Role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide utilizing chromium salen complexes

The mechanism of the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexylene)carbonate catalyzed by (salen)CrN3 (H2salen = N,N,'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylene-diimine) in the presence of a broad range of cocatalysts has been studied. We have previously established the rate of copolymer formation to be very sensitive to both the electron-donating ability of the salen ligand and the [cocatalyst], where N-heterocyclic amines, phosphines, and ionic salts were effective cocatalysts. Significant increases in the rate of copolymerization have been achieved with turnover frequencies of approximately 1200 h(-1), thereby making these catalyst systems some of the most active and robust thus far uncovered. Herein we offer a detailed explanation of the role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide catalyzed by chromium salen derivatives. A salient feature of the N-heterocyclic amine- or phosphine-cocatalyzed processes is the presence of an initiation period prior to reaching the maximum rate of copolymerization. Importantly, this is not observed for comparable processes involving ionic salts as cocatalysts, e.g., PPN+ X-. In these latter cases the copolymerization reaction exhibits ideal kinetic behavior and is proposed to proceed via a reaction pathway involving anionic six-coordinate (salen)Cr(N3)X- derivatives. By way of infrared and 31P NMR spectroscopic studies, coupled with in situ kinetic monitoring of the reactions, a mechanism of copolymerization is proposed where the neutral cocatalysts react with CO2 and/or epoxide to produce inner salts or zwitterions which behave in a manner similar to that of ionic salts.     

α‐Diimine nickel catalyst for copolymerization of hexene and acrylate monomers activated by different cocatalysts

Bulk homopolymerization and copolymerization of 1‐hexene (H) with polar monomers including butyl acrylate (B) and methyl methacrylate (M) in the presence of 1,4‐bis (2,6‐diisopropylphenyl) acenaphthene diimine nickel (II) dibromide catalyst were investigated. Two cocatalysts, including diethyl aluminium chloride (DEAC) and ethyl aluminium sesqui chloride (EASC), were used to activate the catalyst at ambient temperature. In both the homopolymerization and copolymerization of 1‐hexene with polar monomers, the catalyst activity resulted from EASC as cocatalyst was higher than that resulted from DEAC. ¹HNMR analysis was used in order to determine incorporation level of polar monomers and branching density of the synthesized polymers. A highest incorporation level of 13.3% mol was obtained using monomer B in the presence of the cocatalyst EASC. In addition, the influence of polar monomers on molecular weight and molecular weight distribution (PDI) was studied for both the homo‐ and co‐polymerizations of 1‐hexene in the presence of various cocatalysts. A higher molecular weight and narrower PDI were obtained by using the DEAC cocatalyst compared to the EASC cocatalyst. Glass transition temperature (Tg) and melting point (Tm) of the synthesized polymers were found to be dependent on the cocatalyst type and comonomer incorporation level. The addition of dichloromethane solvent into reaction medium showed a positive effect on comonomer incorporation which could not be seen in bulk polymerization. However, the presence of dichloromethane led to decrease the catalyst activity and molecular weight of the polymers.     

光催化全解水助催化剂的设计与构建

能源和环境危机是当今社会面临的两大关键课题,利用太阳光驱动化学反应、将太阳能转化为化学能是解决上述问题的重要措施。通过光催化分解水是直接利用太阳能生产氢燃料的有效策略。光催化水分解过程可以分为三个基元步骤:光吸收、电荷分离与迁移、以及表面氧化还原反应。助催化剂可有效提高电荷分离效率、提供反应活性位点并抑制催化剂光腐蚀的发生,进而提高水分解效率。助催化剂也可以通过活化水分子以提高表面氧化还原动力学,进而提升整体光催化反应的太阳能转换效率。本文综述了助催化剂在光催化反应中的重要作用以及目前常用的助催化剂类型,详细说明了在光催化全解水过程中双助催化剂体系的构建及作用机理,并根据限制全解水的关键因素提出了新型助催化剂的设计策略。     

Effective, selective coupling of propylene oxide and carbon dioxide to poly(propylene carbonate) using (salen)CrN3 catalysts

The copolymerization of propylene oxide and CO2 has been investigated employing Cr(salen)N3 complexes as catalysts. Unfortunately the reaction could not be studied in real time via in situ IR spectroscopy, thereby obtaining detailed kinetic data, because of the copolymer limited solubility in most solvents. Investigations employing batch reactor runs concentrating on varying the cocatalyst, the equivalents of cocatalyst, and the steric and electronic structure of the catalyst through modification of the salen ligand were undertaken. It was discovered that the optimal catalyst for copolymer selectivity vs the monomeric propylene carbonate was one that contained a salen ligand with an electron-withdrawing phenylene backbone and electron-donating tert-butyl groups in the phenolate rings. This catalyst was used to investigate the effect of altering the nature of the cocatalyst and its concentration, the three cocatalysts being tricyclohexylphosphine (PCy3), PPN+ N3(-), and PPN+ Cl-, where PPN+ is the large very weakly interacting bis(triphenylphosphoramylidene)ammonium cation. By utilization of more or less than 1 equiv of PCy3 as cocatalyst, the yield of polymer was reduced. On the other hand, the PPN+ salts showed the best activity when 0.5 equiv was employed, and produced only cyclic when using over 1 equiv.     

Cyclohexene oxide/CO2 copolymerization catalyzed by chromium(III) salen complexes and N-methylimidazole: effects of varying salen ligand substituents and relative cocatalyst loading

A detailed mechanistic study into the copolymerization of CO2 and cyclohexene oxide utilizing CrIII(salen)X complexes and N-methylimidazole, where H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine and other salen derivatives and X = Cl or N3, has been conducted. By studying salen ligands with various groups on the diimine backbone, we have observed that bulky groups oriented perpendicular to the salen plane reduce the activity of the catalyst significantly, while such groups oriented parallel to the salen plane do not retard copolymer formation. This is not surprising in that the mechanism for asymmetric ring opening of epoxides was found to occur in a bimetallic fashion, whereas these perpendicularly oriented groups along with the tert-butyl groups on the phenolate rings produce considerable steric requirements for the two metal centers to communicate and thus initiate the copolymerization process. It was also observed that altering the substituents on the phenolate rings of the salen ligand had a 2-fold effect, controlling both catalyst solubility as well as electron density around the metal center, producing significant effects on the rate of copolymer formation. This and other data discussed herein have led us to propose a more detailed mechanistic delineation, wherein the rate of copolymerization is dictated by two separate equilibria. The first equilibrium involves the initial second-order epoxide ring opening and is inhibited by excess amounts of cocatalyst. The second equilibrium involves the propagation step and is enhanced by excess cocatalyst. This gives the [cocatalyst] both a positive and negative effect on the overall rate of copolymerization.     

Catalyst/cocatalyst nuclearity effects in single-site polymerization. Enhanced polyethylene branching and alpha-olefin comonomer enchainment in polymerizations mediated by binuclear catalysts and cocatalysts via a new enchainment pathway

The binuclear "constrained geometry catalyst" (CGC) (mu-CH2CH2-3,3'){(eta5-indenyl )[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2] and the trityl bisborate dianion (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2) have been synthesized to serve as new types of multicenter homogeneous olefin polymerization catalysts and cocatalysts, respectively. Additionally, the complex [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) was synthesized as a mononuclear control. For the bimetallic catalyst or bisborate cocatalyst, high effective local active site concentrations and catalyst center-catalyst center cooperative effects are evidenced by bringing the catalytic centers together via either covalent or electrostatic bonding. For ethylene homopolymerization at constant conversion, the branch content of the polyolefin products (primarily ethyl branches) is dramatically increased as catalyst or cocatalyst nuclearity is increased. Moreover, catalyst and cocatalyst nuclearity effects are approximately additive. Compared to the catalyst derived from monometallic Zr1 and monofunctional Ph3C+B(C6F5)4- (B1), the active catalyst derived from bimetallic Zr2 and bifunctional B2 produces approximately 11 times more ethyl branches in ethylene homopolymerization via a process which is predominantly intradimer in character. Moreover, approximately 3 times more 1-hexene incorporation in ethylene + 1-hexene copolymerization and approximately 4 times more 1-pentene incorporation in ethylene + 1-pentene copolymerization are observed for Zr2 + B2 versus Zr1 + B1.     

Direct synthesis of diphenyl carbonate by oxidative carbonylation of phenol using Pd–Cu based redox catalyst system

A catalyst system was designed for direct synthesis of diphenyl carbonate by oxidative carbonylation of phenol. Besides Pd carbonylation catalyst, inorganic and organic redox cocatalysts were included in the catalyst system for in situ regeneration of active Pd species. Copper(II) acetate was used as inorganic redox cocatalyst and hydroquinone was found to give good results as organic redox cocatalyst. Efficiency of various bases, effect of a drying agent, and optimum reaction conditions for achieving high catalytic activity were also investigated in detail. Using suitable components of catalyst system and under optimum reaction conditions, a Pd turnover number of 250 could be obtained.     

SalenAl(O~iPr)催化CO_2与氧化环己烯共聚及影响因素研究

以SalenAl(OiPr)为催化剂,分别加入各种路易斯碱作为共催化剂催化二氧化碳与氧化环己烯共聚,发现共聚催化效率与共催化剂的供电子能力有关.分别研究催化剂浓度、共催化剂的用量、反应时间、反应温度、CO2压力等各种因素对该共聚反应的影响,发现SalenAl(OiPr)浓度为2 g/L时,以等摩尔量的二甲氨基吡啶(DMAP)作为共催化剂,在4 MPa的CO2和80℃下反应32 h,可得到碳酸酯键含量>99%的共聚产物,其催化效率高达495 g/g,13C-NMR检测表明共聚物为无规立构聚合物,GPC测得分子量Mn为55900,分子量分布比较窄(PDI=1.32).DSC得到共聚物的玻璃化转变温度为136℃,热重分析(TGA)可以看出共聚物在350℃可完全分解,具有优良的热分解性.     

(Tetramethyltetraazaannulene)chromium chloride: a highly active catalyst for the alternating copolymerization of epoxides and carbon dioxide

A tetramethyltetraazaannulene complex incorporating a chromium(III) metal center has been shown to be highly active toward the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexene carbonate) in the presence of [PPN]N3 [PPN+=bis(triphenylphosphoranylidene)ammonium] as a cocatalyst. An asymptotical rate increase was observed, leveling at 2 equiv of cocatalyst with a maximum turnover frequency of 1300 h(-1) at 80 degrees C. A benefit of this new catalyst system over that of the previously studied less-active (salen)CrX system is that the (tmtaa)CrCl catalyst has a much lower propensity toward the formation of a cyclic carbonate byproduct throughout the copolymerization reaction.     

Effect of Alkylaluminum Activators on Ethylene Trimerization Based on 2,5‐DMP/Cr(III)/TCE Catalyst System

Ethylene trimerization toward 1‐hexene catalytic system with 2,5‐dimethylpyrrole (2,5‐DMP)/Cr(III)/alkylaluminum/tetrachloroethane (TCE) was investigated. The effects of various cocatalysts on catalytic activity and product selectivity were discussed. The results showed that triethylaluminum (TEA), trimethylaluminum (TMA), tri‐n‐hexylaluminum (TNHA) and tri‐isobutylaluminum (TIBA) were all effective cocatalysts for ethylene trimerization toward 1‐hexene. 2,5‐DMP/Cr(III)/TEA/TCE catalytic system afforded the best results for ethylene trimerization, while reducing the level of by‐product formation. Some specific interaction modes of alkylaluminum with active Cr species in the catalytic cycle were proposed to explain the effect of cocatalyst on catalytic activity and 1‐hexene selectivity.     

氯化稀土乙二醇二甲醚配合物催化丁二烯聚合

以Nd2O3、(CH3)3SiCl和乙二醇二甲醚(DME)为原料,合成了NdCl3·2DME配合物,并将其用于催化丁二烯聚合。 考察了助催化剂种类与用量、陈化温度和聚合时间对聚合的影响。 结果表明,以烷基铝与MAO共同作为助催[JP2]化剂时具有高聚合活性,而单独以烷基铝或甲基铝氧烷(MAO)为助催化剂时聚合活性很低。 当n(Nd)∶n(AlR3)∶n(MAO)=1∶30∶45时,催化活性最高。 陈化温度对聚合活性、聚合物结构及相对分子质量均有较大的影响。 陈化温度过低或者过高,聚合活性、聚丁二烯cis-1,4含量和相对分子质量均降低;陈化温度为50 ℃时,具有最高聚合活性和最高cis-1,4含量。 NdCl3·2DME催化体系所得聚丁二烯的cis-1,4含量高达98.7%（IR）,而1,4-结构总含量高达99.6%（1H NMR）。     

Synergetic Effect of Dual Co‐catalysts on the Activity of p‐Type Cu2O Crystals with Anisotropic Facets

Spatial separation of reduction sites and oxidation sites to inhibit the recombination of photogenerated electrons and holes plays a vital role in improving the efficiency of photocatalyst systems. It is very challenging to rationally deposit cocatalysts on the right facets (sites), namely, the reduction cocatalyst on the reduction facets (sites) and the oxidation cocatalyst on the oxidation facets (sites). Herein, we report that the reduction and oxidation cocatalysts can be selectively constructed on the different facets of p‐type Cu₂O crystals with anisotropic facets, but not on the Cu₂O crystal with isotropic facets. The deposition of dual cocatalysts on the different facets resulted in a remarkable synergetic effect in the photocatalytic performance, which could be attributed to the spatial separation of the photogenerated charges between facets. Our work reports an instructive strategy for constructing high‐efficiency photocatalyst systems for solar energy conversion.     

Crystalline titanium dichloride—an active catalyst in ethylene polymerization. I. Catalyst activation

Crystalline titanium dichloride, in the absence of organometallic cocatalyst, is a very poor catalyst for the polymerization of ethylene. It is transformed into a very active catalyst through mechanical activation (ball-milling). This catalyst is active in the absence not only of organometallic cocatalysts, but also metals and compounds (such as aluminium and AlCl₃) capable of forming organometallic compounds in situ (i.e., with ethylene, before polymerization starts). Ball-milling causes not only the expected increase in surface area but also disproportionation of Ti⁺⁺ to Ti⁺⁺⁺ and metallic titanium, as well as a crystal phase change to a structure not previously identified with those of TiCl₂ or TiCl₃. Catalyst activity (polymerization rate) is shown to be proportional to surface area and a direct function of Ti⁺⁺ content of the catalyst; an empirical equation relates catalyst activity to surface area and to Ti⁺⁺ lost through disproportionation. Titanium trichloride was found to be inactive in the absence of organometallic cocatalyst, even after ball-milling. The difference in structure of the catalytically active species in the conventional Ziegler (organometallic cocatalyst) and in the titanium dichloride catalyst are discussed. The mechanism of polymerization is compared with that of the supported (CrO₃ on SiO₂/Al₂O₃ and MoO₃ on Al₂O₃) catalyst systems.     

Reversed configuration of photocatalyst to exhibit improved properties of basic processes compared to conventional one

Performances of semiconductor photocatalysts are integrally determined by efficiencies of basic processes such as light absorption, charge separation and surface catalysis, but conventional configurations of photocatalysts normally suffers from the competition of light absorption originating from cocatalyst deposition and limited interface charge separation between the photocatalyst and cocatalyst. Herein we give the first proof-of-concept illustration that a reversed configuration of photocatalysts with a core/shell structure of microsized Mo_2N cocatalysts and nanosized CdS photocatalysts, which exhibits superior solar hydrogen production to the conventional configuration with nanosized Mo_2N cocatalysts deposited on the surface of CdS photocatalysts. It is revealed that the reversed configuration outperforms the conventional one in all areas of light absorption,charge separation and surface catalysis. Strikingly, the special core/shell structure introduced here can well avoid the competition of light absorption by cocatalysts and make an effective confinement effect to promote the surface catalysis of Mo_2N. Our finding provides an alternative strategy to improve performances of photocatalysts.     

Catalysis of olefin isomerizations by boron trifluoride

The kinetics of isomerization of butenes by boron trifluoride with acetic acid or methanol as cocatalysts have been re-examined. The results, over a wide concentration range, and at temperatures from ?20 to +20°C, are consistent with previous data, but it is shown that the previously suggested mechanism cannot apply. By using deuterated acetic acid as cocatalyst it has been found that isomerization exactly parallels protonation, which is consistent with a mechanism involving a classical carbonium ion.     

“Smart” Catalysis with thermoresponsive ruthenium catalysts for miniemulsion ru‐mediated reversible deactivation radical polymerization cocatalyzed by smart iron cocatalysts

This work reports the use of cocatalysts in addition to “smart” ruthenium catalysts for Ru‐mediated reversible deactivation radical polymerization (RDRP) in miniemulsion, allowing for the synthesis of final products with significantly reduced residual metal. Using amine cocatalysts in miniemulsion allows for high conversions (> 90%) in under 10 h. Two forms of ferrocene cocatalysts are also used, including “smart” thermoresponsive PEGylated ferrocene derivatives (FcPEG) and ferrocene containing surfactants (FcTMA). Using “smart” thermoresponsive cocatalyst at low concentrations, rate enhancements in BMA and BzMA polymerizations are observed, with good catalyst removability. Using the FcTMA cocatalyst surfactant, increasing monomer hydrophobicity is shown to increase the polymerization rate and initiator efficiency. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 305–312     

Cationic Copolymerization of Propylene Oxide with Tetrahydrofuran. XI. Calculation of Individual Rate Constants

Rate constants (k₁ k₁₁, k₁₂, k₂₂, k₂₁ and k_t) for various steps involved in the copolymerization of propylene oxide (PO) with tetrahydrofuran (THF) have been calculated from reaction rate data obtained with the following catalyst system: (a) triphenyl-methyl cations ((C₆H₅)₃C⁺) associated with hexafluorophosphate (PF₆ ^?), hexafluoroarsenate (AsF₆ ^?) and hexafluoroantimonate (SbF₆ ^?) gegenions; (b) antimony pentachloride (SbCl₅); and, (c) boron trifluoride etherate, BF₃:(C₂H₅)₂O. The latter two systems were studied in the presence of cocatalysts. The effects of several parameters (the cocatalyst concentration and bulk size, the nature of the solvent, and the reaction temperature) on the rate constants are highlighted. The role of entropy in the initiation, propagation and termination steps is discussed in terms of solvation and desolvation processes. Based on termination activation energy considerations, the order of stability for the gegenions used in the copolymerization of PO with THF was found to be: AsF₆ ^? > SbF₆ ^? > HOBF₃ ^?PF₆ ^? > SbCl₆ ^?     

8-苯胺-1-萘磺酸钛配合物的合成及催化乙烯与降冰片烯共聚合反应

合成了新型催化剂8-苯胺-1-萘磺酸钛配合物, 并应用于乙烯与降冰片烯的共聚合反应中. 分别考察了助催化剂种类[甲基铝氧烷(MAO)和三乙基铝(TEA)]、 降冰片烯浓度、 Al/Ti摩尔比、 聚合温度和聚合压力对催化活性与共聚性能的影响. 通过核磁共振、示差扫描量热和凝胶渗透色谱等对所制备的共聚物进行了表征. 结果表明, 在相同条件下, 以MAO为助催化剂时, 共聚催化活性更高, 催化剂为单活性中心, 可得到分子量分布较窄(PDI≈3)的共聚产物, 其共聚反应机理为加成聚合. 另外, 随着降冰片烯浓度的升高, 共聚物中降冰片烯单元的摩尔比呈线性上升趋势, 所得共聚物的熔点随之降低.     

Cationic polymerization of isobutene initiated by stannic chloride and phenols: Polymer endgroup studies

Low molecular weight polymers of isobutene produced with stannic chloride as catalyst and phenols as cocatalysts have been subjected to ultraviolet and NMR analysis. A high proportion of the endgroups are derived from the phenol cocatalyst when the concentration of free phenol in the reaction mixture at ?78.5°C is fairly large. At low concentrations of free phenol, termination to give vinylidene endgroups becomes more significant. The results lend support to the suggestion that an important mode of termination in this polymerization system involves the reaction between a growing carbonium ion and the phenol cocatalyst.     

(Salen)CrIIIX catalysts for the copolymerization of carbon dioxide and epoxides: role of the initiator and cocatalyst

The copolymerization of CO(2) and cyclohexene or propylene oxide has been examined employing (salen)Cr(III)Nu complexes (Nu = Cl or N(3)) as catalysts. The addition of various cocatalysts, including phosphines and PPN+ or Bu4N+ Cl- salts serves to greatly enhance the rate of copolymer production. In these instances, the mechanism of the initiation step appears to be unimolecular in catalyst concentration, unlike the bimolecular process cocatalyzed by N-methylimidazole. The copolymers were produced with >95% carbonate linkages with TOFs in the range 39-494 mol epoxide consumed/mol Cr.h. In the presence of phosphine cocatalysts, no cyclic carbonate was produced as a byproduct.