymers, developed herein, afforded universal access to various morphologies, including lamella, gyroid, and cylinder, in addition to the precision control of the domain spacing at the 0.01 nm level.

Lamellar Microphase Separation and Phase Transition of Hydrogen-Bonding/Crystalline Statistical Copolymers: Amide Functionalization at the Interface.

Microphase separation of random copolymers, as well as that of high χ-low N block copolymers, is promising to construct sub-10-nm structures into materials. Herein, we designed statistical copolymers consisting of 2-hydroxyethyl acrylate (HEA) and N-octadecylacrylamide (ODAAm) to produce crystallization and hydrogen bond-assisted lamellar structure materials. The copolymers not only formed a crystalline lamellar structure with 3-4 nm domain spacing but also maintained an amorphous lamellar structure via phase transition above the melting temperature up to approximately 100 °C. The key is to introduce hydrogen-bonding amide junctions between the octadecyl groups and the polymer backbones, by which the polymer chains are physically fixed at the interface of lamellar structures even above the melting temperature. The stabilization of the lamellar structure by the amide units is also supported by the fact that the lamellar structure of all-acrylate random copolymers bearing hydroxyethyl and crystalline octadecyl groups is disordered above the melting temperature. By spin-coating on a silicon substrate, the HEA/ODAAm copolymer formed a multilayered lamellar thin film consisting of a hydrophilic hydroxyethyl/main chain phase and a hydrophobic octadecyl phase. The structure and order-disorder transition were analyzed by neutron reflectivity.

Cation Template-Assisted RAFT Cyclopolymerization of Hexa(Ethylene Glycol) Di(meth)acrylates to Thermoresponsive Pseudo-Crown Ether Polymers.

Cation template-assisted reversible addition fragmentation/chain transfer (RAFT) cyclopolymerization of hexa(ethylene glycol) diacrylate (PEG6DA) or hexa(ethylene glycol) dimethacrylate (PEG6DMA) is developed as a versatile system to produce large in-chain ring cyclopolymers, thermoresponsive pseudo-crown ether polymers. For an efficient synthesis, potassium hexafluorophosphate (KPF6 ) is employed as a cation template; PEG6DA as well as PEG6DMA recognizes the potassium cation with the hexa(ethylene glycol) spacer to dynamically form a pseudo-cyclic divinyl monomer. Those monomers interacting with the potassium cations are efficiently polymerized with RAFT agents and radical initiators into cyclopolymers comprising 24-membered hexa(ethylene glycol) rings. The cation template-assisted RAFT cyclopolymerization is also effective for the synthesis of amphiphilic random cyclocopolymers bearing hydrophilic hexa(ethylene glycol) rings and hydrophobic butyl groups. Cyclopolymers of PEG6DA and PEG6DMA further show thermoresponsive solubility in water. The cloud point temperature of cyclopoly(PEG6DA)s is higher than that of a cyclopoly(PEG6DMA).

Multilayered Lamellar Materials and Thin Films by Instant Self-Assembly of Amphiphilic Random Copolymers.

Making ordered nanostructures in polymers and their thin films is an important technique to produce functional materials. Herein, we report instant yet precise self-assembly systems of amphiphilic random copolymers to build multilayered lamellar structures in bulk materials and thin films. Random copolymers bearing octadecyl groups and hydroxyethyl groups induced crystallization-driven microphase separation via simple evaporation from the solutions to form lamellar structures in the solid state. The domain spacing was controlled in the range between 3.1 and 4.2 nm at the 0.1 nm level by tuning copolymer composition. Interestingly, just by spin-coating the polymer solutions onto silicon substrates, the copolymers autonomously formed thin films consisting of multilayered lamellar structures, where amorphous/hydrophilic parts and crystalline octadecyl domains are alternatingly layered from a silicon substrate to the air/polymer interface at regular intervals. The lamellar domain spacing was tunable by selecting hydrophilic pendants.

Controlled Self-Assembly of Amphiphilic Random Copolymers into Folded Micelles and Nanostructure Materials.

In this review, we report controlled self-assembly systems of amphiphilic random copolymers in aqueous or organic media and the solid state to produce folded micelles, related nanoaggregates, vesicles, and microphase separation materials. The key features of random copolymer self-assemblies are 1) self-folding of polymer chains, 2) precision self-assembly of side chains, and 3) dynamic self-sorting and selective recognition. Typically, random copolymers bearing hydrophilic poly(ethylene glycol) and hydrophobic alkyl groups folded into small unimer micelles (~10 nm) via the association of the hydrophobic groups in water. Importantly, those random copolymers afforded precision intermolecular self-assembly into multichain micelles; the size, aggregation number, and thermoresponsive properties can be controlled as desired by tuning their molecular weight, composition, and side chains. The binary mixture of different random copolymers further self-sorted via chain exchange in water to simultaneously form discrete micelles. Namely, amphiphilic random copolymers can dynamically recognize themselves in complex media like natural biomolecules and proteins. Amphiphilic random copolymers opened new ways to create self-assembled materials with well-defined nanostructures and compartments, dynamic recognition properties, and functions.

Selective Coupling and Polymerization of Folded Polymer Micelles to Nanodomain Self-Assemblies.

Herein, we developed selective coupling and polymerization systems of folded polymer micelles via physical interaction in water. The polymer micelles serve as nanodomains to provide double core micelles, alternating necklace micelles, and micelle-connected hydrogels. For this, cation- or anion-tail unimer micelles and amine- or carboxy-tail unimer micelles were designed; the unimer micelles consist of folded amphiphilic random copolymers carrying hydrophilic poly(ethylene glycol) and hydrophobic or hydrogen-bonding pendants. Mixing a cation-tail micelle and an anion-tail micelle, and even the combination of a double cation-tail micelle and a double anion-tail micelle, selectively provided double-core micelles in water without forming large aggregates. Double core micelles afforded structural transformation into linear or cyclic polymers and dynamic exchange of the micelle domains. In contrast, mixing amine-tail micelles and carboxy-tail micelles gave an alternating necklace micelle or a hydrogel. The controlled connection of polymer micelles was achieved by designing suitable physical interaction. This technique opened new ways to create various nanodomain self-assemblies with controlled higher-order structure.

Orthogonal Folding of Amphiphilic/Fluorous Random Block Copolymers for Double and Multicompartment Micelles in Water.

Here, we report orthogonal folding and self-assembly systems of amphiphilic/fluorous random block copolymers for double core and multicompartment micelles in water. For this, we developed the precision folding techniques of polymer chains via the selective self-assembly of the pendant groups. Typically, A/C-B/C random block copolymers were designed: Hydrophobic dodecyl groups (A) and fluorous fluorinated octyl groups (B) were introduced into the respective blocks, while hydrophilic poly(ethylene glycol) chains (C) were randomly incorporated into all the segments. By controlling the chain length and composition of the respective blocks, the copolymers induce orthogonal single-chain folding in water to form double-compartment micelles comprising hydrophobic and fluorous cores. The copolymers were site-selectively folded in a fluoroalcohol to result in tadpole unimer micelles comprising a hydrophobic A/C unimer micelle and an unfolded fluorous B/C chain. Additionally, asymmetric A/C-B/C random block copolymers with short and highly hydrophobic or fluorous segments were effective for multicompartment micelles in water.

Unnatural Oligoaminosaccharides with N-1,2-Glycosidic Bonds Prepared by Cationic Ring-Opening Polymerization of 2-Oxazoline-Based Heterobicyclic Sugar Monomers.

Glycooligomers and glycopolymers (glycocompounds) play important roles in maintaining homeostasis in biological systems. Glycobiology is a burgeoning area in the elucidation of biological systems for which the molecular design of glycocompounds requires further diversification, including both natural and unnatural glycocompounds. Herein, we proposed a synthesis strategy based on the chain polymerization of deliberately designed sugar monomers. Unnatural oligoaminosaccharides comprising N-1,2-glycosidic bonds were synthesized without enzymes through the cationic ring-opening polymerization of 2-oxazoline-based heterobicyclic sugar monomers. To achieve this, a heterobicyclic monomer [Glc(MeOx)], comprising protected glucosamine (GlcN) and 2-methyl-2-oxazoline (MeOx) rings, was designed. This monomer was polymerized using a binary initiating system of tert-butyl iodide (t-BuI) and GaCl3 to afford oligo[Glc(MeOx)]. The resulting structure corresponded to the condensation product of GlcN with N-1,2-glycosidic bonds. After deprotection of oligo[Glc(MeOx)], the resulting oligoaminosaccharide had a secondary structure different to that of protected oligo[Glc(MeOx)]. Owing to the N-1,2-glycosidic bonds, the oligoaminosaccharide was not degraded by chitinase, which hydrolyzes the condensation product of GlcN with O-1,4-glycosidic bonds.

Self-Sorting of Amphiphilic Copolymers for Self-Assembled Materials in Water: Polymers Can Recognize Themselves.

Amphiphilic random copolymers bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic alkyl pendants showed dynamic self-sorting behavior, that is, self-recognition, under competitive conditions in aqueous media. The self-sorting universally takes place not only in water but also in hydrogels and on the material surfaces, according to encoded information originating from the primary structure of composition and pendants. Binary blends of the copolymers with different composition or alkyl pendants readily induced composition- or alkyl pendant-dependent self-sorting to simultaneously provide discrete and size-controlled micelles with hydrophobic cores. Surprisingly, the micelles reversibly keep exchanging polymer chains exclusively between identical polymer micelles even in the presence of different counterparts. Owing to the dynamic self-sorting behavior, ABA-triblock copolymers comprising the amphiphilic random copolymer A segments and a hydrophilic PEG chain B segment further provided hydrogels with self-healing yet selectively adhesive properties.

Nanostructured Materials via the Pendant Self-Assembly of Amphiphilic Crystalline Random Copolymers.

Versatile self-assembly systems to nanostructured materials in both solid and solution were developed with common amphiphilic random copolymers bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic crystalline octadecyl pendants. The copolymers efficiently induced precision self-assembly of the pendants to provide not only core-crystalline, thermoresponsive micelles and vesicles in water and reverse micelles in hexane but also sub-10 nm lamellar or spherical microphase separation structure in solid. Typically, the solid random copolymers with 50-80 mol % octadecyl units formed lamellar structure of a hydrophilic PEG layer and a hydrophobic, crystalline octadecyl layer. Importantly, the domain spacing is about 5 nm, much smaller than that generally obtained with conventional block copolymers. The domain structure is controlled by composition, independent of chain length. The copolymers further gave various thermoresponsive, compartmentalized materials in aqueous and organic media, where the 3D structure can be also controlled by the composition and sample preparation protocols.

Acrylate-Selective Transesterification of Methacrylate/Acrylate Copolymers: Postfunctionalization with Common Acrylates and Alcohols.

Acrylate-selective transesterification of methacrylate/acrylate copolymers with alcohols was developed for a site-selective postfunctionalization technique of polymers without using specific monomers. Importantly, a common methyl acrylate efficiently works as a selective modification unit via transesterification coupled with a titanium alkoxide catalyst. The acrylate-selective transesterification is achieved owing to less steric hindrance of the carbonyl groups that are attached to the main chain without an α-methyl group. Typically, the acrylate pendants of dodecyl methacrylate/methyl acrylate (MA) random copolymers were selectively transesterified with benzyl alcohol (BzOH). The conversion of the pendent esters into benzyl esters proportionally increased with MA contents. Additionally, various alcohols were applicable to this MA-selective transesterification system.

Compartmentalization Technologies via Self-Assembly and Cross-Linking of Amphiphilic Random Block Copolymers in Water.

Orthogonal self-assembly and intramolecular cross-linking of amphiphilic random block copolymers in water afforded an approach to tailor-make well-defined compartments and domains in single polymer chains and nanoaggregates. For a double compartment single-chain polymer, an amphiphilic random block copolymer bearing hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dodecyl, benzyl, and olefin pendants was synthesized by living radical polymerization (LRP) and postfunctionalization; the dodecyl and benzyl units were incorporated into the different block segments, whereas PEG pendants were statistically attached along a chain. The copolymer self-folded via the orthogonal self-assembly of hydrophobic dodecyl and benzyl pendants in water, followed by intramolecular cross-linking, to form a single-chain polymer carrying double yet distinct hydrophobic nanocompartments. A single-chain cross-linked polymer with a chlorine terminal served as a globular macroinitiator for LRP to provide an amphiphilic tadpole macromolecule comprising a hydrophilic nanoparticle and a hydrophobic polymer tail; the tadpole thus self-assembled into multicompartment aggregates in water.

Terminal-Selective Transesterification of Chlorine-Capped Poly(Methyl Methacrylate)s: A Modular Approach to Telechelic and Pinpoint-Functionalized Polymers.

Terminal-selective transesterification of chlorine-capped poly(methyl methacrylate)s (PMMA-Cl) with alcohols was developed as a modular approach to create telechelic and pinpoint-functionalized polymers. Being sterically less hindered and electronically activated, both the α-end ethyl ester and ω-end methyl ester of PMMA-Cl were efficiently and selectively transesterified with diverse alcohols in the presence of a titanium alkoxide catalyst, while retaining the pendent esters intact, to almost quantitatively give various chlorine-capped telechelic PMMAs. In sharp contrast to conventional telechelic counterparts, the telechelic polymers obtained herein yet carry a chlorine atom at the ω-terminal to further work as a macroinitiator in living radical polymerization. The iterative process of living radical polymerization and terminal-selective transesterification successfully afforded unique pinpoint-functionalized polymers where a single functional monomer unit was introduced into the desired site of the polymer chains.

Star Polymer Gels with Fluorinated Microgels via Star-Star Coupling and Cross-Linking for Water Purification.

Two types of star polymer gels containing perfluorinated microgels were created as purification materials to separate polyfluorinated surfactants (e.g., perfluorooctanoic acid) from water. One macrogel is prepared by the radical coupling of fluorine and/or amine-functionalized microgel star polymers alone, while another is done by the radical cross-linking of the star polymers with poly(ethylene glycol) methyl ether methacrylate. Importantly, the reactive olefin remaining within the microgel cores was directly employed for both coupling and cross-linking reactions. Swelling properties of star polymer gels were effectively controlled by the latter cross-linking technique. Analyzed by small-angle X-ray scattering, a star-star coupling gel typically consists of a three-dimensional network where star polymers are sequentially connected with the microgels at the constant interval of about 20 nm. Owing to the fluorous and acid/base cooperative interaction, star polymer gels carrying fluorine/amine-functionalized microgels efficiently captured polyfluorinated surfactants in water and successfully afforded the removal from water via simple mixing and filtration.

LCST-Type Phase Separation of Poly[poly(ethylene glycol) methyl ether methacrylate]s in Hydrofluorocarbon.

Poly[poly(ethylene glycol) methyl ether methacrylate]s [poly(PEGMA)s] sharply and reversibly exhibited lower critical solution temperature (LCST)-type phase separation in 2H,3H-perfluoropentane (2HPFP). The cloud points decreased from 52 to 41 °C with increasing the PEG pendant length [-(CH2CH2O)mCH3: m = 4.5, 9, 19]. The cloud point was precisely controlled via the addition of perfluoroalkanes (e.g., perfluorooctane) to the 2HPFP solution: typically, it was inversely proportional to the amount of perfluorooctane in the mixture. The unique thermoresponsive solubility further afforded the temperature-mediated micellization of a block copolymer of PEG19MA and methyl methacrylate (MMA) in 2HPFP to uniquely give a PEG-core micelle with PMMA shell. Therefore, the LCST phase separation properties in the hydrofluorocarbon would open new vistas for thermoresponsive polymeric materials.

Single Chain Polymeric Nanoparticles as Selective Hydrophobic Reaction Spaces in Water.

A Ru(II)-based catalyst trapped within an amphiphilic, folded polymer is employed for the oxidation of secondary alcohols to their corresponding ketones using tBuOOH as the oxidant. Under the applied catalytic conditions, the polymer catalyst forms a compartmentalized structure with a hydrophobic interior. We selected secondary alcohols that differ in hydrophobicity, reactivity, and steric hindrance as substrates, with the aim to elucidate how this affects the rate and the end conversion of the oxidation reaction. Our investigations show that the Ru(II)-based catalyst is very efficient for oxidation reactions in water. Moreover, high selectivity toward the more hydrophobic substrate is observed, which originates from the hydrophobic interior of the compartmentalized catalyst system. This hydrophobic selectivity is also observed in the reverse reaction, the transfer hydrogenation.

Utilization of star-shaped polymer architecture in the creation of high-density polymer brush coatings for the prevention of platelet and bacteria adhesion.

We demonstrate utilization of star-shaped polymers as high-density polymer brush coatings and their effectiveness to inhibit the adhesion of platelets and bacteria. Star polymers consisting of poly(2-hydroxyethyl methacrylate) (PHEMA) and/or poly(methyl methacrylate) (PMMA), were synthesized using living radical polymerization with a ruthenium catalyst. The polymer coatings were prepared by simple drop casting of the polymer solution onto poly(ethylene terephthalate) (PET) surfaces and then dried. Among the star polymers prepared in this study, the PHEMA star polymer (star-PHEMA) and the PHEMA/PMMA (mol. ratio of 71/29) heteroarm star polymer (star-H71M29) coatings showed the highest percentage of inhibition against platelet adhesion (78-88% relative to noncoated PET surface) and Escherichia coli (94-97%). These coatings also showed anti-adhesion activity against platelets after incubation in Dulbecco's phosphate buffered saline or surfactant solution for 7 days. In addition, the PMMA component of the star polymers increased the scratch resistance of the coating. These results indicate that the star-polymer architecture provides high polymer chain density on PET surfaces to prevent adhesion of platelets and bacteria, as well as coating stability and physical durability to prevent exposure of bare PET surfaces. The star polymers provide a simple and effective approach to preparing anti-adhesion polymer coatings on biomedical materials against the adhesion of platelets and bacteria.

Fluorous microgel star polymers: selective recognition and separation of polyfluorinated surfactants and compounds in water.

Immiscible with either hydrophobic or hydrophilic solvents, polyfluorinated compounds (PFCs) are generally "fluorous", some of which have widely been employed as surfactants and water/oil repellents. Given the prevailing concern about the environmental pollution and the biocontamination by PFCs, their efficient removal and recycle from industrial wastewater and products are critically required. This paper demonstrates that fluorous-core star polymers consisting of a polyfluorinated microgel core and hydrophilic PEG-functionalized arms efficiently and selectively capture PFCs in water into the cores by fluorous interaction. For example, with over 10 000 fluorine atoms in the core and approximately 100 hydrophilic arms, the fluorous stars remove perfluorooctanoic acid (PFOA) and related PFCs in water from 10 ppm to as low as a parts per billion (ppb) level, or an over 98% removal. Dually functionalized microgel-core star polymers with perfluorinated alkanes and additional amino (or ammonium) groups cooperatively recognize PFOA or its ammonium salt and, in addition, release the guests upon external stimuli. The "smart" performance shows that the fluorous-core star polymers are promising PFC separation, recovery, and recycle materials for water purification toward sustainable society.

Arm-cleavable microgel star polymers: a versatile strategy for direct core analysis and functionalization.

Arm-cleavable microgel star polymers were developed, where the arm chains can readily be cleaved by acidolysis after the synthesis, allowing isolation of the core, direct analysis of its structure, and also the creation of functional nanometer-sized microgels. The key is to employ a macroinitiator (PEG-acetal-Cl) that carries an acetal linkage between a poly(ethylene glycol) arm chain and a chloride initiating site. From this, star polymers were synthesized via the linking reaction with a divinyl monomer and a ruthenium catalyst in living radical polymerization. The arms were subsequently cleaved by acidolysis of the acetal linker to give soluble microgels (cores free from arms). Full characterization revealed that the microgel cores are spherical, nano-sized (<20 nm), and of relatively low density. Amphiphilic, water-soluble, and thermosensitive arm-free microgels can be obtained by additionally employing functional methacrylate upon arm linking.