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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 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...

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.

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.

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.

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.

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.

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.

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.

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.

Available experimental data for several metallocenes indicate that the ethene/propene copolymerization ratio rc can be much more temperature dependent than would be expected if competing insertion transition states (TS) are rate limiting. Detailed exploration of the reaction paths reveals in several cases the existence of a “capture-like” transition state before the actual insertion, with free energies close to the insertion TS. Movement around these transition states does not just involve monomer and chain, but also clear distortion of the ligand skeleton to allow entry of the monomer. Taking these additional TSs into account leads to much improved agreement with experiment for a series of metallocenes and a constrained geometry catalyst system. Depending on catalyst and temperature, selectivity is determined by competing insertion/insertion, capture/insertion or capture/capture. It seems likely that this is a common situation especially for highly efficient catalysts, complicating (but not preventing) prediction of copolymerization performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

High-throughput-screening (HTS) tools and methods are used more and more, especially in industry, in the search for new, selective organometallic catalysts. In most cases, the approach is, in essence, empirical, and the strategy is to increase the number of experiments that can be run at a given place in a given time. Highly miniaturized, parallel reaction setups have been implemented for the rapid assessment of whether novel catalysts resulting from the structural amplification of a basic framework are "good" or "bad" with respect to the properties of interest, and, depending on the response, worthy of a subsequent, more-careful evaluation. In this article, we demonstrate that it is possible to utilize these state-of-the-art HTS platforms with a different strategy: the rapid generation of reliable kinetic data for mechanistic studies in view of a thorough understanding and rational catalyst design. Ziegler-Natta-type catalytic olefin polymerization will be used throughout as an example.

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.

A study was conducted to improve the behavior of bis(phenoxyamine) group 4 metal catalysts for controlled alkene polymerization. The performance of bis(phenoxyamine)Hf homologues was investigated, assuming that a higher chain propagation to transfer ratio was associated with a longer duration of the controlled regime. The bis(phenoxyamine)Hf-dibenzyl complexes Hf-1 and Hf-2 were prepared similarly to the Zr-1 and Zr-2 Hf homologues. The complexes were activated with methylalumoxane (MAO) in combination with 2,6-di-tert-bytylphenol (TBP) to scavenge free trimethylaluminum and inhibit chain transfer through trans-alkylation. The complexes were also tested comparatively with Zr-1 and Zr-2 in ethene and propene polymerization at 25 °C in toluene solution.

Suitably activated, (Cp*){N( t Bu)C(Me)N(Et)}ZrMe2 is known to initiate the 'living' and isotactic-selective polymerization of alk-1-enes, and it can be used to synthesize block copolymers and stereoblock polymers. We report a full molecular kinetic investigation of propene, but-1-ene, and hex-1-ene polymerization with a MAO-activated catalyst system. By combining NMR microstructural polymer analysis with QM modeling of the active species, the complicated regio- and stereochemistry of the polyinsertion process, as well as the active chain-transfer pathways, are investigated. The perspectives and limitations of this catalyst for application in (stereo)block polymerizations are discussed.

In this paper, we report the full resonance assignments in the olefinic region of the 600 MHz 1H NMR spectra of a typical ethene/1-octene copolymer (4.0 mol % of octene units) produced with a metallocene catalyst at high temperature, and of an ethene homopolymer prepared under similar conditions for comparison. The assignments were based on a thorough analysis of 1D (1H, 1H{1H}, 13C) spectra and of 2D 1H−1H (COSY, TOCSY) and 1H−13C (HSQC, DEPT-HSQC) maps. Thirteen different olefinic structures were identified, and their formation explained in terms of plausible mechanistic paths associated with the high polymerization temperature. The fraction of internal chain unsaturations traceable to allylic activation turned out to be comparable to that of terminal ones in both samples. Despite its complexity, the said spectral region is now amenable to deconvolution and full simulation in terms of known and recognizable patterns, and usable for fast (possibly on-line) measurements of chain unsaturation degree, as we...

Based on the recent literature, the synthesis of novel polyolefin architectures via Ziegler-Natta chemistry under living conditions appears to be a promising and fast-growing area. In this paper, the appropriateness of the term living as applied to the said context is discussed in mechanistic terms, and the potential and the limitations of the field are outlined with the aid of two paradigmatic case histories (i.e., propene polymerization in the presence of bis(phenoxy-imine)Ti and bis(phenoxy-amine)Zr catalysts).

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.

The propene/ethene-[1-C-13] Copolymerization method has been applied to measure precisely the regioselectivity in propene polymerization of the "simple" MgCl2/TiCl4 catalyst (activated with Al((i)-Bu)(3)) which can be viewed as the ancestor of all present-day MgCl2-supported industrial catalysts containing Lewis bases as selective modifiers. It has been found that the average content of 2,1-regiomistakes in the polypropylene produced is fairly high (ca. 0.7 mol %), although-not unexpectedly, due to the multisite catalyst nature-these are distributed rather nonuniformly throughout the polymer. The difficulty in detecting the 2,1-units, despite their relatively high concentration, in the C-13 NMR spectra of homopolymer samples at natural C-13 abundance has been traced to their occurrence in a variety of stereochemical environments, which leads to splitting and broadening of the corresponding resonances. The fraction of "dormant" polypropylene chains with a 2,1-last-inserted unit was estimated to be in the 20-50% range. QM calculations of regioselectivity on classical Corradini-type models of epitactic octahedral catalytic Ti species turned out to be in basic agreement with the experiment.

The field of stereoselective propene polymerization has been dramatically innovated by the discovery of homogeneous metallocene-based catalysts with well-defined and tunable molecular structure. Of all, "oscillating" metallocenes are probably the most ingenious and challenging example of catalyst design. Their catalytic species were built to "flip-flop" between a chiral and an achiral conformation, at a rate intermediate between those of monomer insertion and chain transfer. The result of this molecular switching would be a polypropylene with an isotactic/atactic stereoblock structure, performing as a thermoplastic elastomer. This essay discusses how the real polymerization mechanism differs from what the catalyst inventors had in mind, but also how - through fortunate circumstances their optimism has been rewarded.

The microstructure of a number of ethene/propene copolymers prepared with three representative C2-symmetric ansa-metallocene catalysts was investigated by means of high-field (150 MHz) 13C NMR. Thanks to the exceedingly high resolution of the spectra, an unprecedented experimental basis for the constitutional sequence analysis was achieved, which enabled us to test complex statistical models of chain propagation in a significant manner. In particular, the need for a 2nd-order Markov model, previously suggested by some authors on the basis of triad data, was confirmed by our study at pentad/heptad level. Some correlations between metallocene structure and copolymerization statistics were also identified. Expansion of the methyl region of the 13C NMR spectrum of an E/P copolymer obtained at 30 °C with catalyst system I/MAO (150 MHz).

Chemo-, regio- and enantioselectivities in the (co-)polymerization of propene and ethene have been measured (in some cases for the first time) for a number of prototypical metallocene catalysts (in combination with methylalumoxane), and compared with those calculated at the DFT level for gas-phase cationic systems. In this paper, the first part of the study, we discuss the achiral catalysts Cp2TiCl2, Cp2ZrCl2, and Me2SiCp2ZrCl2. All three catalysts were confirmed to be highly regioselective for propene, in favor of 1,2-insertion. The performance of the titanocene in this respect is truly remarkable (only two misinsertions in 10 000 at -15 degreesC), whereas that of the two zirconocenes turned out to be worse than generally assumed (two to three misinsertions in 1000), and-quite unexpectedly-nearly identical (both in experiment and calculations), despite the decidedly more open structure of the Si-bridged system. Understandably, in all cases ethene was found to insert faster than propene, but the difference in relative reactivity of the two monomers is particularly dramatic when the growing chain is secondary, which confirms the "dormant" character of the latter for propene. The fair correlation between observed and calculated selectivities suggests that, at least for the systems considered here, solvent and counterion effects, though undoubtedly important, are rather indiscriminate and that olefin insertion is indeed the rate-determining step.

A microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C2-symmetric and Cs-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M], because so is the rate of polyinsertion, 〈vp〉. Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of 〈vp〉 can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reprod...

Results of propene polymerization in the presence of two known octahedral C2-symmetric Zr complexes bearing tetradentate [ONNO]-type ligands are reported for the first time. Depending on the steric hindrance at the active metal, isotactic site-controlled or weakly syndiotactic chain-end-controlled polymers were obtained, in both cases via highly regioselective 1,2 (primary) monomer insertion. In this respect, the complexes mimic the behavior of the active Ti species on the surface of the heterogeneous Ziegler-Natta catalysts of which they might represent good structural models.

PROPENE POLYMERIZATION

A study has focused on finding intrinsic properties of Hafnocenes and methylalumoxane (MAO) based catalysts. The researchers have evaluated more carefully the behavior of HF-based homologues 3 and 4 with yield polymers with much higher average molecular masses rate, in propene polymerization with different activators. Analysts have used MAO or [Ph3C] [B(C 6F5)4]/Al(iso-butyl)3 (TrB/TIBA). TIBA was considered for its role as a scavenger and also as a convenient alkylating agent of metallocene dichlorides. The observation presented in the study highlighted a hidden layer of complexity underneath the seemingly straightforward picture of propene polymerization mediated by group 4 metallocene catalysts. Proposed structure demonstrated that catalyst behavior is strongly idiosyntcratic and that changing the transition metal from Zr(IV) to Hf(IV) in an otherwise identical ligand framework can change dramatically.

The stereosequence distribution of the “atactic” and “isotactic” fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution 13C NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of “routine” 13C NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic (“isotactoid”) and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the ...

The kinetics of ethene and propene polymerization at 20–60°C in the presence of the homogeneous catalyst system rac-Me2Si(2-methyl-4-phenyl-1-indenyl)2ZrCl2/methylaluminoxane was investigated by means of stopped-flow techniques. The specific rate of chain propagation, measured at the very short reaction times typical of this method, turned out to be ≈102 times higher for ethene than for propene; this suggests that diffusion limitations through the poly(ethylene) precipitating at longer reaction times may be responsible for the fact that the two monomers polymerize instead at comparable rates under “standard” conditions. It was also found that the concentration of active sites is significantly lower than the analytical Zr concentration.

In this paper, we report the results of a 150 MHz 13C NMR characterization of ethene-1-13C/propene copolymers at low (

The distribution of configurations in (predominantly) isotactic polypropylene samples obtained with heterogeneous Ziegler-Natta catalyst systems, investigated by means of high-field 13C NMR, reveals deviations from that predicted by the simple "enantiomorphic-site" model commonly used to describe it. Such deviations suggest that the "active socket" in these systems is similar to that of the new homogeneous isotactic-selective catalysts based on C1-sym. ansa-metallocenes of the 4th column. This provides, inter alia, a common explanation for the increase of stereoselectivity with decreasing monomer concn. obsd. for both catalyst classes.

1-Butene polyinsertion promoted by two typical C 2 -symmetric zirconocene catalysts (i.e. rac-ethylenebis(1-indenyl)ZrCl 2 (1) and rac-dimethylsilylbis(1-indenyl)ZrCl 2 ) (2) was investigated under both hydrooligomerization and polymerization conditions and compared with that of propene. It was found that, in spite of a similar regiospecificity for the two monomers, the fraction of dormant sites is higher in 1-butene polymerization, as a result of a (much) lower reactivity of a 2,1 last-inserted unit. Accordingly, in the investigated cases, 2,1 units of 1-butene were not incorporated in the growing chains but either isomerized to 4,1 units or remained at the chain end until they underwent chain transfer via β-hydrogen elimination. Experimental evidence is also presented of an epimerization reaction of the growing poly(1-butene) chain with a 1,2 last-inserted unit, leading to a decrease of polymer stereoregularity with decreasing monomer concentration.

In this presentation, some mechanistic aspects of isotactic propene polymerization in the presence of homogeneous Group IV metallocene catalysts are analyzed in the light of new (and still largely unpublished) experimental results. In particular, the regiospecificity of catalyst systems based on two typical C2-symmetric ansazirconocenes (i.e. rac-ethylenebis(1-indenyl) ZrCl2 and rac-dimethylsilylbis(1-indenyl) ZrCl2; co-catalyst, methylalumoxane) is discussed comparatively. An influence of propene concentration on the ratio between 2,1 and 1,3 monomer enchainments and surprisingly - on the stereoregularity of the produced polymers is pointed out. Tentative kinetic explanations of the observed polymerization behaviours are also proposed.

The productivity of a number of bis(phenoxyamine)Zr(IV) -based catalysts (bis(phenoxyamine) = N,N-biS (3-R-1-5-R-2-2-O-C6H2-CH2)-N-N'(R-3)(2)-(NCH2CH2N)) in ethene and propene polymerization was evaluated for different R-1/R-2/R-3 combinations. In previous studies on this class we demonstrated that the cations that form upon precatalyst activation (e.g., by methylalumoxane) adopt a "dormant" mer-mer geometry, and an endothermic isomerization to the active fac-fac geometry is the necessary first step of the catalytic cycle. Herewith we report a clear correlation between catalyst activity and the DFT-calculated energy difference Delta E-i between the active and dormant state. The correlation only holds when the calculations are run on ion pairs, which is less obvious than it may appear because the anion in these systems is not at the catalyst front. This finding provides a comparatively simple and fast method to predict the activity of new catalysts of the same class.

The synthesis of olefin block copolymers and stereoblock polymers in the presence of Ziegler−Natta (and related) catalysts is a long-standing important industrial target, in view of the peculiar physical properties anticipated for the resulting materials (e.g., thermoplastic elastomers). The results reported in this Communication, demonstrate the wide scope of chain shuttling in catalytic olefin polymerization but also a fairly narrow operation window. In particular, for a given catalyst/CSA combination, the precondition that the alkyl-bridged, hetero-bimetallic intermediates assumed to be involved are labile seems to be exquisitely dependent (inter alia) on the chemical identity of the monomer (and therefore of the growing polymeryl), of the catalyst activator, andlast but not leastof the reaction solvent.

The block copolymerization polypropylene under highly isotactic stereocontrol, using a new catalyst obtained by means of rational ligand design, was discussed. Moderately isotactic polypropylenes of very low average molecular mass in reasonable yields was obtained. NMR analysis of samples prepared at different propene concentrations revealed a 1,2 insertion regiochemistry and pointed to trans-alkylation by the Al-alkyl and β-H transfer to the monomer as the dominating chain transfer pathways. The results are in agreement with quantum mechanics/molecular mechanics (QM/MM) calculations.

Fundamental implications of occasional secondary (2,1-) insertions in propene polymerization with different MgCl2-supported Ziegler-Natta catalysts are discussed, with reference to the properties of the polymers obtained. The relatively narrow molecular weight distribution of polypropylene prepared using catalysts of type MgCl2/TiCl4/diether-AlR3 is ascribed to the fact that the active species in this system are relatively uniform, in the sense that significant 2,1-insertion takes place at both highly isospecific and weakly isospecific active species. In contrast, the isospecific species in the catalyst system MgCl2/TiCl4/diisobutyl phthalate-AlEt3-alkoxysilane undergo less 2,1-insertion and are therefore less responsive to chain transfer with hydrogen. The presence of (some) highly regiospecific active sites in such catalysts will therefore lead to the formation of a high molecular weight polymer fraction and, overall, a broad molecular weight distribution. The presence of high molecular weight chains leads to relatively slow molecular relaxation and a more rapid onset of crystallization of the polymer from the melt, as evidenced by rheological studies of polypropylenes prepared using different catalysts and having different molecular weight distributions.

The propene/ethene-[1-13C] copolymerization method has been applied to measure precisely the regioselectivity of two modern MgCl2-supported Ziegler-Natta catalyst systems for the industrial production of isotactic polypropylene, namely MgCl2/di(isobutyl)-o-phthalate/TiCl4-cyclohexylmethyldimethoxysilane/AlR3 (1) and MgCl2/2,2-di(isobutyl)-1,3-dimethoxypropane/TiCl4-AlR3 (2). System 2 turned out to be slightly less regioselective than system 1 (on average, 0.26% 2,1 misinsertions instead of 0.18%), but the distribution of the regiodefects throughout the polymer was found to be more uniform, most likely as a result of a lower differentiation of the active species. This provides a simple explanation for the different response of the two catalysts to molecular hydrogen as a chain transfer agent. In system 1, indeed, part of the active species are almost completely regioselective, which means that H2 finds very few preferential "cutting points" in the form of growing chains with a sterically hindered 2,1 last-inserted unit; therefore, a higher H2 partial pressure is neededcompared with system 2in order to achieve the same decrease of average polymer molecular mass.

The synthesis of PE-block-iPP with a Zr-based postmetallocene catalyst was carried out. 13C NMR provided direct and complete evidence of a true block nature for a polyolefin-based material containing a polypropylene block of any tacticity.

In a previous letter, we proposed the synthesis and C-13 NMR characterization of propene/ethene-[1-C-13] copolymers at low ethene-[1-C-13] content as a convenient tool for measuring the regioselectivity of coordinated propene polyinsertion, in case 13C NMR fails to detect the regiomistakes in propene homopolymers. In the present article, we introduce the foundations of the approach in more detail, and illustrate how its scope can be extended to the evaluation of catalyst "dormancy" due to regiochemical monomer inversion. The method was calibrated for two well-known C-2-symmetric ansa-metallocene catalysts (namely, rac-Me2Si(1-indenyl)(2)ZrCl2 and rac-Me2Si(2-Me-4-phenyl-1-indenyl)(2)ZrCl2), by means of a cross-check with results of a C-13 NMR characterization of samples of poly(propene-[3-C-13]) prepared under corresponding experimental conditions. In particular, it was proved that the fraction of consecutive 2,1 insertions in propene homopolymerization promoted by the quoted catalysts, although nonzero, is low enough not to invalidate the propene/ethene-[1-C-13] copolymerization approach.

For over thirty years after the discovery of stereoselective olefin polymerization in 1954, the word ‘polypropylene’ has meant, both scientifically and commercially, little else than the predominantly isotactic polymer achievable with the classical heterogeneous Ziegler–Natta catalysts. The picture, however, has changed dramatically in the last 15 years, due to the development of a variety of new transition metal catalysts with tunable structures and selectivities. Nowadays, ‘polypropylene’ means a whole world of innovative polymers with novel and well-controlled microstructures, and — as a result — tailored properties and applications. With the powerful aid of high-field 13 C NMR data, this review aims at introducing the reader to this fascinating and challenging frontier of macromolecular stereochemistry.

The potentialities of this approach are illustrated for propene polymn. promoted by two well-known ansa-metallocene catalysts (cocatalyst: methylaluminoxane, MAO): the isotactic-specific rac-Me2Si(1-indenyl)2ZrCl2 (I), and the syndiotactic-specific (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (II). A series of copolymn. expts. at variable ethene-1-13C-propene feeding ratios in the presence of I/MAO or II/MAO was carried out to locate the value of ethene incorporation above which "all" 2,1-propene units are followed by an ethene one. The higher regioselectivity and regioirregular sequences of the copolymers were clearly detected by 13C NMR. The 13C NMR spectra of an isotactic polypropylene prepd. with I/MAO at 10°, a copolymer of propene contg. 2.9 mol% of ethene-1-13C prepd. with I/MAO, and an ethene-1-13C-propene copolymer (ethene content 2.5 mol%) prepd. with II/MAO, were presented.

3-Methyl-1-butene was polymerized in the presence of a number of homogeneous metallocene catalysts (co-catalyst methylalumoxane). Contrary to literature reports, it was found that even the simplest C2-symmetric metallocenes promote the isotactic polymerization of this monomer with reasonable productivities. Quite surprisingly, a prevailingly isotactic polymer was also obtained in the presence of Cs-symmetric metallocenes, which are instead syndiotactic-specific in propene polymerization.

The stereosequence distribution in the heptane-insoluble fraction of polypropene samples prepared with a number of MgCl 2 -supported catalysts was analyzed in detail by means of 13 C NMR. The results indicate that what is conventionally taken as «isotactic» polypropene is normally a stereoblock polymer, in which at least part of the macromolecules contain long isotactic blocks spaced by much shorter «syndiotactoid» sequences. This implies that the active sites responsible for the formation of the two types of stereosequences are able to interconvert reversibly on the surface of the MgCl 2 support in times which may be shorter than the average growth times of the polymer molecules. Possible models of active sites apt to explain the observed polymerization behavior are also briefly discussed