Poly(urethane-norbornene) Aerogels via Ring Opening Metathesis Polymerization of Dendritic Urethane-Norbornene Monomers: Structure-Property Relationships as a Function of an Aliphatic Versus an Aromatic Core and the Number of Peripheral Norbornene Moieties.

We report the synthesis and characterization of synthetic polymer aerogels based on dendritic-type urethane-norbornene monomers. The core of those monomers is based either on an aromatic/rigid (TIPM/Desmodur RE), or an aliphatic/flexible (Desmodur N3300) triisocyanate. The terminal norbornene groups (three at the tip of each of the three branches) were polymerized via ROMP using the inexpensive 1st generation Grubbs catalyst. The polymerization/gelation conditions were optimized by varying the amount of the catalyst. The resulting wet-gels were dried either from pentane under ambient pressure at 50 °C, or from t-butanol via freeze-drying, or by using supercritical fluid (SCF) CO2. Monomers were characterized with high resolution mass spectrometry (HRMS), ¹H- and solid-state 13C-NMR. Aerogels were characterized with ATR-FTIR and solid-state 13C-NMR. The porous network was probed with N2-sorption and SEM. The thermal stability of monomers and aerogels was studied with TGA, which also provides evidence for the number of norbornene groups that reacted via ROMP. At low densities (<0.1 g cm−3) all aerogels were highly porous (porosity > 90%), mostly macroporous materials; aerogels based on the aliphatic/flexible core were fragile, whereas aerogels containing the aromatic/rigid core were plastic, and at even lower densities (0.03 g cm−3) foamy. At higher densities (0.2⁻0.7 g cm−3) all materials were stiff, strong, and hard. At low monomer concentrations all aerogels consisted of discrete primary particles that formed spherical secondary aggregates. At higher monomer concentrations the structure consisted of fused particles with the size of the previous secondary aggregates, due to the low solubility of the developing polymer, which phase-separated and formed a primary particle network. Same-size fused aggregates were observed for both aliphatic and aromatic triisocyanate-derived aerogels, leading to the conclusion that it is not the aliphatic or aromatic core that determines phase separation, but rather the solubility of the polymeric backbone (polynorbornene) that is in both cases the same. The material properties were compared to those of analogous aerogels bearing only one norbornene moiety at the tip of each branch deriving from the same cores.

Exploring the Reactivity of Na[W2(µ-Cl)3Cl4(THF)2]∙(THF)3 towards the Polymerization of Selected Cycloolefins.

The bimetallic compound Na[W2(µ-Cl)3Cl4(THF)2]·(THF)3 (1, {W (3) W}(6+), a'2e'4) is a highly efficient room-temperature initiator for ring opening metathesis polymerization (ROMP) of norbornene (NBE) and some of its derivatives. In most cases, addition of phenylacetylene (PA) as co-initiator improves the catalytic activity and retains the high cis-stereoselectivity. On the other hand, 1 can polymerize cyclopentadiene (CPD), not via a metathetic, but rather, via a cationic mechanism. Here, we present a comparison of the reactivity of the two catalytic systems (1 and 1/PA) between themselves and with other systems reported in the literature, the characterization of the polymers formed and mechanistic aspects of the corresponding reactions.

Statistical Ring Opening Metathesis Copolymerization of Norbornene and Cyclopentene by Grubbs' 1st-Generation Catalyst.

Statistical copolymers of norbornene (NBE) with cyclopentene (CP) were prepared by ring-opening metathesis polymerization, employing the 1st-generation Grubbs' catalyst, in the presence or absence of triphenylphosphine, PPh3. The reactivity ratios were estimated using the Finemann-Ross, inverted Finemann-Ross, and Kelen-Tüdos graphical methods, along with the computer program COPOINT, which evaluates the parameters of binary copolymerizations from comonomer/copolymer composition data by integrating a given copolymerization equation in its differential form. Structural parameters of the copolymers were obtained by calculating the dyad sequence fractions and the mean sequence length, which were derived using the monomer reactivity ratios. The kinetics of thermal decomposition of the copolymers along with the respective homopolymers was studied by thermogravimetric analysis within the framework of the Ozawa-Flynn-Wall and Kissinger methodologies. Finally, the effect of triphenylphosphine on the kinetics of copolymerization, the reactivity ratios, and the kinetics of thermal decomposition were examined.

A versatile tripodal Cu(I) reagent for C-N bond construction via nitrene-transfer chemistry: catalytic perspectives and mechanistic insights on C-H aminations/amidinations and olefin aziridinations.

A Cu(I) catalyst (1), supported by a framework of strongly basic guanidinato moieties, mediates nitrene-transfer from PhI═NR sources to a wide variety of aliphatic hydrocarbons (C-H amination or amidination in the presence of nitriles) and olefins (aziridination). Product profiles are consistent with a stepwise rather than concerted C-N bond formation. Mechanistic investigations with the aid of Hammett plots, kinetic isotope effects, labeled stereochemical probes, and radical traps and clocks allow us to conclude that carboradical intermediates play a major role and are generated by hydrogen-atom abstraction from substrate C-H bonds or initial nitrene-addition to one of the olefinic carbons. Subsequent processes include solvent-caged radical recombination to afford the major amination and aziridination products but also one-electron oxidation of diffusively free carboradicals to generate amidination products due to carbocation participation. Analyses of metal- and ligand-centered events by variable temperature electrospray mass spectrometry, cyclic voltammetry, and electron paramagnetic resonance spectroscopy, coupled with computational studies, indicate that an active, but still elusive, copper-nitrene (S = 1) intermediate initially abstracts a hydrogen atom from, or adds nitrene to, C-H and C═C bonds, respectively, followed by a spin flip and radical rebound to afford intra- and intermolecular C-N containing products.

Radioactivity/Radionuclide (U-232 and Am-241) Removal from Waters by Polyurea-Crosslinked Alginate Aerogels in the Sub-Picomolar Concentration Range.

The removal of radionuclide/radioactivity from laboratory and environmental water samples under ambient conditions was investigated via batch-type experiments using polyurea-crosslinked calcium alginate (X-alginate) aerogels. Water samples were contaminated with traces of U-232 and Am-241. The removal efficiency of the material depends strongly on the solution pH; it is above 80% for both radionuclides in acidic solutions (pH 4), while it decreases at about 40% for Am-241 and 25% for U-232 in alkaline solutions (pH 9). This is directly associated with the presence of the radionuclide species in each case; the cationic species UO22+ and Am3+ prevail at pH 4, and the anionic species UO2(CO3)34- and Am(CO3)2- prevail at pH 9. Adsorption on X-alginate aerogels is realized by coordination of cationic species on carboxylate groups (replacing Ca2+) or other functional groups, i.e., -NH and/or -OH. In environmental water samples, i.e., ground water, wastewater and seawater, which are alkaline (pH around 8), the removal efficiency for Am-241 is significantly higher (45-60%) compared to that for U-232 (25-30%). The distribution coefficients (Kd) obtained for the sorption of Am-241 and U-232 by X-alginate aerogels are around 105 L/kg, even in environmental water samples, indicating a strong sorption affinity of the aerogel material for the radionuclides. The latter, along with their stability in aqueous environments, make X-alginate aerogels attractive candidates for the treatment of radioactive contaminated waters. To the best of our knowledge, this is the first study on the removal of americium from waters using aerogels and the first investigation of adsorption efficiency of an aerogel material at the sub-picomolar concentration range.

Uranium Removal from Aqueous Solutions by Aerogel-Based Adsorbents-A Critical Review.

Aerogels are a class of lightweight, nanoporous, and nanostructured materials with diverse chemical compositions and a huge potential for applications in a broad spectrum of fields. This has led the IUPAC to include them in the top ten emerging technologies in chemistry for 2022. This review provides an overview of aerogel-based adsorbents that have been used for the removal and recovery of uranium from aqueous environments, as well as an insight into the physicochemical parameters affecting the adsorption efficiency and mechanism. Uranium removal is of particular interest regarding uranium analysis and recovery, to cover the present and future uranium needs for nuclear power energy production. Among the methods used, such as ion exchange, precipitation, and solvent extraction, adsorption-based technologies are very attractive due to their easy and low-cost implementation, as well as the wide spectrum of adsorbents available. Aerogel-based adsorbents present an extraordinary sorption capacity for hexavalent uranium that can be as high as 8.8 mol kg−1 (2088 g kg−1). The adsorption data generally follow the Langmuir isotherm model, and the kinetic data are in most cases better described by the pseudo-second-order kinetic model. An evaluation of the thermodynamic data reveals that the adsorption is generally an endothermic, entropy-driven process (ΔH0, ΔS0 > 0). Spectroscopic studies (e.g., FTIR and XPS) indicate that the adsorption is based on the formation of inner-sphere complexes between surface active moieties and the uranyl cation. Regeneration and uranium recovery by acidification and complexation using carbonate or chelating ligands (e.g., EDTA) have been found to be successful. The application of aerogel-based adsorbents to uranium removal from industrial processes and uranium-contaminated waste waters was also successful, assuming that these materials could be very attractive as adsorbents in water treatment and uranium recovery technologies. However, the selectivity of the studied materials towards hexavalent uranium is limited, suggesting further developments of aerogel materials that could be modified by surface derivatization with chelating agents (e.g., salophen and iminodiacetate) presenting high selectivity for uranyl moieties.

Polylactide-Grafted Metal-Alginate Aerogels.

Τhis work describes the synthesis of PLA-grafted M-alginate (g-M-alginate; M: Ca2+, Co2+, Ni2+, Cu2+) aerogels. DL-lactide (LA) was attached on the surface of preformed M-alginate beads and was polymerized, using stannous octoate as catalyst and the -OH groups of the alginate backbone as initiators/points of attachment. The material properties of g-M-alginate aerogels were not affected much by grafting, because the linear PLA chains grew on the M-alginate framework like a brush and did not bridge their points of attachment as in polyurea-crosslinked M-alginate aerogels. Thus, all g-M-alginate aerogels retained the fibrous morphology of their parent M-alginate aerogels, and they were lightweight (bulk densities up to 0.24 g cm-3), macroporous/mesoporous materials with high porosities (up to 96% v/v). The BET surface areas were in the range of 154-542 m2 g-1, depending on the metal, the nature of the alginate framework and the PLA content. The latter was found at about 15% w/w for Ca- and Ni-based materials and at about 29% w/w for Co- and Cu-based materials. Overall, we have demonstrated a new methodology for the functionalization of alginate aerogels that opens the way to the synthesis of polylactide-crosslinked alginate aerogels with the use of multifunctional monomers.

Efficient Removal of Polyvalent Metal Ions (Eu(III) and Th(IV)) from Aqueous Solutions by Polyurea-Crosslinked Alginate Aerogels.

The removal of polyvalent metal ions Eu(III) and Th(IV) from aqueous solutions using polyurea-crosslinked calcium alginate (X-alginate) aerogels has been investigated by batch-type experiments under ambient conditions and pH 3. The material presents relatively high sorption capacity for Eu(III) (550 g kg-1) and Th(IV) (211 g kg-1). The lower sorption capacity for Th(IV) compared to Eu(III) is attributed to the net charge of the dominant species in solution under the given experimental conditions, which is Eu3+ for Eu(III), and Th(OH)22+ and Th(OH)3+ for Th(IV). Generally, the sorption is an endothermic and entropy-driven process, and it follows the Langmuir isotherm model. According to the FTIR spectra, sorption occurs via formation of inner-sphere complexes between the surface functional groups and the f-metal cationic species. The presence of europium and thorium in the adsorbent material was confirmed and quantified with EDS analysis. To the best of our knowledge, this is the first report of an aerogel material used as an adsorbent for Eu(III). Compared to other materials used for the sorption of the specific ions, which are mostly carbon-based, X-alginate aerogels show by far the highest sorption capacity. Regarding Th(IV) uptake, X-alginate aerogels show the highest capacity per volume (27.9 g L-1) among the aerogels reported in the literature. Both Eu(III) and Th(IV) could be recovered from the beads by 65% and 70%, respectively. Furthermore, Th(VI) could also be quantitatively removed from wastewater, while Eu(III) could be removed by 20%. The above, along with their stability in aqueous environments, make X-alginate aerogels attractive candidates for water treatment and metal recovery applications.

Noninvasive Detection, Tracking, and Characterization of Aerogel Implants Using Diagnostic Ultrasound.

Medical implants are routinely tracked and monitored using different techniques, such as MRI, X-ray, and ultrasound. Due to the need for ionizing radiation, the two former methods pose a significant risk to tissue. Ultrasound imaging, however, is non-invasive and presents no known risk to human tissue. Aerogels are an emerging material with great potential in biomedical implants. While qualitative observation of ultrasound images by experts can already provide a lot of information about the implants and the surrounding structures, this paper describes the development and study of two simple B-Mode image analysis techniques based on attenuation measurements and echogenicity comparisons, which can further enhance the study of the biological tissues and implants, especially of different types of biocompatible aerogels.

Evaluation of Polyurea-Crosslinked Alginate Aerogels for Seawater Decontamination.

Polyurea-crosslinked Ca-alginate (X-Ca-alginate) aerogel beads (diameter: 3.3 mm) were evaluated as adsorbents of metal ions, organic solvents, and oils. They were prepared via reaction of an aromatic triisocyanate (Desmodur RE) with pre-formed Ca-alginate wet gels and consisted of 54% polyurea and 2% calcium. X-Ca-alginate aerogels are hydrophobic nanoporous materials (90% v/v porosity), with a high BET surface area (459 m2/g-1), and adsorb PbII not only from ultrapure water (29 mg/g-1) but also from seawater (13 mg/g-1) with high selectivity. The adsorption mechanism involves replacement of CaII by PbII ions coordinated to the carboxylate groups of the alginate backbone. After treatment with a Na2EDTA solution, the beads can be reused, without significant loss of activity for at least two times. X-Ca-alginate aerogels can also uptake organic solvents and oil from seawater; the volume of the adsorbate can be as high as the total pore volume of the aerogel (6.0 mL/g-1), and the absorption is complete within seconds. X-Ca alginate aerogels are suitable for the decontamination of aquatic environments from a broader range of inorganic and organic pollutants.

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents.

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co(II) sites (S = 3/2) feature a minimal [N3N] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative L4Co catalyst (-COCMe3 arm) is operating faster than the L8Co analogue (-COCF3 arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C b atom of styrene, followed by product-determining ring-closure), the L4Co catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [L4Co]NTs and [L8Co]NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for L4Co (due to the polarizability of the tert-Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

Synthesis and characterization of a series of structurally and electronically diverse Fe(II) complexes featuring a family of triphenylamido-amine ligands.

A family of triphenylamido-amine ligands of the general stoichiometry L(x)H(3) = [R-NH-(2-C(6)H(4))](3)N (R = 4-t-BuPh (L(1)H(3)), 3,5-t-Bu(2)Ph (L(2)H(3)), 3,5-(CF(3))(2)Ph (L(3)H(3)), CO-t-Bu (L(4)H(3)), 3,5-Cl(2)Ph (L(5)H(3)), COPh (L(6)H(3)), CO-i-Pr (L(7)H(3)), COCF(3) (L(8)H(3)), and i-Pr (L(9)H(3))) has been synthesized and characterized, featuring a rigid triphenylamido-amine scaffold and an array of stereoelectronically diverse aryl, acyl, and alkyl substituents (R). These ligands are deprotonated by potassium hydride in THF or DMA and reacted with anhydrous FeCl(2) to afford a series of ferrous complexes, exhibiting stoichiometric variation and structural complexity. The prevalent [(L(x))Fe(II)-solv](-) structures (L(x) = L(1), L(2), L(3), L(5), solv = THF; L(x) = L(8), solv = DMA; L(x) = L(6), L(8), solv = MeCN) reveal a distorted trigonal bipyramidal geometry, featuring ligand-derived [N(3,amido)N(amine)] coordination and solvent attachment trans to the N(amine) atom. Specifically for [(L(8))Fe(II)-DMA](-), a N(amido) residue is coordinated as the corresponding N(imino) moiety (Fe-N(Ar) horizontal lineC(CF(3))-O(-)). In contrast, compounds [(L(4))Fe(II)](-), [(L(6))(2)Fe(II)(2)](2-), [K(L(7))(2)Fe(II)(2)](2)(2-), and [K(L(9))Fe](2) are all solvent-free in their coordination sphere and exhibit four-coordinate geometries of significant diversity. In particular, [(L(4))Fe(II)](-) demonstrates coordination of one amidato residue via the O-atom end (Fe-O-C(t-Bu) horizontal lineN(Ar)). Furthermore, [(L(6))(2)Fe(II)(2)](2-) and [K(L(7))(2)Fe(II)(2)](2)(2-) are similar structures exhibiting bridging amidato residues (Fe-N(Ar)-C(R) horizontal lineO-Fe) in dimeric structural units. Finally, the structure of [K(L(9))Fe](2) is the only example featuring a minimal [N(3,amido)N(amine)] coordination sphere around each Fe(II) site. All compounds have been characterized by a variety of physicochemical techniques, including Mossbauer spectroscopy and electrochemistry, to reveal electronic attributes that are responsible for a range of Fe(II)/Fe(III) redox potentials exceeding 1.0 V.

Isolation, characterization, and computational studies of the novel [Mo3(mu3-Br)2(mu-Br)3Br6]2- cluster anion.

The novel trimolybdenum cluster [Mo(3)(mu(3)-Br)(2)(mu-Br)(3)Br(6)](2-) (1, {Mo(3)}(9+), 9 d-electrons) has been isolated from the reaction of [Mo(CO)(6)] with 1,2-C(2)H(4)Br(2) in refluxing PhCl. The compound has been characterized in solution by electrospray ionization mass spectrometry (ESI-MS), UV-vis spectroscopy, cyclic voltammetry, and in the solid state by X-ray analysis (counter-cations: (n-Bu)(4)N(+) (1), Et(4)N(+), Et(3)BzN(+)), electron paramagnetic resonance (EPR), magnetic susceptibility measurements, and infrared spectroscopy. The least disordered (n-Bu)(4)N(+) salt crystallizes in the monoclinic space group C2/c, a = 20.077(2) A, b = 11.8638(11) A, c = 22.521(2) A, alpha = 90 deg, beta = 109.348(4) deg, gamma = 90 deg, V = 5061.3(9) A(3), Z = 4 and contains an isosceles triangular metal arrangement, which is capped by two bromine ligands. Each edge of the triangle is bridged by bromine ions. The structure is completed by six terminal bromine ligands. According to the magnetic measurements and the EPR spectrum the trimetallic core possesses one unpaired electron. Electrochemical data show that oxidation by one electron of 1 is reversible, thus proceeding with retention of the trimetallic core, while the reduction is irreversible. The effective magnetic moment of 1 (mu(eff), 1.55 mu(B), r.t.) is lower than the spin-only value (1.73 mu(B)) for S = 1/2 systems, most likely because of high spin-orbit coupling of Mo(III) and/or magnetic coupling throughout the lattice. The ground electronic state of 1 was studied using density functional theory techniques under the broken symmetry formalism. The ground state is predicted to exhibit strong antiferromagnetic coupling between the three molybdenum atoms of the core. Moreover, our calculated data predict two broken symmetry states that differ only by 0.4 kcal/mol (121 cm(-1)). The antiferromagnetic character is delocalized over three magnetic orbitals populated by three electrons. The assignment of the infrared spectra is also provided.

Large, Rapid Swelling of High-cis Polydicyclopentadiene Aerogels Suitable for Solvent-Responsive Actuators.

High-cis polydicyclopentadiene (PDCPD) aerogels were synthesized using ring opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD) with a relatively air-stable ditungsten catalytic system, Na[W2(-Cl)3Cl4(THF)2]·(THF)3 (W2; (W 3 W)6+, a΄2e΄4), and norbornadiene (NBD)as a co-initiator. These aerogels are compared in terms of chemical structure and material properties with literature PDCPD aerogels obtained using well-established Ru-based alkylidenes as catalysts. The use of NBD as a co-initiator enhances the degree of crosslinking versus the more frequently used phenylacetylene (PA), yielding materials with a controlled molecular structure that would persist solvent swelling. Indeed, those PDCPD aerogels absorb selected organic solvents (e.g., chloroform, tetrahydrofuran) and swell rapidly, in some cases up to 4 times their original volume within 10 min, thus showing their potential for applications in chemical sensors and solvent-responsive actuators. The advantage of aerogels versus xerogels or dense polymers for these applications is their open porosity, which provides rapid access of the solvent to their interior, thus decreasing the diffusion distance inside the polymer itself, which in turn accelerates the response to the solvents of interest.

A Cellular Automata Approach for the Modeling of a Polyamide and Carbon Aerogel Structure and Its Properties.

In this work, a cellular automata (CA) approach was used to generate 3D structures of polyamide and carbon aerogels. Experimental results are used as initial data for materials' digital representations and to verify the developed CA models. Based on the generated digital structures, a computer study of aerogels' mechanical properties was conducted. The offered CA models can be applied for the development of new nanoporous materials such as aerogels of different nature and allow for a reduction in the amount of required full-scale experiments, consequently decreasing development time and costs of new material formulations.

Synthetic Polymer Aerogels in Particulate Form.

Aerogels have been defined as solid colloidal or polymeric networks of nanoparticles that are expanded throughout their entire volume by a gas. They have high surface areas, low thermal conductivities, low dielectric constants, and high acoustic attenuation, all of which are very attractive properties for applications that range from thermal and acoustic insulation to dielectrics to drug delivery. However, one of the most important impediments to that potential has been that most efforts have been concentrated on monolithic aerogels, which are prone to defects and their production requires long and costly processing. An alternative approach is to consider manufacturing aerogels in particulate form. Recognizing that need, the European Commission funded "NanoHybrids", a 3.5 years project under the Horizon 2020 framework with 12 industrial and academic partners aiming at aerogel particles from bio- and synthetic polymers. Biopolymer aerogels in particulate form have been reviewed recently. This mini-review focuses on the emerging field of particulate aerogels from synthetic polymers. That category includes mostly polyurea aerogels, but also some isolated cases of polyimide and phenolic resin aerogels. Particulate aerogels covered include powders, micro granules and spherical millimeter-size beads. For the benefit of the reader, in addition to the literature, some new results from our laboratory concerning polyurea particle aerogels are also included.

Metalloradical complexes of manganese and chromium featuring an oxidatively rearranged ligand.

Redox events involving both metal and ligand sites are receiving increased attention since a number of biological processes direct redox equivalents toward functional residues. Metalloradical synthetic analogues remain scarce and require better definition of their mode of formation and subsequent operation. The trisamido-amine ligand [(RNC6H4)3N]3-, where R is the electron-rich 4-t-Bu Ph, is employed in this study to generate redox active residues in manganese and chromium complexes. Solutions of [(L1)Mn(II)-THF]- in THF are oxidized by dioxygen to afford [(L1re-1)Mn(III)-(O)2-Mn(III)(L1 re-1)]2-as the major product. The rare dinuclear manganese (III,III) core is stabilized by a rearranged ligand that has undergone an one-electron oxidative transformation, followed by retention of the oxidation equivalent as a pi radical in ano-diiminobenzosemiquinonate moiety. Magnetic studies indicate that the ligand-centered radical is stabilized by means of extended antiferromagnetic coupling between the S ) 1/2 radical and the adjacent S ) 2 Mn(III) site, as well as between the two Mn(III) centers via the dioxo bridge. Electrochemical and EPR data suggest that this system can store higher levels of oxidation potency. Entry to the corresponding Cr(III) chemistry is achieved by employing CrCl3 to access both[(L1)Cr(III)-THF] and [(L1re-1)Cr(III)-THF(Cl)], featuring the intact and the oxidatively rearranged ligands, respectively. The latter is generated by ligand-centered oxidation of the former compound. The rearranged ligand is perceived to be the product of an one-electron oxidation of the intact ligand to afford a metal-bound aminyl radical that subsequently mediates a radical 1,4-(N-to-N) aryl migration.

Millimeter-Size Spherical Polyurea Aerogel Beads with Narrow Size Distribution.

We report the room temperature synthesis of spherical millimeter-size polyurea (PUA) aerogel beads. Wet-gels of said beads were obtained by dripping a propylene carbonate solution of an aliphatic triisocyanate based on isocyanurate nodes into a mixture of ethylenediamine and heavy mineral oil. Drying the resulting wet spherical gels with supercritical fluid (SCF) CO2 afforded spherical aerogel beads with a mean diameter of 2.7 mm, and a narrow size distribution (full width at half maximum: 0.4 mm). Spherical PUA aerogel beads had low density (0.166 ± 0.001 g cm⁻3), high porosity (87% v/v) and high surface area (197 m² g⁻1). IR, ¹H magic angle spinning (MAS) and 13C cross-polarization magic angle spinning (CPMAS) NMR showed the characteristic peaks of urea and the isocyanurate ring. Scanning electron microscopy (SEM) showed the presence of a thin, yet porous skin on the surface of the beads with a different (denser) morphology than their interior. The synthetic method shown here is simple, cost-efficient and suitable for large-scale production of PUA aerogel beads.

Poly(Urethane-Acrylate) Aerogels via Radical Polymerization of Dendritic Urethane-Acrylate Monomers.

The purpose of this work was to investigate the effect of multifunctionality on material properties of synthetic polymer aerogels. For this purpose, we present the synthesis and characterization of monolithic dendritic-type urethane-acrylate monomers based on an aliphatic/flexible (Desmodur N3300), or an aromatic/rigid (Desmodur RE) triisocyanate core. The terminal acrylate groups (three at the tip of each of the three branches, nine in total) were polymerized with 2,2'-azobis(isobutyronitrile) (AIBN) via free radical chemistry. The resulting wet-gels were dried with supercritical fluid (SCF) CO2. Aerogels were characterized with ATR-FTIR and solid-state 13C NMR. The porous network was probed with N2-sorption and scanning electron microscopy (SEM). The thermal stability of aerogels was studied with thermogravimetric analysis (TGA). Most aerogels were macroporous materials (porosity > 80%), with high thermal stability (up to 300 °C). Aerogels were softer at low monomer concentrations and more rigid at higher concentrations. The material properties were compared with those of analogous aerogels bearing only one acrylate moiety at the tip of each branch and the same cores, and with those of analogous aerogels bearing norbornene instead of acrylate moieties. The nine-terminal acrylate-based monomers of this study caused rapid decrease of the solubility of the growing polymer and made possible aerogels with much smaller particles and much higher surface areas. For the first time, aliphatic/flexible triisocyanate-based materials could be made with similar properties in terms of particle size and surface areas to their aromatic/rigid analogues. Finally, it was found that with monomers with a high number of crosslinkable groups, material properties are determined by multifunctionality and thus aerogels based on 9-acrylate- and 9-norbornene-terminated monomers were similar. Materials with aromatic cores are carbonizable with satisfactory yields (20⁻30% w/w) to mostly microporous materials (BET surface areas: 640⁻740 m² g-1; micropore surface areas: 360⁻430 m² g-1).

Copolymerization of Norbornene and Norbornadiene Using a cis-Selective Bimetallic W-Based Catalytic System.

The bimetallic cluster Na[W2(µ-Cl)3Cl4(THF)2]·(THF)3 ({W2}, {W 3 W}6+, a'²e'4), which features a triple metal-metal bond, is a highly efficient room-temperature initiator for ring opening metathesis polymerization (ROMP) of norbornene (NBE) and norbornadiene (NBD), providing high-cis polymers. In this work, {W2} was used for the copolymerization of the aforementioned monomers, yielding statistical poly(norbornene)/poly(norbornadiene) PNBE/PNBD copolymers of high molecular weight and high-cis content. The composition of the polymer chain was estimated by 13C CPMAS NMR data and it was found that the ratio of PNBE/PNBD segments in the polymer chain was relative to the monomer molar ratio in the reaction mixture. The thermal properties of all copolymers were similar, resembled the properties of PNBD homopolymer and indicated a high degree of cross-linking. The morphology of all materials in this study was smooth and non-porous; copolymers with higher PNBE content featured a corrugated morphology. Glass transition temperatures were lower for the copolymers than for the homopolymers, providing a strong indication that those materials featured a branched-shaped structure. This conclusion was further supported by viscosity measurements of copolymers solutions in THF. The molecular structure of those materials can be controlled, potentially leading to well-defined star polymers via the "core-first" synthesis method. Therefore, {W2} is not only a cost-efficient, practical, highly active, and cis-stereoselective ROMP-initiator, but it can also be used for the synthesis of more complex macromolecular structures.