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Ethylene/1-octene statistical multiblock copolymers (OMBCs) consisting of the alternation of crystalline, hard blocks and amorphous, soft blocks, with ≈0.5 and 20 mol % of 1-octene units, respectively, are subjected to thermal fractionation resorting to successive self-nucleation and annealing (SSA). The study is extended to random copolymers (RCs) of high (44 kDa) and low (3.4 kDa) number average molecular mass Mn, mimicking in 1-octene content the crystalline hard blocks and to the OMBC fractions extracted in boiling n-hexane and cyclohexane through a suitable solution fractionation protocol. For all the samples, the melting endotherms are well resolved in a multiplicity of peaks corresponding to the melting of crystals of different thicknesses generated in the SSA protocol that reflect the distribution of the methylene sequence length (MSL) in between consecutive interruptions along the chains. It is shown that, regardless of molecular mass, the MSL distributions of the RC samples are shifted toward greater values than those of the OMBC samples, and that also the shapes of the distributions are different. Since the MSL distribution depends on the frequency and distribution of the defects along the chains, and the defects act as interruption points, the higher fraction of long crystallizable sequences in the RC samples suggests that whereas for the RC samples the interruptions are merely represented by the 1-octene units that are rejected outside the crystals, for the OMBCs, the interruption of the regular methylene sequences belonging to the crystalline hard blocks due to the amorphous soft blocks linked to them should also play a role. Indeed, due to the partial miscibility of the hard and soft blocks, the hard blocks of major length tend to crystallize in a confined environment. This prevents the formation of thicker crystals and induces decrease of the MSL values as well as changes in the shape of the MSL distributions (topological confinement). On the other hand, the hard blocks of shorter length tend to crystallize, crossing the hard block rich regions and causing melting point depression and consequent shift of the MSLs toward lower values with respect to RCs (diluent effect). It is shown that differences in the MSL distribution of OMBCs are the result of the interplay between topological confinement and diluent effect, which in turn reflects differences in the OMBC chain microstructure, that is, differences in the distribution of block length at the inter- and intra-chain level.

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 dinuclear aluminum salt {[iBu2(DMA)Al]2(μ-H)}+[B(C6F5)4]− (AlHAl; DMA = N,N-dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant (s-AlHAl) bearing p-hexadecyl-N,N-dimethylaniline (DMAC16) units, which enhances solubility in aliphatic hydrocarbons. The novel s-AlHAl was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process.

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.

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.

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).

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.

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.

Measuring accurate translational self-diffusion coefficients (Dt ) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (VH ) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the Dt values of three isosteric Cp2 MCl2 complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that VH is the critical molecular property affecting Dt . This central concept served to clarify the assumptions behind the derivation of Dt =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (Dt ) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules.

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...

Syndiotactic propene–hexene (sPPC6) and propene–octene (sPPC8) copolymers have been synthesized in a wide range of compositions with a syndiospecific metallocene catalyst. The samples are crystallized up to a comonomer content of nearly 20 mol% with crystallinity, melting and glass transition temperatures that decrease with increasing concentrations of comonomers. The copolymers show elastic properties that depend on the crystallization behavior. The as-prepared samples with a comonomer content lower than 2 mol% crystallize in mixtures of the helical form I and form II of syndiotactic polypropylene (sPP), the latter containing conformational kink band defects and the former containing a b/4 shift disorder. Both sPPC6 and sPPC8 copolymers crystallize from the melt in disordered modifications of form I characterized by disorder in the alternation of right- and left-handed two-fold helical chains along the axes of the orthorhombic unit cell of form I of sPP. At high crystallization temperatures, more ordered modifications of form I develop for samples with a low comonomer concentration and form II surprisingly crystallizes in samples with a high comonomer content. The crystallization of form II rather than the ordered form I in melt-crystallized samples is a valuable result because it indicates that it is possible to tune the polymorphic transformations and the elastic properties of these copolymers through tuning of the branches concentrations.

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.

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.

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.

The correlations between the thermal behavior and the crystal morphology of isotactic propene-pentene copolymers were studied through wide-angle (WAXS) and small-angle (SAXS) X-ray diffraction. Copolymers with pentene concentration lower than 11 mol% crystallize in the α form of isotactic polypropylene (iPP) and a concomitant decrease of melting temperature and of the thickness of crystalline lamellae with increasing pentene concentration has been observed. At higher pentene concentrations the trigonal form of iPP crystallizes and a neat increase crystalline lamellar thickness and of the long period, with a slower decrease of melting temperature and crystallinity have been observed. These results have been treated in the general framework of copolymer crystallization theories, using a method proposed by Crist and correlated with the different level of inclusion of pentene co-units in the crystals of α and trigonal forms. For copolymers with pentene concentration lower than 11 mol% pentene co-units are in part incorporated in the crystals of α form. For higher pentene concentrations the decrease of melting temperature coupled with the increase of lamellae thickness with increasing comonomer content is the hallmark of the almost total inclusion of pentene co-units in the crystals of the trigonal form.

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.

The crystallographic micromechanical model (CMM) for prediction of yield stress of semicrystalline polymers, based on the thermally activated nucleation of screw dislocations at the boundary of lamellar crystals, is employed to interpret the yield behavior by effect of uniaxial drawing of some isotactic copolymers of propylene with different comonomeric units such as ethylene, 1-butene, 1-pentene, 1-hexene, and 1-octadecene (iPPEt, iPPBu, iPPPe, iPPHe and iPPOc, respectively). The samples are characterized by a random distribution of the comonomeric units. The CMM predicts that the values of stress at yield depend on the thickness of the lamellar crystals and relies on two parameters, i.e. the critical value of the free energy needed for nucleation and activation of a screw dislocation in crystallographic planes parallel to the chain axes, and the shear modulus relative to the planes of slip for the dislocations, whereas the role of the interlamellar amorphous phase is neglected. The aim of this study is to analyze to which extend the thickness of the lamellar crystals influences the yield stress for a series of propylene-based copolymers having a well-defined chain microstructure and crystallized in the α and/or γ forms, but possessing different degree of crystallinity, thickness of the lamellar crystals, stability of the crystals and also intrinsic flexibility of the portions of chains belonging to the amorphous regions, in relation with the degree of inclusion (exclusion) of the co-monomers in (from) the crystals. It is shown that, in the case of copolymers with a comonomer concentration lower than a threshold, the yield stress increases with increasing the lamellar thickness regardless of type of comonomer, the relative amount of the two polymorphs (α and γ forms), and the degree of inclusion, in good agreement with the predictions of the CMM approach. For copolymers with comonomer concentration higher than a threshold and bulky side groups, which are excluded from the crystals, the thickness of the lamellar crystals becomes low and the role of the chains in the amorphous regions becomes not negligible. In the case of copolymers with high butene concentration, instead, the stress at yield decreases with increasing the thickness of the lamellar crystals, because the high degree of inclusion of butene units in the crystals induces not only an increase in the thickness of the lamellar crystals, but also a decrease in the stability of the crystals, and the role of the chains located in the interlamellar amorphous layers acting as tie-molecules may not be disregarded.

“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.

Accurately measuring mol. kinetics of catalytic olefin polymn. has been a challenging objective. Many methods have been proposed in the literature, but all of them have drawback(s)​. In this paper, we introduce a labeling method to count active sites employing Me propargyl ether (MPE) as the quench-​labeling agent. It is com. available, does not react with Al-​alkyl species, and has a labeling efficiency close to 100​%. The labeling reaction was evidenced by a mechanistic study on the reaction between the model system Cp2ZrMe2/MAO (Cp = cyclopentadienyl) and MPE that it may occur through a coordination-​insertion mechanism without noticeable multiple insertions. The method was benchmarked by studying a MgCl2-​supported Ziegler-​Natta catalyst in 1-​hexene polymn. The fraction of the active transition metal χ* is found to be

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.

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.

Statistical multiblock copolymerization of ethene and higher alpha-olefins via tandem catalysis under reversible trans-alkylation conditions, commonly known as polyolefin chain shuttling, was discovered by means of a High Throughput Experimentation (HTE) approach.(1) The rapid screening of catalyst pairs in the presence of a proper chain shuttling agent (CSA) was key to identify well-working catalyst formulations (something that had been pursued in vain for decades with conventional methods).(2) In recent years, we spent a considerable effort to extend the scope of HTE from discovery to mechanistic investigations.(3) 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 usable in QSAR studies. The approach was successfully applied to Ziegler-Natta and molecular olefin polymerization catalysts.(3,4) 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 even more problematic, and requires a very long time. In the present talk, we will present for the first time the results of a HTE study of the chain shuttling formulation originally disclosed by Dow Chemical(1), as applied to ethene homopolymerization and ethene/1-hexene copolymerization. Molecular kinetic insight into the process, with special focus on the relationship between trans-alkylation statistics and polymer microstructure, will be provided. References (1) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D. Science 2006, 312, 714-719. (2) Wenzel, T. T.; Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L. Top. Organomet. Chem. 2009, 26, 65-104. (3) Busico, V.; Cipullo, R.; Mingione, A.; Rongo, L. Ind. Eng. Chem. Res. 2016, 55, 2686-2695. (4) Busico, V.; Pellecchia, R.; Cutillo, F.; Cipullo, R. Macromol. Rapid Commun. 2009, 30, 1697-1708.