New multinuclear half-titanocene catalysts in the syndiospecific polymerization of styrene

The syndiospecific polymerization of styrene with a new class of multinuclear transition metal catalysts in the presence of methylalumoxane and triisobutylaluminum has been investigated. The new multinuclear catalysts [(η⁵-C₅Me₅)Ti]₄(μ-O)₆ and [(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ were received by reaction of the corresponding mononuclear compounds with water and characterized by X-ray crystal structure analysis. The molecular structure of both complexes is tetrameric with six bridging oxygen atoms between the four titanium atoms, forming an adamantane-like cage structure with a substituted cyclopentadienyl ligand remaining η⁵-bonded to each titanium atom.The bulky [(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ shows higher polymerization conversions than [(η⁵-C₅Me₅)Ti]₄(μ-O)₆. The polymerization activity is significantly increased by an enhancement of the MAO concentration after a short retardation period and levels off at MAO/[(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ molar ratios above about 600. Triisobutylaluminum increases the polymerization yield to a maximum at a TIBA/[(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ molar ratio of about 30-100, but considerably decreases it at higher molar ratios below the polymerization conversion reached without any additional aluminum alkyl. Both compounds affect molecular weight and molecular weight distribution without any influence on the stereospecificity of the different catalytic sites active in polymerization reactions.The new multinuclear transition metal catalysts reach about 30-50% of the polymerization activity of the mononuclear catalysts on a molar basis and show a remarkably high catalytic activity in complex-coordinative polymerizations even after storage in non-inert-atmosphere conditions. The active polymerization sites of the multinuclear catalysts are not as uniform as the active sites of the mononuclear catalysts are and provide polystyrenes of a slightly lower syndiospecificity, but do not significantly influence the weight average molecular weights.     

Polymerization of vinyl chloride with half‐titanocene/methylaluminoxane catalysts

The polymerization of vinyl chloride (VC) with half‐titanocene /methylaluminoxane (MAO) catalysts is investigated. The polymerization of VC with the Cp*Ti(OCH₃)₃/MAO catalyst (Cp* = η⁵‐pentamethylcyclopentadienyl) afforded high‐molecular‐weight poly(vinyl chloride) (PVC) in good yields, although the polymerization proceeded at a slow rate. With the Cp*TiCl₃/MAO catalyst, the polymer was also obtained, but the polymer yield was lower than that with the Cp*Ti(OCH₃)₃/MAO catalyst. The polymerization of VC with the Cp*Ti(OCH₃)₃/MAO catalyst was influenced by the MAO/Ti mole ratio and reaction temperature, and the optimum was observed at the MAO/Ti mole ratio of about 10. The optimum reaction temperature of VC with the Cp*Ti(OCH₃)₃/MAO catalyst was around 20 °C. The stereoregularity of PVC obtained with the Cp*Ti(OCH₃)₃/MAO catalyst was different from that obtained with azobisisobutyronitrile, but highly stereoregular PVC could not be synthesized. From the elemental analyses, the ¹H and ¹³C NMR spectra of the polymers, and the analysis of the reduction product from PVC to polyethylene, the polymer obtained with Cp*Ti(OCH₃)₃/MAO catalyst consisted of only regular head‐to‐tail units without any anomalous structure, whereas the Cp*TiCl₃/MAO catalyst gave the PVC‐bearing anomalous units. The polymerization of VC with the Cp*Ti(OCH₃)₃/MAO catalyst did not inhibit even in the presence of radical inhibitors such as 2,2,6,6,‐tetrametylpiperidine‐1‐oxyl, indicating that the polymerization of VC did not proceed via a radical mechanism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 248–256, 2003     

C-NMR study of Ti(IV) species formed by Cp*TiMe3 and Cp*TiCl3 activation with methylaluminoxane (MAO)

Using ¹³C- and ¹H-NMR spectroscopy, titanium(IV) species formed in the catalytic systems Cp*TiMe₃/MAO and Cp*TiCl₃/MAO (Cp*=C₅(CH₃)₅) in toluene and chlorobenzene were studied within the temperature range 253-293 K and at Al/Ti ratios 30-300. It was shown that upon activation of Cp*TiMe₃ with methylaluminoxane (MAO) mainly the ‘cation-like’ intermediate Cp*Me₂Ti⁺←Me⁻Al(MAO) (2) is formed. Three types of titanium(IV) complexes were identified in Cp*TiCl₃/MAO catalytic system. They are methylated complexes Cp*TiMeCl₂ and Cp*TiMe₂Cl, and the ‘cation-like’ intermediate 2. Complex 2 dominates in Cp*TiCl₃/MAO system in conditions approaching to those of practical polymerization (Al/Ti ratios more than 200). According to the EPR measurements, the portion of EPR active Ti(III) species in the Cp*TiCl₃/MAO system is smaller than 1% at Al/Ti=35, and is about 10% at Al/Ti=700.     

Kinetic studies on the syndiotactic propylene polymerization catalyzed over iPr(Cp)(Flu)ZrCl2

Kinetic studies on the syndiospecific polymerizations of propylene with iPr(Cp)(Flu)ZrCl₂/methylaluminoxane (MAO) were performed at 20, 40 and 70 °C and at 5 atm with various Al/Zr molar ratios. The average polymerization activity for 60 min decreased, and the time to reach a maximum activity (t_max) decreased as Al/Zr molar ratio increased. However, at Al/Zr molar ratio of 10,000, catalytic activity decreased rapidly and became the smallest among any other Al/Zr molar ratios after 20 min of polymerization. At higher Al/Zr molar ratio, methylation and cationization progress rapidly, but its polymerization rate decayed quickly due to strong interaction between MAO and metallocene, resulting in less active species. Regardless of change in polymerization temperature, t_max was maintained around 15 min. Stereoregularity was strongly dependent on the polymerization temperature, and active site isomerization was dominant source for stereoirregularity, and it was strongly influenced by polymerization temperature.     

Transition Metal Compounds in Combination with Alkylaluminoxanes as Catalysts for Olefin and Diene Polymerization

Catalytic systems based on Zr, Co, Ti, V, and Nd in combination with –(–RAlO–)– _n alkylaluminoxanes, where R = Me or iso-Bu, were used for the polymerization of olefins and dienes. The structure of methylaluminoxane (MAO) was studied with the use of theoretical calculations and experimentally by vibrational spectroscopy. It was found that only cis and trans conformations of linear MAO oligomers actually occurred at the active centers of olefin polymerization. The stereospecificity of catalytic systems for diene polymerization depends on the nature of the substituent at aluminum, as well as on the nature of the transition metal and its ligand environment.     

Comparison of Syndiotactic Polystyrene Morphology Obtained Via Heterogeneous and Homogeneous Polymerization with Metallocene Catalyst

Summary: Supported catalyst system for the slurry phase polymerization of styrene in toluene was prepared by the immobilization of 2-methylindenyltrichlorotitanium(2-MeIndTiCl₃) on silica and activation of this catalyst was performed by methylaluminoxane(MAO) in polymerization media. Homogeneous polymerization of styrene with 2-methylindenyltrichlorotitanium activated by MAO was performed in toluene. The morphology of obtained syndiotactic polystyrene (sPS) via heterogeneous and homhgeneous catalyst system was compared. Polymerization of styrene by homogeneous catalyst lead to formation of gel and resultant polymers presented a compact and dense texture while the global gelation do not occur with silica supported catalyst at different Ti/SiO₂ mol ratios and sPS was obtained as separated particles. Unlike to the homogeneous catalyst, obtained polymers showed a porous texture. Highly porous texture of sPS was obtained with Ti/SiO₂ = 0.5% mol ratio.     

Syndiospecific polymerization of styrene with BzCpTiCl3 and methylaluminoxane as cocatalysts

Benzyl cyclopentadienyl titanium trichloride (BzCpTiCl₃) was synthesized from benzyl bromide, cyclopentadienyl lithium, and titanium tetrachloride and used in combination with methylaluminoxane (MAO) for the syndiospecific polymerization of styrene. Kinetic measurements of the polymerization were carried out at different temperatures. The polymerization with BzCpTiCl₃/MAO differs from the polymerization with cyclopentadienyl titanium trichloride in its behavior toward the Al/Ti ratio. In addition, high activities are observed at high Al/Ti ratios. By analyzing the polymerization runs and the physical properties of the polymers with differential scanning calorimetry, ¹³C NMR spectroscopy, wide‐angle X‐ray scattering measurements, and gel permeation chromatography, we found that the phenyl ring coordinates to the titanium atom during polymerization. Other known substitutions of the cyclopentadienyl ring (V. Scholz, Dissertation, University of Hamburg, 1998) in principle influence the polymerization activity. The physical properties of the polymers produced by the catalysts already known are nearly identical. BzCpTiCl₃ is the first catalyst that leads to polystyrene obviously different from the polystyrene produced by other highly active catalysts. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2805–2812, 2001     

Heat Stabilizers as Activators in Coordination Polymerization

Summary: The solvent‐free syndiospecific styrene polymerization as an example of a coordination polymerization has been investigated with a catalyst system consisting of η⁵‐octahydrofluorenyl titanium trimethoxide as a transition metal catalyst, MAO as a cocatalyst, and TIBA, in the presence of reaction products of sterically hindered phenolic compounds, usually applied as heat stabilizers of polymers. Unexpectedly, such reaction products led to a significant increase in polymerization activity of the catalyst system. Second, after deactivation of the catalyst system, such activators result in a significantly enhanced thermal stability of the syndiotactic polymers received.

Effect of the P₈‐activator on polymerization activity in dependence on polymerization time (molar ratio–styrene:MAO:TIBA:P:Ti = 700 000:50:25:25:1; molar ratio–phenolic compound:TIBA = 1:3.2; polymerization temperature: 50 °C).     

Hydrogen transfer reactions of supported metallocene catalysts

The evolution of methane from methylaluminoxane (MAO) solutions is enhanced in the presence of homogeneous metallocenes. This reaction serves as a model for the deactivation of metallocene catalysts. By supporting different metallocenes on a silica/MAO carrier the deactivation reaction by alpha-hydrogen transfer among metallocene active sites and aluminum alkyls can be suppressed. The suppression of alpha-hydrogen transfer is proven for different Al/Zr ratios and by near independence of the polymerization activity on the catalyst aging time, after reaching maximum activity. Aluminum alkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization if MAO is present.     

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

The structure of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by Raman and in situ IR spectroscopy, polymerization experiments, and density functional calculations. From experimental results, a number of quantum chemical calculations, and bonding properties of related compounds, we have suggested a few Me₁₈Al₁₂O₉ cage structures, including a highly regular one with C_3h symmetry, which may serve as models for methylaluminoxane solutions. The cages themselves are rigid but may contain up to three bridging methyl groups on the cage surfaces that are labile and reactive. Bridging methyls were substituted with Cl atoms to form a compound otherwise similar to MAO. Chlorinated MAO is unable to activate a metallocene catalyst, even in the presence of trimethylaluminum (TMA), but allows subsequent activation by regular MAO. With bis(pentamethylcyclopentadienyl)zirconium dichloride, MAO and TMA seem to influence chain termination independently. Several findings previously poorly explained are rationalized with the new model, including the observed lack of reaction products with excess TMA. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3106–3127, 2000     

Mechanistic aspects of the olefin polymerization with metallocene catalysts

With C₁-, C₂- or C_s-symmetric metallocenes, different intermediates and types of copolymers can be obtained from randomly distributed to alternating structures. Substitution of the Cp-ring in [Me₂C-(tert-Bu Cp)(Flu)]ZrCl₂ yields ethene/norbornene copolymers with an alternating structure, because the rigid norbornene can only be inserted from the open side of the metallocene. By variation of the polymerization parameters, copolymers with glass transition temperatures above 180°C and molecular weights > 100 000 are synthesized. By supporting different metallocenes on a silica/methylaluminoxane (MAO) carrier the deactivation reaction under electron and hydrogen transfer can be suppressed. This is proved for different Al/Zr ratios when trimethylaluminum (TMA) is used as cocatalyst by the lack of methane evolution by metallocenes and by near independence of the polymerization activity on the prereaction time, after reaching maximum activity. Aluminumalkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization by adding MAO.     

Novel Complex Catalyst of η3‐(Pentamethylcyclopentadienyl)dimethylaluminum/Titanium(IV) Butoxide (Cp*AlMe2/Ti(OBu)4) for Syndiotactic Polystyrene

A novel metallocene catalyst was prepared from the reaction of (η³‐pentamethylcyclopentadienyl)dimethylaluminum (Cp*AlMe₂) and titanium(IV) n‐butoxide Ti(OBu)₄. The resulting titanocene Cp*Ti(OBu)₃ was combined with methylaluminoxane (MAO)/tri‐iso‐butylaluminum (TIBA) to carry out the syndiotactic polymerization of styrene. The resulting syndiotactic polystyrene (sPS) possesses high syndiotacticity according to ¹³C NMR. Catalytic activity and the molecular weight of the resulting sPSs were discussed in terms of reaction temperature, concentration of MAO, amounts of scavenger TIBA added, and the hydrogen pressure applied during polymerization.     

Polymerization of 1,3-butadiene, 4-methyl-1,3-pentadiene and styrene with catalyst systems based on biscyclopentadienyl derivatives of titanium

1,3-Butadiene, 4-methyl-1,3-pentadiene and styrene were polymerized with dicyclopentadienyltitanium dichloride/methylaluminoxane (Cp₂TiCl₂/MAO) and dicyclopentadienyltitanium chloride/MAO (Cp₂TiCl/MAO). These systems are less active than cyclopentadienyltitanium trichloride/MAO (CpTiCl₃/MAO), but show the same stereospecificity as the latter; they give predominantly cis-1,4-polybutadiene, 1,2-syndiotactic poly(4-methyl-1,3-pentadiene) and syndiotactic polystyrene. Cp₂TiCl/MAO is much more active than Cp₂TiCl₂/MAO; this is probably due to the fact that in the reaction of Cp₂TiCl₂ with MAO, only a small amount of Ti(IV) is reduced to Ti(III), which is the active species in the polymerization of styrene and 1,3-dienes. An interpretation of the structure of the active species in Cp₂TiCl/MAO is reported.     

Polymerization of norbornene with Co(II) complexes

The norbornene polymerization was studied in the presence of 6 pyridine bis(imine) cobalt(II) complexes activated with methylaluminoxane (MAO). Norbornene was also polymerized with CoCl₂ associated to MAO. All these catalytic systems generate an addition polymerization of norbornene, yielding fully saturated polymers. It was shown that the polymerization yield and the molar masses are highly dependant on several reaction parameters (monomer concentration, [Al]/[Co] ratio, polymerization temperature and time) and the frame of the ligand.     

Donor- and acceptor-modified metallocene-based homogeneous Ziegler-Natta catalysts

The polymerization kinetics of propene polymerization using metallocene/methylaluminoxane (MAO) homogeneous catalysts have been investigated to explore the role of donor/acceptor interactions and to enhance the catalyst productivities. In the case of the non-stereospecific Cp₂ZrCl₂/MAO model system it has been demonstrated that, in addition to the well known irreversible deactivation, reversible deactivations, which are second order relative to the zirconium active site concentration, account for the decay of the polymerization rate. While MAO injection during polymerization enhances the polymerization rate, zirconocene addition deactivates the catalyst which can be reactivated by injecting additional MAO. A sequence of dynamic equilibria involving the formation of active cationic metallocene intermediates as well as inactive zirconocene species, e.g., zirconocene dimers, is proposed. Lewis base and Lewis acid additives have been added as probes to examine the role of such equilibria in the case of metallocene-based catalyst systems such as MAO-activated Cp₂ZrCl₂, racemic ethylenebisindenyl zirconium dichloride (EBIZrCl₂), and racemic ethylenebis (4,5,6,7-tetrahydroindenyl) zirconium dichloride (EBTHIZrCl₂). While the conventional donors such as 2,6-ditert.butyl-4-methylphenol (BHT) and 2,2,6,6-tetramethylpiperidine (TMP) reduce catalyst productivities, even at very low donor/Al molar ratios, increasing propene concentration and addition of trimethylboroxine (TMB) substantially enhance catalyst productivities and affect molecular weights of the polypropylene produced with metallocene/MAO catalysts.     

Control of molecular weight in polymerization of vinyl chloride with Cp*Ti(OPh)3/MAO catalyst

Polymerization of vinyl chloride (VC) with titanium complexes containing Ti‐OPh bond in combination with methylaluminoxane (MAO) catalysts was investigated. Among the titanium complexes examined, Cp*Ti(OPh)₃/MAO catalyst (Cp*; pentamethylcyclopentadienyl, Ph; C₆H₅) gave the highest activity for the polymerization of VC, but the polymerization rate was slow. From the kinetic study on the polymerization of VC with Cp*Ti(OPh)₃/MAO catalyst, the relationship between the M_n of the polymer and the polymer yields gave a straight line, and the line passed through the origin. The M_w/M_n values of the polymer gradually decrease as a function of polymer yields, but the M_w/M_n values were somewhat broad. This may be explained by a slow initiation in the polymerization of VC with Cp*Ti(OPh)₃/MAO catalyst. The results obtained in this study demonstrate that the molecular weight control of the polymers is possible in the polymerization of VC with the Cp*Ti(OPh)₃/MAO catalyst. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3872–3876, 2007     

Routes to reduce aluminium co-catalyst content for zirconocene activation in olefin polymerization

In this paper, we investigate different routes to lower drastically the amount of methylaluminoxane (MAO) required to activate racEt(Ind)₂ZrX₂ catalysts towards olefin polymerization. A first approach consists in replacing Cl ligands by more easily extractable X groups such as Me, CH₂Ph or NMe₂ groups. A second method focuses on the preparation of TMA-depleted MAO either by pumping off TMA from commercial MAO or by exploring new synthetic source to MAO via non-hydrolytic processes such as the reaction of TMA with benzophenone. Both methods allowed us to produce polyolefins with a maximal catalytic activity for Al/Zr ratios not exceeding 150, i.e. ratio 20 times lower than those required in the presence of commercial MAO.     

Ethylene Polymerization Catalyzed by Monocyclopentadienyl Titanium Complex Containing 8-Quinolinolato Ligand and ADF Study on the Formation Mechanism of Active Species

A monocyclopentadienyl titanium complex containing 8-quinolinolato (QCpTiCl_2) was synthesized. Its activities in ethylene polymerization at various Al/Ti molar ratios, different temperatures and activation time were investigated. The activity with a Al/Ti molar ratio of 500 exhibited a maximum of 2.8×10~5 g/(mol.h) at 30℃. The activation time of QCpTiCl_2 with MAO before polymerization also plays a role on the activity. The structural properties of the produced polyethylene (molecular weight, molecular weight distribution and melting point) were discussed. Kinetic behaviors of ethylene polymerization with the QCpTiCl_2/MAO system at different Al/Ti molar ratios were studied. For the QCpTiMeCl/MAO system and the CpTiMe_2Cl/MAO system, binding energies of the examined intermediates were calculated by quantum-mechanical method based on ADF program, respectively. It is confirmed that the chlorinebridged adduct formed by the reaction of QCpTiMeCl with MAO is thermodynamically steady. In the case of the QCp     

Stereospecific and molecular weight‐controlled polymerization of 1,3‐butadiene with Co(acac)3‐MAO catalyst

The polymerization of butadiene (Bd) with Co(acac)₃ in combination with methylaluminoxane (MAO) was investigated. The polymerization of Bd with Co(acac)₃‐MAO catalysts proceeded to give cis‐1,4 polymers (94 – 97%) bearing high molecular weights (40 × 10⁴) with relatively narrow molecular weight distributions (M_w's/M_n's). The molecular weight of the polymers increased linearly with the polymer yield, and the line passed through an original point. The polydispersities of the polymers kept almost constant during reaction time. This indicates that the microstructure and molecular weight of the polymers can be controlled in the polymerization of Bd with the Co(acac)₃‐MAO catalyst. The effects of reaction temperature, Bd concentration, and the MAO/Co molar ratio on the cis‐1,4 microstructure and high molecular weight polymer in the polymerization of Bd with Co(acac)₃‐MAO catalyst were observed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2793–2798, 2001     

五甲基茂基三(对氯苯氧基)钛/甲基铝氧烷体系催化丙烯聚合反应与动力学的研究

采用茂钛配合物五甲基茂基三(对氯苯氧基)钛[Cp- Ti(O- C6H4Cl -P)3]甲基铝氧烷(MAO)体系进行丙烯聚合反应,考察了聚合温度和Al- Ti摩尔比(nAl- nTi)对聚合活性和产物分子量的影响.研究发现在合适的聚合条件下,聚合初期产物的分子量随聚合时间线性增大,并保持较窄的分子量分布(Mw Mn=1.5～1.8),表现出准活性聚合的行为,而聚合物的GPC曲线呈双峰分布,表明聚合初期的体系存在多活性中心.另外,初步提出了衰减动力学方程以探讨聚合速率衰减期的丙烯聚合反应规律,求出了不同条件下丙烯聚合的衰减系数,从而可计算出衰减期内任一时间的丙烯聚合速率.