Focus Review in 2019

We have developed sequential multicomponent reaction and interconvertible hybrid copolymerization methods to prepare sequence-controlled polymers. The sequential multicomponent reaction combines more than two different multicomponent reactions in one pot to prepare periodic sequence˗controlled polymers with sufficient molecular diversity. The interconvertible hybrid copolymerization method uses trithiocarbonate compounds to combine radical polymerization of vinyl monomers and anionic ring-opening polymerization of thiirane monomers, resulting in hybrid multiblock sequence-regulated polymers.

Our recent studies on the polymerization of diazocarbonyl compounds to afford functional polymers are reviewed. In the first part of this article, Pd-mediated polymerization of various functionalized diazoacetates and unique properties of the resulting polymers due to the effects of the dense packing of the functional groups in the polymers are described. The second part of this report summarizes our recent results on the polycondensation of bifunctional diazocarbonyl compounds as monomers.

Sequence precise regulation provides the possibility to probe the fundamental relationship between structure and properties in artificial polymers. Recent progress in sequence regulation through maleimide chemistry is focused on in this review, which put emphasis on the development of three strategies taking advantages of the versatile chemical properties of maleimide.

A new type of asymmetric chirogenic polymerization by asymmetric allylic substitution catalyzed by planar–chiral ruthenium complexes was designed. The polymerization systems function in a highly stereoselective manner. The asymmetric carbon in the main chain is precisely controlled. Each monomer unit of the polymer has a potentially reactive terminal olefin, which can be used for further transformations, ring-closing metathesis and thiol–ene reaction. Additionally, the resulting polymer can be transformed poly “arylopeptide”, through reductive cleavage of the N–O bond in N-alkoxyamide, which adopts a one-handed stable helical conformation in solution.

The implementation of Direct Heteroarylation Polymerization (DHAP) in the synthesis of well-defined and defect-free materials for organic electronics is still in its early stages but giant leaps have been made over the past few years to improve and stir the selectivity of the reaction to avoid unwanted side-reaction such as β-branching or homocoupling. DHAP has proven to be an efficient, cost-effective, green and scalable polymerization method now competing or even surpassing well-established Suzuki-Miyaura or Migita–Stille cross-coupling polymerization methods. This focus review puts emphasis on the synthetic strategies that made DHAP successful.

This focus review provides an overview of the formation and functions of surface-segregated monolayers (SSMs) on organic semiconductor films. Careful design of SSM molecules allows the formation of highly ordered surface monolayers with high coverage densities. SSMs can be used to induce changes in the molecular structure and the orientation in the bulk film. SSMs can be used to change the interfacial properties of organic/metal and organic/organic interfaces in various organic electronic devices. These modifications can drastically alter the performance of devices, demonstrating the general importance of interfaces.

Melanin, a black component of human hair, plays an important role in bright structural colorations in nature. Polydopamine-based artificial melanin particles, inspired by natural melanin and its important role in structural coloration, were used to produce bright structural colors by controlling light reflection and absorption. A variety of structural coloration characteristics have become possible through our investigations of the assembly structure, composition, shape, and application of artificial melanin particles.

Novel network structures were applied to crosslinked liquid-crystalline polymers (CLCPs) with photomechanical effects to enhance their performance and function. CLCPs with rearrangeable networks could be reshaped into 3D architectures, in contrast with conventional crosslinked polymers memorizing permanent shapes. The reshaped samples showed various photoinduced motions depending on their initial shapes. Furthermore, interpenetrating polymer networks (IPNs) were developed to improve mechanical and photoresponsive properties of CLCPs. These strategies of controlling the network structures of CLCPs could enable the versatile design of photomobile polymer materials as soft actuators.

Bipolar electropolymerization is a powerful method for wirelessly achieving site-selective anisotropic modification of bipolar electrodes (BPEs) with conducting polymers. In addition, alternating current (AC) bipolar electropolymerization was developed to induce conducting polymer fibers from the terminals of BPEs that propagate parallel to the direction of the electric field. Bipolar electropolymerization is a class of next-generation electropolymerization for obtaining hybrid materials of conducting polymers and conductive objects.

Functionalized colloidal particles are fascinating due to their huge range of potential and reported applications. In this focus review, the recent development of a novel “interfacial photoreaction” for the preparation of functional polymer particles is summarized. This approach allows for the direct fabrication of hollow, capsular, non-spherical polymer particles from spherical polymer particles. Furthermore, the ability of this approach to synthesize morphology controlled particles possessing sophisticated functions, such as stimuli responsiveness and molecular recognition capability, was also described in this paper.

Biodegradable injectable polymer systems exhibiting temperature-triggered formation of chemically cross-linked hydrogel were developed by “mixing strategy”. The mixture of triblock copolymer of poly(ε-caprolactone-co-glycolic acid) and poly(ethylene glycol) (PCGA-b-PEG-b-PCGA, tri-PCG) solution containing hexafunctional polythiol (DPMP) and tri-PCG with terminal acryloyl groups (tri-PCG-Acryl) solution showed temperature-responsible sol–gel transition, and the gelation was irreversible. Duration time of the gel state under physiological condition and physical property of the hydrogel could be controlled by a simple method, just changing the mixing ratio of tri-PCG-Acryl to tri-PCG.

The effect of polymer design on the interfacial structure and physical properties of polymer films in water based on a poly(vinyl ether) platform with hydrophilic side-chains was examined to construct bioinert interfaces. Hydrophilic surfaces were prepared by utilizing the preferential segregation of a rubbery component in a diblock copolymer film with a glassy component, crosslinking a hydrophilic polymer, and designing an interfacial modifier with a special architecture. Interfacial structure and physical properties controlled by polymer design play important roles in determining bioinert properties.

Welcoming more participants in a polymer synthesis; recent years, multicomponent reactions (MCRs) have been gradually integrated into synthetic polymer chemistry, which led to a new synthetic toolbox for a range of interdisciplinary applications. In this focus review, this new trend in polymer chemistry has been summarized with including contributions from the author.

A method to control on-demand phase transition and alignment of organic–inorganic hybrid containing lyotropic liquid crystals (LLCs) have been developed. By using polysiloxane containing hygroscopicity and photocrosslinking groups, the phase transition of the LLCs can be induced by changes in humidity. The phase-transition behavior of the LLCs in polysiloxane can be suppressed by light irradiation. Further, the author proposed new methods for vertically aligning nanochannels by using surface interaction or the spontaneous vertical-aligned nature, and photopatterning of chromonic dye aggregates.

The development of organic semiconductors (OSCs) applicable to organic field-effect transistors is crucial to printed and flexible electronics. OSCs must meet several prerequisites: 1) high chemical stability, 2) charge carrier mobility exceeding 10 cm²/Vs, 3) appropriate solubility, and 4) high thermal durability. The author and collaborators developed state-of-the-art OSCs based on bent-shaped π-electron cores (π-cores) that satisfy the requirements for printed semiconductor devices. In this focused review, the chemistry and device engineering are introduced with respect to sulfur-bridged V-shaped and N-shaped π-cores among a series of bent-shaped π-cores.

We successfully controlled birefringence and its wavelength dispersion of cellulose esters by choosing different ester substitution groups and sites, controlling the orientation of the crystal, adding low-mass molecules, and forming microscale porous structures. The substitution of two ester groups could provide extraordinary wavelength dispersion of birefringence, which is important for wide-range retardation films. The low-mass additives enhance birefringence values due to the intermolecular orientation correlation with matrix cellulose esters. The anisotropic microporous structure produced by thermoinduced phase separation generates form birefringence, which improves the total birefringence of cellulose esters.

To develop highly functional gel materials, the precision construction of the network structure is required. This focus review outlines the structural design of polymer gels by utilizing precision radical polymerization techniques with a focus on the authors’ recent research. In particular, the synthesis of homogeneous networks using precise radical polymerization and the control of the swelling characteristics by tuning the monomer sequence in the gel network chain are described.

This focus review describes that “elastic organic crystals of π-conjugated molecules”. The author developed the field of innovative materials for flexible optoelectronics. Intentional design of intramolecular interaction in π-system furnish the flexible crystals. These crystals show not only elastic bending flexibility and shape tunability but also functionality for mechanical sensors and flexible optical waveguide.

Categorization of molecular assemblies of primary to quaternary structures is herein proposed with citing typical examples of peptide molecular assemblies for each category. Primary structure; tubes and vesicles, Secondary structure; chimeric morphology, Tertiary structure; phase-separated tubes, Quaternary structure; hierarchical organization of molecular assemblies.

Microstructured surfaces have been attracting a considerable amount of attention for many practical applications, such as superhydrophobic materials. In this report, vulcanized rubber was focused on as a flexible and durable hydrophobic material for the fabrication of microstructured superhydrophobic surfaces. Superhydrophobic spike-array microstructures were simply prepared from a vulcanized rubber by using a silicon micromold, and the arrangements were reversibly deformed by repeated stretching without destruction of the spikes. Surface wettability can be controlled by stretching the vulcanized rubber, and the phenomenon was theoretically explained by the wettability transition from a Cassie–Baxter to a Wenzel state.