Versatile Production of Multivariate, Hyperdimensional End Group and Main Chain Functionalized Polyolefins.

The (stereoselective) living coordinative copolymerization of 1-alkenes with 4-aryl-1,6-heptadienes, in both the absence and presence of multiple equivalents of a reversible chain transfer agent, is established as a highly versatile strategy for production of multivariate hyperdimensional functionalized semi-crystalline or amorphous polyolefins that optionally possess either mono- or difunctionalized (telechelic) end-groups in combination with a programmable level of incorporation of orthogonal functional groups within the main-chain. The non-conjugated diene comonomers are readily obtained from a diverse range of aryl carboxaldehyde precursors through a one-step bis-allylation process. These results serve to provide a new platform for exploring the science and technology of a vast new landscape of functionalized classes of polyolefins that are now accessible in practical and scalable quantities.

Rapid Phase Transitions of Thermotropic Glycolipid Quasicrystal and Frank-Kasper Mesophases: A Mechanistic Rosetta Stone.

Experimental results are presented that serve to lower the barrier for developing the science and technology of non-classical thermotropic glycolipid mesophases, which now include dodecagonal quasicrystal (DDQC) and Frank-Kasper (FK) A15 and σ mesophases that can be produced under mild conditions from a versatile class of sugar-polyolefin conjugates. By employing "alloys" comprised of mono- and disaccharide-polyolefin conjugates, and optionally with vitamin E as a small molecule phase modulator, we report the spontaneous formation of stable A15 mesophases at ambient temperature. We further document a rich thermotropic phase map that includes DDQC, A15, and σ mesophases of tunable periodicity that are connected through rapid thermotropic phase transitions as a function of increasing temperature in the order: liquid-like packing (LLP)→DDQC → A15→σ→ disorder. This first direct observation of a rapid thermotropic A15→σ phase transition provides support for a diffusionless martensitic process proceeding through strain-induced introduction of planar defects into the A15 lattice.

Tailoring Glass Transition Temperature in a Series of Poly(methylene-1,3-cyclopentane-stat-cyclohexane) Statistical Copolymers.

A systematic investigation of the synthesis and characterization of a new class of amorphous atactic cis, trans poly(methylene-1,3-cyclopentane-stat-cyclohexane) statistical copolymers (I) is reported. Production of different grades of I that vary with respect to the ratio of 5- and 6-membered cycloalkane repeat units was achieved through the living coordinative chain transfer cyclopolymerization of different initial feed ratios of 1,5-hexadiene and 1,6-heptadiene comonomers. It was determined that the glass transition temperature, Tg, of I can be systematically increased from -16 to 100 °C as a function of increasing 6-membered ring content, although not in a strictly linear fashion. It was further determined that a small level of 6-membered ring content is sufficient to disrupt the crystallinity of the limiting atactic cis, trans poly(methylene-1,3-cyclopentane) (PMCP) homopolymer that possesses a melting temperature, Tm, of 98 °C. These results establish a foundation for future potential technological applications of this unique class of polyolefin copolymers.

Stereomodulation of Poly(4-methyl-1-pentene): Adoption of a Neglected and Misunderstood Commercial Polyolefin.

A toolbox of living coordination polymerization methods based on CPAM group 4 metal initiators has been applied for the stereomodulation of poly(4-methyl-1-pentene) (PMP) tacticity to provide new fundamental forms in which thermal phase transitions, Tg and Tm , can be adjusted over a wide range. Also reported for the first time are the synthesis and properties of authenticated 1,2-regioregular atactic PMP, which serves to correct past errors regarding this fundamental polyolefin.

Highly Versatile Strategy for the Production of Telechelic Polyolefins.

A general and versatile synthetic strategy for producing practical quantities of a wide range of phenyl-group-terminated hetero- and homotelechelic semicrystalline polyethenes and amorphous atactic and semicrystalline isotactic poly(α-olefins) is reported. The phenyl groups serve as synthons for functionalities of additional classes of telechelic polyolefins that can be "unmasked" through simple high yielding postpolymerization reactions. A demonstration of the value of these materials as building blocks for structural classes of polyolefin-based synthetic polymers was provided by syntheses of well-defined polyolefin-polyester di- and triblock copolymers that were shown to adopt microphase-segregated nanostructured mesophases in the condensed phase.

Temporal Control over Two- and Three-State Living Coordinative Chain Transfer Polymerization for Modulating the Molecular Weight Distribution Profile of Polyolefins.

A highly versatile new strategy for manipulating the molecular weight profiles, including breadth, asymmetry (skewness) and modal nature (mono-, bi-, and multimodal), of a variety of different polyolefins is reported. It involves temporal control over two- and three-state living coordinative chain transfer polymerization (LCCTP) of olefins in a programmable way. By changing the identity of the R' groups of the chain transfer agent, ER'n , with time, different populations of chains within a bi- or multimodal polyolefin product can be selectively tagged with different end-groups. By changing the nature of the main-group metal of the CTA, programmed manipulation of the relative magnitudes of the dispersities of the different maxima that make up the final MWD profile can be achieved. This strategy can be implemented with existing LCCTP materials and conventional reactor methods to provide access to scalable and practical quantities of an unlimited array of new polyolefin materials.

Stable Thermotropic 3D and 2D Double Gyroid Nanostructures with Sub-2-nm Feature Size from Scalable Sugar-Polyolefin Conjugates.

Ultra-low molecular weight disaccharide-polyolefin conjugates with cellobiose, lactose and maltose head groups and atactic polypropene tails, such as 1, undergo a series of irreversible thermotropic order-order transitions with increasing temperature to provide nanostructured phases in the sequence: lamellar (L), hexagonal perforated lamellar (HPL), double gyroid (DG) and hexagonal cylindrical (C). The DG phase displays exceptional stability at ambient temperature and features two interpenetrating sugar domain networks having a sub-2-nm strut width and a lattice parameter, aDG , of 13.1 nm. The unique stability of this DG phase extends further within ultrathin films all the way down to the two-dimensional limit of 15 nm in which film thickness, l, is now less than the surface-oriented unit cell height, hDG . In addition to raising the fundamental question of what minimally constitutes a Schoen triply periodic minimal surface and DG lattice, these results serve to establish the class of sugar-polyolefin conjugates as a new material platform for nanoscience and nanotechnology.

Small-Molecule Modulation of Soft-Matter Frank-Kasper Phases: A Method for Adding Function to Form.

Incorporation of small amounts of α-tocopherol (vitamin E) in blends with the cellobiose-triazole-linked atactic poly(4-methyl-1-pentene) (CB-aPMP) sugar-polyolefin conjugate can be used to exert external control over thermotropic phase behavior and provide access to non-canonical soft matter Frank-Kasper A15 and σ phases. These results establish a paradigm that can be used for the further design and development of scalable quantities of soft matter FK phases of increased structural complexity and functional capability.

An Exceptionally Stable and Scalable Sugar-Polyolefin Frank-Kasper A15 Phase.

"One-component" soft material Frank-Kasper (FK) phases are an intriguing structural form of matter that possess periodically ordered structures arising from the self-reconfiguration and close packing of an initial assembly of identical "deformable" spheres into two or more size- or shape-distinct sets of particles. Significant challenges that must still be addressed to advance the field of soft matter FK phases further, however, include their rare and unpredictable occurrence, uncertain mechanisms of solid-state assembly, and low thermodynamic stability. Here we show that a readily-accessible sugar-polyolefin conjugate quantitatively produces an exceptionally stable solid-state FK A15 phase through a rapid and irreversible thermotropic order-order transition, which contrary to other prevailing proposed mechanisms, does not require mass transfer between particles or large structural reorganization in the bulk to establish unit cell non-equivalency. Our results provide the basis for a realistic strategy for obtaining practical and scalable quantities of a diverse range of sugar-polyolefin FK A15 phases with unique intrinsic physical properties and chemical reactivities not previously seen in such systems.

Closing the Loop on Transition-Metal-Mediated Nitrogen Fixation: Chemoselective Production of HN(SiMe3)2 from N2, Me3SiCl, and X-OH (X = R, R3Si, or Silica Gel).

Treatment of the Mo(IV) terminal imido complex, (η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo(NSiMe3) (3), with a 1:2 mixture of iPrOH and Me3SiCl resulted in the rapid formation of the Mo(IV) dichloride, (η5-C5Me5)[N(Et)C(Ph)N(Et)]MoCl2 (1), and the generation of 1 equiv each of HN(SiMe3)2 and iPrOSiMe3. Similarly, a 1:2 mixture of Me3SiOH and Me3SiCl provided 1, HN(SiMe3)2, and O(SiMe3)2. Finally, silica gel, when coupled with excess equivalents of Me3SiCl, was also effectively used as the X-OH reagent for the generation of 1 and HN(SiMe3)2. A proposed mechanism for the 3 → 1 transformation involves formal addition of HCl across the Mo═N imido bond through initial hydrogen-bonding between X-OH and the N-atom of 3 to form the adduct IIIb, followed by chloride delivery from Me3SiCl to the metal center via a six-membered transition state (IV) that leads to the intermediate, (η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo(Cl)(NHSiMe3) (V), and XOSiMe3 as a co-product. Metathetical exchange of the new Mo-N amido bond of V by a second equivalent of Me3SiCl then generates 1 and HN(SiMe3). These results serve to complete a highly efficient chemical cycle for nitrogen fixation that is mediated by a set of well-characterized transition-metal complexes.

Dynamic Sub-10-nm Nanostructured Ultrathin Films of Sugar-Polyolefin Conjugates Thermoresponsive at Physiological Temperatures.

Spin-casting of a cellobiose-atactic polypropene (CB-aPP) conjugate (1) from a 0.1% (w/w) n-butanol/hexane solution onto highly oriented pyrolytic graphite (HOPG) and carbon-coated Si(100) spontaneously produced microphase-separated sub-10-nm nanostructured ultrathin films in the form of alternating CB and aPP lamellar domains (d = 6.60 ± 0.68 nm) that are oriented perpendicular to the substrate surface. Thermal annealing at modest temperatures (e.g., 50-100 °C), and as low as the physiologically relevant temperature of 38 °C, serves to drive a structural transition that yields a parallel stacked bilayer assembly as the thermodynamically favored nanostructure. These results establish the advantage of low molecular weight, narrow polydispersity, and amorphous, low Tg, poly(α-olefinate)s (xPAOs) as a new class of hydrophobic building block for amphiphilic materials, and sugar-PAO conjugates in particular, for the development of stimuli-responsive, nanostructured materials for technological applications at physiological temperatures.

Steric Switching from Photochemical to Thermal Reaction Pathways for Enhanced Efficiency in Metal-Mediated Nitrogen Fixation.

Programmed manipulation of the subtle interplay of nonbonded steric interactions within a supporting ligand environment has been used for the conversion of a photochemically driven chemical cycle for group 6 metal-mediated nitrogen fixation into a thermally promoted process with increased energy efficiency and atom economy for key transformations involving N≡N bond cleavage and N-atom functionalization of coordinated N2.

End-Group-Functionalized Poly(α-olefinates) as Non-Polar Building Blocks: Self-Assembly of Sugar-Polyolefin Hybrid Conjugates.

Living coordinative chain-transfer polymerization of α-olefins, followed by chemical functionalization of a Zn(polymeryl)2 intermediate, provides entry to end-group functionalized poly(α-olefinates) (x-PAOs) that can serve as a new class of non-polar building block with tailorable occupied volumes. Application of these x-PAOs for the synthesis and self-assembly of sugar-polyolefin hybrid conjugates demonstrate the ability to manipulate the morphology of the ultra-thin film nanostructure through variation in occupied volume of the x-PAO domain.

Metal-Mediated Production of Isocyanates, R3 EN=C=O from Dinitrogen, Carbon Dioxide, and R3 ECl.

A highly efficient and versatile chemical cycle has been developed for the production of isocyanates through the molecular fixation of N2 , CO2 and R3 ECl (E=C, Si, and Ge). Key steps include a 'one-pot' photolytic N-N bond cleavage of a Group 6 dinuclear dinitrogen complex with in situ trapping by R3 ECl to provide a metal terminal imido complex that can engage in simultaneous nitrene-group transfer and oxygen-atom transfer to generate an intermediate metal terminal oxo complex with release of the isocyanate product. Reaction of the oxo complex with additional equivalents of R3 ECl regenerates a metal dichloride that is the precursor for dinuclear dinitrogen starting material.

Metal-catalyzed "on-demand" production of carbonyl sulfide from carbon monoxide and elemental sulfur.

The group 6 molybdenum(II) cyclopentadienyl amidinate (CPAM) bis(carbonyl) complex [Cp*Mo{N(iPr)C(Ph)N(iPr)}(CO)2] (Cp*=η(5)-C5Me5) serves as a precatalyst for the high-yielding photocatalytic production of COS from CO and S8 under near-ambient conditions (e.g., 10 psi, 25 °C). Further documented is the isolation and structural characterization of several key transition-metal intermediates which collectively support a novel molybdenum(IV)-based catalytic cycle as being operative. Finally, in the presence of an excess amount of a primary amine, it is demonstrated that this catalytic system can be successfully used for the "on-demand" generation and utilization of COS as a chemical reagent for the synthesis of ureas.

De Novo Design of a New Class of "Hard-Soft" Amorphous, Microphase-Separated, Polyolefin Block Copolymer Thermoplastic Elastomers.

Sequential cyclic/linear/cyclic living coordination polymerization of 1,6-heptadiene (HPD), propene, and HPD, respectively, employing the well-defined and soluble group 4 transition-metal initiator, {(η5-C5Me5)Hf(Me)[N(Et)C(Me)N(Et)]}[B(C6F5)4], provides the stereoirregular, amorphous poly(1,3-methylenecyclohexane)-b-atactic polypropene-b-poly(1,3-methylenecyclohexane) (PMCH-b-aPP-b-PMCH) polyolefin triblock copolymer (I) in excellent yield. By varying the weight fraction of the end group, minor component "hard" PMCH block domains, fPMCH, relative to that of the midblock "soft" aPP domain, three different compositional grades of these polyolefin block copolymers, Ia-c, were prepared and shown by AFM and TEM to adopt microphase-separated morphologies in the solid state, with spherical and cylindrical morphologies being observed for fPMCH = 0.09 (Ia) and 0.23 (Ic), respectively, and a third more complex morphology being observed for Ib (fPMCH = 0.17). Tensile testing of Ia-c served to establish these materials as a new structural class of polyolefin thermoplastic elastomers, with Ia being associated with superior elastic recovery (94 ± 1%) after each of several stress-strain cycles.

Fine-tuning the energy barrier for metal-mediated dinitrogen N≡N bond cleavage.

Experimental data support a mechanism for N≡N bond cleavage within a series of group 5 bimetallic dinitrogen complexes of general formula, {Cp*M[N((i)Pr)C(R)N((i)Pr)]}2(µ-N2) (Cp* = η(5)-C5Me5) (M = Nb, Ta), that proceeds in solution through an intramolecular "end-on-bridged" (µ-η(1):η(1)-N2) to "side-on-bridged" (µ-η(2):η(2)-N2) isomerization process to quantitatively provide the corresponding bimetallic bis(µ-nitrido) complexes, {Cp*M[N((i)Pr)C(R)N((i)Pr)](µ-N)}2. It is further demonstrated that subtle changes in the steric and electronic features of the distal R-substituent, where R = Me, Ph and NMe2, can serve to modulate the magnitude of the free energy barrier height for N≡N bond cleavage as assessed by kinetic studies and experimentally derived activation parameters. The origin of the contrasting kinetic stability of the first-row congener, {Cp*V[N((i)Pr)C(Me)N((i)Pr)]}2(µ-η(1):η(1)-N2) toward N≡N bond cleavage is rationalized in terms of a ground-state electronic structure that favors a significantly less-reduced µ-N2 fragment.

Regio- and Stereospecific Cyclopolymerization of Bis(2-propenyl)diorganosilanes and the Two-State Stereoengineering of 3,5-cis,isotactic Poly(3,5-methylene-1-silacyclohexane)s.

Transition-metal-mediated coordination cyclopolymerization of bis(2-propenyl)dimethylsilane (1a) using the C1-symmetric, group 4 metal preinitiator, (η5-C5Me5)Zr(Me)2[N(Et)C(Me)N(tBu)] (I), in combination with 1 equiv of the borate coinitiator, [PhNHMe2][B(C6F5)4] (II), proceeds in a regio- and stereospecific manner to provide highly stereoregular 3,5-cis,isotactic poly(3,5-methylene-1,1-dimethyl-1-silacyclohexane) (2a). Successful stereoengineering of 2a to eliminate undesirable crystallinity while preserving a high Tg value of >120 °C was subsequently accomplished by employing a "two-state" propagation system that uniquely produces an isotactic stereoblock microstructure of decreasing stereoblock length with decreasing percent level of "activation" of I with II. The controlled character of cyclopolymerization of 1a using the less sterically encumbered preinitiator, (η5-C5Me5)Hf(Me)2[N(Et)C(Me)N(Et)] (III), and 1 equiv of II was used to prepare well-defined poly(1-hexene)-b-poly(3,5-methylene-1-silacyclohexane) block copolymers through sequential monomer additions.

N-N bond cleavage of mid-valent Ta(IV) hydrazido and hydrazidium complexes relevant to the Schrock cycle for dinitrogen fixation.

Chemical reduction of the Ta(V) hydrazido chloride 1 generates the open-shell, mononuclear Ta(IV) hydrazido complex 2, which upon N-methylation yields the corresponding structurally characterized Ta(IV) hydrazidium 6. Chemical reduction of 6 results in N-N bond cleavage to generate a cis/trans mixture of the [Ta(V),Ta(V)] bis(µ-nitrido) product 7 in tetrahydrofuran and the mononuclear Ta(V) parent imide 8 in toluene. These results serve to establish an important foundation for the pursuit of a group-5 metal variant of the Schrock cycle for dinitrogen fixation.

Stereoengineering of poly(1,3-methylenecyclohexane) via two-state living coordination polymerization of 1,6-heptadiene.

External control over the rate of dynamic methyl group exchange between configurationally stable active species and configurationally unstable dormant species with respect to chain-growth propagation within a highly stereoselective and regiospecific living coordination polymerization of 1,6-heptadiene has been used to generate a spectrum of different physical forms of poly(1,3-methylenecyclohexane) (PMCH) in which the stereochemical microstructure has been systematically varied between two limiting forms. The application of this strategy to manipulate the bulk properties of PMCH and the solid-state microphase behavior of well-defined PMCH-b-poly(1-hexene) block copolymers is further demonstrated.