Modulating Thermostability and Productivity of Benzhydryl-Substituted Bis(imino)pyridine-Iron C2H4 Polymerization Catalysts through ortho -CnH2n−1 (n=5, 6, 8, 12) Ring Size Adjustment

Four types of benzhydryl-containing bis(arylimino)pyridine-iron(II) chloride complex, [2-{CMeN(2,4,6-Me3C6H2)}-6-{CMeN(Ar)}C5H3N]FeCl2 (Ar=2,4-(CHPh2)2-6-(C5H9)C6H2 Fe1, 2,4-(CHPh2)2-6-(C6H11)C6H2 Fe2, 2,4-(CHPh2)2-6-(C8H15)C6H2 Fe3 and 2,4-(CHPh2)2-6-(C12H23)C6H2 Fe4), discriminable by the ring size of the ortho-CnH2n−1 (n=5, 6, 8, 12) group, have been prepared in good yield (>85 %) via the reaction of the corresponding N,N,N′-ligand, L1–L4, with ferrous chloride tetrahydrate. All new complexes were characterized by FT-IR spectroscopy and elemental analysis. The molecular structure of Fe3 emphasizes the distorted pseudo-square pyramidal geometry bestowed to the metal center and the steric unevenness provided by the inequivalent N-mesityl and N-2,4-dibenzyhydryl-6-cyclooctylphenyl groups. Extremely high activities for ethylene polymerization were achieved at temperatures between 60 °C and 70 °C following pre-treatment of the iron(II) precatalyst with either MMAO or MAO, with the relative performance being: Fe2>Fe1>Fe3>Fe4. Notably, ortho-cyclohexyl Fe2, under activation with MMAO, afforded the highest level of performance of this study reaching 2.82×107 g PE per mol (Fe) per h at 70 °C forming strictly linear and narrowly dispersed polyethylene (Mw/Mn=2.0) with moderate molecular weight (11.3 kg per mol). Furthermore, runs performed using Fe1/MMAO at 90 °C saw the catalytic activity drop by around only 40 % highlighting the remarkable thermostability of these catalysts. By comparison, polymerizations performed using MAO as co-catalyst were less controlled with broader dispersities (Mw/Mn range: 4.8–10.1), while the molecular weights were generally higher (55.7–150.9 kg per mol).</p

Ring size enlargement in an ortho-cycloalkyl-substituted bis(imino)pyridine-cobalt ethylene polymerization catalyst and its impact on performance and polymer properties

Four unsymmetrical examples of bis(arylimino)pyridines that each possess an ortho-cycloalkyl substituent of unique ring size, 2-{CMeN(2,4,6-Me3C6H2)}-6-{CMeN(2,4-(CHPh2)2-6-RC6H2)}C5H3N (R = cyclopentyl L1, cyclohexyl L2, cyclooctyl L3, cyclododecyl L4), have been prepared and employed as reactants in the formation of the corresponding cobaltous chloride complexes, (N,N,N′)CoCl2 (Co1–Co4). Structural characterization of Co2 and Co3 spotlights the five-coordinate geometry and the steric disparity imposed on the metal center by the inequivalent N-mesityl and N-2,4-dibenzyhydryl-6-cycloalkylphenyl groups. Co1–Co4 all proved productive precatalysts for ethylene polymerization achieving optimal performance at a temperature of 60°C on activation with either MAO or MMAO. Cyclopentyl-containing Co1 displayed the highest level (up to 10.3 × 106 g PE per mol [Co] per h), whereas its cyclododecyl counterpart Co4 the lowest (down to 0.14 × 106 g PE per mol [Co] per h). Strictly linear polyethylenes were produced with molecular weights spanning the range 22.0–36.0 kg per mol for Co1–Co4/MAO and 22.6–33.6 kg per mol for Co1–Co4/MMAO, with cyclohexyl Co2 producing the highest values with either activator. With the exception of the polymer produced using Co4, all catalysts afforded narrow unimodal distributions (Mw/Mn range: 1.7–2.2) for the polyethylenes in line with single site active species. By contrast, Co4 bearing the largest and conformationally flexible cycloalkyl group formed polyethylenes that were distinctly bimodal, which would imply the presence of two active species. End-group analysis of lower molecular weight polymer samples identified vinyl groups congruent with a termination step involving β-H elimination

Exceptionally high molecular weight linear polyethylene by using N,N,N′-Co catalysts appended with a N′-2,6-bis{di(4-fluorophenyl)methyl}-4-nitrophenyl group

The bis(arylimino)pyridines, 2-[CMeN{2,6-{(4-FC6H4)2CH}2–4-NO2}]-6-(CMeNAr)C5H3N (Ar = 2,6-Me2C6H3 L1, 2,6-Et2C6H3 L2, 2,6-i-Pr2C6H3 L3, 2,4,6-Me3C6H2 L4, 2,6-Et2–4-MeC6H2 L5), each containing one N′-2,6-bis{di(4-fluorophenyl)methyl}-4-nitrophenyl group, have been synthesized by two successive condensation reactions from 2,6-diacetylpyridine. Their subsequent treatment with anhydrous cobalt (II) chloride gave the corresponding N,N,N′-CoCl2 chelates, Co1 – Co5, in excellent yield. All five complexes have been characterized by 1H/19F NMR and IR spectroscopy as well as by elemental analysis. In addition, the molecular structures of Co1 and Co3 have been determined and help to emphasize the differences in steric properties imposed by the inequivalent N-aryl groups; distorted square pyramidal geometries are adopted by each complex. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), precatalyts Co1 – Co5 collectively exhibited very high activities for ethylene polymerization with 2,6-dimethyl-substituted Co1 the most active (up to 1.1 × 107 g (PE) mol−1 (Co) h−1); the MAO systems were generally more productive. Linear polyethylenes of exceptionally high molecular weight (Mw up to 1.3 × 106 g mol−1) were obtained in all cases with the range in dispersities exhibited using MAO as co-catalyst noticeably narrower than with MMAO [Mw/Mn: 3.55–4.77 (Co1 – Co5/MAO) vs. 2.85–12.85 (Co1 – Co5/MMAO)]. Significantly, the molecular weights of the polymers generated using this class of cobalt catalyst are higher than any literature values reported to date using related N,N,N-bis (arylimino)pyridine-cobalt catalysts

Phenoxy-imine/-amide aluminum complexes with pendant or coordinated pyridine moieties: Solvent effects on structural type and catalytic capability for the ROP of cyclic esters

Depending on the solvent employed, the dimeric aluminum phenoxyamide complexes [2-O,4-R4C6H3CHMeN(3′-R1,4′-R2,5′-R3C5HN)]2Al2Me2 (R1 = Me, R2 = R3 = R4 = H Al1; R2 = Me, R1 = R3 = R4 = H Al2; R3 = Me, R1 = R2 = R4 = H Al3; R1 = R2 = R3 = R4 = H Al4; R1 = Me, R4 = OMe, R2 = R3 = H Al5) or their monoaluminum phenoxyimine counterparts, [2-O-C6H4CH = N(3′-R1,4′-R2,5′-R3C5HN)]AlMe2 (R1 = Me, R2 = R3 = H Al6; R2 = Me, R1 = R3 = H Al7; R3 = Me, R1 = R2 = H Al8; R1 = R2 = R3 = H Al9), were obtainable by the treatment of the corresponding 2-pyridyl substituted salicylaldimine pro-ligand with AlMe3. Structural characterization of Al1 – Al4 highlights the Npy,N,O-chelation and bridging capacity of the dianionic pyridyl substituted phenoxyamide ligand. By contrast, the monoanionic phenoxyimine ligand in Al8 serves as an N,O-bidentate ligand with the Npy unit pendant. In the presence of benzyl alcohol (BnOH), all nine complexes exhibited high efficiency for the ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL), in which the activity displayed by dinuclear Al1 – Al5 in general exceeding that seen by mononuclear Al6 – Al9. Analysis of the polycaprolactone (PCL) generated using Al1/BnOH by 1H NMR spectroscopy and MALDI-TOF mass spectrometry showed the polymer to adopt mainly a linear structure with BnO groups constituting the end groups. By contrast, when Al1 was used in the absence of BnOH, the PCL was mainly cyclic in nature. For the ROP of L-LA or rac-LA good efficiency was again achieved albeit at a lower level than that seen for ϵ-CL. In common with that seen with ϵ-CL, the amount of BnOH employed proved crucial in determining both the linearity and end group composition of the polylactide (PLA)

Optimizing planting density for production of high-quality apple nursery stock in China

<div><p>In China, apple (<i>Malus</i> × <i>domestica</i> Borkh.) nursery stock is generally of low quality because of extremely high planting density. The objective of this study was to determine the optimum planting density of 2-year-old grafted apple trees. Tree growth (height, trunk diameter, leaf area index) increased as density decreased. Trees grown at high densities (14.3–50 plants/m²) were the shortest with the smallest trunk diameters and leaf areas, whereas trees grown at lower densities (4.8–10 plants/m²) were generally largest in terms of height, diameter and leaf area. Trees grown at lower densities tended to have higher bud dry weight, leaf dry weight, nitrogen content, total soluble sugar concentration and total non-structural carbohydrate content. Higher levels of these parameters were generally observed with tree densities at or below 10 plants/m². Therefore we conclude that 10 plants/m² is the optimum density for maximizing the number of trees produced per unit land area while maintaining tree quality of nursery stock.</p></div

Manganese(I)-catalyzed asymmetric (transfer) hydrogenation of ketones: An insight into the effect of chiral PNN and NN ligands

A new type of (RC,SP)-1-(2-diphenylphosphino)ferrocenylethylamine N-substituted with a (RC)-5,6,7,8-tetrahydroquinolinyl group (LPNN-1) was successfully employed as a chiral chelating ligand in both Mn-catalyzed asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH) of a broad range of ketonic substrates (39 examples), leading to high conversions and excellent enantioselectivities for their product alcohols. In particular, PNN-pincer complex Mn-1 and its NN-bidentate analogue Mn-2 have been isolated and their comparative performance as catalysts studied with Mn-1 proving more effective in both ATH and AH. Moreover, Mn-1 generally imparted higher degrees of enantiomeric excess (ee) in both hydrogenation processes which can reach up to 99% in ATH and 93% in AH for propiophenone-type substrates. DFT calculations highlight the importance of π-π interactions and steric hindrance between catalyst and substrate which manifests itself in enhancements in ee for propiophenone over acetophenone substrates. Furthermore, a possible mechanism for the Mn-catalyzed ATH has been proposed on the basis of a joint DFT and experimental investigation.</p

A surface-tethered dopant method to achieve 3D control over the growth of a nanometers-thin and intrinsically transparent polypyrrole film

The electrochemical growth of conductive polymer films is a convenient synthesis route but challenging to control due to local variability in the reaction kinetics. Here we report a new method for electropolymerizing highly reproducible conductive polypyrrole films that are just nanometers thick, highly conductive and possess intrinsic optical transparencies comparable to ITO. The synthesis method utilizes a surface-tethered dopant molecule, in this case a self-assembled monolayer of the highly anionic protein lubricin (LUB), to template and thus control the 3-dimensional growth of the polypyrrole when the electrochemical polymerization reaction is performed in a pyrrole monomer solution containing no additional dopant molecules or ions. Because the tethered dopant controls where and how much polypyrrole growth occurs, this method effectively decouples the fine film morphology, thickness, and spatial-growth from the polymerization reaction kinetics and represents a paradigm shift in the electrochemical polymerization of conductive polymer films