Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me₃Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]⁻) and [Me₂Al]⁺ abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me₂Al]⁺ abstraction, while [Me]⁻ abstraction from Me₃Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)_n, suggest that anions such as [(MeAlO)₇(Me₃Al)₄Me]⁻=[ 7,4 ]⁻ are especially stable compared to higher homologues, even though their neutral precursors are unstable.     

Real-time analysis of methylalumoxane formation

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H₂O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)₁₆(Me₃Al)₆Me]⁻, and we have computationally located a new sheet structure for this ion 94 kJ mol⁻¹ lower in Gibbs free energy than any previously calculated.

The activator methylaluminoxane is made by hydrolysis of trimethylaluminum. Analysis using ESI-MS reveals rapid formation of small oligomers is followed by slower aggregation to the larger precursors most capable of releasing [Me₂Al]⁺.     

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators

Hydrolysis of trimethylaluminum (Me₃Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me₃SiO)₂SiMe₂, OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h-MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non-hydrolytic MAO was prepared using trimethylboroxine. The properties of this material, which contains incorporated boron, differ significantly from h-MAO. In the case of MAO prepared by direct hydrolysis, oligomeric anions are observed to rapidly form, and then more slowly evolve into a mixture dominated by an anion with m/z 1375 with formula [(MeAlO)₁₆(Me₃Al)₆Me]⁻. Theoretical calculations predict that sheet structures with composition (MeAlO)_n(Me₃Al)_m are favoured over other motifs for MAO in the size range suggested by the ESI-MS experiments. A possible precursor to the m/z 1375 anion is a local minimum based on the free energy released upon hydrolysis of Me₃Al.     

Characterization of Macrocycles Formed from Cationic Copolymerization of Tetrahydrofuran with Propylene Oxide by GPC and GC/MS

Abstract

The copolymers obtained from a THF/PO-BF₃O(C₂H₅)₅-glycerin-ethylene chloride cationic copolymerization system were analyzed by GPC. The chromatograms show two groups of peaks, one located in the high molecular weight (MW) region and the other in the low MW region. In order to examine the low MW region in detail, the oligomers were separated from the copolymer by distillation under high vacuum, and further identified by GC/MS, NMR. It was found that these were an expected series of cyclic oligomers, i.e., PO:THF crown ethers: 1:2, 3:1, 2:2, 1:3, 3:2, 2:3, 1:4, 4:2, 3:3, 2:4, 4:3. Through the study of the elution behavior of these crown ethers in toluene with GPC, a linear relationship of the elution time against the logarithm of their MW's was obtained. Under the same GPC conditions, the relationship between the elution time and MW of n-alkanes was also determined. It was found that the hydrodynamic volume of the crown ethers is less than that of the n-alkanes with same MW, and they can be treated as homologous series as in the case of n-alkanes on the study of GPC elution behavior ignoring the influence of PO:THF composition ratios in crown ether rings.     

Olefin terpolymerizations. III. Symmetry,sterics, and monomer structure in ansa-zirconocenium catalysis of EPDM synthesis

Ethylenebis (η⁵-fluorenyl) zirconium dichloride ( 1 ) and rac-dimethylsilylene bis (1-η⁵-in-denyl) zirconium dichloride ( 2 ) were activated with methylaluminoxane (MAO) to catalyze ethylene (E) propylene (P) copolymerizations. The former produces high MW copolymer at 20°C rich in ethylene with reactivity ratio values of r_E = 1.7 and r_P <0.01, whereas the latter produces lower MW random copolymers with r_E = 1.32 and r_p = 0.36. Ethylidene norbornene (ENB) complexes with 1/MAO but does not undergo insertion in the presence of E and P. In contrast, 2/MAO catalyzes terpolymerization incorporating 9-15 mol % of ENB with slightly lower MW and activity than the corresponding copolymerizations. In comparison, 1,4–hexadiene was incorporated by 2/MAO with much lower A and MW . Terpolymerizations were also conducted with vinylcyclohexene using both catalyst systems. The steric and electronic effects in these processes were discussed. © 1995 John Wiley & Sons, Inc.     

Effects of dual initiators and catalytic additives on atom transfer radical polymerization of styrene

The effect of two initiators, so-called dual initiators system on atom radical transfer polymerization (ATRP), were studied with dimethyl-2,6-dibromohepanedioate (DMDBHD) and ethyl-2-bromoisobutyrate (EBIB). Cu(I)Br as catalyst and N,N,N??,N??,N??-pentamethyl-diethylenetriamine as ligand were employed for styrene ATRP. Interestingly, bimodal MWD were shown for the dual initiator system, and one of the peaks had higher molecular weight (MW) and the other had lower MW compared to a one-initiator system. The lower MW peak in bimodal peaks seemed to be mainly resulting from EBIB and the higher MW peak from DMDBHD. Furthermore, methylaluminoxane (MAO) was fed into the ATRP reaction to observe the effect of it on ATRP. As the MAO/CuBr molar ratio in feed increased from 0 to 1, the molecular weight and conversion increased without a notable change in PDI and curve shape of GPC. The conversion in the presence of MAO was also increased with the increase in MAO/CuBr molar ratio. The effect of Cp^*TiCl₃ on the ATRP was opposite to that of MAO. As the Cp^*TiCl₃/CuBr molar ratio increased from 0 to 1, the conversion of polymerization was down from 56 to 35%. Furthermore, the molecular weight was drastically decreased from 10,000 to 5,500, but their PDI did not show a significant change. These results can elucidated by the retarding effect of Cp^*TiCl₃ on the propagation of polymerization.     

Ionization of Cp2ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2’-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)₁₆(Me₃Al)₆ (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me₃Al prior to methide abstraction by 16,6 to form the ion-pairs [Me₂Al(κ²-L)][ 16,6 ] ( 5 : L=OMTS, 6 : L=bipy, [ 16,6 ]⁻=[(MeAlO)₁₆(Me₃Al)₆ Me]⁻) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp₂ZrMe₂ with 16,6 to form both neutral adducts 1 Cp₂ZrMe₂ ⋅ 16,6 and contact ion-pairs 4 and 7 , both with formula [Cp₂ZrMe][μ-Me(MeAlO)₁₆(Me₃Al)₆], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp₂ZrMe₂ or Me₃Al was also studied, forming outer-sphere ion-pairs [(Cp₂ZrMe)₂μ-Me][ 16,6 ] ( 2 ) and [Cp₂Zr(μ-Me)₂AlMe₂][ 16,6 ] ( 3 ). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp₂ZrMe₂ and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.     

Preparation of a PE/MT Composite by Copolymerization of Ethylene with In‐Situ Produced Ethylene Oligomers under a Dual Functional Catalyst System Intercalated into MT Layer

The catalyst dichlorobis(acetylacetone)zirconium was intercalated into montmorillonite (MT) together with AlEt₂Cl as a cocatalyst. Reacting ethylene in the presence of this catalyst yields α‐olefin oligomers with over 80% selectivity. [rac‐Et(Ind)₂ZrCl₂]/MAO was then added to form a dual functional catalyst system. An MT particle‐dispersed polyethylene composite was prepared by copolymerization of the in‐situ produced oligomers with ethylene using this dual system.     

Mechanism of emulsion polymerization of styrene in soap-free systems

The formation and growth of monodisperse polystyrene latex particles in the absence of added surfactant has been studied by sampling polymerization reactions at different times and determining the surface and bulk properties of the latex. A large number of nuclei in excess of 5 × 10¹²/ml were generated during the first minute of reaction, but this fell due to coagulation until a constant number (10¹¹?10¹²/ml) was reached. The rate of polymerization per particle was then found to be proportional to the particle radius. Gel-permeation chromatography has shown that the initial particles consist mainly of material of MW 1000 with a small amount of polymer up to MW 10⁶, and the presence of this low molecular weight polymer, which in many cases can still be detected after 100% conversion, is taken as being indicative of particle formation via a micellization-type mechanism involving short-chain (MW 500) free-radical oligomers. M?_n values determined for the latex particles throughout the course of reactions show that the molecular weight increases to a maximum of about 10⁵ as the particles grow. The presence of anomalous regions within the particles has been confirmed by transmission electron microscopy, scanning electron microscopy, and gas adsorption studies. It has also been found possible to re-expose these regions within apparently homogeneous particles by stirring with styrene monomer; this is indicative of a molecular weight heterogeneity within the latex particles. The presence of sulfate, carboxyl, and hydroxyl groups upon the latex particle surfaces has been determined by conductometric titration.     

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)_n(AlMe₃)_m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co‐catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis‐acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO‐derived AlMe₂⁺ cation. Thermodynamics suggest that activation through AlMe₂⁺ transfer is the dominant mechanism because sites that are able to release AlMe₂⁺ are more abundant than Lewis‐acidic sites. The model catalyst system is demonstrated to polymerize ethene.     

Exploration on the microwave‐assisted synthesis and formation mechanism of polyaniline nanostructures synthesized in different hydrochloric acid concentrations

Under microwave‐assisted synthesis, polyaniline (PANI) products with multiple nanostructures were synthesized by the oxidative polymerization of aniline and ammonium peroxodisulfate in the different concentrations of hydrochloric acid solutions. The structural analysis of PANI using FTIR, UV, and XPS indicated that phenazine‐like oligomers were produced in acid‐free and low acidic systems. Moreover, long linear PANI chains were obtained in the presence of highly acidic solutions. The morphology of PANI observed by SEM and TEM showed that nanoscale structures, including stacked sheets, nanotubes, branched nanofibers, and uniform nanofibers, occurred respectively in acid‐free solution, low acidity, medium and high acidity systems, effectively regulating by acidity. The formation mechanism of PANI nanostructures was explored here. The sheets were formed by the oligomers containing flat phenazine rings that can be stacked together with strong π–π interactions. Furthermore, nanotubes were fabricated by the self‐curling of thin sheets consisted of phenazine‐like oligomers with numerous linear units in the chains. The nanofibers are supposed to form by the linear PANI chains and the secondary growth during aniline polymerization caused the branch formation on the nanofibers. All results indicate that acidity, rather than microwave assistance, is the critical factor that determines the polymerization mechanism and the final nanostructure. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3357–3369     

Properties of networks based on poly(alkylene glycol) diacrylates

Three series of poly(alkylene glycol) diacrylates, namely poly(ethylene glycol), poly(propylene glycol) and poly(tetramethylene glycol) α,ω-diacrylates have been prepared by acryloylation of the corresponding bis-OH-terminated oligomers. Their MW was in the range 200–1000 and their acrylic functionality was close to two. They were subjected to UV curing as thin films obtaining, by FTIR analysis, complete terminal double bond conversion in all cases with the exception of the lowest MW oligomers. Thermal and mechanical properties of the networks were investigated by means of DSC, TMA and DMTA analyses. Concerning the PPGDA oligomers, products totally amorphous and rubbery were obtained; PEGDA and PTMGDA oligomers gave similar amorphous structures with the exception of the highest MWs which showed partial crystallinity. By means of thermal and DMTA measurements, T_g values and E' moduli were obtained and correlated to the MW of the oligomers used. In particular, T_g values, as function of the crosslinking density, agreed with those calculated by the Nielsen equation for the three series investigated. The equilibrium swelling in water was determined for various networks as a function of the MW of the oligomers. The values obtained were compared and discussed in terms of the solubility parameters and the crosslinking density of various structures.     

Separation of Polybutylene Glycols Widely Differing in Molecular Weight on Different Stationary Phase by Gradient RP-HPLC and Detection by Evaporative Light Scattering

Polybutylene glycols (PBG’s) substantially differing in molecular weight (MW), i.e., PBG 650, PBG 1000, PBG 2000 and PBG 3000 were subjected to separation on monolithic, i.e., Chromolith SpeedRod RP-18 and Chromolith Performance RP-18, polymeric, i.e., Polymer C₁₈ and silica-based, i.e., Nucleosil 5C₄ chromatographic supports. Different eluent systems each consisting of a ternary gradient built up from acetonitrile, tetrahydrofuran and water were applied and monitoring of signal responses was accomplished by evaporative light scattering detection (ELSD). Fine-tuning of the gradient profiles for the different PBG’s yields optimum separation of the individual oligomers of the low MW samples PBG 650 and PBG 1000 and no marked differences seem to occur on the four stationary phases used within the study. However in contrast, pronounced deviations between them were observable when applied for separation of PBG 2000 and PBG 3000. Surprisingly in the latter case the Nucleosil 5C₄ support proved to be better suited for separation of sample constituents of higher MW. Although better resolution into individual oligomers, in particular of PBG 2000 and PBG 3000, was achieved on both Chromolith SpeedRod RP-18 and Chromolith Performance RP-18 when compared to the Polymer C₁₈ support, the good separation efficiency of the latter support is nevertheless quite remarkable making this material an additional valuable alternative for chromatographic fingerprinting at least for PBG 650 and PBG 1000.     

Polyethylene/graphite nanocomposites obtained by in situ polymerization

The synthesis of polyethylene/graphite nanocomposites by in situ polymerization was achieved using the catalytic system Cp₂ZrCl₂ (bis(cyclopentadienyl)zirconium(IV) dichloride)/methylaluminoxane (MAO). Graphite with nano dimensions, previously treated with MAO, was added into the reactor as filler at percentages of 1, 2, and 5% (w/w). XRD analysis showed that the chemical and thermal treatments employed preserve the structure of the graphite sheets. The formation of graphite nanosheets and nanocomposites was confirmed by TEM and AFM. TEM micrographics showed that the polyethylene grew between the graphene nanosheets, giving intercalated and exfoliated graphite nanocomposites. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 692–698, 2010     

Coordination polymerization at very low amounts of methylaluminoxane as cocatalyst

The effect of very low amounts of methylaluminoxane as an activating cocatalyst in the coordination polymerization has been investigated in the syndiospecific polymerization of styrene with a half-sandwich metallocene catalyst in the presence of triisobutylaluminum at molar ratios of methylaluminoxane/transition metal from 0/1 to about 20/1 in comparison to the polymerization behavior at high molar methylaluminoxane (MAO)/metal ratios.As a result, there cannot be observed any polymerization reaction below a true molar ratio MAO/Ti of 6:1. At higher molar ratios until about 20, the polymerization conversion is increasing significantly with the MAO/Ti molar ratio.These observations and the results of the determination of the kinetic reaction order can be explained with Barron’s tert-butyl aluminoxane based model of MAO as a cage of six monomeric MAO units (AlOMe)₆ in contrast to Sinn’s MAO model of a cage of twelve monomeric units (AlOMe)₁₂ and are discussed with the results received at usually applied much higher MAO/transition metal ratios leading to a first-order dependence of the polymerization rate on the MAO concentration.From the thermal behavior of the syndiotactic polystyrenes synthesized it can be concluded, that the stereospecificity of the polymerization reaction is not affected by MAO at low MAO concentrations.     

Facile and efficient synthesis of star-shaped oligomers from a triazine core

A facile and efficient synthetic approach to tripodal star-shaped oligomers is described. Several generations of hydroxyl-terminated tripodal star-shaped oligomers were prepared in high yield from 1,3,5-triacryloylhexahydro-1,3,5-triazine (TAT) as a core by alternating amine-catalyzed thiol-ene and acrylate esterification reactions. The compounds were fully characterized by 1D and 2D NMR spectroscopy, ESI-MS, and elemental analysis. The combination of thioether groups and hydrogen bonding moieties suggests that these products can be used as metal chelating ligands.     

Iron(III) complexes bearing 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines: Synthesis, characterization and their catalytic behaviors towards ethylene oligomerization and polymerization

A series of iron(III) complexes ligated by 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines was synthesized and examined by ¹H NMR, ESI-MS, IR spectroscopic, elemental analysis and X-ray photoelectron spectroscopy (XPS). Activated with methylaluminoxane (MAO), all ferric complexes exhibited good activities (up to 5.38 × 10⁶ g mol⁻¹(Fe) h⁻¹) of ethylene oligomerization and polymerization, and resultant oligomers and polyethylene waxes showed high α-olefin feature, meanwhile the distribution of oligomers mostly resembled Schulz-Flory rules. The various reaction parameters were investigated in detail, and the less bulky and electron-withdrawing substituents of ligands could enhance the catalytic activities of their ferric complexes. The observations explain the cause for unstable activities performed by stored iron(II) complexes.     

Counterion-dependent proton-driven self-assembly of linear supramolecular oligomers based on amino-calix[5]arene building blocks

Self-assembly of a calix[5]arene bearing a 12-aminododecyl pendant group on the lower rim into supramolecular oligomers through intermolecular iterative inclusion events is readily triggered by contact with acid solutions and is reversed to the amino monomer precursor by treatment with a base. 1H NMR data are consistent with the formation of head-to-tail assemblies derived from endo-cavity inclusion of the alkylammonium moiety. Diffusion NMR and light-scattering studies provide evidence for the presence of oligomers in solution and show that different counterions and concentrations result in different oligomer sizes, whereas ESI-MS and SEM investigations, respectively, indicate that self-assembly also takes place in the gas phase and in the solid state. The growth of these supramolecular oligomers is concentration-dependent; however, as a consequence of the saline nature of the monomer, it also shows a distinct counterion-dependence owing to ion-pairing/solvation effects.     

Heterogeneous Copolymerization of Ethylene and α-olefins Using Aluminohydride-Zirconocene/SiO2/MAO by High-Throughput Screening

Copolymerizations of ethylene and α-olefins (1-hexene and 1-octene) using a supported catalyst derived from the activation of a zirconocene aluminohydride complex with PMAO and MMAO are reported. The supported (nBu-Cp₂ZrH₃AlH₂)/SiO₂/MAO system was evaluated by high-throughput techniques, in order to find approaches to the optimal copolymerization conditions. The polymerization reactions were carried out in a parallel polymerization reactors system (PPR) by Symyx Technologies, Inc. The screening of the activity of the supported system and the molecular weight (MW) of the polymers and copolymers obtained in the PPR, allowed us to optimize copolymerization conditions, like hydrogen (H₂) addition to control MW and molecular weight distribution (MWD), polymerization temperature, cocatalyst ratio, and solvent type. The copolymerization reactions were scaled-up in order to validate the performance of the catalytic system at higher polymerization scales, according to the results obtained in the combinatorial phase. The scaled-up copolymerizations of ethylene with 1-hexene and 1-octene, showed high activities and MW, and low comonomer incorporation (from 0.3 to 1.3 mol-%, determined by ¹³C NMR). However, the crystallinity (X_c), thermal properties (T_c and T_m) and densities of the polyethylenes obtained with the supported (nBu-Cp₂ZrH₃AlH₂)/SiO₂/MAO system, were significantly modified, approaching those of metallocene linear low-density polyethylenes (mLLDPE).     

Bidentate iron complexes based on hyperbranched salicylaldimine ligands and their catalytic behavior toward ethylene oligomerization

A series of bidentate iron complexes based on hyperbranched salicylaldimine ligands were synthesized and characterized by spectroscopic and analytical methods. Upon activation with methylaluminoxane (MAO), the complexes showed good activities [up to 8.17 × 10⁴ g/(mol Fe h)] for ethylene oligomerization. Activation of the bidentate iron complex with a 1-octadecyl moiety in the ligand backbone (complex C3) with Et₂AlCl produced higher catalytic activity than C3 with MAO, although the selectivity for C₈₊ oligomers was lower. The choice of solvent and reaction parameters significantly affected both the activities and selectivities of these complexes. Under the conditions ([Fe] = 5 μmol; temperature = 25 °C; toluene = 50 mL; time = 30 min; ethylene pressure = 0.5 MPa; MAO as cocatalyst), complex C3 gave high activity [7.46 × 10⁴ g/(mol Fe h)] with better selectivity for C₈₊ oligomers (26.58%). The catalytic activities and selectivities were also influenced by the ligand structure and choice of metal. The catalytic activities declined with increasing alkyl chain length of the ligand backbone. Compared to the nickel complex with 1-tetradecyl as core in the ligand backbone (C4), the iron complexes exhibited lower catalytic activities but the better selectivities for C₁₀₊ oligomers.