Chenxi Bai

Synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture in the presence of bifunctional catalyst

Abstract Ethylene carbonate was rapidly synthesized from supercritical carbon dioxide/ethylene oxide mixture by using as catalyst the system of tetradentate schiff-base aluminum complexes (designated as SalenAlX) coupling with a quaternary ammonium or phosphonium salt. The high rate of reaction was attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Various reaction periods present different formation rate of ethylene carbonate, mainly due to the existence of phase change during the reaction. The synergistic effect of the binary catalyst for ring-opening of ethylene oxide results from nucleophilicity of highly reactive anions of quaternary salts and the electrophilic interaction of SalenAlX with ethylene oxide. The activation of CO 2 was generally initiated by nucleophilic attack of the alcoholate(OCH 2 CH 2 BrNBu 4 ) at the carbon atom of CO 2 , and weak interaction between the central metal ion of SalenAlX and the lone pairs of one oxygen atom of CO 2 . It resulted in the insertion of CO 2 to AlO bond of Salen(X)AlOCH 2 CH 2 BrN( n -Bu) 4 or SalenAlOCH 2 CH 2 X to form linear carbonate which was transformed into ethylene carbonate by intramolecular substitution of halides. The experimental results demonstrate that supercritical carbon dioxide could be used as not only an environmentally benign solvent but also a carbon precursor in synthesis.

Highly Active and Cis-1, 4 Selective Polymerization of 1, 3-Butadiene Catalyzed by Cobalt(II) Complexes Bearing Α-Diimine Ligands

A series of cobalt(II) complexes bearing α‐diimine ligands were synthesized and characterized by elemental and spectroscopic analysis. These complexes had the general formulas [ArN = C(Me)-(Me)C = NAr]CoCl2 (Ar = C6H5, 3a; 4‐MeC6H4, 3b; 4‐MeOC6H4, 3c; 4‐FC6H4, 3d; 4‐ClC6H4, 3e; 2‐MeC6H4, 3f; 2‐EtC6H4, 3g; 2‐iPrC6H4, 3h; 2, 4, 6‐Me3C6H2, 3i; 2, 6‐Et2C6H3, 3j; 2, 6‐iPrC6H3, 3k). 2,6‐Bis[(2,6‐diisopropylphenylimino)ethyl]pyridine CoCl2 (4a) was also synthesized for comparison. The structures of complexes 3i, 3k, and 4a were further analyzed by X‐ray crystallography. When the Co(II) complexes were activated with ethylaluminum sesquichloride, they exhibited high catalytic activity for 1,3‐butadiene polymerization. The polymers produced have high cis‐1,4 stereoregularity (up to 98.0%) and high molecular weights (Mn = 1×10^4 - 1×10^5). The substituent ligand affected both catalytic activity and stereoselectivity through an electronic effect while steric hindrance by the substituent was not important. The effects of the polymerization conditions, such as polymerization time, temperature, different alkylaluminum compounds used as cocatalyst, and [Al]/[Co] molar ratio, on polymerization behavior were investigated.

Highly trans-1,4-stereoselective coordination chain transfer polymerization of 1,3-butadiene and copolymerization with cyclic esters by a neodymium-based catalyst system

trans-1,4-Selective coordination chain transfer polymerization of 1,3-butadiene was achieved by using a Nd(Oi-Pr)3/Mg(n-Bu)2 catalyst, affording polybutadienes having high trans-1,4 contents (ca. 96%), moderate molecular weight (Mn = 1.0–2.3 × 104), and narrow polydispersity (Mw/Mn ∼ 1.7). In the system, Mg(n-Bu)2 acted as both a co-catalyst and a chain transfer agent, and the calculated transfer efficiencies of Mg(n-Bu)2 were 27–34%. The produced living polybutadiene could further initiate the ring-opening polymerization of e-CL/lactide to give TPB-b-PCL/PLA copolymers in a controlled fashion. The crystalline amphiphilic copolymers (TPB-b-PCL/PLA) were subsequently applied to investigate their self-assembly behavior by adding a selective solvent into a polymer/co-solvent solution. The polymer plates composed of a crystallized TPB core and PCL/PLA brushes were obtained by the crystallization-driven self-assembly. Moreover, the morphology of the polymers underwent change from nano-sized plates to micro-sized plates with increasing addition of the selective solvent.

Iminopyridine-Based Cobalt(II) and Nickel(II) Complexes: Synthesis, Characterization, and Their Catalytic Behaviors for 1,3-Butadiene Polymerization

A series of iminopyridine ligated Co(II) (1a–7a) and Ni(II) (1b–7b) complexes were synthesized. The structures of complexes 3a, 4a, 5a, 7a, 5b, and 6b were determined by X-ray crystallographic analyses. Complex 3a formed a chloro-bridged dimer, whereas 4a, 5a, and 7a, having a substituent (4a, 5a: CH3; 7a: Br) at the 6-position of pyridine, producing the solid structures with a single ligand coordinated to the central metal. The nickel atom in complex 5b features distorted trigonal-bipyramidal geometry with one THF molecule ligating to the metal center. All the complexes activated by ethylaluminum sesquichloride (EASC) were evaluated in 1,3-butadiene polymerization. The catalytic activity and selectivity were significantly influenced by the ligand structure and central metal. Comparing with the nickel complexes, the cobalt complexes exhibited higher catalytic activity and cis-1,4-selectivity. For both the cobalt and nickel complexes, the aldimine-based complexes showed higher catalyst activity than their ketimine counterparts.

Polymerization of 1,3-butadiene catalyzed by cobalt(II) and nickel(II) complexes bearing pyridine-2-imidate ligands

Cobalt and nickel complexes (1a-1d and 2a-2d, respectively) supported by 2-imidate-pyridine ligands were synthesized and used for 1,3-butadiene polymerization. The complexes were characterized by IR and element analysis, and complex 1a was further characterized by single-crystal X-ray diffraction. The solid state structure of complex 1a displayed a distorted tetrahedral geometry. Upon activation with ethylaluminum sesquichloride (EASC), all the complexes showed high activities toward 1,3-butadiene polymerization. The cobalt complexes produced polymers with high cis-1,4 contents and high molecular weights, while the nickel complexes displayed low cis-1,4 selectivity and the resulting polymers had low molecular weights. The catalytic activities of the complexes highly depended on the ligand structure. With the increment of polymerization temperature, the cis-1,4 content and the molecular weight of the resulting polymer decreased.

Study on Dynamic Performance of Novel Isoprene Rubber Synthesized with Homogeneous Rare-Earth Catalyst System

The homogeneous rare-earth catalytic system was used to initiate the polymerization of isoprene, and a novel IR was synthesized with high molecular weight and narrow molecular weight distribution. According to the self-developed rare-earth IR properties, a study on IR and IR/NR blend was made. The effect of JiHua rare-earth IR on the vulcanization characteristics and dynamic properties in IR/NR blend was investigated and compared with IR (SKI-3) synthesized with titanium catalyst system and rare-earth isoprene rubber (SKI-5) made in Russia. The results showed that JiHua rare-earth IR to be developed independently was a novel synthetic rubber with excellent comprehensive performance, and it can be used independently and partly substitute natural rubber to make tire. In the IR/NR blend, Not only vulcanization characteristics and the safety in operation for NR was greatly improved, But also dynamic properties of vulcanizates such as dynamic cutting resistance, abrasion resistance, wet-skid resistance and aging resistance were improved to different degrees. These properties reach or surpass the level of the foreign similar products like Russia SKI-3 and Russia SKI-5.