Routes to covalent catalysis by reactive selection for nascent protein nucleophiles

Reactivity-based selection strategies have been used to enrich combinatorial libraries for encoded biocatalysts having revised substrate specificity or altered catalytic activity. This approach can also assist in artificial evolution of enzyme catalysis from protein templates without bias for predefined catalytic sites. The prevalence of covalent intermediates in enzymatic mechanisms suggests the universal utility of the covalent complex as the basis for selection. Covalent selection by phosphonate ester exchange was applied to a phage display library of antibody variable fragments (scFv) to sample the scope and mechanism of chemical reactivity in a naive molecular library. Selected scFv segregated into structurally related covalent and noncovalent binders. Clones that reacted covalently utilized tyrosine residues exclusively as the nucleophile. Two motifs were identified by structural analysis, recruiting distinct Tyr residues of the light chain. Most clones employed Tyr32 in CDR-L1, whereas a unique clone (A.17) reacted at Tyr36 in FR-L2. Enhanced phosphonylation kinetics and modest amidase activity of A.17 suggested a primitive catalytic site. Covalent selection may thus provide access to protein molecules that approximate an early apparatus for covalent catalysis.     

Role of ion-pair equilibria in homogeneous Ziegler-Natta olefin polymerization catalysis

By means of a multinuclear NMR study of the complexes formed between AlCl₃ and either Cp₂TiCl₂ or Cp₂Ti(CH₂SiMe₃)Cl in methylene chloride solution, isomeric forms of the resulting 1:1 complexes have been detected. The influence of temperature, concentration, ratio of the titanocene chloride to aluminum chloride and nature of the solvent upon the ¹H, ¹³C and ²⁷Al NMR spectra has been investigated. The spectral changes caused by the foregoing factors give compelling evidence for a equilibrium in such Cp₂Ti(R)Cl · AlCl₃ complexes (R = Cl, CH₂SiMe₃) between contact ion pairs, Cp₂TiR · Cl · AlCl₃, and solvent-separated ion pairs, Cp₂TiR⁺ AlCl₄⁻. Upon experimental variations in temperature, concentration, solvent and ratio of R_nAlCl_3−n to the titanium catalyst, the polymerization activity of the catalyst system towards ethylene was markedly altered. Such changes in activity support the conclusion that the most active sites for polymerization in such systems are the solvent-separated ion pairs.     

Stereochemical regulation of polypropylenes prepared with various Ziegler-Natta catalysts

The 220-MHz proton magnetic resonance and infrared spectra of stereoregular polypropylenes polymerized with a number of Ziegler-Natta catalysts and isotactic polymers of low molecular weight obtained by thermal degradation of a highly isotactic polypropylene were measured in an attempt to obtain some information on the local regularity. The fraction of thermally degraded polymer soluble in diethyl ether shows stereorandomness (tactie sequence length is quite short), and the portion soluble in n-pentane has stereoblock character. The results so obtained provide strong evidence that racemic dyads of whole polymer consist of two models of racemic dyad isolated and racemic dyads in groups. The polymers prepared with vanadium catalyst systems show stereorandom character and these polymers have \documentclass{article}\pagestyle{empty}\begin{document}$\hbox{-\hskip-1pt-}\hskip-4pt({\rm CH}_2 \rlap{--} )$\end{document} groups formed by two propylene units in a tail-to-tail linkage. Syndiotactic polypropylene has head-to-head and tail-to-tail arrangements of two propylene units and this is the origin of randomness of syndiotacticity.     

Functionalized polyolefins prepared by the combination of borane monomers and transition metal catalysts

This paper summarizes the new developments in the functionalization of polyolefins by borane monomers and transition metal catalysts. It is becoming clear that the borane groups are stable to transition metal catalysts and borane groups in polyolefins are valuable intermediates which not only can be quantitatively converted to a wide range of functional groups but also can be used as initiator for free radical graft-from copolymerizations. Some interesting functionalized polymers, such as polyethylene (PE) and polypropene (PP) with -OH, -NH₂ and halide groups, and graft copolymers of polypropene-graft-poly(methyl methacrylate) (PP-g-PMMA), polypropene-graft-polycapolactone (PP-g-PCL) and PE-g-PMMA, have been synthesized with controllable compositions and molecular microstructures. Most of them would be otherwise very difficult to prepare by the other existing methods.     

Morphology of polytetrafluoroethylene prepared by radiation-induced emulsion polymerization

The original morphology of polytetrafluoroethylene prepared by radiation-induced emulsion polymerization was studied by electron microscopy. The morphology depends on molecular weight, which in turn depends on polymerization conditions, especially the emulsifier concentration. The molecular weight decreases with increasing emulsifier concentration. The morphology changes with molecular weight roughly as follows: fibrils below 10⁵, rods between 10⁵ and 5 × 10⁵, and granular particle above 10⁶. The crystallinity is high for all morphologies.     

Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler-Natta catalysis

The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler- Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back-skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.     

Physically crosslinked elastomers prepared from oligomeric polyolefins with mesogenic units

Two ABA‐type liquid crystalline oligomers were newly synthesized, where A was a mesogenic group and B was polyolefin whose molecular mass was 2470. The A segment was prepared from p‐hydroxyl benzoic acid and terephalic acid. The elastomeric films, whose moduli at 20% elongation were 0.4–1.0 MPa, were obtained by solution casting of the ABA‐type oligomers. Dynamic mechanical analysis and differential scanning calorimetry measurement showed the glass transition of amorphous polyolefin segments, the melting of mesogenic groups, and the meso‐to‐isotropic transition of liquid crystalline phase. The formation of microphase‐separated structures was confirmed by a small‐angle X‐ray scattering (SAXS) measurement. The presence of hexagonal cylinder domains, which were attributed to the aggregation of mesogenic groups in the polyolefin matrix, was also detected by SAXS. These liquid crystalline oligomers showed anisotropy under the crossed Nicoles, and the textures were observed to be nematic. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2247–2253, 2000     

Morphology study of Nafion membranes prepared by solutions casting

The structures of Nafion membranes prepared by solutions casting from low aliphatic alcohols/water mixture solvents and N,N′‐dimethyl formamide (DMF) solvent were investigated using differential scanning calorimeter and small angle X‐ray scattering. The aggregation behavior of Nafion molecules in the casting solutions was also investigated using dynamic light scattering. We show that the morphology of membranes was strongly influenced by the conformations of Nafion molecules in the solutions. In aliphatic alcohol/water mixture solvents, which have a worse compatibility with Nafion backbones, the Nafion molecules aggregate and form fringed rod‐like structures. These primary rod‐like structures then aggregate again through fringed side chains to form secondary ionic aggregations. In DMF solvent, owing to its better compatibility with Nafion backbones, less Nafion molecules aggregate. The high degree of Nafion molecular aggregations in aliphatic alcohol/water mixture solvents leads to a high degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/aliphatic alcohol/water solutions. However, the lower degree of molecular aggregations in DMF solvent results in a lower degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/DMF solution. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3044–3057, 2005     

Morphology and melting behavior of nascent ultra-high molecular weight polyethylene

The nascent morphology of UHMW PE exhibits high melting point, high crystallinity, and increased WAXS line breadth relative to samples formed by melt crystallization. Different empirical relationships between crystal size and melting point are observed for nascent and molded samples. This differentiation is removed following nitric acid treatment of the nascent flake. Solid-state annealing behavior is differentiated by several regimes. Regime I is characterized by increasing crystallite dimensions and crystallinity at low annealing temperatures. Regime II[a] and II[b] is identified by double melting in DSC scans of moldings and nascent flake, respectively. The double melting is due to partial melting with incomplete recrystallization. Regime II[a] of moldings is differentiated from Regime II[b] of flake by an increase in melting point of the higher melting endotherm. Within Regime II[b], the partial melting of the nascent structure is sensitive to the distribution of morphological stability. Regime III is initiated at annealing temperatures approaching the zero heating rate melting point, and shows melting kinetics by DSC or time-resolved WAXS using synchrotron x-ray radiation. The superheat, partially associated with Regime III behavior, is sensitive to morphological heterogeneity and annealing history. Morphological models are discussed which highlight the role of noncrystalline regions and melting kinetics on the melting behavior of nascent form crystallinity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 495–517, 1998     

Morphology of optically anisotropic agarose hydrogel prepared by directional freezing

Agarose hydrogels which showed optical anisotropy were obtained by the directional freezing of starting isotropic gels under a temperature gradient. The directional freezing caused a crystallization of many isolated ice crystal phases, leaving a honeycomb-like gel phase with a higher polymer content. The crystallographic c-axis of the ice crystals was directed to the temperature gradient. X-ray and optical analyses showed that agarose chains had a strong planar orientation along the walls'side surfaces, which were parallel to the equatorial planes of the ice crystals.Scanning electron microscopy showed that the wall consisted of a large number of sheets stacked along the wall thickness; in each sheet, agarose fibrillar structures were found to be densely aligned. With the application of repeated freezing and thawing, the anisotropy of the segregated gel phases increased.     

Theoretical investigations on Ziegler-Natta catalysis: Alkylation of the TiCl4 catalyst

Theoretical ab initio methods have been used to study the alkylation reaction of the MgCl₂-supported TiCl₄ catalyst. Investigation of the reaction path indicated that the Al(CH₃)₃ co-catalyst inserts into the coordination sphere of the titanium atom of the catalyst so that a methyl group of Al(CH₃)₃ interacts with the titanium. The methyl group migrates from Al(CH₃)₃ to the vacant coordination site of the catalyst. When the catalyst loses one of its chlorine atoms to the co-catalyst during this reaction, the catalyst maintains its vacant site, but the position of the vacant site changes. The presence of the external electron donor coordinated to the co-catalyst makes the alkylation reaction energetically more favourable.     

Functionalization of polyolefins by reactive processing

From the study of the modification of PE and PP with maleic anhydride, maleic acid and an acid salt of maleic acid (sodium maleate) in a liquid or solid state, it was shown that the main controlling factor in the solid state modification of polyolefins is the joint plastic deformation of the system components. The changes of molecular characteristics were investigated by gel permeation chromatography. It was shown, that the molecular weight of modified PP samples increases at times by linking with intermolecular associates in the solutions. The formation of associates plays essential role in the behaviour of solutions of modified PP. The morphology of modified PP samples was investigated by scanning electron microscopy. It was shown, that intermolecular association strongly influences on the processes of formation and growth of nucleus and the structure of generated polymeric materials.     

Morphological characterization of poly(phenylacetylene) nanospheres prepared by homogeneous and heterogeneous catalysis

Scanning electron microscopy characterization of the materials obtained by homogeneous and heterogeneous catalytic polymerization of phenylacetylene is described. The catalysts used are β‐dioxygenato rhodium(I) complexes. The effects of the reaction medium, presence of a cocatalyst and the type of catalysis (homogeneous or heterogeneous) on the morphology of the polymers obtained have been studied and discussed. Using a supported complex at 0 °C, nanoparticles with a diameter distribution as narrow as 30 to 70 nm were obtained. Polymer nanopowders were found to be unaffected by ageing. Copyright © 2003 John Wiley & Sons, Ltd.     

Characterizations of lanthanum trivalent ions/TiO2 nanopowders catalysis prepared by plasma spray

Lanthanum trivalent ions (La(3+)) doped titanium dioxide (TiO(2)) nanopowders in the range of 20-60 nm were prepared successfully by plasma spray in the self-developed plasma spray equipment. The photocatalytic activity of samples at different doping concentrations in photocatalytic degradation of methyl orange was discussed. The nanopowders prepared were characterized by transmission electron microscopy, X-ray diffraction, ultraviolet-visible spectra, photoluminescence (PL) and X-ray photoelectron spectroscopy. The results show that La(3+) doping increased the photocatalytic activity of TiO(2) greatly, the optimal doping concentration was 0.5 at%. The La(3+) doping decreases the particle size and the distribution of particle sizes becomes more uniform. The doped powders were the mixture of anatase and rutile phase. The contents of anatase phase decreased firstly and then increased with an increase in the contents of La(3+). The intrinsic absorption band of La(3+) doped TiO(2) nanopowders appears red shift from that of pure TiO(2) nanopowders. The intensity of PL spectra increases and then decreases with increasing the content of La(3+). The PL spectral intensity reaches its peak when the ratio of La(3+)/TiO(2) is 0.2 at%. There are O, Ti, C and La elements in the prepared La(3+) doped TiO(2) nanopowders, La element still exists in trivalent and Ti element always exists in tetravalent.     

Degenerative transfer living Ziegler-Natta polymerization: application to the synthesis of monomodal stereoblock polyolefins of narrow polydispersity and tunable block length

The neutral dimethyl pentamethylcyclopentadienylzirconium acetamidinate, (eta(5)-C(5)Me(5))ZrMe(2)[N(t-Bu)C(Me)N(Et)], can serve as a highly active initiator for the living Ziegler-Natta polymerization of alpha-olefins to produce polyolefins of narrow polydispersity (D < or = 1.05) when "activated" through mono demethylation by a substoichiometric amount of the borate, [PhNMe(2)H][B(C(6)F(5))(4)]. The mechanism by which this living polymerization proceeds is through a process of degenerative transfer involving rapid and reversible methyl group exchange between cationic (active) zirconium propagating centers and neutral (dormant) methyl, polymeryl zirconium end groups. Facile metal-centered epimerization of the dormant species is responsible for a loss of stereocontrol during propagation that produces iso-rich material in contrast to the pure isotactic polymer microstructure obtained when degenerative transfer is not present. By turning degenerative transfer "on" and "off" between successive monomer polymerizations, a successful strategy for the production of monomodal stereoblock polyolefins of narrow polydispersity and tunable block length has been demonstrated.     

Morphology of calcium carbonate prepared via homogeneous synthesis

The morphology of calcium carbonate prepared via homogeneous synthesis from carbonate–chloride solutions was studied. The precipitates were investigated by scanning electron microscopy (SEM) and Xray diffraction. The effects of the ratio between the chloride and carbonate components of a solution on the morphology, crystal structure, and particle size of precipitated calcium carbonate were illustrated.