Reactivity Of Active Centers Research Articles

Simple/commercially‐available Lewis acid in anionic ring‐opening polymerization: powerful compounds with multiple applications in macromolecular engineering

Simple and commercially‐available Lewis acids (LA) are commonly used catalysts in anionic ring‐opening polymerization (AROP) reactions. In particular, for the AROP of epoxides, the addition of a Lewis acid allows the transition from a so‐called end‐chain mechanism to a monomer‐activated mechanism. The presence of the LA simultaneously leads to a decrease in the reactivity of active centers through the formation of a three‐species ate complex and to the activation of the monomer by LA coordination to the oxygen atom of the oxirane ring. These two effects result in both an increase in propagation kinetics and a decrease in transfer reactions, which has enabled the synthesis of high molecular weight polyethers. However, the impact of Lewis acids goes far beyond these classic effects. They have indeed enabled the polymerization of new functional monomers as well as the synthesis of heterotelechelic macromolecules. Also widely used as catalysts in copolymerization reactions (statistical, sequential, or alternating) Lewis acids can strongly influence the composition and sequence of monomer units in macromolecules. Finally, Lewis acids can also significantly influence the architecture of the obtained macromolecules. This review aims to list the various contributions of Lewis acids to macromolecular engineering and illustrate them with well‐chosen examples.

Effects of Internal Electron Donor on Distribution and Reactivity of Active Centers in Ethylene/1‐Hexene Copolymerization with MgCl2‐Supported Ziegler‐Natta Catalyst

AbstractEthylene/1‐hexene copolymerization with two MgCl2‐supported Ziegler‐Natta catalysts containing no internal electron donor or diethylphthalate (DEP) is conducted for different polymerization time . Effects of DEP on active center distribution are studied by fractionating each copolymer sample into boiling n‐heptane soluble (C7‐sol) and insoluble (C7‐ins) fractions, and counting the number of active centers in the copolymer fractions . The main effect of introducing DEP in the catalyst are reduction in the Ti content and significant increase in the proportion of active centers producing C7‐ins fraction. The propagation rate constants of ethylene insertion (kpE) and 1‐hexene insertion (kpH) are respectively estimated by linear fitting/extrapolating the change of apparent propagation rate constants (kpi)a with polymer yield according to a simplified multi‐grain particle model. In both catalysts, kpE in the C7‐ins fraction is 9–12 times larger than that in the C7‐sol fraction, and kpH in the C7‐ins fraction is 3–4 times larger than that in the C7‐sol fraction. The two groups of active centers have distinctly different catalytic properties. Introducing DEP reduced the kpE and kpH values and the extent of diffusion limitation . In summary, addition of electron donor in MgCl2‐supported Z‐N catalyst significantly changed the active center distribution and catalytic properties of its two groups of active centers.

Evaluation of Pd/ZSM-5 catalyst for simultaneous reaction of transesterification and partial catalytic transfer hydrogenation of soybean oil under supercritical methanol

Biodiesel, termed as Fatty Acid Methyl Esters (FAMEs), has an essential role in reducing greenhouse gases by replacing fossil fuel and is needed to be partially hydrogenated to enhance its fuel properties. In this study, biodiesel was synthesized by the simultaneous reaction of transesterification and partial catalytic transfer hydrogenation (CTH) of soybean oil under supercritical methanol over Pd/ZSM-5 or commercial Pd/Al2O3. At 300 °C, 10 MPa, 45:1 of the molar ratio of methanol: oil with 0.5 mg Pd/g oil for 30 min, the FAMEs yield for Pd/ZSM-5 was 97.1%. On the other hand, the FAMEs yield for Pd/Al2O3 was 58.0% under the same condition. Pd/ZSM-5 was less active for CTH despite the higher dispersion of Pd nanoparticles (NPs) than Pd/Al2O3. The reactivity of Pd/ZSM-5 for CTH would be affected by the unique pore structure of ZSM-5 that seems to inhibit the hydrogenation of FAMEs on Pd NPs by confining the reactant diffusion. Consequently, biodiesel obtained by Pd/ZSM-5 had satisfactory fuel properties to the biodiesel standard specification at once. This work highlights that the highly efficient catalyst for producing the upgraded biodiesel could be developed by manipulating the reactivity of active centers via shape selectivity.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

AbstractKinetics of propylene polymerization with two TiCl4/MgCl2 model catalysts of different Ti load (Cat‐1: Ti 0.1 wt%; Cat‐2: Ti 1.0 wt%) was investigated in this work. Time‐dependent variations of active centers fraction ([C*]/[Ti]) and active center distribution of the two catalysts were compared. In propylene polymerization with Cat‐1, [C*]/[Ti] was only 7.8 % in the initial stage, but increased for more than three times in reaction period of 600 s. Propylene polymerization with Cat‐2 had much lower [C*]/[Ti]. Active center distributions of the two catalysts were markedly different. Cat‐1 has evidently higher proportion of aspecific active centers and less isospecific ones than that of Cat‐2. These catalysts also showed different time‐dependent variations of active center distribution. The chain propagation rate constant of isospecific centers in Cat‐2 was evidently higher than that in Cat‐1, but propagation rate constants of either aspecific and medium‐isospecific centers were only slightly changed by enhancing Ti content of the catalyst. The results suggest that isospecific centers are mainly originated from clustered Ti species, but non‐clustered Ti species are the main source of active centers with lower stereoselectivity. By considering factors like dispersion states of Ti species and release of shielded active species through disintegration of catalyst particles, active center models have been proposed to explain the different kinetic behaviors of the two model catalysts.

The Kinetic Study of Ethylene Polymerization over Titanium–Magnesium Catalysts in the Presence of Hydrogen: The Number and Reactivity of Active Centers

Using the inhibition of polymerization by radioactive carbon monoxide 14СО, the effect of hydrogen (chain transfer agent) on the number of active centers and propagation rate constant in ethylene polymerization over modern highly active titanium–magnesium catalysts with different composition is studied. It is found that a decrease in the rate of ethylene polymerization in the presence of hydrogen is associated primarily with reduction in the number of active centers and these changes are reversible at the introduction/removal of hydrogen. The scheme of reversible temporary deactivation of active center precursors is advanced to explain the experimental results. According to the data on the effect of hydrogen concentration on the molecular weight of polyethylene and the values of propagation rate constants, the rate constant of polymer chain transfer by hydrogen is calculated for catalysts with different composition.

Introducing electron-donating substituents on ligand backbone of α-diimine nickel complex and the effects on catalyst thermal stability in ethylene polymerization

A novel α-diimine nickel pre-catalyst Cat.1 with two electron-donating methyl substituents on the ligand backbone was synthesized and investigated for ethylene polymerization using methylaluminoxane as cocatalyst. Cat.1 exhibited higher polymerization activity and produced highly branched polyethylene with slightly higher molecular weight than that of the typical Brookhart catalyst (B-Cat) at elevated temperature. By tracing the changes of active center number in the polymerization process, Cat.1 was found to have higher ratio of activated nickel ([C∗]/[Ni]), longer catalyst lifetime and larger chain propagation rate constant (kp) as compared with B-Cat. The computational studies demonstrated the electron-donating methyl substituents can make a more electron-rich ligand to influence the catalytic properties of Cat.1. Stronger interactions between the electron-rich ligand and the nickel center should be the main reasons for longer catalyst lifetime and enhanced thermal stability of Cat.1. The production of polyethylene with higher molecular weight by Cat.1 can be explained by its larger kp value and more stable transition state for ethylene insertion than for chain transfer reaction due to the electron-donating ligand. Overall, the results of this work clearly support the importance of ligand electronic effects on the thermal stability and intrinsic reactivity of active centers in α-diimine Ni(II) catalysts.

Ethylene polymerization reactions with multicenter Ziegler‐Natta catalysts—Manipulation of active center distribution

AbstractThis article describes ethylene/1‐hexene copolymerization reactions with a supported titanium‐based, multicenter Ziegler‐Natta catalyst. The catalyst was modified by pretreating its solid precursor with AlEt2Cl and with similar organoaluminum chlorides, Al2Et3Cl3, AlEtCl2, and AlMe2Cl. Testing of the untreated and the pretreated catalysts in copolymerization reactions under standard reaction conditions demonstrated that the modifying agents produce two changes in the catalyst. First, the pretreatment significantly reduces the reactivity of active centers that produce high molecular weight, highly crystalline copolymer components with a low 1‐hexene content. Second, the pretreatment noticeably increases the reactivity of active centers that produce low molecular weight copolymer components with a high 1‐hexene content. The first effect is caused by Lewis acid‐base interactions of the modifiers with the active centers, whereas the second (activating) effect is due to the removal of catalyst poisons (organosilicon compounds generated in the process of the catalyst synthesis) by AlEt2Cl. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4219–4229, 2010

Distribution of active centers on TiCl4/MgCL2 catalyst for olefin polymerization

1-Octene was polymerized with TiCl4/MgCl2—AlEt3 and the polymerization was quenched with CH3COCl to introduce a CH3CO— group onto each propagation chain. The polymer was fractionated by fractional precipitation, and the number of active centers in each fraction was determined by measuring the CH3CO— content of the fraction. Distributions of the number and reactivity of active centers among the fractions were determined and discussed based on these experiments. The active center distributions were also studied by fitting the molecular-weight distribution (MWD) curve from GPC with multiple Schulz-Flory most-probable distributions. The uncontinuous reactivity distribution of active centers reveals that there exist limited types of active centers on the catalyst. Five types of active center were distinguished by the MWD fitting treatment. The correlations between the active center distributions and catalyst structures are discussed. © 1996 John Wiley & Sons, Inc.

Electronic structure, conformation and reactivity of active centres of anionic polymerization with an aluminium counterion

The quantum-chemical CNDO/2 method was used in connection with the anionic polymerization of unsaturated compounds initiated by organoaluminium compounds. Comparison of the results for model compounds Al(XH n) 3 H 2AlXHP n (X = O, N, C) with the data for analogous lithium and magnesium derivatives showed that the polarity of the MtX bond (calculated taking account of the metal valency) and the order of this bond are reactivity indices. According to these indices, amido- and alkoxy-alkylaluminium derivatives should be reactive in initiation. Absent of activity for the alkoxy derivatives is probably due to the stability of their aggregates. The optimization of the geometry of the molecule CH 3CH(COOCH 3)AlH 2, modelling the propagation active centre, showed that, as a result of the competing interaction of the Al atom with the growing chain and the side radical, the AlC bond is greatly weakened. Its reactivity evaluated from the above mentioned indices exceeds that of alkylaluminium amide initiators. The geometry of the complex of the model compound, CH 3AlH 2, with the methyl acrylate molecule was favourable for the subsequent insertion of the monomer into the chain. Possible reasons for the discrepancy between the experimental and calculated data are considered for acrylonitrile.

The structure and reactivity of active centres of ionic polymerization of heterocyclic compounds. Review

Problems of the structure and reactivity of active centres of cationic, anionic and coordination-anionic polymerization of heterocyclic compounds are discussed. Reliable information on the structure of these active centres can be obtained from the kinetics of the isotopic effect with respect to 13C and 14C. By this method experimental proof was obtained for the first time, that the active centre in cationic polymerization of dioxolan is a cyclic oxonium ion. The polymer chain has a substantial effect on the structure, stability and activity of the centres of ionic polymerization of α-oxides, α-thio-oxides and N-carboxy-α-amino acid anhydrides. Around the metal (Zn, Mg, Cd, Al, Fe) or cation (Na +, K +, Rb +, Cs + the last units of the polymer chain form an asymmetric coordination sphere, like the right and left hand α-helices of polypeptides. This gives rise to enantiomorphic active centres, the stability and activity of which is dependent on the nature of the metal or cation and the structure of the last six to ten polymer chain units attached to it, especially their microstructure the donating power of the heteroatoms and the nature of the side groups. The occurrence of such active centres explains the occurrence of stereoelective as well as stereoselective polymerization of these heterocyclic compounds in coordination-anionic and anionic polymerization.

Effect of the initiator solvation on the change in polymerization rates of dioxolane and tetrahydrofuran

AbstractThe cocatalytical effects of water and ethanol on the cationic polymerization of dioxolane and tetrahydrofuran were studied. The ion pair or were used for initiating the polymerization. The dependence of the polymerization rate on the concentration of cocatalyst was examined with various temperatures, concentrations of monomer, solvents (heptane, tetrachloroethane, tetrachloroethane‐heptane mixtures, and 1,4‐dioxane), and concentrations of initiator. The abscissa of the maximum of the reaction rate in the dependence mentioned above was the criterion for evaluation of the effect of the reaction variables. The changes observed are small; nevertheless, they prove the share of all components of the polymerizing system in establishing solvation equilibria, which determine the number and the reactivity of active centers and, in fact, the reaction rate.