Metallocene Catalysts for 1‐Butene Polymerization

Summary: Isotactic polybutenes of variable isotacticity and melting points of form I in the range 100–125 °C have been prepared with both C₂‐ and C₁‐symmetric zirconocenes. The C₁‐symmetric zirconocenes bearing the bilateral symmetric 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand connected by a dimethylsilandiyl bridge to a substituted indenyl ligand produce iPB with higher molecular mass, up to 400 000 at polymerization temperature of 70 °C in liquid butene. The degree of isotacticity depends on the substitution pattern of the indenyl ligand. The correlations between microstructure and melting points of the crystalline forms I and II of iPB have been defined. Some relevant differences in catalyst selectivity between propylene and 1‐butene polymerizations have been identified.

Linear correlation of melting points of form I and form II in isotactic poly(1‐butene)s of different chain regularities.     

Synthesis of Silica‐Supported Metallocene Catalysts for Olefin Polymerization

Four supported metallocene catalysts were synthesized directly on silica [Si(CpZrCp), Si(CpZrInd), Si(IndZrCp), and Si(IndZrInd)]. The first ligand was chemically tethered to silica (internal ligand) while the second was free from silica (external ligand). The polymerization of ethylene and the copolymerization of ethylene with hexene were investigated. The activity of the catalysts and the molecular weights of the polymers were found to depend on the position of the ligands. In all cases, high‐molecular‐weight polyethylene with moderate polydispersity was obtained. Moreover, the copolymerization of ethylene with hexene provided a homogeneous polymer.

Reaction of ZrCl₄(OC₄H₈)₂ with modified silica (SiCp).     

Ethylene/10‐Undecenoic Acid Copolymers Prepared with Different Metallocene Catalysts

The synthesis and characterisation of several functionalised polyethylenes, obtained by direct copolymerisation of ethylene with 10‐undecenoic acid (UA), have been carried out. Two metallocene complexes of C_s symmetry with different bridges were used: a dimethylsilyl bridge complex Me₂Si(Cp)(Flu)ZrCl₂ and a phenylidene bridge complex Ph₂C(Cp)(Flu)ZrCl₂. For comparison, results obtained with the Cp₂ZrCl₂/MAO system are also reported. The copolymerisation activity depends strongly on the metallocene complex and on the added amount of the UA comonomer. High incorporation levels of up to 7.3 mol‐% are achieved when using the phenylidene bridged catalyst presenting a rigid structure and a wide bite angle. The effect of comonomer content and catalyst geometry on the crystalline structure and mechanical behaviour of the different copolymers are comprehensively discussed.

     

Effects of the Reaction Conditions on the Syndiospecific Polymerization of Propene Promoted by Bis(phenoxyimine) Titanium Catalysts

Summary: The propene polymerization behavior of two typical bis(phenoxyimine) titanium catalysts has been investigated by varying reaction conditions, such as the monomer concentration, the solvent, and the cocatalyst. The experimental results indicate that the stereoregularity and regioregularity of the obtained poly(propylene)s are significantly affected by the reaction conditions. Fractionation of some poly(propylene) samples indicates the formation of macromolecules of different stereoregularity in the same run, suggesting that different active complexes can be generated in situ from these bis(phenoxyimine) titanium precatalysts.

     

Chemistry and Mechanism of Alkene Polymerization Reactions with Metallocene Catalysts

The structures of oligomers formed in ethylene polymerization reactions catalyzed by metallocene complexes are described. Ethylene was homopolymerized and copolymerized with four 1‐alkenes—propene, hex‐1‐ene, hept‐1‐ene, and 3,3‐dimethylbut‐1‐ene—using as catalysts (n‐Bu‐Cp)₂ZrCl₂ and (Me‐Cp)₂ZrCl₂ complexes activated with MAO, both in the absence and in the presence of H₂. Oligomers generated in these reactions are the low molecular weight “tails” of the molecular weight distributions of the respective homopolymers and copolymers. GC analysis affords structural identification of each polymer molecule, albeit a very short one, individually. Analysis of the co‐oligomer structures provides explicit confirmation of the standard mechanisms and kinetics of chain growth and chain transfer reactions. The kinetic features include primary and secondary insertion reactions of 1‐alkene molecules into Cp₂Zr? C and Cp₂Zr? H bonds, chain transfer reactions to AlMe₃, independence of chain growth rate constants on the size of polymer chains attached to active centers, etc. GC analysis also yields detailed information on an unusual feature of metallocene catalysis, namely the formation of saturated macromolecules in ethylene polymerization reactions performed in the absence of H₂.

     

Homogeneous and Heterogeneous Polymerization of ε‐Caprolactone by Neodymium Alkoxides Prepared In Situ

The synthesis of lanthanide(III) alkoxides has been described as a series of reactions for which the control of product purity is difficult. The possibility of using amide complexes, in the presence of alcohol, to produce alkoxides was investigated in homogeneous and heterogeneous media. The polymerization of ε‐caprolactone with alkoxides formed in situ was performed with and without excess alcohol. The latter system was efficient since the polymers were obtained in a short time period, with a well‐controlled molecular weight and low polydispersity. The alcohol functionalized all the polymer chains.     

Synthesis of Heterogeneous Catalysts for Gas Phase Olefin Polymerization: Ziegler‐Natta Catalysts Modified with Indenyl Ligands

A Ziegler‐Natta catalyst was first prepared by reacting a silica/magnesium chloride support with ZrCl₄ in the gas phase, then modified by contacting the supported zirconium with one equivalent of indenyl lithium salt. This new catalyst exhibits high activity when activated with methylaluminoxane both in slurry and gas phase homo‐ and copolymerizations; the polyethylene produced possesses a narrow molar mass distribution. The behavior of such a heterogeneous catalyst has been investigated under different experimental conditions, and its behavior in gas phase copolymerization (with butene) has been compared to that of Ind₂ZrCl₂.     

Effects of Internal and External Donors on the Regio‐ and Stereoselectivity of Active Species in MgCl2‐Supported Catalysts for Propene Polymerization

Poly(propylene)s prepared using MgCl₂‐supported catalysts containing different electron donors have been characterized using temperature rising elution fractionation (TREF), gel permeation chromatography (GPC) and ¹³C NMR analysis. In addition, the regio‐ and stereochemical composition of oligomeric fractions present in selected polymers has been determined. The results indicate that catalysts in which the internal donor is a diether have a relatively narrow distribution of active species, for which the effects of regioselectivity on chain transfer with hydrogen are particularly prominent. The regio‐ and stereoselectivity of active species in catalysts in which the internal donor is diisobutyl phthalate is dependent on the nature of the alkoxysilane external donor used in polymerization. The effects of different alkoxysilanes on polymer tacticity and molecular weight distribution are interpreted in terms of lability of donor coordination in the vicinity of active species, a labile as opposed to a stable donor coordination giving rise to a higher proportion of defect‐rich sequences in the polymer chain. Such species will also give relatively low molecular weight as a result of an increased probability of chain transfer with decreasing regio‐ and stereoselectivity. A broad tacticity and molecular weight distribution in poly(propylene) prepared using monoesters as internal and external donors indicates that these systems may contain not only a significant proportion of labile active species, but also species that do not require the presence of an electron donor in their immediate vicinity for high selectivity.     

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.

     

Living‐Like Polymerization of Propylene with Mixed Metallocene Catalyst Systems

Propylene polymerization was conducted with the Cp₂ZrHCl/B(C₆F₅)₃/[tBuNSiMe₂Flu]TiMe₂ catalyst system with AlOct₃ as a scavenger at –50°C. The polymer obtained displayed a bimodal molar mass distribution. It could be confirmed that the polymer with higher M̄_n was produced from Zr active sites and the polymer with lower M̄_n resulted from Ti active sites. In both fractions, M̄_n was increased linearly with increasing polymerization time. The MWD (M̄_w/M̄_n) values of each fraction were around 1.1. Thus, it could be said that propylene polymerization proceeded in a living manner even with zirconocene active species by using the mixed metallocene system. The living‐like polymerization of propylene with Cp₂ZrMe₂/B(C₆F₅)₃ /Cp*TiCl₃ was also demonstrated at –50°C. Under the reaction between carbon monoxide (CO) and this living polypropylene (PP) at –78°C, it could be found that CO was quantitatively incorporated into living PP.     

Vinylic Polymerization and Copolymerization of Norbornene and Ethene by Homogeneous Chromium(III) Catalysts

Homogeneous chromium(III) catalysts of the type [Cp′CrMeCl]₂/MAO (Cp′ = cyclopentadienyl, pentamethylcyclopentadienyl, indenyl, fluorenyl) have been synthesized to investigate the influence of the electronic nature and the sterical demand of different Cp′‐ligands on the polymerization and copolymerization of norbornene and ethene. In the case of norbornene polymerization the catalyst activity is increased by intensifying the electron donating character of the Cp′‐ligand whereas the sterical demand affects the crystallinity of polynorbornene. The use of the [Cp*CrMeCl]₂ /MAO and the [FluCrMeCl]₂ /MAO catalyst for norbornene‐ethene copolymerization under various polymerization conditions led to the formation of norbornene‐ethene copolymers with different microstructures ranging from statistical norbornene‐ethene copolymers over vinyl‐terminated norbornene macromonomers to norbornene‐ethene block copolymers. Furthermore highly linear, ultra high molecular polyethylene can be obtained using the [FluCrMeCl]₂/MAO catalyst.     

Ethene Co‐ and Terpolymerizations with TIBA‐Protected Norbornenemethanol and TIBA‐Protected Norbornenecarboxylic Acid Using Homogeneous Metallocene/MAO Catalyst Systems

Ethene copolymerizations were carried out with triisobutylaluminum (TIBA)‐protected norbornenemethanol and norbornenecarboxylic acid, respectively, using homogeneous metallocene/MAO catalyst systems. Moreover, ethene terpolymerizations with both polar norbornene derivatives were investigated for the first time. The metallocenes utilized, such as iPr[CpInd]ZrCl₂, iPr[(3‐iPr‐Cp)Ind]ZrCl₂, and iPr[(3‐tert‐But‐Cp)Ind]ZrCl₂ contain ligand frameworks of various sterical demands. The incorporation of polar monomers into the polymer chain was determined by NMR spectroscopic investigations. Kinetic and analytical results of the polymerization experiments revealed increasing activities and a decreasing comonomer incorporation into the polymer chain with an increasing sterical demand of the metallocene ligand. Additionally, the TIBA‐protected norbornenecarboxylic acid shows a lower incorporation rate into the copolymer chain in comparison with the TIBA‐protected norbornenemethanol.     

Access to Heterogeneous Atom‐Transfer Radical Polymerization (ATRP) Catalysts Based on Dipyridylamine and Terpyridine via Ring‐Opening Metathesis Polymerization (ROMP)

A series of dipyridylamine and terpyridine derivatives, norborn‐2‐ene‐5‐di(pyrid‐2‐yl)carbamide ( 1 ), 4′‐(norborn‐2‐ene‐5‐ylmethyleneoxy)terpyridine ( 2 ), di‐(pyrid‐2‐yl)acetamide ( 3 ) and bis(4′‐(norborn‐2‐ene‐5‐ylmethyleneoxy)‐2,2′:6′,2″‐terpyridine)iron(II) hexafluorophosphate ( 4 ) have been synthesized. Linear homopolymers, poly‐ 1 and poly‐ 2 , as well as grafted and coated silica‐ and PS‐DVB‐based polymer supports have been prepared therefrom via ring‐opening metathesis polymerization (ROMP). Monomer 3 , poly‐ 1 , poly‐ 2 have been loaded with Cu (I), Cu (II), Fe (II) as well as Hg (II) salts, respectively. These metal complexes as well as metal loaded supports based on compounds 1 , 2 and 4 have subsequently been used for homogeneous and heterogeneous atom‐transfer radical polymerization (ATRP) of styrene. Systems based on poly‐ 2 coated silica loaded with Cu (I) were found to work best. Under optimum conditions, polystyrene was obtained in 30% yield with polydispersity indices < 1.6 within 2 h and with a remaining metal content in the ng/g (ppb)‐range.     

Stereoblock Copolymerization of Propylene and Methyl Methacrylate with Single‐Site Metallocene Catalysts

Sequential stereoblock copolymerization of propylene (P) and methyl methacrylate (MMA) using Group IV single‐site metallocene catalysts efficiently produces PP‐b‐PMMA stereodiblock copolymers. When activated with B(C₆F₅)₃, C₂‐symmetric rac‐Et(Ind)₂ZrMe₂ yields isotactic‐PP‐b‐isotactic‐PMMA diblock copolymer, whereas C_s‐symmetric Me₂Si(C₅Me₄)(tBuN)TiMe₂ affords atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer. In the copolymerization catalyzed by the C₂‐symmetric catalyst, a very small amount of PMMA homopolymer formed can be removed from the copolymer by extracting the bulk polymer product with boiling methylene chloride. However, separation of isotactic PP formed if any from the copolymer product approves very difficult, due to very similar solubility between the diblock copolymer and isotactic PP homopolymer in various high‐boiling chlorinated solvents. On the other hand, in the copolymerization catalyzed by the C_s‐symmetric catalyst, both PMMA and atactic PP homopolymers formed in small weight fractions during the copolymerization can be successfully removed from the predominant copolymer product by solvent extraction using boiling heptane. After successful removal of both homopolymers, for example, an atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer has high molecular weight (M _n = 21 100), narrow molecular weight distribution (PDI = 1.08), high PMMA incorporation (33.8 mol‐% of PMMA), and moderate syndiotacticity for the PMMA block ([rr] ≈ 80%). Furthermore, the comonomer composition in the copolymer can be controlled by the time for propylene polymerization and the conversion of MMA. A pronounced activator effect is observed; when the same C_s‐symmetric catalyst is activated with Ph₃CB(C₆F₅)₄, formation of homopolymers is predominated.

     

Reaction Calorimetric Investigation of the Propylene Slurry Phase Polymerization with a Silica‐Supported Metallocene/MAO Catalyst

The slurry phase polymerization of propylene with a silica‐supported metallocene/MAO catalyst was kinetically investigated by two different methods at several polymerization conditions. Heat flow calorimetry on the one hand as an innovative method and flowmeter‐technique on the other hand as a well established classical method led to consistent kinetic profiles. Thus, the principle applicability of the heat flow calorimetry for the kinetic investigation of the described polymerization process was demonstrated, contrary to some expectations in literature. Furthermore, the sensitivity of the calorimetric method is shown and polymerization heats for the slurry process have been determined.     

Homogeneous metallocene/MAO‐catalyzed polymerizations of polar norbornene derivatives: copolymerizations using ethene,and terpolymerizations using ethene and norbornene

Terpolymerizations of norbornene derivatives containing different functional substituents were carried out with ethene and norbornene using the homogeneous catalyst system iPr[CpInd]ZrCl₂/MAO. The norbornene derivatives 5‐norbornene‐2‐methanol and 5‐norbornene‐2‐carboxylic acid were prereacted with triisobutylaluminium to prevent the deactivation of the catalyst. ¹³C NMR studies revealed the composition of the polymer. The incorporation rate was 5–12 mol‐% at a content of 50 mol‐% of the norbornene derivative in the monomer feedstock. IR‐GPC coupled experiments confirmed the homogeneous composition of the polymer. In addition, we investigated the ethene copolymerization and the ethene/norbornene terpolymerization using the trialkylsilyl protected norbornene derivates such as 5‐norbornene‐2‐methyleneoxytriethylsilane and 5‐norbornene‐2‐methyleneoxy‐tert‐butyldimethylsilane. These norbornene derivatives reveal an incorporation rate of 5–6 mol‐% in the polymer at a content of 20 mol‐% in the monomer feedstock.     

Novel Zirconocene Hydride Complexes in Homogeneous and in SiO2‐Supported Olefin‐Polymerization Catalysts Modified with Diisobutylaluminum Hydride or Triisobutylaluminum

Reactive species in SiO₂‐supported, zirconocene‐based olefin‐polymerization catalysts have been characterized by comparison of their UV‐vis spectra with those of related, NMR‐spectroscopically identified catalyst species in homogeneous solution. Neutral zirconocene dihydride complexes are found to arise in hydrocarbon solutions as well as on SiO₂ supports when catalyst systems that contain rac‐Me₂Si(ind)₂ZrCl₂ and methylaluminoxane (MAO) are modified by addition of diisobutylaluminum hydride or triisobutylaluminum. These complexes, tentatively formulated as adducts with Lewis‐acidic alkylaluminum species AlR₂X, rac‐Me₂Si(ind)₂ZrH₂ · {nAlR₂X}, are reconverted into the initial reactive zirconocene cations upon addition of isobutene to these reaction systems.

     

Synthesis and Properties of Syndiotactic Poly(propylene)/Carbon Nanofiber and Nanotube Composites Prepared by in situ Polymerization with Metallocene/MAO Catalysts

Summary: The preparation of syndiotactic poly(propylene) (sPP) nanocomposites with multi‐walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), and carbon black (CB) as fillers was accomplished by the in situ polymerization of propylene with a metallocene/methylalumoxane (MAO) catalyst. Different pre‐treatments were applied to achieve a homogeneous dispersion of the fillers in the matrix. The resulting nanocomposites were investigated with respect to their properties, which were then compared to those of the pure polymer and among each other. The thermal stability of the nanocomposites was slightly enhanced compared to the pure polymer. In addition, the yield strength of the nanocomposites could be slightly raised in comparison to the neat sPP. The most significant influence of the nanofillers was detected on the crystallization behavior. The crystallization temperature was increased with rising filler content in all cases. Moreover, the half‐time of crystallization was significantly reduced in the nanocomposites. The rate constant of crystallization was also greatly enhanced for all nanocomposites as compared to the neat sPP. The nanofillers acted, therefore, as nucleating agents. This effect was most pronounced in the case of MWNTs as fillers.

Influence of the type of filler on the degradation temperature (temperature of maximum weight loss).     

A Study of Copolymerization of 1‐Hexene with 2,5‐Norbornadiene Using Metallocene Catalysts

Summary: Copolymerization of 1‐hexene with a symmetrical diene, namely 2,5‐norbornadiene was studied using four different metallocene catalysts. Copolymerization was found to occur exclusively through one of the two equally reactive endocyclic double bonds with all the four catalysts. Copolymerization results in low molecular weight oligomers with the number average molecular weight ( ) varying from 1 000–3 000. End group analysis of the co‐oligomers revealed that the β‐hydrogen transfer after 2,1 insertion also occurs in the presence of highly regiospecific catalysts. The regio errors were also found to depend on various reaction parameters such as polymerization time, Al/Zr mol ratio, metallocene concentration and polymerization temperature.

Plots of variation in end groups and NBD incorporation with time.