Tingcheng Li

β-diiminato titanium complexes with varying fluorine substitution patterns on the N-aryl moiety: probing the effect of ligand substitution on ethylene polymerization

A series of six mono β-diiminato titanium complexes with varying fluorine-substitution patterns on the iminophenyl moiety were designed and synthesized, and the ethylene polymerization activities were studied with modified methylaluminoxane (MMAO) as cocatalyst. The introduction of fluorine substituents significantly increased the catalytic activity of the β-diiminato titanium complexes compared with their alkyl-substituted counterparts. Furthermore, both the position and the amount of fluorine substitution greatly influenced the catalytic behavior. The activity was increased when the ortho-positions of the N-aryl were substituted by fluorine, and the more ortho-positions were substituted, the higher the activity. All of the three o-fluoro substituted complexes (2a, 2e and 2f) exhibited very high activity (up to 106gmol−1xa0h−1xa0atm−1); whereas the m- and p-fluoro derivatives (2b and 2c) were much less active. The high activity of the o-fluoro-substituted complexes is suggested to be due to the formation of non-covalent interactions between the o-fluorine atom and the ethylene monomer, which may assist in the enrichment and coordination of ethylene to the central metal, thus increase the catalytic activity.

Synthesis and structures of mono(β-diiminato) copper complexes and their catalytic performances for homo- and copolymerizations of methyl acrylate

A series of mono(β-diiminato) copper complexes bearing different coordinating anions and with different fluorine substitution patterns on the β-diiminato ligands were synthesized. All of the complexes were characterized by physicochemical and spectroscopic methods, and in addition, the structures of two of them were confirmed by X-ray diffraction. The crystal structures revealed a four-coordinate mode around the copper center in a slightly distorted square planar geometry, with the β-diimine and the carboxyl groups coordinated to give six- and four-membered chelate rings. The conjugation of C=C and C=N in the β-diiminato ligands and the resonance between C–O and C=O in the carboxyl groups resulted in extensive delocalization of electrons and the formation of conjugated N–C–C–C–N and O–C–O π-bonds, respectively. When activated by modified methylaluminoxane (MMAO), these complexes can effectively catalyze methacrylate (MA) polymerization with activities up to 54.5xa0kg/mol Cuxa0h, as well as MA/1-hexene copolymerization with the incorporation of MA over 80xa0%.

Synthesis and structural characterization of trifluoromethyl-substituted β-ketoimino copper(II) complex and its catalytic behavior for methyl acrylate polymerization

A new trifluoromethyl-substituted bis(β-ketoimino)copper complex (II) was synthesized and characterized by FTIR, elemental analysis, and X-ray diffraction (CIF file CCDC no. 1023178). XRD refinement revealed that the copper complex adopted a perfect central symmetric square planar structure with the copper center coordinated by two trans β-ketoimine ligands with delocalized double bonds. On activation with modified methylaluminoxane (MMAO), the bis(β-ketoimino)copper complex can polymerize methyl acrylate effectively. Introduction of trifluoromethyl group into the N-aryl ring of the ligands, which leads to strong electron-withdrawing effect, can improve significantly the catalytic activities. The activity of II/MMAO can reach 89.4 kg/mol Cu h, which is among the highest values reported for copper complexes in acrylic monomer polymerization.

Binuclear Titanium Catalysts Based on Methylene-Bridged Tridentate Salicylaldiminato Ligands for Ethylene Homo- and Copolymerization

A series of methylene-bridged salicylaldiminato tridentate [ONS] ligands bearing different alkylthio sidearms and the corresponding binuclear titanium complexes (Ti2a, Ti2b, Ti2c) were synthesized and characterized by elemental analysis, ESI-MS, FT IR, 1H and 13C NMR. To the best of our knowledge, these were the first non-metallocene tridentate binuclear Ti complexes reported. When activated by modified methylaluminoxane (MMAO), these binuclear Ti complexes displayed extremely high activities in the range of 106xa0gxa0mol−1xa0h−1xa0atm−1 for ethylene polymerization and ethylene/α-olefins copolymerization at atmospheric pressure, producing high molecular weight polymers with narrow polydispersity. Both the catalytic behavior and the spectroscopic characterization indicated that the catalysts assumed symmetric structure and contained single-site active species. Most importantly, the catalytic activities towards ethylene (co)polymerization as well as comonomer incorporation ratios were much higher than that of the corresponding mononuclear complexes. Meanwhile, the properties of polymers and the comonomer incorporation ratios can be effectively tuned by the reaction conditions and the alkylthio side group on ligands. Complex Ti2c bearing the long octylthio sidearm showed significantly higher activity for ethylene polymerization than the methylthio functionalized Ti2a, presumably due to the increased solubility of the catalyst in the reaction medium. While for copolymerization, complex Ti2a demonstrated the highest catalytic activity and comonomer incorporation ratio, due the small steric hindrance of the methylthio side group.Graphical Abstract

Synthesis, characterization and olefin polymerization behaviors of phenylene-bridged bis-β-carbonylenamine binuclear titanium complexes

Binuclear and multinuclear complexes have attracted much attention due to their unique catalytic performances for olefin polymerization compared with their mononuclear counterparts. In this work, a series of phenyl-bridged bis-β-carbonylenamine [O−NSR] (R = alkyl or phenyl) tridentate ligands and their binuclear titanium complexes (Ti2L1–Ti2L5) were synthesized and characterized by 1H NMR, 13C NMR, FTIR and elemental analysis. The molecular structure of ligand L2 (R = nPr) and its corresponding Ti complex Ti2L2 were further investigated by single-crystal X-ray diffraction, which showed that each titanium coordinated with six atoms to form a distorted octahedral configuration along with the conversion of the ligand from β-carbonylenamine to β-imino enol form. Under the activation of MMAO, these complexes catalyzed ethylene polymerization and ethylene/α-olefin copolymerization with extremely high activity (over 106 g mol (Ti)−1 h−1 atm−1) to produce high molecular weight polyethylene. At the same time, wider polydispersity as compared with the mononuclear counterpart TiL6 was observed, indicating that two active catalytic centers may be present, consistent with the asymmetrical crystal structure of the binuclear titanium complex. Furthermore, these complexes possessed better thermal stability than their mononuclear analogues. Compared with the complexes bearing alkylthio sidearms, the complex Ti2L5 bearing a phenylthio sidearm exhibited higher catalytic activity towards ethylene polymerization and produced polyethylene with much higher molecular weight, but with an appreciably lower 1-hexene incorporation ratio. Nevertheless, these bis-β-carbonylenamine-derived binuclear titanium complexes showed much higher ethylene/1-hexene copolymerization activity and 1-hexene incorporation ratios as compared with the methylene-bridged bis-salicylaldiminato binuclear titanium complexes, and the molecular weight and 1-hexene incorporation ratio could be flexibly tuned by the initial feed of α-olefin commoners and catalyst structures.

A Methylene-bridged salicylaldiminato tridentate [ONS] binuclear titanium complex for ethylene-norbornene copolymerization

ABSTRACT A binuclear titanium complex Ti2La with methylene-bridged salicylaldiminato tridentate [ONS] ligand bearing octylthio sidearm was synthesized and used for the copolymerization of ethylene and norbornene (E-NB). The complex exhibited activity over 106 g/mol(Ti).h.atm and high degree incorporation of co-monomer (up to 49.9 mol %), affording high-molecular-weight copolymers with narrow molecular weight distribution. The E-NB copolymers produced by Ti2La/MMAO contained NN dyad and NNN triad sequences even at low norbornene feeds, in contrast to the observation of such sequences only at high level of NB incorporation for most other catalyst systems. The monomer reactivity ratios were calculated to be rE = 14.62 and rN = 0.08, of which the rN value was much larger than that from non-metallocene titanium catalyst systems. The catalytic performances of the binuclear complex Ti2La and its mononuclear analogue TiLb were also compared, with the binuclear complex exhibiting higher catalytic activity and NB incorporation ratio due to the binuclear cooperative effect, and producing much higher molecular weight copolymer due to the increased steric hindrance caused by close proximity of two growing chains. To the best of our knowledge, this is one of the few examples of binuclear catalyst for E-NB copolymerization with high activity and efficient incorporation of norbornene. GRAPHICAL ABSTRACT A non-metallocene binuclear titanium complex was prepared and used for ethylene-norbornene (E-NB) copolymerization. The complex demonstrated higher catalytic activity and incorporation of norbornene in comparison with those of its mononuclear analogue, showing apparent positive “comonomer effect”. The insertion ratios of NB could be flexibly tuned through changing of the NB feed. The E-NB copolymers contained NN dyad and NNN triad sequences even at low NB feeds and incorporation due to the high activity of the catalyst system.