Analyses of Propene and 1‐Hexene Oligomers from Zirconocene/MAO Catalysts – Mechanistic Implications by NMR,SEC, and MALDI‐TOF MS

A series of Cp′(C₅H₅)ZrCl₂ and Cp′₂ZrCl₂ pre‐catalysts (Cp′ = C₅HMe₄, C₄Me₄P, C₅Me₅, C₅H₄^tBu, C₅H₃‐1,3‐^tBu₂, C₅H₂‐1,2,4‐^tBu₃) together with (C₅H₅)₂ZrCl₂ was used for the directed oligomerization of propene and 1‐hexene in comparative experiments. Oligomer characterization was carried out by ¹H NMR, SEC (GPC), MALDI‐TOF MS, cryoscopy and Raman spectroscopy. From ¹H NMR the nature and relative ratio of the double‐bond end group is determined together with M̄_n if every oligomer molecule contains such a double‐bond end group. Normally vinylidene double bonds (from β‐hydrogen elimination) are found. From ¹H NMR and MALDI‐TOF MS also vinyl end groups (from β‐methyl elimination) are observed in the case of oligopropenes with (C₄Me₄P)‐ (C₅H₅)ZrCl₂ and with the symmetrical methyl containing Cp′₂ZrCl₂ pre‐catalysts. The vinylidene/vinyl ratio depends on the ligand and increases from C₅HMe₄ (65/35) over C₄Me₄P (61/39) to C₅Me₅ (9/91). A comparison of M̄_n from ¹H NMR and SEC together with MALDI‐TOF MS shows that the phospholyl‐zirconocenes and (C₅HMe₄)₂ZrCl₂ also exhibit chain transfer to aluminium, thereby giving saturated oligomers.     

Ethylene/1-hexene copolymerization with Ph2C(Cp)(Flu)ZrCl2 derivatives: correlation between ligand structure and copolymerization behavior at high temperature

Ethylene homo- and copolymerization with 1-hexene were performed in the presence of diphenylmethylidene(cyclopentadienyl)(fluorenyl)zirconium dichloride (Ph₂C(Cp)(Flu)ZrCl₂) derivatives activated with dimethylanilinium tetrakis(pentafluorophenyl)borate (Me₂PhNH·B(C₆F₅)₄)/triisobutylaluminium (i-Bu₃Al) or methylaluminoxane (MAO) to study the role of the substituent on activity, comonomer incorporation and molecular weight. C₁ symmetric metallocenes which have several substituents in β-position of the cyclopentadienyl ligand produce lower molecular weight copolymers than the Ph₂C(Cp)(Flu)ZrCl₂ catalyst at 200°C, whereas the copolymerization reactivity is significantly influenced by the volume of the substituent: the trimethylsilyl substituted derivative produces ethylene/1-hexene copolymers with a broad chemical composition distribution. Polyethylene obtained with the diphenylmethylidene(cyclopentadienyl)(indenyl)zirconium dichloride (Ph₂C(Cp)(Ind)ZrCl₂) based catalyst is branched, and the molecular weight distribution and the chemical composition distribution are significantly affected by the cocatalyst. C_s symmetric metallocenes which have alkyl substituents in 2,7 position of the fluorenyl ligand produce higher molecular weight copolymers than the Ph₂C(Cp)(Flu)ZrCl₂ catalyst with equal copolymerization reactivity.     

1H and 13C NMR Study of the Intermediates Formed by (Cp‐R)2ZrCl2 Activation with MAO and AlMe3/[CPh3][B(C6F5)4]. Correlation of Spectroscopic and Ethene Polymerization Data

Using ¹H and ¹³C NMR spectroscopy, cationic intermediates formed by activation of (Cp‐R)₂ZrCl₂ (R = nBu, tBu and 1,2,3‐Me₃) with MAO in toluene were monitored at Al/Zr ratios from 50 to 1 000. The catalysts (Cp‐R)₂ZrCl₂/AlMe₃/CPh₃⁺B(C₆F₅)₄^? (nBu, tBu and 1,2,3‐Me₃) were also studied for comparison of spectroscopic and polymerization data with MAO based systems. Complexes of type (Cp‐R)₂ZrMe⁺ ← Me^?‐Al?MAO ( IV ) with different Me‐MAO^? counter anions have been identified in the (Cp‐R)₂ZrCl₂/MAO systems at low Al/Zr ratios. At Al/Zr ratios of 200–1 000, the complex [(Cp‐R)₂Zr(μ‐Me)₂AlMe₂]⁺ Me‐MAO^? ( III ) dominates in all MAO‐based reaction systems. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200–1 000) for the systems (Cp‐nBu)₂ZrCl₂/MAO and (Cp‐tBu)₂ZrCl₂/MAO, while it is virtually constant in the same range of Al/Zr ratios for the catalytic system (Cp‐1,2,3‐Me₃)₂ZrCl₂/MAO. The data obtained are interpreted on the assumption that complex III is the actual precursor of active centers of polymerization in MAO based systems.

Formation of cationic intermediates by activation with MAO.     

Propylene polymerization with Ph2C(3-RCp)(Flu)ZrCl2 [R = Me,i-Pr,PhCH2, Me3Si] catalysts activated with MAO and Me2PhNH·B(C6F5)4/i-Bu3Al

Propylene polymerization was conducted with Ph₂C(R-Cp)(Flu)ZrCl₂ [R = Me, i-Pr, PhCH₂, Me₃Si] catalysts in combination with methylaluminoxane (MAO) and dimethylanilinium tetrakis(pentafluorophenyl)borate (Me₂PhNH·B(C₆F₅)₄) as cocatalyst; the dependence of the stereoregularity of poly(propylene) on cocatalysts and bulkiness of the substituents in β-position of the cyclopentadienyl ligand was studied. Methyl and i-propyl substituted metallocene catalysts produce hemi-isotactic poly(propylene). These results are in good agreement with the results of the isopropylidene bridged metallocene analogue. The benzyl substituted metallocene catalyst produces syndiotactic poly(propylene) regardless of the cocatalyst. This means that this substituent group does not affect migration insertion of propylene. Stereoregularity of poly(propylene) obtained with diphenylmethylidene(3-trimethylsilylcyclopentadienyl)(fluorenyl)zirconium dichloride (Ph₂C(Me₃SiCp)(Flu)ZrCl₂) as a catalyst was significantly influenced by the cocatalyst. Me₂PhNH· B(C₆F₅)₄/triisobutylaluminium(i-Bu₃Al) produces poly(propylene) with 65% racemic and 23% meso pentads at 40°C, whereas the MAO activated catalyst produces isotactic rich poly(propylene). Fractionation experiments indicated that Me₂PhNH·B(C₆F₅)₄/i-Bu₃Al forms two active sites, one of them being the same as that of the MAO activated catalyst, the other one producing syndiotactic rich poly(propylene).     

Polymerization of Methyl Methacrylate with Chloro Zirconocene Enolates

Summary: The polymerization of methyl methacrylate (MMA) in the presence of the neutral chloro zirconocene enolates, Cp₂ZrCl[OC(OMe)?CMe₂] ( 1 ), Me₄C₂(Cp)₂ZrCl[OC(OMe)?CMe₂] ( 2 ), and Me₂C(Cp)₂ZrCl[OC(OMe)?CMe₂] ( 3 ), was investigated. None of these compounds is catalytically active on its own. They could be activated by adding Ph₃CB(C₆F₅)₄ ( 4 ); however, only if the initial concentration of enolate is higher than that of the perfluorated phenyl borate. Polymers were produced with a number average molar mass of up to 100 000 g/mol and a dispersion index of 1.1–1.3 with mixtures comprising 1 and 4 or 2 and 4 . The degree of polymerization depends only on the quantity of excess enolate and not on the absolute concentration of the initiator components. In contrast, a mixture of 3 and 4 was not very active. Kinetic modeling of the systems as well as NMR spectroscopic investigations indicate that a bimetallic mechanism can describe chain growth. The activity of these catalytic systems depends on the structure of the ligand. The two zirconocenes with bridged cyclopentadienyl ligands, ( 2 ) and ( 3 ), convert MMA considerably slower. In addition to the methyl zirconocene enolates with unbridged cyclopentadienyl ligands, the corresponding chloro zirconocene enolates are also suitable for the polymerization of MMA. The latter compounds offer the advantage that preparations for the production of the initiators are comparatively simple.

Structures of investigated initiators.     

Propene polymerization with zirconocene catalysts supported on siloxane copolymers

A supported version of zirconocene catalysts on siloxane copolymers having 1,2,3,4-tetramethylcyclopentadienyl (Cp″)-fluorenyl (Flu) (I) or cyclopentadienyl (Cp)-fluorenyl (II) groups as substituents was prepared and applied to propene polymerization using methylalumoxane (MAO) or [Ph₃C][B(C₆F₅)₄] as cocatalyst. Catalyst (I), which was soluble in toluene, exhibited very low activity, whereas catalyst (II), which was composed of a toluene soluble (Cat-A) and an insoluble fraction (Cat-B), displayed a fairly high activity. Both catalysts (A and B) give a mixture of syndiotactic, atactic and a small quantity of isotactic polypropene, with a fraction of syndiotactic pentads ranging from about 50–75%. In contrast, the corresponding non-supported catalyst gives almost completely atactic polypropene.     

Copolymerization of ethylene with 1-hexene and 1-octene: correlation between type of catalyst and comonomer incorporated

The behaviour of catalytic systems based on zirconium compounds for the copolymerization of ethylene with 1-hexene and 1-octene is reported. The metallocenes (CH₃)₂SiCp₂ZrCl₂, Cp₂ZrCl₂ (Cp = η⁵-cyclopentadienyl), C₂H₄[Ind]₂ZrCl₂ and (Ind = η⁵-indenyl) were chosen for this study. The bridged catalysts, (CH₃)₂SiCp₂ZrCl₂ and C₂H₄[Ind]₂ZrCl₂, and the metallocene Cp₂ZrCl₂ showed similar catalytic activities for home- and copolymerization of ethylene with 1-hexene. ¹³C NMR analysis showed that the composition of copolymerization products depends on the catalytic system, in other words, on the ligand structure of the transition metal. Copolymers obtained using the bridged catalysts have greater incorporation of comonomer. Thermal analysis and viscosity measurements demonstrated that an increase of the comonomer concentration reduces the melting point, the crystallinity and the molecular weight of the copolymer. Results from infrared spectroscopy showed that β-elimination is one of the possible termination reactions. The monomer reactivity ratios r were determined for all catalytic systems using Fineman-Ross and ¹³C NMR methods. The values of r₁ (M₁ = ethylene) and r₂ (M₂ = α-olefin) showed an effect of the type of metallocene and of α-olefin on the structure of the copolymer obtained.     

Polymerization of epoxides by zirconocene dichloride

Epoxides are polymerized homogeneously by some metallocene halides. Cp₂TiCl₂ is ineffective, Cp₂ZrCl₂ and Cp₂ZrI₂ are active as catalysts (Cp denotes the cyclopentadienyl group). Epichlorohydrin is polymerized to high conversions. The resultant polymers are amorphous and of low mol. wt. because of transfer reactions. The bridged compound (Cp₂ZrCl)₂O is also active and gives higher mol. wt. Phenyl glycidyl ether polymerizes well. The formation of oligomers from propylene oxide is also catalyzed by Cp₂ZrCl₂. A cationic mechanism is suggested for the polymerizations.     

Influence of batch reaction conditions on norbornene/ethylene copolymers made using C2v- and Cs-symmetric metallocene/MAO catalysts

The norbornene/ethylene copolymerization was studied using C_2v- and C_s-symmetric metallocene catalysts in the presence of methylaluminoxane as a cocatalyst. The batch reactions were carried out at four different temperatures copolymerizing a constant amount of ethylene with the appropriate amount of norbornene. The activity decreases for the Cp₂ZrCl₂ catalyst and increases to a constant level for the [Ph₂C(Flu)(Cp)]ZrCl₂ catalyst when the norbornene/ethylene ratio in the reaction increased. The highest activities and the highest norbornene contents in the copolymers were achieved using the [Ph₂C(Flu)(Cp)]ZrCl₂. The microstructure of the copolymer determined with ¹³C NMR was observed to depend on catalyst, temperature and monomer ratio. Cp₂ZrCl₂ produces more norbornene blocks as a function of norbornene content in the copolymer and longer norbornene sequences than [Ph₂C(Flu)(Cp)]ZrCl₂. The correlation between the increasing norbornene content in the copolymer and the increasing glass transition temperature is more obvious for [Ph₂C(Flu)(Cp)]ZrCl₂.     

Semi‐Batch Polymerisations of Ethylene with Metallocene Catalysts in the Presence of Hydrogen, 3. Correlation Between Hydrogen Sensitivity and Molecular Parameters

Semi‐batch ethylene polymerisations have been carried out with heterogenised Cp₂ZrCl₂/MAO, Ind₂ZrCl₂/MAO, Cp(Ind)ZrCl₂/MAO and Cp(Flu)ZrCl₂/MAO (MAO, methylaluminoxane) catalysts where hydrogen was added as the chain‐transfer agent. Modelling of molecular weight distributions of the polymer formed gave estimates of the relative rate constant for the chain transfer to hydrogen and the rate constant for propagation, k/k_p. Values of 0.7(2), 22(7), 13(3) and 35(4) were obtained for the Cp₂, Ind₂, (Cp,Ind) and (Cp,Flu) catalysts, respectively. The observed order of reactivity towards hydrogen has been correlated with chemical shifts from ⁹¹Zr NMR and with the atomic charges of zirconium from DFT calculations for a series of metallocene complexes. The efficiency of hydrogen as a chain‐transfer agent is larger the more electron deficient the zirconium atom in the catalyst is.     

Ligand effects on olefin polymerizations with multinuclear zirconocene catalysts having dimethylsilylferrocene as a cyclopentadienyl substituent

Olefin polymerizations were conducted with novel multimetallic zirconocene catalyst systems: R? CpCpZrCl₂ ( 1 ) (R = dimethylsilylferrocene; Cp = cyclopentadienyl) and (R? Cp)₂ZrCl₂ ( 2 ) in combination with methylaluminoxane or tetrakis(pentafluorophenyl) borates. In ethylene polymerization, the maximum catalyst activity was observed at a lower temperature (30–45°C) when using complex 1 or 2 , compared with Cp₂ZrCl₂. It was also revealed that complex 1 is a more effective catalyst than Cp₂ZrCl₂ for copolymerization of ethylene and propene.     

Variation of molecular weight distribution (MWD) and short chain branching distribution (SCBD) of ethylene/1‐hexene copolymers produced with different in‐situ supported metallocene catalysts

Copolymerization of ethylene and 1‐hexene was carried out with different catalysts: homogeneous Et[Ind]₂ZrCl₂, Cp₂HfCl₂ and [(C₅Me₄)SiMe₂N(tert‐Bu)]TiCl₂, the corresponding in‐situ supported metallocenes, and combined in‐situ supported metallocene catalysts (mixtures of Et[Ind]₂ZrCl₂ and Cp₂HfCl₂, and Et[Ind]₂ZrCl₂ and [(C₅Me₄)SiMe₂N(tert‐Bu)]TiCl₂). Carbon‐13 nuclear magnetic resonance spectroscopy (¹³C NMR), gel permeation chromatography (GPC) and crystallization analysis fractionation (CRYSTAF) studies showed that, compared to the corresponding homogeneous metallocenes, in‐situ supported metallocenes produced polymers having different 1‐hexene fractions, molecular weight distributions (MWD) and short chain branching distributions (SCBD). It was also demonstrated that polymers with broader MWD and SCBD can be produced by combining two different in‐situ supported metallocenes.     

13C NMR Studies of Ethene/Norbornene Copolymers using 13C‐Enriched Monomers: Signal Assignments of Copolymers Containing Norbornene Microblocks of up to a Length of Three Norbornene Units

Ethene and norbornene were copolymerized at 0.8 bar ethene absolute pressure using metallocene catalysts that produce microstructures containing norbornene blocks, particularly rac‐Me₂Si[Ind]₂ZrCl₂/MAO, iPr[CpFlu]ZrCl₂/MAO and iPr[CpInd]ZrCl₂/MAO. Intensive ¹³C NMR spectroscopic investigations of the obtained copolymers were performed and norbornene di‐ and triblock sequences detected clearly. Several copolymerization experiments were carried out by using ¹³C₁‐enriched ethane, or ¹³C_5/6‐enriched norbornene, respectively. Thus, resonances of the norbornene carbon atoms C5/C6 and the ethene carbon atoms Ca/Cb, which overlap extensively in the ¹³C NMR spectra, could clearly be differentiated. In addition, further copolymerizations with natural monomers were performed at 2 bar ethene excess pressure to investigate the influence of the monomer concentration on the microstructure. By comparing the ¹³C NMR spectra of copolymers obtained under different conditions, assignments of most resonances were achieved. These ¹³C NMR studies made it possible to assign norbornene triblock resonances with differing stereochemical connections such as meso,meso and rac,meso which have not been mentioned in previous works. The results led to new insights about the copolymer microstructure and allow development of a scheme showing the ethene/norbornene microstructures depending on the various catalyst systems utilized, including the metallocenes iPr[(3‐iPr‐Cp)Flu]ZrCl₂ and iPr[(3‐tert‐But‐Cp)Ind]ZrCl₂ which have been applied in previous publications.     

Effect of Metallocene Structures on their Performance in Ethene Polymerisation

Six (Cp)(4‐phenyl‐indenyl)MCl₂ (A) and four (Cp)(1‐phenyl‐indenyl)MCl₂ (C) (M = Zr/Hf) based complexes were tested in ethene homo‐ and copolymerisation under different conditions to explain how the differences in the complex structure affect the polymerisation process and the formed polymer. Polymerisation experiments reveal that hafnocene catalysts need a higher amount of MAO to reach the maximum activity than zirconocenes. Hafnocenes also incorporate better 1‐hexene. Catalysts with the Cp* or 1,2,4‐Me₃Cp ligand show higher activity, and work well with [HNMe₂Ph][B(C₆F₅)₄]/TIBA as cocatalyst, but produce polymers with lower than the corresponding catalyst with a plain Cp ligand. The Cp* ligand seems to prevent 1‐hexene incorporation. A methyl group in the 2‐position of (Cp)(4‐PhInd)ZrCl₂ decreases the activity and but favours 1‐hexene insertion. Polymerisation activity is higher with catalysts with a 1‐PhInd‐ligand than with a 4‐PhInd‐ligand.

     

Elementarprozesse der ziegler-katalyse, 5. Lösungsmitteleinflüsse am Beispiel stereorigider Zirkonocen/methylaluminoxan Ziegler-Katalysatoren

The syndiotactic propene polymerization was investigated by means of the C_s-symmetric isopropylidene(fluorenyl)(cyclopentadienyl)zirconium dichloride/methylaluminoxane (iPr(Flu)-(Cp)ZrCI₂/MAO) catalyst system in toluene/methylene chloride solvent mixtures. The main result is that with increasing dielectric constant of the solvent mixture the polymerization rate increases linearly but the stereospecificity of the catalyst decreases strongly; this is revealed through a drastic decay of the rrrr-pentads and of the melting point of the polymers. Hence, the stereospecificity of this catalyst system is connected with the existence of a polarized Zr-CI-AI-complex (in toluene) or at least of a tight contact ion pair with a stereoregulating role of the counter ion. The loss of the stereospecifity in polar solvents is caused by an isomerization reaction of the solvent-separated free zirconocene species via migration of the growing polymer chain before the next monomer insertion. But in the case of the isotactic catalyst system dimethyl-silyl(bisindenyl)zirconium dichloride/methylaluminoxane (Me₂Si(Ind)₂ZrCl₂/MAO) with a C₂-symmetric π-ligand system the migration of the growing polymer chain in polar solvents causes no isomerization.     

Synthesis of Ethylene‐co‐Norbornene Polymers Promoted by MAO Free Catalysts Based on Group 4 Dicarbollide Complexes

Ethylene‐co‐norbornene polymers [P(E‐co‐N)s] have been synthesized with the dicarbollide catalysts (η⁵‐C₂B₉H₁₁)M(NEt₂)₂(NHEt₂) [M = Ti ( 1 ), Zr ( 2 )] activated with methylaluminoxane (MAO) or alkylaluminium compounds, as AlMe₃ (TMA), Al(ⁱBu)₃ (TIBA), AlH(ⁱBu)₂ (DIBALH), AlEt₂Cl (DEAC). Polymerization results by 1 ‐TIBA, 2 ‐TIBA, 1 ‐MAO and 2 ‐MAO catalysts have been preliminary compared with those of cyclopentadienyl derivatives of the group 4 metals, (η⁵‐C₅R₅)TiX₃ [R = H, X = Cl ( 3 ); R = Me, X = Cl ( 4 ), Me( 5 )], (η⁵‐C₅H₅)ZrCl₃ ( 6 ), ethylenebis(1‐indenyl) zirconium dichloride ( 7 ) and “Cp‐free” compounds M(NEt₂)₄ [M = Ti ( 8 ), Zr ( 9 )] precursors of the dicarbollide compounds 1 – 2 , under the same conditions (T_p = 50°C, [M] = 1×10^–3 M ; P_E = 1 atm; [N] = 1.75 M ). The 1 ‐TIBA and 2 ‐TIBA catalysts exhibit productivity values greater than the corresponding MAO activated system and incorporate high concentration of the cyclic monomer in the copolymer products (N mol‐% = 38–45). Crystalline blocks of isotactic alternating NENE sequences were identified in the P(E‐co‐N)s copolymers produced by these catalysts by means of ¹³C NMR and DSC analysis. The dicarbollide derivatives 1 and 2 were also efficiently activated with the alkyl aluminium compounds TMA, DIBALH and DEAC at very low Al/M molar ratio (Al/M = 10); the titanium and zirconium cyclopentadienyl derivatives 3 , 4 , 6 , 7 are inactive under these conditions.     

Structures of polyethylene and copolymers of ethylene with 1-octene and oligoethylene produced with the Cp2ZrCl2 and [(C5Me4)SiMe2N(t-Bu)]TiCl2 catalysts

Polymerization of ethylene and copolymerizations of ethylene with 1-octene and oligoethylene having a vinyl end group were conducted with Cp₂ZrCl₂ and [(C₅Me₄)SiMe₂N(t-Bu)]TiCl₂^a as catalysts, and the structures of the resulting polymers were analyzed in detail. The Cp₂ZrCl₂ catalyst combined with methylalumoxane (MAO) gives polyethylene with vinyl, vinylidene and trans-vinylene groups. The use of Al(iC₄H₉)₃/(C₆H₅)₃C · B(C₆F₅)₄ as cocatalyst produces polyethylene predominantly containing trans-vinylene groups. Polyethylene with vinyl end groups was obtained selectively with [(C₅Me₄)SiMe₂N(t-Bu)]TiCl₂ MAO as catalyst. The types and contents of CC double bonds in polyethylene markedly depend upon the metallocene compound as well as as the cocatalyst. In the copolymerizations the [(C₅Me₄)SiMe₂N(t-Bu)]TiCl₂ MAO catalyst system, on the other hand, gives poly[ethylene-co-(1-octene)] with a high content of 1-octene and a polyethylene containing an appreciable amount of long-side-chain oligoethylene.     

Effect of operating conditions on the molecular weight distribution of polyethylene synthesized by soluble metallocene/methylaluminoxane catalysts

Investigations of the effects of polymerization conditions on the molecular weight distribution (MWD) of polyethylene synthesized with soluble metallocene/methylaluminoxane (MAO) catalysts have been performed. The following variables were investigated in this study: catalyst type, polymerization temperature, catalyst concentration, MAO concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MAO with trimethylaluminium (TMA), and of different catalyst activities of polyethylene. Similarities and differences with other published results are highlighted. In all cases, an effort was made to illustrate the significance of the effects by presenting replicate measurements. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂ ( 1 )), its titanium and hafnium analogues (Cp₂TiCl₂ ( 2 ) and Cp₂HfCl₂ ( 3 )), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)₂ZrCl₂ ( 4 ) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(H₄Ind)₂ZrCl₂ ( 5 )). According to a 2² factorial experiment, independent increases in the concentrations of catalyst or MAO causes a decrease in average molecular weight, with no interaction between these two factors. Replacing MAO with TMA at constant overall aluminium concentration causes a drastic decrease in average molecular weight. Extremely high polymerization rates were observed to impart only a slight increase in the breadth of the MWD. The effects of ethylene partial pressure suggest that for the zirconium catalysts, transfer to monomer is the main chain transfer mechanism, while for hafnium catalysts, this is not the case.     

Novel zirconocene catalysts for the production of high molecular weight LLDPE in high-temperature polymerization

Ethylene polymerization and ethylene/α-olefin copolymerization were conducted using diphenylmethylidene(cyclopentadienyl)(fluorenyl)zirconium dichloride (Ph₂C(Cp)(Flu)ZrCl₂) as a catalyst activated with dimethylanilinium tetrakis(pentafluorophenyl)borate (Me₂PhNH·B(C₆F₅)₄)/triisobutylaluminium (i-Bu₃Al) at high temperature and different ethylene pressure. This catalyst produces high molecular weight polyethylene with high activity. The molecular weight of the copolymers hardly decreases with increasing amount of comonomer in the feed. This is attributed to the control of β-H transfer from the propagating chain containing primary inserted comonomer. The occurrence of inner trisubstituted double bonds was confirmed. These bonds are probably formed by dehydrogenation reactions after β-H transfer from the propagating chain followed by ethylene insertion. Therefore, this reaction might play an important role in the production of high molecular weight ethylene/1-hexene copolymers at high temperature. At high ethylene pressure, isomerization reactions from inserted ethylene or primary inserted α-olefin as terminal units, which were observed under low ethylene pressure, can be controlled at low level.     

Polymerization of Methyl Methacrylate with Homogeneous Organopalladium/MMAO Systems

A series of Cp′Pd(η³‐C₃H₅)/MMAO systems was found to proceed in the syndio‐rich polymerization of methyl methacrylate in high yield, while these initiators were completely inert for polymerizations of styrene, ethylene, and isoprene. Among these complexes trimethylsilylated complexes such as (Me₃SiC₅H₄)Pd(η³‐C₃H₅) 4 and [1,3‐(Me₃Si)₂C₅H₃]Pd(η³‐C₃H₅) 5 coupled with MMAO operate the polymerization to result in polymers with narrow molecular weight distributions and high syndiotacticities. In sharp contrast to these systems, alkyl substituted complexes like (tBuC₅H₄)Pd(η³‐C₃H₅) and (tBu₂C₅H₃)Pd(η³‐C₃H₅) produced the polymers in poor yields with low syndiotacticities.