Nanoparticle-Based Tough Polymers with Crack-Propagation Resistance.

Although thin elastomer films of polymer nanoparticles are regarded as environmentally friendly materials, the low mechanical strength of the films limits their use in various applications. In the present study, we investigated the fracture resistance of latex films composed of acrylic nanoparticles where a small quantity of a rotaxane crosslinker was introduced. In contrast to conventional nanoparticle-based elastomers, the latex films composed of the rotaxane-crosslinked nanoparticles exhibited unusual crack propagation behavior; the direction of crack propagation changed from a direction parallel to the crack to one perpendicular to the crack, resulting in an increase in tear resistance. These findings will help to broaden the scope of design of new types of tough polymers composed of environmentally friendly polymer nanoparticles.

The Effect of the Axle End Structure and Number of Through-Space Bonds on the Properties of Rotaxane Crosslinked Polymers.

A rotaxane crosslinker (RC) is known to toughen the resulting rotaxane crosslinked polymer (RCP) via a stress dispersion effect that is attributed to the movable nature of the crosslinking structure. To evaluate this toughening mechanism in detail, a series of structure-definite RCs equipped with different axle end structures or different numbers of wheel components were synthesized, and subjected to free radical polymerization with a vinyl monomer to obtain RCPs. Analyses of the obtained RCPs revealed that the size of the axle end structure should be well-balanced to produce a strong toughening effect, and a [3]rotaxane crosslinker works more effectively than [2]rotaxane to toughen RCPs. The mobility of the crosslinking points, in terms of rotational and flipping movements, was more crucial to toughening the RCP than that of translational movement along the axle. The first observation of the above crucial findings proved the utility of the systematic molecular design used in this study.

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate.

Post-synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long-standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle-metal-complex-catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd- and Pt-macrocycle-catalyzed systems. The most fascinating feature of the both reactions is that higher-molecular-weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.

Rotaxanes with dynamic mechanical chirality: Systematic studies on synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion.

Dynamic mechanical chirality of [2]rotaxane consisting of a C s symmetric wheel and a C 2v symmetric axle is discussed via the synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion. This [2]rotaxane is achiral and/or prochiral when its wheel locates at the center of the axle, but becomes chiral when the wheel moves from the center of the axle. These were proved by the experiments on the enantiomer separation and racemization. The racemization energy of the isolated single enantiomers was controlled by the bulkiness of the central substituents on the axle. Furthermore, the chiral-prochiral interconversion was achieved by relative positional control of the components. The present systematic studies will provide new insight into mechanically chiral interlocked compounds as well as the utility as dynamic chiral sources.

Topology-transformable block copolymers based on a rotaxane structure: change in bulk properties with same composition.

The topology of polymers affects their characteristic features, i.e., their microscopic structure and macroscopic properties. However, the topology of a polymer is usually fixed during the construction of the polymer chain and cannot be transformed after its determination during the synthesis. In this study, topology-transformable block copolymers that are connected via rotaxane linkages are introduced. We will present systems in which the topology transformation of block copolymers changes their 1) microphase-separated structures and 2) macroscopic mechanical properties. The combination of a rotaxane structure at the junction point and block copolymers that spontaneously form microphase-separated structures in the bulk provides access to systems that cannot be attained using conventional covalent bonds.

An air-stable organometallic polymer containing titanafluorene moieties obtained by the Sonogashira-Hagihara cross-coupling polycondensation.

The synthesis of a polymer containing alternating titanafluorene and arylene ethynylene moieties is described. The polymerization of a 2,7-dibromo-9-titanafluorene derivative with 1,4-dioctyloxy-2,5-diethynylbenzene is carried out at 70 °C for 48 h in tetrahydrofuran (THF) in the presence of palladium dichloride/4,5-bis(diphenylphosphino)-9,9-dimethylxanthene as a catalyst and diisopropylamine as a base to produce a dark red polymer. The polymer thus obtained is soluble in organic solvents and stable towards both air and moisture. In the UV-vis absorption spectrum of the polymer, the absorption maxima (λmax) are observed at 321 nm and 395 nm, which are bathochromically shifted compared to those of a model compound of the repeating unit, a 2,7-bis(phenylethynyl)titanafluorene derivative (λmax = 309 nm and 364 nm). The optical band gap (Eg) of the polymer is estimated to be 2.8 eV on the basis of the absorption onset, which is narrower than that of the model compound (3.1 eV).

Rotaxane-Based Difunctional Nitrile N-Oxide Crosslinker: Synthesis and Direct Introduction of Movable Crosslinking Points into Ethylene-Propylene-Butadiene Monomer (EPDM) Rubber.

Incorporation of rotaxane scaffolds into the crosslinking points of polymer networks significantly affects their rheological and mechanical properties. The present study involves the synthesis of a new rotaxane-type crosslinker containing nitrile N-oxide functional groups on both the axle and wheel components. The prepared crosslinker is highly reactive; however, it can be isolated and applied in the crosslinking reaction of a commercially important polymer, namely ethylene-propylene-butadiene monomer rubber (EPDM), in the absence of additives and catalysts. Tensile tests reveal that compared to a network containing conventional crosslinking points, both breaking strength and strain of the network structure prepared herein are improved due to the incorporation of movable crosslinking points. The synthesized network structure also exhibits five times higher fracture energy. The developed post-crosslinking methodology for the direct introduction of movable crosslinking points into pre-formed polymers will be valuable in the production of rotaxane materials for various applications.

Crystal structure of 2,6-bis-(3-hy-droxy-3-methyl-but-1-yn-1-yl)pyridine monohydrate.

In the title pyridine derivative, C15H17NO2·H2O, the two OH groups are oriented in directions opposite to each other with respect to the plane of the pyridine ring. In the crystal, hydrogen bonds between the pyridine mol-ecule and the water mol-ecule, viz. Ohy-droxy-H⋯Owater, Ohy-droxy-H⋯Ohy-droxy, Owater-H⋯Ohy-droxy and Owater-H···Npyridine, result in the formation of a ribbon-like structure running along [011].

A Pyridine-Acetylene-Aniline Oligomer: Saccharide Recognition and Influence of this Recognition Array on the Activity as Acylation Catalyst.

In order to create new functions of foldamer-type hosts, various kinds of recognition arrays are expected to be developed. Here, a pyridine-acetylene-aniline unit is presented as a new class of a saccharide recognition array. The conformational stabilities of this array were analyzed by DFT calculation, and suggested that a pyridine-acetylene-aniline oligomer tends to form a helical structure. An oligomer of this array was synthesized, and its association for octyl ß-D-glucopyranoside was confirmed by 1 H NMR measurements. UV/Vis, circular dichroism, and fluorescence titration experiments revealed its high affinity for octyl glycosides in apolar solvents (Ka =104 to 105  M-1 ). This oligomer was relatively stable under basic conditions, and therefore this array was expected to be applied to the derivatization of saccharides. A 4-(dialkylamino)pyridine attached pyridine-acetylene-aniline oligomer proved to catalyze the acylation of the octyl glucoside.

Syntheses of polynorbornadienes by ring-opening metathesis polymerizations of symmetric and non-symmetric 2,3-bis(alkoxycarbonyl)norbornadienes and their conversion to half-ester derivatives.

Libraries of polynorbornadienes were synthesized with good yields with a ruthenium-containing 2nd generation Grubbs catalyst by ring-opening metathesis polymerization (ROMP) of a variety of symmetric and non-symmetric 2,3-bis(alkoxycarbonyl)norbornadiene monomer units prepared from the half-esters obtained efficiently by the selective monohydrolysis reactions of symmetric diesters we reported earlier. Among these polymers, the polynorbornadienes with t-butoxycarbonyl groups derived from non-symmetric monomer units were converted to the half-ester derivatives by deprotection with trifluoroacetic acid, yielding amphiphilic polymers. The hydrogenation reactions of the obtained polymers were carried out to yield polymers having saturated structures in the main chains for improvement of the thermal stabilities. All these polymers were characterized by their molecular weights and thermal properties along with the spectroscopic data. Our selective monohydrolysis reactions have been proven to be a versatile tool for production of relatively homogeneous polymer libraries.

Macrocyclic Dinuclear Palladium Complex as a Novel Doubly Threaded [3]Rotaxane Scaffold and Its Application as a Rotaxane Cross-Linker.

A dinuclear PdII complex possessing a cyclic ligand was developed as a novel doubly threaded [3]rotaxane scaffold and applied as a rotaxane cross-linker reagent. The dinuclear complex (PdMC)2 was prepared by one-step macrocyclization followed by the double palladation reaction. 1 H NMR analysis and UV/Vis measurements revealed the formation of a doubly threaded pseudo[3]rotaxane by the complexation of (PdMC)2 with 2 equivalents of 2,6-disubstituted pyridine 3 through double metal coordination. The treatment of (PdMC)2 with 2 equivalents of 4-vinylpyridine (VP) afforded a doubly threaded [3]rotaxane cross-linker (PdMC-VP)2 . Radical co-polymerization of VP and t-butylstyrene in the presence of (PdMC-VP)2 afforded a stable rotaxane cross-linked polymer (RCP). An elastic RCP was also prepared by using n-butyl acrylate as a monomer. The obtained RCPs exhibited higher swelling ability and higher mechanical toughness compared with the corresponding covalent cross-linked polymers.

Quantification for the Mixing of Polymers on Microspheres in Waterborne Latex Films.

Although tremendous efforts have been devoted to the structural analysis and understanding of the toughness of latex films, in which soft elastomer microspheres are interpenetrated, a method to quantitatively analyze the mixing of polymer chains at the microsphere surface, i.e., delocalization of hydrophilic charged group on the polymer chains by aging, has not yet been established. In this study, small-angle X-ray scattering was applied to characterize latex films by assuming a pseudo-two-phase system, which consists of an average-electron density microsphere core and a high-electron density interphase between the microsphere interfaces due to localized charged groups. The thus obtained parameter, i.e., the characteristic interfacial thickness (tinter), quantitatively reflects the degree of mixing of polymer chains on the microsphere surface. We found that tinter is strongly correlated to the fracture energy of the latex films. The proposed analysis method for the microscopic mixing of polymers on the microsphere surface in the film can thus be expected to shed light on design guidelines for industrial latex films and on the understanding of the mechanical properties of such films.

Switchable Polymer Materials Controlled by Rotaxane Macromolecular Switches.

The synthesis and dynamic nature of macromolecular systems controlled by rotaxane macromolecular switches are introduced to discuss the significance of rotaxane linking of polymer chains and its topological switching. Macromolecular switches have been synthesized from macromolecular [2]rotaxanes (M2Rs) using sec-ammonium salt/crown ether couples. The successful synthesis of M2Rs possessing a single polymer axle and one crown ether wheel, constituting a key component of the macromolecular switch, has allowed us to develop various unique applications such as the development of topology-transformable polymers. Polymer topological transformations (e.g., linear-star and linear-cyclic) are achieved using rotaxane-linked polymers and rotaxane macromolecular switches. The pronounced dynamic nature of these polymer systems is sufficiently interesting to design sophisticated stimuli-responsive molecules, polymers, and materials.

Visualization of the slide-ring effect: a study on movable cross-linking points using mechanochromism.

To evaluate the 'slide-ring' effect in a rotaxane cross-linked network, we incorporated mechanochromophores into static and rotaxane cross-linking points and compared the mechanochromisms exhibited by the obtained polymers. This novel strategy reveals a molecular-level phenomenon with visible color changes as well as enabling quantitative electron paramagnetic resonance measurements.

Open clamshell dinuclear palladium(ii) complexes possessing out-of-plane anisotropy.

Double cyclometalation of planar, oligomeric phenylpyridines yielded dinuclear palladium(ii) complexes with novel out-of-plane anisotropy. An X-ray crystal structure analysis revealed that the complexes exhibit concave-convex geometry, and 1H NMR measurement evidenced the occurrence of stable out-of-plane anisotropy. The dipole moment and Pd-Pd interaction were investigated by theoretical calculations.

Macrocyclic Metal Complexes Bearing Rigid Polyaromatic Ligands: Synthesis and Catalytic Activity.

We synthesised palladium and platinum complexes possessing cyclic and acyclic pincer-type polyaromatic ligands and investigated their structural effect on the catalysis. The pincer-type bis(6-arylpyridin-2-yl)benzene skeleton was constructed via Kröhnke pyridine synthesis under transition metal-free conditions on gram-scale quantity. Ligand structure significantly influenced catalytic activity toward the platinum-catalysed hydrosilylation of diphenyl acetylenes, despite the ligand-independence of the conformations and electronic properties of these complexes.

Phase Transition Behaviors and Nanoscale Film Morphologies of Poly(δ-valerolactone) Axles Bearing Movable and Fixed Rotaxane Wheels.

In this study, poly(δ-valerolactone) (PVL) axles bearing movable and fixed dibenzo-24-crown-8-ether wheels (rot-M and rot-F) are investigated for the first time in the terms of phase transition and nanoscale film morphology: PVL-rot-M and PVL-rot-F. Interestingly, the PVL axles reveal a strong tendency to form a horizontal lamellar structure with three different rotational crystal lattice domains in nanoscale films. The morphological structural parameters are discernibly varied by the movable and fixed rotaxane wheels. In particular, the rot-M wheel tends to be populated in both the interfacial and amorphous layers. The rot-M wheel is found to significantly influence the phase transition characteristics of the PVL axle because of its movability along the polymer backbone chain. In contrast, the rot-F wheel tends to be more localized in the interfacial layer rather than in the amorphous layer because of its immovability constrained at the polymer chain end. The rot-F wheel causes severe thermal instability in the PVL axle, which can be attributed mainly to the presence of its counter anion (PF6 - ).

33S nuclear quadrupole resonance study of dibenzyl disulfide toward understanding of cross-linked structures in rubber.

Experimental and theoretical investigations of a sulfur-33 electric-field-gradient (EFG) tensor of disulfide bonds in 33S-labled dibenzyl disulfide have been presented. Temperature dependence of quadrupolar frequencies, νQ, is observed in the temperature range between 80 and 280 K, in which single peaks appear in all the 33S nuclear quadrupole resonance (NQR) spectra. Analysis of nutation echo 33S NQR spectra at 200 K yields the quadrupolar coupling constant, CQ value, of 46.8(6) MHz and the asymmetry parameter, ηQ, of 0.48(7). The orientation of the 33S EFG tensor of the disulfide is obtained by quantum chemical calculations: the largest EFG tensor component, VZZ, is approximately perpendicular to the molecular plane (C-S-S), and the smallest component, VXX, is approximately 41° off the C-S bond. Extensive quantum chemical calculations are systematically performed to investigate dependences of 33S EFG tensors on changes of torsion angles in disulfide and trisulfide bonds, indicating that analysis of νQ and CQ values potentially makes it possible to assign the secondary structures of cross-linking in a rubber.

A supramolecular network derived by rotaxane tethering three ureido pyrimidinone groups.

A supramolecular network and its film were prepared via intermolecular hydrogen bonding of a [2]rotaxane with three ureido pyrimidinone groups. Adding less polar solvents such as CHCl3 to the rotaxane afforded a swollen rotaxane-cross-linked network. The properties of the film obtained by drying were evaluated to characterize it.

A Guiding Principle for Strengthening Crosslinked Polymers: Synthesis and Application of Mobility-Controlling Rotaxane Crosslinkers.

Three component mobility controlling vinylic rotaxane crosslinkers with two radically polymerizable vinyl groups (RC_Rs) were synthesized to prove that the mobility of the components of the RC_Rs plays a crucial role in determining the properties of rotaxane-crosslinked polymers (RCPs). RC_Rs (R=H, Me, or Et) were obtained from living ring-opening polymerization. RCP_Et was prepared using RC_Et, which exhibits the lowest component mobility. The low component mobility is reflected in inferior mechanical strength and stretching ability in tensile stress tests compared to components with good (R=Me) and high (R=H) mobility. However, RCP_Et exhibited significantly higher stress and strain values than the corresponding covalently crosslinked polymers (CCP_Rs). These results indicate that a suitable component mobility substantially enhances the mechanical strength of RCPs. This behavior could serve as a guiding principle for the molecular design of advanced RCs.