No abstract

Accurate in silica prediction of copolymerization performance of olefin polymerization catalysts is demonstrated. It is shown by the example of 19 metallocene and post-metallocene group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X(PCM)/TZ//TPSSTPSS/DZ) can accurately describe the copolymerization factor r(e): i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental r(e) values were computationally reproduced with a mean average deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol, respectively. Both dispersion and solvent corrections play a crucial role in achieving this accuracy. Ethene insertion is found to be entropically favored for all catalysts due to a combination of symmetry factors and less congested insertion geometries. The enthalpic preference for either ethene or propene is catalyst dependent. The predictions are based on straightforward calculation of relevant insertion transition state energies; there are no indications for a shift in rate-limiting step from insertion to e.g. olefin capture or chain rotation

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al + [B­(C6F5)4]−, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al + = [iBu2(DMA)­Al]2(μ-H)+). Unlike the simpler [AliBu2]+[B­(C6F5)4]−, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al + [B­(C6F5)4]− can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al + is the true activating species formed in situ in the binary cocatalyst [PhMe2NH]+[B­(C6F5)4]−/AliBu3. As little as 50 equiv of Al-H-Al + [B­(C6F5)4]− are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu3. The high, yet “tamed,” Lewis acidity of cationic Al-H-Al + is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al + [B­(C6F5)4]− offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.

Propene polymerization using Ti Cp-phosphinimide catalysts in toluene and related aromatic solvents leads to the formation of benzyl-terminated polymer chains. End-group analysis suggests that these are formed after a 2,1-insertion event; density functional theory (DFT) studies support a mechanism involving homolysis of a Ti-sec-alkyl bond. This reaction could enable the catalytic formation of chain-end functionalized polyolefins. More importantly, it demonstrates that Ti–C homolysis might limit activity but does not necessarily constitute an irreversible deactivation mechanism.

Available exptl. data for several metallocenes indicate that the ethene​/propene copolymn. ratio rc can be much more temp. dependent than would be expected if competing insertion transition states (TS) are rate limiting. Detailed exploration of the reaction paths reveals in several cases the existence of a "capture-​like" transition state before the actual insertion, with free energies close to the insertion TS. Movement around these transition states does not just involve monomer and chain, but also clear distortion of the ligand skeleton to allow entry of the monomer. Taking these addnl. TSs into account leads to much improved agreement with expt. for a series of metallocenes and a constrained geometry catalyst system. Depending on catalyst and temp., selectivity is detd. by competing insertion​/insertion, capture​/insertion or capture​/capture. It seems likely that this is a common situation esp. for highly efficient catalysts, complicating (but not preventing) prediction of copolymn. performance

The MAO/BHT (MAO = methylaluminoxane; BHT = 2,6-di-tert-butyl-4-methylphenol) cocatalyst for olefin polymerization has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe 0.9 (bht) 0.1 ] n cages and monomeric MeAl(bht) 2 (bht = deprotonated BHT). Diffusion NMR indicates an average n for Al clusters of 62−96, i.e., 2−3 times higher than that estimated for unmodified MAO under analogous conditions (n ≈ 26− 41). The reactivity of MAO/BHT has been explored by monitoring the activation of the Cp*−phosphinimide titanium dichloride precatalyst Cp*(tBu 3 PN)TiCl 2 . Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixtures obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu 3 PN)TiX] 2 (μ-Y) + (X, Y = Me or Cl) forming outer sphere ion pairs (OSIPs) with MAO/ BHT-derived anions are dominant at low Al/Ti ratios, whereas mononuclear inner sphere ion pairs [Cp*(tBu 3 P N)TiX] + [MAO/BHT] − are formed at high Al/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analogue Cp*(tBu 3 PN)TiBn 2 results in the clean formation of [Cp*(tBu 3 PN)Ti-Bn] + [MAO/BHT] − OSIP, giving sharp 1 H and 31 P NMR signals; this reaction was exploited to quantify the amount of strongly acidic sites on Al clusters, shedding further light on the structure and properties of MAO/BHT.

Highly active molecular catalysts for olefin polymerization are extremely difficult to run in high-throughput experimentation (HTE) platforms. With common activators like methylaluminoxane (MAO) or a combination of tri-iso-butylaluminum and trityl tetrakis(perfluorophenyl)borate (TIBA/TTB), the necessary downscaling ends up with (sub)nanomole precatalyst loadings and poorly reproducible results due to the presence of adventitious impurities in similar amounts. Unexpectedly, we have now discovered that a convenient solution to this problem is provided by TIBA/AB (AB = N,N-dimethylanilinium tetrakis(perfluorophenyl)borate), a long-known but relatively uncommon protic activator. Indeed, with a proper operating protocol, a tunable precatalyst activation delay (minutes to hours) can be achieved, and even at high (≥10 nmol) catalyst loadings, a transient phase of well-controlled activity can be maintained long enough to produce the polymer amounts necessary for the characterizations under highly reproducible conditions. Importantly, polymer properties were not affected by choice of the activator, provided that the polymerization was kinetically controlled, which makes TIBA/AB the best option for HTE screenings of industrially relevant catalysts. Article pubs.acs.org/IECR

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)2(μκ 3 -O,N,O-H4-EDTMP)] (Ir-H4-EDTMP, H4-EDTMP 4− = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)2(μ-κ 3 -O,N,O-EDTA)] (Ir-EDTA, EDTA 4− = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR