Ethylene polymerization using N-Heterocyclic carbene complexes of silver and aluminum.

Various transition metal catalysts have been utilized for ethylene polymerization. Silver catalysts have attracted less attention as the catalysts, but are potential for production of high molecular weight polyethylene. Herein, we report that silver complexes with various N-heterocyclic carbene (NHC) ligands in combination with modified methylaluminoxane (MMAO) afford polyethylene with high molecular weight (melting point over 140°C). SEM observation showed that the produced polyethylene has ultra-high molecular weight. NMR investigation of the reaction between the silver complexes with organoaluminums indicate that the NHC ligands transfer from the silver complex to aluminum to produce NHC aluminum complexes. Ph3C[B(C6F5)4] abstract methyl group from the NHC aluminum complex to afford cationic aluminum complex. The NHC aluminum complex promoted ethylene polymerization in the presence of Ph3C[B(C6F5)4] and organoaluminums. NHC ligand also promoted ethylene polymerization in combination with MMAO to produce polyethylene with high melting point (140.7°C). Thus, the aluminum complexes are considered to be the actual active species in silver-catalyzed ethylene polymerization.

Preparation of Various Nanomaterials via Controlled Gelation of a Hydrophilic Polymer Bearing Metal-Coordination Units with Metal Ions.

We investigated the gelation of a hydrophilic polymer with metal-coordination units (HPMC) and metal ions (PdII or AuIII). Gelation proceeded by addition of an HPMC solution in N-methyl-2-pyrrolidone (NMP) to a metal ion aqueous solution. An increase in the composition ratio of the metal-coordination units from 10 mol% to 34 mol% (HPMC-34) increased the cross-linking rate with AuIII. Cross-linking immediately occurred after dropwise addition of an HPMC-34 solution to the AuIII solution, generating the separation between the phases of HPMC-34 and AuIII. The cross-linking of AuIII proceeded from the surface to the inside of the HPMC-34 droplets, affording spherical gels. In contrast, a decrease in the ratio of metal-coordination units from 10 mol% to 4 mol% (HPMC-4) decreased the PdII cross-linking rate. The cross-linking occurred gradually and the gels extended to the bottom of the vessel, forming fibrous gels. On the basis of the mechanism for the formation of gels with different morphologies, the gelation of HPMC-34 and AuIII provided nanosheets via gelation at the interface between the AuIII solution and the HPMC-34 solution. The gelation of HPMC-4 and PdII afforded nanofibers by a facile method, i.e., dropwise addition of the HPMC-4 solution to the PdII solution. These results demonstrated that changing the composition ratio of the metal-coordination units in HPMC can control the gelation behavior, resulting in different types of nanomaterials.

Synthesis of Nanosheets Containing Uniformly Dispersed PdII Ions at an Aqueous/Aqueous Interface: Development of a Highly Active Nanosheet Catalyst for Mizoroki-Heck Reaction.

A method was developed to synthesize a nanosheet at the interface of an aqueous layer of PdII ions and an aqueous layer of hydrophilic polymer bearing a metal coordination unit (HPMC). The nanosheet was synthesized through generation of an interface by the addition of an aqueous solution of PdII ions with a low specific gravity (1.03 g/cm3) to a dispersed aqueous solution of HPMC with a high specific gravity (1.50 g/cm3), resulting in rapid cross-linking at the interface. An electron probe microanalysis mapping image showed that the PdII ions were uniformly dispersed in the nanosheet. The nanosheet showed a high catalyst activity for the Mizoroki-Heck cross-coupling reaction with a turnover number (TON) and turnover frequency (TOF) greater than 3,333,333 and 138,889 h-1, respectively. These are the greatest TON and TOF values reported for heterogeneous polymeric catalysts for the Mizoroki-Heck reaction.

2,4,5,7,9,10-Hexaethynylpyrenes: Synthesis, Properties, and Self-Assembly.

A series of 2,4,5,7,9,10-hexaethynylpyrenes was synthesized using 2,7,9,10-tetrabromopyrene-4,5-dione as the key intermediate. The effects of the position and number of the ethynyl groups on the physicochemical properties of the corresponding pyrenes were clarified by comparison with 4,5,9,10-tetraethynylpyrene and 2,7-diethynylpyrene derivatives. The prepared hexaethynylpyrenes that bear benzene moieties self-assemble via π-π stacking in solution and/or the condensed phase.

Trithiazolyl-1,3,5-triazines bearing decyloxybenzene moieties: synthesis, photophysical and electrochemical properties, and self-assembly behavior.

We report the synthesis, photophysical properties, redox characteristics, and self-assembly behavior of disk-shaped trithiazolyl-1,3,5-triazines that bear decyloxybenzene moieties. These compounds were synthesized by a Migita-Kosugi-Stille coupling reaction of 1,3,5-trichlorotriazine with three different tributyltin(thiazoles) as the key step. The structure-property relationships, namely the effects of the incorporation of thiazole units into the triazine unit, the conjugation connectivity between the thiazole and triazine units, and the insertion of ethynylene spacers between the thiazole and decyloxybenzene moieties on the properties of the trithiazolyl-1,3,5-triazines were comprehensively investigated. Binding of the triazine core at the 5-position of the thiazole moieties effectively extended the π-conjugation and afforded high fluorescence quantum yields. The ethynylene spacers substantially lowered the LUMO level relative to the HOMO level. The prepared trithiazolyl-1,3,5-triazines self-assembled in solution, and the introduction of thiazole units at the 5-position enhanced this behavior. Detailed thermodynamic studies on the self-association behavior were conducted, and the formation of self-assembled 1D clusters is disclosed.

Tetraalkoxyphenanthrene-Fused Hexadecadehydro[20]- and Tetracosadehydro[30]annulenes: Syntheses, Aromaticity/Antiaromaticity, Electronic Properties, and Self-Assembly.

Tetraalkoxyphenanthrene-fused hexadecadehydro[20]- and tetracosadehydro[30]annulenes possessing octatetrayne linkages were synthesized and their properties together with those of phenanthrene-fused octadehydro[12]- and dodecadehydro[18]annulenes have been investigated. Various spectroscopic and electrochemical measurements as well as quantum chemical calculations support that planar [20]- and [30]annulenes are weakly antiaromatic and nonaromatic, respectively. The detailed concentration- and temperature-dependent 1H NMR and UV-vis data of present dehydroannulenes provided evidence for the enhancement of π-π stacking interactions by extension of the acetylenic linkages. Dehydroannulenes formed self-assembled clusters and their morphology and crystallinity proved to depend on the length of acetylenic linkages, the shape of dehydroannulene core, and the bulkiness of alkoxy groups appended to the phenanthrene moieties.

Crystallization and hardening of poly(ethylene-co-vinyl acetate) mouthguards during routine use.

Mouthguards (MGs) made from poly(ethylene-co-vinyl acetate) (EVA) are widely used in contact sports to prevent injuries such as breaking teeth and lip lacerations and to reduce brain concussion. However, the changes in morphology and the molecular mobility of EVA, which can affect its physical properties during practical usage, have not been precisely examined. Therefore, we attempted to determine the main factors which lead to changes in MG performance after one season of practical use by high school rugby players. Solid-state nuclear magnetic resonance (NMR) and pulse NMR measurements showed the hardening of MGs, which was associated with an increased crystallinity of the EVA resulting from prolonged usage. Furthermore, our data indicated that the increase in the relative amount of the crystalline phase may be primarily attributed to temperature fluctuations and repeated changes in pressure, which could cause the hardening of EVA and eventually diminish the protective ability of MGs.

Tetraalkoxyphenanthrene-Fused Thiadiazoloquinoxalines: Synthesis, Electronic, Optical, and Electrochemical Properties, and Self-Assembly.

π-Extended thiadiazoloquinoxaline (TQ) derivatives 1a,b-3a,b, in which a tetraalkoxyphenanthrene moiety is annulated with the TQ core and benzene rings are incorporated via the ethynylene spacer, were synthesized. They display absorption bands reaching into 750 nm and possess the electron-affinity comparable to [60]fullerene. The CF3- and OMe-substituents on the benzene rings have moderate effects on modulation of the HOMO and LUMO levels. Tetraalkoxyphenanthrene-fused TQs 1a,b-3a,b aggregate in the solid state and assemble in solution through π-π stacking interactions. The self-assembly of 1a,b-3a,b into 1D superstructures was confirmed, and the difference in the alkoxy groups and the solvents for self-assembly proved to change their morphology. Comparison of the properties of 1a and those of reference compounds 4 and 5 clarified the effects of both the fusion of the phenanthrene moiety and the introduction of ethynylene spacers on the properties.

Phase Transition during Heating of Nanostructured Ultrahigh Molecular Weight Polyethylene Membranes.

Ultrahigh molecular weight polyethylene (UHMW-PE) membranes were prepared using biaxial melt-drawing and subsequent melt-shrinking. Electron microscopy observations indicate that the former membrane has more extended-chain crystals (ECCs), whereas the latter is mainly composed of folded-chain crystals (FCCs). Corresponding double-melting endotherms are recorded on differential scanning calorimetry (DSC) measurements. Detailed assignments of such double-melting components are performed using in situ X-ray measurements during heating. Wide- and small-angle X-ray diffraction and scattering (WAXD/SAXS) images were simultaneously recorded at SPring-8. Changes in WAXD images indicate that the orthorhombic reflection peak begins to decrease at 130 °C, followed by the appearance of the hexagonal reflection peak beyond 145 °C for both membranes, but the latter melt-shrunk membrane exhibits weaker hexagonal reflection intensity. Simultaneous SAXS results indicate that FCCs rapidly disappear at 135 °C for the melt-shrunk membrane, resulting in a sharper endotherm. In contrast, residual ECCs restrict the melting of FCCs for the melt-drawn membrane, resulting in a broader endotherm of FCC melting spread to a slightly higher temperature position.

In situ analysis of melt-drawing behavior of ultrahigh molecular weight polyethylene films with different molecular weights: roles of entanglements on oriented crystallization.

Ultrahigh molecular weight polyethylene (UHMW-PE) films having different molecular weights (MWs) were melt-drawn at 150 °C. The stress-strain curve for higher-MW film exhibits higher stress on the characteristic plateau region and a subsequent steeper increase of stress due to strain hardening. Structural changes during such melt-drawing were analyzed using in situ wide-angle X-ray diffraction measurements. Hexagonal crystallization occurs at the beginning of the plateau region, independent of the sample MW. Once this hexagonal reflection intensity is saturated, it remains constant even at the later stage of draw. In contrast, orthorhombic reflection intensities gradually increase with increasing draw strain. Both of these oriented crystallizations into plateau hexagonal and increasing orthorhombic forms are accelerated with increasing MW. Correspondingly, the higher amount of extended chain crystals (ECCs) was confirmed from morphological observation for the resultant melt-drawn films of the higher-MW sample. Deep entanglements can effectively transmit the applied stress; thus, the oriented amorphous melts induce rapid hexagonal crystallization with disentangling shallow entanglements, which subsequently transforms into orthorhombic form. Such hexagonal crystallization plays the role of a thermodynamic pathway for growing such ECCs, where the stable orthorhombic form gradually accumulates with increasing draw strain.

Property development for biaxial drawing of ethylene-tetrafluoroehtylene copolymer films and resultant fractural behavior analyzed by in situ X-ray measurements.

The property development of the ethylene-tetrafluoroethylene copolymer (ETFE) membrane induced by simultaneous biaxial drawing was investigated. Commonly, tensile strength can be increased by drawing; conversely, tear resistance decreases. In this study, the balance between tensile strength and tear resistance for the resultant ETFE membrane was optimized achieved by a combination of lamination of low molecular weight (LMW) and high molecular weight (HMW) layers and subsequent biaxial drawing. The structural factor determining tear resistance of these biaxially drawn membranes was determined based on in situ small-angle X-ray scattering (SAXS) measurement during tensile deformation simulating tearing tests. Lozenge shaped scattering, which indicated inclined lamellae, was observed during such tensile deformation of the resultant membranes. Remarkably, this inclined lamellar structure was observed for the pure LMW membrane; however, it also appeared at the interface between LMW and HMW layers within biaxially drawn membranes. For the membrane exhibiting the highest tearing strength, the fraction of such inclined lamella increased up to the critical strain corresponding to the actual sample breaking. These results confirm that the inclined lamellar structure absorbed strain during membrane tearing.

Modification of physical properties of poly(L-lactic acid) by addition of methyl-ß-cyclodextrin.

Poly(L-lactic acid) (PLLA) is a biodegradable plastic and one of the most famous plastics made from biobased materials. However, its physical strength is insufficient compared to general-purpose plastics. In this study, the effect of methylcyclodextrin (MeCD) addition on the structure and physical properties, especially the drawing behavior, of PLLA was investigated. Through thermal analysis, it was found that MeCD addition lowers the crystallinity and enhances the mobility of PLLA. The sample containing approximately 17% MeCD was drawn to more than 1000% at 60 °C, although PLLA fractured at a strain of less than 100%. Differential scanning calorimetry (DSC)-Raman in situ measurements also revealed decreases in the glass transition temperature (T g), cold crystallization temperature (T c), and melting point (T m), and improvement in structural distribution with temperature. DSC-Raman measurements simultaneously supplied information about crystallinity and thermal properties. Thus, it was concluded that MeCD had high affinity for PLLA, and the addition of MeCD increased the amorphous component of PLLA and enhanced the drawability.

Tetraalkoxyphenanthrene-fused dehydroannulenes: synthesis, self-assembly, and electronic, optical, and electrochemical properties.

Novel tetraalkoxyphenanthrene-fused dehydro[12]-, [18]-, and [24]annulenes 1-3 were synthesized by using Cu-mediated or Pd-catalyzed oxidative macrocyclization reactions as key steps, and their electronic, optical, and electrochemical properties have been investigated in detail. X-ray crystallographic analysis of a single crystal of 1 a demonstrated that the molecules were arranged longitudinally in a slipped π-stacked fashion to form a 1D column. (1)H NMR and UV/Vis spectroscopic and cyclic voltammetric analysis in conjugation with nucleus-independent chemical shift (NICS) calculations for 1-3 support that the annulation at the 9,10-positions of phenanthrene to the dehydroannulene ring enhances the tropicity and decreases the HOMO-LUMO gaps of the molecules relative to the benzannulation and that 1 possesses an antiaromatic character. Self-association behavior due to π-π stacking in CDCl3 was observed for 1 and 2 and was quantified by concentration-dependent (1)H NMR spectroscopic measurements. The self-assembly of 1 and 2 into well-defined 1D superstructures with high aspect ratios were obtained, and the morphology and crystallinity of these compounds were investigated by means of SEM and wide-angle X-ray diffraction (WAXD) measurements. Furthermore, it was shown that 1 b and 2 b display liquid-crystalline phases by means of differential scanning calorimetry, polarizing optical microscopy, and variable-temperature WAXD measurements.

Stereo-complex crystallization of poly(lactic acid)s in block-copolymer phase separation.

Block-copolymer containing a poly(L-lactic acid) (PLLA) segment was blended with pure poly(D-lactic acid) (PDLA) chain in chloroform solution and casted into the dried film. This method could form the stereocomplex (Sc) crystal of PLLA/PDLA within the nanometer-sized phase separation self-assembled by block-copolymer. Differential scanning calorimetry measurements of the prepared and annealed films gave the maximum achievable melting temperature of 245 °C, which was highest among those previously reported for PLA Sc-crystals. Surprisingly, all of added PDLA chains were Sc-crystallized, depending on the blend composition with block-copolymer containing PLLA.

Phase transitions during heating of melt-drawn ultrahigh molecular weight polyethylenes having different molecular characteristics.

Changes in the crystalline structure during heating of melt-drawn ultrahigh molecular weight polyethylenes (UHMW-PEs) having different molecular characteristics were analyzed by in situ wide-angle X-ray diffraction measurements. A phase transition from the orthorhombic into the hexagonal phase was observed for all samples, but the perfection was enhanced and the possible temperature window for the hexagonal phase was greater for the sample containing only a higher molecular weight component. In contrast, an increase in retractive stress during heating was confirmed for the sample containing a lower molecular weight component, reflecting melting of the folded-chain crystal (FCC). Differential scanning calorimetry and transmission electron microscopy revealed the dependency of the molecular characteristics of the sample on the resultant morphologies. These results demonstrate that the existence of FCC determines both the quality and the width of the temperature window for the hexagonal phase during heating of melt-drawn UHMW-PEs.

Molecular weight segregation on surfaces of polyethylene blended films as estimated from nanoscratch tests using scanning probe microscopy.

Nanoscratch tests using scanning probe microscopy (SPM) were performed on films prepared from two polyethylene (PE) materials polymerized by using a metallocene catalyst system with different molecular weights (MWs). Blended samples were prepared by dissolving both PE materials at various ratios in hot p-xylene. The pure and blended samples were compression molded into films at 180 degrees C for different holding times in the molten state. The results of SPM nanoscratch tests with an applied load of 30 nN indicated that the lower-MW surface could be easily plowed with wear debris but the higher-MW surface was less deformed. However, the deformation pattern of the blended film surface was similar to that of the lower-MW surface. These results suggest that MW segregation occurs during holding in the molten state as lower-MW components rise to the film surface.

Surface-deformation characteristics of uniaxially oriented poly(ethylene terephthalate) film as evaluated from nanoscratch tests with scanning probe microscopy.

The surface-deformation characteristics of uniaxially drawn poly(ethylene terephthalate) (PET) film were successfully evaluated with multiline scratch tests using scanning probe microscopy (SPM) on a nanometer scale. The PET film was prepared by compression molding from the melt, followed by quenching in ice water. The obtained amorphous film was drawn uniaxially below its glass-transition temperature, and the resultant surface roughness could be reduced to within 5 nm. A multiline scratch with the Si(3)N(4) tip of an SPM on the oriented PET surface was made parallel and perpendicular to the drawing axis under applied loads of 5-30 nN. The perpendicular scratching generated a characteristic periodic pattern on the film surface, but the parallel scratching induced a tearing of the surface. These results suggest that surface-deformation mechanisms were dominated by molecular anisotropy. The surface-deformation properties, as evaluated from scratch-angle dependences on morphological changes on a nanometer scale, were similar to the mechanical properties of the bulk.

Morphology of melt-crystallized syndiotactic polypropylene.

A transmission electron microscopy (TEM) study was made to investigate the mechanism of isothermal crystallization from melt for syndiotactic polypropylene (sPP) with a syndiotactic pentad fraction of 0.92. The TEM morphologies of RuO4-stained and ultrathin-sectioned specimens revealed a clear assignment of lamellar crystals with different crystal thicknesses to their corresponding melting temperatures and types of crystal structures. It has been found that isolated, single crystal lamellae with cell III structure were grown at 140 degrees C by a plausible crystal growth mechanism with regime I. Fringed-micellar type of lamellae with cell II structure as thin as c. 4 nm were formed in the interstices of the thick chain-folded lamellae by the subsequent quenching.