Olefin Polymerization with Supported Catalysts ... - ACS Publications

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Olefin Polymerization with Supported Catalysts as an Exercise in Nanotechnology Markus Klapper,* Daejune Joe, Sven Nietzel, Joseph W. Krumpfer, and Klaus Müllen* Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany ABSTRACT: Research concerning polyolefin synthesis is often focused on the particular catalyst being used, and the importance of the support material is generally not fully appreciated. Only a few common inorganic carriers, such as SiO2 or MgCl2, are typically described in literature, with new developments in support materials rarely found. Acknowledging this lack in fundamental research on support materials, numerous cases in which the catalyst support, particularly organic nanoparticles, plays a critical role in the formation of polyolefins are described. Here, new organic supports for both Ziegler−Natta and metallocene catalysts are described under the same conditions used for inorganic supports. In these cases, similarly high activities can be achieved, while also offering additional features based on designed polymer architectures which inorganic supports cannot provide. These features produce polyolefin fibers and core−shell structures directly from the reactor. Due to the broad synthetic variety, nanoparticles can be optimized for morphology control of the polyolefin products, such as shape and bulk density. The introduction of nucleophilic groups can further improve the binding strength between the catalyst and the support. For the design of new support systems, many of the central concepts found in nanotechnology (e.g., size control, surface properties, or nanoparticle interactions) are extremely important, and it is surprising how much modern nanotechnology has to offer such a mature field as polyolefin synthesis. KEYWORDS: polyolefin, nanotechnology, supported catalyst, morphology control, metallocene, Ziegler−Natta

1. INTRODUCTION Polyolefins are the most widely used class of polymer with a global consumption of approximately 130 million metric tons in the year 2012 and are forecast to grow 4% per year over the next 5 years.1 While initially believed to be limited to only low cost applications, such as plastic bags, polyolefin materials have recently been demonstrated to be direct competitors to much more expensive high performance polymers such as polyetherketones, polyethersulfones, or polyaramides. As a result, predictions in the 1980s claiming that high performance polymers will acquire market shares of more than 10% have become obsolete. Polyolefins can be found everywhere in daily life, as they are included in textiles, housing, medicine, and automotive industries. They can be used in package materials, in bullet proof vests, or even as hip implants.2 The reasons for this omnipresence are the extremely cheap production via various catalytic polymerizations such as the Ziegler−Natta-, Phillips-, and metallocene-catalyzed processes and the widely adjustable polymer properties. With nearly 80 years of olefin polymerization history, one may conclude, perhaps justly, that this process is quite mature. This article, however, will show that while certainly well-established, there is still much room for new developments toward polymerization methods and new polyolefin materials. Herein, the interplay of polymerization catalysts and supports is particularly highlighted. Prior to this, © XXXX American Chemical Society

though, it would seem appropriate to provide a brief historical background on the development of catalysts. The first catalysts for olefin polymerization were developed in the 1950s by Phillips Petroleum using chromium oxides supported on either silica or alumina (generally known as Phillips catalysts).3 These catalysts gave polyethylenes with moderate molecular weights in the range of 10−20 kg·mol−1. This polymerization process involves the reduction of Cr(VI) complexes to Cr(II) by ethylene. Replacement of one of the coordinated tetrahydrofuran (THF) molecules by one ethylene molecule initiates the polymerization (Figure 1). Although the Phillips catalysts are capable of producing various types of high density polyethylene (HDPE) with particularly high molar mass, they are incapable of polymerizing propylene. In 1953, in part to overcome some of the shortcomings of the Phillips catalysts, Karl Ziegler discovered that by combining catalysts like TiCl3 and Et2AlCl, a production of polyethylene with molecular weights exceeding 20 kg·mol−1 in larger quantities could be prepared (Figure 2).4 This catalyst showed high ethylene polymerization activity under even mild Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 11, 2013 Revised: October 23, 2013

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dx.doi.org/10.1021/cm402309z | Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. Activation and polymerization of ethylene in the presence of Phillips catalysts.

A major breakthrough in olefin polymerization was the discovery of active metallocenes. In the case of bis(cyclopentadienyl)titanium (or zirconium) dichloride, uniform catalytic sites (single-site catalysts) were formed yielding polyolefins with narrow molecular weight distributions (MWD) lower than 2. Metallocenes became particularly relevant for the industry due to their possible activation by methylaluminoxane (MAO), which was introduced by Sinn and Kaminsky in 1980. Under these conditions, polyolefins can be achieved with a high productivity.6 Similar to the Ziegler catalyst, the aluminum compound (MAO) abstracts one chloride ion and substitutes the second with an alkyl group (Figure 3). The first addition of monomer occurs at the free coordination site (vacant site). Insertion of monomer between cationic metal center and alkyl group leads to initiation of the polymerization. This illustration, however, is highly simplified since a cage of MAO is additionally formed around the active site during the activation process.7 Metallocenes have gained importance by substitution of the η5cyclopentadienyl unit(s) with larger aromatic ligands, such as fluorenyl or indenyl, and furthermore by bridging these ligands with silylene or alkylene moieties (Figure 4).8 This transformation results in chiral catalysts which are capable of adjusting the stereochemistry of polymerized substituted olefins, such as propylene or hexene. Particularly, control of the interaction between monomer and catalytic site prior to monomer insertion is paramount.9 Depending on the nature of the catalysts, atactic, isotactic, syndiotactic, or hemitactic polymers can be produced (Figure 4).10 For example, while bis(cyclopentadienyl)zirconium dichloride gives only atatic polypropylene, isopropylidene(9-fluorenyl)(1cyclopentadienyl)zirconium dichloride and rac-dimethylsilylbis(1-indenyl)zirconium dichloride produce syndiotactic propylene and isotactic propylene, respectively. The control of tacticity is important as it affects the crystallization behavior and the thermal properties of the obtained polyolefins.10 Practically speaking, the materials are rather flexible when the degree of

Figure 2. Activation and polymerization of Ziegler−Natta catalysts.

conditions. The catalytic system was further developed by Natta who used crystalline α-TiCl3 in combination with Al(C2H5)3. In this way, the synthesis of high density polyethylene and, for the first time, isotactic polypropylene was successful.5 As shown in Figure 2, the active site of the titanium complex is formed through the replacement of one of the chloride ions in a penta-coordinated titanium chloride by an alkyl group originating from Al(C2H5)3 (step A). The polymerization subsequently starts after formation of an alkene complex at the still uncoordinated position (step B) by complex rearrangement at the titanium center (step C) and insertion of the next monomer at the new coordination site.

Figure 3. Activation and polymerization in the case of metallocenes. B



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complexes of early transition metals (Ti, Zr, Hf).11d,14 Furthermore, the development of group IV transition metal complexes having bis(phenoxyimine) ligands, known as FI catalysts, has been reported.15 These catalysts yield polyethylenes having extremely high molecular weights (UHMWPE) (Mw > 2 million) and narrow MWDs due to their “living polymerization” character, leading to polymers with the highest reported mechanical properties among polyolefins.2b,16 From the previous examples, it is clear that the nature of the metallocenes and postmetallocenes or the titanium and chromium complexes in a Ziegler−Natta catalyst is decisive for the properties of the polyolefin, as it determines molecular weight, MWD, and branching. Also important for the industrial production of polyolefins is the heterogenization of the catalysts. Under equivalent homogeneous polymerization conditions, the polymers are often obtained as materials with a low bulk density and amorphous shape.17 For transport and processing, high density spherical olefin particles in a size range of 0.5−3 mm are desired since the ability to transport more material at once significantly lowers shipping costs and these particles possess a high flowability for easy processing in an extruder or a film blowing reactor.18 For economic reasons, such morphology control of the products must be obtained during the polymerization process itself, which is most recently achieved by supporting the catalysts. Thereby, the carrier acts as a template for the product particle formation, since the shape of the supports is replicated during polymerization.3,17 This effect has been demonstrated by electron microscopy studies.19 In order to achieve higher bulk density, the supports should be adjusted to the needs of specific catalysts. In industrial processes, MgCl2 is used for Ziegler−Natta catalysts since it is isomorphous with TiCl3 and therefore an excellent scaffold for the strong adsorption of the active sites.3 Both Phillips and Ziegler−Natta catalysts, however, do not consist of a single catalytically active species. Since the catalyst may adsorb differently to various support crystal phases, these catalyst exhibit multisite active species (Figure 6). This difference in catalytic centers and activities results in polymers with rather broad MWD.20 Over the years, MgCl2-based supports have been modified by combination with various additives (internal donors), in particular electron donors, resulting in an increase of both catalytic activity and tacticity control for propylene polymerizations. While initial experiments in the 1940s gave only atactic, low molecular weight waxy polypropylene, nowadays, the Ziegler−Natta systems based on mixtures of TiCl3, MgCl2, and polydimethylsiloxane exhibit extremely high activities (>20 000 kg PE/g cat × bar) and yield polyethylene with adjustable high molecular weights in the range of 300−1000 kg·mol−1.21 For Phillips catalysts, silica particle supports are favored as chromium forms a covalent bond with the silanol groups on the surface.22 Silica is also used for metallocenes as it strongly interacts with the MAO-cage which is formed around the metallocene catalyst. A stable immobilization of catalyst that avoids dust formation or reactor fouling is only one of the criteria for a support to be excellent. Supports should also be spherical since this is the only way of obtaining spherical products due to the replication effect.23 In addition, silica particles should be large enough to be capable of immobilizing sufficient amounts of catalyst to yield the required product size.23,24 For example, in order to generate polyolefin product particles having a size in the range of several 100 μm to a few

Figure 4. Tacticity control in polypropylene by metallocene catalysts.

tacticity is low. On the other hand, isotactic polymers are generally more mechanically stable, but brittle, due to the strong tendency toward crystallization. The most recent advances in the catalytic polymerization of olefins have emerged from the development of “non-metallocene” single site catalysts (known as postmetallocenes) based on diimine complexes of nickel and palladium and phenoxyimine complexes of zirconium and nickel (Figure 5).11

Figure 5. Postmetallocenes based on diimine and phenoxy-imine ligands.

Discovery of highly active α-diimine nickel catalysts, which produce branched polyethylene without using comonomers via a “chain-walking” mechanism,12 has resulted in more intensive research of postmetallocenes in recent years.13 Interestingly, postmetallocene complexes bearing late transition metals (Ni, Pd) are capable of synthesizing functionalized polyolefins due to their higher resistance against polar groups, compared to C



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Figure 6. (a) Adsorption of TiCl3 to different crystal phases of MgCl2 and (b) resulting differences in activity yield broader MWD.

Figure 7. Fragmentation processes in the case of Silica and MgCl2 supports.

if fragmentation is too slow, the fragments of the supports will not be small enough to avoid scattering effects.24 The fragmentation process works nicely for MgCl2 supports in the Ziegler−Natta case while incomplete fragmentation is a continuing concern in the use of silica for metallocene-catalyzed polymerizations. For silica, the fragmentation process starts from the outer layer and slowly proceeds toward the inside, whereas in the case of MgCl2 fragmentation occurs throughout the whole particle right from the start (Figure 7).23a,25 Oftentimes, this “layer-by-layer” process of silica supports leads to not only an incomplete fragmentation but also an induction period delaying the polymerization process. At the initial stage, a dense crystalline polyolefin layer is formed on the silica support and results in a diffusion barrier for the monomer (Figure 8). Only after a certain reaction time, the layer breaks apart, and subsequently, microchannels are generated which grant access toward the inner reactive sites to yield the full fragmentation of the support.27 Despite its many advantages, the necessity of supporting catalysts has several drawbacks in terms of activity for olefin

millimeters, the supports must typically possess a size of 15−60 μm.3 However, the support material often cannot be isolated from the products, due to their low relative concentration. Therefore, scattering of visible light by micrometer-sized support particles incorporated in the polyolefin particles can thus be expected. As a result, a loss of transparency of such products is observed.23a This problem can be overcome by designing supports with a fragile structure. Typically, these supports are comprised of nanoparticles (primary particles) which are “glued” together to form porous micrometer-sized aggregate particles (secondary particles) and loaded with an activated catalyst. In such a scenario, the increasing mechanical stress within the pores during polyolefin formation causes the secondary particles to break apart into the initial nanometer-sized primary particles. Control of the fragmentation process of microparticle aggregates into nanoparticles is one of the primary concerns for industrial polymerization processes.24 If fragmentation occurs too early during polymerization, only fluffy materials are obtained since there is no replication effect. On the other hand, D



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Figure 8. Kinetic investigation of the ethylene polymerization by bis(benzindenylcyclopentadienyl)zirconium dichloride supported on silica. Reproduced with permission from ref 26, copyright 2001 Wiley-VCH Verlag GmbH, Weinheim, Germany.

polymerization and accessibility of long chain α-olefins. The activity of supported metallocene catalysts is typically lower by a factor of 10 compared to homogeneous polymerizations.28,29 While under homogeneous conditions the monomer is able to reach the catalyst from all directions, in the supported case the catalyst is partially shielded by the rigid support. Additionally, the rigidity of inorganic surfaces affects the conformation of catalytically active sites and therefore decreases the stereo control in cases of higher α-olefin polymerizations. In this regard, “softer” organic microparticles prepared by emulsion polymerization of styrene, vinyl chloride, and other monomers came into focus as new supports for metallocenes.30 Organic supports were expected to more closely match the properties of a homogeneous polymerization as they offer a flexible surface and surrounding for the catalyst. The diffusion was believed to be less hampered by an organic support as it is swellable by monomer. The flexibility of a polymeric material should also allow faster monomer diffusion toward the catalyst without the activity being negatively influenced. However, these concepts only partially fulfill the expectations for a new support. While they allowed stable immobilization of the catalyst, the fragmentation of the supports was not considered. This resulted in low activities and inhomogeneities in polyolefin products.30c,31 From these early studies, therefore, the primary consideration must shift to the design of organic microparticles which are able to undergo fragmentation and show excellent binding abilities toward the catalysts without compromising activity. Herein, the fulfillment of these requirements will be demonstrated using organic supports based on microgels of reversibly cross-linked polystyrene polymers and with microparticles formed by self-assembly of nanoparticles. As an outline, the following points will be the focus of different sections: the stable covalent and noncovalent loading of metallocenes on linear polymers (Section 2); organic nanoparticles based on chemically cross-linked (Section 3.1)

and physically aggregated (Section 3.2) polystyrene, pyridinecontaining nanoparticles (Section 3.3), and nanoporous polyurethane microparticles (Section 3.4) for polymerizations using postmetallocenes; shape anisotropic supports formed by polystyrene nanoparticles to yield polyolefin fibers directly out of the reactor (Section 4); the formation of complex polymer architectures such as polyolefin core−shell structures in one step triggered by spatially resolved loading of catalysts on supports (Section 5) and morphology control by polymerizing in confined geometries such as micelles or silica hollow sphere (Section 6); and organic supports for Ziegler−Natta systems (Section 7). This review will show that organic nanoparticle supports can be effectively used as replacements for traditional inorganic supports. It will also demonstrate the advantages of organic supports based on aggregates of latex particles for opening new avenues to new materials by controlling surface polarity, shape, and catalyst interactions of the support. Of crucial importance is the fragmentation of the support. In a sense, it must respond to the formation of the polymer and only occur during the polymerization itself. Thereby, a precise control over binding strength among the fragments is required. When dealing with these characteristics, it becomes obvious that olefin polymerization can be rightfully described as a special case of applied nanotechnology which will be addressed in more detail in Section 8.

2. LINEAR POLYMERS AS SUPPORT FOR METALLOCENE CATALYSTS The development of new supports is very much affected by the applied processes in already existing industrial production plants. One cannot ignore that a drop-in technology, which is able to use existing equipment, has the highest potential of being transferred to industrial production processes. Therefore, new approaches should consider the above-mentioned aspects E



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Figure 9. Cyclopentadienyl functionalized polystyrene as catalyst support.

Damaged structures cannot be eliminated by purification methods and may potentially affect the polymerization process. To overcome these limitations, a process is needed which allows the separate synthesis of both the catalysts and the supports. In this approach, already existing metallocenes and postmetallocenes are loaded on organic supports via a noncovalent binding which does not affect the polymerization activity by interaction with catalytically active centers. This can be achieved by utilizing the specific characteristics of the MAOactivated catalyst itself. The metal species is immured by a cage of MAO (Figure 10).7,37 This cage shields the catalyst from attack by nucleophilic groups and is highly oxophilic due to the involved aluminum species.

like immobilization, fragmentation of the catalyst, and morphology control, and concepts that are established for inorganic supports should also be adopted to polymeric systems, bearing in mind that organic materials are more easily varied in their shape (e.g., linear polymers, single particles, or networks) and potential anchor groups for catalysts (alcohol, amides, and amines like pyridine or polyether). In addition, the preparation of organic materials by radical polymerization in solution or emulsion is well-known, fairly easy, and inexpensive.32 The key parameter for the design of new supports is the formation of reversibly cross-linked materials which can mimic the fragmentation process discussed above. The formation of such nanoparticles will require techniques to synthesize functional nanoparticles and to postmodify them without precipitation. In our first approach, we attempt to achieve fragmentation by using polystyrene which is functionalized with cyclopentadiene units (Figure 9).30a This polymer is easily accessible by copolymerization of styrene and chloromethylstyrene, followed by nucleophilic substitution of the chlorine with cyclopentadiene. The postfunctionalization has a twofold role regarding the formation of metallocene-based catalytic system: it allows a reversible intermolecular cross-linking via Diels− Alder reaction which yields gel-like microparticles in the range of 10−100 μm, and the cyclopentadiene can be used as a ligand to covalently bind a zirconium catalyst. Under industrial conditions (70 °C and 40 bar), these supported catalysts achieve activities of more than 11 000 kg/ (mol Zr × bar), and spherical particles with high bulk densities are isolated.33 Subsequently, Wang et al. have confirmed the above-mentioned results in similar studies by using the same catalytic system.34 In addition, these studies have shown that the catalytic activity and the product morphology can be improved by introducing spacer chains between the support and metallocene.31,35 Nevertheless, this process reveals several drawbacks. First, the polymerization under higher pressure with more reactive monomers such as propylene only yields very fluffy materials.36 Ruling out catalyst leaching which is prevented by the covalent bonding, premature fragmentation of the support can be assumed. As a result the generated polymer does not retain the spherical shape of the initial support.36 Second, the covalent binding of the catalyst to the polymeric backbone limits the choice of catalysts as one ligand must be a cyclopentadiene unit. The broad variety of available metallocenes, which are able to control the tacticity, cannot be used. Additionally, reactions with polymeric species often cause undesired side-reactions or incomplete functionalization of the polymer backbone.

Figure 10. Proposed structure for an activated MAO/metallocene complex.7,37

To improve the stable adsorption of different MAO/ zirconium complexes, ether linkages are introduced into the polymers. The formation of this polymer is similar to the previous case, with the exception that methoxystyrene is added as an additional comonomer (Figure 11).32d By separating the supporting particle formation and the immobilization of metallocenes, the design of support/catalyst systems becomes more flexible, as loading of supports with any metallocene catalysts such as (BenzInd)2ZrCl2 is feasible. The still-existing cyclopentadiene units were only incorporated for the crosslinking reaction. The methoxy groups, however, were not sufficiently nucleophilic to strongly bind the catalyst by noncovalent interactions, resulting in reactor fouling. Only after substitution of the methoxy groups by more nucleophilic oligoethylene oxide groups could a satisfactory increase in the binding of the MAO/catalyst complex be achieved (Figure 12).38 These catalytic systems are again highly active, but they still do not solve the problem of premature fragmentation of the support in the case of propylene polymerization.38 This is F



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increase in the distance between different catalytic sites and, therefore, in less densely packed products. From these studies, linear polymers do not seem to be qualified as supports for metallocene catalysts. To overcome their drawbacks, the design of new supports must incorporate more rigid structures. Therefore, spherical organic nanoparticles will be used rather than linear polymers. Due to the large surface area of nanoparticles, the number of potential cross-linking sites is increased holding promise for a more stable network.

3. ORGANIC NANOPARTICLES AS SUPPORTS 3.1. Chemically Cross-Linked Polystyrene Nanoparticle Network. As stated in the introduction, one of the decisive properties of silica supports is the fragmentation of secondary particles into nanoparticles during polymerization.23a In designing new organic supports, the concepts which were already established for the preparation of silica supports can be adopted. For silica nanoparticles, the primary particles are aggregated by treatment with trace amounts of TEOS (tetraethoxysilane), resulting in fragile microporous networks (secondary particles). The surface area of these porous structures is homogeneously covered with MAO-activated metallocenes. For organic substrates, polystyrene nanoparticles are introduced as “primary particles”. Therefore, the synthesis of organic nanoparticles with defined functionalities on the surface is crucial for controlling self-assembly toward major aggregates in the micrometer range. Via emulsion polymerization, polystyrene latex particles having functional groups such as cyclopentadiene and phenol groups on the surface were prepared. Again, the cyclopentadienyl groups are built-in for

Figure 11. Polystyrene bearing nucleophilic functionalities as catalyst support.

attributed to an insufficient degree of cross-linking via the Diels−Alder reaction resulting in less robust particles. Furthermore, due to their flexibility, the metallocene-bearing polystyrene chains may also adopt extended linear conformations after a retro-Diels−Alder reaction. This results in an

Figure 12. Noncovalent interaction between a MAO/metallocene complex and ether functionalized polymers. G



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Figure 13. Synthesis of polystyrene nanoparticles with cyclopentadiene units on the surface.

Figure 14. PEO-functionalized nanoparticles as supports for metallocenes: synthesis and supporting process.

(80−300 nm) were reversibly aggregated by the interaction of PEO chains with the MAO/zirconocene clusters. During polymerization the aggregated supports are completely and homogeneously fragmented into the initial nanometer-sized particles within the final product due to the mechanical stress from the formation of polyolefins between latex particles. This fragmentation is possible due to the weaker cohesive forces of the support aggregates, which was not possible with the covalently cross-linked particles. Such a fragmentation, as proven for silica based supports, is considered to be essential for the control of morphology in polyolefin polymerization. This approach has been exemplified by Bouilhac et al. where physically aggregated supports based on star-like polystyrenes functionalized with short ethylene oxide units were used.41 The advantage of using aggregated nanoparticles becomes obvious by comparison with similar experiments performed with cross-linked polystyrene microparticles (Merrifield resins) or polydimethylsiloxane microgels.31 The activities in the case of nanoparticles are higher by a factor of 10. The primary reason for this improved activity is the loading of catalysts inside the resin microparticles. It can be assumed that only metallocenes adsorbed or covalently linked on the surface are catalytically active throughout the whole polymerization process, since catalytic sites inside the particles lose their activity as the diffusion pathways for the olefin monomer are blocked by formed polyolefins.30a,32d Aggregates of nanoparticles have the potential to solve this issue by the fragmentation process; in contrast, the required fragmentation of micro- into nanoparticles did not occur due to crosslinking.19b Obviously, aggregation of nanoparticles and their induced fragmentation by formed polyolefins define the fundamental guidelines for the design of new supports for

cross-linking via Diels−Alder reactions (Figure 13). However, like the previously discussed microresins, the networks of the nanoparticles worked well in the case of the ethylene polymerization but failed to display any morphology control using propylene as monomer. Again, this is attributed to insufficient cross-linking to allow for a controlled particle growth.39 3.2. Physically Aggregated Polystyrene Nanoparticles. As previously discussed, the Diels−Alder reaction is not suitable for forming networks which can undergo controlled fragmentation and yield polymers with the desired morphology. While in the previous case, covalent bonds were required to break for fragmentation of supports to occur, now the focus is on physically aggregated organic nanoparticles. Again, polystyrene nanoparticles were prepared by emulsion processes in a similar way as shown in Section 3.1. Such particles can easily be obtained by miniemulsion or emulsion polymerization. The use of suitable emulsifiers (Lutensols, pyridine, PEO) allows for tailoring of surface nucleophilicity for subsequent immobilization of the catalysts. Latex particles based on polystyrene were selected for such supports as this polymer does not contain any functionalities which may interfere with the active catalyst.16a Carriers based on polystyrenes containing methoxy groups or PEO chains, which can immobilize active MAO/metallocene complexes through noncovalent bonding with nucleophilic groups, have also been reported.32d,38 These catalysts have shown high activities and productivities and form distinct polymer particles with a high bulk density of ∼400 g/L. This concept was further developed by applying polystyrene-based nanoparticles as catalyst carriers functionalized with polyethylene oxide (PEO) or polypropylene oxide (PPO) chains on the surface (Figure 14).40 In this manner, uniform and well-defined carrier particles H



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Figure 15. Decomposition of postmetallocenes in the presence of trimethylaluminium and scavenger function of the pyridine units in the support.

be embedded in a polymer matrix and subsequently sliced. For a detailed study of fragmentation, as performed for silica, a large number of slices need to be measured. Furthermore, to obtain a reliable overview of the process, several particles must also be measured. Therefore, we have introduced two optical methods: video microscopy for obtaining deeper insights in the loading of the catalysts and laser scanning confocal fluorescence microscopy (LSCFM) for observing the fragmentation process.19b Video microscopy allows direct visualization of particle growth during polymerization, and the increase of several particle diameters can be compared to determine reaction rates. For example, the rate of growth for pyridine functionalized particles was found to be uniform by video microscopy.19b This very versatile technique clearly indicates that all catalyst particles have the same relative loading of catalyst and similar activities.19b,42 As a second tool, laser scanning confocal fluorescence microscopy is considered. Using dye-labeled supports, LSCFM can be applied as a very rapid tool to study fragmentation. This is demonstrated by 3D-images showing the distribution of different support fragments in the polyethylene product particles (Figure 16). To utilize this technique, polystyrene nanoparticles and Merrifield resin particles that contained the highly fluorescent tetraphenoxy divinylperylene diimide as a comonomer were synthesized.40a For comparison, silica supports tagged with tetrasulfonato phenoxy diphenyl perylene diimide were also prepared. On all systems, bis(indenyl)zirconium(IV) dichloride was immobilized. The supported catalyst showed different kinetic and fragmentation behavior during ethylene polymerization under the same reaction conditions depending on the support material. While the resins had a relatively low activity, which drops with time, the silica supported catalysts displayed an induction period prior to polymerization. In contrast, the catalysts supported on polystyrene nanoparticles immediately demonstrated a very high activity with no induction period. To explain this difference in behavior, the fragmentation of supports with the reaction times was examined by LSCFN. For the Merrifield resin particles, fragmentation does not occur, and fluorescence images reveal the support remains unaffected during the polymerization. In the silica case, a layer by layer model is observed. The fluorescence images at different stages show that the support slowly fragments beginning with the outermost layer followed by subsequent inner layers in an “onion-like” fashion. Due to the generated channels, more and more catalyst centers became involved in the polymerization process resulting in an apparent increase in the polymerization rate. The secondary polystyrene latex particles, formed by nanoparticle aggregates, behaved differently. From the beginning fragmentation started throughout the whole particle, and

olefin polymerization catalysts. This aspect will now be elaborated for another class of nanoparticles. 3.3. Pyridine Functionalized Nanoparticles. In recent years, postmetallocenes, mainly titanium bisphenoxyiminecatalysts (FI-catalysts),15 have gained increasing industrial importance for producing UHMWPE. Due to molecular weights above 2000 kg·mol−1, these polymers have extreme mechanical stability.2d Supporting the FI catalysts on the abovementioned physically aggregated polystyrene nanoparticles failed as only low activities could be achieved. The main reason for this is the decomposition reaction of the postmetallocene caused by trimethylaluminium (TMA) which is a typical byproduct of the MAO production.16a This problem is solved by using particles containing vinylpyridine as a comonomer along with styrene. Again, these particles are easily accessible through a miniemulsion process using monomer mixtures of styrene and vinylpyridene and block copolymer emulsifiers of oligoethylene-PEO (Lutensols).16a Due to the polarity of the pyridine units, the vinylpyridine is mainly located at the surface of the nanoparticles. Similar to the PEO chains, the pyridyl groups contribute to the interaction with the aluminum compound in the formation process of the secondary particles. More important, however, is the role of pyridine as a scavenger for TMA. Due to a strong interaction between both compounds, as shown by NMR model studies, the poisoning compound is removed from the system (Figure 15), and an active catalyst system is obtained.16a By varying the concentration of the pyridyl groups on the surface and degree of support cross-linking, the reaction can be optimized resulting in polyethylene having a Mw exceeding 7 000 000 g/mol and a narrow MWD of 2.8 ± 0.4. Excellent and constant activities in ethylene polymerization are observed for more than six hours of polymerization. Productivities of up to 15 kg PE/g cat have been measured.16a Clearly, latex particles not only act as supports but they also improve the activity of the FI catalyst by trapping poisoning compounds like TMA. This distinctive property cannot be achieved by common inorganic supports like silica or MgCl2. At this point, one should also consider mechanistic and kinetic aspects of the polymerization in the presence of new supports. While the kinetics and the morphology have been widely investigated for inorganic supports, systematic reports for organic materials are largely missing. The characteristics of the polymerization process should not only consider activity and productivity but also the loading of a catalyst occurring within the secondary particles. Also, the fragmentation of the support, which has been addressed in the previous chapters, requires further attention. Both distribution and catalyst fragmentation can be visualized by tunneling electron microscopy (TEM) combined with elemental mapping. However, these kinds of TEM studies are often very complex and time-consuming as the particles must I



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Figure 16. Schematic description of a LCSFM and example of the catalyst distribution in a PE particle after 15 min of polymerization.19b

emulsions consisting of alkanes as a continuous phase and DMF as the dispersed phase. Block copolymers, such as polyisoprene-b-PMMA, are added to better stabilize the droplets.44 By that, dense latex particles of polyester and polyurethanes (PU) can be obtained. Porous polyurethane particles, however, are obtained by a controlled addition of H2O during the polymer formation (Figure 17). The water

an induction period was not observed. This behavior is attributed to the unpolar organic particles which can be swollen by the monomer. As the monomer is distributed throughout the whole carrier, the polymerization can directly start at all catalytic centers regardless of location within or on the particle.19c The fragmentation studies clearly demonstrate the difference between a more flexible organic material and the rigid silica supports. In the inorganic case, the support cannot swell which limits the diffusion and uptake of the monomer. Thus, fragmentation is mandatory in order to form channels for transport to the inner catalytic centers. This conclusion further emphasizes the importance of the control of aggregation and separation of nanoparticles over the course of the polymerization reaction. Fragmentation is required for yielding transparent products as previously mentioned and, furthermore, for achieving high activities and polymerization rates as demonstrated by the kinetic studies. 3.4. Nanoporous Supports Based on Polyurethanes. Up to now, self-assembly of small nanoparticles has been used to create the larger secondary particles for supporting of the catalysts. However, this self-assembly process holds a major drawback of not forming very uniform supports since the aggregation of particles is not precisely controlled. As a result, the supports have neither a spherical shape nor a narrow size distribution. This problem can be partially solved by sieving the organic as well as the inorganic supports. However, it would be more desirable to avoid the aggregation step altogether and rather produce spherical supports in a simple one-pot procedure. Therefore, the synthesis of narrowly distributed micrometersized porous polyurethane particles by using a nonaqueous emulsion polymerization was developed.43 In recent years, this technique has allowed the polymerization of water-sensitive monomers, such as acid dichlorides and isocyanates, in

Figure 17. SEM micrographs of (a) nonporous and (b−d) porous PU particles. The amount of water during the polymerization varied (a−d: 0, 0.78, 1.67, and 3.27 mmol of water).

leads to a slow decomposition of isocyanate functions into amino groups, accompanied by the formation of CO2, which acts as a blowing agent during the polymer formation. Depending on the amount of water and type of tertiary amine, the pore size can be easily adjusted. Obtained pore sizes range between 50 and 100 nm, and surface areas of approximately 15 m2/g are found. This relatively low porosity J



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Figure 18. Schematic representation of the preparation of PE fibers by supported metallocene catalysts on electrospun fibers.

4. SHAPE-ANISOTROPIC SUPPORTS While the current discussion has been largely occupied with the formation of spherical olefin particles, there is also the question of whether shape anisotropic polyolefin particles, particularly fibers, can be prepared in a supported metallocene-catalyzed polymerization. Considering the templating effect of the supports, this requires the formation of shape anisotropic carriers. Typically, fibers in the nanometer range are produced via electrospinning. However, this method has been used mainly for formation of fibers from polar monomers. As seen in Section 2, supports of linear polymers do not guarantee morphology control in the olefin polymerization. Consequently, a method for colloidal electrospinning using organic nanoparticles is required. Similar to the previous approaches, PEO-functionalized nanoparticles which were stained with BODIPY (difluoro-boron-dipyrromethene) as a fluorescent dye for better visualization can be used. In the presence of polyvinyl alcohol, which acts as an adhesive between the nanoparticles, very homogeneous fibers can be obtained through electrospinning which is subsequently used as a shape-anisotropic support (Figure 18).47 The support is then loaded with metallocenes, activated with MAO, and used for ethylene polymerization. In this way, polyolefins grow homogeneously around the supporting fiber, as shown by SEM and laser scanning confocal fluorescence microscopy (Figure 19). The thickness can be easily tuned by reaction time and applied pressure. Directly out of the reactor, fiber mats of polyolefin are obtained in an easy fashion. Such materials might be of particular use as fillers in composite materials and to substitute other expensive materials like carbon or glass fibers in the future. Therefore, highly mechanically stable materials are required to become comparable with corresponding inorganic additives. For these applications UHMWPE, which is very difficult to process due to the extremely high molecular weights (Mn > 3 000 000), is a viable alternative. In addition, depending on the particular

and large pore sizes are desirable since a metallocene surrounded by a MAO-cage cannot enter the pores.7 Furthermore, pore sizes in this range allow fast monomer transport to reactive centers in the interior. After loading of the PU particles with bis(methylcylcopentadienly)zirconium(IV) dichloride (MCP) and activation with MAO, ethylene polymerization was performed. The obtained polyethylene products were spherical, again replicating the initial shape of the supports. The porous structure and complete fragmentation were also demonstrated by LSCFM staining of the PU particles with Rhodamine B prior to activation.45 The fluorescence images show the dye homogeneously distributed throughout the PU particles, including the interior, indicating that the pores have an open structure. Such morphology allows metallocenes to be absorbed not only on the surface but also in the voids of the porous PU particles. This is mandatory as otherwise fragmentation would not occur as was the case for Merrifield resins and large PU particles would remain in the polyolefin product. LSCFM of the products confirms the decomposition of PU-based supports from a micrometer-sized carrier into nanometer particulates, which is only possible when the polymerization also occurs within the interior. The kinetic studies performed in a similar way as for the pyridine functionalized supports also reveal a homogeneous loading from particle to particle. Like the latex particles,46 they show relatively similar activities (∼200 × kgPE/(mole metallocene × bar × h)) in the ethylene polymerization and prevent reactor fouling. Furthermore, they can be easily prepared in a one-pot reaction via a nonaqueous emulsion. Pore sizes, key parameters for catalyst loading and mass transport, can be adequately tuned. Finally, due to their spherical shape they can also serve as a template in the olefin polymerization. On the basis of these characteristics, PU-based supports have many superior qualities over the previously discussed supports. K



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5. HYBRID SUPPORTS FOR THE FORMATION OF POLYOLEFIN CORE−SHELL PARTICLES The previous chapters described polymerizations in the presence of a single catalyst which yielded only particles and fibers of a single type of polyolefin. The preparation of more complex architectures having, for example, two types of polyolefin in one particle with a spatial resolution, as in the case of core−shell particles, is a more challenging endeavor.50 The primary method for obtaining core−shell structures industrially is through cascade processes, such as the Spheripol technology which combines two different reaction steps.50a,51 In the cascade process, the catalyst particles polymerize for a certain period of time in a first reactor containing a monomer or monomer mixture. According to the layer-by-layer model, the reaction is performed until the catalysts of only the outer part of the secondary particles have been involved in the polymerization process.28 Afterward, the intermediate product is transferred into a second reactor containing a different monomer.52 The polymerization is finalized when the support is completely fragmented and the active sites in the interior have started to participate in the polymerization. As a result, a core−shell structure is formed containing exclusively the monomer(s) used in step 2 in the core, while the shell is formed by a mixture of both the monomers used in step 1 and in step 2 as well. A major drawback of this process is the difficulty of avoiding partial decomposition of the active centers due to the presence of moisture and oxygen during the reactor transfer. Additionally, the cascade process does not allow the synthesis of two totally different polymers with complete spatial resolution. At least the shell contains a blend of the polymers formed in the two stages. Therefore, it would be preferable to form core− shell polyolefin particles in a one-pot reaction and in a more defined way. This problem can be approached by loading two catalysts on the same support having different polymerization abilities, thereby producing different types of polypropylene. There have already been numerous examples where mixtures of catalysts were supported in such a way; however, only mixtures were obtainedso-called reactor blends. In this approach, two catalysts are loaded and spatially resolved in the shell and in the core of a hybrid support formed from silica and latex particles (Figure 20).50a In particular, MCP (A) is immobilized in the presence of MAO on the silica particles. In a second step, organic nanoparticles, which have already been used in Section 3.1 to immobilize rac-(dimethylsilyl-bis(2-methylbenzindenyl) zirconium dichloride (MBI) (B), are added. Both supported catalysts are combined so that a shell of organic particles surrounds the silica. This new type of supported catalyst was applied for propylene polymerization. By a variety of methods, such as 13C NMR spectroscopy, GPC, and TEM, at different reaction times, different polymers could be characterized. In the beginning solely isotactic polypropylene was formed indicating an exclusive polymerization in the outer layer where catalyst B is located. Only after a certain period of time, propylene reached the silica core and metallocene A started to produce atactic PP. This was verified by electron microscopy which demonstrated the formation of noncrystalline atactic polymer in the core and partially crystalline polypropylene in the shell. An additional proof of the replication effect in the presence of spatially resolved catalysts was again obtained by LSCFM. By selective staining of core and shell with either perylene- or

Figure 19. SEM micrographs of electrospun nanofibers with ethylene polymerized in a gas phase reactor for 0 min (a), 10 min (b), 30 min (c), and 60 min (d) under 3.0 bar of ethylene at 40 °C, 25 μmol Zr/(g fiber), Al:Zr 170:1.

process, fiber spinning of UHMWPE is nearly impossible due to its extremely high viscosity, very limited solubility, and high melting temperature. Obtaining fibers directly out of the reactor offers a variety of advantages: it simplifies the processing of polyolefins and reduces the costs as spinning processes can be avoided. Thickness of the fibers can be controlled directly during the polymerization. The obtained fibers might find their value in a number of applications. They offer the opportunity to generate composite materials of different polyolefins by coextrusion or by melting of a low molecular or branched polyolefin, e.g., linear low density polyethylene (LLDPE) in the presence of UHMWPE fibers. As such, hybrid materials would only consist of a similar polymer, and the compatibilization between matrix polymer and reinforcing fiber would not be necessary. Just this argument alone might be of decisive importance considering the tremendous amount of work put into homogeneous incorporation of inorganic as well as carbon fibers in commodities. Also, the formation of polar fibers on shape anisotropic supports should be possible by metallocenecatalyzed polymerization. The copolymerization of polar monomers such as norborneneol derivatives or undecenol as a comonomer should yield hydrophilic fibers which are suitable for incorporation into more polar polymers. Post-modification with acid or amino functions can yield membrane materials for fuel cells or water purification with metal ion adsorption. These few cases highlight the many new opportunities of making polyethylene fibers. Entry into this new field of polyolefins is possible by using organic support systems. The formation of shape-anisotropic nanoobjects is common in nature. A prominent example is the crystallization of CaCO3 in coral to yield very unique scaffolds and is related to the process of biomineralization in the presence of peptides.48 Inorganic chemistry achieves control over anisotropic crystallization of oxides and sulfides, e.g., ZnO or CdS by double hydrophilic block copolymers.49 For polymers this is rarely possible, again highlighting the fact that anisotropic materials become accessible by the template effect of the support. L



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6.1. Polymerization in Hollow Silica Spheres. In classical silica and MgCl2 supports, the catalyst carrier consists of an amorphous porous particle which can be fragmented into smaller particles. Therefore, it would be desirable to minimize the amount of support and, thereby, decrease the amount of potential scattering materials in the formed polyolefin particle. To achieve this minimization, hollow silica spheres which consist of a thin porous silica shell are utilized. This eggshelllike structure serves as a reservoir for the catalyst. Hollow spheres are easily accessible by a template process. In the first step, an aqueous, self-stabilizing emulsion using polystyrene and acrylic acid carboxylate is used to obtain functionalized polystyrene particles. By a modified Stöber process, silica nanoparticles are generated by hydrolysis of tetraethoxysilane (TEOS) on the surface of these polystyrene particles.53 After removal of the polystyrene core, the silica particles form a stable porous egg-like shell (Figure 21).

Figure 20. Synthesis of polyolefin core−shell particles by spatially resolved catalysts. Reproduced with permission from ref 48a, copyright Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2009.

terrylene diimide dye, we were able to simultaneously visualize the two applied carriers of the hybrid systems before and after polymerization. The separated location of both dyes in the core and in the shell of the product particle provides evidence that both catalysts were not mixed during polymerization. Consequently, different polymers were formed in different areas of the hybrid catalyst. The concept of spatially resolved catalysis paves new pathways for the preparation of core−shell structures and can be considered as a new concept of nanotechnology. While up to now such morphologies were only accessible in stepwise protocols such as surrounding inorganic nanoparticles by an organic shell or by subsequent polymerization of different monomers in an emulsion, this new approach proceeds in onepot. The concept applied herein for metallocenes is not limited to olefin polymerizations, but also other combinations of catalytic polymerizations such as ATRP, ADMET, or ROMP should become feasible. However, to achieve this, the catalysts must be selectively loaded in different areas of a support and the monomers must not obstruct each other or poison the catalysts.

Figure 21. (a) SEM and (b) TEM micrographs of silica hollowspheres.

Through the pores the metallocenes can be incorporated into the hollow sphere and activated by MAO (Figure 22). While it is probable that much of the catalyst is adsorbed on the surface as well, a significant portion is simply trapped in the core after activation, as the large MAO/catalyst complexes are not able to pass through the pores of the silica shell. The SEM micrographs reveal extremely uniform PE particles obtained by this technique. The spherical shape of the silica supports is replicated in this process. Clearly, the polymerization proceeded only within the interior of the hollow spheres, since silica fragments which are not covered by polyolefin are detectable on the surface (Figure 23). In comparison to conventional silica or MgCl2 supports which are frequently not uniform, the use of hollow spheres is an excellent solution to get polyolefin products with a spherical morphology. Such an approach is totally different from conventional inorganic or organic supports as the catalyst does not necessarily have to be bound on the exterior surface but can be located inside the hollow sphere. Instead, due to the size of the MAO cage surrounding the metallocene, the catalyst is trapped within the confined geometry regardless of covalent or noncovalent binding. 6.2. Olefin Polymerization in Nonaqueous Emulsions. A second approach for creating nanoparticles in a confined geometry utilizes an emulsion process. Here, ambipolar structures also play a decisive role. Normally, emulsion polymerizations are performed in the presence of water as either the continuous or dispersed phase. Due to the sensitivity of the catalysts and the aluminum-based activators, protic solvents must be avoided. Therefore, an emulsion of perfluoroalkanes and cyclohexane has been utilized (Figure 24).50a Both solvents are immiscible and inert to activated metallocene and postmetallocene catalysts.

6. POLYMERIZATION IN CONFINING GEOMETRIES In this chapter the immobilization of catalysts by entrapment within confined geometries is described. In all of the previously given examples, a minimum size of the support in the micrometer range was required to yield sufficiently large particles for further processing and to prevent reactor fouling. Additionally, the catalysts were typically loaded in a very complex way in order to ensure fragmentation. Because of these limitations, these procedures have only yielded particles approaching the millimeter range; smaller particles with higher densities were obviously not accessible by this method. Here, two examples are given to show how these limitations can be overcome by polymerizations in confined geometry. M



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Figure 22. Immobilization of a metallocene inside a hollow silica sphere and fragmentation during ethylene polymerization.

which are not accessible by supported catalysts can be obtained (Figure 24). The particle size can be increased with the reaction time from 50 nm up to several micrometers. However, the activity is comparably low. It can be increased, however, by using propylene as monomer at pressures above 6 bar.50a Under these conditions, the monomer can be used as solvent in the dispersed phase and the mechanism of the polymerization is switched from emulsion to miniemulsion. The main difference in this case is the high concentration of the monomer in the droplets and the avoidance of any diffusion limitation by the monomer transport through the fluorinated phase. These new emulsions based on perfluorinated solvents allow for the synthesis of polyolefins with a diameter range different from processes based on supported catalysts.44b While only particles in the range of several hundred micrometers to millimeters have been accessible, the nonaqueous emulsions yield polyolefin nanoparticles with diameters below 100 nm. Potential applications of polyolefins are thus dramatically broadened; polyolefin nanoparticles can enable applications which have before been restricted to acrylate or styrene particles. Indeed, dispersed nanoparticles provide the basis for several huge markets such as paints or powder coatings.

Figure 23. SEM micrographs of polyethylene particles after gas phase polymerization using hollow silica particles.

Essential for the formation of the emulsion is the design of suitable stabilizers. As low molecular weight compounds containing alkyl and perfluoroalkyl groups were not sufficient for stabilization, block and statistical copolymers of styrene and perfluorostyrene were synthesized and used. The fluorophilicity of the stabilizer was further enhanced by matching perfluoroalkylether groups in a nucleophilic substitution to the paraposition of the perfluorostyrene moieties. As the metallocene and MAO are exclusively soluble in the alkane droplets, the polymerization can proceed in the presence of gaseous ethylene. Spherical particles in the nanometer range

7. LATEX PARTICLES AS SUPPORTS FOR ZIEGLER−NATTA CATALYSTS In the previous chapters, metallocenes and postmetallocenes were used. Consequently, the question arises whether these concepts can be adopted for Ziegler−Natta systems and what potential benefits could arise. In particular, can organic supports overcome one of the most significant disadvantages of inorganic

Figure 24. Emulsion and miniemulsion polymerization of olefins in a mixture of alkanes/perfluoraoalkanes. N



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ability to control MWD is a significant advantage for overcoming the problem of extremely broad MWDs in the Ziegler−Natta polymerizations. This achievement may be regarded as a technological breakthrough as it eliminates the need for blending with another narrowly distributed polyolefin and the combination of different catalyst systems in order to furnish polymers with lower melt viscosities. Furthermore, this might significantly simplify the production of many polyolefin materials.

support systems for Ziegler−Natta (ZN) catalysts which is the lack of control over molecular weight (Mw) and MWD.20 Due to the absence of defined catalyst structures on the surface of the TiCl4 and MgCl2, the values of MWD for polyethylene produced by classical titanium−magnesium based ZN-catalysts are very broad with a range of 4−12, and large amounts of oligomers are formed which negatively affect the mechanical properties. On the other hand, it should be noted that the very narrow MWDs obtained by metallocenes are not necessarily always favorable.32d,38,40c Low MWD values between 1.6 and 3 lead to extremely high melt viscosities which dramatically hamper processing by extrusion. Currently, industry solves this problem by blending different types of polyolefins to yield MWDs in the desired range of 3−5. This, however, results in an additional blending step during processing. Additionally, in some cases phase separation occurs due to the differences in molecular weight and intramolecular structure.54 Therefore, more defined catalyst structures in comparison to the existing systems have been attempted. In the Ziegler−Natta case, due to the rigidity and differences in the crystal faces of the MgCl2 support, the immobilized titanium species often have different activities which greatly affect the ultimate product properties.20 It was expected that a more flexible substrate, such as organic particles, would allow for greater uniformity of the active species and lead to a more uniform product structure, i.e., a narrower MWD. While in the previous cases mainly noncovalent binding was used in the immobilization process, a covalent binding is used here to better mimic the interaction between a silica or MgCl2 surface with the titanium complex. Therefore, nanoparticles which contain hydroxyl groups on the surface are used as carriers. For example, cross-linked poly(styrene-co-vinylbenzyl alcohol) nanoparticles were prepared by emulsion polymerization (Figure 25). The amount of hydroxyl groups on the surface is easily altered by varying the amount of vinylbenzyl alcohol (0.0 mol % to 5.0 mol %).55

8. ORGANIC SUPPORTS AS APPLIED NANOTECHNOLOGY Polyolefins are considered a rather mature topic of polymer science, and industrial approaches have been developed over many years, making the synthesis of polyolefins cheap and versatile. From the lessons learned from these industrial approaches, the rational development of new supports has been presented in Sections 2−7. Admittedly, some of these approaches may appear exotic and far from finding any practical industrial application, but, nevertheless, they open pathways to new strategies for morphology and topology control. When considering these strategies, a striking connection appears between olefin polymerization and modern nanotechnology. At first glance, this opinion might appear surprising as nanotechnology, even being only softly defined, is widely considered an area dealing with the synthesis and treatment of tailored nanoobjects. In general, nanotechnology covers the preparation and use of inorganic or organic particles in the range of a few to several hundred nanometers by methods like precipitation in the gas phase,56 laser ablation,57 and precipitation in confining geometries, e.g., polymerization or redispersion in emulsion.58 Prominent examples of designed nanoobjects are gold or silver nanoparticles for biosensors,59 highly functionalized latex particles for drug delivery and diagnostics, or inorganic hybrid materials, such as ZnO/silica core-nanoparticles dispersed in a polymer matrix for UVshielding.60 Due to such special applications, nanotechnology is often associated with costly and very specialized materials61 and less so with low-cost products like polyethylene or polypropylene. Throughout this review, the importance of many features, like size control, interaction between particles and catalysts, or the fragmentation process for the design of organic supports for olefin polymerization, has been discussed. In fact, many of these features are quintessential aspects of nearly all applied nanotechnology. Here, five of these “connection points”, for example, synthesis, size control, and self-assembly, show that organic supports are, indeed, nanotechnology in application. First, the design of organic supports requires the careful synthesis of nanoparticles. As previously discussed, the methods most commonly employed to achieve this, and herein focused on, are aqueous emulsions or miniemulsions. These techniques are probably the most important workhorses for the fabrication of both organic and inorganic nanoparticles, which are one of the fundamental building blocks of nanotechnology. Furthermore, many particles used in nanocomposites of polystyrene, poly(methyl methacrylate), or other polymers are derived from emulsions. A common example of such systems is the inclusion of silica nanoparticles into a polystyrene or PMMA matrix to increase impact toughness or MgCO3 for flame retardancy. In a similar fashion, organic nanoparticles based on vinyl monomers are widely prepared and used throughout industry. In Sections 2 and 3, the process to form polystyrene and

Figure 25. Synthesis of hydroxyl functionalized nanoparticles as supports for Ziegler−Natta catalysts.

After removing traces of moisture with triisobutylaluminum (TIBA) as a scavenger, the catalyst is formed by the addition of TiCl4. Typically, a concentration of titanium is chosen in the range of 0.2 mmol/g catalyst, and activation is performed by the addition of 40 mmol of triethylaluminum. The obtained catalysts are then used for ethylene polymerization at 60 °C under 3 bar of ethylene pressure. Remarkably, the immobilized catalysts show reasonable activities in the range of 100−300 kg PE/(mol Ti × h × bar). From these results, it can be seen that the organic surfaces do not poison the catalytic sites. More remarkable are the MWDs, which are obtained in the range of 3−5. This narrow MWD indicates that the more flexible organic supports lead to more uniform catalytic sites. These results highlight the importance of flexible substrate surfaces on the catalytic behavior for polymerization. The O



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There are some cases, however, where reversibility of a defined, stable structure is important. In the presented approaches, the formation of stable aggregate structures is necessary to provide suitable supports, but this aggregation must also be reversible to allow for the proper fragmentation of these supports. Like in other nanotechnologies, this is achieved through the careful tuning of the particle−particle interactions. The binding between the particles must be weak enough that the mechanical stress caused by the expansion of the formed polymers causes fragmentation but also strong enough that they can form aggregates which do not instantly fall apart. Here, these interaction forces can be carefully tuned through the number of polar groups in a PEO chain attached to the nanoparticle surface (Chapter 3). Finally, the defined adsorption of nanoparticles on larger microparticles becomes of particular importance when aiming for supports which allow for the formation of more complex polyolefin structures, such as core−shell or fibril, directly during the polymerization process. Core−shell particles, wherein a flexible shell and stiff core are combined, are often utilized in paints and coatings.50a,63 The most prominent examples are polybutylacrylate/polystyrene lattices which can be easily prepared by radical polymerization in emulsion. The flexible polybutylacrylate shell establishes the film-forming properties while the mechanical stability is provided by the polystyrene. Similar structures are difficult to achieve using polyolefins as spatially resolved polymerizations of different polyolefins in the core and the shell cannot be performed. The strategy presented here to adsorb organic particles on inorganic microparticles solves the problem as different catalysts can be selectively loaded either on the silica core or in the nanoparticle shell. Since both of them produce different types of polyolefins in different areas of the support, the desired core−shell structures were obtained (Section 5).

vinylpyridine nanoparticles was discussed. It should be noted that similar processes are continually at the forefront of nanotechnology. Recently, a nonaqueous process based on mixtures of aprotic solvents, such as DMF/hexane or acetonitrile/cyclohexane, with poly(isoprene-b-methylmethacrylate) was used to form stable emulsions.43,44,62 These systems allow for the polymerization of water-sensitive monomers in droplets to form nanoparticles of polyurethanes, polyamides, polyesters, and other polycondensates. The nonaqueous emulsion has also provided the basis for fabricating polyolefin nanoparticles in mixtures of perfluoroalkanes and hydrocarbons (Section 6). Size control of particles is a second key factor in all nanotechnologies. Size matters, for example, when considering transparency of hybrid materials such as UV absorbers based on ZnO in PMMA. The diameter of the particles must be below 200 nm particles to avoid optical scattering. This requirement is also true for the organic supports used in polyolefin synthesis. If the fragmentation of the support yields fragments (primary particles) which are too large, transparent materials cannot be produced. The (mini)emulsion process allows for precise control over the size of the primary particles through simple variations of the procedure, e.g., emulsifier concentration of ultrasonication conditions (Section 6). The third issue regards the stabilization of the latex or inorganic nanoparticles which is definitely another key consideration in nanotechnology. Without the ability to make distinct and stable particles, uncontrollable aggregation often occurs. Typically, stabilization is achieved through the use of amphiphilic statistical and block copolymers or low molecular weight surfactants. These stabilize the droplets during emulsion polymerization or adsorb on preexisting particles like silica. The most common of amphiphilic stabilizers possess polar groups which anchor to the particle or droplet and nonpolar groups to stabilize the particle by either steric or electrostatic effects. For metallocene supports, the requirements for appropriate stabilizers are more complex. Not only must they be able to stabilize the nanoparticles, but they must also interact enough to form stable, controlled aggregates (secondary particles) during the loading of the catalysts (Sections 2−5 and 7). This stabilization and controlled aggregation are typical goals found in nanotechnology. The controlled aggregation of nanoobjects to form larger structures represents a fourth connection point between organic supports and nanotechnology. One should distinguish between an aggregation process forming undefined, uncontrolled microparticles and the purposeful formation of stable, well-defined supramolecular structures. Here, as is the case for most nanoapplications, the careful tuning of the interactions of the nanoscopic building blocks is crucial. In fact, the adjustment of this interaction can be considered as one of the most important problems for nanoscientists, since nowadays most functional materials are obtained from processing in solution or nanoprinting. In many cases, undefined aggregation is not favorable for the fabrication of complex structures. The formation of defined 1D, 2D, or 3D structures plays an important role in electronics and diagnostics. Typical examples are multilayers of donor and acceptor polymers by orthogonal processing of polymers with different polarities or the design of a lab-on-a-chip in diagnostics. In almost all of these cases, stable, ordered structures are required which are often achieved by electrostatic interactions or hydrogen bonding between particles.

9. CONCLUSION AND OUTLOOK There is still a tremendous amount of new catalysts being synthesized with the aim to adjust the polymer properties through the alteration of macromolecular structure, such as tacticity, branching, or comonomer incorporation. However, the importance of the supports is largely ignored by the academic community. Typical support materials, like silica or MgCl2, are routinely applied, but conceptually new developments can rarely be found. In this review organic particles with specially designed surfaces have been demonstrated to equal or even surpass the performance of their inorganic counterparts for the following reasons: (i) They can be more easily synthesized and modified in size and surface functionality than silica or MgCl2. (ii) Due to the introduction of nucleophilic groups on the primary organic particles, the aggregation to secondary particles can be optimized as the binding strength of their interaction can be varied. (iii) Similar to inorganic supports, precise morphology control is obtained and low MAO amounts are needed. In contrast to silica supports, an induction period is not observed. In comparison to the MgCl2 supported polymerization of Ziegler-catalysts, polymers with a narrower MWD are obtained. (iv) Industrial inorganic supports do not give access to complex architectures, such as core−shell particles or fibers, in a simple one-step procedure; i.e., only cascade processes can be applied. A combination of inorganic and organic supports can overcome this limitation as it allows for a spatially resolved loading of different catalysts. This results in the formation of core−shell particles due to the replication P



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effect. (v) Organic nanoparticles are not only advantageous in comparison to inorganic supports but also to many other organic supports. Carriers based on linear polymers only work under very low pressures. Under industrial conditions (40 bar and higher), leaching, reactor fouling, and formation of fluffy polymers with low bulk densities are observed due to uncontrolled fragmentation of the supports during the olefin polymerization. Catalysts supported on microparticles, in particular, formed by cross-linked polysiloxanes and polystyrene, show only a low activity and yield nontransparent products, as fragmentation becomes impossible due to the high degree of cross-linking. As summarized in Section 8, many concepts of modern nanotechnology dealing with the control over particle size and surface functionality can be used to form new supports based on nanoparticles for metallocenes and Ziegler−Natta catalysts. While still far from finding universal industrial application, organic particles offer a wide-range of enticing new avenues as support materials. Organic support materials are still a relatively young field with a largely untapped potential. However, with recent advances and the introduction of new techniques, there is still plenty of new science to be done regarding organic support materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by Lyondell-Basell, Mitsui Chemicals, KACST, IUPAC, and DFG is gratefully acknowledged.

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REFERENCES

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