Comparison of zirconocene and hafnocene catalysts for the polymerization of ethylene and 1-butene

In polymerizations with ethylene and copolymerizations with ethylene and 1-butene the catalyst system rac-ethylene(bisindenyl)zirconium dichloride (rac-Et(Ind)₂ZrCl₂)/methylaluminoxane (MAO) shows lower activities than the analogous Hafnium catalyst. The polymers obtained with the hafnium-catalyst have average molar masses up to ten times higher than those synthesized with the zirconium compound. It is remarkable that rac-Et(Ind)₂HfCl₂ is much more able to incorporate the comonomer. This is expressed by the copolymerization parameters. At a polymerization temperature of +30°C, e.g., the r₁-parameter of the hafnium-catalyst is 5,4, the corresponding parameter of the zirconium catalyst is 19,4. Polymerizations with a catalyst mixture rac-Et(Ind)₂(Hf, Zr)Cl₂ (containing 95% rac-Et(Ind)₂HfCl₂ and 5% rac-Et(Ind)₂ZrCl₂) show that each catalyst compound produces its own polymer independently.     

The effect of temperature on the polymerization of propene with dimethylsilylbis(1-indenyl)zirconium dichloride/methylaluminoxane and dimethylsilylbis(2-methyl-1-indenyl)zirconium dichloride/methylaluminoxane. Modeling of kinetics

Propene polymerizations were performed using the two ansa-zirconocene catalyst systems dimethylsilylbis(1-indenyl)zirconium dichloride/methylaluminoxane and dimethylsilylbis(2-methyl-1-indenyl)zirconium dichloride/methylaluminoxane. The polymerization rate was observed by continuously monitoring the monomer consumption. Reaction rate profiles were obtained in the temperature range from 40°C to 130°C at pressures between 1 and 2.5 bar and catalyst concentrations from 4.6 · 10⁻⁶ M to 4.2 · 10⁻⁵ M. Isotacticity, as measured by NMR, melting point and molecular weight decreases markedly at higher temperatures. Small amounts of 1,3-inserted monomer (<1 mol-%) was observed at polymerization temperatures above 80°C. No 2,1-inserted monomer was detected. A kinetic model was developed that describes the polymerization rate for Me₂Si(Ind)₂ZrCl₂ as the catalyst over the entire observed temperature range, and the polymerization rate for Me₂Si(2-Me-Ind)₂ZrCl₂ in a limited temperature range. The model includes an activation reaction, latent sites that may revert to active sites and a permanent deactivation that is second order with respect to the active sites. The activation energy for the propagation reaction was found to be 37 kJ/mol for Me₂Si(Ind)₂ZrCl₂ and 32 kJ/mol for Me₂Si(2-Me-Ind)₂ZrCl₂/MAO. Several kinetic models are compared and discussed.     

Polymerization behavior of α-olefins with rac- and meso-type ansa-metallocene catalysts: Effects of cocatalyst and metallocene ligand

Polymerizations of propene, 1-butene and 1-hexene were conducted with a mixture of rac- and meso-[dimethylsilylenebis(2-methylindenyl)]zirconium dichloride ( 1 ) combined with methylaluminoxane (MAO), triethylaluminium (AlEt₃)/triphenylcarbenium tetrakis(pentafluorophenyl)borate ( 2 ) or triisobutylaluminium (AliBu₃)/ 2 as a cocatalyst. The polymerization profiles of propene with rac- 1 and meso- 1 were determined from the rate of overall propene consumption and the fractions of isotactic and atactic polymers which were sampled during polymerization. An induction time to reach the maximum R_p (rate of polymerization) followed by gradual decay was observed in the case of using the systems rac-, meso- 1 –MAO and rac- 1 –AliBu₃/ 2 . Besides, a rapid drop of R_p from the initial value was found when using AlEt₃/ 2 . Molecular weights of the isotactic and atactic polymers sampled do not change during polymerization, and it is suggested that the change of [C*] (number of active centers) is reflected in the profiles of R_p. The rate ratio of rac- 1 to meso- 1 (R_p(rac)/R_p(meso)) in propene and 1-butene polymerizations decreases in the following order: AlEt₃/ 2 > AliBu₃/ 2 > MAO. In the case of 1-hexene polymerization, the highest R_p(rac)/R_p(meso) value was obtained. This result indicates that the coordination of 1-hexene to the sterically hindered site of meso- 1 is difficult compared with propene and 1-butene.     

Influence of Temperature on the Regioselectivity of Highly Isospecific C2‐Symmetric Zirconocenes in Propene Polymerization

The influence of polymerization temperature (T_p) on the isoselectivity of the C₂‐symmetric zirconocene catalysts rac‐Me₂Si(2‐Me‐4‐PhInd)₂ZrCl₂/MAO ( 1 /MAO), rac‐Me(CyHex)Si[2‐Me‐4‐(4′‐^tBuPh)Ind]₂ZrCl₂/MAO ( 2 /MAO), and rac‐Me₂Si(2‐Me‐4‐Ph‐5‐OMe‐6‐^tBuInd)₂ZrCl₂/MAO ( 3 /MAO) in propene polymerization, has been studied in the range 30–85 °C in liquid propene. All three catalysts show similar polymerization activity versus T_p profiles, with a maximum at about 60 °C ( 1 /MAO), 80?85 °C ( 2 /MAO), and 70?80 °C ( 3 /MAO). 3 /MAO shows the highest activity at any T_p. The isoselectivity of the three catalysts remains high and approximately constant in the whole T_p range investigated, as indicated by the roughly constant isotactic triad content (mm > 99%) of the produced isotactic poly(propylene)s (iPP). Surprisingly, the melting and crystallization temperatures of iPP increase by increasing T_P. This behavior is due to an unexpected increase in catalyst regioselectivity with T_p, as indicated by the decrease in the content of 2,1‐erythro regiodefects (2,1e) at increasing T_p. A mechanism involving a metal cation–2,1‐coordinated propene slow state is proposed to account for this so far unnoticed behavior. The correlations between T_p, T_m, and [2,1e] are defined for these three catalysts.

     

Kinetic Studies of the Copolymerization of Ethylene with Norbornene by ansa‐Zirconocene/Methylaluminoxane Catalysts: Evidence of a Long‐Lasting “Quasi‐Living” Initial Period

Copolymers of ethylene (E) with norbornene (N) were synthesized using the catalysts rac‐Et(Ind)₂ZrCl₂/MAO ( 1 ), 90%rac/10%meso‐Et(4,7‐Me₂Ind)₂ZrCl₂/MAO ( 2 ), and rac‐H₂C(3‐t‐BuInd)₂ZrCl₂/MAO ( 3 ). Catalyst activity, molar mass (MM), and copolymer composition were studied as a function of time. The polymers showed an unusually narrow molar mass distribution (MMD) and a significant increase of their MM with time for up to one hour, suggesting a “quasi‐living” polymerization at 30 °C. The experimental data were fitted to kinetic equations and the propagation and transfer reactions were described in quantitative terms. Norbornene greatly depressed the propagation rate, along with the chain transfer rate. The more sterically hindered catalysts of the series showed lower propagation and chain transfer turnover frequency than 1 and yielded polymers with a low ( 2 ) to very low ( 3 ) norbornene content. The presence of norbornene in solution seemed to be one of the main factors responsible for the observed “quasi‐living” character of the copolymerization, probably due to coordination of norbornene to the active site. Time‐resolved kinetic studies also allowed for the calculation of the fraction of active metal centers, ranging from 56% ( 3 ) and 66–68% ( 1 ) to 94% ( 2 ) of the total zirconium present, depending on catalyst structure.

Left: molar mass (top) and polydispersity (bottom) as a function of the normalized polymer yield. The dashed line is the theoretical curve for ideal living polymerization. Catalysts 1 (□), 2 (?), and 3 (○) at N/E ratio 12.5 and catalyst 1 (?) at N/E ratio 28.4. Right: enlargement of the low yield section.     

Microstructure of Metallocene‐Catalyzed Propene/1‐Pentene Copolymers

The microstructure of propene/1‐pentene copolymers prepared with the metallocene catalysts rac‐Et(Ind)₂ZrCl₂ and rac‐Me₂Si(2‐MeBenz‐[e]Ind)₂ZrCl₂ was studied by ¹³C NMR spectroscopy. Both catalysts lead to the formation of random copolymers although rac‐Me₂Si(2‐MeBenz‐[e]Ind)₂ZrCl₂ favors a somewhat higher incorporation of 1‐pentene than rac‐Et(Ind)₂ZrCl₂. Surprisingly, the presence of 1‐pentene has a significant influence on the stereoregularity of the copolymers formed with rac‐Me₂Si(2‐MeBenz‐[e]Ind)₂ZrCl₂. Propene/1‐pentene copolymers produced with rac‐Et(Ind)₂ZrCl₂ retained the isotacticity of polypropene and featured only the stereoerror pentads mmmr, mmrr and mrrm typical for enantiomorphic site control. However, propene/1‐pentene copolymers obtained with rac‐Me₂Si(2‐MeBenz‐[e]Ind)₂ZrCl₂ are characterized by a gradual loss of tacticity which is concurrent with an increase in the amount of all irregular pentads. It is proposed that the presence of 1‐pentene can lead to a reversible loss of stereocontrol over a whole sequence of inserted monomer units. Subsequently, more or less atactic blocks are formed besides isotactic blocks.

Percentage of 1‐pentene in the copolymer as function of the feed composition for 1‐pentene/propene copolymers prepared with EI and MBI .     

Metallocene-catalyzed propene/1-hexene copolymerization: Influence of amount and bulkiness of cocatalyst and of solvent polarity

An investigation on the effect of the variation of the amount and the structure of cocatalyst and of polarity of the solvent on propene/1-hexene copolymerization with the isospecific catalyst rac-[ethylenebis(1-indenyl)]zirconium dichloride (rac-(EBI)ZrCl₂) was performed. A noticeable enhancement of copolymerization activity and a moderate increase of the molecular weights were observed with increasing MAO/Zr ratio as well as upon mixing methylaluminoxane (MAO) and AliBu₃, while isotacticity and comonomer composition are not affected by such variations. On the other hand, a gradual increase in CH₂Cl₂ content in the CH₂Cl₂/toluene solvent mixture, which greatly increases the copolymerization activity, noticeably influences the molecular weight and sensibly enhances the 1-hexene comonomer content. The formation of cationic active species, separated from their counterions, might be the cause of the better accessibility of the active sites for the bulkier comonomer.     

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.     

Polymerization and asymmetric oligomerization of allylsilanes using chiral ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium and -hafnium complexes

Stereospecific polymerization and asymmetric oligomerization of allylsilanes were investigated by using C_2- and C_s- symmetric zirconocene catalysts. Isotactic and syndiotactic poly(allylsilane)s were produced with rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride ( 1 ) and diphenylmethylene(η⁵-cyclopentadienyl)(η⁵-fluorenyl)zirconium dichloride ( 6 ), respectively. Bulky allylsilanes afforded optically active oligomers in the reactions using optically active ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium and -hafnium complexes.     

Effect of Different Aluminum Alkyls on the Metallocene/Methylaluminoxane Catalyzed Polymerization of Higher α‐Olefins and Styrene

The replacement of MAO by different aluminum alkyls in the polymerization of higher α‐olefins catalyzed by rac‐EtInd₂ZrCl₂ and rac‐(CH₃)₂CInd₂ZrCl₂ and in the syndiospecific polymerization of styrene with CpTiCl₃ as a catalyst was studied. An activating effect for all catalysts investigated was found when adding TIBA, while TEA and THA have a deactivating effect. It was found that more than half of the expensive methylaluminoxane can be successfully replaced by TIBA without deleterious effects on the catalyst activity and the properties of the polymers synthesized during higher α‐olefin polymerization catalyzed by rac‐EtInd₂ZrCl₂ and rac‐(CH₃)₂CInd₂ZrCl₂.

     

Polymerization of 5-vinyl-2-norbornene with TiCl3 and alkylaluminium catalysts

The polymerization of 5-vinyl-2-norbornene (VNB) with TiCl₃-alkylaluminium catalysts proceeded selectively via the vinyl group; the vinylene group in the bicyclic ring did not participate in the polymerization. The rate of polymerization of VNB was slow due to the steric effect of the side chain, but effects of the types of TiCl₃ and alkylaluminiums on the polymerization were observed. Hydrogen-activated TiCl₃ plus Al(C₂H₅)₃ revealed the highest catalytic activities among the catalysts examined. The monomer-isomerization copolymerization of VNB and trans-2-butene with TiCl₃/Al(i-C₄H₉)₃ afforded a copolymer consisting of VNB and 1-butene units. The rate of copolymerization of VNB and 1-butene was larger than the monomer-isomerization copolymerization of VNB and trans-2-butene. Copolymers rich in VNB units were produced easily by monomerisomerization copolymerization of VNB and trans-2-butene.     

U.V./visible spectroscopic study of the rac-Et(Ind)2ZrCl2/MAO olefin polymerization catalytic system, 1. Investigation in toluene

The U.V./visible spectrum of the rac-Et(Ind)₂ZrCl₂/methylaluminoxane (MAO) catalytic system in toluene at 20°C, which is related to the ‘ligand to metal charge transfer’ bands (LMCT), varies greatly with the MAO/Zr ratio (mole ratio Al/Zr). Indeed, for MAO/Zr ratios <30, a hypsochromic shift of the absorption band is observed and is explained by monomethylation of the metallocene dichloride. Conversely, a progressive bathochromic shift of the absorption band is observed for MAO/Zr ratios ranging from 30 to 2000. This behavior is interpreted by the formation of an increasing proportion of dissociated to associated zirconocenium species [L₂ZrX]⁺ [MAOCl]⁻. The λ_max changes of the U.V./visible spectrum of the rac-Et(Ind)₂ZrCl₂/MAO system were correlated with its catalytic activity for the hex-1-ene polymerization. Moreover, a conductimetric study performed on the rac-Et(Ind)₂ZrCl₂/MAO system for different MAO/Zr ratios is also in agreement with the formation of ionic species with different nature and ionization degrees.     

Activation of iPr(CpFluo)ZrCl2 by methylaluminoxane, 4. UV/visible spectroscopic study in hydrocarbon and chlorinated media

The olefin polymerization catalytic system iPr(CpFluo)ZrCl₂/MAO was investigated by UV/visible absorption spectroscopy in solvents of various polarity at 20°C. It was shown that the UV/visible main absorption band of the zirconocene, which can be related to the Ligand to Metal Charge Transfer bands (LMCT), varies greatly upon incremental addition of MAO or AlMe₃. For low [AlMe₃]/[Zr] or [MAO]/[Zr] ratios (Al : Zr < 20), a hypsochromic shift of the initial iPr(CpFluo)ZrCl₂ absorption band, corresponding to the monomethylation of the metallocene dichloride, is observed. On the contrary, for higher Al/Zr ratios (with MAO uniquely), a bathochromic shift of the metallocene absorption band proceeding in two distinct steps is observed. These two steps are interpreted by the successive formation of associated (or complexed) and dissociated active zirconocenium species. The changes in the absorption spectra of iPr(CpFluo)ZrCl₂/MAO are correlated with the initial activity observed for hex-1-ene polymerization in the various solvents examined. In the same way, the catalytic activity loss with polymerization time observed at elevated Al/Zr ratios (>1000) with this system is attributed to a reduction of active species concentration, as indicated by the intensity decrease of their UV/visible absorption band. This reveals the low stability of active species formed from iPr(CpFluo)ZrCl₂ compared to those from rac-Et(Ind)₂ZrCl₂.     

ω-Chloro-α-olefins as co- and termonomers for the synthesis of functional polyolefins

This paper deals with the use of ω-chloro-α-olefins as monomers for the synthesis of chloro-functional homo-, co- and terpolyolefins in the presence of (IVb) metallocene/MAO catalytic systems. Two ω-chloro-α-olefins, 11-chloroundec-1-ene and 5-chloropent-1-ene, were tested. It was found that only the functional olefin containing the longer aliphatic spacer between the alkene function and the halide polymerizes readily in the presence of the rac-Et(Ind)₂ZrCl₂/MAO catalyst. The drastic influence of the reaction solvent on the chloro-olefin reactivity and (co)polymerization behavior is examined and discussed. Finally, the terpolymerization of 11-chloroundec-1-ene with ethylene and propene is investigated. In a second part, the quantitative preparation, by derivatization of 11-chloroundec-1-ene units, of functional polyolefins bearing benzoate, hydroxy or azido functions as side groups is described. The corresponding functional polymers are characterized by IR and ¹H NMR spectroscopy.     

Polymerization of bicyclic acetals, 18. Propagation processes in the cationic ring-opening polymerization of 6,8-dioxabicyclo[3.2.1]octane derivatives having a 3(e)-benzyloxy group as a common substituent

Polymerizations of two bicyclic acetals, rac-2(a), 3(e)-bis(benzyloxy)-6,8-dioxabicyclo-[3.2.1]octane ( 1d ) and the D -enantiomer of 2(a),3(e),4(a)-tris(benzyloxy)-6,8-dioxabicyclo[3.2.1]octane (1,6-anhydro-2,3,4-tri-O-benzyl-β-D -allopyranose) ( 1e ) were investigated in dichloromethane as solvent at different temperatures ranging from ?90 to ?30°C. The polymerization of 1d gave polyacetals containing both the trans- and cis-2,6-linked tetrahydropyran units ( 2d and 3d ) over the entire temperature range. The proportion of 3d increased up to 73% with decreasing initial monomer concentration at higher polymerization temperature. On the other hand, the polymerization of 1e gave rise to polymers exclusively or predominantly composed of trans units ( 2e ), the content of cis units ( 3e ) being at most 25%. On the basis of these results, along with the previous experimental results on the polymerizations of three other relevant bicyclic acetals ( 1a – c ), the marked substituent effect as well as the influence of polymerization temperature and initial monomer concentration on the stereochemical course of the polymerization are discussed in terms of a propagation mechanism involving an oxonium exchange reaction at the penultimate unit of a growing chain.     

Ethene polymerization with a poly(styrene-co-divinylbenzene) beads supported rac-Ph2Si(Ind)2ZrCl2 catalyst

Poly(styrene-co-divinylbenzene) beads supported rac-Ph₂Si(Ind)₂ZrCl₂ was prepared and tested as a catalyst for ethene polymerization using methylaluminoxane (MAO) as a cocatalyst. At a polymerization temperature below 100°C, the catalyst showed pretty high activity to give polyethene beads replicating the shape of the carrier. With increasing polymerization temperature up to 150°C, the catalyst activity increased drastically but the spherical shape of polyethene disappeared due to the melting. From the plots of apparent activity against polymerization temperature, it was suggested that the polymerization below 100°C is more or less controlled by monomer diffusion through the crystalline polyethene films.     

New Group IV Metallocene Systems Active in the Copolymerization of α‐Olefins and Conjugated Dienes

Summary: Rac‐[CH₂(2,4‐di‐tert‐butyl‐cyclopentadienyl)₂]ZrCl₂ ( 1 ), rac‐[CH₂(3‐tert‐butyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 2 ), rac‐[CH₂(3‐isopropyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 3 ), and rac‐[CH₂(3‐methyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 4 ) were synthesized and characterized by ¹H NMR analysis. These C₂ symmetric ansa‐zirconocenes, activated with methylaluminoxane (MAO), were able to promote copolymerization of ethene and 1,3‐butadiene. The structures of the copolymers are strongly influenced by zirconocene utilized as catalytic precursor. Depending on the reaction conditions polyethenes containing cyclopropane and cyclopentane rings, or 1,1‐ 1,3‐butadiene inserted constitutional units, were obtained.

Structures of the synthesized catalytic precursors.     

Homo- and copolymerization of ethylene at high temperature with cationic zirconocene catalysts

Homo- and copolymerization of ethylene with 1-hexene were conducted at different temperature and ethylene pressure with several zirconocenes activated with dimethylanilinium tetrakis(pentafluorophenyl)borate (Me₂PhNH·B(C₆F₅)₄)/triisobutylaluminium (i-Bu₃Al) to study the effect of ligand structure and polymerization conditions on catalytic activity, molecular weight and chain transfer reactions. At high temperature and low ethylene pressure, rac-ethylene(bisindenyl)zirconium dichloride (rac-Et(Ind)₂ZrCl₂) activated with Me₂PhNH·B(C₆F₅)₄/i-Bu₃Al initially gives a highly active catalyst that is rapidly deactivated. trans-Vinylene double bonds, which were not formed at low temperature, were detected in polyethylene synthesized at high temperature and low ethylene pressure. They reasonably arise from β-H transfer after isomerization reaction. The molecular weight of ethylene/1-hexene copolymers decreases with increasing 1-hexene feed, followed by the formation of vinylidene end groups. This reveals that β-H transfer from propagating chains containing primary inserted 1-hexene as a terminal unit is predominant. This reaction is influenced by the ligand structure. At high temperature and high ethylene pressure, trans-vinylene and vinylidene contents decrease and the vinyl content increases, indicating that the high ethylene pressure controls β-H transfer after isomerization reaction.     

Addition of Lewis bases and acids. Effect on α‐olefins polymerization with soluble metallocenes, 1 Ethylene

In Part 1 of this series the soluble system rac‐EtInd₂ZrCl₂/methylaluminoxane (MAO) was studied in ethylene polymerization with additives. Lewis acids and bases such as AlCl₃ and ethyl benzoate (EB) were used to evaluate their effects on polymerization kinetics, catalyst activities, and polymer properties. The results obtained showed a clear trend to improve the performance when AlCl₃ was added. The Lewis base might be acting in different ways on the active sites. The activity is strongly decreased with EB. To further elucidate some of the different aspects of metallocene activation, a UV/Visible spectroscopic study was carried out, with toluene as solvent. It was possible to correlate the catalytic activity with AlCl₃ as additive to changes in the intensity of the ligand to metal charge transfer (LMCT) bands of EtInd₂ZrCl₂ when MAO/AlCl₃ was the coactivator.     

Influence of indenyl ligand substitution pattern on metallocene-catalyzed propene copolymerization with 1-octene

Copolymerization of propene with 1-octene (1 mol/1 mol) was performed in toluene at 40°C in the presence of homogeneous methylaluminoxane (MAO)-activated ansa-metallocenes in order to study the role of benzannelation and 2-methyl-substitution of the silylene-bridged bisindenyl ligand on comonomer incoporation, molecular mass, molecular mass distribution, and end groups. While 2-methyl-substitution promoted higher degree of polymerization without affecting copolymerization parameters, benzannelation improved markedly 1-octene incorporation. Only with MAO-activated rac-Me₂Si(2-MeBenz[e]Ind)₂ZrCl₂ catalysts vinylidene end groups were formed exclusively. Molecular weight distribution remains narrow in all experiments.