The Relationship between Catalyst Precursors and Chain End Groups in Homogeneous Propene Polymerization Catalysis

The chain transfer to monomer reactions promoted by primary and secondary growing chains in the propene polymerization promoted by ansa-zirconocenes and postmetallocene precursors are studied by using DFT methods. From the theoretical results it comes out that the prevalence of propene insertion over b-hydrogen transfer to the monomer decreases drastically in the presence of a secondary chain. Furthermore, we explained the reason why C2-symmetric metallocene catalysts promote the selective formation of cis but-2-enyls end group after a 2,1 inserted unit whereas for octahedral bis(phenoxy- imine)titanium-based catalysts, chain release promotes exclusively the formation of allyl terminated chain end. These results might be useful to design ligand precursors able to obtain not only high Mn PP polymers but also tuned chain end groups to build new polymer architectures. Overall, a more general picture of the enantioselectivity of the chain transfer to monomer processes is reported

The complete mitochondrial genome of Pseudocellus pearsei (Chelicerata: Ricinulei) and a comparison of mitochondrial gene rearrangements in Arachnida

<p>Abstract</p> <p>Background</p> <p>Mitochondrial genomes are widely utilized for phylogenetic and population genetic analyses among animals. In addition to sequence data the mitochondrial gene order and RNA secondary structure data are used in phylogenetic analyses. Arachnid phylogeny is still highly debated and there is a lack of sufficient sequence data for many taxa. Ricinulei (hooded tickspiders) are a morphologically distinct clade of arachnids with uncertain phylogenetic affinities.</p> <p>Results</p> <p>The first complete mitochondrial DNA genome of a member of the Ricinulei, <it>Pseudocellus pearsei </it>(Arachnida: Ricinulei) was sequenced using a PCR-based approach. The mitochondrial genome is a typical circular duplex DNA molecule with a size of 15,099 bp, showing the complete set of genes usually present in bilaterian mitochondrial genomes. Five tRNA genes (<it>trnW</it>, <it>trnY</it>, <it>trnN</it>, <it>trnL</it>(CUN), <it>trnV</it>) show different relative positions compared to other Chelicerata (e.g. <it>Limulus polyphemus</it>, <it>Ixodes </it>spp.). We propose that two events led to this derived gene order: (1) a tandem duplication followed by random deletion and (2) an independent translocation of <it>trnN</it>. Most of the inferred tRNA secondary structures show the common cloverleaf pattern except tRNA-Glu where the TψC-arm is missing. In phylogenetic analyses (maximum likelihood, maximum parsimony, Bayesian inference) using concatenated amino acid and nucleotide sequences of protein-coding genes the basal relationships of arachnid orders remain unresolved.</p> <p>Conclusion</p> <p>Phylogenetic analyses (ML, MP, BI) of arachnid mitochondrial genomes fail to resolve interordinal relationships of Arachnida and remain in a preliminary stage because there is still a lack of mitogenomic data from important taxa such as Opiliones and Pseudoscorpiones. Gene order varies considerably within Arachnida – only eight out of 23 species have retained the putative arthropod ground pattern. Some gene order changes are valuable characters in phylogenetic analysis of intraordinal relationships, e.g. in Acari.</p

High-Field 13C NMR Characterization of Ethene-1-13C/Propene Copolymers Prepared with Cs-Symmetric ansa-Metallocene Catalysts: A Deeper Insight into the Regio- and Stereoselectivity of Syndiotactic Propene Polymerization

In this paper, we report the results of a 150 MHz 13C NMR characterization of ethene-1- 13C/propene copolymers at low (<5 mol %) ethene content prepared in the presence of the syndiotacticselective ansa-metallocene catalyst (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (cocatalyst, MAO). In particular, from the fine structure of the resonances of the ethene-1-13C units we conclude that the enantioselectivity of 1,2-propene insertion is substantially lower and the probability of chain back-skip substantially higher after an ethene insertion than after a propene one. Moreover, we find that the regioirregular 2,1-propene units (whose concentration is higher than claimed in the literature) are also substantially stereoirregular

Sensing the underground – ultrastructure and function of sensory organs in root-feeding Melolontha melolontha (Coleoptera: Scarabaeinae) larvae

Eilers E, Talarico G, Hansson BS, Hilker M, Reinecke A. Sensing the underground – ultrastructure and function of sensory organs in root-feeding Melolontha melolontha (Coleoptera: Scarabaeinae) larvae. PLoS ONE. 2012;7(7): e41357.Introduction Below ground orientation in insects relies mainly on olfaction and taste. The economic impact of plant root feeding scarab beetle larvae gave rise to numerous phylogenetic and ecological studies. Detailed knowledge of the sensory capacities of these larvae is nevertheless lacking. Here, we present an atlas of the sensory organs on larval head appendages of Melolontha melolontha. Our ultrastructural and electrophysiological investigations allow annotation of functions to various sensory structures. Results Three out of 17 ascertained sensillum types have olfactory, and 7 gustatory function. These sensillum types are unevenly distributed between antennae and palps. The most prominent chemosensory organs are antennal pore plates that in total are innervated by approximately one thousand olfactory sensory neurons grouped into functional units of three-to-four. In contrast, only two olfactory sensory neurons innervate one sensillum basiconicum on each of the palps. Gustatory sensilla chaetica dominate the apices of all head appendages, while only the palps bear thermo-/hygroreceptors. Electrophysiological responses to CO2, an attractant for many root feeders, are exclusively observed in the antennae. Out of 54 relevant volatile compounds, various alcohols, acids, amines, esters, aldehydes, ketones and monoterpenes elicit responses in antennae and palps. All head appendages are characterized by distinct olfactory response profiles that are even enantiomer specific for some compounds. Conclusions Chemosensory capacities in M. melolontha larvae are as highly developed as in many adult insects. We interpret the functional sensory units underneath the antennal pore plates as cryptic sensilla placodea and suggest that these perceive a broad range of secondary plant metabolites together with CO2. Responses to olfactory stimulation of the labial and maxillary palps indicate that typical contact chemo-sensilla have a dual gustatory and olfactory function

Polymorphism and structural disorder in melt-crystallized and fiber samples of syndiotactic copolymers of propane with 1-butene

The structural characterization of melt-crystallized samples and oriented fibers of syndiotactic copolymers between propene and 1-butene is reported. Melt-crystallized samples of copolymers are crystallized in the form I of syndiotactic polypropylene (s-PP) up to a Content of 1-butene of 60-70 mol %, although disorder in the alternation of right- and left-handed helical chains along both axes of the unit cell is present. More ordered modifications, close to the limit ordered, fully antichiral, form I of s-PP are obtained by crystallization at high temperatures only for samples containing small contents of 1-butene (1-2 mol %). The presence of 1-butene prevents that the order in the alternation of right- and left-handed helical chains develops at high crystallization temperatures. Samples with contents of 1-butene higher than 70 mol % crystallize in structures similar to that of form I of syndiotactic poly(1-butene). Fiber samples of copolymers with small content of 1-butene (1-2 mol %) present the same behavior of syndiotactic polypropylene. Stretched fibers are in the trans planar form III of s-PP,which transforms into the isochiral helical form II of s-PP upon the release of the tension. Mixtures of crystals in forms I and II of s-PP are obtained by annealing. With increasing the 1-butene content only the antichiral helical form I of s-PP is observed in the stretched fibers of the copolymers, as well as upon the release of the tension. The presence of 1-butene units, for contents higher than 4 mol %, prevents the formation of the trans planar form III of s-PP by stretching. For these samples the formation of form I of s-PP, instead of the isochiral form II, either in the stretched fibers or upon the release of the tension, is a further evidence that the isochiral helical form II of s-PP can be obtained only from fibers initially in the trans planar form III, through a cooperative conformational transformation which induce the formation of helical chains having the same chirality

Structural Analysis of Copolymers of Syndiotactic Polypropylene with 13C-Enriched Ethylene

A structural analysis of copolymers of syndiotactic polypropylene with small amounts of 13 C-enriched ethylene, in the range 0.4-2.6 mol %, is reported. X-ray diffraction and solid-state C-13 NMR CPMAS data indicate that the as-prepared copolymer samples are crystallized in the conformationally disordered modification of form II of sPP containing kink bands. The disorder corresponds to the presence of portions of chains in the trans-planar conformation in chains having a prevailing 2-fold helical conformation. A direct evidence of the partial inclusion of the ethylene units in the crystalline regions of the copolymers is provided

Breaking Symmetry Rules Enhance the Options for Stereoselective Propene Polymerization Catalysis

An example of breaking "Ewen's symmetry rule" for olefin catalysis polymerization is proposed by DFT calculations. Catalyst precursors with Cs symmetry are suggested to promote the isotactic propene polymerization by a modification of the active site geometry obtained via coordination with AlH-alkyl species in solution. The origin of stereocontrol in olefin polymerization is due to a dual mechanism dictated by the chiral catalyst. These findings may expand the toolbox for promoting stereoselective olefin polymerization by transition metal catalysts

Melt-Crystallizations of α and γ Forms of Isotactic Polypropylene in Propene-Butene Copolymers

Random isotactic propene-butene copolymers (iPPC4) of different stereoregularity have been synthesized with three different homogeneous single center metallocene catalysts having different stereoselectivity. All samples crystallize from the polymerization solution in mixtures of α and γ forms, and the relative amount of γ form increases with increasing concentrations of butene and of rr stereodefects. All samples crystallize from the melt in mixtures of α and γ forms and the fraction of γ form increases with decreasing cooling rate. At high cooling rates, the crystallization of the α form is always favored, even for samples that contain high total concentration of defects that should crystallize in the γ form. The results demonstrate that in iPPs containing significant concentrations of defects, such as stereodefects and comonomeric units, the γ form is the thermodynamically stable form of iPP and crystallizes in selective conditions of very slow crystallization, whereas the α form is the kinetically favored form and crystallizes in conditions of fast crystallization

Ring-Opening Polymerization of L-Lactide Catalyzed by Potassium-Based Complexes: Mechanistic Studies

Two non-toxic potassium compounds, 1 and 2, with a commercial oximate ligand have been prepared and fully spectroscopically characterized. Their activity as catalysts for the ring-opening polymerization (ROP) process of LLA has been studied, showing that they are extremely active and able to polymerize the monomer in a few minutes. For derivative 2, the presence of a crown ether in the potassium coordination sphere affects the nuclearity of the compound and consequently its solubility, with both aspects having an influence in the polymerization process. Detailed studies of the polymerization mechanism have been performed, and an unusual anionic mechanism was observed in absence of a co-initiator. Indeed, the monomer deprotonation generates a lactide enolate, which initiates the polymerization propagation. On the contrary, when a 1:1 ratio of cat:BnOH is used, a mixture of mechanisms is observed, the anionic mechanism and the activated monomer one, while from a cat:BnOH ratio of 1:2 and over, only the activated monomer mechanism is observed