Observation of different catalytic activity of various 1-olefins during ethylene/1-olefin copolymerization with homogeneous metallocene catalysts

This research aimed to investigate the copolymerization of ethylene and various 1-olefins. The comonomer lengths were varied from 1-hexene (1-C?) up to 1-octadecene (1-C??) in order to study the effect of comonomer chain length on the activity and properties of the polymer in the metallocene/MAO catalyst system. The results indicated that two distinct cases can be described for the effect of 1-olefin chain length on the activity. Considering the short chain length comonomers, such as 1-hexene, 1-octene and 1-decene, it is obvious that the polymerization activity decreased when the length of comonomer was higher, which is probably due to increased steric hindrance at the catalytic center hindering the insertion of ethylene monomer to the active sites, hence, the polymerization rate decreased. On the contrary, for the longer chain 1-olefins, namely 1-dodecene, 1-tetradecene and 1-octadecene, an increase in the comonomer chain length resulted in better activity due to the opening of the gap aperture between C(p)(centroid)-M-C(p)-(centroid), which forced the coordination site to open more. This effect facilitated the polymerization of the ethylene monomer at the catalytic sites, and thus, the activity increased. The copolymers obtained were further characterized using thermal analysis, X-ray diffraction spectroscopy and 13C-NMR techniques. It could be seen that the melting temperature and comonomer distribution were not affected by the 1-olefin chain length. The polymer crystallinity decreased slightly with increasing comonomer chain length. Moreover, all the synthesized polymers were typical LLDPE having random comonomer distribution.     

Development of the mesomorphic phase in isotactic propene/higher α-olefin copolymers at intermediate comonomer content and its effect on properties

Several isotactic propene-α-olefin copolymers using 1-octene, 1-dodecene and 1-tetradecene as comonomers (CiPO, CiPDD and CiPTD, respectively) have been synthesized and their structure and thermal/mechanical/viscoelastic properties have been evaluated. At intermediate comonomer molar content ranged from around 4 to 9, the mesomorphic phase is developed under rather mild quenching conditions and independently of length of the comonomer. Thermal and mechanical parameters diminish as α-olefin content increases and their dependence on composition changes as function of existence of three-dimensional monoclinic crystallites or less ordered entities.     

Main kinetic features of ethylene polymerization reactions with heterogeneous Ziegler–Natta catalysts in the light of a multicenter reaction mechanism

A previously developed kinetic scheme for ethylene polymerization reactions with heterogeneous Ziegler–Natta catalysts (see Y. V. Kissin, R. I. Mink, & T. E. Nowlin, J Polym Sci Part A: Polym Chem 1999, 37, 4255 and Y. V. Kissin, R. I. Mink, T. E. Nowlin, & A. J. Brandolini, J Polym Sci Part A: Polym Chem 1999, 37, 4273, 4281) states that the catalysts have several types of active centers that have different activities and different stabilities, produce different types of polymer materials, and respond differently to reaction conditions. Each type of center produces a single polymer component (Flory component), a material with a uniform structure (copolymer composition, isotacticity, etc.) and a narrow molecular weight distribution (weight-average molecular weight/number-average molecular weight = 2.0). This article examines several previously known features of ethylene polymerization and copolymerization reactions on the basis of this mechanism. The discussed subjects include temperature and cocatalyst effects on the polymerization kinetics and molecular weight distribution of polymers and reaction parameter effects (temperature, ethylene and hydrogen partial pressures, and α-olefin and cocatalyst concentrations) on the molecular weights of Flory components. The results show that the formulation of the multicenter kinetic scheme and the development of kinetic tools necessary for the application of this scheme significantly expand our understanding of the working of heterogeneous polymerization catalysts and provide additional means for their control. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1681–1695, 2001     

Ethylene polymerization reactions with Ziegler–Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

Kinetics of ethylene homopolymerization reactions and ethylene/1-hexene copolymerization reactions using a supported Ziegler–Natta catalyst was carried out over a broad range of reaction conditions. The kinetic data were analyzed using a concept of multicenter catalysis with different centers that respond differently to changes in reaction parameters. The catalyst contains five types of active centers that differ in the molecular weights of material they produce and in their copolymerization ability. In ethylene homopolymerization reactions, each active center has a high reaction order with respect to ethylene concentration, close to the second order. In ethylene/α-olefin copolymerization reactions, the centers that have poor copolymerization ability retain this high reaction order, whereas the centers that have good copolymerization ability change the reaction order to the first order. Hydrogen depresses activity of each type of center in the homopolymerization reactions in a reversible manner; however, the centers that copolymerize ethylene and α-olefins well are not depressed if an α-olefin is present in the reaction medium. Introduction of an α-olefin significantly increases activity of those centers, which are effective in copolymerizing it with ethylene but does not affect the centers that copolymerize ethylene and α-olefins poorly. To explain these kinetic features, a new reaction scheme is proposed. It is based on a hypothesis that the Ti—C₂H₅ bond in active centers has low reactivity due to the equilibrium formation of a Ti—C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4255–4272, 1999     

Ethylene polymerization reactions with Ziegler–Natta catalysts. III. Chain-end structures and polymerization mechanism

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of α olefins: (1) a relatively low ethylene reactivity, (2) markedly higher polymerization rates in the presence of α olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions¹ provided the basis for a new kinetic scheme that postulates the equilibrium formation of Ti C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any α-olefin molecule into the Ti H bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/α-olefin copolymer chains should contain α-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4281–4294, 1999     

用于NH3选择性催化还原NOx的钒基催化剂

在富氧且有催化剂存在条件下以NH3或尿素为还原剂选择性地还原NOx为N2的技术,即NH3/Urea-SCR技术,是去除固定源和移动源NOx最为有效且应用最广的技术之一,其中最重要的催化剂体系是钒基催化剂。本文从钒基催化剂的组成及其NH3-SCR反应性能、钒基催化剂的活性改进以及钒基催化剂上的NH3-SCR反应机理三个方面对该领域的研究进展做了较为全面的综述,并对NH3-SCR领域可能的发展方向做了展望。传统的V2O5-WO3(MoO3)/TiO2催化剂以及改性后的钒基催化剂在中温段具有优异的NOx净化效率和抗SO2中毒性能,其中高分散的V5+物种以及多聚的钒酸盐物种为NH3-SCR反应的活性中心。针对采用不同方法制备的或具有不同组成的钒基催化剂体系,多数学者认为NH3-SCR反应按照Eley-Rideal(E-R)机理进行,部分学者认为按照Langmuir-Hinshelwood(L-H)机理进行,这可能与催化剂的钒负载量以及反应温度区间相关。在后续工作中研究者应结合多种测试手段,具体问题具体分析,综合考虑温度的动态影响以及表面酸碱性对反应物的吸附活化,以得出更为全面、真实的反应机理。系统了解前人在钒基NH3-SCR催化剂领域的研究进展有助于现阶段开发高效稳定、可适应复杂工作条件的钒基SCR催化转化器,同时也对设计合成新型高效、环境友好且抗中毒的非钒基SCR催化剂体系具有一定的参考价值。     

Further Studies on Homo- and Copolymerization of Styrene through Metallocenic Initiator Systems

Summary: Binary metallocene-MAO and ternary diphenylzinc-metallocene-MAO initiator systems have been tested as initiators in the homopolymerization of styrene and also in its copolymerization with several diverse comonomers including substituted styrenes, styrene derivatives, α-olefins and dienes. Various titanocenes and zirconocenes and some exploratory experiment with hafnocene were carried out. The results indicate that titanocenes were more effective than zirconocenes in the homopolymerization of styrene while zirconocenes did better in α-olefin polymerization. It was found that titanocenes generated mainly syndiotactic polystyrene, s-PS, while zirconocenes yielded atactic polystyrene or, depending on the zirconocene, a low percentage of s-PS. For these types of initiators the polymerization process depends largely on the inductive effect of the substituents linked to the benzene ring of styrene and on its position (ortho, meta or para). Substituent multiplicity reduced markedly the effectiveness of these initiator systems. Styrene/isoprene polymerization was also studied using binary zirconocene-MAO initiator systems that yielded low conversions and also low molecular weight polymers.     

Ethylene polymerization and ethylene/hexene copolymerization with vanadium(III) catalysts bearing heteroatom‐containing salicylaldiminato ligands

A series of novel vanadium(III) complexes bearing heteroatom‐containing group‐substituted salicylaldiminato ligands [RN?CH(ArO)]VCl₂(THF)₂ (Ar = C₆H₄, R = C₃H₂NS, 2a ; C₇H₄NS, 2c ; C₇H₅N₂, 2d ; Ar = C₆H₂tBu₂ (2,4), R = C₃H₂NS, 2b ) have been synthesized and characterized. Structure of complex 2c was further confirmed by X‐ray crystallographic analysis. The complexes were investigated as the catalysts for ethylene polymerization in the presence of Et₂AlCl. Complexes 2a–d exhibited high catalytic activities (up to 22.8 kg polyethylene/mmol_V h bar), and affording polymer with unimodal molecular weight distributions at 25–70 °C in the first 5‐min polymerization, whereas produced bimodal molecular weight distribution polymers at 70 °C when polymerization time prolonged to 30 min. The catalyst structure plays an important role in controlling the molecular weight and molecular weight distribution of the resultant polymers produced in 30 min polymerization. In addition, ethylene/hexene copolymerizations with catalysts 2a–d were also explored in the presence of Et₂AlCl, which leads to the high molecular weight and unimodal distributions copolymers with high comonomer incorporation. Catalytic activity, comonomer incorporation, and polymer molecular weight can be controlled over a wide range by the variation of catalyst structure and the reaction parameters, such as comonomer feed concentration, polymerization time, and polymerization reaction temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3573–3582, 2009     

巴豆醛气相选择性加氢催化剂

巴豆醛是α, β-不饱和醛中最具代表性的一类有机化合物,采用气相催化巴豆醛选择加氢制备巴豆醇符合原子经济和绿色化学要求,具有重要的工业应用和学术价值。本文综述了近十年国内外巴豆醛气相选择性加氢合成巴豆醇的负载型催化剂的研究成果,评述了贵金属催化剂(铂、金、铱、银、钯)和非贵金属催化剂(钴、铜)上巴豆醛选择性加氢性能,分析了活性组分、载体、助剂以及活性组分粒径对催化剂性能的影响,探讨了巴豆醛选择性加氢的反应机理和失活机理。最后,对气相巴豆醛选择性加氢催化剂所存在的问题进行总结,并对催化剂的发展趋势作出了展望。指出了非贵金属催化剂的巴豆醛选择性加氢性能因具有价廉易得等优势,将是该领域的研究方向之一。催化剂失活是巴豆醛气相选择性加氢工业化的最大障碍,因此研究和认识反应机理,解决催化剂失活问题是重点研究方向。     

Polymerization of ethylene at high temperature by vanadium-based heterogeneous Ziegler–Natta catalysts. I. Study of the deactivation process

Kinetics of the polymerization of ethylene initiated by heterogeneous vanadium-based Ziegler-Natta catalysts (VCI₃-1/3 AICI₃) have been studied at high temperature (160°C, 5 bars) and compared with a titanium-based system. For the V catalyst, the dependence of the polymerization activity versus time, with the nature and the concentration of the associated aluminum alkyl, has been investigated. Kinetic results have also been correlated with the oxidation state of vanadium in the polymerization conditions. Despite the relatively high initial activity a low productivity is obtained; it can be attributed to a very fast deactivation of the active sites due to the reduction of vanadium III into vanadium II. The effect of the nature of the alkyl aluminum component of the catalytic system on the reduction process is shown. A kinetic model for the polymerization is proposed. © 1993 John Wiley & Sons, Inc.     

Novel vanadium(III) complexes with tridentate phenoxy‐phosphine [O,P(O),O] ligands: Synthesis,characterization, and catalytic behavior of ethylene polymerization and copolymerization with 10‐undecen‐1‐ol

A series of novel vanadium(III) complexes bearing tridentate phenoxy‐phosphine [O,P,O] ligands and phosphine oxide‐bridged bisphenolato [O,P?O,O] ligands, which differ in the steric and electronic properties, have been synthesized and characterized. These complexes were characterized by Fourier transform infrared spectroscopy (FTIR) and mass spectra as well as elemental analysis. Single‐crystal X‐ray diffraction revealed that complexes 3c and 4e adopt an octahedral geometry around the vanadium center. In the presence of Et₂AlCl as a cocatalyst, these complexes displayed high catalytic activities up to 22.8 kg PE/mmol_V.h.bar for ethylene polymerization, and produced high‐molecular‐weight polymers. Introducing additional oxygen atom on phosphorus atom of [O,P,O] ligands has resulted in significant changes on the aspect of steric/electronic effect, which has an impact on polymerization performance. 3c and 4c /Et₂AlCl catalytic systems were tolerant to elevated temperature (70 °C) and yielded unimodal polyethylenes, indicating the single‐site behavior of these catalysts. By pretreating with equimolar amounts of alkylaluminums, functional α‐olefin 10‐undecen‐1‐ol can be efficiently incorporated into polyethylene chains. 10‐Undecen‐1‐ol incorporation can easily reach 14.6 mol % under the mild conditions. Other reaction parameters that influenced the polymerization behavior, such as reaction temperature, Al/V (molar ratio), and comonomer concentration, are also examined in detail. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Cross-Linked Polyphosphazene Hollow Nanosphere-Derived N/P-Doped Porous Carbon with Single Nonprecious Metal Atoms for the Oxygen Reduction Reaction

Heteroatom-doped polymers or carbon nanospheres have attracted broad research interest. However, rational synthesis of these nanospheres with controllable properties is still a great challenge. Herein, we develop a template-free approach to construct cross-linked polyphosphazene nanospheres with tunable hollow structures. As comonomers, hexachlorocyclotriphosphazene provides N and P atoms, tannic acid can coordinate with metal ions, and the replaceable third comonomer can endow the materials with various properties. After carbonization, N/P-doped mesoporous carbon nanospheres were obtained with small particle size (≈50 nm) and high surface area (411.60 m² g⁻¹). Structural characterization confirmed uniform dispersion of the single atom transition metal sites (i.e., Co-N₂P₂) with N and P dual coordination. Electrochemical measurements and theoretical simulations revealed the oxygen reduction reaction performance. This work provides a solution for fabricating diverse heteroatom-containing polymer nanospheres and their derived single metal atom doped carbon catalysts.     

Some newer concepts on ethylene/α-olefin copolymerization on heterogeneous Ziegler-Natta catalysts

Unsteady diffusion kinetics, recently advanced by this laboratory, is applied to the examination of some polymerization and molecular chain structure problems. Hitherto deemed “anomalous” phenomena, such as the faster rate of copolymerization of ethylene/α-olefin than the homopolymerization of ethylene and the enrichment in the incorporation of a higher α-olefin in its copolymerization with ethylene by a lower α-olefin, are reasonably explained by unsteady diffusion of monomers. Molecular chain structure of copolymers, such as compositional heterogeneity and its dependence on comonomer incorporation originates from the difference in diffusion coefficients of the monomers. A copolymer composition equation taking into consideration the unsteady diffusion was developed. In cases where simulated curves were compared with experimental curves, good agreements were found.     

Ethylene copolymers with Fischer-Tropsch olefins

The properties of ethylene copolymers, terpolymers and multipolymers prepared with even and uneven carbon number linear and branched α-olefins were compared. The most likely microstructures of ethylene/linear α-olefin copolymers was assigned by considering co-unit bulkiness, average crystallizable sequence lengths and thermal properties. The higher α-olefins were found to be more effective at decreasing density, but peak melting temperatures were higher. In terpolymers where lower α-olefins such as 1-butene and 1-pentene were used as comonomers, density was decreased more than the mathematical average expected from the ratio of comonomers in the terpolymers. Peak melting temperatures were also lower. Based on NMR evidence and the microstructures of the different copolymers the rationale for this occurrence could be ascribed to decreased clustering for these terpolymers. Branched α-olefins produced ethylene co- and terpolymers with significantly decreased densities as compared to the linear α-olefins. Impact strength of these polymers was also substantially higher, even at low comonomer content. Thermal evidence indicates that the microstructure of the co- and terpolymers containing branched α-olefins are very similar to that of the copolymers prepared with linear α-olefins of the same carbon number.     

Synthesis and properties of propene copolymers with ether comonomers

The direct copolymerization of propene with polar comonomers using metallocene catalysts in solution was investigated. As comonomers, two ether compounds were used in comparison to 10-undecene-1-ol as well-investigated comonomer. The ether comonomers were diethylene glycol mono-10-undecenyl ether (DEGUE) and octaethylene glycol-10-undecenyl methyl ether (OEGUME). The influence of the different comonomers on the copolymerization behavior was studied. The copolymers were characterized with respect to their comonomer contents, molar masses, and thermal properties. The incorporation rate of DEGUE and OEGUME into the propene copolymers did not exceed 1.6 mol% for DEGUE and 0.31 mol% for OEGUME and was thus considerably lower than in the reference propene copolymerization with 10-undecene-1-ol. An uncompleted shielding of the oxygen atoms of the ether groups by triisobutyl aluminum (TIBA) to the metallocene catalyst is assumed to be responsible for this behavior. The crystallization kinetics in the copolymers with comparable molar masses is mainly influenced by the side chain density per 1000 propene units, n₁₀₀₀.The incorporation of hydrophilic comonomers into polypropene was expected to alter the surface properties. The slightly lowered water contact angles found on films of copolymers with higher comonomer content indicated the enhanced hydrophilicity of the polypropene copolymer surfaces compared to polypropene (PP).     

Ethylene–propylene terpolymers based on methylcyclopentadienyl-5-norborn-2-enylmethane derivatives

The terpolymerization of ethylene, propylene, and methylcyclopentadienyl-5-endonorborn-2-enylmethane (III) by means of different vanadium-based coordination catalysts was investigated. The structure of III and its isomeric composition was studied by using vapor-phase chromatography, mass spectrometry, and NMR, infrared, and ultraviolet spectroscopy. The catalyst system V Acac₃–Et₂AlCl was used under different conditions, and the influence of several variables regulating the terpolymerization process were related to the properties of the resulting terpolymers (EPTM). The insertion of III into EPTM chains takes place randomly and does not influence the random distribution of comonomers. The selective opening of the norbornene double bond of III has been demonstrated by use of 2,3-dihydro-III as model compound. Tritiated III gave a radiochemical titer of unsaturation in excellent agreement with the value deduced from ultraviolet measurements. The influence of Lewis bases added to VAcac₃–Et₂AlCl catalyst is discussed.     

Emulsifier-free emulsion polymerization with cationic comonomer

An earlier article¹ described the emulsion polymerization of styrene and various anionic comonomers, together with an anionic initiator, to give uniform latices at ca. 35% solids content. This article extends the work to cationic systems. Cationic comonomers 1,2-dimethyl 5-vinylpyridinium methylsulfate and 1-ethyl 2-methyl 5-vinylpyridinium bromide were synthesized and used with azobis(isobutyramidine hydrocholoride) initiator in the emulsifier- free emulsion polymerization of styrene. Recipes and results were generally comparable to those of the anionic systems, excepts for the dependence of particle diameter on comonomers concentration. Here the initial decrease was followed by an increase in particle diameter at higher comonomer content. The surface charge increased sharply with comonomer content.     

Polymeric Catalysts

The present-day position in the field of polymeric catalysts is outlined. The following selected groups of polymeric catalysts are discussed: synthetic hydrolases, immobilized enzymes, phase-transfer catalysts, nucleophilically active bases, polymers with conjugated π-systems, photosensitizers, polymers as carriers for catalytically active metals or ions, and immobilized homogeneous catalysts. Polymeric catalysts have the following valuable properties: insoluble polymeric catalysts are readily separable from reaction solutions and can often be re-used without loss of activity; a hydrophobic matrix protects the organometallic active center from deactivation by oxygen and water; by fixation of finely divided metals on an ion exchanger, multistage reactions may be effected successively in one reactor. Polymeric carriers may influence the catalytic properties; for example, in the case of immobilized enzymes on polyionic carriers the pH of the activity maximum may be shifted.     

Advances in homogeneous and heterogeneous catalysis with metal-containing silsesquioxanes

Metal-containing silsesquioxane derivatives provide new catalysts with both homogeneous and heterogeneous applicability. The steric and electronic properties of silsesquioxane silanolate ligands render metal centers more Lewis acidic than conventional alkoxide or siloxide ligands do. This concept has been exploited in newly developed catalysts for alkene metathesis, polymerization, epoxidation, and Diels-Alder reactions of enones. Other applications are envisioned in the near future.     

Cationic β-Scission of C−H and C−C Bonds for Selective Dimerization and Subsequent Sulfur-Free RAFT Polymerization of α-Methylstyrene and Isobutylene

A series of exo-olefin compounds ((CH₃)₂C(PhY)−CH₂C(=CH₂)PhY) were prepared by selective cationic dimerization of α-methylstyrene (αMS) derivatives (CH₂=C(CH₃)PhY) with p-toluenesulfonic acid (TsOH) via β-C−H scission. They were subsequently used as reversible chain transfer agents for sulfur-free cationic RAFT polymerization of αMS via β-C−C scission in the presence of Lewis acid catalysts such as SnCl₄. In particular, exo-olefin compounds with electron-donating substituents, such as a 4-MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo-olefin terminals. Other exo-olefin compounds (R−CH₂C(=CH₂)(4-MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur-free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo-olefin αMS dimer ((CH₃)₂C(Ph)−CH₂C(=CH₂)Ph). Furthermore, telechelic poly(IB) with exo-olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo-olefins. Finally, block copolymers of αMS and methyl methacrylate (MMA) were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo-olefin terminals containing 4-MeOPh groups as common sulfur-free RAFT groups for both cationic and radical polymerizations.