Magnesium chloride supported high-mileage catalysts for olefin polymerization. XVII. Effect of lewis base on propylene polymerization

A systematic study has been made on the functions of external Lewis base (B_e, methyl-p-toluate, MPT) and internal Lewis base (B_i, ethyl benzoate, EB) for the CW-catalyst system MgCI₂/EB/PC/AlEt₃/TiCl₄–AlEt₃/MPT (PC, p-cresol). B_i is a nonstereoselective modifier. It increases the active site concentrations and rate constants of propagation, k_p, of both the isospecific and nonspecific sites, and thus the productivities of the stereoregular and irregular polypropylenes by five- to tenfold. It seems that B_i complexes with the MgCl₂ support to lower the electronegativity of the surface Mg atoms. It also acts to lower the rate constant of chain transfer to aluminum alkyl, k, by two- to fourfold. The action of B_e is highly stereospecific. The isotacticity index of polypropylene is ? 95% in the presence of B_e but ? 68% without it. Addition of B_e decreases nonspecific [Ti^*]_a by about (11 ± 2)-fold; there is only about a twofold reduction of the isospecific [Ti^*]_i. It decreases k_p,a about three times but has no effect on k_p,i, so that the latter is (21 ± 4) times the former. B_e decreases k for transfer with aluminum alkyl much more than it does to k; but it does not affect the rates of chain transfer with monomer and by β-hydride elimination or the rate of catalyst deactivation. The number of active sites without B_e is [Ti^*]_i = 15% and [Ti^*]_a = 55% for a total of 70%. In the presence of B_e they are both about 6%. Optimum performance in propylene polymerizations requires both B_i and B_e in the case of the CW-catalyst.     

Magnesium chloride supported high mileage catalyst for olefin polymerization. VII. Determinations of concentrations of titanium–polymer and aluminum–polymer bonds

Two methods were used in an attempt to determine by radioquenching the active site concentration, [Ti*], in a MgCl₂ supported high activity catalyst. For the reactions of tritium labelled methanol, the kinetic isotope effects were first determined: k_H/k_T = 1.63 for the total polymer and 1.67 for the isotactic polypropylene fraction. Polymerizations were quenched with an excess of isotopic CH₃OH after various lengths of time, at different A/T (amount of AlEt₃ with 0.33 equivalent of methyl-p-toluate to amount of Ti in the catalyst) ratios, and temperatures. From the known specific activity of tritium in CH₃OH and radioassay of the polymer, value of the total metal polymer bond, [MPB], can be obtained. [MPB] increases linearly with polymerization time. Extrapolation to t = 0 gives [MPB]₀, which should be close to [Ti*] because chain transfer with aluminum alkyls to produce Al–P bonds is negligible during very early stage of the polymerization. The values of [MPB]₀ range from 7–30% of the total Ti; the number of MPB is nearly equally distributed in the amorphous and isotactic fractions of polypropylene in most runs. The rate of incorporation of radioactive CO into polymers produced by the MgCl₂ supported high mileage catalyst is far slower than that claimed by some investigators for TiCl₃ type catalysts. There is an initial rapid phase of incorporation of CO which lasts for about 1 hr of contact time. The subsequent rate of CO incorporation steadily declines, yet there is no constant maximum value of radioactivity even after 48 h of reaction in the absence of monomer. Radioquenching of polymerizations with CO was also performed at several temperatures and A/T ratios. In all cases, the maximum [Ti–P] was reached after 30–40 min of polymerization, whereas the maximum rates of polymerization, R_p,m, occurred within 3–10 min. In fact, the rate of polymerization decays to a small fraction of R_p,m after 30–40 min. Furthermore, this maximum value of [Ti–P] remains constant until the end of polymerization (t = 90 min). Therefore, isotopic CO is not reacting with the initially formed active sites Ti₁*, but only with those sites, Ti₂*, which predominate during the later stage of polymerization.     

Magnesium-chloride-supported high-mileage catalysts for olefin polymerization. III. Electron paramagnetic resonance studies

Electron paramagnetic resonance (EPR) was used to study a MgCl₂-supported, high-mileage olefin polymerization catalyst. Anhydrous Toho MgCl₂ was the starting material. Treatment with HCl at an elevated temperature, ethyl benzoate by ball-milling, p-cresol, AlEt₃, and TiCl₄produced a catalyst that contained a single EPR observable Ti⁺³ species A, which was strongly attached to the catalyst surface, had a D_3h symmetry, and no other Ti⁺³ ion in an immediately adjacent site. Species A constitutes only 20% of all the trivalent titaniums; the remainder is EPR-silent and may be attributed to those Ti⁺³ ions that have adjacent sites occupied by one or more Ti⁺³ ions. Activation with preformed AlEt₃/methyl-p-toluate complexes produced a single Ti⁺³ species (C) with rhombic symmetry and displaying ²⁷Al superhyperfin splitting which has attributes for a stereospecific active site. This species is unstable under polymerization conditions and is transformed to another species with axial symmetry and solubilization. Both processes could lead to catalyst deactivation and loss of stereospecificity. Catalysts activated by AlEt₃ and methyl-p-toluate separately in various sequential orders produced a multitude of EPR-observable Ti⁺³ species with varying degrees of motional freedom deemed detrimental to stereospecific polymerization of α-olefins.     

Magnesium chloride supported high mileage catalysts for olefin polymerization. XII. Polymerization of ethylene

Polymerizations of ethylene by the MgCl₂/ethylbenzoate/p-cresol/AlEt₃ TiCl₄-AlEt₃/methyl-p-toluate (CW-catalyst) have been studied. The initially formed active site concentration, [Ti] has a maximum value of 50% of total titanium at 50°C and lower values at other temperatures. The Ti decays rapidly to Ti sites with conc. ca. 10 mol %/mol Ti. The rate constants for four chain transfer processes have been obtained at 50°C: for transfer with AlEt₃, k = 2.1 × 10^?4 s^?1 and k = 4.8 × 10^?4 s^?1; for transfer with monomer, k = 3.6 × 10^?3 (M s)^?1 and K = 8.3 × 10^?3 (M s)^?1; for β-hydride transfer, k = 7.2 × 10^?4 s^?1 and k = 4.9 × 10^?4 s^?1; and transfer with hydrogen, k = 4.0 × 10^?3 torr^1/2 s^? and k = 5.1 × 10^?3 torr^1/2 s^?1. The rate constants for the termination assisted by hydrogen is k = 1.7 (M^1/2 torr^1/2 S)^?1. If monomer is assisting termination as was observed for propylene polymerization, then k = 7.8 (M^3/2 s)^?1. Values of all the rate constants can be higher or lower at other temperatures. Detailed comparisons were made with the results of propylene polymerizations. There are more than four times as many Ti active sites for ethylene polymerization than there are for stereospecific polymerization of propylene; the difference is more than a factor of two for the Ti sites. Certain rate constants are nearly the same for both monomers while others are markedly different. Some of the differences can be explained by stereoelectronic effects.     

Kinetics and mechanism of acetylene polymerization

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, R_p, [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.     

Mechanism of olefin polymerization on supported Ziegler-Natta catalysts based on data on the number of active centers and propagation rate constants

The number of active centers (C _g) and propagation rate constants (k _g) for the polymerization of propylene and ethylene on highly active titanium-magnesium catalysts (TMCs) of different compositions at 70°C were determined using the method of ¹⁴CO inhibition of polymerization. In the polymerization of propylene on the TiCl₄/D₁/MgCl₂-AlEt₃/D₂ system (D₁ is dibutyl phthalate or 2,2-diisobutyl-1,3-dimethoxypropane; D₂ is a silane), the effects of D₁ and D₂ donors on the values of C _g and k _g were studied. It was found that the donors decreased the values of k _g for nonstereospecific centers, had no effect on the values of k _g for stereospecific centers, and increased the fraction of stereospecific centers, as well as the fraction of sleeping centers regardless of their stereospecificity. The rate constants of isotactic-chain transfer with C₃H₆, AlEt₃, H₂,and ZnEt₂ were determined. In the polymerization of ethylene, a number of TMCs exhibited strong diffusion limitations, which manifested themselves in a dramatic decrease in the determined values of k _g. It was demonstrated that diffusion limitations can be removed by decreasing the particle size and the concentration of active centers and by performing prepolymerization with propylene. The values of k _g in ethylene polymerization were similar for stereospecific and nonstereospecific centers.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 180–190.Original Russian Text Copyright © 2005 by Bukatov, Zakharov, Barabanov.     

Magnesium chloride supported catalysts for olefin polymerization. XIX. Titanium oxidation states,catalyst deactivation,and active site structure

A precise method for the determinations of Ti⁺², Ti⁺³ and Ti⁺⁴ was developed. The CW-procatalyst before activation contains mostly Ti⁺⁴ ions with 6% Ti⁺³ and 4% Ti⁺² ions. Activation with AlEt₃ alone at room temperature reduced all the titaniums to lower valence states consisting of 71% Ti⁺³ and 29% Ti⁺². Reduction is incomplete when methyl-p-toluate was present as external Lewis base during activation: at 25°C the distribution of Ti⁺⁴ : Ti⁺³ : Ti⁺² is 36% : 25% : 38%; the distribution at 50°C is 37% : 22% : 40%. Aging of the activated catalyst caused little or no changes in the distribution of [Ti⁺ⁿ]; whereas the catalytic activity decays rapidly with aging. The aged catalysts have polymerization activity comparable to the decreased activity of the catalyst during a polymerization. The [Ti⁺ⁿ] was determined for the CW-catalyst during the course of a decene polymerization; they were found to be Ti⁺⁴ : Ti⁺³ : Ti⁺² = 30% : 27% : 43%, which did not change with polymerization time. These results suggest that the reducibility of Ti⁺⁴ species by AlEt₃ or 3AlEt₃/MPT to different valence states is predicated by their structures. These species do not undergo further changes in their oxidation states during either aging or polymerization. Their decays probably involve nonreductive metathesis reactions like those known for zirconium alkyls. Possible structures for the stereospecific and nonspecific sites are proposed.     

Metallocene-methylaluminoxane catalysts for olefin polymerization. V. Comparison of Cp2ZrCl2 and CpZrCl3

The ethylene polymerization by Cp₂ZrCl₂/MAO (Cp = η⁵: cyclopentadienyl; MAO = methyl aluminoxane) and CpZrCl₃/MAO have been studied. The MW and PD (= M _w/M _w) of polymers obtained after 2.5-60 min are the same, which indicate short chain lifetime. The values of rate constants for Cp₂ZrCl₂ at 70°C are: k_p = 168?1670 (M s)^?1 and k_tr^A1 = 0.012-0.81 s^?1 depending upon [Zr] and [MAO,] k_tr^β = 0.28 s^?1, and k_tr^H = 0.2 M^?1 torr^?1/2 s^?1. These chain transfer rate constant values are two to three orders of magnitude greater than the corresponding values found for MgCl₂ supported titanium catalysts. One significant difference between the heterogeneous and homogeneous catalysts is that the former decays according to an apparent second order kinetics, whereas the latter decay is simple first order at 0°C and biphasic first order at higher temperatures. The productivity of the catalysts depends weekly on temperature while the MW decreases strongly with increase of temperature above 30°C. All the active species were formed upon mixing Cp₂ZrCl₂ with MAO while it took up to 20 min for the CpZnCl₃/MAO system. The productivity of the former increase more strongly with the decrease of [Zr] than the latter. Otherwise, the two catalyst systems have all their kinetic parameters differing less than a factor of two.     

Olefin copolymerization with metallocene catalysts. II. Kinetics,cocatalyst, and additives

The kinetics of ethylene/propylene copolymerization catalyzed by (ethylene bis (indeyl)-ZrCI₂/methylaluminoxane) has been investigated. Radiolabeling found about 80% of the Zr to be catalytically active. The estimates for rate constants at 50°C are k₁₁ = 1104 (M_s)^?1, k₁₂ = 430 (M_s)^?1, k₂₂ = 396 (M_s)^?1,k₂₁ = 1020 (M_s)^?1, and k^A_tr,1 + k^A_tr.2 = 1.9 × 10^?3 s^?1. Substitution of trimethylaluminum for methylaluminoxane resulted in proportionate decrease in polymerization rate. The molecular weight of the copolymer is slightly increased by loweing the [Al]/[Zr] ratio, or addition of Lewis base modifier but at the expense of lowered catalytic activity and increase in ethylene content in the copolymer. Lowering of the polymerization temperature to 0°C resulted in a doubling of molecular weight but suffered 10-fold reduction in polymerization activity and increase of ethylene in copolymer.     

Solvay type VCl3 isospecific catalysts

Solvay type S –VCl₃ catalyst has 7% of catalytically active vanadium sites ([C^*]) with k_p (rate constant of propagation) = 31 (M s)^?1 for ethylene polymerization. Addition of a comonomer, propylene of 4-methylpentene-1 (4-MP) significantly raised the ethylene polymerization activity. S –VCI₃ catalyst has very small amounts of catalytically active vanadium for propylene polymerizations: [C] = 0.19% with k_p,i = 857 (M s)^?1 and [C] = 0.45% with k_p,a = 23 (M s)^?1 for isospecific and nonspecific sites, respectively. Addition of a conomer, ethylene or 4-MP. lowered the propylene polymerization activity. S –VCI₃ is more easily reduced to the divalent ion by AIR₃ than S –TiCl₃. Methyl-p-toluate moderates the reducting power of AIR₃; it increase the productivity and stereoselectivity of the S –YiCl₃ catalyst, VCI₃ supported on MgCl₂ (CW–V catalyst) has enhanced rate constant of propylene polymerization but has the opposite effects on the S –TiCl₃ Catalyst. VCI₃ supported on MgCl₂ (CW–V catalyst) has enhances rate constant of propylene polymerization but only a minute fraction of the supported vanadiums are catalytically active: [C] = 0.019% and k_p,i = 1580 (Ms)^?1, [C] = 0.057% and k_p,i = 58 (M s)^?1. This is compared with far greater number of catalytically active titanium sites in the TiCl₃ supported on MgCl₂ catalyst: [C] = 6% and k_p,i = 200 (M s)^?1, [C] = 6% and k_p,a = 16(M s)^?1. Therefore, both the S –VCI₃ and CW–V catalysts are highly stereoselective but low in efficiency with respect to the utilization of the vanadium ion in the catalysis.     

Magnesium chloride supported high-mileage catalysts for olefin polymerizations. XVIII. Effect of hydrogen and lewis bases

Hydrogen has been found earlier to increase the initial rate of polymerization by MgCl₂/EB/PC/AlEt₃/TiCl₄-AlEt₃/MPT, CW-catalyst (+B_i, +B_e) (EB, ethyl benzoate; PC, p-cresol; MPT, methyl-p-toluate), but decays more rapidly as compared to polymerizations in the absence of H₂. In this study the effect of H₂ was studied when either the internal Lewis base, EB B_i, or the external Lewis base, MPT B_e, or both are deleted from the CW-catalyst. H₂ does not affect the stereospecificity of all the catalysts, but causes a slight increase of polymer yield, whereas the yield is virtually unchanged by H₂ for the catalysts activated with B_e. Unlike the catalyst (+B_i, +B_e) where H₂ increases active site concentrations [Ti^*] about threefold, it affects [Ti^*] negligibly when B_e is absent. The rate constants of propagation is about the same with or without H₂ for the CW-catalyst (+B_i, –B_e) or (–B_i, –B_e); the same statement can be said about the rate constant of chain transfer with AlEt₃ or with H₂. Hydrogen increases the rate of catalyst site deactivation for the various catalysts in the order of(+B_i, +B_e) > (–B_i, –B_e) > (+B_i, –B_e).     

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.     

Magnesium chloride supported high mileage catalysts for olefin (decene-l) polymerization XIV. Propagation and chain transfer

Decene-l was polymerized with the MgCl₂/ethylebenzoate/p-cresol/AIEt₃/TiCl₄-AlEt₃/methyl-p-toluate catalyst at 50° using an A/T ratio of 167 and a range of monomer concentration. The concentration of the two kinds of active sites are [Ti] = 12% and [Ti] = 4% of the total titanium. The rate constants of propagation are 24 M^?1 s^?1. Chain transfers to AIEt₃, monomer, and by β-hydride elimination have rate constant values of 1.7 × 10^?3 M^?1 s^?1, 1.34 × 10^?2 M^?1 s^?1, and 1.7 × 10^?2 s^?1, respectively. Poly(decene-l) have relatively narrow MW which are unchanged during the course of a polymerization. Therefore, the active site concentrations in the CW catalyst for propylene and decene polymerization are identical and their rate constant values agree within a factor of 2. However, the rate of decene polymerization depends on fractional order of monomer concentration and decreases with the increase of activator concentration. Furthermore, the formation of metal polymer bonds has a rate independent of these concentrations. These kinetic behaviors are a manifestation of absorption processes of these species which are not seen in propylene polymerizations.     

Kinetic Study of the Bromine-Catalyzed Isomerization of Maleic Acid in Aqueous Ce(IV)-Br−-H2SO4 Medium

The presence of ceric and bromide ions catalyzes the isomerization of maleic acid (MA) to fumaric acid (FA) in aqueous sulfuric acid. A kinetic study of this bromine-catalyzed reaction was carried out. The reaction between ceric ion and maleic acid is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M (adjusted by NaClO₄), and [MA]₀=(0.5–1.0)M, the observed pseudo-first-order rate constant (k₀₃) at 25° is k₀₃=7.622×10^?5 [MA]₀/(1+0.205[MA]₀). The reaction between ceric and bromide ions is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀=(0.025–0.150)M, the pseudo-first-order rate constant (k₀₂) at 25° is k₀₂= (4.313±0.095)x10^?2[Br^?]²+(2.060±0.119)x10^?3[Br^?]. The reaction of Ce(IV) with maleic acid and bromide ion is also first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [MA]₀=0.75 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀= (0.025–0.150)M, the pseudo-first-order rate constant (k₀₃) at 25° is k₀₃= (5.286±0.045)x10^?2[Br^?]²+(3.568±0.056)x10^?3[Br^?]. For [Ce(IV)]₀=5.0 × 10^?4 M, [Br^?]₀=0.050 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [MA]₀=(0.15–1.0)M at 25°, k₀₃=(2.108×10^?4+2.127×10^?4[MA]₀)/(1+0.205[MA]₀). A mechanism is proposed to rationalize the results. The effect of temperature on the reaction rate was also studied. The energy barrier of Ce(IV)—Br^? reaction is much less than that of Ce(IV)—MA reaction. Maleic and fumaric acids have very different mass spectra. The mass spectrum of fumaric acid exhibits a strong metastable peak at m/e 66.5.     

Metallocene–methylaluminoxane catalysts for olefin polymerizations. IV. Active site determinations and limitation of the 14CO radiolabeling technique

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^*.     

Magnesium chloride supported high-mileage catalysts for olefin polymerization. I. Chemical composition and oxidation states of titanium

The chemical composition of a MgCl₂-supported, high-mileage catalyst has been determined at every stage of its preparation. Ball milling of MgCl₂ with ethyl benzoate (EB) resulted in the incorporation of 95% of the EB present to give MgCl₂·EB_0.15. A mild reaction with a half-mole equivalent of p-cresol (PC) at 50°C for 1 h resulted in near quantitative retention of p-cresol by the support. The composition is now approximately MgCl₂·EB_0.15P?_0.5. Addition of an amount of AlEt₃ corresponding to half-mole equivalent of p-cresol liberated one mole of ethane per mole of p-cresol, thus signaling quantitative reaction between the two components. The support contains on the average one ethyl group per Al. Further reaction with TiCl₄ resulted in the incorporation of titanium of approximately 8, 38, and 54% in the oxidation states of +2, +3, and +4, respectively. The ratio of Al to Ti in the catalyst lies in the range of 0.5–1.0. Only 19% of all the Ti⁺³ species in the catalyst can be observed by electron paramagnetic resonance (EPR); these are attributable to isolated Ti⁺³ complexes. The remaining EPR silent Ti⁺³ species are believed to be bridged to another Ti⁺³ by Cl ligands. The total Cl content is equal to the sum of 2 × Mg + 3 × Al + 3.5 × Ti. Most of the p-cresol moiety apparently disappeared from the support, leaving much of ethyl benzoate in the catalyst. Activation with AlEt₃/methyl-p-toluate complex reduces 90% of the Ti⁺⁴ in the catalyst to lower oxidation states. The ester apparently moderates the alkylating power of AlEt₃ to avoid excessive formation of divalent titanium sites. There appears to be a constant fraction of 1/4–1/5 of the titanium which is isolated and the remainder is in bridged clusters independent of the oxidation states of titanium.     

Aluminum Complexes Stabilized by Piperazidine‐Bridged Bis(phenolate) Ligands: Syntheses,Structures, and Application in the Ring‐Opening Polymerization of ε‐Caprolactone

The synthesis and characterization of aluminum alkoxide and alkyl complexes stabilized by piperazidine‐bridged bis(phenolate) ligands are described. Treatment of ligand precursors H₂[ONNO]¹ {H₂[ONNO]¹=1,4‐bis(2‐hydroxy‐3‐tert‐butyl‐5‐methylbenzyl)piperazidine} and H₂[ONNO]² {H₂[ONNO]²=1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)piperazidine} with AlEt₂(OCH₂Ph) and AlEt₂(OPr‐i), which were generated in situ by the reactions of AlEt₃ with equivalent of the corresponding alcohols, in a 1:1 molar ratio in THF gave the corresponding aluminum alkoxide complexes [ONNO]¹Al(OCH₂Ph) ( 1 ) and [ONNO]²Al(OPr‐i) ( 2 ), respectively. The reaction of H₂[ONNO]¹ with AlEt₂(OCH₂Ph) in a 1:2 molar ratio in THF afforded a mixture of monometallic aluminum ethyl complex [ONNO]¹AlEt ( 3 ) and complex 1 , which can be isolated by stepwise crystallization. Similarly, H₂[ONNO]² reacted with AlEt₂(OPr‐i) in a 1:2 molar ratio in THF to give a mixture of aluminum ethyl complex [ONNO]²AlEt ( 4 ) and complex 2 . Complexes 1 and 2 were also available via treatment of complexes 3 and 4 with 1 equiv. of benzyl alcohol and isopropyl alcohol, respectively. All of these complexes were fully characterized including X‐ray structural determination. It was found that complexes 1 to 4 can initiate the ring‐opening polymerization of ε‐caprolactone, and complexes 1 and 2 showed higher catalytic activity in comparison with complexes 3 and 4 .     

A novel tridentate nitrogen donor as ligand in copper catalyzed ATRP of methyl methacrylate

A tridentate ligand, BPIEP: 2,6‐bis[1‐(2,6‐diisopropyl phenylimino) ethyl] pyridine, having central pyridine unit and two peripheral imine coordination sites was effectively employed in controlled/“living” radical polymerization of MMA at 90°C in toluene as solvent, Cu^IBr as catalyst, and ethyl‐2‐bromoisobutyrate (EBiB) as initiator resulting in well‐defined polymers with polydispersities M_w/M_n ≤ 1.23. The rate of polymerization follows first‐order kinetics, k_app = 3.4 × 10^?5 s^?1, indicating the presence of low radical concentration ([P*] ≤ 10^?8) throughout the reaction. The polymerization rate attains a maximum at a ligand‐to‐metal ratio of 2:1 in toluene at 90°C. The solvent concentration (v/v, with respect to monomer) has a significant effect on the polymerization kinetics. The polymerization is faster in polar solvents like, diphenylether, and anisole, as compared to toluene. Increasing the monomer concentration in toluene resulted in a better control of polymerization. The molecular weights (M_n,SEC) increased linearly with conversion and were found to be higher than predicted molecular (M_n,Cal). However, the polydispersity remained narrow, i.e., ≤1.23. The initiator efficiency at lower monomer concentration approaches a value of 0.7 in 110 min as compared to 0.5 in 330 min at higher monomer concentration. The aging of the copper salt complexed with BPIEP had a beneficial effect and resulted in polymers with narrow polydispersitities and higher conversion. PMMA obtained at room temperature in toluene (33%, v/v) gave PDI of 1.22 (M_n = 8500) in 48 h whereas, at 50°C the PDI is 1.18 (M_n = 10,300), which is achieved in 23 h. The plot of lnk_app versus 1/T gave an apparent activation energy of polymerization as (ΔE^≠_app) 58.29 KJ/mol and enthalpy of equilibrium (ΔH⁰_eq) to 28.8 KJ/mol. Reverse ATRP of MMA was successfully performed using AIBN in bulk as well as solution. The controlled nature of the polymerization reaction was established through kinetic studies and chain extension experiments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4996–5008, 2005     

Site‐Selective Ligand Exchange on a Heteroleptic TiIV Complex Towards Stepwise Multicomponent Self‐Assembly

A series of heteroleptic [Ti 1 ₂X]^? complexes have been selectively constructed from a mixture of Ti^IV ions, a pyridyl catechol ligand (H₂ 1 ; H₂ 1 =4‐(3‐pyridyl)catechol), and various bidentate ligands (HX) in the presence of a weak base, in addition to a previously reported [Ti 1 ₂(acac)]^? (acac=acetylacetonate) complex. Comparative studies of these Ti^IV complexes revealed that [Ti 1 ₂(trop)]^? (trop=tropolonate) is much more stable than the [Ti 1 ₂(acac)]^? complex, which allows the replacement of acac with trop on the [Ti 1 ₂(acac)]^? complex. This Ti^IV‐centered site‐selective ligand exchange reaction also takes place on a heteronuclear Pd^II? Ti^IV ring complex with the preservation of the Pd^II‐centered coordination structures. Intra‐ and intermolecular linking between two Ti^IV centers with a flexible or a rigid bis‐tropolone bridging ligand provided a tetranuclear and an octanuclear Pd^II? Ti^IV complex, respectively. These higher‐order structures could be efficiently constructed only through a stepwise synthetic route.