Search

Stoichiometric reactions of YCl3(THF)3withpotassium 2-((arylimino)methyl)quinolin-8-olates or 2-(1-(arylimino)ethyl)quinolin-8-olatesin THF solution gave the mononuclear LYCl2(DMSO)2complexes 1–5in the presence ofDMSO and a representative dinuclear complex 6in theabsence of DMSO. All yttrium complexes were fully characterized byNMR measurements and elemental analysis, and the crystal structuresof complexes 1and 4–6were determined by single-crystal X-ray diffraction. The structuresindicate coordination number seven around the yttrium center and pentagonalbipyramidal geometries. The complexes all feature diapical YCl2moieties and one tridentate organic ligand in the equatorialplane. Upon reaction of the yttrium precatalysts 1–6with LiCH2Si(CH3)3aloneor with LiCH2Si(CH3)3together withBnOH, the ring-opening polymerization (ROP) of ε-caprolactone(ε-CL) occurred with high efficiency. Depending on conditions,the ROP of ε-CL produced polycaprolactone with narrow moleculardistribution and in a living manner. Theoretical studies of the chlorine/CH2SiMe3and Me3SiCH2/BnO ligandexchange reactions suggest that the replacement of the apical ligandscan proceed without significantly affecting the equatorial ligands.These results suggest that one of the apical Y–CH2SiMe3bonds within the LY(CH2SiMe3)2intermediate catalyzes the polymerization in the BnOH-freeprocess. Most polymers generated by BnOH-assisted catalysis possess Mnvalues that are similar to Mn,calvalues based on Y–OBn, suggesting that oneapical Y–OBn bond of the diapical LY(OBn)(CH2SiMe3) intermediate catalyzes most or all of the ring polymerizationof ε-CL. [ABSTRACT FROM AUTHOR]

To gain a deeper knowledge of the nature of the species formed in situ by reaction of a catalyst precursor complex and typical cocatalysts used for ethylene oligomerization or polymerization, the Cr(III) complex fac-[Cr(NPN)Cl3] (1; NPN = bis(2-picolyl)phenylphosphine) was reacted with MAO and partial methylation to give the mononuclear complex fac-[Cr(NPN)Cl2.23Me0.77] (2) was observed, whereas reaction with AlMe3in toluene led to partial halide abstraction and produced the dinuclear, chloride-bridged dicationic complex [{fac-Cr(NPN)Me(μ-Cl)}2][AlMexCl4–x]2(3·[AlMexCl4–x]2), in which the metal centers are bridged by two chloride ligands. The complex fac-[Cr(NPN)Cl2Et] (4) was isolated in high yield from the reaction of 1with AlEt3and contains a rare example of CrIII–Et function. Upon treatment of 1with EtAlCl2, the dinuclear Cr(III) complex [{fac-Cr(NPN)}2(μ-Cl)3][AlCl4]3(5·[AlCl4]3) was obtained, which contains three bridging chlorides. Thus, under the conditions investigated, MAO and AlEt3behaved as alkylation agents, whereas AlMe3and EtAlCl2reacted as cationization agents. On activation with MAO and under comparable conditions, 1–5exhibited similar catalytic properties, with activities between 2700 and 2800 g of C2H4/((g of Cr) h), but the major oligomer produced using the 4/MAO system was 1-decene. The neutral alkyl complexes 2and 4and the cationic complexes 3and 5did not exhibit catalytic activity for ethylene oligomerization in the absence of MAO. The structures of the complexes 2, 3·[AlMexCl4–x]2, 4, and 5·[AlCl4]3have been determined by single-crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Synthesis, Characterization, and Heterobimetallic Cooperation in a Titanium−Chromium Catalyst for Highly Branched Polyethylenes," by Shaofeng Liu, Alessandro Motta, Massimiliano Delferro, and Tobin J. Marks from volume 135, 2013.

Remote manipulation of a micromachine under an external magnetic field is significant in a variety of applications. However, magnetic manipulation requires that either the target objects or the fluids should be ferromagnetic or superparamagnetic. To extend the applicability, we propose a versatile optical printing technique termed femtosecond laser-directed bubble microprinting (FsLDBM) for on-demand magnetic encoding. Harnessing Marangoni convection, evaporation flow, and capillary force for long-distance delivery, near-field attraction, and printing, respectively, FsLDBM is capable of printing nanomaterials on the solid-state substrate made of arbitrary materials. As a proof-of-concept, we actuate a 3D polymer microturbine under a rotating magnetic field by implementing γ-Fe 2 O 3 nanomagnets on its blade. Moreover, we demonstrate the magnetic encoding on a living daphnia and versatile manipulation of the hybrid daphnia . With its general applicability, the FsLDBM approach provides opportunities for magnetic control of general microstructures in a variety of applications, such as smart microbots and biological microsurgery.

Organic small-molecule-based photothermal agents such as cyanine dyes have received increasing attention in developing novel cancer therapies with potential clinical utility but suffer from poor stability, low photothermal efficiency, and limited accumulation at tumor sites in molecular forms. Self-assembly of small-molecule dyes into supramolecular assemblies may address these concerns by controlling the molecular organization of dye monomers to form structures of a higher order. Among them, H-aggregates of dyes favor face-to-face contacts with strongly overlapping areas, which always have a negative connotation to exhibit low or no fluorescence in most cases but may emanate energy in nonradiative forms such as heat for photothermal cancer therapy applications. Here, the synergistic self-assembly of cyanine dyes into H-aggregates is developed as a new supramolecular strategy to fabricate small-molecule-based photothermal nanomaterials. Compared to the free cyanine dyes, the H-aggregates assembled from pyrene or tetraphenylethene (TPE) conjugating cyanine exhibit the expected absorption spectral blue shift and fluorescence self-quenching but unique photothermal properties. Remarkably, the obtained H-aggregates are saucer-shaped nanoparticles that exhibit passive tumor-targeting properties to induce imaging-guided photothermal tumor ablation under irradiation. This supramolecular strategy presented herein may open up new opportunities for constructing next-generation small-molecule-based self-assembly nanomaterials for PTT cancer therapy in clinics.

Phosphazene base is an important organocatalyst in polymer chemistry owing to its high activity and versatility. In this contribution, we demonstrate that cyclic trimeric phosphazene base ( CTPB ) can catalyze stereoselective ring-opening polymerization (ROP) of rac -lactide ( rac -LA) to produce isotactic stereoblock PLA ( P i up to 0.93). The polymerizations are highly controlled, as evidenced by linear relationship between molecular weights (MW) and monomer conversions and the narrow dispersity ( Đ = M w / M n ) of the resulted polymers with high fidelity of end groups. The investigations on polymerization parameters show that the tacticity of produced PLA depends on the polymerization temperatures and solvents, while the kinetic studies reveal a faster rate for ROP of l-LA as compared to rac -LA under same conditions. Based on these results, the chain end control mechanism is proposed to explain the production of isotactic stereoblock PLA from rac -LA by an achiral catalyst.

The heterobimetallic complexes, (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-CnH2n-[N,N-bis(2-(ethylthio)ethyl)amine]CrCl3 (n = 0, Ti-C0-Cr(SNS); n = 2, Ti-C2-Cr(SNS); n = 6, Ti-C6-Cr(SNS)), (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-C2H4-[N,N-bis((o-OMe-C6H4)2P)amine]CrCl3 (Ti-C2-Cr(PNP)), and (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-C2H4-[N,N-bis((diethylamine)ethyl)-amine]CrCl3 (Ti-C2-Cr(NNN)), are synthesized, fully characterized, and employed as olefin polymerization catalysts. With ethylene as the feed and MAO as cocatalyst/activator, SNS-based complexes Ti-C0-Cr(SNS), Ti-C2-Cr(SNS), and Ti-C6-Cr(SNS) afford linear low-density polyethylenes (LLDPEs) with exclusive n-butyl branches (6.8-25.8 branches/1000 C), while under identical polymerization conditions Ti-C2-Cr(PNP) and Ti-C2-Cr(NNN) produce polyethylenes with heterogeneous branching (C2, C4, and C≥6) or negligible branching, respectively. Under identical ethylene polymerization conditions, Ti-C0-Cr(SNS) produces polyethylenes with higher activity (4.5× and 6.1×, respectively), Mn (1.3× and 1.8×, respectively), and branch density (1.4× and 3.8×, respectively), than Ti-C2-Cr(SNS) and Ti-C6-Cr(SNS). Versus a CGC(Et)Ti + SNSCr tandem catalyst, Ti-C0-Cr(SNS) yields polyethylene with somewhat lower activity, but with 22.6× higher Mn and 4.0× greater branching density under identical conditions. In ethylene +1-pentene competition experiments, Ti-C0-Cr(SNS) yields 5.5% n-propyl branches and 94.5% n-butyl branches at [1-pentene] = 0.1 M, and the estimated effective local concentration of 1-hexene is ∼8.6 M. In contrast, the tandem CGC(Et)Ti + SNSCr system yields 91.0% n-propyl branches under identical reaction conditions. The homopolymerization and 1-pentene competition results argue that close Ti···Cr spatial proximity together with weak C-H···Ti and C-H···S interactions significantly influence relative 1-hexene enchainment and chain transfer rates, supported by DFT computation, and that such effects are conversion insensitive but cocatalyst and solvent sensitive.

A heterobimetallic catalyst, {Ti--Cr}, consisting of a constrained-geometry titanium olefin polymerization center (CGC(Et)Ti) covalently linked to a chromium bis(thioether)amine ethylene trimerization center (SNSCr) was synthesized and fully characterized. In ethylene homopolymerizations it affords linear low-density polyethylene with molecular weights as high as 460 kg·mol(-1) and exclusively n-butyl branches in conversion-insensitive densities of ~18 branches/1000 carbon atoms, which are ~17 and ~3 times (conversion-dependent), respectively, those achieved by tandem mononuclear CGC(Et)Ti and SNSCr catalysts under identical reaction conditions.