The initial active site concentrations, [C*]0, have been determined with CH3OT radiolabeling for the Cp2ZrCl2/MAO and CpZrCl3/MAO catalysts (Cp = η5 : cyclopentadienyl, MAO = methyl aluminoxane). Almost all the Zr are found to be catalytically active in 70°C ethylene polymerizations; [C*]0 = [Zr] and [C*]0 = 0.8[Zr] at Al/Zr ratios of 104 and 103, respectively. Lowering the temperature to 50°C and Al/Zr to 5.5 × 102 reduces [C*]0 to 0.2[Zr]. The rate constant of propagation at 70°C was calculated to be 1.6 × 103(M s)?1 for both catalysts at Al/Zr = 1.1 × 104; the values are decreased fivefold and tenfold, respectively, for the CpZrCl3 and Cp2ZrCl2 systems. The usage of 14CO to determine the propagating Zr–P species was investigated. With regard to the time of reaction of 14CO with the polymerization mixture, the initial phase is attributed to reversible CO complexation and reversible migratory insertion. The second slower phase may be due to the formation of enediolate. During the course of a batch polymerization the 14C radioactivity incorporated is small compared to the number of active sites found by CH3OT determination; it is only ca. 10% of [C*]0 at maximum rate of polymerization. Therefore, 14CO radiolabeling cannot be used to count C*. 相似文献
The kinetics of acetylene polymerization initiated by Ti(OBu)4/4AlEt3 catalyst was studied by radioquenching with C*O to count the number of active sites [C] and by CH3OT* to determine the total metal polymer bonds [MPB] and M?n of the polymer. The amount of quenching agent and time of reaction required and the kinetic isotope effect for CH3OT* were determined. The effects of Al/Ti ratio, catalyst aging, catalyst concentration, temperature, and monomer pressure on the polymerization were investigated. Detailed kinetic data on the variation of rate of polymerization, Rp, [C] [MPB], and M?n with time were obtained at 298 and 195°K. The results required the assumption that the catalytic species C, is initially active and within less than 30 min all are converted by bimolecular kinetics to a far less active species. Analysis of the data yielded rate constants of propagation and termination and their energies of activation. Estimates of chain transfer efficiency were obtained. The mechanisms for the propagation, termination, and transfer processes were discussed. By drawing on our earlier EPR results we propose probable structures for the catalytically active species. 相似文献
Propene polymerization was conducted with the [t‐BuNSiMe2Flu]TiMe2/B(C6F5)3 catalyst at –50°C at various concentrations of propene. The polymerization proceeds in a living manner regardless of the propene concentration employed. Molecular weight of polymer obtained in a certain polymerization time with low monomer conversion increases with increasing concentration of propene. The logarithmic plot of molecular weight against propene concentration gives a straight line with a slope of 1.8, which indicates that the propagation rate is almost of second order with respect to propene concentration. 相似文献
The gas-phase, ion molecule reaction between C3O2+. and C3O2 has been studied by both double-focusing and ion trap mass spectrometry, rationalized by the formation of a dimeric, odd electron cation [C6O4]+. which decomposes extensively through sequential CO losses giving rise to [C5O3]+. and [C4O2]+. ions. The thermodynamics of this process have been investigated by means of ab initio calculation performed on the above species using different basis sets (STO-3G, 3-21G and 6-31G*). 相似文献
The dependence of the steric microstructure of cationically polymerized poly(N-vinylcarbazole) (PVK) upon catalyst, polymerization temperature, and polymerization solvent has been investigated. The effect of polymerization temperature variation was found to be small, whereas the choice of catalyst and polymerization solvent was found to have a strong influence upon the PVK steric microstructure. A correlation was found between the syndiotacticities Xs and the π* solvent polarities of the polymerization solvents for a given catalyst. A decrease in Xs with increasing π* solvent polarity was observed using BF3OEt2 and AlEt2Cl catalysts and has been interpreted in terms of propagation via contact ion-pair ring structures reversibly formed between the active end group and a preceding repeating unit. The increase in Xs with increasing π* solvent polarity observed with several of the catalysts investigated has been interpreted in terms of chain ion pairs whose separation increases with increasing π* solvent polarity. The influence of the various Lewis acid catalysts upon the steric microstructures of cationically polymerized PVK allowed the following order of nucleophilicity to be established: 相似文献
Syndiospecific polymerization of styrene was catalyzed by monocyclopentadienyltributoxy titanium/methylaluminoxane [CpTi (OBu)3/MAO]. The atactic and syndiotactic polystyrenes were separated by extracting the former with refluxing 2-butanone. The activity and syndiospecificity of the catalyst were affected by changes in catalyst concentration and composition, polymerization temperature, and monomer concentration. Extremely high activity of 5 × 107 g PS (mol Ti mol S h)?1 with 99% yield of the syndiotactic product were achieved. The concentration of active species, [C*], has been determined by radiolabeling. The amount of the syndiospecific and nonspecific catalytic species, [C] and [C] respectively, correspond to 79 and 13% of the CpTi(OBu)3. The rate constants of propagation for C and C at 45°C are 10.8 and 2.0 (M s)?1, respectively, the corresponding rate constants for chain transfer to MAO are 6.2 × 10?4 and 4.3 × 10?4s?1. There was no deactivation of the catalytic species during a batch polymerization. The rate constant of chain transfer with monomer is 6.7 × 10?2 (M s)?1; the spontaneous β-hydride transfer rate constant is 4.7 × 10?2 s?1. The polymerization activity and stereospecificity of the catalyst are highest at 45°C, both decreasing with either higher or lower temperature. The stereoregular polymer have broad MW distributions, M?w/M?n = 2.8–5.7, and up to three crystalline modifications. The Tm of the s-PS polymerized at 0–90°C decreased from 261.8 to 241°C indicating thermally activated monomer insertion errors. The styrene polymerization behaviors were essentially insensitive to the dielectric constant of the medium. 相似文献
Unlike isoelectronic 6-indenyl ruthenium, manganese, and chromium complexes, heating of Mn(CO)5OTf, [Ru(Cp*)Cl4], and Cr(C10H8)(CO)3 with isomeric 2-ethyl-5-methylcyclopenta[b]thiophenes leads to the corresponding 5-complexes where the metal is coordinated at the C5-carbocycle. The structure of the complex Mn(Th)(CO)3 (Th = 2-ethyl-5-methylcyclopenta[b]thienyl) was proved by X-ray analysis. 相似文献
The “borohydride/alkyl” (B/A) route initially reported for isoprene has been applied successfully to the polymerization of styrene. This method provides via an in situ approach an interesting tool for the assessment of the influence of a ligand on the performance of half-lanthanidocene catalysts. All systems lead to well-controlled oligomerization/polymerization processes. This method is thus a convenient tool for the controlled polymerization of styrene starting from a common trisborohydride precursor and commercial ligands. The influence of the nature of several ligands on the activity could be established, with trends corresponding to those obtained starting from the isolated precursors: HCpHCpPh3>HCp*(Cp=C5H5,CpPh3=1,2,4-Ph3C5H2,Cp*=C5Me5). These results suggest an influence of the electron donating ability of the ligand rather than steric requirements. 相似文献
A series of smectic C* liquid-crystalline (LC) block copolymers were successfully synthesized via the living anionic polymerization of polystyrene with optically active methacrylate monomers containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl-4′-carboxylate mesogens. These materials are the first reported smectic C* block copolymers. Anionic polymerization in tetrahydrofuran (THF) at −70°C leads to LC block molecular weights of approximately twenty repeating units. The number-average molecular weight of the polystyrene block was varied from 7000 to 20000 to adjust the composition in the block copolymers. Differential scanning calorimetry and optical microscopy indicate that the smectic C* phase is present in the systems over broad temperature ranges. 相似文献
Living polymerization of propene with alkyltitanium-based catalysts is described with emphasis on the role of the activators employed. (1 : 3-tert-Butyl(dimethylfluorenylsilyl)amido)dimethyltitanium activated by tris(pentafluorophenyl)borane (B(C6F5)3) catalyzes the living polymerization at -50 °C. The use of dried methylaluminoxane (MAO) in place of B(C6F5)3 raises the living polymerization temperature to 0 °C and improves the syndiospecificity. A chelating diamide dimethyltitanium activated by dried modified MAO (MMAO) catalyzes the living polymerization at 0 °C to give a statistically atactic polymer. The heterogenization of the living systems is attempted by supporting MAO, MMAO and dried MMAO on SiO2 as solid activators. 相似文献
Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VOx/TiO2 were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes Cβ–H bond activation of propane followed by the formation of a surface hydroxyl group V–OtH and a propyl radical n-C3H7. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging Ob oxygen atom with the methylene C–H bond of propyl radical n-C3H7 followed by the formation of a hydroxylated surface site HOt–V4+–ObH and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V5+–(OtOb)? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity. 相似文献
The kinetics of ethene and propene polymerization at 20–60°C in the presence of the homogeneous catalyst system rac‐Me2Si(2‐methyl‐4‐phenyl‐1‐indenyl)2ZrCl2/methylaluminoxane was investigated by means of stopped‐flow techniques. The specific rate of chain propagation, measured at the very short reaction times typical of this method, turned out to be ≈102 times higher for ethene than for propene; this suggests that diffusion limitations through the poly(ethylene) precipitating at longer reaction times may be responsible for the fact that the two monomers polymerize instead at comparable rates under “standard” conditions. It was also found that the concentration of active sites is significantly lower than the analytical Zr concentration. 相似文献
Summary: A binaphthyl‐bridged salen dichlorozirconium (IV ) complex that displays an octahedral structure with a trans‐O, cis‐N, and cis‐Cl arrangement was synthesized and tested as a precatalyst for ethylene and α‐olefin polymerization. While use of methylaluminoxane (MAO) cocatalyst afforded poor catalytic activity, activation by mixtures of aluminium alkyls such as AliBu3 and either MAO or [CPh3][B(C6F5)4] resulted in reasonable polymerization activities for ethylene, propene, and higher α‐olefins. Quite unexpectedly, while the polymerization of propene results in the production of a high‐molecular‐weight stereoirregular polymer, highly isotactic polymers are obtained under similar conditions from polymerization of 1‐butene, 1‐pentene, and 1‐hexene.
Polymerization employing the binaphthyl‐bridged salen dichlorozirconium (IV ) complex gave unexpected different stereospecificities for the polymerization of propene and higher α‐olefins, to yield ultrahigh‐molecular‐weight atactic poly(propylene) and highly isotactic polymers, respectively. 相似文献