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High Throughput Experimentation (HTE) was originally introduced in olefin polymerization R&D as a discovery tool. In recent years, we spent a considerable effort to extend the scope of the HTE approach to mechanistic investigations. 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 resulting workflow was optimized for the fast acquisition of large structure-properties databases usable in QSAR studies. The workflow was successfully applied to Ziegler-Natta and molecular olefin polymerization systems and products thereof.(1) In this presentation, we will illustrate case histories of increasing complexity, covering molecular kinetics as well as polymer characterization aspects.

A general model of the catalytic species on the surface of heterogeneous Ziegler-Natta catalysts (either TiCl3-based or MgCl2-supported), based on the statistical analysis of their high-field 13C NMR "fingerprint" in the polymerization products, is presented and discussed.

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 (

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.

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.

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.

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.

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

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.

A study was conducted to demonstrate in situ diversification of (pyridylamide)hafnium(IV) catalysts. An extensive density functional theory (DFT) study was conducted for several C 1-symmetric and C s-symmetric precatalysts to investigate the diversification of the (pyridylamide)hafnium(IV) catalysts. The study also investigated propylene polymerizations were investigated with two catalysts to find evidence of multisite behavior. It was observed that catalyst systems 1/Ph 3C][B-(C 6F 5) 4] and 2/Ph 3C][B(C 6F 5) 4] produced polypropylene samples with broad molecular weight distributions.

The origin of the syndiotactic selectivity in propene polymerization of a typical bridged fluorenyl-amido catalyst, namely [Me 2 Si(3,6-di-tert-butyl-9-fluorenyl)(N-tert-butyl)]TiCl 2 (1), activated with methylaluminoxane or [HMe 2 N(C 6 H 5 )][B(C 6 F 5 ) 4 ]/Al(i-butyl) 3 , was investigated by means of 150 MHz 13 C NMR spectroscopic microstructural polymer analysis. The asymmetric induction was traced unambigously to enantiotopic-sites control. Compared with the better-known cyclopentadienyl-fluorenyl ansa-zirconcenes, 1 turned out ot be almost identically enantioselective (in agreement with computer modeling), but less stereoselective because of a higher propensity to undergo site epimerization. This results in a chain microstructure with large predominance of m over mm stereodefects, deceptively similar to that of syndiotactic poly(propylene) produced under chain-end control.

This communication describes the microstructure of "stereoblock isotactic" polypropylene obtained with sterically hindered "oscillating" metallocene catalysts, points out in which respects it differs from that of "chain-end-control led isotactic" polypropylene, and explains why the two definitions cannot be used as synonymous (as is commonly found in the literature).

PROPENE POLYMERIZATION

The new octahedral column 4 catalysts bearing phenoxy-amine or phenoxy-imine ligands have opposite regioselectivities in propene polymerization. In this Communication, we report on a QM/MM investigation indicating that one of the key factors controlling the regiochemistry of propene insertion is the nature of the N atoms: steric and electronic effects related to the different hybridization synergically favor 1,2 or 2,1 insertion when the said N's are respectively of amine or imine type.

We have studied the balance between olefin insertion and beta -hydrogen transfer to monomer for all "well-defined" aluminum polymerization catalysts reported to date. Consistently, the balance is predicted to be significantly worse than for Me2AlEt, implying that none of the proposed active species, should give a high-molecular-mass polymer. A more systematic analysis of ligand effects allows a rationalization of these results and shows that small modifications to the proposed active species are unlikely to solve the problem. We conclude that olefin polymerization at a single aluminum center is, rather unlikely. Alternative interpretations of the experimental data are discussed.

Studies on the effects of hydrogen, monomer concentration and temperature in propene polymerization using high-activity MgCl2-supported catalysts have revealed that the effects of the catalyst on polymer yield, isotacticity and molecular weight are interrelated and dependent on both regio- and stereoselectivity. The high hydrogen sensitivity of catalysts containing a diether donor is mainly due to chain transfer following the occasional regioirregular (2,1-) monomer insertion. The probability of regio- and stereoirregular insertion decreases with monomer concentration, indicating the presence of nonsymmetric active centres. The stereoregularity of the isotactic polymer also increases with polymerization temperature, indicating a greater relative increase in productivity for highly isospecific as opposed to moderately isospecific centres.