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A full elucidation of chemisorbed donor behavior is of fundamental importance for tailoring the active species but has not yet been achieved for Ziegler–Natta catalysts. Herein, by exploiting the s...

The effect of nanosize and structural disorder on the MgCl2 support of Ziegler–Natta catalysts has been investigated in terms of induced changes to their vibrational spectroscopic fingerprint. In p...

Herein, we study comparatively melt-memory effects on recrystallization of two sets of linear low-density polyethylenes (LLDPE) synthesized with Ziegler–Natta (ZN-LLDPE) and with metallocene cataly...

In the field of Ziegler–Natta catalysis for olefin polymerization, carbon monoxide (CO) is used in the industrial practice to quench the reaction when it proceeds too fast, approaching critical levels for the plant safety. The quenching effect is explained as due to the reversible coordination of CO to the titanium active sites, but no direct evidence has been ever reported. In this work, we designed a series of experiments to monitor CO adsorption at variable temperatures on a model Ziegler–Natta catalyst by means of FT-IR spectroscopy. For the first time, we have been able to spectroscopically detect CO coordinated to alkylated Ti3+ sites and the Ti–acyl species formed upon the subsequent insertion of CO into the Ti3+–alkyl bond, both in the absence and in the presence of the olefin monomer. In perspective, this has important implications for the characterization of the active sites in industrial Ziegler–Natta catalysts, even under working conditions.

1,9-decadiene/propylene copolymerization is assessed as a way for Ziegler-Natta catalysts to access long chain-branched polypropylene. A MgCl2/9,9-bis-(methoxymethyl)fluorine/TiCl4 catalyst with tr...

This study reported novel eco-friendly Ziegler-Natta catalysts containing bio-derived maleic rosinate tri-n-butyl and maleic rosinate tri-n-octyl as internal electron donors for propylene polymeriz...

Two series of polypropylene samples (PP) were prepared with two MgCl2/TiCl4-type catalysts with diisobutyl phthalate and 9,9-bis(methoxymethyl)fluorine as internal donors (named as cat1 and cat2), ...

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.

Chain entanglement was very important for adjusting the processability and mechanical property of nascent ultrahigh molecular weight polyethylene (UHMWPE). So far, it is still a mystery to unravel ...

A new multidentate carbonate (3,3,3′,3′-tetramethyl-2,2′,3,3′-tetrahydro-1,1′-spirobiindane-5,5′,6,6′-tetracarbonate) is studied as a potential internal electron donor for Ziegler–Natta (Z–N) catal...

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.

Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1-butene with 11-bromo-1-undecene using Ziegler–Natta polymerization. The resulting bromoalkyl-fun...

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of “active sites” of ZN catalysts in the absence of ethylene, referred to as “dormant active sites”, is elucidated from magnetic resonance experiments, carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using EPR and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by DFT calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl2-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene, and enables us propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations

A “flexible cluster” model approach to Ziegler− Natta catalysts for the production of isotactic polypropylene, allowing the use of realistically sized MgCl2 monolayer clusters (up to 38 MgCl2 units) without any constraints, was employed to investigate the formation of adducts between the MgCl2 support and three industrially relevant internal donor classes, namely phthalates, succinates, and 1,3-dimethoxypropanes. The calculated adsorption modes and thermochemical data for adducts of single-donor molecules confirmed earlier literature trends only in part. Results for adducts with multiple donor molecules, in turn, did not confirm the indications of periodic models about steric repulsion between neighboring adsorbates hampering high degrees of surface coverage; as a matter of fact, such repulsions seem to be largely traceable to unnecessary constraints inherent in periodic calculations.

In this work, we have addressed the question of the location of electron donors (we chose ethylbenzoate, EB, for practical reasons) on the surface of MgCl2-supported Ziegler–Natta catalysts by syne...

Density functional theory (DFT) has been used for the study of ethylene polymerization in the Ziegler–Natta (ZN) olefin polymerization system for eight different alkoxy group containing titanium catalysts (Cat-A–H), Ti(III)Et(OR)(OR′) (where R = −CH3, – Et, −tert-butyl, −cyclohexane, R′ = −CH3, −Et, −tert-butyl, −cyclohexane). What is of significance is that the catalysts studied were all considered to be tethered to the (104) MgCl2 surface, which has traditionally been considered a “dormant” surface in ZN catalysis systems, in contrast to the “more active” (110) MgCl2 surface. Our calculations indicate that the binding of all the catalysts to the (104) surface is favorable, even after taking entropic effects into account. For purposes of comparison, ethylene polymerization has been investigated for the Cat-C (TiEt(OEt)2) and the Cat-H (TiEt(Cl)(OC4H8Cl)) (OC4H8Cl = the chlorobutoxy group) cases, for both the (i) (110) and the (ii) (104) MgCl2 surfaces. It has been seen that for both (i) and (ii) the ener...

Polyethylenimines, polymers bearing amino functionalities, are studied for the first time as internal electron donors for Ziegler–Natta catalysts. An advantage of polyethylenimines (PEIs) compared to the conventional phthalate electron donors is their relative harmlessness. Interaction of PEI with MgCl2 support was studied using computational (DFT; M06-2X) and experimental (PXRD, DRIFT, CP/MAS 13C NMR) methods. Quantum chemical calculations suggest that the structural variations in PEIs significantly affect their ability to stabilize the catalytically relevant MgCl2 surfaces. Coordination on the (104) surface seems to be favored upon consideration of the layered structure of MgCl2. The surface stabilization energies of branched PEIs are of the same magnitude with a phthalate electron donor reference. Experimental results indicate, in agreement with theoretical results, a strong coordination ability of branched PEI through nitrogen atoms to MgCl2. Based on spectroscopic data, nitrogen atoms of primary, sec...

The electron paramagnetic resonance (EPR) method was used to investigate the formation of alkylated Ti(III) species in superactive titanium–magnesium catalysts with a low titanium content during their interaction with an organoaluminum activator (AlMe3), as well as the interaction of alkylated Ti(III) surface species with carbon monoxide. EPR data on the content of alkylated Ti(III) species in these catalysts agree well with the number of Ti–R bonds that are determined after the interaction of radioactive carbon monoxide (14CO) with catalyst activated by triethylaluminum in the absence of monomer. Parameters of EPR spectra of the Ti(III) species having different structure and composition on the surface of titanium–magnesium catalysts were calculated by quantum-chemical simulations. The calculated g-values are consistent with the g-values observed in EPR spectra of the catalysts. Analysis of the literature data and results of our study made it possible to propose the parameters of EPR spectra characterizin...

The structural, vibrational, and electronic properties of silica-supported Ziegler–Natta catalysts industrially relevant for polyethylene production were investigated in detail by means of a multitechnical approach at each step of catalyst preparation, including precatalyst activation. In the precatalyst, the TiClx phase is grafted mainly to the silica surface and almost independent of the supported MgClx phase. However, the subsequent activation by means of an aluminum–alkyl compound causes important changes to both the supported MgClx phase and the TiClx phase. The resulting catalyst is entirely reconstructed so that most of the titanium sites are detached from the silica surface and in interaction with a highly dispersed MgCl2 phase, thus mimicking the most famous and highly investigated Ziegler–Natta catalysts (not silica-supported). For the first time, the catalyst performances were monitored by means of in situ FT-IR spectroscopy in transmission mode, simulating industrially significant polymerizati...

Five new alkoxysilanes with different sizes of hydrocarbon substituents were first synthesized and employed as external donors for propylene polymerization with a Ziegler–Natta catalyst. The effects of these and industrial alkoxysilanes with different sizes of hydrocarbon substituents on the microstructure of polypropylene were studied by SSA and 13C NMR. The results showed that isotactic sequence length of polypropylene increased with the size of R2 substituent on alkoxysilanes in the five donor systems, similar to the regularity of molecular weight, isotacticity, and thermal properties of polypropylene. Although the polypropylene produced by double cycloalkane substituents on alkoxysilanes had lower lamellae thickness L1, its contents were the highest. Moreover, excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to improving isotactic sequence length of polypropylene. Polypropylene prepared by MIPDMS had a more uniform distribution of the stereodefects, indicating that...

In this work, we present a systematic DFT analysis of the effect of surface coverage on the coordination properties of alkoxysilanes to the (104) and (110) surfaces of MgCl2. Furthermore, we invest...

The influence of the structure of external alkoxysilane donors on stereoslectivity and molecular weight distributions in MgCl2-supported Ziegler–Natta catalysis has been examined. We shall demonstr...

In this work, we report on the structure and formation energy of uncovered MgCl2 crystallites of different shapes (hexagonal and square), sizes (up to crystallites composed of 157 MgCl2 units), and edges (crystallites presenting the (104) and (110) edges). Both uncovered crystallites and crystallites covered by dimethyl ether were considered. Our results indicate that the formation energy of uncovered crystallites, irrespective of shape, size, and edges, linearly depends on the density of vacancies (measured as the ratio between the number of Mg vacancies and the number of MgCl2 units in the crystallite) and that larger crystallites that present (104) edges are favored. In the case of crystallites completely covered by dimethyl ether, our results indicate that the formation energy of crystallites, again irrespective of shape, size, and edges, inversely depends on the dimethyl ether/Mg ratio. As opposed to uncovered crystallites, in the presence of dimethyl ether, smaller crystallites presenting (110) edge...

We present a computational study, using density functional theory, of the role of internal and external donors in supported heterogeneous Ziegler−Natta polymerization catalyst systems. The focus of the investigation is the ability of the donors to stabilize the MgCl2 support through the zip mode of coordination. Phthalates and alkoxy benzoates have been considered as representative internal and external donors, respectively. Models for the α (ccp) and the β (hcp) phases of the (110) MgCl2 lateral cut have been considered. Studies were first done with a “Fully Fixed Model”, where the atoms in the MgCl2 lateral cuts were kept fixed. The studies indicated that the phthalate donors would preferentially stabilize the α phase of MgCl2, while the alkoxy benzoate donors would stabilize the β phase more, corroborating experimental results. A comparison of the zip coordination mode with other modes of coordination (mono, chelate, and bridge) indicates that it would be preferred over the mono and chelate coordinatio...

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl2 surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti2Cl6 complexes residing on the dominant (104) MgCl2 surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl2 is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl4/MgCl2 + AlR3 system.

All types of chemical reactions (reduction, alkylation, and complexation of Ti species by organoaluminum compounds) leading to formation of active sites in heterogeneous Ziegler−Natta catalysts have been studied within density functional theory. A generally accepted mechanism of active site formation was found to be less preferable as compared to the alternative ways considered. Based on the calculation implemented, a whole active site formation scenario was postulated for the different active center precursors on the MgCl2 surface. The mechanism proposed allows us to rationalize the domination of Ti3+ over Ti2+ under reduction of Ti4+ surface species by AlR3, the absence of an electron spin resonance signal for the TiCl4/MgCl2 + AlR3 system with low Ti content, the stronger reduction ability of Al(i-Bu)3 than AlEt3, the deactivation effect of AlR2Cl, and the reactivation ability of AlR3.

Polypropylene/ethylene−propylene rubber (PP/EPR) reactor alloys were prepared with a metallocene/Ziegler−Natta hybrid catalyst system (rac-Et(Ind)2ZrCl2/TiCl4/MgCl2) using a process composed of three stages: propylene homopolymerization, metallocene activation, and ethylene−propylene copolymerization. A series of alloy samples were produced and characterized at various copolymerization conditions by varying the methylaluminoxane (MAO)/Zr ratio and monomer composition. It was shown that the metallocene/Ziegler−Natta hybrid system exhibited the features of both metallocene and Ziegler−Natta catalysts during copolymerization. The hybrid catalyst had better ability in incorporating α-olefin than the Ziegler−Natta catalyst owing to the action of metallocene active sites. DSC and IR analyses suggested that EPR in the alloys became random with increased MAO/Zr ratio. In addition, reducing the ethylene content in the feed decreased the activity and promoted the production of random copolymers. An operation window...

"Classical" MgCl2-supported Ziegler-Natta catalysts (ZNCs) continue to dominate the industrial prodn. of isotactic polypropylene. There is a growing awareness of the inherent competitive edge of these low-cost systems over single-center (primarily metallocene) catalysts and of the potential for further improvement, particularly if deeper insight into the structure of the catalytic surfaces and the mechanisms of their modification by electron donors can be achieved. In the framework of a project ultimately aiming at the implementation of ZNCs with known and controlled surface structures, we are revisiting this whole area by using a combination of advanced computational (periodic DFT) and spectroscopic (high-resoln. magic-angle-spinning 1H NMR spectroscopy) tools. In this article, we report on the neat MgCl2 matrix and on model MgCl2/electron-donor adducts. The (104) surface, with five-coordinate Mg cations, is the dominant lateral termination in well-formed large crystals, and in highly activated MgCl2 samples prepd. by ball-milling. In the latter case, a minor fraction of surface Mg sites with a higher extent of coordinative unsatn. [e.g., four-coordinate Mg cations on (110) edges and/or at crystal corners or other defective locations] also appear to be present. RMe2Si(OMe) (R=octadecyl) binds to both types of Mg sites, albeit with different strengths resulting in different mobilities. The less-electron-donating RMeSi(OMe)2, in contrast, binds to the more unsatd. Mg sites only. The approach described herein is currently being extended to MgCl2/TiCln systems, and to their adducts with internal and external donors of different natures, strengths, and steric demands.

The morphologies of 1-octene-based linear low-density polyethylenes (LLDPEs) prepared with single-site (ss) or Ziegler−Natta (ZN) catalysts were investigated using solid-state 13C NMR spectroscopy. For each type of LLDPE, two samples, containing either approximately 10 or approximately 30 hexyl branches per 1000 backbone carbons, were studied. Mass fractions of their crystalline and amorphous phases as well as the interphase were quantified; a significant amount of LLDPE exists in the interphase for both types of samples, with their relative amounts decreasing with increasing branch content. Hexyl branches are approximately evenly distributed between the two noncrystalline phases for all samples except the high branch content ZN LLDPE, whose branches tend to cluster in the amorphous phase. The latter observation is attributed to the fact that most of the branched molecules in ZN LLDPE are in the low molar mass fraction and when the branch content is high these chains cannot fold into ordered structural un...

Molecular dynamics (MD) simulations were carried out to study the solid-state structures of single-site (ss) and Ziegler−Natta (ZN) linear low-density polyethylenes (LLDPE) at a temperature slightly below their melting temperatures. The two bulk state models, used to represent the polymers, possessed the same average branch content (10 hexyl branches per 1000 backbone carbons) but with different degrees of interchain branch distribution homogeneity. Both models were first equilibrated at 463 K (i.e., 190 °C) for several nanoseconds, and the resultant structures, which were found to be representative of the corresponding liquid-state structures, were then used as the initial structures for the subsequent quenching process. The quenching temperature was 373 K (i.e., 100 °C), and the structures were then equilibrated at the same temperature for a period of about 10 ns. The structures of the two polymers formed after the low-temperature equilibrations were considerably different. In particular, the ZN-LLDPE m...

A new series of cationic zirconium amidinates, [(η5-C5Me5)ZrMe{N(Et)C(R3)N(tBu)}][B(C6F5)4] (R3 = Me, H, Ph, and tBu) (1a−d), were synthesized and their ability to function as initiators for the stereoselective living Ziegler−Natta polymerization of 1-hexene evaluated. Whereas 1a is highly active for the isospecific living polymerization of 1-hexene as previously reported, polymerizations conducted with 1b and 1c both display a significant loss of stereocontrol, and in the case of 1b, the polymerization is no longer living. Further, 1d was found to be inactive for polymerization. The Golidlocks effect observed for the distal R3 substituent in this series, i.e., 1b “too small”, 1c “too large”, 1a “just right”, appears to be steric in origin. Solution and solid state structural studies suggest, however, that two different mechanisms are most likely operative for the loss of stereocontrol observed for 1b and 1c: a low barrier to metal-centered epimerization in the case of 1b and a lack of steric discriminat...

A compared analysis of the polymorphic behavior of samples of isotactic polypropylene (iPP) prepared with heterogeneous Ziegler-Natta catalysts and with a single center homogeneous metallocene catalyst is presented. Different samples of Ziegler-Natta iPP samples, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the gamma form. The more stereoregular fractions crystallize instead basically in the alpha form. This confirms that, even in the case of Ziegler-Natta iPP samples the gamma form may develop by melt-crystallization at atmospheric pressure in fractions containing high concentration of defects. The relative amount of gamma form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the gamma form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler-Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler-Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler-Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the alpha form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the gamma form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of gamma form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler-Natta iPP are characterized by chains with a stereo-block microstructure, consisting in regular isotactic sequences linked to more irregular sequences. The latter contain the major part of stereodefects mainly consisting in isolated rr triads, r diads and longer …rrrr… syndiotatic sequences. The hypothesis of a stereo-block microstructure for some of these less stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.

The hydroalumination of the allyl ethers CH2CHCH2OR with iBu2AlH yields the (3-alkoxypropyl)diisobutylaluminum complexes iBu2Al(CH2)3OR (R = Me (1), Et (2), Bu (3)). Et2AlCl and iBu2AlCl react with...

The miscibility of metallocene linear low-density polyethylene (m-LLDPE) and Ziegler- Natta LLDPE (ZN-LLDPE) with linear high-density polyethylene (HDPE) was studied. The influences of composition distribution (CD) and branch content (BC) on the miscibility of m-LLDPE and ZN-LLDPE with HDPE were investigated with rheological methods. The m-LLDPEs (BC ) 13.2 CH3/1000 C) and ZN-LLDPE (BC ) 14.5 CH3/1000 C) of similar molecular weights were paired to study one molecular variable at a time. Melt blending was carried out in a Haake PolyDrive at 190 °C in the presence of 1000 ppm of antioxidants. Dynamic and steady shear measurements were performed in a Rheometrics ARES at 190 °C. The miscibilities of blends were revealed by the dependence of their o, ', N1 and Gon blend composition and from predictions of rheological models. The CD was found to have no effect on the miscibility of low-BC ZN-LLDPE and m-LLDPE blends with linear HDPE, and both blends were miscible at all compositions. Increasing the branch content (BC ) 42.0 CH3/1000 C) resulted in an increased immiscibility of m-LLDPE-rich blends with linear HDPE. The rheology of immiscible blends suggests a layered morphology and agreement with Bousmina-Palierne-Utracki's and Lin's models was obtained with slip parameters k ) 2.85 10 -5 and I ) 0.72, respectively.

Two and three-state statistical models, which are mathematical devices used to partition observed poly(propylene) sequence distributions, are usually evaluated on how well they reproduce observed overall pentad/heptad distributions from 13C NMR. In this study, sequence distribution curves vs crystallization rates are also employed to judge the behavior of a particular model. A supercritical fluid fractionation of a poly(propylene) prepared with a MgCl2-supported Ziegler−Natta catalyst provided a series of samples that had progressively decreasing racemic contents as the molar masses increased. A modification was made to an existing three-state statistical model that gave sequence distribution curves that satisfied the fractionation behavior as well as linear spherulitic growth rates and polymorphic behaviors observed for the series of Ziegler−Natta poly(propylene) fractions. The results indicate that MgCl2-supported Ziegler−Natta isotactic poly(propylene)s contain significant amounts of a high molar mass,...

We present a Car−Parrinello investigation of various active surfaces and catalytic sites in a realistic Ziegler−Natta heterogeneous system. We examine the (100), (110), and (104) surfaces of the MgCl2 support and the related binding of the possible mononuclear and dinuclear catalyst configurations. Relaxation and/or reconstruction processes affect these surfaces in varying degrees, according to the different Miller indexes. We find that TiCl4 and Ti2Cl6 species can bind as stable adducts, depending on the morphology of the surface considered. However, the activation and polymerization phases show that destabilization phenomena can affect the dinuclear species during the catalysis reaction. This provides a new insight into the ability of the different centers to give rise to the real polymerization process. Finally, we present a first attempt to address the role of a typical donor phthalate at a fully first principles level.

Several new, novel bridged and unbridged bis-tetrahydrofluorenyl zirconocenes were synthesized and evaluated as Ziegler−Natta type catalysts for ethylene and propylene polymerization. The complexes were found to be highly stable compared to some bis-fluorenylzirconocenes. Polypropylene and polyethylene were produced in good yield using the new catalyst precursors with MAO or Trityl/TIBA as cocatalysts. Both the unsubstituted and substituted bridged bis-tetrahydrofluorenyl zirconocenes were isolated as 50:50 mixtures of rac and meso isomers, which produced mostly atactic polypropylene. Partially crystalline polypropylene was obtained at low temperatures with bis(3-methyltetrahydrofluorenyl)zirconium dichloride.

The relative energetic stability of the “rac” and the “meso” rotational isomers of metallocene-based Ziegler−Natta (ZN) catalysts plays an important role in determining the structural, physical and...

As part of our continued investigation of the polymerization behavior of substituted asymmetric indenyl−fluorenyl metallocenes, three new zirconocenes bearing 2,4-disubstitution on the indenyl moiety have been synthesized and evaluated as α-olefin polymerization catalysts. For the new complex ethylene-1-(9-fluorenyl)-2-[1-(2-methyl-4-phenyl)indenyl]zirconium dichloride (11), highly isotactic polypropylene with [mmmm] = 91% and Mw 1.1 × 105 at Tp = 40 °C was produced in very high yield. The dimethylsilylene analogue (13) gave similar results in terms of stereoregularity with a 2-fold increase in Mw. In addition, dimethylsilylene(9-fluorenyl)[1-(2,4-dimethyl)indenyl]zirconium dichloride (12) was prepared and found to be an effective catalyst precursor, although the smaller methyl substituent at the 4-position resulted in polypropylenes of lower stereoregularity.

Energy calculations relative to the adsorption of several substituted 1,3-diethers on the unsatured (100) and (110) lateral cuts of MgCl2 are presented. Independent of the particular approach used, coordination of the diethers on the (110) cut is always preferred. The energy difference favoring the diether coordination on the (110) cut depends on the substituents on carbon 2 of the 1,3-alkoxypropane skeleton. These calculated energy differences are able to rationalize the observed dependence on the chemical structure of the 1,3-diethers of the stereoregulating ability for propene polymerization of MgCl2/TiCl4 catalytic systems. In fact, a semiquantitative relationship has been found in the assumption that xylene-soluble (essentially atactic) and -insoluble (essentially isotactic) fractions of polypropylene obtained by these catalytic systems are prevailingly produced by Ti catalytic species adsorbed on (100) and (110) cuts, respectively.

In this work we address the problem of isotacticity in a realistic heterogeneous Ziegler−Natta system by means of ab initio molecular dynamics. We focus on a previously identified 5-fold catalytic center, and we inspect its ability to select the appropriate olefin enantioface in the chain growth process. We study the first steps in the propene polymerization process and determine the energetics of the initial complexation phase for the different stereochemical orientations of the incoming propene. Then we analyze the subsequent insertions, which represent the crucial issue for the formation of a stereospecific polymer chain, and we find that the 5-fold catalytic center possesses a high degree of stereoselectivity. We examine the role of the agostic interaction, which can switch from α to β and allow, even in the presence of a substrate, processes that can lead to chain termination.

This contribution reports efficient and selective organotitanium-mediated silanolytic (PhSiH3, PhMeSiH2, Me2SiH2, Et2SiH2) chain transfer in the homogeneous polymerization and copolymerization of a variety of α-olefins. The result is several broad classes of silyl-capped and silyl-linked polyolefins which have been characterized by 1H/13C/29Si NMR and IR spectroscopy as well as by GPC. “Single-site” Ziegler−Natta catalysts effect this chain transfer with significant efficiency and scope. For [Me2Si(Me4C5)tBuN]TiMe+B(C6F5)4--mediated propylene polymerization, the mechanism of chain transfer is supported by the observation that Mn of the capped atactic polypropylenes produced at constant [catalyst], [PhSiH3], and [propylene] is inversely proportional to [PhSiH3]. Using the same catalyst, this process also efficiently produces silyl endcapped poly(1-hexene), ethylene + 1-hexene copolymers, ethylene + styrene copolymers (both with high degrees of comonomer incorporation), and (at low temperatures) polyethylen...

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 ...

During the structural evolution of MgCl2 Lewis base adducts for the industrial preparation of heterogeneous Ziegler–Natta catalysts, disordered polynuclear (MgCl2)x species are formed that explain the high degree of nanoporosity of the active δ form of MgCl2 obtained in the process.