A highly active Fe(I) catalyst for radical polymerisation and taming the polymerisation with iodine

A fast and controlled radical polymerisation of acrylates and acrylamides can be achieved with a combination of a highly active metal catalyst, Fe2Cp2(CO)4, and a mild radical scavenger, iodine, in the presence of an iodide initiator.     

Discovery and evaluation of highly active imidotitanium ethylene polymerisation catalysts using high throughput catalyst screening

A family of ca. 50 imidotitanium precatalysts [Ti(NR)(Me(3)[9]aneN(3))Cl(2)](R = alkyl or aryl; Me(3)[9]aneN(3)= 1,4,7-trimethyltriazacyclononane) were prepared in good yields using semi-automated procedures; high-throughput screening techniques identified seven highly active ethylene polymerisation precatalysts with activities in the range ca. 3 400 to 10 000 kg(PE) mol(-1) h(-1) bar(-1).     

Beta-H transfer from the metallacyclobutane: a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst

The surface complex [([triple bond]SiO)Re([triple bond]CtBu)(=CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After beta-H transfer, the most accessible pathway is the insertion of ethene in the Re-H bond. The resulting pentacoordinated trisperhydrocarbyl complex rearranges via either (1) alpha-H abstraction yielding the unexpected 1-butene byproduct and the regeneration of the catalyst or (2) beta-H abstraction leading to degrafting. These deactivation and byproduct formation pathways are in full agreement with the experimental data.     

Development of highly active and selective novel Pd based acetoxylation catalysts and prevention of catalyst deactivation by Bi modification

A first approach to successful prevention of catalyst deactivation while simultaneously achieving extremely high selectivity of benzyl acetate (> or = 95%) at significantly high toluene conversion (> 70%) by gas phase acetoxylation over novel Pd-Sb-Bi/TiO2 catalysts.     

Highly branched polyethylene graft copolymers prepared by means of migratory insertion polymerization combined with TEMPO‐mediated controlled radical polymerization

New families of highly branched polyethylenes containing alkyl short chain branches as well as polar and non‐polar long‐chain branches were prepared by combining migratory insertion copolymerization with controlled radical graft copolymerization. Key intermediate was a novel alkoxyamine‐functionalized 1‐alkene which was copolymerized with ethylene using a palladium catalyst. The resulting highly branched polyethylene with alkoxyamine‐functionalized short chain branches was used as macroinitiator to initiate controlled radical graft copolymerization of styrene and styrene/acrylonitrile. Novel polyethylene graft copolymers with molecular masses of M_w >100 000 g/mol and narrow polydispersities were obtained. Transmission electron microscopic studies (TEM) and the presence of two glass transition temperatures at –67 and +100°C indicated microphase separation.     

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.     

“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     

Reactions of mixtures in supported catalysts: I. Models for catalyst deactivation and support pore size change and a single pellet behavior

A general model is presented for reaction mixtures undergoing catalytic reactions that are affected by catalyst deactivation. A continuum approach is used to represent the mixtures in terms of molecular size. The model accounts for the pore size exclusion effect, the change of pore size with deactivation along the entire pore length, and the effect of multilayer deposition of coke or metals on residual activity. The model also accounts for all possible reaction paths leading to all products and intermediates. Simulation results are presented for typical reaction mixtures. The results show that a larger pellet gives a higher residual activity for smaller molecules whereas a smaller pellet yields a higher activity for larger molecules. They also show that pellet size has little effect on the change of pore diameter with time.     

The interaction mechanisms between aqueous solutions of137Cs and134Ba radionuclides and local natural zeolites for an deactivation scenario

¹³⁷Cs and¹³⁴Ba were removed from synthetic aqueous solutions by means of natural zeolites of Slovakian origin. The equilibrium sorption behavior of Cs and Ba ions onto clinoptilolite and mordenite were studied under static as well as dynamic experimental conditions. Both Freundlich and Langmuir isotherms describe satisfactory by Cs and Ba adsorption on the zeolites studied. The elution of Cs and Ba ions from zeolite columns after the loading cycle was undertaken additionaly, in order to compare column operating runs of various exchanged zeolite forms.     

Thermal deactivation mechanism and the effect of Nd addition on the deactivation of the Pt/SiO<Subscript>2</Subscript> catalyst for NO oxidation reaction

Pt-based catalysts cannot be used permanently for the diesel after-treatment system because the catalytic activity is decreased due to coarsening of Pt particles at high temperature of the exhaust gas. In this study, to prevent Pt-based catalyst from deactivation, Nd was added to the Pt/SiO₂ catalyst, and the effect of the Nd addition on the catalytic activity was investigated. The Pt/SiO₂ catalyst showed a high catalytic activity for the oxidation of NO but was severely deactivated after the fast thermal aging process. Pt crystallite size was increased and some Pt particles were buried in the SiO₂ pore during the fast thermal aging process, which led to the decrease of catalytic activity. Nd-added Pt/SiO₂ catalyst showed lower activity than Pt/SiO₂ catalyst, but Pt–Nd/SiO₂ catalyst maintained its catalytic activity after fast thermal aging process. It can be postulated that a stable Nd silicate, on which Pt particle is placed, protects SiO₂ pores from destruction and so the number of the catalytically active sites remains nearly unchanged. As a result the Pt–Nd/SiO₂ catalyst maintained its catalytic activity after fast thermal aging process.     

Synthesis, reaction, and recycle of light fluorous Grubbs-Hoveyda catalysts for alkene metathesis

Light fluorous versions of first- and second-generation Grubbs-Hoveyda metathesis catalysts are introduced. These exhibit the expected reactivity profile, are readily recovered from reaction mixtures by fluorous solid-phase extraction, and can be routinely reused five or more times. The catalysts can be used in a stand alone fashion, or supported on fluorous silica gel.     

Heterogeneous catalysts for the controlled ring-opening polymerisation of rac-lactide and homogeneous silsesquioxane model complexes

In this communication we report, for the first time, the ring-opening polymerisation in the melt of rac-lactide with heterogeneous catalysts. Homogeneous model systems based on silsesquioxanes have been prepared for comparison.     

Chromium imine and amine complexes as homogeneous catalysts for the trimerisation and polymerisation of ethylene

Cr(III) complexes of tridendate imine and amine ligands with N, P, O, S donor atoms 1 and 2 have been prepared and tested as catalysts in the oligomerisation and polymerisation of ethylene giving excellent selectivity towards 1-hexene and polymerisation to polyethylene when activated with cocatalysts. X-ray structure analyses of the precatalysts 1a-c, 1i, and 2b are investigated. The metal-ligand binding in 1a and 1b is nearly the same, which leads to similar catalytic activities of these precatalysts.     

Evaluation of ruthenium‐based catalytic systems for the controlled atom transfer radical polymerisation of vinyl monomers

Only [RuCl₂(p‐cymene)(PR₃)] complexes where the phosphine ligand, PR₃, is both strongly basic and bulky proved to be effective catalysts for the controlled atom transfer radical polymerisation (ATRP) of methyl methacrylate and styrene. The best phosphine ligands were typically P(i‐Pr)₃, P(cyclohexyl)₂Ph, P(cyclohexyl)₃, and P(cyclopentyl)₃. Less basic and/or bulky phosphines led to ineffective systems for ATRP. Tricyclohexylarsine gave rise to a highly efficient catalyst system. However, related complexes in which the phosphine ligand was replaced by tricyclohexylstibine, nitrogen (piperidine and 4‐cyanopyridine) and carbon ligands (alkyl isocyanides) proved to be inefficient. The observation of a direct relationship between the p‐cymene lability (measured by TGA) and catalyst activity suggests that p‐cymene release is a prerequisite for the polymerisation process.     

Stereospecific reversible deactivation radical polymerization of biomass‐based acrylates for precise control of tacticity and molecular weight

Reversible deactivation radical polymerization (RDRP) of biomass‐based acrylates, (S )‐ and (R )‐2‐isopropyl‐5‐methylene‐1,3‐dioxolan‐4‐ones (S‐MiPDO and R‐MiPDO), was successfully performed to produce a well‐defined polymer with simultaneously controlled tacticity and molecular weight, and low dispersity (? < 1.3). In particular, meso triad (mm ) of the polymer was continuously controlled as designed from 28.1% to about 100% by changing the molar ratio of S‐MiPDO/R‐MiPDO in feed. In kinetic studies, the rate of RDRP was strongly influenced by the stereostructures of the propagating radical, and it was much lower in isospecific RDRP than atactic one in reversible chain transfer catalyzed polymerization (RTCP) in contrast to atom transfer radical polymerization (ATRP) where the rate would not change regardless of the tacticity. Increase of molecular weight and low ? of the polymer were also observed in reversible addition‐fragmentation chain transfer (RAFT) polymerization of MiPDO. In addition, block copolymers including stereoblock copolymers were feasibly synthesized by RTCP of styrene and methyl methacrylate using poly(MiPDO) prepolymer. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 445–456     

Structure-reactivity relationships in SHOP-type complexes: tunable catalysts for the oligomerisation and polymerisation of ethylene

For over 30 years complexes with the general formula [NiPh(P,O)L] (L = tertiary phosphine; P,O = chelating phosphanylenolato ligand) have been used as highly efficient oligomerisation catalysts suitable for the production of linear alpha-olefins. The same complexes, which are usually referred to as SHOP-type catalysts (SHOP = Shell Higher Olefin Process) can also be used as ethylene polymerisation catalysts, provided they are treated with a phosphine scavenger that selectively removes the tertiary phosphine ligand (L). This Perspective examines the impact of various parameters (influence of the substituents, backbone size, solvent, use of co-catalysts, etc.) on the catalytic outcome of the complexes. Overall, this review shows that the selectivity and activity of the catalyst may be tuned efficiently through directed modification of the P,O chelator.     

Kinetic study of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al2O3 catalyst

The kinetics of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al2O3 catalyst were studied.Performance test runs were carried out in a fixed-bed integral reactor.Using a power-law rate expression for the surface reaction kinetics and independent law for deactivation kinetics,the experimental data were analyzed both by integral and a novel differential method of analysis and the results were compared.To avoid fluctuation of time-derivatives of conversion required for differential analysis,the conversion-time data were first fitted with appropriate functions.While the time-zero and rate constant of reaction were largely insensitive to the function employed,the rate constant of deactivation was much more sensitive to the function form.The advantage of the proposed differential method,however,is that the integration of the rate expression is not necessary which otherwise could be complicated or impossible.It was also found that the reaction is not limited by external and internal mass transfer limitations,implying that the employed kinetics could be considered as intrinsic ones.     

Chiral Jacobsen's catalyst immobilized on zinc poly(styrene‐phenylvinylphosphonate)‐phosphate functionalized by diamine as highly efficient and reusable catalyst for alkene epoxidation

Chiral Jacobsen's catalyst anchored on zinc poly(styrene‐phenylvinylphosphonate)‐phosphate (ZnPS‐PVPA) functionalized by diamines shows superior catalytic activities (conversion up to 99%; enantiomeric excess up to 99%) in the enantioselective epoxidations of unfunctional olefins with m ‐chloroperoxybenzoic acid and NaIO₄ as oxidants. The whole chiral salen Mn(III) catalyst, including the ZnPS‐PVPA support and the linker as well as chiral salen Mn ligand together contribute to the chirality of products. The heterogeneous catalyst has the potential for use in industry owing to superior stability (recycling nine times) and activity in large‐scale reactions (such as 200 times).     

Dendritic ruthenium porphyrins: a new class of highly selective catalysts for alkene epoxidation and cyclopropanation

Attachment of Fréchet-type poly(benzyl ether) dendrons [G-n] to carbonylruthenium(II) meso-tetraphenylporphyrin (5) using covalent etheric bonds forms a series of dendritic ruthenium(II) porphyrins 5-[G-n](m) (m=4, n=1, 2; m=8, n=0-2). The attachment was realized by treating the carbonylruthenium(II) complex of 5,10,15,20- tetrakis(4'-hydroxyphenyl)porphyrin or 5,10,15,20-tetrakis(3',5'-dihydroxyphenyl)porphyrin with [G-n]OSO(2)Me in refluxing dry acetone in the presence of potassium carbonate and [18]crown-6. Complexes 5-[G-n](m) were characterized by UV/Vis, IR, and NMR spectroscopy and mass spectrometry. All of the dendritic ruthenium porphyrins are highly selective catalysts for epoxidation of alkenes with 2,6-dichloropyridine N-oxide (Cl(2)pyNO). The chemo- or diastereoselectivity increases with the generation number of the dendron and the number of dendrons attached to 5, and complex 5-[G-2](8) exhibits remarkable selectivity or turnover number in catalyzing the Cl(2)pyNO epoxidation of a variety of alkene substrates including styrene, trans-/cis-stilbene, 2,2-dimethylchromene, cyclooctene, and unsaturated steroids such as cholesteryl esters and estratetraene derivative. The cyclopropanation of styrene and its para-substituted derivatives with ethyl diazoacetate catalyzed by 5-[G-2](8) is highly trans selective.     

Well-dispersed catalysts of supported chromium,molybdenum and tungsten carbonyls which are highly active for alkene hydrogenation

Traditional catalysts (prepared by the reduction of a salt) of supported Cr, Mo, and W compounds have low activities for the hydrogenation of alkenes. This low activity is partly due to the difficulty in fully reducing these catalysts. The use of the zero-valent complexes Cr(CO)₆, Mo(CO)₆, and W(CO)₆ for catalyst synthesis is described. Proper activation can yield well-dispersed and low-valent catalysts primarily containing subcarbonyl complexes which are far more active than traditional catalysts of these elements for the hydrogenation of alkenes.