Kinetic study of ethylene polymerization with chromium oxide catalysts by a radiotracer technique

The quenching of polymerization with a chromium oxide catalyst by radioactive methanol ¹⁴CH₃OH enables one to determine the concentration of propagation centers and then to calculate the rate constant of the propagation. The dependence of the concentration of propagation centers and the polymerization rate on reaction time, ethylene concentration, and temperature was investigated. The change of the concentration of propagation centers with the duration of polymerization was found to be responsible for the time dependence of the overall polymerization rate. The propagation reaction is of first order on ethylene concentration in the pressure range 2–25 kg/cm². For catalysts of different composition, the temperature dependence of the overall polymerization rate and the propagation rate constant were determined, and the overall activation energy E_ov and activation energy of the propagation state E_p were calculated. The difference between E_ov and E_p is due to the change of the number of propagation centers with temperature. The variation of catalyst composition and preliminary reduction of the catalyst influence the shape of the temperature dependence of the propagation center concentration and change E_ov.     

The number of active centers and propagation rate constant in ethylene polymerization with a homogeneous catalyst based on cobalt 2,6-bis[imino]pyridyl complex with methylaluminoxane activator

The number of active centers C _p and propagation rate constant k _p upon ethylene polymerization with a homogeneous catalyst based on a cobalt complex with bis[imino]pyridyl ligands (LCoCl₂, where L is 2,6-(2,6-(Me)₂C₆H₃N=CMe)₂C₅H₃N) using methylaluminoxane as an activator was determined by quenching by radioactive carbon monoxide (¹⁴CO). It was found that the drop in activity during polymerization on the above catalyst is due to the decreasing number of active centers (from 0.23 to 0.14 mol/mol Co within 15 min of polymerization); the propagation rate constant remained unchanged, 3.5 × 10³ l/(mol s) at 35°C, which is substantially lower than for a catalyst based on an iron complex with analogous bis[imino]pyridyl ligands. It follows from the data on molecular mass characteristics of the produced polymer that the homogeneous catalyst LCoCl₂/methylaluminoxane is of monocenter type, and the obtained value of the propagation rate constant reflects the true reactivity of its active centers.     

Polymerization of tetrahydrofuran by AlEt3–H2O promoter system: Rate of propagation reaction

Kinetic studies were carried out on the polymerization of tetrahydrofuran with catalyst systems of aluminum alkyl–epichlorohydrin. As aluminium alkyl species AlEt₃, AlEt₃–H₂O (1:0.1 to 1:1.0), and “oxyaluminum ethyl” were employed. The polymerizations with these catalysts are characterized by a mechanism of stepwise addition without chain transfer or termination, which is expressed by the kinetic relation R_p = K_p[P*] ([M]–[M]_e), where [M] and [M]_e are the instantaneous and equilibrium concentrations of monomer and [P*] is the concentration of propagating species calculated from the amount and molecular weight of the product polymer. The determination of the rate constant k_p for these catalysts has shown that the polymerization rate varied considerably with the change of aluminum alkyl species, i.e., with the water-to-aluminum ratio, but the propagation rate constant itself varied very little. The variation of polymerization rate was, therefore, attributed primarily to the differences in concentration of the propagating species, i.e. the efficiency of the catalyst in forming propagating species. The catalyst efficiency was closely related to the acid strength of the aluminum alkyl species, which was estimated from the magnitude of shift of the xanthone carbonyl band in the infrared spectrum of its coordination complex with aluminum alkyl. The maximal catalyst efficiency was attained at about [H₂O]/[AlEt₃] = 0.75.     

Polymerization of oxiranes by polymeric organoantimony oxides

Polymeric arylantimony(V) oxides [poly(ArSbO₂), Ar = phenyl, p-chlorophenyl (CPh), and p-methylphenyl (Tol)] were employed as catalysts for the polymerization of oxirane [ethylene oxide (EO)] and also substituted oxiranes [propylene oxide (PO), 1,2-butylene oxide (BO), and epichlorohydrin (ECH)]. The polymerization of EO by ArSbO₂s proceeded 3–60 times faster than that by the other organoantimony and -tin compounds such as triphenylstibine oxide (Ph₃SbO) and arenestannoic acids (ArSnO₂H), respectively. Apparent activation energy for the polymerization of EO was estimated as 13.7, 13.3, and 13.6 kcal/mol for PhSbO₂, TolSbO₂, and CPhSbO₂, respectively. The results of the polymerization as well as ¹H-, ¹³C-, and ¹⁷O-NMR spectroscopy suggested that the polymerization was initiated by ArSbO₂ or Ar₂Sb₂O₄ fragments, which was derived from a nucleophilc solvation of the polymeric ArSbO₂ by oxiranes in situ.     

Ethylene polymerization behavior of Cr(III)‐containing montmorillonite: Influence of chromium compounds

Montmorillonite was treated with Cr(NO₃)₃, Cr(acetate)₃, and Cr(acac)₃ to give three catalyst precursors, Cr‐MMT‐1 , Cr‐MMT‐2 , and Cr‐MMT‐3 , respectively. Application of these catalysts to the ethylene polymerization reaction revealed Cr‐MMT‐1 to be much more reactive than the other two while the molecular weight distributions of the polymers were practically the same. Elemental analysis, XRD, and TEM measurements suggested that chromium occupied the interlayer section in Cr‐MMT‐1 and mostly the outer surface region for the other two catalysts. Aluminosilicate‐supported Cr catalysts exhibited reactivity similar to that of Cr‐MMT‐2 and Cr‐MMT‐3 . However, more of the low‐molecular‐weight polymer was formed. These data suggested that there is a relationship between the sites of the Cr ions and catalytic reactivity, and between supporting solid identity and molecular weight distribution of the polymer. The use of n‐Bu₂Mg and Et₂Zn in the place of Et₃Al led to lower activity but gave polymers of narrower molecular weight distribution, with more of the high‐molecular‐weight material. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2272–2280, 2009     

Reactivity of Metal Catalysts in Glucose–Fructose Conversion

A joint experimental and computational study on the glucose–fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl₃, AlCl₃, CuCl₂, FeCl₃, and MgCl₂) was analyzed. Experimentally, CrCl₃ and AlCl₃ achieved the best glucose conversion rates, CuCl₂ and FeCl₃ were only mediocre catalysts, and MgCl₂ was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen‐atom transfer; the latter was the rate‐determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pK_a=4–6), coordination with either water or weaker σ donors, energetically low‐lying unoccupied orbitals, compact transition‐state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity.     

Active center determinations on MgCl2‐supported fourth‐ and fifth‐generation Ziegler–Natta catalysts for propylene polymerization

Active center determinations on different Ziegler–Natta polypropylene catalysts, comprising MgCl₂, TiCl₄, and either a diether or a phthalate ester as internal donor, have been carried out by quenching propylene polymerization with tritiated ethanol, followed by radiochemical analysis of the resulting polymers. The purpose of this study was to determine the factors contributing to the high activities of the catalyst system MgCl₂/TiCl₄/diether—AlEt₃. Active center contents (C*) in the range 2–8% (of total Ti present) were measured and a strong correlation between catalyst activity and active center content was found, indicating that the high activity of the diether‐containing catalysts is due to an increased proportion of active centers rather than to a difference in propagation rate coefficients. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1635–1647, 2006     

High activity Ziegler–Natta catalysts for the preparation of ethylene copolymers

High activity ethylene polymerization catalysts have been prepared by the interaction of ethylmagnesium chloride in tetrahydrofuran with high surface area silica, followed by reaction with excess titanium tetrachloride in heptane. The catalysts were tested in ethylene—hexene copolymerization reactions in the presence of AlEt₃ at 80°C. For comparison purposes, the copolymerization properties of a similar catalyst prepared without silica were also evaluated. Preparative conditions were identified which provide catalysts that possess high reactivity towards 1-hexane. The silica and the amount of magnesium used in catalyst preparation strongly affect the copolymerization properties of the catalysts. Generally, catalysts prepared with silica showed much higher sensitivity to 1-hexene (effective reactivity ratio r₁ = 25–60) while a similar catalyst prepared without silica exhibited an r₁ value of 125. Fractionation of the copolymer with a series of boiling solvents showed that all the catalysts exhibit a wide distribution of active centers with respect to reactivity ratios, with the r₁ values varying from 5–7 to ca. 200. The width of a the center distribution depends on catalyst composition—it is the narrowest for the catalyst prepared without silica and is the widest for the catalysts with intermediate Ti : Mg ratios.     

Polymerization of α-methylstyrene by living polymer in tetrahydrofuran

Kinetics of polymerization of α-methylstyrene by poly-α-methylstyrylsodium (a “living” polymer) has been studied in tetrahydrofuran at ?78°C. Complex dependences were established: that of the conversion X on reduced time φ and that of the apparent rate constant for the polymer chain propagation on conversion X and on the concentration of living polymers and monomer. The experimental data obtained were explained by assuming a coordination mechanism of anionic polymerization including the following elementary reaction: (a) generation of active polymerization centers (K₁) by interaction of the living polymer with the monomer; (b) propagation of the polymer chains (K₂); (c) monomolecular (K₃₁) and bimolecular (K₃₂) reactions of isomerization of active centers resulting in the formation of high molecular weight living polymers capable of again becoming active centers of polymerization. Approximate derivation of kinetic equation was carried out and the constants of elementary reactions were determined (K₁ = 0.15, K₂ = 24, K₃₂ = 14.1./mole-min and K₃₁ = 0.05 min^?1). The coincidence of the expected dependencies X = F(τ; φ) K_p = F(X; n₀^?1/2); dx/dτ = F(n₀) with the experimental ones was followed with the aid of computers. The expected change in the values of X and K_p depending on the contribution of each elementary reaction to the overall polymerization process was analyzed.     

Heterodinuclear Mg(II)M(II) (M=Cr,Mn, Fe,Co, Ni,Cu and Zn) Complexes for the Ring Opening Copolymerization of Carbon Dioxide/Epoxide and Anhydride/Epoxide

The catalysed ring opening copolymerizations (ROCOP) of carbon dioxide/epoxide or anhydride/epoxide are controlled polymerizations that access useful polycarbonates and polyesters. Here, a systematic investigation of a series of heterodinuclear Mg(II)M(II) complexes reveals which metal combinations are most effective. The complexes combine different first row transition metals (M(II)) from Cr(II) to Zn(II), with Mg(II); all complexes are coordinated by the same macrocyclic ancillary ligand and by two acetate co-ligands. The complex syntheses and characterization data, as well as the polymerization data, for both carbon dioxide/cyclohexene oxide (CHO) and endo-norbornene anhydride (NA)/cyclohexene oxide, are reported. The fastest catalyst for both polymerizations is Mg(II)Co(II) which shows propagation rate constants (k_p) of 34.7 mM⁻¹ s⁻¹ (CO₂) and 75.3 mM⁻¹ s⁻¹ (NA) (100 °C). The Mg(II)Fe(II) catalyst also shows excellent performances with equivalent rates for CO₂/CHO ROCOP (k_p=34.7 mM⁻¹ s⁻¹) and may be preferable in terms of metallic abundance, low cost and low toxicity. Polymerization kinetics analyses reveal that the two lead catalysts show overall second order rate laws, with zeroth order dependencies in CO₂ or anhydride concentrations and first order dependencies in both catalyst and epoxide concentrations. Compared to the homodinuclear Mg(II)Mg(II) complex, nearly all the transition metal heterodinuclear complexes show synergic rate enhancements whilst maintaining high selectivity and polymerization control. These findings are relevant to the future design and optimization of copolymerization catalysts and should stimulate broader investigations of synergic heterodinuclear main group/transition metal catalysts.     

The Catalytic Dehydrogenation of c2h6 to c2h4 under non-Oxidative Conditions over the 6cr/g-al2o3Catalyst

In the present paper, the catalytic dehydrogenation of C₂H₆ to C₂H₄ under non-oxidative conditions was investigated in a fixed-bed micro-reactor under ambient pressure at 823 - 923 K. The 6Cr/g-Al₂O₃ catalyst was found to be the best catalyst among the g-Al₂O₃, SiO₂, MCM41, MgO and Si-2 supported chromium oxide catalysts. The features of the 6Cr/g-Al₂O₃ catalyst for the reaction could be listed as follows: (1) At 823 - 923 K, the C₂H₄ selectivity of 92.5-78.6% at a C₂H₆ conversion of 9.5-29.8% could be obtained. (2) The catalyst had the good regeneration performance, i.e., could be regenerated by air repeatedly. (3) The main products were C₂H₄, CH₄, H₂ and coke. No carbon oxides were identified.     

Homopolymerization of propylene oxide and copolymerization of propylene oxide and carbon dioxide with metal salts of acetic acid

The homopolymerization of propylene oxide was first conducted at 80°C in the absence of any solvent by using various metal salts of acetic acid and it was found that Mg(OAc)₂, Cr(OAc)₃, Mn(OAc)₂, Co(OAc)₂, Ni(OAc)₂, Zn(OAc)₂, and Sn(OAc)₂ were effective for the polymerization. The copolymerization of propylene oxide and carbon dioxide was next examined by using these effective metal salts of acetic acid as catalysts. Most of these were effective also for the copolymerization. The nature of the polymer obtained was strongly dependent on the catalyst used. Co(OAc)₂ and Zn(OAc)₂ gave an alternate copolymer of propylene oxide and carbon dioxide, Mg(OAc)₂, Cr(OAc)₃, and Ni(OAc)₂ gave a random copolymer, while Sn(OAc)₂ gave a homopolymer of propylene oxide. Then the copolymerization of propylene oxide and carbon dioxide was kinetically investigated in some detail by using Co(OAc)₂ as a catalyst. On the basis of the results obtained, a plausible mechanism was proposed for both the homopolymerization of propylene oxide and copolymerization of propylene oxide and carbon dioxide.     

碱金属助剂类型及前驱物对改性NiAl复合氧化物催化分解N2O活性的影响

制备了不同Ni/Al原子比的NiAl类水滑石样品,焙烧获得NiAl复合氧化物,用于N₂O分解反应,研究了NiAl复合氧化物组成对催化活性的影响。在活性较高的NiAl复合氧化物表面浸渍碱金属碳酸盐溶液,制备改性NiAl复合氧化物,考察了碱金属类型（Na、K、Cs）和钾前驱物（K₂CO₃、K₂C₂O₄、CH₃COOK、KNO₃）对改性催化剂活性的影响。用XRD、ICP-AES、FT-IR、BET、H₂-TPR、XPS技术表征了催化剂的组成结构。结果表明,Ni/Al原子比为2.7的NiAl复合氧化物催化活性较高;Na、K、Cs碳酸盐改性NiAl复合氧化物均提高了催化剂活性,其中K的助剂效应最强。钾前驱物对K改性NiAl复合氧化物的催化活性有显著影响,其中碳酸钾、醋酸钾、草酸钾的加入明显提高了改性催化剂的催化活性,而加入硝酸钾反而降低了催化剂活性。     

Homo- and copolymerization of butadiene and styrene with neodymium tricarboxylate catalysts

Homo- and copolymerizations of butadiene (BD) and styrene (St) with rare-earth metal catalysts, including the most active neodymium (Nd)-based catalysts, have been examined, and the cis-1,4 polymerization mechanism was investigated by the diad analysis of copolymers. Polymerization activity of BD was markedly affected not only by the ligands of the catalysts but also by the central rare-earth metals, whereas that of St was mainly affected by the ligands. In the series of Nd-based catalysts [Nd(OCOR)₃:R = CF₃, CCl₃, CHCl₂, CH₂Cl, CH₃], Nd(OCOCCl₃)₃ gave a maximum polymerization activity of BD, which decreased with increasing or decreasing the pKa value of the ligands. This tendency was different from that for Gd(OCOR)₃ catalysts, where the CF₃ derivative led to the highest polymerization activity of BD. For the polymerization of St and its copolymerization with BD, the maximum activities were attained at R = CCl₃ for both Nd- and Gd-based catalysts. The copolymerization of BD and St with Nd(OCOCCl₃)₃ catalyst was also carried out at various monomer feed ratios, to evaluate the monomer reactivity ratios as r_BD = 5.66 and r_St = 0.86. The cis-1,4 content in BD unit decreased with increasing St content in copolymers. From the diad analysis of copolymers, it was indicated that Nd(OCOCCl₃)₃ catalyst controls the cis-1,4 structure of the BD unit by a back-biting coordination of the penultimate BD unit. Furthermore, the long range coordination of polymer chain by the neodymium catalyst was suggested to assist the cis-1,4 polymerization. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 241–247, 1998     

STUDIES ON THE KINETICS OF PROPYLENE POLYMERIZATION WITH THE COMPLEXTYPE TiCl_3 CATALYST

The active center concentration C_p, the rate constant k_p, and the activation energy of chain propagation E_p in the polymerization of propylene with complex-type TiCl_3-(C_2H_5)_2AlCl catalyst system were studied. The Mn was corrected by (?) value determined by GPC. The values thus obtained for C_p, k_p, and E_p at 50℃were 3.01 mol/mol Ti, 6.27 1/mol·sec, and 5.10 Kcal/mol respectively.The kinetic parameters were compared with those obtained from conventional TiCl_3·AlCl_2 catalyst, showing that the higher activity of the complex-type catalyst over the conventional catalyst is not only due to the higher C_p of the former, but to a greater extent due to the increase of the k_p value.     

Kinetics of ethylene polymerization in the presence of a homogeneous catalyst based on a bis(phenoxyimine) complex of zirconium(IV)

Changes in the molecular-weight characteristics of the product of ethylene polymerization in the course of reaction in the presence of a homogeneous catalytic system and in the number and reactivity of catalyst active sites were studied. The catalytic system consisted of bis[N-(3-tert-butylsalicylidene)anilinato]zirconium dichloride and methylalumoxane as an activator. This catalytic system exhibited the signs of unsteady-state conditions: the rate of polymerization dramatically decreased as the reaction time increased. At the onset of polymerization (to 5 min), the catalyst was single-site, and it produced low-molecular-weight polyethylene with M _w = (4–10) × 10³ g/mol. The fraction of active sites at the initial point in time was as high as 11% based on the initial amount of the zirconium complex. The reactivity of these centers was very high (the rate constant of polymer chain growth was 5.4 × 10⁴ l mol⁻¹ s⁻¹ at 35°C). As the polymerization time increased, the number of active sites decreased and the molecular-weight distribution of polyethylene broadened because of the decay of a portion of initial centers and the formation of new centers that produced high-molecular-weight polyethylene with M _w to 130 × 10⁴ g/mol. The propagation rate constant measured at a sufficiently long polymerization time (20 min) was lower than that at the initial point in time; this fact suggests the much lower reactivity of the new active sites.     

Phase-Transfer Catalysis: Free-Radical Polymerization of Methyl Methacrylate using K2S2O8-Quaternary Ammonium Salt Catalyst System. A Kinetic Study

Abstract

The kinetics of phase-transfer-agent-assisted free-radical polymerization of methyl methacrylate using K₂S₂O₈ as the water-soluble initiator and triethylbenzylammonium chloride (TEBA) as the phase-transfer catalyst (PTC) was investigated in toluene-water biphase media at 60°C. The effect of varying [MMA], [K₂S₂O₈], [TEBA], [H⁺], the ionic strength of the medium, and the temperature on the rate of polymerization (R _p) was studied. R _p was found to be proportional to [MMA]², [K₂S₂O₈]¹, and [TEBA]^0.5. Based on the kinetic results, a mechanism involving initiation of polymerization by phase-transferred S₂O₈ ^2- and termination by Q⁺ (quaternary ammonium ion) is proposed.     

Probing the steric limits of rhodium catalyzed hydrophosphinylation. P-H addition vs. dimerization/oligomerization/polymerization

The reactivity of secondary phosphine oxides containing bulky organic fragments in hydrophosphinylation reactions has been investigated using several rhodium based catalysts. Upon heating in a focused microwave reactor, HP(O)(2-C₆H₄Me)₂ adds to prototypical terminal alkynes affording a complex mixture containing 1,2 and 1,1-addition products. Addition of a second ortho-substituent (HP(O)Mes₂) completely suppresses the hydrophosphinylation reaction for alkyl and aryl substituted alkynes. Variations in the temperature, catalyst loading, solvent, and microwave power were unable to induce an addition reaction in the case of HP(O)Mes₂. While this secondary phosphine oxide did not participate in the hydrophosphinylation reaction, it promoted the polymerization of phenylacetylene. HP(O)R₂ substrates are not commonly thought of as innocent ligands for rhodium complexes in reactions involving alkynes due to facile hydrophosphinylation. While this is certainly true for diphenylphosphine oxide, the chemistry presented herein suggests that HP(O)Mes₂ and related bulky secondary phosphine oxides have great potential as valuable ligands for rhodium catalyzed transformations involving alkynes due to their lack of reactivity towards the addition reaction.     

Ethanol to Acetaldehyde Conversion under Thermal and Microwave Heating of ZnO-CuO-SiO2 Modified with WC Nanoparticles

The nonoxidative conversion of ethanol to acetaldehyde under thermal and microwave heating was studied on mixed oxide ZnO-CuO-SiO₂ catalysts modified with additives of tungsten carbide nanoparticles. The results revealed that the WC-modified catalyst exhibited superior activity and selectivity under microwave heating conditions. It is assumed that when microwave heating is used, hot zones can appear at the contact points of WC nanoparticles and active centers of the mixed oxide ZnO-CuO-SiO₂ catalyst, which intensively absorb microwave energy, allowing the more efficient formation of acetaldehyde at moderate temperatures. Thermodynamic calculations of equilibrium concentrations of reagents and products allowed us to identify the optimal conditions for effective acetaldehyde production. The initial catalyst and the catalyst prepared by the coprecipitation of the oxides with the addition of WC were characterized by physicochemical methods (TPR-H₂, XRD, DRIFTS of adsorbed CO). The active centers of the oxide catalyst can be Cu⁺ cations.