Summary: Branched polyethylene/linear polyethylene blends (BPE/LPE) were prepared using the combined Ni(α‐diimine)Cl2 ( 1 ) (α‐diimine = 1,4‐bis(2,6‐diisopropylphenyl)acenaphthenediimine) and {TpMs*}TiCl3 ( 2 ) (TpMs* = hydridobis(3‐mesitylpyrazol‐1‐yl)(5‐mesitylpyrazol‐1‐yl)) catalysts supported in situ on methylaluminoxane (MAO)‐modified silica (4.0 wt.‐% Al/SiO2). The polymerization reactions were performed in toluene at two different polymerization temperatures (0 and 30 °C) and several nickel molar fractions (xNi), using MAO as external cocatalyst. At all temperatures, the activities show an approximate linear correlation with xNi, indicating a non‐synergistic effect between the nickel and the titanium species. Higher activities were found at 0 °C. The melting temperatures for the polyethylene blends produced at 0 °C decrease as xNi increases in the medium, indicating good compatibility between the polyethylene phases made by both catalysts. The melting temperature (Tm) of the polyethylene blends was shown to depend on the order in which the catalysts were immobilized on the MAO‐modified silica support. The initial immobilization of 1 on the support ( 2 / 1 /SMAO‐4) affords polymers with a lower Tm than those produced with 1 / 2 /SMAO‐4. In addition, scanning electron microscopy (SEM) studies revealed that the spherical morphology of the supported catalyst is replicated in the polyethylene particles.
Influence of polymerization temperature on the activity of 1 / 2 /SMAO‐4 with varying xNi. 相似文献
The present study showed enhanced activities of ethylene/1-octene copolymerization via TiO2–SiO2 mixed oxides-supported MAO with a zirconocene catalyst. It was proposed that titania was decorated on silica surface and acted as a spacer to anchor MAO to the silica support resulting in less steric hindrance and less interaction on the support surface. 相似文献
Here, we revealed the effect of particle size of the nanoscale SiO2 on catalytic and characteristic properties of LLDPE/nano-SiO2 composites synthesized via the in situ polymerization with a zirconocene/MAO catalyst. In the experiment, SiO2 (10 and 15 nm) was first impregnated with MAO. Then, copolymerization of ethylene/1-hexene was performed in the presence of nano-SiO2/MAO to produce LLDPE/nano-SiO2 composites. It was found that the larger particle exhibited higher polymerization activity due to fewer interactions between SiO2 and MAO. The larger particle also rendered higher insertion of 1-hexene leading to decreased melting temperature (Tm). There was no significant change in the LLDPE molecular structure by means of 13C NMR. 相似文献
The effect of Al content on MAO-modified silicas was evaluated on catalyst activity, on polymer properties and on residual metal content in the resulting polyethylenes. MAO-modified silicas were prepared by impregnating MAO toluene solutions in concentration range between 0.5 and 20.0 wt% Al/SiO2. Commercial MAO-modified silica (Witco) containing 24.4 wt% Al/SiO2 was used for comparative reasons. The resulting modified-silicas were employed as supports for grafting (nBuCp)2ZrCl2. Using external MAO as cocatalyst (Al/Zr=2000) no difference in catalyst activity was observed. Nevertheless, for Al/Zr=500, catalyst activities were shown to be higher for supported zirconocene systems containing 0.0-2.0 wt% Al/SiO2 range. According to DSC analysis, one Tm peak was detected for polymer obtained with catalyst prepared with 0.5 wt% Al/SiO2 (135 °C), but two Tm peaks were observed for polymers obtained with catalysts prepared with 10.0 wt% Al/SiO2 (136 and 141 °C) and 20.0 wt% Al/SiO2 (133 and 141 °C). 相似文献
The effect of a series of hydrotris(pyrazolyl)borate ligands (Tpx) in the regioselectivity of the catalytic functionalization of the secondary carbon-hydrogen bonds of hexane with ethyl diazoacetate and TpxCuL as the catalysts has led to the definition of the relative steric parameter SPR. The validity of the parameter has been assessed upon its correlation with the regioselectivity observed in several transformations (olefin cyclopropanation, C−H functionalization, C−H amidation), in all cases a good degree of fitting being observed. 相似文献
Silica-supported titanium dioxide (TiO2/SiO2) and Ti-MCM 41 catalysts have been used for transesterification of dimethylcarbonate (DMC) and phenol to methylphenylcarbonate (MPC). The structure and the chemical state of titanium species in TiO2/SiO2 and Ti-MCM 41 have been investigated by means of X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) for Ti K-edge and X-ray photoelectron spectroscopy (XPS). To understand the role of pore size on the activity of catalysts, different pore size silica supports (Q-series) were utilized in TiO2/SiO2 catalysts. Similarly, to understand the effect of Ti symmetry on the activity of catalysts, Ti-MCM 41 was used with different Ti-loadings. It was observed that the Ti surface area was an only important factor to achieve highest activity. In case of Ti-MCM 41 catalysts, as the Ti-loading increased octahedral symmetry increased and tetrahedral symmetry decreased. But, turnover rates based on the surface Ti atoms were independent of the Ti symmetry. They are also similar to those obtained for TiO2/SiO2 catalysts. Showing that transesterification of DMC and phenol over Ti-based catalysts is a structure insensitive reaction. 相似文献
Summary
The polymerization of styrene with catalysts based on Ni(acac)2 supported on SiO2 and Al2O3 was investigated. Using catalysts based on MAO supported on silica, a highly isotactic polystyrene was obtained. Nevertheless,
the Al2O3-supported catalyst can promote isospecific polymerization activated by common. alkyl aluminum compounds even by any prior
support treatment with MAO.
Received: 3 March 1998/Revised version: 14 April 1998/Accepted: 14 April 1998 相似文献
Basic catalysts for carbon?Ccarbon addition reactions were synthesized by immobilization of amine species on silica supports. Tetraethylenepentamine was impregnated and immobilized on amorphous silica (SiO2) and SBA-15 using an epoxy resin. The basicity of the catalysts was determined by adsorption?Cdesorption of CO2 and the degree of immobilization was evaluated by FTIR. The catalytic activity towards the Claisen condensation reaction of methyl benzoate and methyl ethyl ketone was evaluated by an in-situ FTIR micro-scale reactor. A mechanism is proposed to show that the catalysts promote the formation of ??-diketone and methanol; the effects of the support and amine immobilization degree are discussed. 相似文献