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In this study, a state-of-the-art high-throughput experimentation (HTE) workflow for catalytic olefin polymerization, covering an unprecedented wide part of the polymer knowledge and value chains from catalytic synthesis all the way down to "engineering"microrheology, was thoroughly assessed with respect to its ability to prepare new materials and produce large and accurate databases for the investigation of quantitative structure-property relationships (QSPRs). Olefin blocks copolymers (OBCs) produced under chain-shuttling polymerization conditions were used as a demonstration case. The results of a thorough microstructural, structural, mechanical, morphological, and rheological characterization of OBC replicas prepared with the HTE synthetic platform and a commercial sample, chosen as a benchmark, demonstrate the robustness of the approach. The proposed workflow can become a paradigm for the high-throughput synthesis and investigation of novel materials, thus reducing the time to market of new products. In our opinion, this opens the door to integrated HTE and artificial intelligence approaches to QSPR problem solving in the numerous cases for which a thorough understanding of the theory is not sufficient to deterministically unravel the complexity of practical applications.

The nonuniform molecular architecture of ethylene/1-octene multiblock copolymers (O-MBCs) synthesized by the chain shuttling technology is investigated. The samples consist of chains characterized by alternating hard (crystalline) and soft (amorphous) blocks, corresponding to random ethylene/1-alkene copolymers with a low and high comonomer content, respectively. The chains are nonuniform as the distribution in the length and number of blocks per chain are statistical and vary from chain to chain. A clear-cut investigation of the inter-and intrachain constitutional heterogeneity of O-MBCs is performed by carrying out, at first, a sequential and exhaustive solvent fractionation procedure in boiling solvents, that is, diethyl ether, n-hexane, and cyclohexane. Successively, the unfractionated samples and the corresponding fractions are subjected to analytical crystallization elution fractionation (aCEF), solution 13C NMR, differential scanning calorimetry, and wide-and small-angle X-ray scattering (SAXS) analyses. Four fractions of increasing average ethylene content, hard block content, and degree of crystallinity are obtained, that is, a fraction soluble in diethyl ether (sEE), a fraction insoluble in diethyl ether/soluble in n-hexane (iEE-sC6), a fraction insoluble in n-hexane/soluble in cyclohexane (iC6-sCC6), and a fraction insoluble in cyclohexane (iCC6). The results of aCEF and 13C NMR analysis highlight that the multiblock chain microstructure of the O-MBCs corresponds to a statistical distribution of the length of hard and soft blocks that occurs not only at an interchain level but also at an intrachain level. SAXS measurements essentially confirm the results of the microstructural analysis and allow achieving a quantitative description of the constitutional heterogeneity affecting O-MBCs at the intramolecular level. In particular, it is shown that for the inferior fractions (sEE and iEE-sC6), the chains include hard blocks of low molecular mass (

Highly accurate high-throughput experimentation (HTE) data for a set of 21 silicon-bridged C2-symmetric ansa-zirconocenes in propene homopolymerization were collected and were used to develop quant...

Surface modification of MgCl2-supported Ziegler–Natta catalysts (ZNCs) by means of organic Lewis bases, either used as precatalyst components (“internal donors”, ID) or in combination with the AlEt3 cocatalyst (“external donors”, ED), is key for achieving a high stereoselectivity in propene polymerization. In fourth-generation systems, which are the working horses of this important catalyst class, the ID is an (ortho-)­dialkyl phthalate; under polymerization conditions, this reacts with AlEt3 and must be replaced by an ED (typically an alkoxysilane) in order to obtain the desired performance. In a previous study, we investigated the molecular kinetics of the reaction between dibutyl phthalate and AlEt3 in solution by means of integrated experimental and computational protocols. A similar approach has now been applied to monitor the progress of the reaction for the complete catalyst system. Compared with solution (ΔH# ≈ 15 kcal mol–1; ΔS# ≈ −28 cal mol–1 K–1), the activation parameters in heterogeneous phase (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −49 cal mol–1 K–1) indicate that phthalate reduction is less activated, but suffers from an augmented entropic penalty. This suggests that the ID reacts with AlEt3 while still on the MgCl2 surface, rather than in the liquid phase after desorption, and that the surface is not innocent. Whether or not an alkoxysilane ED was present in the system turned out to be immaterial on reaction kinetics. High-level DFT calculations on a well-established MgCl2/dibutyphthalate model cluster reproduced the experimental data with a remarkably good agreement (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −46 cal mol–1 K–1). Experiments and calculations agree on the first ethyl transfer to the reacting ester group representing the rate-determining step, in solution as well in heterogeneous phase. In both cases, a four-membered transition state (TS) appears to be involved; in heterogeneous phase, though, the ester carbonyl is extra-activated by the interaction with a Lewis-acidic surface Mg center, rather than a second AlEt3 molecule. In our opinion, the interest of the proposed approach goes beyond the present study; indeed, further applications can be proposed for the design and engineering of novel ZNCs with tailored behaviors.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure–activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations (Me2Si(2-methyl-4-phenyl-6-tert-butylindenyl)2ZrMe+, [1Me]+ Me2Si(2,4-dimethylindenyl)2ZrMe+, [2Me]+ Me2Si(indenyl)2ZrMe+, [3Me]+) paired with the weakly coordinating B(C6F5)4- anion have been investigated by NMR spectroscopy in aromatic hydrocarbons with different polarities (toluene, ϵr = 2.38; chlorobenzene, ϵr = 5.69; 1,2-difluorobenzene (ODFB), ϵr = 13.38). These highly electrophilic cations are stabilized by solvent coordination, as evidenced by the unequivocal identification of [1Me·C7H8]+B(C6F5)4-, a rare example of a toluene-stabilized ion pair that has been fully characterized in solution. Combining spectroscopic and DFT methods allowed us to evaluate how solvent coordination modulates the dynamic processes typical of this class of catalysts. An exchange between the two faces of coordinated toluene (face inversion, FI) occurs without solvent decomplexation (ΔH⧧FI = 14.6 kcal mol-1 ΔS⧧FI = 3 cal mol-1 K-1 ΔG⧧FI(298) = 13.7 kcal mol-1) and is about 20 times faster than the exchange between coordinated and free solvent (solvent decomplexation, SD, ΔH⧧SD = 17.9 kcal mol-1 ΔS⧧SD = 10 cal mol-1 K-1 ΔG⧧SD(298) = 14.9 kcal mol-1) and ion pair symmetrization (IPS, ΔH⧧IPS = 18.6 kcal mol-1 ΔS⧧IPS = 12 cal mol-1 K-1 ΔG⧧IPS(298) = 14.9 kcal mol-1). For the ion pairs [1-3Me·solvent]+B(C6F5)4-, IPS rate constants (kIPS) do not correlate with the solvent polarity (kIPS(ODFB) > kIPS(toluene) > kIPS(chlorobenzene)). For the more coordinating toluene and chlorobenzene solvents, ΔG⧧IPS nicely tracks with the amount of positive charge at the metal, increasing as the positive charge increases (qZrMe2,CM5; 1Me2 > 3Me2 > 2Me2). In contrast, in the less coordinating ODFB, differences in the IPS barriers for [1-3Me·ODFB]+B(C6F5)4- appear to correlate better with steric parameters, as measured by the percent buried volume on the open quadrants (%VBur,open) for the corresponding neutral precursors (%VBur,open = 34.8, 35.5, 34.6% for 1Me2, 2Me2, and 3Me2, respectively).

A series of 20 bis(phenolate-ether) ligands, in combination with Zr and Hf, was tested for performance in propene polymerization, focusing on molecular weight, stereoregularity and regioregularity of the resulting polymer. Ligand variation covers length of the aliphatic linker between the ligand halves, as well as steric bulk of the groups ortho to the phenolate oxygen. The linker length has a dramatic effect on MW: two-carbon linkers produce oligomers (Mn < 2.5 kDa) while three- and four-carbon linkers generate much higher MW (Mn typically 50–500 kDa). Stereoselectivity can be tuned using large, flat substituents in the o-phenolate position; tuning of regioselectivity is much harder. Hf catalysts are slower than their Zr analogs and do not work well with MAO/BHT (BHT = 2,6-di-tert-butyl-4-methylphenol); they are generally more selective (MW, stereo and regio). Density functional calculations agree fairly well with observed selectivities, supporting the involvement of a fac/fac coordinated active species. These O4 catalysts are considerably more flexible than e.g. metallocenes, making accurate prediction of PP microstructure a significant challenge.

In the search for more efficient single-center ethene/α-olefin copolymerization catalysts, metallocenes bearing a 2-indenyl substituent pattern have largely been ignored in the past. Here, we show that such a structural motif yields competent linear low-density polyethylene (LLDPE) catalysts. They are also relatively easy to synthesize, allowing for a wide structural amplification. A screening of 28 catalysts reveals that the lead catalyst in this study displays high comonomer affinity and molecular weight capability at industrially relevant temperatures. QSAR models show that steric factors likely contribute stronger than electronic factors to the observed substituent trends, both for comonomer affinity and MW capability.

Advanced High Throughput Experimentation (HTE) methods were applied to quantify the regioselectivity and the so-called "hydrogen response" in propene polymerization of four Ziegler-Natta catalysts representative of the three latest industrial generations (i.e., modified with a dialkyl phthalate, 1,3-dimethoxypropane or dialkyl succinate Internal Donor), used in combination with AlEt3 and a library of eight alkoxysilane External Donors with large structural diversity. At odds with stereoselectivity, regioselectivity turned out to be only modestly impacted by the chemical structure of the electron donors used as surface modifiers. A clear and simple relationship between regioselectivity and "hydrogen response", and, therefore, polymer molecular weight modulation was highlighted.

This paper introduces a high throughput experimentation method for fast and accurate evaluations of regioselectivity in Ziegler–Natta (ZN) propene polymerizations. With a simple protocol, the (very low) fraction of regio-irregular 2,1 monomeric units in the polymers can be quantitated by means of 13C NMR chain-end analyses on single H2-terminated polypropylene samples. The method, that was successfully validated for three representative ZN catalyst systems, also provides information on catalyst “dormancy” and propensity to undergo chain hydrogenolysis. This opens the door to the rapid and accurate implementation of quantitative structure–activity relationship (QSAR) databases of regioselectivity and “hydrogen response” in this important industrial catalysis.

The performance of C2-symmetric ansa-hafnocene catalysts for isotactic polypropylene typically deteriorates at increasing temperature much faster than that of their zirconium analogues. Herein, we analyze in detail a set of five Hf/Zr metallocene pairs—including some of the latest generation catalysts—at medium- to high-polymerization temperature. Quantitative structure–activity relationship (QSAR) models for stereoselectivity, the ratio allyl/vinyl chain ends, and 2,1/3,1 misinsertions in the polymer indicate a strong dependence of polymerization performance on electrophilicity of the catalyst, which is a function of the ligand framework and the metal center. Based on this insight, the stronger performance decline of hafnocenes is ascribed to electrophilicity-dependent stabilization effects.

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 Bronste...

Olefin coordinative chain-transfer polymerization (CCTP) under concurrent tandem catalysis conditions is a smart and valuable synthetic approach to statistical olefin block copolymers (OBC). This process, better known as chain shuttling (co)polymerization, was disclosed by Dow Chemical in 2006. Many new polyolefin materials with unique properties and applications can be produced, and as a matter of fact some processes have reached the commercial stage. Several OBC can act as phase compatibilizers in immiscible polyolefin blends, for which a high demand is anticipated in a circular economy. On the other hand, the number of catalyst systems amenable to this polymerization chemistry is very limited, and it is clear now that several literature claims of CCTP are questionable. In particular, the low average molar mass of polyolefin samples produced in the presence of the typical transalkylating agents used for CCTP (such as diethyl-Zn, DEZ) often hampers effective separations of the different components of physical blends, which can be mistaken as an indication of a block copolymer nature. On top of that, the mechanical properties of intimate polyolefin physical blends can resemble, at least at a superficial characterization, those of true OBC. A thorough understanding of such processes is a mandatory requirement towards a rational catalyst design leading to improvements of existing materials or, ultimately, the synthesis of new/better ones. In this respect, a smart use of High Throughput Experimentation (HTE) methodologies represents the best possible approach to address complex and multivariable dilemmas. The present talk will focus on a recent case history: despite several claims on the amenability for binary metallocene systems to undergo chain-shuttling polymerization producing stereoblock isotactic/syndiotactic polypropylene materials, we demonstrated that the isotactic-selective C2-symmetric rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 catalyst is not prone to propene CCTP, and therefore does not yield stereoblock PP materials when used in racemic form or in tandem with other metallocenes. The proposed approach can be easily extended, providing irrefutable evidence for CCTP, as long as the average polymer molar mass is not low to the point that it largely dictates physical properties and flaws the determination of the 13C NMR microstructural fingerprint.

Olefin coordinative chain-transfer polymerization (CCTP) under concurrent tandem catalysis conditions is a smart and valuable synthetic approach to statistical olefin block copolymers (OBC). This process, better known as chain shuttling (co)polymerization, was disclosed by Dow Chemical in 2006. Many new polyolefin materials with unique properties and applications can be produced, and as a matter of fact some processes have reached the commercial stage. Several OBC can act as phase compatibilizers in immiscible polyolefin blends, for which a high demand is anticipated in a circular economy. On the other hand, the number of catalyst systems amenable to this polymerization chemistry is very limited, and it is clear now that several literature claims of CCTP are questionable. In particular, the low average molar mass of polyolefin samples produced in the presence of the typical transalkylating agents used for CCTP (such as diethyl-Zn, DEZ) often hampers effective separations of the different components of physical blends, which can be mistaken as an indication of a block copolymer nature. On top of that, the mechanical properties of intimate polyolefin physical blends can resemble, at least at a superficial characterization, those of true OBC. A thorough understanding of such processes is a mandatory requirement towards a rational catalyst design leading to improvements of existing materials or, ultimately, the synthesis of new/better ones. In this respect, a smart use of High Throughput Experimentation (HTE) methodologies represents the best possible approach to address complex and multivariable dilemmas. The present talk will focus on a recent case history: despite several claims on the amenability for binary metallocene systems to undergo chain-shuttling polymerization producing stereoblock isotactic/syndiotactic polypropylene materials, we demonstrated that the isotactic-selective C2-symmetric rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 catalyst is not prone to propene CCTP, and therefore does not yield stereoblock PP materials when used in racemic form or in tandem with other metallocenes. The proposed approach can be easily extended, providing irrefutable evidence for CCTP, as long as the average polymer molar mass is not low to the point that it largely dictates physical properties and flaws the determination of the 13C NMR microstructural fingerprint.

Absolute rigidity is rare in the "soft" world of organometallics. Here we introduce two cyclopenta[a]triptycyl ansa-zirconocene catalysts for isotactic-selective propene polymerization, designed by means of an integrated high-throughput experimentation/quantitative structure-activity relationship modeling approach. An ultrarigid ligand precisely wrapped around the Zr center enforces an enzyme-like lock and key fit, effectively hampering undesired reactive events, even at high temperature. Stereodefective units are hardly detectable by 13C NMR in the polymer produced at 120 °C; this corresponds to an enantioselectivity exceeding 6-7 kcal/mol: i.e., less than 1 propene misinsertion every 4000 (and at room temperature, one every ∼40000!).

For a set of 40 silicon-bridged C2-symmetric ansa-zirconocenes, reactivity ratios in ethene/1-hexene copolymerization were experimentally determined by means of an accurate high-throughput experimentation (HTE) approach, and used to develop quantitative structure-activity relationship (QSAR) models for comonomer affinity using chemically meaningful descriptors. These QSAR models rely almost exclusively on steric descriptors, with the single most important descriptor being the 'openness' of the open quadrants. Catalysts with an unobstructed main insertion pathway, i.e. without substituents affecting the open quadrant, show a remarkable insensitivity to further substituent effects, be it in 2-, 4-, 5-, 6- or 7-position or the bridge. We attribute this insensitivity to a shift in rate-limiting step for the comonomer incorporation, from insertion to olefin capture, with the latter being much less sensitive to modulation of the active pocket than the former. This indicates that our best incorporators are already close to the upper limit for comonomer affinity within this catalyst class. This journal is

Compared to heterogenous Ziegler–Natta systems (ZNS), ansa-metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the C2-symmetric bis(indenyl) ansa-zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution. Herein, we illustrate a Quantitative Structure – Activity Relationship (QSAR) model trained on a robust and highly accurate HTE database. The clear-box QSAR model utilizes, in particular, a limited number of chemically intuitive 3D geometric descriptors that screen various regions of space in and around the catalytic pocket in a modular way thus enabling to quantify individual substituent contributions. The main focus of the paper is on the methodology, which should be of rather broad applicability in molecular organometallic catalysis. Then again, it is worth emphasizing that the specific application reported here led us to identify in a comparatively short time novel zirconocene catalysts rivaling or even outperforming all previous homologues which strongly indicates that the metallocene story is not over yet.

A set of 19 silicon-bridged C2-symmetric zirconocenes rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 of varying steric demand in position 4 were synthesized and screened in propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability (R2 ≈ 0.8−0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure−activity relationship based on one single computational descriptor, Δ%VBur using the difference in the percent of buried volume between the “blocked” and “open” quadrants of the catalyst precursoris established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG‡ enantio), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (Mw > 1 MDa) and relatively high regioselectivity.

Chemistry in general is not an exact science. Chemical catalysis, moreover, is a purely kinetic phenomenon. This translates into the fact that discovering and even optimizing a catalyst for a desired application heavily relies on trial-and-error, and serendipitous advances are not rare. In recent years, we spent a considerable effort to extend the scope of HTE from discovery to mechanistic investigations. The general objective was to introduce protocols for ‘smart’ applications of an existing HTE workflow to complex chemical problems in polyolefin catalysis. In particular, methods for the rapid and accurate determination of the Quantitative Structure-Activity Relationship (QSAR) of representative molecular or heterogeneous catalyst formulations were implemented as the basis for statistical modeling with predictive ability. A high-performance polymerization platform (Freeslate PPR48) was integrated with state-of-art polymer characterization tools (including GPC, 13C NMR and A-CEF), and the workflow was optimized for the fast acquisition of large structure-properties databases. The approach was successfully applied to Ziegler-Natta and molecular olefin polymerization catalysts. Chain shuttling chemistry represented an extreme challenge, because operating reliably in solution at high temperature under controlled kinetic conditions in HTE scale is highly demanding; on the other hand, exploring exhaustively the complex variables hyperspace of this chemistry with conventional methods is also challenging, and requires a very long time. In the present talk, we want to share the idea that innovation in chemistry is not over even in areas that are considered mature, like e.g. catalytic olefin polymerization. Many chemical problems, including long standing ones, can rapidly find a solution as soon as adequate information becomes available. This simple and – in a way – trivial concept is often overlooked because the actual complexity of chemical systems tends to be under-estimated.

A computational exploration of the predicted molecular weights for Ti- and Zr-catalyzed olefin polymerizations shows that there are considerable opportunities for electronic tuning. Ligand variation mainly affects the propagation rate, whereas chain transfer to the monomer is hardly affected by electronic factors. The results are analyzed in terms of the effects of ligand variation on the relative energies of “connected couples” of reactant local minima and the corresponding transition states on the basis of the Hammond postulate and the Curtin–Hammett principle. For the constrained-geometry catalysts (CGCs) and bis(amido) systems studied, better donating ligands increase the preference for a “planar” metal environment and produce higher molecular weights.

Quantitative agreement between experimentally determined M–C bond dissociation energies (BDE) and DFT predictions (M06-2X/TZ//TPSSTPSS/DZ) can be reached by choosing the correct anchor for experimentally derived BDE. For the example of the archetypical metallocene catalyst Cp 2 TiCl 2 , it is shown that titanium–carbon bonds are very weak under polymerization conditions and fluctuate; steric strain is introduced after 2,1 insertion and via olefin capture. Thus, homolysis can become competitive with chain propagation. Depending on the catalyst and temperature, 2,1 insertion can be only a temporary inconvenience (dormancy) or a definitive decay event. It is then shown for a set of nine common Ti and Zr polymerization catalysts how ligand variation affects the metal–carbon BDE. Predicted stabilities of the M(IV) oxidation state with respect to homolysis are in nice agreement with the experimentally observed temperature tolerance of the various catalysts: homolysis is easier for Ti than for Zr, and cyclopentadienyl groups in particular facilitate homolysis, especially in bis-cyclopentadienyl systems.

In the search for more efficient single-center ethene/α-olefin copolymerization catalysts, metallocenes bearing a 2-indenyl substituent pattern have largely been ignored in the past. Here, we show that such a structural motif yields competent linear low-density polyethylene (LLDPE) catalysts. They are also relatively easy to synthesize, allowing for a wide structural amplification. A screening of 28 catalysts reveals that the lead catalyst in this study displays high comonomer affinity and molecular weight capability at industrially relevant temperatures. QSAR models show that steric factors likely contribute stronger than electronic factors to the observed substituent trends, both for comonomer affinity and MW capability.

“Structural” AlMe2(bht) was found to be an integral building block of the “TMA-free” olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6-di-tert-butyl-4-methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N-donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'-bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al-cages) not only when unmodified MAO is exploited but also in the case of “TMA-free” BHT-modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe2(bht) units.

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.

The crystalline morphology of ethylene/1-octene (EO-BC) statistical multiblock copolymers is investigated by transmission electron microscopy (TEM). EO-BCs are an important class of thermoplastic elastomers characterized by alternating crystalline, octene-poor, hard blocks and amorphous, octene-rich, soft blocks and statistical distribution of block length (BL) and number of blocks/chain (NB). Adopting similar crystallization conditions, samples with only subtle differences in the chain microstructure, are shown to display different solid-state morphology as probed by TEM and different melt rheology. In particular, the homogeneous or heterogeneous lamellar morphology which develops in the solid state reflects the tendency of the samples to show homogeneous or heterogeneous melt-rheology, respectively. As EO-BCs are a reactor blend of non-uniform chains, it is argued that the solid state and melt phase behavior of EO-BCs is controlled by differences in the relative abundance of chains including a prevalence of hard blocks of short and/or high length.

New ansa-zirconocene with bis(inden-2-yloxy)dimethylsilane ligand was synthesized and characterized by NMR spectroscopy and X-ray crystallography. The zirconocene was found to be highly active catalyst of ethylene polymerization and ethylene/hex-1-ene copolymerization upon methylaluminoxane activation.

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A–C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators., A model cluster for methylaluminoxane (MAO) was used to study the tendency of model Lewis acidic sites to release neutral Al fragments or transient aluminum cations both for unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol). Albeit only sites of lower Lewis acidity are found in TMA depleted modified MAOs, they can release cationic Al fragments when treated with certain N-donors. This shows how TMA depleted MAOs can still be potent activators.

Compared to heterogenous Ziegler&ndash, Natta systems (ZNS), ansa-metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the C2-symmetric bis(indenyl) ansa-zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution. Herein, we illustrate a Quantitative Structure &ndash, Activity Relationship (QSAR) model trained on a robust and highly accurate HTE database. The clear-box QSAR model utilizes, in particular, a limited number of chemically intuitive 3D geometric descriptors that screen various regions of space in and around the catalytic pocket in a modular way thus enabling to quantify individual substituent contributions. The main focus of the paper is on the methodology, which should be of rather broad applicability in molecular organometallic catalysis. Then again, it is worth emphasizing that the specific application reported here led us to identify in a comparatively short time novel zirconocene catalysts rivaling or even outperforming all previous homologues which strongly indicates that the metallocene story is not over yet.

Isotactic polypropylene (i-PP) is mainly produced with heterogeneous Ziegler-Natta (ZN) catalyst systems. Direct structural information on the active species is very difficult to achieve; as a matter of fact, the 13C NMR microstructure of the polymer has long been used as an indirect "fingerprint". For a long time, however, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy hampered a complete evaluation of the microstructure, and many rare albeit important details escaped detection with standard tools and methods. The most prominent case is that of the regiodefects, which are specially important because they dictate ZN catalyst response to molecular hydrogen used as a chain transfer agent. In recent papers we have reported on the quantification of regioirregular 2,1 units in ZN i-PP by means of high-temperature cryoprobe 13C NMR. Here we shift the focus onto the stereochemical environment of such units, which was highlighted by comparing the spectra of suitable ZN PP fractions with those of model PP samples made with molecular catalysts of different structures and symmetries. The results lend support to our three-site model of ZN catalytic species, originally based on the stereochemistry of regioregular PP chains/blocks.

Zr and Hf complexes of bis(phenolate-ether) (“O4”) ligands feature high activity, stereoselectivity and molecular weight capability for propene polymerization at high temperature. Here we report a simplified ligand synthesis and several new examples of O4 ligands. The formation of precatalysts LMR2 (M = Zr, Hf; R = Bn, Me) from LH2 and MR4 was found to be accompanied in some cases by the formation of dimers (μ-L)2[MR2]2, and X-ray structures of two such dimers have been determined. Treatment of LMMe2 with [Ph3C]+[B(C6F5)4]– produces fairly clean cationic species LMMe+ which were studied by 1H NMR. 2D ROESY data, in particular, suggest that for “smaller” O4 ligands the LMMe+ cation reversibly rearranges from the active (fac/fac) form to a presumably inactive fac/mer or mer/mer form; more bulky substituents appear to suppress this rearrangement. Implications for polymerization catalysis are discussed.

MAO​/BHT (MAO = methylalumoxane; BHT = 2,​6-​di-​tert-​butyl-​4-​methylphenol) cocatalyst for olefin polymn. has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)​0.1]​n cages and monomeric MeAl(bht)​2. Diffusion NMR indicates an av. n for Al-​clusters of 62-​96, i.e. 2-​3 times higher than that estd. 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*(tBu3P=N)​TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixts. obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=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 (ISIPs) [Cp*(tBu3P=N)​Ti-​X]​+[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 analog Cp*(tBu3P=N)​TiBn2 results in the clean formation of [Cp*(tBu3P=N)​Ti-​Bn]​+[MAO​/BHT]​-​ OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amt. of strongly acidic sites on Al-​clusters, shedding further light on the structure and properties of MAO​/BHT.

Four Cl/Me substituted [ONNO] Zr-catalysts have been tested in ethene/&alpha, olefin polymerization. Replacing electron-donating methyl with isosteric but electron-withdrawing chlorine substituents results in a significant increase of comonomer incorporation. Exploration of steric and electronic properties of the ancillary ligand by DFT confirm that relative reactivity ratios are mainly determined by the electrophilicity of the metal center. Furthermore, quantitative DFT modeling of propagation barriers that determine polymerization kinetics reveals that electronic effects observed in these catalysts affect relative barriers for insertion and a capture-like transition state (TS).

The 13C NMR microstructure of a polypropylene (PP) sample is a fundamental source of information on its properties, and also a &lsquo, fingerprint&rsquo, of the catalytic species used to produce it. Likely due to a much greater technological importance, isotactic polymers (i-PP) have been more thoroughly investigated that syndiotactic ones (s-PP). In this paper, we report the first full assignment of regioirregular sequences in s-PP samples made with two well-known molecular catalysts, namely a Cs-symmetric (cyclopentadienyl)(fluorenyl) ansa-zirconocene and a fluxional bis(phenoxyimine)Ti species. The results shed more light on the mechanism of chain propagation at the two catalysts, and open the door to the investigation of more elusive cases like the formation of s-PP blocks in the presence of multi-sited heterogeneous Ziegler-Natta systems.

Quenched-flow data for propene polymerization with rac-Me2Si(2-Me-4-Ph-1-indenyl)(2)ZrCl2/MAO support a picture where removal of MAO qualitatively changes the kinetic profile from a mainly enthalpic to a mainly entropic barrier. DFT studies suggest that a not previously recognized singly-bridged end-on coordination mode of Me6Al2 to catalytically active centers may be kinetically relevant as a resting state. In contrast, the more traditional doubly-bridged complex of Me3Al is proposed to be more relevant to chain transfer to cocatalyst.

Ziegler–Natta catalysis is a very important industrial process for the production of polyolefins. However, the catalysts are not well-understood at the molecular level. Yet, atomic-scale structural information is of pivotal importance for rational catalyst development. We applied a solid-state NMR/density functional theory tandem approach to gain detailed insight into the interactions between the catalysts’ support, MgCl2, and organic electron donors. Because of the heterogeneity of the samples, large line widths are observed in the carbon spectra. Despite this, good agreement between experimental and computational values was reached, and this shows that 1,3-diether based donors coordinate at (110) surface sites, while phthalates are less selective and coordinate at both (104) and (110) surface sites.

The typical activation of a fourth generation ZieglerNatta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti3+ centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti3+ species have been identified. A comparison of the experimentally derived g tensors and 35,37Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti3+ species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

A simple method for the counting and structural elucidation of the different active sites in an olefin polymn. catalyst system is introduced, based on a reaction quench by O2 under controlled kinetics, followed by NMR anal. of the OH-​labeled chain end-​groups formed upon acidic work-​up of the polymer. The method is successfully benchmarked using a mol. bis(phenoxyamino)​Zr(IV)​-​based catalyst characterized by a comparatively long-​lasting controlled kinetic transient at room temp.

NMR spectroscopy and DFT studies indicate that the Symyx/Dow Hf(IV)–pyridylamido catalytic system for olefin polymerization, [{N−,N,CNph−}HfMe][B(C6F5)4] (1, Nph=naphthyl), interacts with ERn (E=Al or Zn, R=alkyl group) to afford unusual heterobimetallic adducts [{N−,N}HfMe(μ-CNph)(μ-R)ERn−1][B(C6F5)4] in which the cyclometalated Nph acts as a bridge between Hf and E. 1H VT (variable-temperature) EXSY NMR spectroscopy provides direct evidence of reversible alkyl exchanges in heterobimetallic adducts, with ZnR2 showing a higher tendency to participate in this exchange than AlR3. 1-Hexene/ERn competitive reactions with 1 at 240 K reveal that the formation of adducts is strongly favored over 1-hexene polymerization. Nevertheless, a slight increase in the temperature (to >265 K) initiates 1-hexene polymerization.

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.