Silolene-Bridged zirconocenium polymerization catalysts

A new silolene-bridged compound, racemic (1,4-butanediyl) silylene-bis (1-η⁵-in-denyl) dichlorozirconium ( 1 ) was synthesized by reacting ZrCl₄ with C₄H₈Si (IndLi)₂ in THF. 1 was reacted with trialkylaluminum and then with triphenylcarbenium tetrakis (penta-fluorophenyl) borate ( 2 ) to produce in situ the zirconocenium ion ( 1 ⁺). This “constraint geometry” catalyst is exceedingly stereoselective for propylene polymerization at low temperature (T_p = ?55°C), producing refluxing n-heptane insoluble isotactic poly(propylene) (i-PP) with a yield of 99.4%, T_m = 164.3°C, δH_f = 20.22 cal/g and M?_w = 350 000. It has catalytic activities of 10⁷?10⁸ g PP/(mol Zr · [C₃H₆] · h) in propylene polymerization at the T_p ranging from ?55°C to 70°C, and 10⁸ polymer/(mol Zr · [monomer] · h) in ethylene polymerization. The stereospecificity of 1 ⁺ decreases gradually as T_p approaches 20°C. At higher temperatures the catalytic species rapidly loses stereochemical control. Under all experimental conditions 1 ⁺ is more stereospecific than the analogous cation derived from rac-dimethylsilylenebis (1-η⁵-indenyl)dichlorozirconium ( 4 ). The variations of polymerization activities in ethylene and in propylene for T_p from ?55°C to +70°C indicates a Michaelis Mention kinetics. The zirconocenium-propylene π-complex has a larger insertion rate constant but lower thermal stability than the corresponding ethylene π-complex. This catalyst copolymerizes ethylene and propylene with reactivity ratios of comparable magnitude r_E ? 4r_p. Furthermore, r_E.r_p ? 0.5 indicating random copolymer formation. Both 1 and 4 activated with methylaluminoxane (MAO) exhibit much slower polymerization rates, and, under certain conditions, a lower stereo-selectivity than the corresponding 1 ⁺ or 4 ⁺ system. © 1994 John Wiley & Sons, Inc.     

Isoprene polymerization on titanium-magnesium catalysts

The high activity of a titanium-magnesium catalyst in the polymerization of isoprene with formation of a unique thermoplastic material, synthetic gutta percha, was shown. It is established that a change in polymerization conditions over a wide range has no effect on the content of trans-1,4 units in the polymer. Unlike natural gutta percha with the crystalline phase containing a mixture of α-and β-crystalline modifications, the synthetic trans-1,4-polyisoprene crystallizes only in an α-monoclinic form, the melting temperature of which is close to 70°C. The melting followed by crystallization results in formation of a stable β-crystalline modification with a melting temperature approximating 50°C.     

Ring opening metathesis polymerization catalysts

Over the past eight years, early transition metal catalysts for the ring opening metathesis polymerization of cyclic olefins have been developed. These catalysts are simple organometallic complexes containing metal carbon multiple bonds that in most cases polymerize olefins by a living process. These catalysts have been used to prepare a family of near monodispersed and structurally homogeneous polymers. A series of group VIII ROMP catalysts that allow a wide range of functionality to be incorporated into the polymer side chains have recently been prepared. This most important members of this family of complexes are the bisphosphinedihaloruthenium carbene complexes. These catalysts show excellent functional group stability and can be used to prepare well defined telechelic polymers, polyolefins with ordered functionality, and highly functionalized block copolymers.     

Phospholyl catalysts for olefin polymerization

Seventeen different phospholyl ligands were incorporated in a total of 22 zirconium complexes, (Phos)₂ZrCl₂, (Phos)(C₅H₅)ZrCl₂, investigated in propylene polymerization catalysis using methylaluminoxane as cocatalyst. Atactic polypropylene with M_n varying from 450 to >20 000 and vinylidene end groups (CH₂C(Me)R) was obtained with activities up to 170 kg/g Zr·h. For the 11 diphospholyls of structure (2,5-R₂C₄H₂P)₂ZrCl₂, catalytic activity was highest with substituents of moderate bulk adjacent to phosphorus (e.g., c-Pr, Ph), whereas complexes with two small (H) or two large (CMe₃, SiMe₃) ligand substituents were inactive. It is hypothesized that optimum activity with MAO requires selective blocking of phosphorus lone pair coordination to aluminum, whilst allowing free propylene approach to the active site. The degree of polymerization increased steadily in the series of 2,5-disubstituted phospholyl complexes, dialkyl<alkyl-phenyl<diphenyl, suggesting that electronic factors are more important than steric factors in determining M_n.     

Chromocene catalysts for ethylene polymerization: Scope of the polymerization

Chromocene deposited on silica supports of high surface area forms a highly active catalyst for polymerization of ethylene. Polymerization is believed to occur by a coordinated anionic mechanism previously outlined. The catalyst formation step liberates cyclopentadiene and leads to a new divalent chromium species containing a cyclopentadienyl ligand. The catalyst has a very high chain-transfer response to hydrogen which permits facile preparation of a full range of molecular weights. Catalyst activity increases with an increase in silica dehydration temperature, chromium content on silica, and ethylene reaction pressure. The temperature-activity profile is characterized by a maximum near 60°C, presumably caused by a deactivation mechanism involving silica hydroxyl groups. A value of 72 was estimated for the ethylene–propylene reactivity ratio (r₁). Linear, highly saturated polymers are normally prepared below 100°C. By contrast with other commercial polyethylenes, the chromocene catalyst produces polyethylenes of relatively narrow molecular weight distribution. Above 100°C, unsaturated, branched polymers or oligomers are formed by a simultaneous polymerization–isomerization process.     

Redox control within single-site polymerization catalysts

The activity of a single-site titanium-based lactide polymerization initiator supported by a ferrocenyl-derivatized salen ligand is shown to be modulated by a chemical redox switch; a substantially higher rate of propagation is found for the neutral catalyst compared to its oxidized dicationic ferrocenium counterpart.     

Heterogeneous metallocene catalysts for ethylene polymerization

The catalyst precursor 9-fluorenylidene-1-cyclopentadienylidene-2-hex-5-enylidene zirconium dichloride proved to be highly active in the heterogeneously catalyzed polymerization of ethylene using silica gel/partially hydrolyzed trimethylaluminum (PHT) as cocatalyst. The substitution of position 4 of the fluorenylidene fragment and position 3 of the cyclopentadienylidene ring improves the catalyst activity. The introduction of a phenyl group into the bridge increases the catalyst activity and the molecular weight of the polymer. The prepolymerization of this catalyst system leads to a major change in catalyst and polymer properties. A significant increase in catalyst activity and a lower molecular weight of the produced polyethylene is observed. The presence of hydrogen during prepolymerization or polymerization of ethylene produces a broader molecular weight distribution indicating a higher number of different active centers.     

Recent progress on olefin polymerization catalysts

Recent progress on metallocene catalysts is reviewed. This consists for the main part of research activities in metallocene catalysts and their polymerization performances (ethylene polymerization, propylene polymerization, styrene polymerization). In addi-tion, the computational design of metallocene catalysts and the commercial status of metallocene technologies in Japan are described.     

Supported titanium-magnesium catalysts for propylene polymerization

The results of studies of the synthesis and properties of supported titanium-magnesium catalysts for propylene polymerization performed at the Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, are considered. The composition of the catalysts is TiCl₄/D₁/MgCl₂-AlEt₃/D₂, where D₁ and D₂ are stereoregulating donors. With the use of the procedure proposed for the synthesis of titanium-magnesium catalysts, the morphology of catalyst particles depends on the stage of the preparation of a Mg-containing support. The titanium-magnesium catalysts developed afforded polypropylene (PP) in a high yield; this PP was characterized by high isotacticity and excellent morphology. The controllable fragmentation of the catalyst by the polymer is of crucial importance for the retention of the morphology of titanium-magnesium catalyst particles in PP. The fragmentation of catalyst particles to microparticles occurred in the formation of more than 100 g of PP per gram of the catalyst. The surface complexes were studied by DRIFT and MAS NMR spectroscopy and chemical analysis. It was shown that the role of internal donors is to regulate the distribution of TiCl₄ on different MgCl₂ faces and, thereby, to influence the properties of PP. It was found that chlorine-containing complexes of aluminum compounds were formed on the catalyst surface by the interaction of the catalyst with AlEt₃; these complexes can block the major portion of titanium chloride. Data on the number of active sites and the rate constants of polymer chain propagation (k _p) at various sites indicate that donor D₁ increases the stereospecificity of the catalyst because of an increase in the fraction of highly stereospecific active sites, at which k _p is much higher than that at low-stereospecificity active sites. Donor D₂ enhances the role of D₁. Similar values of k _p at sites with the same stereospecificity in titanium-magnesium catalysts and TiCl₃ suggest that the role of the support mainly consists in an increase in the dispersity of titanium chloride.     

Living polymerization with well-defined alkylidene catalysts

Alkylidene complexes of the type Mo(CH-t-Bu)(NAr)(OR)₂ (Ar = 2, 6-diisopropylphenyl; OR = O-t-Bu, OCMe₂(CF₃), OCMe(CF₃)₂, etc.) can be prepared from a “universal precursor”, Mo(CH-t-Bu)(NAr)(triflate)₂(1, 2-dimethoxyethane),¹ which can be prepared in three high-yield steps from ammonium dimolybdate.² These complexes serve as initiators for the ring opening metathesis of norbornenes and substituted norbornadienes,³ or for the polymerization of acetylenes to give polyenes,⁴ all in a living manner. The organic polymer can be cleaved upon treatment with an aldehyde. Norbornenes that contain a variety of organic or inorganic functionalities can be polymerized to give essentially monodisperse homopolymers and block copolymers. Recently chiral catalysts have been prepared that will polymerize norbornenes and norbornadienes stereoselectively to give all cis isotactic polymers.⁵ Molybdenum catalysts also have been found that will cyclopolymerize dipropargyl derivatives or substituted phenylacetylenes in a living manner.⁶ The stereospecific synthesis of all cis, or all trans, tactic polynorbornadienes and polynorbornenes will be discussed in detail, including a proof of tacticity.⁷     

Stereospecific post-metallocene polymerization catalysts: the example of isospecific styrene polymerization

In the context of developing single-site stereoselective post-metallocene catalysts, the case for isospecific styrene polymerization catalysts based on methylaluminoxane-activated group 4 metal bis(phenolato) complexes is summarized. Ligands derived from the 1,4-dithiabutanediyl-linked bis(phenol)s have been found to induce stereochemical rigidity by the presence of the hemi-labile sulfide donor functions. Isospecific styrene polymerization was achieved using easily accessible catalyst precursors of the type [MX₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] (M = Ti, Zr, Hf; X = Cl, OⁱPr, CH₂Ph). Activating the dibenzyl titanium complex [Ti(CH₂Ph)₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] with B(C₆F₅)₃ and AlⁱBu₃, controlled isotactic polymerization became possible at lower temperatures. A remarkable dependence of both the activity and stereoselectivity on the ligand substitution pattern was observed. Analogous precursors with the 1,5-dithiapentanediyl-linked bis(phenolato) ligand gave syndiotactic polystyrene with lower activity.     

Comparative ethylene polymerization via imino-quinolinol catalysts

A series of zirconium catalysts based on tridentate 8-hydroxyquinoline Schiff base ligands were prepared and successfully used for polymerization of ethylene. The highest activities of the prepared catalysts were obtained at polymerization temperatures about 30–45ºC. By increasing the [Al]/[Zr] molar ratio productivity of all the catalysts enhanced to an maximum value then decrease at higher [Al]/[Zr] molar ratio with the exception of catalyst 4, which showed no optimum activity in the range studied. Also, the activities and selectivities to produce low-carbon olefins were profoundly influenced by the catalysts structure indicating the dramatic effects of the substitution on the polymerizations behavior. Fouling of the reactor was strongly related to polymerization parameters like as monomer pressure and [Al]/[Zr] ratio in the homogeneous polymerization. Heterogeneous polymerization of ethylene using the catalysts and the MAO modified silica decreased the fouling. The obtained polyethylenes have a melting point of about 125–130°C, crystallinities of about 45–55% and PDI of 2.45–3.45.     

Fracturing silica-based catalysts during ethylene polymerization

The rate and extent to which silica-based catalysts fracture during the polymerization of ethylene in a slurry process were examined. Reactions were stopped at various times to yield as little as 20 g of polymer per gram of catalyst to more than 20,000 g/g. The catalyst residue was then collected and analyzed. Fragmentation of the catalyst was complete within the first minute or two of polymerization, whereas the rate of reaction continued to increase for more than an hour. Therefore fragmentation cannot be the rate-controlling step. A comparision of the kinetics of Cr/silica and Ti-Mg/silica led to the same conclusion. The most active silica gels also tended to be the most porous, which suggests fragility as a necessary requisite. Although 98% of their surface was internal, their activity was comparable to that of nonporous silicas, in which all the surface was external. This suggests that long polymer chains can diffuse out of the pores.     

Novel catalysts for syndiospecific polymerization of styrene

The current development of the metallocene-based catalysts for syndiotactic polystyrene (SPS) has been reviewed. SPS is a new semi-crystalline engineering thermoplastic with a crystalline melting point of 270°C. Because of its crystalline nature, SPS has high heat resistance, excellent chemical resistance and waterysteam resistance. In this review, some mechanistic models for polymerization and stereoregulation, as well as the factors which affect the activity and stereospecificity of the catalysts, are discussed.     

Supported catalysts for stereospecific polymerization of propylene

Tetrabenzyltitanium (B₄Ti), tribenzyltitanium chloride (B₃TiCl), tetra(p-methylbenzyl)titanium (R₄Ti) and tri(p-methylbenzyl)titanium chloride (R₃TiCl) have been used as catalysts for ethylene and propylene polymerization activated by AlEt₂Cl. B₄Ti-AIEt₂Cl in solution polymerizes ethylene readily but its activity decays rapidly. B₄Ti was also supported on Cab-O-Sil, Alon C, and Mg(OH)Cl. The last support was found to give catalyst with longest lifetime with a rate of polymerization, R_p = 7.0 g/hr-mmole Ti-atm ethylene. ¹⁴CO counting techniques gave 1.13 × 10^?3 mole of propagating center per mole of B₄Ti; the rate constant of propagation, k_p = 540 l./mole-sec. None of the tetravalent titanium compounds polymerize propylene in solution. However, when supported on Mg(OH)Cl, Cab-O-Sil, Alon C, Cab-O-Ti, and charcoal, they all polymerize propylene. In this work the supports were characterized by various techniques, including the paramagnetic probe method, to determine the concentration and nature of surface hydroxyls. Those factors controlling the rate and stereospecificity of propylene polymerization were investigated. The system B₃TiCl–Mg(OH)Cl–AlEt₂Cl is the most active with R_p = 2.89 g/hr-mmole Ti-atm propylene. The concentration of propagation center is 0.9 × 10^?3 mole per mole of B₃TiCl; k_p = 32 l./mole-sec. This catalyst gave only about 70% stereoregular polymer. Diethyl ether is found to raise stereospecificity to 100%, but there is a concommittent tenfold decrease of activity. Other interesting catalyst systems are: (π-C₅H₅)TiMe₃–Mg(OH)Cl–AlEt₂Cl (1.56, 89.5); (π-C₅H₅)TiMe₂–Mg(OH)Cl–AlEt₂Cl (0.075, 94.5); and (π-C₅H₅)TiMe₃–Alon C–Al-Et₂Cl (0.08,97.2), where the first number in the parenthesis is R_p in g/mmole Ti-hr-atm and the second entry corresponds to percentage yield of stereoregular polypropylene. Hafnocene and titanocene supported on Mg(OH)Cl produce only oligomers of propylene.     

Developments of chiral metallocenes as polymerization catalysts

This review article describes developments in chiral metallocenes as polymerization catalysts focusing on C2 symmetric ansa-zirconocene complexes. Selective synthesis of rac-isomers of ansa-zirconocenes are surveyed. Isospecific polymerizations of propylene catalyzed by chiral zirconocenes are summarized. Advanced series of polymerizations by chiral metallocenes such as asymmetric polymerization and polymerization of polar monomers are also introduced.     

Progress of olefin polymerization by metallocene catalysts

New C₁‐symmetric metallocenes such as [Me₂C(PhCp)(Flu)ZrCl₂, [Me₃Pen(Flu)]ZrCl₂, [PhMe₃Pen(Flu)]ZrCl₂ were synthesized and used for the polymerization of propene by higher polymerization temperatures. Different polypropylene micro structures were obtained. Important for industrial processes are the high molecular weights of the polymers produced by the pentalenelike catalysts, which are very stable by higher temperatures. For synthesis of syndiotactic polystyrene and new substituted half‐sandwich titanocenes are used such as 1,3‐Me₂‐CpTiCl₃, Me₄CpTiCl₃, PhCpTiCl₃, cyclohexyl‐CpTiCl₃. If they are fluorinated, the activity for the production of syndiotactic polystyrene can be increased 10 times. The synthesized polymer shows a high melting point of 275°C.     

Modification of metallocene catalysts for olefin polymerization

The metallocene catalyst developed by Kaminsky and Sinn has been demonstrated to permit the synthesis of any kind of stereoregular polymers as well as uniform copolymers with very narrow compositional and molecular weight distributions. The catalyst is, thus, expected to comprise a revolution in the polyolefin industry. More recently, a great deal of research effort has been devoted to modify it for practical applications, which has yielded a new generation of metallocene catalysts. This paper summarizes the results reported so far in the field. Some of our original data will be also reported.     

Cyclic ruthenium-alkylidene catalysts for ring-expansion metathesis polymerization

A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through (1)H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP.     

Propylene polymerization with catalysts containing divalent titanium

The behavior in propylene polymerization of divalent titanium compounds of type [η⁶-areneTiAl₂Cl₈], both as such and supported on activated MgCl₂, has been studied and compared to that of the simple catalyst MgCl₂/TiCl₄. Triethylaluminium was used as cocatalyst. The Ti–arene complexes were active both in the presence and in the absence of hydrogen, in contrast to earlier reports that divalent titanium species are active for ethylene but not for propylene polymerization. ¹³C-NMR analysis of low molecular weight polymer fractions indicated that the hydrogen activation effect observed for the MgCl₂-supported catalysts should be ascribed to reactivation of 2,1-inserted (“dormant”) sites via chain transfer, rather than to (re)generation of active trivalent Ti via oxidative addition of hydrogen to divalent species. Decay in activity during polymerization was observed with both catalysts, indicating that for MgCl₂/TiCl₄ catalysts decay is not necessarily due to overreduction of Ti to the divalent state during polymerization. In ethylene polymerization both catalysts exhibited an acceleration rather than a decay profile. It is suggested that the observed decay in activity during propylene polymerization may be due to the formation of clustered species that are too hindered for propylene but that allow ethylene polymerization. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2645–2652, 1997