Organometallics are a family of useful organic chemicals because they play important roles in organic synthesis as reagents and as catalysts.They can be classified according to the number of metals they contain.Bimetallic compounds are important organometallics and they are either homobimetallic or heterobimetallic depending on whether the two metals are the same or different.In this paper,we focus on homobimetallic compounds.Homobimetallic compounds are generally used as dianions to react with electrophiles in organic synthesis.Recently,homobimetallics have also been used as catalysts in organic reactions such as in asymmetric reactions.
WU Wei GU DeLiang WANG ShiMeng NING YingNan MAO GuoLiang
A silica-supported Ziegler-Natta catalyst with dimethyldichlorosilane （DMDS） as modifier and small silica as support was successfully prepared and characterized. Results from pilot screen showed that the new catalyst exhibited higher catalytic activity, better hydrogen response ability and better copolymerization ability than the commercial M catalyst. Pilot screen in ethylene gas phase fluidized bed polymerization, the catalytic activity of the new catalyst was up to 8000 g PE/g cat, which was twice of that of the commercial M catalyst. The bulk density of polyethylene obtained with the new catalyst was 0.38 g/cm^3. The new catalyst is suitable for condensed and super-condensed process in fluidized bed ethylene polymerization.
Late-transition-metal(LTM) catalysts are a family of very flexible ethylene polymerization catalysts because their catalytic performance can be easily adjusted by modifying the ligand structure.Their less oxyphilicity character,which may promote the production of copolymers from ethylene and polar comonomers,is another aspect that attracts much attention in both academic and industrial fields.The immobilization of LTM catalysts on spherical supports is a crucial step prior to their use in the industrial processes of gas-phase or slurry polymerizations.This paper reviews recent developments in supported LTM catalysts for olefin polymerization,and summarizes loading methods and mechanisms of the immobilization of LTM catalysts on inorganic,organic,and inorganic-organic materials,and the effects of immobilization on catalytic activity,polymerization mechanism,and polymer morphology.
Recent progress in the research of radical anion ligands and their complexes with metals were summarized in this review.Radical anions were sorted into several types including iminosemiquinonate and iminoquinonate radical,nitroxide radical,heterocycle radical etc.Structural characteristics and properties of the corresponding complexes were introduced.The complexes exhibited novel properties and possibility for applications in organic magnetic materials and transition metal catalysis.
Magnesium chloride supported vanadium/titanium bimetallic Ziegler-Natta catalysts with di-i-butyl phthalate as internal donor for copolymerization of ethylene and propylene were prepared. The effects of reaction temperature, ethylene/propylene molar ratio, aluminium/vanadium （Al/V） molar ratio and titanium/vanadium molar ratio on the catalytic activity were investigated. The molecular weight, molecular weight distribution, sequence composition and crystallinity of the products were measured by gel permeation chromatography, ^13C-NMR and differential scanning calorimetry analysis, respectively. In comparison to the vanadium and titanium catalysts, the bimetallic catalyst showed higher catalytic activity and better copolymerization performance. The obtained ethylene/propylene copolymers have high molecular weight （105）, broad molecular weight distribution, high propylene content with random or short blocked sequence structures （rErp = 1.919）, low melting temperatures and low crystallinities （Xc 〈 20%）.
Researches of late-transition metal complexes for ethylene polymerization and oligomerization have attracted a lot of attention in the last two decades.A large number of late-transition metal complexes with symmetric diimine ligands have been extensively studied.Although the research of unsymmetric diimine late-transition metal complexes was still in the initial stage,some superior performances were observed in olefin polymerization.Recent developments in the research of ethylene polymerization and oligomerization catalyzed by late-transition metal complexes with unsymmetric diimine ligands were summarized in this review,with the influence of complex structure on the catalytic performance of the catalyst analyzed.
Guoliang MaoYue JiangNing LiQin WangDejin ZhengMeijiao LiYingnan Ning
A series of novel N-halogen-substituted-aryl bisphosphinoamine ligands were synthesized and characterized by elemental analysis,1H NMR and mass spectrometry.The combination of these ligands with Cr(Ⅲ) and activation by methylaluminoxane leads to highly active catalytic systems for the tetramerization of ethylene to form 1-octene.The catalytic activities and product selectivi-ties depend on the aryl ring substituents and the reaction conditions.We found that the location and size of the substituents are important in determining the catalytic activity and selectivity toward 1-octene.This trend is similar to that observed for their alkyl-substituted analogues.
