From Multisite Polymerization Catalysis to Sustainable Materials and

Review pubs.acs.org/CR

From Multisite Polymerization Catalysis to Sustainable Materials and All-Polyolefin Composites Markus Stürzel,† Shahram Mihan,‡ and Rolf Mülhaupt*,†,§ †

Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, Albert-Ludwigs-University Freiburg, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany ‡ Basell Polyolefine GmbH, Catalyst Systems, Industriepark Hoechst, D-65926 Frankfurt am Main, Germany § Sustainability Center Freiburg, Ecker-Strasse 4, D-79104 Freiburg, Germany 1. INTRODUCTION 1.1. Polyolefins and Sustainability

Since the pioneering advances of Karl Ziegler and Giulio Natta during the early 1950s, a continuing series of breakthroughs in research and industrial innovations have greatly simplified polyolefin production by enabling industrial tailoring of advanced polyolefin materials in solvent-free catalytic polymerization processes.1−8 In 2015, polyolefin materials such as polyethylene and polypropylene account for almost half of the 300 million tons of the global plastics production. This outstanding economic success reflects the significant progress made in catalysis, reaction engineering, and polyolefin processing by greatly improving manufacturing, performance, and economy of polyolefin products.8−13 Today, polyolefins are an integral part of daily life. They meet the need of the rapidly growing world population for cost-, resource-, and energyefficient, environmentally benign materials with low greenhouse gas emissions (“carbon footprint”), low weight, and versatility in terms of tailoring properties, applications, and recycling. The highly diversified applications of polyolefins range from packaging, enabling safe supply of food and medicine, to lightweight engineering plastics for automotive and architectural applications, textiles, rubbers, electrical and thermal insulation, as well as earthquake-proof pipes for safe transport of water and gas. Advanced polyolefins meet the demands of both green chemistry and sustainable development in an ideal way.14−16 The life cycle of polyolefins is illustrated in Figure 1. Produced in energy-efficient, exothermic catalytic processes that generate a considerable amount of energy (heat of polymerization is 3.34 MJ/kg), polyolefins are high molar-mass hydrocarbon materials (“solid modification of oil”) that possess an oil-like energy content. Hence, recycled polyolefin wastes serve as valuable sources of energy and hydrocarbon feedstocks. On thermal cleavage of C−C bonds above 300 °C, polyolefins readily degrade to form low molar-mass hydrocarbons, thus recycling oil and gas feedstocks in essentially quantitative yields. Polyolefins meet the demands of sustainable development, which translates to meeting the demands of the present without compromising the ability of future generations to meet their own demands. As summarized in Table 1, progress in

CONTENTS 1. Introduction 1.1. Polyolefins and Sustainability 1.2. Multisite Polymerization Catalysis 2. Homogeneous Multisite Polyolefin Catalysis 2.1. Polyolefins with a Bimodal Molar-Mass Distribution 2.2. Branched Polyethylene Reactor Blends 2.3. Chain Shuttling and Molecular Switching of Sites 2.3.1. Polyethylene Block Copolymers 2.3.2. Stereoblock Polypropylene 3. Heterogeneous Multisite Polyolefin Catalysis 3.1. Ziegler−Natta Hybrids and MgCl 2-Supported Multisite Catalysts 3.2. Silica-Supported Multisite Catalysts 3.3. Tailored Supports for Multisite Catalysts 4. Polyolefin Tandem Catalysis 5. Perspectives of Multisite Olefin Polymerization Catalysis 5.1. High-Performance Commodity Polyethylenes 5.2. Nanostructured Polyolefins as Thermoplastic Elastomers 5.3. All-Polyolefin Composites 6. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments Glossary References © 2015 American Chemical Society

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Special Issue: Frontiers in Macromolecular and Supramolecular Science Received: May 25, 2015 Published: September 16, 2015 1398

DOI: 10.1021/acs.chemrev.5b00310 Chem. Rev. 2016, 116, 1398−1433

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paper and other packaging materials, LLDPE has a lower density, thus reducing weight and carbon dioxide emissions with respect to transportation. Moreover, unlike paper, thin and robust LLDPE packaging films are readily fabricated by blow molding. As illustrated in Figure 2, remarkable progress has been made in achieving control of molecular architectures. In the 20th

Figure 1. Polyolefin life cycle as illustrated for polyethylene.

polyolefin catalysis and reaction engineering brings substantial benefits for both society and sustainable development. Whereas free-radical polymerization of ethylene produces branched polyethylene at temperatures well above 150 °C and a high ethylene pressure exceeding 1000 MPa, the polymerization catalysts developed by Karl Ziegler and the Phillips Petroleum Company during the early 1950s enabled ethylene polymerization at ambient temperature and pressure. In contrast to free-radical polymerization, ethylene homo- and copolymerization afford much better control of chain branching. The spectrum of polyolefin materials produced by catalytic polymerization ranges from linear high-density polyethylene (HDPE) for pipes to linear short-chain-branched polyethylene (LLDPE) for packaging, and ethylene/propylene/diene copolymers (EPDM) for rubbers. During the late 1970s, in the aftermath of the first oil crisis, energy efficiency became a successful marketing tool, employed for the first time by the Union Carbide Corporation, to promote LLDPE as an alternative to LDPE. Within two decades after the first oil crisis, LLDPE production surged and crossed the million ton annual production mark, although LLDPE had been commercially available since the 1950s. In comparison to

Figure 2. Polyolefin catalysis: from microstructures to granules and multiphase polyolefins.

century, the major focus has been placed on controlling molecular architectures and polymer morphology. At the beginning of the 21st century, the focus in polyolefin development is gradually shifting toward the development of cost-efficient high-performance commodity polyolefins by controlling their nanostructures and by building functional structural hierarchies in solvent-free polymerization reactions

Table 1. Milestones and Impact of Advances in Polyolefin Catalysis decade

milestones in catalyst technology

1950s

catalytic low-pressure olefin polymerization

1960s

highly active leave-in catalysts

1960s 1970s

supported catalysts for gas-phase polymerization highly active and stereospecific catalysts based on Lewis-base-modified, supported MgCl2/TiCl4 catalysts linear low-density polyethylene (LLDPE) by copolymerization of ethylene with 1-olefins

1970s 1980s 1980s 1990s 1990s 1990s 2000s 2000s 2000s

tailor-made polypropylene reactor blend technology and cascade reactors particle-forming olefin polymerization and reactor granule technology single-site catalyst technology chain-walking polymerization multizone polymerization reactor polyolefins from bioethanol and polyolefin wastes advanced multisite polyolefin catalysis

impact on sustainability energy- and resource-efficient production of versatile hydrocarbon materials such as polypropylene, polyethylene, and EPDM rubber high yield and high atom economy; catalyst residues can be left in the polymer and do not require removal by solvent extraction solvent-free, environmentally benign catalytic polymerization processes no formation of byproducts such as waxes and atactic polypropylene; no purification by solvent extraction; no landfill of wastes highly energy- and resource-efficient LLDPE packaging materials as alternatives to LDPE, which is produced by free-radical polymerization at elevated temperatures (>150 °C) and high pressure (>1000 MPa) substitution of environmentally less benign polymers and weight savings in automotive engineering improved performance of polyolefins such as PE100 pipes, PE packaging, and PP engineering plastics energy savings by eliminating pelletizing melt extrusion and melt compounding tailor-made polyolefins with unprecedented control of microstructure and properties LLDPE produced from ethylene without requiring 1-olefin comonomer addition tailor-made PE and PP reactor blends in one reactor instead of reactor cascades renewable polyolefins cost-, resource-, eco-, and energy-efficient high-performance polyolefins and all-polyolefin composites 1399


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polyolefin superstructures and structural hierarchies that range from nanometer to millimeter dimensions during polymerization (Figure 2). In principle, olefin monomers are available from virtually all carbon sources ranging from oil, coal, natural gas, and fracking gases to biomass, bioethanol, and even carbon dioxide. As a consequence of the remarkable progress in catalyst and process development, polyolefins meet the demands of green chemistry. The principles of green chemistry, as outlined by Anastas and Warner, include high atom economy, no solvents, ambient polymerization conditions, low byproduct and waste formation, catalysis, no toxic intermediates, safe production, facile product recycling, and even exploitation of renewable resources.44−47 In fact, by meeting all of these demands of green chemistry, today’s advanced polyolefins have a distinct green image.14,48 In terms of facile tailoring of polymer properties in the gas phase and liquid pool, modern polyolefin production is far superior to biosynthesis of natural biopolymers and also biotechnology routes to natural and synthetic biopolymers, which have a high water demand and require extensive purification and modifications for applications in melt processing. In particular and unlike enzyme catalysts, highly active and stereospecific polyolefin catalysts enable facile tailoring of molar mass distribution, branching, and also polymer crystallization, all of which are essential to facilitate polymer melt processing. The term “sustainable” is by no means synonymous with “bio”. For instance, many biopolymer syntheses are not eco-efficient, especially cellulose manufacturing and pulping in paper production, which require chemical separation of cellulose from wood.49 Therefore, it is not surprising that metering of sustainability in 2010 came up with the clear ranking in life cycle assessment listing polypropylene as the number one and polyethylene as the number two, well ahead of all biobased materials, including polylactic acid.50

and during melt processing. In the 1980s, the discovery of single-site catalysts has enabled high-precision polyolefin synthesis with unprecedented control of molecular weight, molecular weight distributions, end groups, short- and longchain branching, stereochemistry, together with polyolefin nanostructures created by self-assembly of tailored polyolefins. In catalytic polymerization on single-site catalysts, polymer performance is primarily governed by the molecular architectures of transition metal complexes such as metallocenes,17−23 constrained-geometry cyclopentadienyl-amido complexes,24−26 phenoxyimine titanium complexes,27−30 and a great variety of postmetallocenes, including novel iron catalysts.13,31−34 Whereas the catalytically active transition metal of conventional Ziegler catalysts is always located at the end of the polyolefin chain, the catalytically active transition-metal alkyls of diimine nickel and palladium complexes, also known as “Brookhart catalysts”, travel up and down the polymer chain during polymerization. Hence, the chain-walking polymerization process produces linear, branched, and even hyperbranched polyethylenes from an exclusively ethylene feedstock without requiring separate production and transport of 1-olefin comonomers.34 Yet another polyolefin and sustainability success story started in 1954 when Giulio Natta discovered stereospecific 1-olefin polymerization and successfully prepared semicrystalline isotactic polypropylene, which had been unknown until then. As the only commodity polyolefin with a heat distortion temperature above 100 °C, isotactic polypropylene serves the needs of highly diversified markets, ranging from lightweight automotive engineering of bumpers to textiles, bottles, packaging, pipes, and even battery cases. In terms of sustainability, it is highly desirable to have such a single organic material exhibiting high versatility in terms of properties and applications combined with high performance, low energy demand during fabrication, oil-like energy content, and facile recycling. Because of its sustainability and versatility, polypropylene has replaced various other less environmentally friendly and more expensive metallic, ceramic, and polymeric materials. Since the 1960s, the better insight into polymerization mechanisms and polyolefin particle formation has led to the development of highly active and stereospecific supported catalysts that have revolutionized and simplified polypropylene production. Among the catalyst families, the development of magnesium chloride-supported titanium catalysts, modified with Lewis bases, has been the most important polyolefin technology pacemaker for polypropylene and polyethylene.35−43 With high catalyst activities exceeding 106 g polyolefin/g, the content of nontoxic titanium catalyst residues falls in the ppm range, thus eliminating the need for polyolefin purification by solvent extraction of catalyst residues. Moreover, modern high-precision catalytic synthesis drastically lowers byproduct formation and does not require removal of wax-like fractions such as amorphous atactic polypropylene. During the 1980s, a new dimension of morphology control entered the industrial scale with the production of pellet-sized polyethylene and polypropylene during olefin polymerization (“reactor granule technology”), using spherical catalyst particles as seeds for controlled polyolefin particle growth by catalyst fragmentation during polymerization. This progress brings considerable additional energy savings because it eliminates melting of polymers in pelletizing melt-extrusion that is typical of many conventional processes. Since then, a special focus in polyolefin development has been placed on controlling

1.2. Multisite Polymerization Catalysis

When considering the outstanding achievements of polyolefin catalysis over the past 60 years, as outlined in Table 1, one could be tempted to conclude that everything has now been done and that the future of commodity polyolefins is merely restricted to optimization of existing processes and cost reduction. The opposite is true. This becomes apparent when comparing the development of traditional organic compounds such as aspirin and polyethylene. Both compounds were discovered during the late 1890s. Today, aspirin is the same compound, produced by the same synthetic route, exhibiting the same properties, and serving the same applications. It is even marketed under the same trade name and sold by the same company. In sharp contrast, modern polyethylene has nothing in common with the polyethylene of the 19th century. In fact, von Pechmann’s polymethylene, which is equivalent to high-density polyethylene and was prepared in 1898 by decomposition of diazomethane, was forgotten and had no industrial impact at all. After Nobel laureate Hermann Staudinger recognized the role of a high molar mass for producing both natural and synthetic polymers,51 breakthroughs in free-radical and especially in catalytic ethylene polymerization catalysis have enabled industrial tailoring of low and high molecular-weight polyethylene materials with unprecedented precision. However, it should be noted that high precision has an entirely different meaning in polyolefin chemistry. In sharp contrast to proteins, in which all of the polymer chains have an identical chain length with precise and 1400


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In principle, there are two different strategies toward meltprocessable UHMWPE/HDPE blends, melt blending and inreactor blending. However, melt blending of UHMWPE is rather difficult because commercial micrometer-sized UHMWPE pellets do not melt during the short residence time typical of melt extrusion. Moreover, efficient dispersion of immiscible polymers, such as rubber, and of nanofillers in viscous polyolefin melts requires high shear forces and special preprocessing, both of which increase the energy demand and impair sustainability. Hence, in-reactor blending offers considerable cost and energy savings with respect to melt compounding. Intimate blending takes place during polymerization at much lower temperatures, thus enabling mixing of immiscible components on a nanometer-scale without requiring high shear forces. As illustrated in Figure 4, polyethylene reactor blends are produced by ethylene polymerization either in reactor cascades or on multisite catalysts in one reactor.

identical placement of the comonomers, such high-precision polyolefins would be catastrophic in polyolefin melt processing. Polyolefin processing by injection molding and blow molding as well as extrusion requires shear thinning resulting from broad molar-mass distributions (MWDs). The attractive combination of good flowability and improved mechanical properties is achieved by tailoring MWDs and also by incorporating longchain branches.52 Low molecular-weight polyethylene has a low viscosity and a fast crystallization rate, accounting for high stiffness, whereas ultrahigh molecular-weight polyolefins exhibit a significantly higher toughness, but a much slower crystallization rate and a lower crystallinity together with a high melt viscosity due to extensive entanglement of the polymer chains. As illustrated in Figure 3, ultrahigh molecular-weight polyethylene (UHMWPE)

Figure 3. Polyethylene (HDPE/UHMWPE reactor blends) with bimodal MWDs containing short-chain-branched UHMWPE as tie molecules linking together polyethylene crystal lamellae.

serves as a tie molecule, linking together polyethylene lamellae.53,54 Hence, incorporation of UHMWPE affords substantial improvement of the fatigue- and stress-crack resistance together with melt strengthening. This MWD control is of interest for producing sustainable materials for damagetolerant pipes and flexible packaging with reduced weight. When 1-olefins are exclusively incorporated as short-chain branches into the UHMWPE fraction, the crystallinity of UHMWPE decreases, thus producing more tie molecules. In particular, long n-alkyl side chains, such as butyl and hexyl, promote the formation of these physical linkers between two polyethylene crystal lamellae. The increase in branching with increasing molar mass is also known as an inverse or orthogonal branching distribution. The development of HDPE/UHMWPE polyethylene reactor blends has led to the industrial production of polyethylene pipes with a substantially improved service life, which is guaranteed to be more than 50 years for certain pipe resins (PE100).55 For many decades and due to massive UHMWPE entanglement, the tolerable amount of UHMWPE was restricted to a few percent in conventional melt processing. In contrast to HDPE and LLDPE, UHMWPE is processed by special techniques, such as sintering and gel-spinning, to produce ultrastrong and ultratough polyethylene materials with a high abrasion resistance and a low friction that are unparalleled by HDPE with a lower molar mass. In view of improving both sustainability and performance of polyethylene commodities, it is an important challenge to explore new routes toward melt-processable UHMWPE or UHMWPE/HDPE blends with a high UHMWPE content without requiring special processing and without sacrificing economic and ecological benefits typical of polyethylene materials. To meet these goals, robust and highly active multisite catalysts and reactor blend technology are in great demand.

Figure 4. Tailored polyolefins with broad MWDs produced by staged olefin polymerization in reactor cascades (above left) or by olefin polymerization on multisite catalysts in one reactor (above right).

The engineering approach toward in-reactor blending exploits reactor cascade technology together with control of the polymer morphology using controlled fragmentation of supported catalysts during polymerization. In sequenced reactors, the production of bimodal polyolefins with an inverse branching distribution demands that the catalyst maintains a high activity over a prolonged period in conjunction with stable polymer particle formation. The impact of reactor blend technology and cascade reactors on the development of advanced polyethylene materials, as illustrated in LyondellBasell’s Hostalen process, was reviewed by Boehm.55,56 In the first reactor, ethylene polymerization in the presence of hydrogen as a chain-transfer agent produces lower molecular-weight HDPE with high flowability. In the second and third reactors, at a much lower hydrogen content, high and ultrahigh molecularweight polyethylenes with much higher melt viscosities are formed. Furthermore, the addition of 1-olefins as a comonomer in the third reactor affords an inverse branching distribution by exclusive incorporation of the comonomer into UHMWPE. Whereas HDPE with a lower molar mass forms the crystalline phase, the short-chain-branched UHMWPE with a much slower crystallization rate yields the amorphous phase with UHMWPE tie molecules linking together the polyethylene lamellae (Figure 3). Despite the rather low UHMWPE content of just a few weight percent, the stiffness, fatigue, and crack resistance simultaneously improve. Because of their improved performance, bimodal polyethylene reactor blends substantially 1401


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contribute to sustainable development of damage-tolerant pipes for the safe transport of potable water, gas, and also sewage. In fact, no polyethylene pipes were damaged during the Kobe earthquake.9 Furthermore, blow-molded containers and packaging films made from bimodal polyethylene ensure a safe food supply together with substantial weight savings and reduced carbon dioxide emissions in the transportation of diverse goods. Because of melt-strengthening, the packaging films are rendered thinner without impairing their excellent mechanical properties. Recent progress in cascade reactor technology has led to the production of polyethylene with a much higher UHMWPE content.57−59 Industrial polypropylene cascade reactor systems such as LyondellBasell’s Catalloy9,60,61 and Borealis’ Borstar process11,62 combine particle-forming liquid-pool stereospecific polymerization of propylene in a loop reactor with subsequent gas-phase polymerization to produce impact-modified polypropylene without requiring solvents and pelletizing meltextrusion (Figure 5, center).

Figure 6. Routes to tailored polyolefins by melt blending together polymers produced on different catalysts (A), by olefin polymerization on binuclear catalysts (B, above) or dual-site catalysts (B, below), and by tandem catalysis (C) producing in situ a 1-olefin comonomer that is copolymerized with ethylene on the second site.

ethylene feedstock and eliminates the need for separate production, transport, and feed of 1-olefin comonomers. Among the multisite catalysts, three-site tandem catalysis combines in situ formation of 1-olefin with its exclusive incorporation into the ultrahigh molecular-weight polyolefin, whereas one site is highly selective with respect to ethylene and produces highly linear HDPE with a high stiffness and a lower molar mass. The ultimate goal of multisite catalysis is to control both reactor blend molecular architectures, MWDs, and nanostructure formation via the molecular architectures and the molar mixing ratio of two single-site catalysts combined in a multisite catalyst. The prime requirement for multisite polymerization catalysis is the presence of robust catalyst systems containing independent stable sites having similar polymerization kinetics. The impact of multisite polyolefin catalysts on sustainability is obvious. On the one hand, they simplify the production of tailored polyolefins in highly energy-, resource-, and eco-efficient processes and improve their performance without sacrificing efficient recycling. On the other hand, they enable the production of novel advanced polyolefin materials ranging from self-reinforced polyolefins (“all-polyolefin composites”) to thermoplastic elastomers. Such high-performance hydrocarbon materials are in great demand for lightweight engineering. These dream concepts relating to multisite polymerizationcatalysis reactions and polyolefin production have been around since the early days of Ziegler−Natta catalysis. Most conventional supported catalysts are in fact multisite catalysts containing different catalytically active sites with a different response to hydrogen, different stereospecificity, and different reactivity regarding comonomer incorporation. However, the individual sites do not exhibit single-site behavior and do not enable facile control of polymerization processes typical of single-site catalysts. Hence, most of them produce polyolefins with a broad molar-mass distribution. Taking into account their rather complex surface chemistry, however, it is difficult to independently tune one site without affecting the other. In fact, early generations of multisite catalysts produced rather illdefined polymer mixtures in polymerization processes that were very difficult to control. Consequently, during the early days of reactor blend technology, catalytic olefin polymerization in reactor cascades was the preferred process with respect to olefin polymerization on multisite catalysts. Since the discovery of single-site catalysts during the 1980s, however, multisite catalysts with much easier process and product control have been entering a new dimension in development. In particular, better insight into reaction mechanisms, interactions of metal alkyls, and correlations of the single-site catalyst architectures with polymer architectures has enabled the development of new generations of robust homogeneous and heterogeneous multisite catalysts for the production of advanced sustainable

Figure 5. Polyolefin reactor blends produced by olefin polymerization on multisite catalysts in one reactor (left), in reactor cascades (center), and in a multizone reactor (right).

From the perspective of both economy and sustainability, it is highly desirable to tailor polyolefin reactor blends in just one reactor to simultaneously lower costs and the energy demand. As illustrated in Figure 5, in-reactor blending is achieved either by olefin polymerization on a multisite catalyst in one reactor (Figure 5, left) or by olefin polymerization using a conventional supported catalyst in one multizone reactor (Figure 5, right). In LyondellBasell’s Spherizone process,9,63−66 launched in 2004, a multizone circulating reactor produces spherical polypropylene particles with a tailored molar-mass distribution in a single-loop reactor. The circulation of growing polyolefin particles between riser and downer zones, which have different polymerization conditions, creates a “one-reactor cascade” that produces polypropylene with broad MWDs and blends together different polypropylenes on a nanometer scale inside the growing particles. This process produces polypropylenes with an ultrahigh melt strength that can substitute conventional materials in rigid packaging. Figure 6 displays the idealized concepts for in-reactor blending on dual-site catalysts, which are also referred to as hybrid, two-site, or binary catalysts and which consist either of blends of two single-site catalysts or of a binuclear complex containing two different sites in the same catalytically active complex. In sharp contrast to polyolefin blends prepared by melt-blending, the close proximity of different sites in a multisite catalyst enables blending of different polyolefins on a nanometer-scale without requiring extensive shearing. Furthermore, tandem catalysis (Figure 6, right) yields 1-olefin comonomers by ethylene oligomerization, followed by copolymerization with ethylene on the second site. Hence, tandem catalysis produces branched polyethylene from one 1402


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that the individual sites do not interact and independently produce polyethylene typical of the corresponding single-site catalyst component. However, the activity of one site relative to the other drastically changed with an increase in temperature. Similar bimodal and temperature-dependent molecular-weight distributions of polyethylene were reported by Woo et al., who polymerized ethylene on MAO-activated Cp 2 TiCl 2 /Et(Ind)2ZrCl2 dual-site catalysts.72 Soares examined ethylene slurry polymerization on homogeneous MAO-activated dual-site catalysts by blending together single-site catalysts such as Cp2MCl2 (M: Zr, Hf, Ti) (1a−c) and Et(Ind)2ZrCl2 (2).73 They found that the polyethylene MWDs of the resulting reactor blends agree very well with MWDs of polyethylenes obtained by solution blending of monomodal polyethylenes prepared with the individual dualsite catalyst components. Again, this finding provides strong experimental evidence that both sites do not interact and retain their single-site nature when combined in multisite catalysts. Hence, combining different single-site catalysts, selected on the basis of their individual performance, offers new opportunities for tailoring reactor blends, going far beyond the very limited design possibilities typical of early Ziegler−Natta catalyst generations. This dual-site catalyst strategy for producing bimodal polyethylene MWDs also applies to other single-site catalysts, such as Fe(II) and Ni(II) dual-site catalysts.74 In 2001, Fukui and Murate used Cp 2 ZrHCl/B(C 6 F 5 ) 3 / [tBuNSiMe2Flu]TiMe2 dual-site catalysts in propylene polymerization. The higher molar mass of the iso-rich PP was attributed to the Zr sites, whereas the lower molecular-weight fraction was synthesized on the Ti sites.75 By combining three different isospecific bridged metallocenes, Paredes varied the MWD of isotactic polypropylene.76 According to studies by Rytter77−79 and Soares80 of ethylene polymerization on homogeneous dual-site catalysts, precise control of polyethylene MWDs via the molar ratios of the catalyst components is possible when taking into account the influence of various other parameters such as monomer type, temperature, pressure, comonomer, hydrogen, the presence of trimethylaluminum, and diffusion problems associated with the precipitation of polyethylene during polymerization. Instead of combining different single-site catalysts, extensive research has been aimed at linking different sites together in biand polynuclear transition-metal complexes. In contrast to solution blends, the design of a spacer enables systematic variation of the distance between two different sites, thus exploiting cooperative effects. The remarkable progress made in the field of multisite catalysts based on bi- and polynuclear complexes was summarized in comprehensive reviews by Marks.81,82 Bridged binuclear metallocenes with flexible (4a− g83and 4h−i,84 Figure 8 and Scheme 1) and rigid spacers (5a,b, Figure 9),85 binuclear constrained-geometry catalysts (6a,b, Figure 10),86,87 and binuclear postmetallocenes (7a,b, Figure 11)88,89 were prepared by several groups. Hence, tailored binuclear dual-site catalysts that exploit cooperative effects enabled broadening of polyethylene MWDs and the formation of bimodal MWDs.86,90−98 Although singlesite catalyst blends afford better MWD control via the site molar ratio, dinuclear complexes are attractive with respect to their use in tandem catalysis (see section 4) and compartmentalized supported catalysts. Regarding their industrial application in slurry polymerization, heterogeneous multisite catalysts are preferred because they prevent formation of fine particles accompanied by severe reactor fouling, which is typical of most

polyolefin materials. The major focus of this Review is the progress made in multisite catalysis by exploiting single-site catalyst technology, with special emphasis on the development of polyolefin materials for sustainable developments.

2. HOMOGENEOUS MULTISITE POLYOLEFIN CATALYSIS 2.1. Polyolefins with a Bimodal Molar-Mass Distribution

Narrow polyolefin MWDs, typically with polydispersities of Mw/Mn = 2, represent the characteristic feature of polyolefins prepared by olefin polymerization on homogeneous single-site catalysts. However, soon it was recognized that improving melt processing by shear thinning and melt strengthening requires broadening of the polyolefin MWDs. Pioneered by Ewen and Kaminsky during the 1980s, different homogeneous single-site metallocene catalysts were blended together to produce homogeneous multisite catalysts that afforded polyolefins with different chain lengths in a single reactor. In view of both economy and sustainability, this multisite catalyst approach is attractive because polyolefins with broad MWDs are produced in one reactor without requiring hydrogen as a chain-transfer agent. An important objective is to control MWDs via the single-site catalyst type and blend ratio. A comprehensive literature survey of the early days of polyolefin dual-site catalysts until 2001 is given by Casagrande.67 In 1984, Ewen claimed the preparation of polyolefins with broad and multimodal molar-mass distributions (Mw/Mn > 5) by means of ethylene polymerization on a methylalumoxane (MAO)activated blend of two or more single-site metallocenes of groups 4, 5, and 6 metals containing mono-, di-, and tricyclopentadienyl ligands.68,69 Kaminsky used MAO-activated binary catalyst blends by combining different metallocenes such as bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) (1a), bis(cyclopentadienyl)hafnium dichloride (Cp2HfCl2) (1b), racemic ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2) (2), and bis(indenyl)zirconium dichloride ((Ind) 2ZrCl 2) (3) to produce broadened polyethylene MWDs as a function of the metallocene types and their molar blend ratio (Figure 7). In contrast, ethylene polymerization on the individual catalyst components gave narrow molar-mass distributions, as expected for single-site catalysts.70,71 In the case of the MAO-activated Cp2HfCl2/Et(Ind)2ZrCl2 dual-site catalyst, Kaminsky et al. succeeded in varying the resulting bimodal polyethylene MWDs as a function of the Hf/ Zr molar ratio. In comparison to Zr catalysts, Hf catalysts gave higher molar mass, but lower catalyst activities. They concluded

Figure 7. Dual-site catalysts combining metallocenes, as proposed by Kaminsky et al.70,71 1403


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Figure 8. Binuclear metallocenes with flexible spacers.83

Scheme 1. Routes to Binuclear Zirconocenes84

Figure 11. Binuclear postmetallocenes.88,89

homogeneous catalysts (section 3). In addition, it should be noted that catalyst immobilization can lead to very significant improvements in catalyst stability, avoiding the rapid deactivation that is obtained during polymerization with many homogeneous catalyst systems. 2.2. Branched Polyethylene Reactor Blends

With increasing short-chain branching, resulting from either ethylene/1-olefin copolymerization or chain-walking-type ethylene homopolymerization, both the crystallinity and the density of polyethylene decrease, whereas a few long-chain branches and tailored bimodal polyethylene MWDs account for improved shear thinning and melt strengthening in blow molding and film extrusion. This balance of tailored MWDs and branching makes low-density polyethylene films thinner and stronger, thus reducing weight in packaging applications,

Figure 9. Binuclear zirconocenes with rigid spacers.85

Figure 10. Binuclear constrained-geometry complexes.86,87 1404


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catalysts combining early and late transition-metal single-site catalysts.108 Whereas ethylene polymerization on MAOactivated diimine nickel (8, 9a, 9b) produced branched LLDPE by chain walking (Scheme 2), MAO-activated metallocenes (10, 11) and the iron bis(imino)pyridyl complex (12) yielded linear HDPE (Figure 12).

lowering carbon dioxide emissions in transportation. Furthermore, this sustainable technology secures a safe food supply by improving the quality of packaging materials used for perishable goods. In the case of bimodal high-density polyethylene pipes, an inverse (“orthogonal”) branching distribution with exclusive short-chain branching of UHMWPE markedly enhances the resistance to fatigue and environmental stress cracking, thus prolonging pipe service life and damage tolerance. As compared to conventional polyethylene cascade reactors, multisite catalysts enable control of MWDs and branching in one reactor. Moreover, the use of tandem catalysis (Section 4) or the combination of polyinsertion on early transition-metal catalysts with chain walking on late transition-metal catalysts does not require separate production, transport, and feed of 1olefin comonomers. This simplifies polyolefin production and further improves its resource-, eco-, and energy-efficiency. To evaluate ethylene/1-olefin copolymerization on multisite catalysts, Soares’ group developed a new methodology that combines high-temperature size-exclusion chromatography and Fourier-transform IR spectroscopy to track the contributions of the individual components of dual-site catalysts.99,100 The resulting polyethylene MWDs, obtained by ethylene polymerization on multisite catalysts combining two different single-site metallocenes and constrained-geometry catalysts, reflect ethylene/1-olefin copolymerization on the individual sites, which do not appear to interact.101−105 The independent nature of the two single-site components in dual-site catalysts was also experimentally verified by Kaminsky et al., who investigated ethylene/1-butene copolymerization on dual-site catalysts combining MAO-activated Et(Ind)2ZrCl2 and its hafnium analogue, and ethylene/propylene copolymerization on dualsite catalysts combining rac-[Me2Si(2-Me-4-(1-Naph)Ind)2]ZrCl2/MAO with [Me2Si(Ind)(Flu)]ZrCl2/MAO.70,106 Surprising deviations from this mixing rule and independent site behavior were observed by Rytter et al. for ethylene/1-hexene copolymerization on MAO-activated two-site catalysts in which (1,2,4-Me3Cp)2ZrCl2 was combined with (Me5Cp)2ZrCl2. According to their hypothesis, polyethylene block copolymers, containing segments with high and low comonomer contents, are accounted for by reversible polymeric transfer between two different sites resulting from aluminum alkyl-mediated transmetalation.107 As illustrated in Scheme 2, to produce HDPE/LLDPE reactor blends from an exclusively ethylene feedstock without requiring 1-olefin comonomer, Mecking developed dual-site

Figure 12. Dual-site catalysts introduced by Mecking et al., combining MAO-activated late transition-metal catalysts (8−9b) for chainwalking ethylene polymerization, producing LLDPE, with MAOactivated zirconocenes (10, 11) and a bis(imino)pyridyl iron catalyst (12), both of which produce highly linear HDPE.108

In the resulting HDPE/LLDPE blend, the Zr/Ni molar ratio correlated with the number of n-alkyl branches, which is reflected by the polyethylene density. However, the branching distribution of methyl and longer n-alkyl side chains and the polyethylene molar masses were unaffected by the Zr/Ni molar ratio. This observation provides experimental evidence for the absence of strong interactions between the two different sites. Moreover, hydrogen as a chain-transfer agent selectively interacted with early transition-metals sites, whereas nickel sites were much less sensitive to hydrogen. Hence, hydrogen addition improved flowability in melt processing and produced polyethylene reactor blends with predominant branching of polyethylene with a higher molar mass. Produced in the presence of hydrogen, these reactor blends were equivalent to ethylene/1-olefin copolymers with inverse comonomer incorporation typical of ethylene polymerization in cascade reactors, although the molar mass of LLDPE failed to fall in the UHMWPE range. To achieve stable slurry polymerization, however, the degree of branching is limited to the formation of semicrystalline LLDPE, which is insoluble and precipitates during polymerization. Whereas 8 produced soluble LLDPE with a high degree of branching (59 branches per 1000 C), both 9a and 9b gave insoluble LLDPE with less branching. Therefore, 9a and 9b represent the preferred diimine nickel catalyst components of these early/late transition-metal dualsite catalysts. It was preferable to use silica supports to heterogenize these dual-site catalysts, thus affording much

Scheme 2. Reactor Blends of Branched and Linear Polyethylene Prepared by Ethylene Polymerization on DualSite Catalysts, Which Combine Chain Walking on Diimine Nickel Complexes (8, 9a, 9b) To Produce LLDPE or with Polyinsertion on Iron and Zirconium Complexes (10, 11, 12) To Produce HDPE108,a

a

In this case, the addition of 1-olefins as comonomers is not required to produce HDPE/LLDPE reactor blends. 1405


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with the more economical continuous polymerization process. Nevertheless, the recent development of catalytic chain-transfer polymerization, also known as chain shuttling,128−131 and molecular switching of sites by catalytic group-transfer polymerization on a single-site catalyst130,132,133 have overcome these shortcomings without sacrificing high precision in block copolymer synthesis. This progress opens a new dimension for tailoring polyolefin block copolymers and also many other hydrocarbon materials. In principle, chain shuttling exploits chain transfer by means of transmetalation reactions between transition metal sites and metal alkyls, including main-group metal alkyl activators. Chain transfer is well-known as a chain-termination reaction in polyolefin catalysis.134 There are two chain-transfer pathways involving either irreversible or reversible transmetalation between transition-metal alkyls and main-group metal alkyls, which are used as activators, impurity scavengers, or chain shuttling agents in olefin polymerization. By irreversible chain transfer, “dead” main-group metal alkyls are formed as polyolefin end groups, thus lowering the polyolefin molar mass. For many years, the efficient chain-transfer reaction with ZnEt2 has been used for polyolefin end-group functionalization and formation of polyolefin block copolymers, both of which employ zinc alkyl-terminated oligo-olefins as intermediates.135,136 Moreover, ZnEt2 represents an efficient molar mass regulator for living ethylene polymerization on MAOactivated bis[N-(3-methylsalicylidene)-2,3,4,5,6pentafluoroanilinato]TiCl2 (FI catalyst, 15) (Figure 14).137

higher polyethylene bulk densities by preventing the formation of fine particles as well as reactor fouling typical of homogeneous catalysts. Casagrande and co-workers prepared homogeneous multisite catalysts by combining bis-1,4-bis(2,6diisopropyl-phenyl)acenaphthenediimine nickel dichloride (13) with racemic ethylenebis(tetrahydroindenyl)zirconium dichloride (14) (Figure 13).

Figure 13. Dual-site catalysts by Casagrande et al. that combine early and late transition-metal complexes.67,109−114

They investigated the influence of catalyst compositions and polymerization parameters on polyethylene compositions and properties, and they thus prepared a wide range of polyethylene reactor blends with variable branching patterns and tailored properties.67,109−114 The last two decades have seen the emergence of several other families of multisite catalysts that combine early and late transition-metals and include binuclear nickel complexes.74,98,115−123 Moreover, bimodal polyethylenes with tunable MWDs were obtained when ZnEt2 was added as a chain-transfer agent to diimine nickel/zirconocene dual-site catalysts.124 2.3. Chain Shuttling and Molecular Switching of Sites

2.3.1. Polyethylene Block Copolymers. Tailored polyolefin block copolymers, containing alternating soft and hard semicrystalline or amorphous segments, respectively, are thermoplastic elastomers that qualify as potential substitutes for flexible PVC and other elastomers containing low molecular-weight plasticizers. Furthermore, polyolefin block copolymers serve the needs of a wide variety of applications ranging from tubes and hoses to flexible films, fibers, foams, and adhesives. As compared to most other thermoplastic elastomers, polyolefin block copolymers combine lower weight with lower cost and better performance. Typically, conventional thermoplastic elastomers such as polystyrene-block-poly(ethylene-co-butene)-block-polystyrene, well-known as SEBS in Shell’s Kraton thermoplastic elastomer family, are produced in a two-step solution process that combines anionic living polymerization with subsequent hydrogenation. In the first step, the living anionic polymerization of styrene with sequenced addition of butadiene and styrene affords polystyrene-block-polybutadiene-block-polystyrene (SBS). In the second step, SBS is hydrogenated to produce SEBS. Despite groundbreaking research on living olefin polymerization during the past two decades,125−127 the elegant high-precision one-step polyolefin-block-copolymer synthesis by living olefin polymerization suffers from an inherent limitation: each chain contains an active transition-metal alkyl as end groups. Hence, the demand and residual content of the transition metal are high, which requires tedious purification that impairs both costs and sustainability. Moreover, conventional living polymerization to produce narrow Poisson-type MWDs with polydispersities Mw/ Mn = 1 is operated as a batch process, which is not competitive

Figure 14. Dual-site catalyst combining a phenoxyimine catalyst precursor (15) with ZnEt2 as a chain-transfer agent.137

As reported by Fan and Li for a staged polymerization process, irreversible chain transfer by adding ZnEt2 during the course of living ethylene polymerization affords polyethylene with precisely controlled bimodal MWDs. Key parameters for MWD control are polymerization time and the Zn/Ti molar ratio. During the initial stage in the absence of ZnEt2, a high polyethylene molar mass is obtained. The molar mass increases with increasing polymerization time. On adding ZnEt2 in the second stage, chain transfer occurs and terminates part of the polyethylene chain growth, thus producing shorter polyethylene chains and decreasing their molar mass with increasing Zn/Ti molar ratio. Catalytic olefin polymerization, which involves exceptionally rapid and reversible exchange of polyolefin chains between active transition metal sites and inactive (“dormant”) maingroup metal sites, is referred to as chain shuttling when using multisite catalysts containing two or more different transition metal sites. Provided that the chain-transfer reaction via transmetalation is much faster than polyolefin chain growth, the resulting controlled olefin polymerization exhibits the high precision typical of living polymerization, but with a drastically 1406


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reduced demand for the transition metal catalyst operating in continuous olefin polymerization processes. Hence, highprecision polymer synthesis by chain shuttling represents a rather versatile “drop-in technology” for efficient continuous catalytic olefin polymerization processes in solution polymerization. Whereas ZnPh2 is inactive in chain transfer, the addition of chain shuttling agents such as ZnEt2 or AlR3 (R = Me, Et, Oct, iBu) to the MAO-activated homogeneous bis(imino)pyridyl iron catalyst [2,6-(MeC = N-2,6iPr2C6H3)2C5H3N]FeCl2 (12) affords an exceptionally rapid reversible exchange of the polymer chains between iron and the chain shuttling agent. This accounts for the formation of poly(1-olefins) with extremely narrow Poisson-type MWDs and polydispersities of Mw/Mn = 1 (Figure 15). Gibson and co-workers attribute the

In chain shuttling ethylene polymerization, dual-site polyolefin catalysts combine two sites having very different reactivities with respect to incorporation of 1-octene in the presence of a chain shuttling agent such as aluminum or zinc alkyls. The exceptionally rapid polymeryl exchange between the one site, producing linear polyethylene by ethylene homopolymerization, and the other site, which copolymerizes ethylene with 1-octene, accounts for the formation of polyolefin block copolymers. They contain alternating segments of hard linear polyethylene (Tm = 135 °C) and flexible branched polyethylene with a low glass-transition temperature (Tg < −40 °C).128,146−150 In 2006, the Dow Chemical Company commercialized polyethylene block copolymers (Infuse) as thermoplastic elastomers produced by the chain shuttling process using tailored dual-site catalysts. Scheme 3 displays the principle of this chain shuttling mechanism. The first site (CAT1, 16) is highly selective and ignores the presence of 1olefin comonomers to produce linear polyethylene without short-chain branching. The second site (CAT2, 17) efficiently copolymerizes ethylene with 1-olefins, thus yielding flexible branched polyethylene elastomers. In the absence of chain shuttling agents (CSA), the dual-site catalysts produce a reactor blend with very broad and also bimodal MWDs containing immiscible linear polyethylene and poly(ethylene-co-1-olefin) elastomers (Figure 16). Only in the presence of CSA such as ZnEt2 does the exceptionally rapid CSA-mediated polymeryl exchange between the two different sites afford growth of the same chain on the two different sites and account for drastic narrowing of MWDs. Because chains formed on 16 continue to grow on 17 until the resulting block copolymer is shuttled back to continue its growth on 16, multiblock copolymers with alternating soft and hard segments are formed without any homopolymer side-products. This is in sharp contrast to most conventional multisite catalysts, which produce complex reactor blends with broad MWDs containing homo- and copolymers together with ill-defined block copolymers as side-products. Chain shuttling preferably operates in a solution process at temperatures above 120 °C because polymer precipitation prevents exchange of polymer chains between sites. Highthroughput screening of the large number of highly active single-site catalysts has enabled identification of robust dual-site catalyst systems comprising two complementary transition metal complexes together with a main-group metal alkyl as a

Figure 15. Influence of ZnEt2 as a chain shuttling agent on polyethylene MWD produced by ethylene polymerization on the MAO-activated bis(imino)pyridyl iron catalyst (12). Reproduced with permission from ref 138. Copyright 2002 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

exceptional performance of this Fe/Zn dual-site catalyst to the relatively weak Zn−C bonds matching the Fe−C bond strength and the presence of monomeric ZnEt2.131 Unlike Ziegler’s ethylene oligomerization on AlR3, known as the “Aufbaureaktion” and which is restricted to ethylene feedstock, catalytic chain-transfer polymerization enables the facile production of a large variety of low and high molecular-weight tailored hydrocarbon materials with precisely controlled molecular architectures by exploiting a large variety of 1-olefin feedstocks.129,131,138−145

Scheme 3. Olefin Block Copolymers Prepared by Chain Shuttling on Dual-Site Catalysts That Combine Two Sites with a Large Reactivity Difference with Respect to the Incorporation of 1-Olefin in Ethylene Polymerization in the Presence of a Chain Shuttling Agent (CSA) Such as Zinc or Aluminum Alkylsa

a

Reproduced with permission from ref 128. Copyright 2006 AAAS. 1407


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produces block copolymers by varying the polymer stereochemistry as a function of the catalyst’s stereocontrol. Provided that at least two isotactic segments are sufficiently long to crystallize, thermoreversible cross-linked elastomeric networks are formed by crystallization. As compared to other thermoplastic elastomers and olefin block copolymers, elastomeric polypropylene has a higher glass transition close to room temperature, which limits its applications in rubber technology. Progress in polyolefin catalysis has led to new generations of elastomeric polypropylene. In the early days of Ziegler−Natta catalysis, amorphous low molecular-weight atactic polypropylene was an undesirable tacky side-product that was removed from isotactic polypropylene by solvent extraction and sent to landfill. Today, as reported by Resconi, progress in single-site polyolefin catalysis has enabled the production of atactic polypropylene with substantially higher molecular weights and longer isotactic polypropylene segments that cocrystallize, thus rendering high molecular-weight atactic polypropylene elastomeric.151,152 Several other single-site catalysts have been developed for producing elastomeric polypropylene by varying the single-site catalyst architectures.153−163 An intriguing “molecular switch” approach was introduced by Waymouth and Coates that converts a single-site metallocene catalyst into a dual-site catalyst to produce stereoblock polypropylene without adding a second metallocene.158,164−168 Their “oscillating” unbridged metallocene catalysts contain cationic complexes of the type (2-Ar-indenyl)2MP]+ (M = Zr, Hf; Ar = aryl; P = polymeryl) (18) as active sites. As illustrated in Scheme 4,

Figure 16. Polyethylene MWD obtained with a homogeneous 16/17 dual-site catalyst, combining 16 and 17, in the absence and in the presence of ZnEt2 as a chain shuttling agent. Reprinted with permission from ref 128. Copyright 2006 AAAS.

chain shuttling agent. For instance, a bisphenoxyimine zirconium catalyst (16), producing linear polyethylene, and a hafnium pyridylamide catalyst (17), producing branched polyethylene by efficient copolymerization of ethylene with 1olefins, combine high activity and rapid ZnEt2 transmetalation with the required drastic difference in monomer selectivity, with matching of the polymerization kinetics of both catalyst components. Only in the presence of ZnEt2 as a chain shuttling agent is the bimodal MWD (Mw/Mn = 13.6), typical of the 16/ 17 dual-site catalysts, rendered monomodal with a distinct narrow MWD (Mw/Mn = 1.33), as expected for exceptionally rapid reversible polymeryl exchange between active Zr/Hf sites and dormant Zn sites.128 Continuous polymerization processes are preferred over batch reactors because the steady-state concentration of zinc-alkyl-terminated polyolefins is always sufficiently high to enable efficient and stable chain shuttling. Moreover, similar to conventional living polymerization, batch reactors produce Poisson-type MWDs with a Mw/Mn around 1, whereas the continuous reactors afford Schulz−Flory-type MWDs with much broader MWDs with a Mw/Mn around 2, without impairing block copolymer molecular architectures. Broadening of MWDs is essential for improving flowability in melt processing by shear thinning. Both molar mass and “blockiness” of the copolymers are varied over a wide range to match the requirements of different applications (section 5.2). Unlike conventional living olefin polymerization, in which each chain contains a transition-metal alkyl at the chain end, the chain shuttling process is a resource- and energy-efficient catalytic process that requires much less transition metal and substantially improves the performance of polyolefin thermoplastic elastomers. 2.3.2. Stereoblock Polypropylene. Among polyolefin materials, polypropylene is outstanding with respect to its high versatility in terms of properties, wide application range, and facile recycling. Whereas highly isotactic polypropylene is a stiff engineering plastic, lowering its stereoregularity renders polypropylene soft, flexible, and even elastomeric. In view of improving resource efficiency, it is attractive to derive a wide variety of polypropylene materials from the same propylene feedstock. Typically, elastomeric polypropylene comprises stereoblock polypropylenes in which low-stereoregular flexible polypropylene chains link together hard isotactic polypropylene segments. The term stereoblock implies that a single monomer

Scheme 4. Converting Unbridged Metallocene Single-Site Catalysts into Dual-Site Catalysts by Molecular Switching (“Oscillation”) between the Two Site Configurations during Polymerizationa

a

Reproduced with permission from ref 164. Copyright 1995 AAAS.

these complexes were thought to “oscillate” exceptionally rapidly and thus switch between their rac-like (isospecific) and meso-like (nonstereospecific) configurations during polymerization, thus producing stereoblock polypropylene containing alternating isotactic and atactic segments. This new concept has inspired several other groups to tailor other bis(2-aryl-indenyl) zirconocenes and hafnocenes with variable aryl substitution patterns.166,169−171 However, as pointed out by Busico, the choice of solvents and counterions can drastically slow ligand rotation with respect to chain growth, thus producing rather complex polypropylene microstructures and reactor blends of polypropylenes with different stereoregularity.172,173 During the late 1970s, the first conventional multisite catalysts, which do not combine single-site catalyst components, were introduced by Du Pont for producing elastomeric polypropylene by propylene homopolymerization.174−177 As verified by their detailed analyses of the polypropylene’s diastereoisomer composition, alumina-supported tetra(neophyl)zirconium and bis(arene)titanium catalysts contain 1408


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different sites having vastly different stereospecificities. This multisite catalyst produces a reactor blend of highly isotactic polypropylene together with high molecular-weight polypropylenes having much lower stereoregularities. Hence, cocrystallization of the isotactic segments in low-stereoregular polypropylene with isotactic polypropylene accounts for the formation of an elastomeric thermally reversible polypropylene network. However, unlike combined single-site catalysts, variation of one site without affecting the other is impossible because their ratio is governed by the surface chemistry of the alumina support. In 1996, Chien prepared thermoplastic polypropylene elastomers by propylene polymerization on homogeneous dual-site catalysts, which combine isospecific C2vsymmetric metallocenes such as rac-ethylenebis(1-indenyl) zirconium dichloride (2) or rac-dimethylsilylbis(1-indenyl)zirconium (19) dichloride, respectively, with a nonstereospecific C 2 -symmetric metallocene such as ethylenebis(9fluorenyl)zirconium dichloride (20) (Figure 17).178−180 On

Figure 18. Catalyst components for the preparation of reactor blends containing iPP-b-sPP stereoblock polypropylene on dual-site catalysts.183

Me) with syndiospecific Ph2C(Flu)(Cp)ZrX2 (X = Cl, Me), produced reactor blends with predominant formation of isotactic and syndiotactic polypropylenes having complex microstrcutures.184 With respect to propylene polymerization on MAO-activated homogeneous dual-site ansa-metallocene catalysts, prepared by combining iso- and syndiospecific metallocenes, Brintzinger gained a better insight into stereoblock polypropylene formation and identified the role of aluminum-alkyl-mediated chain transfer.183 Polymerization with MAO-activated dual-site catalysts, which combined isospecific Me 2Si(2-MeInd) 2ZrCl 2 (24) with nonstereospecific Et(Flu)2ZrCl2 (20), produced completely separable blends of isotactic and atactic polypropylene, as expected for propylene polymerization on independent sites. However, propylene polymerization on the MAO-activated dual-site catalyst, prepared by combining isospecific Me 2 Si(2-Me-4-tBuC5H2)2ZrCl2 (23) with nonstereospecific Et(Flu)2ZrCl2 (20), yielded reactor blends containing isotactic and atactic polypropylene together with stereoblock polypropylene, in which both isotactic and atactic polypropylene chains were inseparably covalently linked. Moreover, MAO-activated dualsite catalysts, combining isospecific Me 2Si(2-Me-4-tBuC5H2)2ZrCl2 (23) and syndiospecific Ph2C(Cp)FluZrCl2 (22), afforded stereoblock polypropylene containing iso- and syndiotactic segments. Both extent and direction of Al-alkylmediated polymeryl transfer via alkyl-bridged heterobimetallic cations depended on steric effects, notably on the degree of ligand substitution. Hence, polymer chains were preferentially transferred from the more hindered to the more open complex species. From this investigation, it became apparent that most dual-site catalysts based on common metallocenes produced polypropylene reactor blends containing stereoblock polypropylene together with significant amounts of other stereoisomers. To produce stereoblock polypropylene with a narrow MWD and without byproducts, these dual-site catalysts would require further refinement to qualify them for chain shuttling propylene polymerization. For the first time, Busico and Stevens succeeded in preparing isotactic stereoblock polypropylenes with narrow MWDs by chain shuttling propylene polymerization on homogeneous MAO-activated racemic (pyridyl-amide)HfMe2 (25, Figure 19), using AlMe3, which is present in the MAO activator, as the chain shuttling agent. Whereas MAO-activated enantiopure 25 produced highly isotactic polypropylene with a polydispersity of Mw/Mn = 2 and isolated false insertions, typical of propylene polymerization on single-site catalysts, racemic 25, which corresponds to a dualsite catalyst containing a 50/50 mixture of the two enantiopure sites, yielded isotactic stereoblock polypropylene (Scheme 5b) with narrow MWDs and a polydispersity around Mw/Mn = 1, typical of chain shuttling. Preferably, polar solvents such as 1,2difluorobenzene enabled rapid AlMe3-mediated polymeryl exchange between the two sites of opposite chirality, producing

Figure 17. Chien’s dual-site catalyst precursors for the synthesis of elastomeric stereoblock polypropylene reactor blends by propylene polymerization on isospecific metallocene 19 combined with nonstereospecific metallocene 20.178−180

activation with triphenylcarbenium tetrakis(pentafluorophenyl)borate and triisobutylaluminum, the resulting highly active dual-site catalysts produce a reactor blend containing isotactic and atactic polypropylene together with stereoblock polypropylene containing alternating isotactic and atactic segments. In the resulting reactor blends, the stereoblock polypropylene compatibilized immiscible isotactic and atactic polypropylene, thus preventing macroscopic demixing. Formation of stereoblock polypropylene was attributed to the transfer of polypropylene chains between stereospecific and nonstereospecific sites. In contrast to the early multisite catalysts for producing elastomeric polypropylene, this dual-site catalyst, which combines two single-site metallocene catalysts, enables facile tailoring of the elastomer properties by varying the C2/C2v metallocene molar ratio. Similarly, Fink prepared iPP-b-sPP stereoblock polypropylene by propylene polymerization on silica/MAO-supported dual-site catalysts that combined isospecific rac-Me2Si[Ind]2ZrCl2 (19) with syndiospecific iPr(FluCp)ZrCl2 (21).181 In another approach toward iPP-b-sPP stereoblock polypropylene, Rytter and co-workers added AlMe3 together with the MAO activator to the homogeneous dual-site catalyst system, which combines the syndiospecific Ph2C(FluCp)ZrCl2 (22) with isospecific rac-Me2Si(4-tBu-2-MeCp)2ZrCl2 (23) (Figure 18).182 Although AlMe3 afforded chain transfer, stereoblock iPP-bsPP was always contaminated with other stereoisomers. Thus, the resulting reactor blends did not exhibit narrow MWDs typical of chain shuttling polymerization. Moreover, on activation with MAO or [PhNMe2H]+[B(C6F5)4]−/iBu3Al, the dual-site ansa-metallocene catalyst system, prepared by combining isospecific Ph2C(Flu)(3-Me3SiCp)ZrX2 (X = Cl, 1409


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the level of substoichiometric activation and transition metal concentration. Because both active and dormant sites are formed on the same catalyst, this methyl group transfer is another approach toward molecular switching of a single-site catalyst, which is converted into a dual-site catalyst. Hence, as illustrated in Scheme 5c, poly(1-olefins) with variable block or gradient architectures are produced in a programmed fashion. Properties of the resulting poly(1-olefin) stereoblock and gradient polymers range from highly stiff to elastomeric and soft, serving the demands of a large variety of applications ranging from engineering plastics to rubbers and thermoplastic elastomers and even lubricants, all of which are derived from one olefin feedstock.

Figure 19. MAO-activated (pyridyl-amide)HfMe2 for producing isotactic stereoblock polypropylene by chain shuttling in the presence of AlMe3.185,186

Scheme 5. Programmed Polypropylene Stereoisomer Synthesis: Isotactic Polypropylene with Isolated Stereodefects (a), Isotactic Stereoblock Polypropylene (b), Gradient (c), and Stereoblock Polypropylenes (d), Which Contain Hard Isotactic and Flexible Atactic Segments

3. HETEROGENEOUS MULTISITE POLYOLEFIN CATALYSIS 3.1. Ziegler−Natta Hybrids and MgCl2-Supported Multisite Catalysts

During the late 1970s, BASF started to develop multisite catalysts for producing polyethylene with a broad MWD in a single stirred tank, which aimed at mimicking the polyethylene MWDs obtained in cascade reactors. Pioneered by Warzelhan and Bachl, this development led to AlEt2Cl-activated silicasupported dual- and triple-site catalysts that combine VCl3 (lower molar mass PE with broad MWD) with TiCl3 (medium molar mass PE and narrow MWD), and ZrCl4 (high and very high molar mass PE with broad MWD).191−195 Typically, silica was impregnated with alcoholic solutions of transition metal halides such as TiCl3/VCl3 and TiCl4 prior to activation with AlEt2Cl in n-heptane. The PE molar mass was controlled by hydrogen pressure and the addition of fluorochloromethane additives, which selectively activate vanadium sites. Successful commercial applications included polyethylene blow molding and films. However, the aspired commercial production of an outstanding film grade was abandoned due to difficulties encountered regarding process control of constant MWDs. Such difficulties were typical of many less robust Ziegler−Natta multisite catalysts developed during the premetallocene age of catalyst development. Following the discovery of highly active and stereospecific Lewis-base-modified MgCl2-supported TiCl4 by Montecatini and Mitsui Chemical in 1968, remarkable progress has been made in catalyst and process development, including tailoring of multisite catalysts. Thus, the eco-, resource-, and energyefficiency of solvent-free polyolefin production have been substantially improved by eliminating numerous process steps such as removal of catalyst residues (deashing), solvent recovery, separation of wax-like byproducts, and even pelletiz-

alternating isotactic blocks with either (R) or (S) configuration of their repeat units.185,186 As an alternative to chain shuttling polymerization and an industrially more viable approach toward molecular switching of a single-site catalyst, Sita and co-workers introduced what they termed “living degenerative group-transfer polymerization”, displayed in Scheme 6. This represents a versatile method for programmable synthesis of poly(1-olefin) stereoisomers.130,132,133,147,187−190 Unlike chain shuttling, polymeryl exchange between two sites does not take place. Instead, highly active and dormant states of a transition metal complex interact via reversible transfer of methyl or chlorine ligands via binuclear complexes as intermediates. Whereas the active cationic transition-metal sites are configurationally stable, producing stereoregular poly(1-olefins), the dormant neutral metal site, formed by methyl group transfer, is configurationally unstable, as reflected by transition-metal-centered epimerization. Epimerization on a dormant site accounts for the formation of flexible poly(1-olefins) with considerably lower stereoregularity. The content of stereodefects in the chains correlates with the content of dormant transition-metal sites, which is affected by

Scheme 6. Molecular Switching of a Single-Site Catalyst by Catalytic Degenerative Group-Transfer Polymerization Involving a Rapid Exchange of X (Methyl, Cl) So That the Configurationally Stable and Highly Active Cationic Site, Which Produces Stereoregular Poly(1-olefins), Is Rendered Neutral and Dormanta

a

Dormant sites are configurationally much less stable, and epimerization accounts for stereodefect formation. Hence, this molecular switching process converts a single-site catalyst into a dual-site catalyst. Reproduced with permission from 130. Copyright 2009 Wiley. 1410


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Figure 20. SEM image of the particle morphology of HDPE obtained by ethylene polymerization on AlEt3-activated Fe (27) single-site (a,b) and (26/27) Ni/Fe dual-site catalysts (c,d), both of which are supported on spherical MgCl2/AlRx(OR′)3−x. Reprinted with permission from ref 207. Copyright 2007 American Chemical Society.

Figure 21. Dual-site catalysts for producing LLDPE/HDPE reactor blends by combining diimine nickel (26) with complexes of iron (27) and chromium (28).

polypropylene-supported metallocene subsequent to liquidpool homopolymerization of propylene in the first step. During the past decade, it was discovered that anhydrous magnesium chloride is an excellent support for a variety of single-site catalysts, including complexes of Ti, Zr, Fe, Ni, Co, and Cr containing diimine, bis(imino)pyridyl, quinolylindenyl, and phenoxyimine ligands.208−220 Effective supports of composition MgCl2/AlRx(OR′)3−x can be prepared via the reaction of an aluminum alkyl with a solid MgCl2/ethanol adduct or with a solution of an adduct of MgCl2 and 2ethylhexanol in decane. This has enabled the design of advanced magnesium chloride-supported multisite catalysts in a one-step process, in which all of the catalyst components exhibit single-site catalyst features without impairing the growth of polyethylene granules during ethylene polymerization. Going well beyond the scope of MWD control and conventional reactor blend formation, Chadwick reported on synergistic effects of diimine nickel (26) in ethylene polymerization on dual-site catalysts. In his dual-site catalysts, single-site catalysts with a Fe- (27) or a Cr- (28) or a Ti-Ziegler catalyst, which produce linear HDPE, were combined with minute amounts of the diimine nickel catalyst (26) to produce branched polyethylene (26 branches per 1000 C) on the same spherical MgCl2/AlRx(OR′)3−x support.

ing extrusion when using particle-forming spherical catalysts.38,39,61 For further improving MWD control and reactor blend formation, conventional supported Ziegler−Natta catalysts are equipped with single-site catalyst components to produce a wide range of Ziegler−Natta hybrid catalysts.186,196−205 Ziegler−Natta hybrid catalysts, in particular, combine the exceptional morphology control typical of spherical MgCl2-supported Ziegler−Natta catalysts with the unique control of polyolefin molecular architectures typical of metallocenes.206 For instance, spherical polypropylene, produced by MgCl2, is used as a support for single-site catalysts to produce Ziegler−Natta hybrid catalysts that, in contrast to multisite catalysts, contain just one single-site catalyst component. Typically, in the first step, propylene polymerization on catalysts prepared from TiCl4 and the AlEt3-activated spherical adduct MgCl2·(H2O)n produces porous polypropylene spheres. In the second step, on deactivation of the Ti catalyst, the porous polypropylene spheres serve as a support for MAO-activated metallocenes such as racemic ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride and mesoethylenebis(4,7-dimethylindenyl)zirconium dichloride. This reactor granule technology affords pellet-sized reactor blends of polypropylene with EPM rubber formed by in-particle gasphase copolymerization of ethylene with propylene on the 1411


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for controlling spherical particle growth during polymerization, thus eliminating pelletizing melt-extrusion, and for improving diffusion of the monomers to the catalytically active sites. Although the support treatment requires optimization, heterogenization of bridged and unbridged metallocene catalysts on MAO-tethered silica does not impair the polymer microstructure, MWDs, and comonomer incorporation typical of homogeneous polymerization.232,233 As shown by Soares, supported dual-site catalysts, prepared by coimmobilization of two different metallocenes on MAO-tethered silica, produced bimodal MWDs and a controlled polyethylene branching distribution in a single polymerization step.103,105,234−238 In accord with olefin polymerization on the corresponding homogeneous single-site catalysts, this indicates the presence of two single-site catalysts that independently polymerize olefins. Because of selectivity differences with respect to comonomer incorporation, it is possible to independently tailor MWDs and the branching distribution. Univation Technologies developed bimodal polyethylene, produced in their Unipol single gas-phase reactor, by exploiting cosupport of different single-site metallocene catalysts on the same silica support.239−242 Further reported combinations are silicasupported Ziegler/metallocene catalysts243,244 and metallocene/postmetallocene combinations.245 Using multisite-catalyzed ethylene polymerization, it became possible to produce bimodal polyethylenes with an inverse branching distribution, as evidenced by the degree of branching increasing with increasing polyethylene molar mass. As compared to the individual components, bimodal polyethylene blends containing slightly branched UHMWPE together with linear HDPE exhibited a considerably higher tear strength. At LyondellBasell, coimmobilizing bis(1-n-butyl-3-methyl-cyclopentadienyl)zirconium dichloride and 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine iron(II) chloride on a silica support has led to the development of butyl-branched LLDPE, produced in a single gas-phase reactor and exhibiting a unique balance of mechanical and optical properties in film extrusion.246−249 Silica supported multisite catalysts containing a silyl-bridged metallocene and tridendate late transition metal catalyst gave interesting PP-HDPE-ethylene-propylene copolymers in a cascaded polymerization process.250 Whereas dual-site metallocene catalysts require 1-olefin as comonomers, similar to the above-mentioned MgCl2-supported dual-site catalysts, coimmobilization of early and late transitionmetal catalysts on an MAO-tethered silica support enables control of branching using ethylene as the exclusive feedstock.108,208,222,251 The resulting branching is attributed to the chain-walking mechanism in which nickel alkyls migrate along the polyethylene chain during polymerization. Several other single-site catalysts were coimmobilized on silica and other metal oxides to prepare polyethylene with a bimodal molarmass distribution.252−257 Among them, Fe/Cr dual-site catalysts are exceptionally robust.258 They are prepared by cosupporting quinolylsilylcyclopentadienyl chromium (29) and bis(imino)pyridyl iron (30) on mesoporous silica supports with tailored pores resulting from emulsion templating. They enable facile control of the polyethylene MWD as a function of the Cr/Fe molar ratio. This indicates independent olefin polymerization on two adjacent sites, producing UHMWPE on Cr sites and HDPE on Fe sites. As shown in Figure 22, the Cr/Fe molar ratio governs the UHMWPE/HDPE weight ratio without affecting the average molar mass of the individual HDPE and UHMWPE

The support was prepared by reacting an aluminum alkyl with a solid MgCl2/ethanol adduct having spherical particle morphology. Neither MAO nor boranes were required for catalyst activation. The activity of the dual-site catalyst significantly increased only in the presence of 26. Because the required amount of the diimine-nickel catalyst component was very small, the properties of the resulting HDPE were retained with respect to the HDPE produced on the individual singlesite catalysts containing exclusively either Fe, Ti, or Cr. Figure 20 shows that the dual-site Fe/Ni catalyst produced more porous HDPE with respect to the dense HDPE granules obtained in the absence of Ni. Obviously, this increased porosity reduced the diffusion limitations typical of diffusion through dense polyethylene, thus improving catalyst activity. By means of ethylene polymerization on coimmobilized [1-(8quinolyl)indenyl]CrCl2 (28) with the iron catalyst 27 on a MgCl2/AlEtx(OEt)3−x support, Chadwick obtained bimodal polyethylenes in which UHMWPE was produced on Cr sites (Figure 21).221 Shear-induced crystallization of such bimodal polyethylene prepared on supported Cr/Fe dual-site catalysts led to the formation of shish-kebab polyethylene fibers when compression molded polyethylene, processed at temperatures near the melting temperature, was drawn in a second processing step. Co-immobilization of early and late transition-metal complexes in combination with polyinsertion and chain walking gave linear and branched polyethylenes without requiring 1-olefin addition.222 In comparison to other supported catalysts, the facile fragmentation of MgCl2supported catalysts gives better control of spherical particle growth during olefin polymerization. Moreover, it is advantageous in terms of both economy and sustainability that neither borate- nor MAO-activation of the support is required to immobilize a large variety of highly active single- and multisite Ti, V, Cr, Fe, and Ni catalysts.208,223 3.2. Silica-Supported Multisite Catalysts

Since the discovery of single-site metallocene catalysts during the 1980s, major emphasis in single-site catalyst development has been placed on heterogenizing homogeneous single-site catalysts without sacrificing their single-site nature to prevent severe reactor fouling typical of slurry and gas-phase polymerization with homogeneous catalysts. Moreover, heterogenization can improve the stability of catalytic sites, thus preventing rapid decay of the polymerization rate observed for many homogeneous single-site catalyst systems. The development of supported multisite catalysts greatly benefits from remarkable progress made in the development of silica-supported singlesite catalysts.208,224−230 Because covalent anchoring of catalysts on silica is rather tedious, few efforts have been made in industry to apply such multisite catalysts.231 An alternative synthetic strategy involves immobilization of the activator prior to addition of the transition metal complex. A comprehensive survey of the different strategies is given by Chadwick.208,229 Preferably, on impregnation of the silica support with MAO, optionally together with the single-site catalysts, MAO is attached to the silica surface by reacting with surface Si−OH groups. On the one hand, the mechanical stability of the supported catalyst should be high enough to prevent its disintegration when encountering shear forces during feed and stirring. On the other hand, the stability should be low enough to enable fragmentation of the catalysts during polymerization. Plugging of the pores with polymers can lead to severe losses of catalyst activity. Moreover, catalyst fragmentation is essential 1412


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In both strategies, it is an important objective to balance the olefin polymerization kinetics on the two different sites and to control particle growth without leaching of catalyst components. In another approach toward bisupported systems, Cho combined MgCl2 as a support for TiCl4 with SiO2 as a support for Cp2ZrCl2.265 Schilling and co-workers studied variation of the support nanostructure. They employed mesoporous silicates with hierarchical structures, such as MCM-41 that contained a regular arrangement of cylindrical nanopores, as supports for Ti/Zr/Fe multisite catalysts. Because metallocenes copolymerize ethylene with 1-olefins formed on iron sites, HDPE/LLDPE reactor blends are obtained from ethylene without requiring 1-olefin addition.266 In contrast to regularly arranged MCM-41, Jongsomjit et al. utilized MCM-41 with a bimodal pore distribution to produce bimodal polyethylene by immobilization of a single-site metallocene.267 Racemic ethylenebis(indenyl)zirconium dichloride was used in combination with modified MAO to obtain bimodal ethylene/1-octene copolymers with good activities. According to Soares, (nBuCp)2ZrCl2-catalyzed ethylene/1-hexene polymerization on silica nanoparticles with pore sizes ranging from 4.6 to 30 nm (SBA-15) produced polyethylene with bimodal MWDs and a variable comonomer distribution typical of supported dual-site catalysts.268 Yamamoto prepared Cr/metallocene dual-site catalysts supported on layered silicates such as montmorillonite. In the first step, the sodium cations of montmorillonite were exchanged for Cr3+ in an aqueous phase. In the second step, the dried Cr montmorillonite was activated with AlEt3 and used to immobilize zirconocenes (Cp 2 ZrCl 2 ), hafnocenes ([nBuCp]2HfCl2), as well as a bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride.269 Because of the presence of two different sites, bimodal polyethylene was obtained by ethylene polymerization on montmorillonite-supported Cr/metallocene dual-site catalysts. As compared to Phillips catalysts, such Cr dual-site catalyst systems were remarkably robust and tolerated aluminum-alkyl activators, which are required for metallocene activation. By combining the effects of dual-site catalyst systems with composite supports and diffusion control, Wang and coworkers developed polymer-coated core/shell-supported dualsite catalysts with selective placement of the transition metal sites in the core and shell.253,270−273 Typically, the transition metal catalysts supported on silica were embedded in a shell comprising a poly(styrene-co-acrylic acid) (PSA) membrane in which the carboxylic acid groups served as anchors for immobilizing the second sites. This core/shell catalyst architecture was produced by means of a phase-inversion process. Preferably, the catalyst component, which is more sensitive to hydrogen, was immobilized in the core, whereas the less hydrogen-sensitive site was incorporated into the shell. Because of their larger size, activator alkyls and comonomers have slower diffusion rates than ethylene and hydrogen and thus require more time to reach the inner sites. Dual-site catalysts such as (nBuCp)2ZrCl2/TiCl4, TiCl3/TiCl4, and Cp2ZrCl2/Fe produced polyethylenes with MWDs varying between 18.1 and 44.5. Li and co-workers developed core/shell supports for dual-site catalysts that contained silica-supported VCl3 as the core and a Fe(acac)3/2,6-bis[1-(2-isopropylanilinoethyl)]pyridine catalyst immobilized in the PSA shell.274 This core/shell dual-site catalyst produced disentangled polyethylenes with tailored MWDs and an inverted distribution of short-chain branches.

Figure 22. Catalyst components of dual- and triple-site catalysts that combine chromium and iron complexes and that are used for the preparation of polyethylene reactor blends.

fractions.259,260 Silica-supported three-site catalysts were prepared by coimmobilizing bis(imino)pyridyl chromium (31), which produces low molecular-weight PE (molar mass of 104 g/mol), with bis(imino)pyridyl iron (30), which produces medium molar-mass PE (105 g/mol), and with quinolylsilylcyclopentadienyl chromium (29), which produces UHMWPE. These catalysts gave trimodal and tailored ultrabroad MWDs, with the MWD varying between 10 and 420, that were readily tailored via the molar ratio of the three precatalysts.261 Similar to the MgCl2-supported Fe/Cr dual-site catalysts, the Fe/Cr dual- and triple-site catalysts were remarkably robust, yielding HDPE/UHMWPE polyethylene reactor blends, in which flow-induced crystallization of UHMWPE led to the formation of shish-kebab polyethylene fibers during classical injection molding. Unlike conventional reactor blends that are restricted to a few wt % UHMWPE, more than 10 wt % UHMWPE was incorporated without impairing melt processing by injection molding. Most likely, nanophase separation of UHMWPE during polymerization reduced the entanglement of UHMWPE, which is well-known to drastically increase viscosity. Rastogi showed that the processability of UHMWPE was markedly improved by lowering chain entanglement during polymerization.262,263 Moreover, Rastogi demonstrated that ethylene polymerization on homogeneous MAO-activated quinolylindenyl chromium (28) gave control of the UHMWPE entanglement density.264 UHMWPE disentanglement is an important prerequisite for producing advanced UHMWPE/HDPE reactor blends on supported multisite catalysts, producing melt-processable polyethylene reactor blends with a high UHMWPE content for injection and blow-molding applications as well as new generations of all-polyolefin composites, obtained by conventional injection and blow molding (see section 5.3). 3.3. Tailored Supports for Multisite Catalysts

Going beyond silica- and MgCl2-supported multisite catalysts, other supported catalyst systems were specifically designed for producing polyethylene reactor blends with bimodal MWDs and a controlled branching distribution on multisite catalysts. In principle, there are two different strategies toward the formation of dual-site catalysts: (i) coimmobilizing two different transition-metal catalyst precursors on the same support, and (ii) creating at least two different compartments for immobilizing one transition-metal catalyst precursor in different environments that have different olefin diffusion rates. 1413


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Whereas the majority of supports for dual-site catalysts are spherical, nanosheets, such as functionalized graphene as 2D carbon nanomaterials, prepared by thermolysis of graphite oxide or mechanochemical graphite functionalization, are attracting growing attention in olefin polymerization, particularly with respect to the in situ formation of graphene/ polyolefin nanocomposites as molecular carbon/polyolefin hybrids.275−284 Unlike the nonfunctionalized graphite, the presence of functional groups enables facile anchoring of MAO and other activators on graphene as well as immobilization of various transition metal catalysts. As illustrated in Figure 23, Mülhaupt and Stürzel prepared

Figure 24. Cr/Fe three-site catalysts, supported on functionalized graphene, produce polyethylene reactor blends with a high UHMWPE content by ethylene polymerization (1). During injection molding, the in situ-formed UHMWPE nanoplatelets (2) melt and form reinforcing shish-kebab UHMWPE fibers (3) by flow-induced crystallization.285

components are carefully matched, tandem polymerization catalysis produces a wide variety of polyethylene materials, ranging from stiff engineering plastics to flexible LLDPE packaging materials and even thermoplastic elastomers, using ethylene as the exclusive feedstock. By substituting cascade reactors with one reactor and by eliminating comonomer feed, tandem catalysis offers obvious benefits with respect to economics, ecology, and sustainability. In particular, elimination of the separate production, storage, transportation, and recycling of 1-olefin comonomers substantially lowers plant investment, operating costs, and emissions. However, to render polyolefin tandem catalysis useful in sustainable development, it is imperative to achieve full conversion of the in situ-formed vinyl-terminated oligo- and polyethylene macromonomers in ethylene copolymerization. Residual macromonomers could impair the mechanical properties and cause unacceptable emission of organic compounds by leaching. Comprehensive reviews on the development of polyolefin tandem catalysis, which also address the role of advanced single-site catalyst technology, have been presented by Bazan, Gibson, and Bianchini.32,286−290 As displayed in Schemes 7 and 8, and

Figure 23. Tailored HDPE/UHMWPE reactor blends prepared on silica-supported Fe/Cr dual-site catalysts. Polyethylene MWD is controlled via the Cr/Fe (29/30) molar ratio without affecting the average molar masses of the individual HDPE and UHMWPE fractions. Reprinted with permission from ref 259. Copyright 2014 Elsevier.

graphene-supported Cr/Fe triple-site catalysts by coimmobilizing quinolylsilylcyclopentadienyl chromium (29), which produces UHMWPE, together with bis(imino)pyridyl complexes of iron (30), which produces HDPE with an intermediate molar mass, and bis(imino)pyridyl chromium (31), which produces polyethylene wax. The molar ratio of the catalytic complexes enabled facile control of the polyethylene MWDs due to independent formation of the different polyethylenes on different sites. During melt-processing by classical injection molding, which is not restricted to processing at temperatures around the polymer melting temperature, the in situ-formed UHMWPE nanoplatelets readily melted and formed UHMWPE shish-kebab fibers by flow-induced crystallization of elongated UHMWPE, which has a lower crystallization rate and nucleates crystallization of HDPE (Figure 24). Simultaneously, graphene was uniformly dispersed in the polyethylene matrix.

Scheme 7. Tandem Polyolefin Catalysis on Dual-Site Catalysts: Ethylene Oligomerization on cat1 Produces 1Olefins or Vinyl-Terminated Polyethylene Macromonomers That Are Copolymerized with Ethylene on cat2 To Produce Short-Chain-Branched (SCB) and Long-Chain-Branched (LCB) Polyethylenes

4. POLYOLEFIN TANDEM CATALYSIS Apart from dual-site catalysts that combine polyinsertion with chain walking, tandem catalysis produces branched polyethylenes with a tailored branch distribution using ethylene as the exclusive feedstock without requiring the addition of 1olefin as a comonomer. Polyethylene tandem catalysis on dualsite catalysts joins together two catalytic cycles in one reactor by combining ethylene oligomerization with ethylene copolymerization of the in situ-formed 1-olefins or vinyl-terminated polyethylene macromonomers, respectively. Provided that the compatibility and the kinetics of the different catalyst 1414


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Scheme 8. Tandem Polyolefin Catalysis on Triple-Site Catalysts That Produce LLDPE with a Bimodal BranchLength Distribution (A) and Polyethylene Reactor Blends (B) Consisting of Slightly Branched UHMWPE and Highly Linear HDPE with a Lower Molar Mass

Figure 25. Molecular architectures of branched polyethylenes prepared by tandem polymerization catalysis: short-chain-branched LLDPE (A), LLDPE with long-chain branches (B), highly branched polyethylene elastomers containing semicrystalline polyethylene side chains (C), and reactor blends consisting of slightly branched UHMWPE together with highly linear HDPE (D).

distribution (Figure 25C), containing short and long alkyl side chains. The ratio of the two different alkyl side chains depends on the ratio of the two different oligomerization catalysts. Moreover, by combining catalysts having very different selectivities with respect to comonomer incorporation, the programmed synthesis of reactor blends containing slightly branched UHMWPE together with highly linear HDPE (Figure 25D) becomes feasible in tandem polymerization catalysis. It should be noted that this dream of tandem polyolefin catalysis has existed for more than 50 years. A closer look at pioneering advances in polyethylene tandem catalysis during the premetallocene era of Ziegler−Natta catalysis reveals the problems that have hampered industrial exploitation of tandem catalysis in the past. Inspired by the Ni-ylide-catalyzed oligomerization of ethylene, which produces 1-olefins in Shell’s Higher Olefin Process (SHOP),291 Ostoja Starzewski at Bayer AG designed a Ni-ylide catalyst family that produced a wide range of low and high molecular-weight, mainly vinylterminated oligo- and polyethylenes in the absence of aluminum alkyl activators. Their molar masses were controlled by varying the Ni-ylide architectures.292−295 Although these Niylide catalysts readily copolymerized ethylene with propylene, the incorporation of 1-olefin drastically decreased with increasing 1-olefin chain length. Typically, it was less than 1 mol % for higher 1-olefins. As a consequence, in ethylene polymerization, most of the unreacted polyethylene macromonomers remained as an undesirable impurity in the product. This behavior is typical of many other early Ziegler−Natta catalyst generations. Because of steric hindrance, insertion of the less reactive long-chain 1-olefins drastically slowed ethylene polymerization. Frequently in early catalyst generations, longchain 1-olefins were incorporated exclusively as end groups as a result of chain termination taking place on dormant sites following comonomer insertion, which drastically lowered the activity of the catalytic site. Hence, due to poor conversion,

depending on the type of oligomerization catalyst, polyethylene tandem catalysis produces LLDPE with short n-alkyl branches (Figure 25A,B) as well as novel families of elastomeric polyolefin graft copolymers containing a flexible, amorphous, highly branched polyethylene backbone and semicrystalline polyethylenes as side chains (Figure 25C). Because conventional gas-phase polymerization does not permit feed and incorporation of semicrystalline vinyl-terminated polyethylene comonomers, it is highly advantageous to produce and copolymerize them on adjacent sites using tandem polymerization catalysis. To improve the mechanical properties, especially the stress-crack resistance, butyl or hexyl branches that result from ethylene trimerization or tetramerization, respectively, are preferred with respect to the shorter ethyl branches that result from ethylene dimerization. In principle, as displayed in Scheme 7, tandem catalysis can combine more than two catalytic cycles, thus producing unique polyolefin architectures and reactor blends that are not readily feasible in conventional ethylene/1-olefin copolymerization. For example, triple-site catalysts can combine a catalyst for ethylene copolymerization with two oligomerization catalysts to produce two 1-olefins with very different chain lengths. Unlike branched polyethylene with a Schulz−Flory-type branch-length distribution produced on dual-site tandem catalysts, such triple-site catalysts yield polyethylene with a bimodal branch-length 1415


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respect to 1-butene incorporation. This lack of robustness, typical of many other first-generation Ziegler−Natta catalysts, made process control in tandem catalysis extremely difficult. Improved control in tandem catalysis, producing LLDPE with ethyl and butyl branches, was achieved by Fink and Denger, who combined ethylene copolymerization on the heterogeneous, activator-free MgH2/TiCl3/Cp2TiCl2 catalyst or homogeneous MAO-activated zirconocenes, such as Cp2ZrCl2, Et(H4Ind)2ZrCl2, and SiMe2(Ind)2ZrCl2, with in situ formation of 1-butene/1-hexene by simultaneous ethylene dimerization/ trimerization on a nickel-ylide catalyst.300 The degree of branching was controlled either by starting the catalysts at different times, by varying the Ni/Ti molar ratio, or by varying the zirconocene architectures. In this pioneering advance, it was demonstrated that the structure of the metallocene catalyst component enables facile variation of comonomer incorporation. During the late 1980s, a new age in polyolefin tandem catalysis started with the discovery of very robust stereorigid single-site catalysts. As compared to conventional Ziegler− Natta catalysts, they are much less sensitive to ligand-exchange reactions and exhibit substantially higher catalyst activities with variable selectivity with respect to the incorporation of low and high molecular-weight 1-olefins. In particular, since 1998, the single-site catalyst family has been expanded by exceptionally robust bis(imino)pyridyl iron and cobalt catalyst systems.31,301 They enable facile tailoring of 1-olefins, including vinylterminated oligo- and polyethylenes, with precise control of molar mass and degree of branching, and they are unaffected by the presence of many other single-site catalysts. Polyethylene tandem catalysis, exploitation of iron-containing dual-site catalysts, and reactor blend technology were pioneered by Du Pont.302−308 Moreover, following early advances at Union Carbide,309 advanced oligomerization catalysts are now available that afford in situ trimerization and tetramerization of ethylene in high yields. In tandem catalysis, this in situ formation of 1-hexene and 1-octene is advantageous with respect to 1-butene because incorporation of higher 1-olefins markedly improves the mechanical properties of branched polyethylene. At the beginning of the 21st century, breakthroughs in single-site catalyst technology make the dream of polyolefin tandem catalysis become an industrial reality. Furthermore, for the first time, it is industrially feasible to efficiently combine more than two catalytic cycles by means of tandem catalysis on triple-site catalysts (Scheme 8). Among the rapidly expanding families of single-site catalysts, constrained-geometry cyclopentadienyl-amido titanium catalysts, such as 32 (CGC), pioneered by Exxon and Dow, are well-known for their high reactivity with respect to incorporation of low and high molecular-weight 1-olefins. CGC has been exploited in ethylene homo- and copolymerization to improve shear thinning by incorporating a few long-chain polyethylene branches via copolymerization of polyethylene vinyl end groups.10,11,24,52,310,311 Here, vinyl-terminated polyethylene and polyethylene-graft-polyethylene are formed on the same catalytically active site. During the past decade, several groups have successfully combined CGC with various oligomerization catalysts in polyolefin tandem catalysis to produce new families of branched polyethylenes containing short-chain and semicrystalline long-chain n-alkyl side chains without requiring comonomer addition. Full conversion of the in situ-formed, high molar-mass 1-olefins has been achieved, regardless of their molar mass. Going beyond conventional

long-chain 1-olefins accumulated as highly undesirable byproducts. In contrast to Ti-based Ziegler catalysts, heterogeneous chromium catalysts, also known as the Phillips catalyst and which do not require main-group metal alkyls as activators, afford significant amounts of long-chain branching by efficient copolymerization of vinyl end groups. As illustrated in Scheme 9, Ostoja Starzewski combined homogeneous Ni-ylide catalysts Scheme 9. Long-Chain-Branched Polyethylene Prepared by Conventional Copolymerization (Two Steps) and by Tandem Polymerization Catalysis (One Step), Which Combines Ni-Ylide-Catalyzed Ethylene Oligomerization with Ethylene Copolymerization on Silica-Supported Cr(II) Catalysts (Phillips Catalyst)296

for ethylene oligomerization with activator-free silica-supported chromium(II) catalysts (Phillips catalyst) for ethylene/1-olefin copolymerization. Conventional copolymerization (two-step process, Scheme 9) in two reactors was compared to tandem catalysis on Ni/Cr dual-site tandem catalysts in one reactor (one-step process, Scheme 9).296 High Cr/Ni molar ratios were required to produce long-chain-branched polyethylene on dualsite catalysts. Typically, the catalyst activity drastically declined with increasing Ni content. It was assumed that either complexation or ligand-exchange reactions blocked the chromium(II) sites. Many other early generations of tandem catalysts share the same fate. Frequently, due to complex interactions between sites and severe catalyst poisoning, they produced ill-defined heterogeneous mixtures containing considerable amounts of undesirable low molecular-weight byproducts. Poor process control was the prime obstacle regarding commercial applications of early tandem catalyst systems. During the early days of polyolefin tandem catalysis, similar problems were encountered with polyethylene tandem catalysis on dual-site Ziegler−Natta catalysts combining ethylene oligomerization with ethylene copolymerization of the in situformed 1-olefin. In 1984, Kissin and Beach at Gulf Research and Development Company evaluated various conventional Ziegler−Natta tandem catalysts, which produced LLDPEs containing between 20 and 30 ethyl branches per 1000 C. Their best tandem catalyst system combined ethylene/1-butene copolymerization on the heterogeneous MgCl2/TiCl4/AlEt3 catalyst with simultaneous ethylene dimerization on a homogeneous AlEt3-activated titanium alkoxide catalyst to produce 1-butene in situ.297−299 The properties of LLDPEs prepared by tandem catalysis were very similar to those obtained by means of conventional ethylene copolymerization with 1-butene comonomer feed. The detailed analysis of polymerization kinetics, however, revealed that addition of the homogeneous dimerization catalysts and increasing concentration of 1-butene severely impaired the activity of the polymerization catalyst and also affected its reactivity with 1416


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metallocenes for ethylene oligomerization, Bazan designed zirconium complexes with boratabenzene ligands as an oligomerization catalyst in dual-site tandem catalysis. Whereas MAO-activated [C5H5B-Ph]2ZrCl2/MAO (33a) produced vinylidene-terminated oligoethylene, which does not copolymerize with ethylene, the corresponding MAO-activated [C5H5B-OEt]2ZrCl2 (33b) afforded vinyl-terminated oligoethylene macromonomers, which readily copolymerize with ethylene on the CGC site (Figure 26).312 Once formed, the

Figure 26. Complexes used by Bazan et al. for dual-site tandem catalysts by combining CGC with boratabenzene zirconocenes.312 Figure 27. Catalyst components of triple-site catalysts and tandem polymerization, as proposed by Bazan et al.316

macromonomers are incorporated into polyethylene exclusively at the CGC site, whereas the Zr sites are not active in copolymerization. In contrast to most other early multisite catalyst generations, polyethylene properties such as melting temperature and crystallinity were readily controlled by the CGC/Zr molar ratio. At ExxonMobile Chemical Co., Weng and co-workers copolymerized ethylene with 1-butene using an MAO-activated CGC/Cp2ZrCl2 dual-site catalyst to produce new families of thermoplastic elastomers containing a flexible poly(ethylene-co1-butene) backbone and pendent rigid semicrystalline polyethylene chains.313 In this process, vinyl-terminated polyethylene macromonomers, formed by ethylene oligomerization on Zr sites that do not incorporate 1-butene, were copolymerized with ethylene and 1-butene on CGC sites. The polymer properties depended on the types of the catalyst components and the process conditions. Kaminsky’s MAOactivated dual-site catalyst combined CGC with [Me2C(Cp)2]ZrCl2 to produce long-chain-branched polyethylene with branch lengths of up to 350 carbons in ethylene homopolymerization.314 Polyethylenes with much shorter side chains were obtained when CGC was combined with Ni-based oligomerization catalysts. For example, Bazan’s dual-site catalyst combined the copolymerization catalyst [(η5-C5Me4)SiMe2(η1-NCMe3)TiMe]+[MeB(C6F5)3]− (32b) with [(C6H5)2PC6H4C(OB(C6F5)3)O−P,O]Ni(η3-CH2CMeCH2) (34) to produce ethylene dimers and trimers. This dual-site catalyst did not require MAO-activators to form LLDPE containing ethyl and n-butyl short-chain branches.315−317 Bazan’s triple-site catalyst combined [(η5-C5Me4)SiMe2(η1-NCMe3)]TiMe+[MeB(C6F5)3]− (32b) with [(C6H5)2PC6H4C(OB(C6F5)3)O-κ2P,O]Ni(η3CH2C6H5) (35), and ((H3C)C[N(C6H5)]C[O−B(C6F5)3][N(C 6H 5)]-κ2 N,N})Ni(η 3-CH 2C 6H5 ) (36) (Figure 27) to produce unique polyolefin microstructures that were not paralleled by polyethylenes produced on dual-site catalysts.316 By balancing the three catalyst components in a homogeneous phase, 1-butene formed on 35 as well as Schulz−Florydistributed 1-olefins produced on (36) were copolymerized with ethylene on 32b, thus producing branched polyethylenes with 1-olefin contents varying between 3% and 46%. Ye and co-workers reported on the preparation of longchain-branched poly(ethylene-co-1-hexene) by ethylene poly-

merization in the absence of 1-hexene on MMAO-activated dual-site catalysts combining CCG with (η5-C5H4CMe2C6H5)TiCl3, which trimerized ethylene to form 1-hexene in situ as a comonomer.318,319 A similar approach toward LLDPE with nbutyl branches was exploited with MAO-activated dual-site catalysts that joined together various ansa-metallocenes, ranked [Me2Si(2-Me-Ind)2]ZrCl2 > [Me2Si(2,3,4,5-Me-Cp)(t-BuN)]TiCl2 > Cp2ZrCl2, with bis(2-decylthioethyl)amine-chromium as an ethylene trimerization catalyst.320 Furthermore, a remarkably wide variety of polyethylenes, ranging from LLDPE to amorphous flexible polymers, were reported by Bazan and Kaminsky, who combined CGC with CoCl2(NBT2) (NBT2) = [1-((6-benzo[b]thiophen-2-yl-pyridin-2-yl)ethylidene)-(2,6diisopropylphenyl)amine]).321,322 Although tailoring of bi- or multinuclear complexes that combine two sites in the same complex requires tedious and challenging multistep syntheses, they offer attractive prospects for applications in tandem catalysis.81,82 As reported by Marks and co-workers, LLDPE with exclusively n-butyl branching was obtained by ethylene polymerization on a heterobimetallic catalyst (37) containing a covalent bond between the CCGtype site for ethylene copolymerization (38) with the chromium bis(thioether)amine site (39) for ethylene trimerization (Figure 28).323 As compared to the performance of the dual-site catalyst, prepared by blending together CCG and Cr catalysts without a covalent linkage, the corresponding heterobimetallic catalyst gave polyethylenes with a considerably higher molar mass and higher branching densities. This result was attributed to the close proximity of the two sites: they were much closer together than in the majority of other dual-site catalysts. Multisite tandem catalyst technology is not restricted to CCG-based catalysts. For instance, coimmobilizing bis(1-nbutyl-3-methyl-cyclopentadienyl)zirconium dichloride as a copolymerization catalyst and 2,6-bis[1-(2,6dimethylphenylimino)ethyl]pyridine iron(II) chloride as a trimerization catalyst on the same support produced butylbranched LLDPE, which is equivalent to poly(ethylene-co-1hexene), but without requiring 1-hexene feed. Produced by tandem catalysis from ethylene in a single gas-phase reactor, these butyl-branched LLDPEs exhibited an attractive combina1417


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energy-intensive special processing, and commodity polyethylenes, which have much lower costs and are processed by extrusion, injection molding, and blow molding. Because of the significant progress made in multisite catalyst and reactor blend technology, this boundary is gradually being shifted by expanding the performance and application range of commodity polyethylenes produced by ethylene polymerization on multisite catalysts in a single reactor. Contrary to the first generation of multisite (“hybrid”) Ziegler−Natta catalysts, wellknown for their problematic process control and complex interplay of sites, advanced supported multisite catalysts contain independent sites and are exceptionally robust. In fact, coimmobilization of different single-site catalysts on the same support without sacrificing their single-site nature offers new options for tailoring polyolefin materials and reactor blends. Advanced multisite catalysts enable facile fine-tuning of polyolefin microstructures, MWDs, and reactor blend compositions with unprecedented precision by simply adjusting the types and molar ratios of coimmobilized single-site catalysts. Moreover, due to the close proximity of coimmobilized single catalysts, olefin polymerization on multisite catalysts produces reactor blends with a much finer dispersion of the different polyolefins, going well beyond the limited scope of high-shear melt blending with a high energy demand. The crucial point in the design of a polyethylene resin is the balance between processability and mechanical performance. It is well-known that control of the polyethylene MWDs can be used to adjust the rheology and flowability of the polymer to the targeted process (injection molding, blow molding, film blowing, pipe extrusion, etc.). However, increasing the molar mass, while suppressing the flowability, can enhance the mechanical strength of the polymer in the solid state, which is manifested in the stiffness, impact strength, and environmental stress-crack resistance (ESCR). Furthermore, the presence of long-chain-branching (LCB) is commonly acknowledged to enhance processability to the cost, however, of the mechanical properties. Finally, it is also a common practice to introduce higher 1-olefins (C4−C8) as a comonomer to enhance the impact strength and ESCR. An optimal incorporation, that is, in the high molar-mass fraction, is desired to maintain stiffness. Several multisite catalysts for olefin polymerization in a single reactor have been developed to match the targeted product properties. However, in the early days, multisite polymerization catalysis has been very challenging for commercial use due to a narrow processing window, low process reliability, stability, and reproducibility, as well as a narrow product range capability. The technical requirements for industrial slurry and gas-phase processes for bimodal or multimodal polyethylene in a single strain reactor are very demanding for the multisite catalyst. Differences in activity behavior, comonomer incorporation, hydrogen response, temperature, and pressure need to be engineered to a well-behaving catalyst. Only a few examples have been reported that have succeeded in this challenge. Among these are combinations of selected iron catalysts,338 which produce polyethylene having a narrow MWD as the low molecularweight fraction, with a second single-site catalyst for ethylene copolymerization, which produces the high molar-mass ethylene copolymers. In these iron-based dual-site catalysts, the major drawbacks typical of iron single-site catalysts, insufficient comonomer incorporation and the critically low hydrogen response, turn into a strong advantage. Hence, dual-site catalysts combining single-site iron with metallocene catalysts

Figure 28. Heterobimetallic tandem catalyst and the individual complexes used by Marks.323

tion of excellent mechanical and optical properties with good processability in film extrusion.246 Unlike most other dual-site catalysts, which require two different transition metal sites to enable tandem catalysis in one reactor, a single metallocene catalyst can be used to simultaneously dimerize ethylene to 1butene, which is then copolymerized with ethylene on the same metallocene catalyst. Researchers at Fina Technology, Inc., disclosed that ethylene polymerization on MAO-activated Me2C(3-tBu-Cp)(Ind)ZrCl2 produced ethyl-branched polyethylene incorporating 0.24−0.49 wt % 1-butene without adding any 1-butene as a comonomer.324 This was attributed to in situ dimerization of ethylene. However, this approach to single-site tandem catalysis remained restricted to a specific metallocene type. In a more general approach toward tandem catalysis on a single-site catalyst, Yan and co-workers exploited single-site catalysts exhibiting a dual oligomerization/polymerization nature, which is governed by the activator type.325 Typically, a zirconocene such as Et(Ind)2ZrCl2 or diketonate zirconium complexes such as (dbm)2ZrCl2 produce 1-olefins in the presence of the AlEt2Cl activator, whereas activation of both complexes with the MAO activator affords linear polyethylene. Hence, on activation of the zirconium catalyst precursor with a blend of AlEt2Cl/MAO, branched polyethylenes with broad MWDs and rather complex compositions are formed, as expected for ethylene copolymerization with the in situ-formed vinyl-terminated oligoethylene macromonomers having broad MWDs. However, both catalyst activity and the polyethylene melting temperature decrease with increasing AlEt2Cl/MAO molar ratio.

5. PERSPECTIVES OF MULTISITE OLEFIN POLYMERIZATION CATALYSIS 5.1. High-Performance Commodity Polyethylenes

Multisite polymerization catalysis offers prospects for improving the economics and sustainability of polyethylene materials and expanding their property and application ranges. This is of particular interest with respect to improving conventional product lines such as pipes and films, but also for developing new products such as all-polyolefin composites. Various Ziegler, Phillips, metallocene, and postmetallocene catalysts were combined to enable multisite ethylene polymerization.326−337 Traditionally, there has been a strict boundary between highperformance polymers such as UHMWPE, which require 1418


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enable the production of a broad range of polyethylenes for a diverse range of applications using a single catalyst in a single reactor.258 In principle, adjustment and fine monitoring of the polymer formation by means of modifying the relative activities of the active sites increases the overall capability of the multisite catalyst to produce well-defined short-chain branching (SCB); sufficient polyethylene molar mass and a tailored MWD with a defined split in the ratio between low molecular weight (LMW) and high molecular weight (HMW) polymer fractions is an essential requirement for product consistency and reproducibility. On the basis of differences in intrinsic properties of the catalyst components, various technical solutions have been described in patent literature to adjust the MWD and split in the desired direction such as (a) selective poisoning of one of the two components with ppm amounts of H2O, CO2, or alcohols,339−343 (b) feeding a second catalyst containing only one of the active site components, mostly LMW, to shift the split in favor of that weight fraction,344,345 (c) feeding two versions of the same cosupported multisite catalyst with different split to compensate catalyst lot-to-lot consistency and to cover a broader product range,346 and (d) modifying reactor parameters like temperature, cocatalyst concentration, H2, and/or ethylene partial pressure to influence the relative activity of one catalyst component.347−351 Additionally, the feeding of two metallocene catalyst solutions independently to a single reactor has been addressed by Phillips Petroleum Co.340,352−356 The polymer powders produced by methods (b) and (c) are heterogeneous and thus require a good post reactor homogenization. In polyethylene injection molding, a significantly higher flowability with respect to conventional cascade Ziegler-based HDPE is achieved by dual-site ethylene polymerization, thus providing a higher impact strength (up to 50%) and outstanding ESCR. Energy savings due to easy processing at lower extruder temperature and pressure as well as shorter cycle times are thus possible (Figure 29).357,358

reported to deliver required product consistency at reduced plant complexity and capital cost.359 The required control of overall MWD, Mw of components, and the split (ratio of HMW/LMW) has been realized by catalyst components ratio and reactor variables (temperature and gas composition). Typically, a combination of a bulky metallocene (e.g., PrCpCp*ZrCl2) and a tridendate postmetallocene (e.g., ([2,3,4,5,6-Me5C6)NCH2CH2]2NH)ZrBn2) was used by Univation to produce bimodal PE grades with inverse SCB distribution.360 Another technical solution is the feed of two different metallocenes to an isobutene suspension in a slurry loop process, which was pioneered by Chevron Phillips Chemical Co. A combination of unbridged and bridged metallocenes was used to produce narrow to broad MWDs suited for film applications.352−356,361 In academia, Hong and co-workers have cosupported a combination of CGC (32) with (nBuCp)2ZrCl2 (10) on silica. In the presence of 1-hexene and using butyloctyl magnesium as cocatalyst, pseudo bimodal MWDs and an inverse comonomer distribution were obtained. According to the polymerization characteristics and rate profiles, the individual catalysts in the hybrid catalyst were independent from each other.362 Substantial and simultaneous improvements of the mechanical properties and processability have been achieved with dualsite catalysts producing PE with a broad MWD and inverse comonomer distribution in a single-reactor. On the basis of a dual-catalyst system by combination of tridendate iron catalysts with metallocene complexes, a very narrow MWD with very pronounced inverse comonomer distribution for rotomolding application was reported. The rotomolding process requires high flowability at low shear-rates throughout a broad temperature spectrum with demanding product application properties. The combination of tridendate iron catalysts with metallocene complexes allows a very narrow MWD with very pronounced inverse comonomer distribution (Figure 30). This translates in turn to critical applications with high cold impact and stress crack resistance.363

Figure 29. Comparison of the flowability of HDPE prepared by multisite catalysis as compared to conventional Ziegler catalysts (data from LyondellBasell).

Figure 30. Advanced multisite technology affords excellent molecular weight control and efficient inverse SCB control (data from LyondellBasell).

Dual-site catalysts, which combine a metallocene and a Ziegler catalyst, were developed by Mobile Univation Technology for film and blow molding applications. Univation Technology has developed and licensed the bimodal catalyst under the trade name Prodigy Catalyst for PE100 pipe applications.239 Successful commercial production has been

The rheological behavior is controlled by the broadness and the content of the high molar-mass polyethylene fraction and the degree of long-chain branching, produced by copolymerizing vinyl end groups of polyethylene. Figure 31 compares a conventional polyethylene reactor blend (PE100), prepared in cascade reactors, to a polyethylene reactor blend obtained by 1419


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Figure 31. Polyethylene MWD and short-chain-branching (SCB) distribution obtained by ethylene polymerization in a conventional reactor cascade (PE100) and by ethylene polymerization on multisite catalysts (data from LyondellBasell).258

incorporated by ethylene polymerization on multisite catalysts without drastically impairing conventional melt processing. This is in sharp contrast to melt-compounding of HDPE with UHMWPE. As is apparent from Figure 33 (right), melt-

ethylene polymerization on multisite catalysts. To achieve a PE100 rating, polyethylene pipes must be able to withstand a minimum circumferential stress of 10 MPa for 50 years at 20 °C. Whereas both bimodal MWD and long-chain branching are essential for improving flowability in melt processing, the predominant incorporation of short-chain branches into UHMWPE produces interlamellar tie molecules that improve toughness, environmental stress-crack resistance, and fatigue resistance.12,55,364 Taking into account the slower crystallization rate and lower crystallinity of UHMWPE with respect to HDPE, the HDPE/UHMWPE balance is crucial for achieving the desired simultaneous improvement of stiffness, strength, stress-crack resistance, and fatigue resistance. Clearly, as is apparent from Figure 31, advanced multisite catalysis affords much better control of the polyethylene MWD and inverse branching distribution as compared to polyethylene produced in conventional cascade reactors. Thus, polyethylene reactor blends tailored by multisite catalysis help to reduce the weight of pipes and improve their durability without impairing facile recycling. In particular, this is essential for further improvement of the sustainability and for meeting society’s urgent demand for safe supplies of water and gas. Figure 32 shows typical applications of tailored polyethylenes, prepared by ethylene polymerization on multisite ethylene catalysts, as pipes for floor heating365−368 and fuel tanks363 for biodiesel. Moreover, due to intense mixing and disentanglement at low process temperatures, much higher UHMWPE contents are

Figure 33. Extruded HDPE/UHMWPE (90 wt %/10 wt %) blends produced by ethylene polymerization on multisite catalysts (left) and by melt-compounding of HDPE and UHMWPE, both of which were prepared on two different catalysts in two separate reactors (with permission of A. Kurek, Freiburg Materials Research Center).369

compounding of 10 wt % micrometer-sized UHMWPE powder together with HDPE in a twin-screw extruder fails to melt UHMWPE. Consequently, large UHMWPE particles are visible as an inhomogeneity, which can seriously impair mechanical properties such as tear resistance and impact strength. In contrast, extrusion of polyethylene, containing an identical amount of UHMWPE (10 wt %) and produced by ethylene polymerization on multisite catalysts, forms fully transparent, tough films. Incorporation of up to 18.6 wt % UHMWPE markedly reduces the abrasion of injection-molded polyethylene reactor blends exposed to a sand slurry (Figure 34). This is beneficial for the sustainable development of pipes and other products used in harsh abrasive environments. 5.2. Nanostructured Polyolefins as Thermoplastic Elastomers

The development of single-site metallocene and constrainedgeometry catalysts has enabled random copolymerization of large amounts of short- and long-chain 1-olefins into the polyethylene chain without encountering molar mass losses typical of many titanium catalysts of the first Ziegler−Natta catalyst generations.24,370 In random poly(ethylene-co-1octene) with a low crystallinity and a low density, marketed

Figure 32. Floor heating (left) and fuel tank based on polyethylene reactor blends produced on multisite catalyts (images from LyondellBasell). 1420


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Figure 35. Photonic polyethylene from self-assembled mesophases of poly(ethylene-co-1-octene)-block-polyethylene, which was prepared by chain shuttling olefin polymerization on dual-site catalysts. Reprinted with permission from ref 371. Copyright 2009 American Chemical Society.

5.3. All-Polyolefin Composites

In engineering applications, most polyolefins are reinforced with alien materials such as fibers, fillers, and nanoparticles that impair the facile recycling typical of pure polyolefins. In view of both the economics and the improved sustainability, it is highly desirable to eliminate alien materials and to produce selfreinforcing polyolefins (“all-polyolefin composites”), in which both the matrix and the reinforcing phases are made of the same polyolefin. In particular, thermoplastic all-polyolefin composites are of interest regarding composite recycling by remelting. Another important driving force for the development of all-polyolefin composites is weight reduction because the polyolefin density is considerably lower with respect to glass fibers and other inorganic fillers. A comprehensive review on self-reinforced polymers has been given by Kmetty, Bárány, and Karger-Kocsis.372 In principle, self-reinforcement requires 1D or 2D alignment of polyolefin chains within the polyolefin matrix to produce embedded, highly oriented polyolefin fiberor sheet-like structures resembling fiber-reinforced polyolefin composites or multilayer composites, respectively. Unlike conventional composites, however, these structures are an integral part of the entire hierarchical polyolefin molecular architecture. This facilitates bonding of the reinforcing phases to the matrix, which is an important prerequisite for efficient stress transfer. In contrast to polyolefin nanocomposites, neither nanometer-scale additives nor special safety precautions and handling procedures are required. In principle, two different strategies are employed for producing self-reinforced polymers. First, 1D structures are generated by self-assembly by flow-induced crystallization in solid-phase extrusion or meltshearing of solidifying melts, preferably at a processing temperature close to that of the polyolefin matrix. Second, prefabricated oriented 2D polyolefin structures, such as stretched tapes and 2D fabrics, are bonded together. Ehrenstein used polyethylene extrusion and injection molding through a convergent die, in which extensional flow results in oriented polyethylene chains, thus nucleating polyethylene crystallization to form cylindrical and shish-kebab-type structures by shear-induced crystallization.373 Although there is an ongoing debate concerning the actual nature of crystallization processes, the presence of high molecular-weight polyethylene clearly favors the development of shish-kebab structures [Figure 24, (3)]. According to Ryan, shear flow leads to stretching of the longest polyethylene chains, thus forming the “shish”, that nucleates crystallization of the lower molar-mass polyethylenes to afford the “kebab”.374 Several other groups have investigated shear-induced 1D crystallization and confirmed the prominent role of bimodal polyolefin MWDs.262,375−388 Special injection

Figure 34. Abrasion of injection-molded polyethylene reactor blends, produced by ethylene polymerization on multisite catalysts, decreases with increasing UHMWPE content (with permission of A. Kurek, Freiburg Materials Research Center).369

by The Dow Chemical Company under the trade name Engage, self-assembly affords nanometer-scale fringed micellar crystals that function as thermo-reversible cross-links between the elastomeric copolymer chains. However, due to the comparatively low copolymer melting temperature, the application range of such thermoplastic elastomers is limited to comparatively low temperatures. Ethylene/1-octene copolymerization by chain shuttling on dual-site catalysts has enabled the production of block copolymers, marketed by The Dow Chemical Company under the trade name Infuse. These copolymers consist of alternating flexible amorphous poly(ethylene-co-1-octene) with a low glass-transition temperature and polyethylene segments that have a much higher melting temperature than semicrystalline poly(ethylene-co-1-octene) (section 2.3.1).147 In poly(ethylene-co-1-octene)-block-polyethylene as well as in poly(ethylene-co-1-butene) with semicrystalline polyethylene side chains, prepared by tandem catalysis (section 4),313 the high precision in dual-site polymerization catalysis enables facile variation of elastomer properties via controlled nanostructure formation. Spherical, cylindrical, lamellar, and cocontinuous nanostructures are formed, depending on the polyethylene chain length. Clearly, the discovery of robust dual-site catalysts for chain shuttling and tandem catalysts has expanded the range of polyolefin block and graft copolymer elastomers that can substitute for less cost-, energy-, and eco-efficient elastomers such as plasticized PVC. The low weight of polyolefin elastomers combined with good performance at elevated temperatures and facile recycling is of particular interest in lightweight engineering applications. Moreover, variation of the polyolefin block copolymers architectures enables self-assembly in melts to produce highly ordered mesophases and photonic polyethylene films (Figure 35).371 Tailor-made polyolefin block copolymers offer prospects for designing a variety of functional polymers derived from olefin feedstocks. 1421


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molding processes, among them “shear-controlled orientation in injection molding” (SCORIM)389,390 and “oscillating packing injection molding” (OPIM),391,392 have been developed to fabricate all-polyolefin composites. Farah and Bretas used a slit die for polypropylene extrusion to prepare “all-polypropylene” composites with 2D self-reinforcement by shear-induced crystallization to form layered structures.393 Little is known with respect to tailoring polyolefin reactor blends by multisite catalysis for producing all-polyolefin composites. Rastogi and Chadwick reported on the formation of shish-kebab polyethylene when bimodal polyethylene, prepared by ethylene polymerization on a MgCl2/AlEtx(OEt)3−x-supported Fe (27)/ Cr (28) dual-site catalyst, was compression molded close to the polyethylene melting temperature and drawn in a subsequent processing step.221 Mülhaupt, Kurek, and Stürzel used silica dual- and triple-site Cr/Fe catalysts to tailor melt-processable bi- and trimodal polyethylenes with a high UHMWPE content. Unlike conventional shear-induced crystallization, the resulting reactor blends produce in situ UMHWPE shish-kebab fibers by classical injection molding and extrusion processes without requiring the narrow processing window and without lowering processing temperature.259−261 As is apparent from Figure 36,

enables simultaneous improvement in stiffness, strength, and toughness, approaching the property range typical of glass fiberreinforced polyethylene (Figure 37).

6. CONCLUSIONS AND OUTLOOK The remarkable progress in olefin polymerization catalysis and polyolefin technology contributes to sustainable development in manifold ways and helps to meet the urgent demand of the growing world population for cost-, resource-, and energyefficient as well as environmentally benign materials with a low carbon footprint and facile recycling. Today, polyolefins such as polypropylene and polyethylene are the clear leaders in both world-scale polymer production and life-cycle assessment, far ahead of biobased plastics. Produced in highly efficient, solventfree catalytic processes, polyolefins are hydrocarbon materials that preserve an oil-like high-energy content and are readily recycled by remelting or by thermal cleavage to recover oil and gas feedstocks (“renewable oil”) in essentially quantitative yield. Because of exceptionally high catalyst activities and precise control of polyolefin molecular architectures, no byproducts need to be removed. The minute amounts of nontoxic catalyst residues are left in the polyolefin. Moreover, morphology control by fragmentation of catalyst particles during polymerization enables the formation of spherical polyolefin granules in the reactor, thus eliminating pelletizing extrusion and saving energy. Modern polyolefin production can now meet the demands of green chemistry. Without any doubt, polyolefin chemistry is the most efficient way of utilizing resources, preserving them for future generations. Despite the great achievements in polyolefin research during the past 60 years, there are many challenges left to be met. Polyolefin chemistry and technology are still far away from maturity. In the past, the focus of catalyst research has been placed on improving catalyst activities and stereocontrol, aiming at controlling molecular architectures. In the future, as illustrated in Figure 2, the new focus will be designing functional hierarchic architectures without impairing cost-efficacy of polyolefins. The development of multisite catalyst and reactor blend technology will continue to play a prominent role. During the 20th century, in the premetallocene age, many attempts toward multisite and tandem catalysis were commercial failures due to poor process control, which frequently led to the formation of rather ill-defined product mixtures. Today, at the beginning of the 21st century, considerably more robust catalyst systems are at hand combining different single-site catalysts in a “single” multisite catalyst. Now it is possible to coimmobilize independent sites, thus preserving their single-site nature, to produce reactor blends in which immiscible polymers are blended together and uniformly dispersed on the nanometer scale. Blend composition and properties are primarily governed by the site mixing ratios and the properties of the individual catalyst components. Furthermore, interaction of sites via chain shuttling or molecular switching of sites enables the formation of a wide variety of segmented polyolefins. High-throughput screening enables identification of complementary single-site catalysts with matched compatibilities and polymerization kinetics. The multisite technologies presented in this Review create a new catalyst toolbox for tailoring advanced polyolefin materials by combining the performance of different single-site catalysts. Instead of designing reactor cascades, it is now possible to develop microreactors, embedded in the catalyst, to create a virtual nanometer-scale cascade within the catalyst and the polyolefin particles. Envisioned applications of multisite

Figure 36. Morphology, as determined by SEM, of injection-molded polyethylene reactor blends with broad MWDs, produced in a cascade reactor (left, Lupolen 4261AG from LyondellBasell) and (right) with a triple-site catalyst (28/29/30, 30 wt % UHMWPE, 200 °C IM temperature). Only polyethylene reactor blends produced with multisite catalysts and high UHMWPE content produce nanostructures during conventional injection molding.

conventional reactor blends with a low UHMWPE content of a few wt %, which are produced in cascade reactors, do not form shish-kebab polyethylene in classical injection molding. However, when the UHMWPE content is increased by designing MWDs via the Cr/Fe molar ratio and increasing the UHMWPE content to 10 wt %, massive shish-kebab formation occurred during conventional injection molding. Obviously, in agreement with earlier observations, the much higher UHMWPE content favors shish-kebab formation. Balancing low, medium, and high molar-mass HDPE content by coimmobilizing Cr-BIP, Fe-BIP, and Cr on silica supports 1422


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Figure 37. Polyethylene reactor blends as all-polyethylene composites prepared by ethylene polymerization using Cr/Fe triple-site catalysts (28/29/ 30), supported on functionalized graphene. The reactor blend (blue line) consists of 14 wt % UHMWPE and 21 wt % PE wax and was processed by injection molding at 200 °C: (a) SEM image of shish-kebab morphology of the all-polyethylene composite; (b) its mechanical properties in comparison to the conventional commercial cascade polyethylene reactor blend (Lupolen 4261, shaded area); and (c) rigid all-polyethylene composites are highly damage-tolerant.

catalysis include segmented polyolefins and tailored reactor blends ranging from thermoplastic elastomers and thinner flexible packaging materials to new generations of highly damage-tolerant lightweight engineering plastics such as allpolyolefin composites, which are much easier to recycle than conventional composites. The progress of multisite olefin polymerization catalysis greatly improves the performance of polyolefin materials without impairing their attractive combination of cost efficiency and high sustainability. Advanced polyolefin materials and tailored reactor blends, exhibiting low carbon footprint and leading position in life cycle assessment, offer great prospects for sustainable development ranging from lightweight engineering to safe transport of food, water, and energy.

Dr. Shahram Mihan studied chemistry in Munich (1988−1993) and received his Ph.D. (1993−1996) for his work on organometallic rhenium carbonyl complexes with Prof. Wolfgang Beck. At BASF he started the development of gas-phase polyethylene processes with special focus on chromium catalysts for blow-molding applications such as pipes and films (1998). After corporate restructuring, he joined the subsequently formed Basell polyolefins, which is now LyondellBasell polyolefins. His expertise includes chromium single-site catalysts, metallocene catalysis, multisite catalysts, and their applications. Since 2013, he has been the managing director of catalyst R&D at LyondellBasell in Frankfurt and the academic project leader of the BMBF multiKAT project. He is the author and coauthor of over 50 publications and 150 patents.

AUTHOR INFORMATION Corresponding Author

*Phone: +49 761 203 6273. Fax: +49 761 203 6319. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Dr. Markus Stürzel studied chemistry in Freiburg (2005−2010) and was honored with the Steinhofer prize. At the beginning of 2015, he finished his Ph.D. supervised by Prof. Rolf Mülhaupt in the field of catalytic ethylene polymerization, self-reinforcing all-PE, and polymeric nanocomposites.

Prof. Rolf Mülhaupt studied chemistry in Freiburg, Germany (1973− 1978) and obtained his Ph.D. at ETH Zürich, Switzerland (1978− 1981), where he started his research on catalytic olefin polymerization 1423


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(10) Chum, P. S.; Swogger, K. W. Olefin polymer technologies History and recent progress at The Dow Chemical Company. Prog. Polym. Sci. 2008, 33, 797−819. (11) Gahleitner, M.; Resconi, L.; Doshev, P. Heterogeneous ZieglerNatta, metallocene, and post-metallocene catalysis: Successes and challenges in industrial application. MRS Bull. 2013, 38, 229−233. (12) Boehm, L. L. The ethylene polymerization with Ziegler catalysts: fifty years after the discovery. Angew. Chem., Int. Ed. 2003, 42, 5010−5030. (13) Baier, M. C.; Zuideveld, M. A.; Mecking, S. Post-Metallocenes in the Industrial Production of Polyolefins. Angew. Chem., Int. Ed. 2014, 53, 9722−9744. (14) Muelhaupt, R. Green Polymer Chemistry and Bio-based Plastics: Dreams and Reality. Macromol. Chem. Phys. 2013, 214, 159−174. (15) Dube, M. A.; Salehpour, S. Applying the Principles of Green Chemistry to Polymer Production Technology. Macromol. React. Eng. 2014, 8, 7−28. (16) Razavi, A. Metallocene catalysts technology and environment. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3, 615−625. (17) Brintzinger, H. H.; Fischer, D.; Muelhaupt, R.; Rieger, B.; Waymouth, R. M. Stereospecific olefin polymerization with chiral metallocene catalysts. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143− 1170. (18) Imanishi, Y.; Naga, N. Recent developments in olefin polymerizations with transition metal catalysts. Prog. Polym. Sci. 2001, 26, 1147−1198. (19) Hamielec, A. E.; Soares, J. B. P. Polymerization reaction engineering - Metallocene catalysts. Prog. Polym. Sci. 1996, 21, 651− 706. (20) Coates, G. W. Precise Control of Polyolefin Stereochemistry Using Single-Site Metal Catalysts. Chem. Rev. 2000, 100, 1223−1252. (21) Gladysz, J. A. Frontiers in Metal-Catalyzed Polymerization: Designer Metallocenes, Designs on New Monomers, Demystifying MAO, Metathesis Déshabillé. Chem. Rev. 2000, 100, 1167−1168. (22) Alt, H. G.; Köppl, A. Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization. Chem. Rev. 2000, 100, 1205− 1222. (23) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Selectivity in propene polymerization with metallocene catalysts. Chem. Rev. 2000, 100, 1253−1345. (24) Chum, P. S.; Kruper, W. J.; Guest, M. J. Materials properties derived from INSITE metallocene catalysts. Adv. Mater. 2000, 12, 1759−1767. (25) Braunschweig, H.; Breitling, F. M. Constrained geometry complexes - Synthesis and applications. Coord. Chem. Rev. 2006, 250, 2691−2720. (26) McKnight, A. L.; Waymouth, R. M. Group 4 ansa-cyclopentadienyl-amido catalysts for olefin polymerization. Chem. Rev. 1998, 98, 2587−2598. (27) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. FI Catalysts for Olefin Polymerization-A COrnprehensive Treatment. Chem. Rev. 2011, 111, 2363−2449. (28) Nakayama, Y.; Kawai, K.; Fujta, T. FI Catalysts: Unique Olefin Polymerization Catalysis for Formation of Value-added Olefin-based Materials. J. Jpn. Pet. Inst. 2010, 53, 111−129. (29) Sakuma, A.; Weiser, M.-S.; Fujita, T. Living olefin polymerization and block copolymer formation with FI catalysts. Polym. J. 2007, 39, 193−207. (30) Makio, H.; Fujita, T. Development and Application of FI Catalysts for Olefin Polymerization: Unique Catalysis and Distinctive Polymer Formation. Acc. Chem. Res. 2009, 42, 1532−1544. (31) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. The search for new-generation olefin polymerization catalysts: Life beyond metallocenes. Angew. Chem., Int. Ed. 1999, 38, 428−447. (32) Gibson, V. C.; Solan, G. A. Iron-based and cobalt-based olefin polymerisation catalysts. Top. Organomet. Chem. 2009, 26, 107−158.

with Prof. Piero Pino. After industrial assignments at Du Pont Central Research in Wilmington, DE (1981−1985) and Ciba-Geigy, Plastics & Additives Research in Marly, Switzerland (1985−1989), in 1989 he was appointed full professor of Macromolecular Chemistry and director of the Institute of Macromolecular Chemistry at the University of Freiburg. Since 1992, he has been the managing director of the Freiburg Materials Research Center (FMF) and has been a member of the Heidelberg Academy of Sciences since 2000. He was awarded the Hermann Staudinger Prize of the GDCh in 2009 and the H. F. Mark Medal in 2013. His research, which has been published in 400 papers in refereed journals and 98 patents, focuses on polymerization catalysis, polymers for sustainable development, blends, nanocomposites, functional processing, and tailored specialty polymers

ACKNOWLEDGMENTS We kindly thank the German Federal Ministry of Education and Research (BMBF, project no. 03X3565C, “multiKAT”). This work was supported by LyondellBasell Polyolefine GmbH. GLOSSARY CGC constrained-geometry catalyst ESCR environmental stress-crack resistance HDPE high-density polyethylene HMW high molecular weight IM injection molding LCB long-chain branches LDPE low-density polyethylene LLDPE linear low-density polyethylene LMW low molecular weight MWD molecular weight distribution MAO methylalumoxane OPIM oscillating packing injection molding PP polypropylene SCORIM shear-controlled orientation in injection molding SCB short-chain branches TMA trimethyl aluminum UHMWPE ultrahigh molecular-weight polyethylene REFERENCES (1) Kaminsky, W. In Polyolefins: 50 Years after Ziegler and Natta I: Polyethylene and Polypropylene; Kaminsky, W., Ed.; Springer: New York, 2013; Vol. 257. (2) Kauffman, G. B. History of Polyolefins: The World’s Most Widely Used Polymers; D. Reidel Publishing Co.: Boston, MA, 1985. (3) Porri, L. The years 1954−1963 at the Milan Polytechnic Institute. Memories and reflections on a period which revolutionized macromolecular chemistry. Polim.: Cienc. Tecnol. 2009, 19, E4−E11. (4) Reetz, M. T. One Hundred Years of the Max-Planck-Institut fuer Kohlenforschung. Angew. Chem., Int. Ed. 2014, 53, 8562−8586. (5) Ziegler, K.; Natta, G. Paving the way to polythene. Chem. World 2013, 10, 50−53. (6) Busico, V. Giulio Natta and the Development of Stereoselective Propene Polymerization. In Polyolefins: 50 Years after Ziegler and Natta I: Polyethylene and Polypropylene; Kaminsky, W., Ed.; Springer: New York, 2013; Vol. 257. (7) Busico, V. Ziegler-Natta catalysis: Forever young. MRS Bull. 2013, 38, 224−228. (8) Chadwick, J. C. Polyolefins - Catalyst and Process Innovations and their Impact on Polymer Properties. Macromol. React. Eng. 2009, 3, 428−432. (9) Qiao, J.; Guo, M.; Wang, L.; Liu, D.; Zhang, X.; Yu, L.; Song, W.; Liu, Y. Recent advances in polyolefin technology. Polym. Chem. 2011, 2, 1611−1623. 1424


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