A new zwitterionic nickel(II) catalyst that comprises a partially fluorinated tetrakis(aryl)borate center in a bidentate phosphine ligand and a cationic Ni center has been developed and studied for CO–ethylene copolymerization in the context of comparison with a previously reported zwitterionic catalyst that carries a nonfluorinated borate anion. The crystal structures of several zwitterionic and related nickel compounds are characterized. Partial fluorination of the tetrakis(aryl)borate only brings a modest increase in productivity (2700 vs 1600 g of polyketone per gram of Ni, g (g of Ni)−1). Like the nonfluorinated catalyst, the fluorinated zwitterionic catalyst is extremely active at the beginning of the polymerization but deactivates rapidly. Deactivation of the two catalysts apparently follows different mechanisms. Stoichometric decomposition studies show that the partially fluorinated tetrakis(aryl)borate in the Ni compounds is stable under acidic conditions either directly introduced by addition of an acid or created by a CO atmosphere. In contrast, the nonfluorinated tetrakis(aryl)borate is readily decomposed by an acid or under acidic conditions created by CO. For the new catalyst system with the partially fluorinated tetrakis(aryl)borate anion, the deactivation likely involves initially redox processes and eventually ligand redistribution around Ni, as inferred from the stoichiometric decomposition studies. It turns out that such a process allows the deactivated catalyst to be reactivated by H2. When the polymerization is carried out in the presence of H2, the productivity of the new zwitterionic catalyst can reach 6400 g (g of Ni)−1. The zwitterionic catalyst with the nonfluorinated tetrakis(aryl)borate anion cannot be reactivated by H2. A cationic analogue of the zwitterionic catalysts is also studied for comparison. Its productivity for CO–ethylene copolymerization (230 g (g of Ni)−1) is about 1 order of magnitude lower than that of the zwitterionic catalysts, demonstrating the critical role of the zwitterionic character in attaining the aforementioned high productivity. At the productivity level of the zwitterionic catalysts, which to our knowledge is among the highest observed for Ni catalysts, an unacceptable amount of residual Ni(II) species is left in the product, causing the alternating CO–ethylene copolymer to begin to decompose near its melting temperature and hence making melt processing difficult.

•Mercapto-functionalized β-alanine has been introduced to serve as the building block for supramolecular filler.•A solution-mixing method has been introduced to mix the supramolecular filler with SBR.•Thiol-ene reaction during mixing and vulcanization improves filler dispersion.•The improved filler dispersion and filler-rubber interaction have resulted in significantly improved reinforcement.A reactive amphiphile, SA1, which comprises a mercaptodecyl oleophilic tail and a β-alanine oleophobic moiety, has been introduced to serve as the building block for supramolecular filler. At least a fraction of SA1 reacts in situ during mechanically mixing and vulcanization with SBR via thiol-ene reaction. Some oxidative dimerization also concomitantly occurs to give DSA1. The chemical coupling between SA1 and SBR results in drastically improved filler domain dispersion. Short fibrous crystalline domains, which are likely composed of SA1, SA1 grafted to SBR, and DSA1, less than 10 nm in width and a few tens to one hundred nanometers in length are observed by TEM. The filler domains are evenly distributed in the SBR. In contrast, the simple filler PA1 forms large irregular aggregates on the length scale of hundreds of nanometers. The improved filler dispersion and filler-rubber interaction allow effective energy dissipation and result in significantly improved mechanical properties.A solution-mixing method proves feasible for mixing SA1 and SBR and actually has resulted in better mechanical properties than the mechanical mixing method. Since the conventional mechanical mixing method for rubber compounding is highly energy-intensive, avoidance of the mechanical mixing method is another practical advantage for the novel strategy of supramolecular reinforcement.Download high-res image (196KB)Download full-size image

•A new paradigm of elastomer reinforcement by in situ formed nanocrystals during processing and vulcanization is demonstrated.•The method involves the use of a β-sheet forming β-alanine amphiphile.•The size of the crystalline domain dispersed in the elastomer matrix is critical for the effectiveness of reinforcement.•A moderate 3-fold gain in toughness is so far achievable.A strategy of supramolecular reinforcement of elastomers is conceptualized and demonstrated. Amphiphile A consisting of a β-alanine oleophobic motif and a pentadecyl oleophilic motif is designed to self-assemble in a continuous rubber phase to form discrete micelle-type structures under high-temperature processing conditions and crystallize upon cooling to form supramolecular reinforcing particles dispersed in the rubber phase. Styrene-butadiene rubber (SBR) composites reinforced by A are prepared and vulcanized using conventional rubber processing methods. The hierarchical structure and morphology of the assemblies of A in the vulcanized SBR composites are studied by Fourier-transform infrared spectroscopy, differential scanning calorimetry, wide-angle X-ray diffraction, and transmission electron microscopy. In the SBR continuous phase, A forms β-sheets, and the β-sheets stack to form crystals. At a filler loading of 5 phr (parts per hundred rubber), the β-sheet crystalline domains of A are whisker-like with the short dimensions on the order of a few to a few tens of nanometers and the long dimension on the order of a few hundred nanometers. The whisker-like crystalline domains aggregate to form globules of the size of hundreds of nanometers to micrometers at higher filler loadings (10–20 phr). Tensile test shows that A does improve strength at break, strain at break, and toughness of the reinforced SBR composites in comparison to gum SBR. The best reinforcement, as measured by toughness of the vulcanized rubber composite, is achieved at 5 phr filler loading, where the toughness is increased to nearly 3 times as high as that of the vulcanized gum SBR.Download high-res image (279KB)Download full-size image

•Thermoplastic elastomers physically crosslinked and reinforced by particulate β-sheet nanocrystals are highly extensible and highly stiff at the same time.•Dynamic mechanical properties of such thermoplastic elastomers are superior to those of its vulcanized analog reinforced by carbon black.•The particulate β-sheet nanocrystals undergo first orientation by shear and then disorientation by load-bearing strands during extension.•Fragmentation of the β-sheet crystals is not detectable in the hydrogen-bonding direction and hence, by inference, must be in the sheet-stacking direction during extension.Tris- and tetra-(β-alanine)-grafted polyisobutylenes (A3 and A4) with the same grafting density have been synthesized and studied. IR, DSC, SAXS, and WAXD show that the β-alanine tetramer segments in A4 form β-sheet crystals similar to those in A3. A3 and A4 are melt-processable TPEs. The less than 5% by volume of β-sheet crystals in A3 and A4 bring about mechanical properties significantly better that those of a vulcanized butyl rubber composite optimally reinforced with carbon black (CBV). A4 is somewhat stiffer and less extensible than A3. Overall, only a small gain in toughness is achieved for A4 over A3. Importantly, A3 and A4 also show hysteretic properties and dynamic mechanical properties superior to CBV. Both A3 and A4 are less hysteretic than CBV at low strains and more hysteretic at high strains. Their loss factors are substantially lower than that of CBV in the temperature range of 60–100 °C, and their modulus dependence on strain amplitude at very low strains (Payne effect) are significantly subdued in comparison to CBV. In fact, A3 is slightly better than A4 in all respects from the view point of hysteresis and dynamic mechanical properties. Dichroic IR study indicates that the oligo(β-alanine) segments in A3 and A4 statistically prefer to orient perpendicular to the extension axis due to shear stress exerted on the elongated β-sheet crystals at <250% strain. At higher strains, the grafting molecular architecture and the random location of the load-bearing chain on the surface of the β-sheet crystals dictate the orientation of the β-sheet crystals to be random. No fragmentation of the β-sheet crystals in the hydrogen-bonding direction is detectable during the entire process of extension until break. The residual preference of the oligo(β-alanine) segments for the orientation perpendicular to the extension axis is attributed to the elongated β-sheet crystal morphology, which prevent the crystals from fully comply to the load-bearing chains.Download high-res image (245KB)Download full-size image

Sulfur was chemically incorporated into urushiol in one step to afford an oligomeric sulfurized urushiol (SU). About 53% of the linkages between urushiol monomer units are cleavable multi-sulfidic bonds. Surface modification of silica with SU can be achieved either in a suspension before rubber compounding or in situ during rubber compounding. SU performs better when added in situ into the rubber compound than when used to pre-modify silica. Mooney viscosity and Payne effect studies of the uncured rubber compounds show that SU is as effective as TESPT to promote filler dispersion during mixing. The increased bound rubber content and suppressed filler flocculation demonstrate that SU does indeed generate covalent filler-rubber interactions during mixing in contrast to our previously reported hydrogenated urushiol, which only act as a covering agent. However, SU is less effective than TESPT as a coupling agent. The dynamic mechanical properties of the SU-containing vulcanizate are intermediate between the TESPT-containing standard and the hydrogenated urushiol-containing standard. When SU is used to replace 50% TESPT, mechanical and dynamic mechanical properties of the two vulcanizates are essentially identical. Noteworthy is that the reaction between SU and silica during rubber compounding produces water instead of ethanol, which the reaction of silica and TESPT produces, as a volatile organic compound.

•A zwitterionic Ni(II) acetyl complex has been shown to be active for althernating copolymerization of CO and ethylene.•Decomposition of the above complex occurs under high pressure CO at room temperature.•C–H activation of acetonitrile is observed at the zwitterionic Ni(II) center to give a 12-membered macrocycle.Zwitterionic nickel(II) compounds with a borate-containing anionic bidentate phosphine ligand (PBP−) have been synthesized and some of them crystallographically characterized. The methyl complex, (PBP−)Ni+CH3(NCCH3), reacts with H2 to form an 12-membered macrocycle containing three Ni nuclei and reacts with carbon monoxide (CO) to afford the acetyl complex (PBP−)Ni+COCH3(CO). The square planar acetyl complex is stable in solution under 1 atm of CO but undergoes irreversible decomposition under high CO pressure at room temperature. X-Ray crystallographic characterization of a decomposition product revealed fragmentation of the anionic borate under high pressure. (PBP−)Ni+COCH3(CO) is an active catalyst for ethylene-CO copolymerization, but its productivity is low likely due to its instability under high CO pressure.

β-Sheet crystals in natural silks are particulate and less than 10 nm in size in all three dimensions. In synthetic supramolecular analogues of natural silks, β-sheet crystals have been found to be fibrous with the longest dimension exceeding 100 nm in the hydrogen-bonding direction. This work demonstrates that particulate β-sheet crystals can be achieved without the use of an elaborate amino acid sequence by simply grafting oligo(β-alanine) segments as pendent side groups to a butyl rubber main chain. The size control in the hydrogen-bonding direction is attributable to an entropic force that opposes the driving force for the self-assembly. The nanocrystals, especially those of trimeric β-alanine segments, display a remarkable ability to simultaneously provide stiffness, extensibility, and strength to the synthetic elastic network and do so highly efficiently at a low volume fraction of the material. The herein studied butyl rubber-based thermoplastic elastomers containing no more than 3.6 vol % of β-sheet nanocrystals are stiffer, stronger, and more extensible than vulcanized butyl rubber reinforced by 20 vol % of carbon black and poly(styrene-b-isobutylene-b-styrene) reinforced by >33 vol % of polystyrene domains. The high reinforcing efficacy of the β-sheet crystals is attributable to two phenomena associated with their small sizes: a stick–slip mechanism for energy dissipation and an auxiliary layer of polymer brush that contributes to increasing the modulus.

A series of novel self-assembling star-blocks consisting of Mw = 29 000 g/mol 3-arm polyisobutylene (PIB) stars and oligo(β-alanine) end segments were synthesized and characterized. Star–blocks containing β-alanine dimers are viscous liquids, while those with tri-, tetra-, and penta(β-alanine)s are elastic solids. According to IR spectroscopy, the β-alanine dimer is partially hydrogen-bonded, while the trimer, tetramer, and pentamer are fully hydrogen-bonded and form β-sheets. DSC suggests crystalline β-alanine trimer tetramer and pentamer domains phase separated from the rubbery PIB. The melting temperature of the crystalline domains increases with the length of the oligo(β-alanine) segment. Transmission electron microscopy, wide-angle X-ray diffraction, and small-angle X-ray scattering of star-blocks containing tetra(β-alanine) indicate stacks of hydrogen-bonded β-sheets dispersed in a soft continuous PIB phase. The crystalline phases form fibrous lamellae with lengths up to ∼200 nm, widths up to ∼20 nm, and thicknesses of ∼2 nm, which is the length of β-alanine tetramer. Although the oligo(β-alanine) contents are very low (from 1.5 to 3.6 wt % in the series), the static and dynamic mechanical properties of the star–blocks are very different. The elastic moduli of the TPEs increase 5-fold with increasing β-alanine content. Evidently, the oligo(β-alanine) domains provide not only physical cross-links but also act as fillers.

A zwitterionic nickel(II) catalyst has been discovered to display an initial catalytic activity comparable to that of cationic palladium catalysts for alternating copolymerization of carbon monoxide and ethylene. This demonstrates the absence of a severe dormant state in the present zwitterionic system, in contrast to the cationic nickel(II) catalysts. However, the highly active catalyst is short-lived. Stoichiometric decomposition of the catalyst under carbon monoxide suggests that the insufficient stability of the tetraphenylborate motif in the ligand framework with respect to electrophilic attack is likely a culprit for catalyst deactivation.

Cobalt-catalyzed cyclization of CO, imine, and epoxide has been developed. A convenient catalyst system composed of Co2(CO)8 and LiCl is identified, and the substrate scope has been explored. The reaction provides an efficient method for the synthesis of substituted 1,3-oxazinan-4-ones.

A bidentate phosphine ligand with two amide arms, designed to form hydrogen bonds with electron-donating moieties, was synthesized and isolated in high diastereomeric excess (95% de). The hydrogen-bonding abilities of the ligand and its diastereomer were demonstrated with two rhodium complexes containing these ligands. The structures of the rhodium compounds are reminiscent of the well-studied picket-fence porphyrin systems.A bidentate phosphine ligand with two amide arms, designed to form hydrogen bonds with electron-donating moieties, was synthesized and isolated in high diastereomeric excess (95% de). The hydrogen-bonding abilities of the ligand and its diastereomer were demonstrated with two ligand-containing rhodium complexes. The structures of the rhodium compounds are reminiscent of the well-studied picket-fence porphyrin systems.

A new type of polymer highly resistant to nonspecific protein adsorption is reported. Poly(N-methyl-β-alanine) (PMeA) and poly(N-ethyl-β-alanine) (PEtA) synthesized via cobalt-catalyzed carbonylative polymerization of N-methylaziridine and N-ethylaziridine were end-functionalized with thiol groups and grafted onto Au surfaces. Protein adsorption was studied by the surface plasmon resonance (SPR) method. The amounts of representative single proteins adsorbed onto the PMeA- and PEtA-grafted surfaces were below the detection limit of SPR at the pg/mm2 level. After exposure to full blood plasma and serum for 10 min, protein adsorption was at the level of ∼100 pg/mm2, similar to the level of protein adsorption on poly(ethylene glycol) surfaces subjected to identical conditions. These poly(β-peptoid)s therefore provide excellent protein resistance comparable to the best antifouling materials known to date. The strong proton-accepting ability when forming hydrogen bonds is suggested to be an important attribute for these poly(β-peptoid)s as well as other poly(tertiary amide)s as antifouling materials.

The titled diblock copolymers are synthesized via cobalt-catalyzed living carbonylative polymerization of N-alkylaziridines under moderate pressures followed by a deprotection step. The poly(β-alanine) block is solubilized by the poly(β-alanoid) block in chloroform and remains fully hydrogen-bonded in the form of a sheet-like assembly.

A colloidal lithography method has been developed for patterning nonplanar surfaces. Hexagonal noncontiguously packed (HNCP) colloidal particles 127 nm−2.7 μm in diameter were first formed at the air−water interface and then adsorbed onto a substrate coated with a layer of polymer adhesive ∼17 nm thick. The adhesive layer plays the critical role of securing the order of the particles against the destructive lateral capillary force generated by a thin film of water after the initial transfer of the particles from the air−water interface. The soft lithography method is robust and very simple to carry out. It is applicable to a variety of surface curvatures and for both inorganic and organic colloidal particles.

Several electron-deficient polymers containing the 9,10-diboroanthracene unit have been synthesized and characterized. Electrochemical study shows that they have high electron affinities. Photoluminescence of rr-P3HT is quenched in the presence of one of the polymers in the solid state, demonstrating the potential utility of this class of polymer as powerful n-type materials in organic photovoltaic devices.

Hexagonal noncontiguously packed (HNCP) arrays of submicrometer-sized particles trapped at an air−water interface are successfully transferred to solid substrates. The long-range order of the hexagonal arrays at the interface can be improved by compression−relaxation cycles. The interparticle distance (i.e., the periodicity of the hexagonal array) can be controlled by varying the degree of compression of the particle film. The critical characteristics of the substrate surface are hydrophobicity (advancing water contact angle of >70°) and a charge complementary to the surface of the particles. Suitable silicon and glass substrates are easily prepared by treatment with commercially available organosilicon compounds. Two transfer processes have been developed. When the parallel transfer process is used, the HNCP arrays are deposited on the solid substrates with minimal pattern distortion. The vertical dipping transfer distorts the pattern and renders a sense of directionality perpendicular to the dipping direction. This surface patterning technique is applied to fabrication of subwavelength grating for antireflection in the visible region. Antireflective HNCP arrays comprising varied particle diameters and pattern periodicities are fabricated on glass substrates to demonstrate the effects of these parameters on the antireflection performance.

We provide a full account on our study of the cobalt-catalyzed carbonylative polymerization of N-alkylazetidines involving three representative monomers. The individual N-alkylazetidine monomers display different characteristics in the polymerization, allowing the incorporation of amine and ester units into the amide-based polymers. We will first present the synthesis and characterization of poly(amide-co-amine) with a gradient amine distribution. Then, we will describe how to control the ester distribution in poly(amide-co-ester)s. Poly(amide-co-ester)s containing multiple segments, within which the ester units distribute in a gradient fashion, will be compared with multiblock poly(amide-co-ester)s. Finally, we report our discovery of a novel chain transfer pathway via N-benzyl abstraction.

The titled diblock copolymers are synthesized via cobalt-catalyzed living carbonylative polymerization of N-alkylaziridines under moderate pressures followed by a deprotection step. The poly(β-alanine) block is solubilized by the poly(β-alanoid) block in chloroform and remains fully hydrogen-bonded in the form of a sheet-like assembly.