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Review: During the 1950s the discoveries of Zieglers organometallic catalyst systems and Nattas stereoselective olen polymerization set the stage for extraordinary progress in polymer science and technology. Today advanced catalyst systems are available for catalytic carbon carbon bond formation by means of polyinsertion, metathesis, coupling reactions, and controlled radical polymerization. Modern catalytic olen polymerization processes and polyolens are environmentally friendly and meet the demands of a sustainable development. The architectures of homo- and copolymers based upon olen, cycloolen, diene, styrene and arene feed stocks are tailored as a function of single site catalysts ligand frameworks. Polymer properties are varied over a very wide range, e.g., liquid/solid, rigid/soft, crystalline/amorphous/rubbery, permeable/impermeable, opaque/ transparent, moldable/thermosetting, insulating/conducting. Advanced polymerization reaction engineering, copolymerization processes, reactor granule and reactor blend technologies, tailor-made single site catalysts, catalyst blends and tandem catalysis improve property proles, stability, morphology, and melt processing. Tuned cycloolen polymers are new functional materials for electronic applications. Catalytic chain transfer (CCT) and atom transfer polymerization (ATRP) produce a variety of new functional polymers, reactive oligomers, and block copolymers. Aqueous polyolen emulsions are obtained when catalytic olen polymerization is performed in nanodroplets of catalyst miniemulsions. The scope of catalytic polymerization and post polymerization catalysis is progressing well beyond the frontiers of commodity polyolen manufacturing. Advanced catalytic processes produce polar polymers such as polyethers, polyketones, polyesters, polycarbonates, polyamides, polyaramids, and polyimides. The new phosgene free polycarbonate syntheses are based upon the palladium catalyzed

oxidative carbonylation. This overview highlights history, recent progress and modern trends in catalytic polymerization and catalytic polymer modication illustrated by selected examples.

Zieglers glass reactor for performing his Mulheim low pressure ethylene polymerization. The reactor is on display at the Max Planck Institut fur Kohlenforschung in Mulheim (this picture was made available by courtesy of G. Fink).

Catalytic Polymerization and Post Polymerization Catalysis Fifty Years After the Discovery of Zieglers Catalysts Rolf Mulhaupt Freiburger Materialforschungszentrum and Institut fur Makromolekulare Chemie der Albert-Ludwigs Universitat, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany E-mail: [email protected]

Keywords: cycloolen polymers; history; olen polymerization; polymer modication; single site catalysts; Ziegler-Natta catalysis

IntroductionDuring the 1920s Hermann Staudinger, who was awarded the Nobel prize in 1953, introduced his revolutionaryMacromol. Chem. Phys. 2003, 204, No. 2

concept of high molecular weight macromolecules and polymerization processes linking together individual small monomer molecules by means of covalent bond formation.[13] It took more than three decades to discover the1022-1352/2003/0201289$17.50.50/0

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very effective catalytic carbon carbon bond forming polymerization reactions exploiting olen feed stocks of the petrochemical industry. The discoveries of organometallic polymerization catalysts and stereoselective polymerization during the 1950s have initiated a chain of innovations promoting the rapid growth of the polyolen industry. Today polyolens such as polyethylene and poly(propylene) are well recognized as economically attractive and environmentally friendly polymeric materials and account for more than half of the annual world-wide polymer production of approximately 200 000 000 metric tons. Their energy demand for catalytic polymerization and melt processing is low due to energy-effective catalytic polymerization and processing temperatures around 200 8C. As hydrocarbon materials they preserve oil-like energy content and are easy to recycle.[4] Upon heating above 400 8C, thermal degradation affords synthetic oil and methane gas without solid residues.[5] This potential is being exploited to achieve effective product life cycle engineering displayed in Figure 1. As a consequence, polyolens meet the demands of sustainable development and help to save resources for the future generations. Poly (propylene)s extraordinary versatility in terms of properties and applications in conjunction with its attractive cost/ performance balance is unparalleled by many natural and other synthetic polymeric materials. The remarkable progress of catalytic polymerization has greatly simplied polyolen production. Catalyst design, polymerization reaction engineering, and polymer processing technologies are being pushed forward to produce novel polyolen materials with tailor-made property proles meeting the demands of highly diversied industries. New catalyst systems are being developed to produce other polymers without hydrocarbon backbones. An important challenge of polymerization catalysis is to facilitate the conversion of crude oil or other affordable feedstocks such as carbon monoxide, oxygen, hydrogen, nitrogen, and methane into versatile polymer products. For many years the progress of polymerization catalysis as well as catalytic polymer modication (post polymerization catalysis) has beneted from the cross-fertilization between the elds of organometallic chemistry, catalysis, reaction engineering, materials and polymer sciences. Comprehensive accounts of the progress in homogeneous

catalysis were published by Herrmann[6] and Parshall and Ittel.[7] On occasion of the 50th anniversary of Zieglers discovery of his catalyst systems, it is purpose of this review to highlight routes and modern trends in polymerization and post polymerization catalysis, illustrated by selected case studies and important milestones. Emphasis is being placed upon new approaches, basic correlations between catalyst and polymer architectures, and new opportunities of producing polymeric materials with new property proles.

Catalytic PolyinsertionThe historic routes to polyethylene date back to more than 100 years, whereas highly crystalline poly(propylene) is a child of the 1950s. The exciting history of polyolens was described by Seymour and Cheng in their book compiling personal views of the leading pioneers in polyolen science and technology.[8] Although polymerization of diazomethane, observed 1898 by von Pechmann,[9] was recognized by several other groups[1012] to afford linear crystalline polyethylene (HDPE for high density polyethylene, cf. Figure 2), this route was never viable for industrial production. The industrial breakthrough in polyethylene manufacturing occurred 1933 when Fawcett and Gibson at ICI discovered the high pressure free radical polymerization of ethylene.[13] At high temperatures around 200 8C and pressures well above 1 000 bar traces of oxygen initiated the free radical polymerization. Due to intra- and intermolecular chain transfer reactions, short and long alkyl branches were formed, thus accounting for reduced density and reduced melting temperature with respect to linear polyethylene (LDPE for low density polyethylene, cf. Figure 2, also referred to as high pressure polyethylene). In 1939 the rst plant was on stream supplying LDPE as a new electrical insulator for coaxial cable and for Britains World War II radar systems. Several groups came very close to the discovery of catalysts for low pressure ethylene polymerization. During 1930, when Marvel and Friedrich attempted the alkylation of As compounds with lithium alkyls, they discovered that BuLi polymerized ethylene in high boiling mineral oil at elevated temperature to produce linear polyethylene with transition metal alkyl free catalysts.[14] This very

Rolf Mulhaupt, born on September 13th, 1954 in Waldshut-Tiengen/Germany, studied chemistry at the Albert-Ludwigs University in Freiburg and got his PhD in 1981 at the Swiss Federal Institute (ETH) in Zurich under the supervision of Prof. Dr. P. Pino. After his industrial assignments at Du Ponts Central Research in Wilmington/Delaware, USA, (19811985) and at Ciba-Geigys Plastics & Additives Research in Marly, Switzerland, (19851989) he took the full professor position for macromolecular chemistry at the Institute for Macromolecular Chemistry of the Albert-Ludwigs University in Freiburg. Since 1992 he is the managing director of the Freiburg Materials Research Center (FMF). His research interests include polymer synthesis, catalysis, reactive processing, nanoparticles, nanocomposites, rapid prototyping and tailor-making of specialty polymers and additives.

Catalytic Polymerization and Post Polymerization Catalysis Fifty Years . . .

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Figure 1.

Life cycle of polyolen materials.

interesting result was not followed-up because at that time BuLi was a lab curiosity and the industrial potential of HDPE was not yet recognized. Max Fischer[15] at BASFAG discovered that a mixture of Al powder with TiCl4 produced predominantly high molecular weight liquid and also solid polymers as by-products. Approaching the end of World War II, the German war effort was in desperate demand for synthetic fuel and liquid lubricants and did not permit to assign more resources for exploring new routes to polyethylene. The breakthroughs in catalytic low pressure ethylene polymerization occurred during the early 1950s in the laboratory of Karl Ziegler at the Mulheim Max Planck Institute for Coal Research and the laboratory of Banks and Hogan at Phillips Petroleum Company. Since the early 1920s Karl Ziegler devoted his research to the investigation of the emerging new class of metal alkyl compounds and their application in carbon carbon bond formation by means of addition of alkali alkyls to olens, styrene, and dienes. This research was stimulated by the search for surrogates of natural rubber which was a vital war time raw material. In 1943 already in Mulheim/Ruhr Ziegler pioneered the stepwise organometallic synthesis converting lithium alkyls into the corresponding higher

Figure 2. Polyethylene molecular architectures: linear high density polyethylene (HDPE) and low density polyethylene (LDPE) containing propyl- and butyl short chain branches as well as a few long chain branches.

straight-chain lithium alkyls. Contrary to the very effective lithium alkyl initiation of butadiene polymerization, the growth of polyethylene was limited due to decomposition of lithium alkyls, forming lithium hydride and olen. This growth or propagation reaction (Ziegler called it Aufbaureaktion which means advancement reaction, cf. Figure 4), leading to higher aluminium alkyls, was much more effective using LiAlH4 or even the novel aluminium alkyls, which were truly exotic lab chemicals at that time. Finally in 1952 Ziegler established the direct synthesis of aluminium alkyls from aluminium, hydrogen and olens, thus converting aluminium alkyls into viable industrial chemicals. His long-term tedious research efforts (cf. Figure 3) were summarized and schematically presented by Ziegler in his Nobel Lecture, held on December 12, 1963 when he received the Nobel prize in chemistry together with Giulio Natta.[16] Today Zieglers Aufbaureaktion is still being applied for manufacturing of 1-olens as well as straight chain alcohols together with high purity alumina, produced by oxidation of the aluminium alkyls. Although Zieglers research effort was clearly based upon basic sciences, serendipity inuenced the choice of his research priorities. In 1952 nickel contamination of an autoclave was recognized to prevent ethylene propagation in the presence of aluminium alkyls and to favor chain termination. In the presence of nickel salts, aluminium alkyls gave exclusively the ethylene dimer 1-butene. In 1953 the more detailed investigation of Zieglers nickel effect ultimately led to the discovery of Zieglers Mulheim low pressure process for the catalytic ethylene polymerization. When zirconium and titanium compounds were added together with aluminium alkyls, high molecular weight linear high density polyethylene was formed at atmospheric pressure and room temperature (cf. Figure 4). His organometallic mixed catalyst systems (metallorganische Mischkatalysatoren), later referred to as Ziegler catalysts by Giulio Natta, was composed of aluminium alkyls in conjunction with titanium halides or other group 4, 5 and 6 transition metal compounds.[17] His rst patent claiming mixed catalysts was conned to ethylene polymerization. This discovery was quite spectacular. As shown in Figure 5 regular glass ware was sufcient to perform catalytic ethylene polymerization at room temperature and atmospheric pressure. In 1954 the copolymerization of ethene and propene, preferably in the presence of vanadium-based catalysts, was recognized to afford rubbery materials.[18] Within very short time Zieglers catalyst was licensed and industrial production started just a few years later. Zieglers story of success is told in an excellent book[19] entitled Polymere and Patente Karl Ziegler, das Team, 1953 1998 (polymers and patents the team of Karl Ziegler) by Heinz Martin who is a coinventor, a long-term member of Zieglers team, and the licensing expert of Ziegler catalysts. Another important discovery was made by Banks and Hogan at Phillips Petroleum Company in 1951. They

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Figure 3. Karl Zieglers research strategy as outlined in his Nobel Award address.

Figure 4. Karl Zieglers Aufbaureaktion (advancement reac tion), the nickel effect and the Mulheim low pressure process for making linear high density polyethylene.

attempted to convert liquid hydrocarbons like ethylene and propylene into high octane synthetic fuel. While nickel oxide on silica-alumina gave only liquid oligomers of ethylene and propylene, NiO-silica-alumina activated with CrO3, gave solid poly(propylene) with a tacky, latex-like nature.[20] This research led to CrO3-silica catalysts, which do not require aluminium alkyl activators, and polymerize ethylene to afford linear high density polyethylene with an attractive combination of properties and easy processing. In 1999 the lab of Hogan and Banks was honored by the American Chemical Society as a National Historic Landmark. On March 11, 1954 the group of Giulio Natta at the Milan Polytechnic succeeded to polymerize propene using Zieglers catalyst system to produce a tacky solid. Natta immediately recognized that the poly(propylene) obtained


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Figure 6. Poly(propylene) stereoisomers as proposed by Giulio Natta: isotactic, syndiotactic and atactic (from above). Figure 5. Zieglers glass reactor for performing his Mulheim low pressure ethylene polymerization. The reactor is on display at the Max Planck Institut fur Kohlenforschung in Mulheim (this picture was made available by courtesy of G. Fink).

synthesizes many stereoregular polymers, for example cellulose and rubber. This ability has so far been thought to be a monopoly of Nature operating with biocatalysts known as enzymes. But Professor Natta has broken this monopoly.

is composed of different diastereoisomers with very different physical properties. Their separation was achieved by means of extraction using boiling solvents. The diethyl ether soluble fraction was amorphous and sticky, whereas the heptane insoluble fraction was crystalline with melting temperature above 160 8C. The small amount of benzene insoluble fraction of Phillips poly(propylene) was less regular with a melting temperature of only 144 to 151 8C. Natta, who was an expert in solids characterization by means of X-ray diffraction, applied X-ray crystal structure analysis to identify the stereochemistry of poly(1-olens). He distinguished between highly crystalline isotactic and syndiotactic poly(1-olens) and amorphous atactic poly (1-olens). His concept of stereoisomers is displayed in Figure 6. Today single-site catalysts are available to produce most of these individual stereoisomers on an industrial scale. Nattas new concept of polymer stereoregularity in conjunction with transition metal catalyzed stereospecic polymerization by means of enantiomorphic catalytically active sites had far-reaching impact on the progress of polymer science and technology.[21] In 1957 Montecatini Company started the industrial, production of poly(propylene) at its Ferrara plant.[22] In his Nobel prize presentation speech in honor of Nattas and Zieglers Nobel prize in chemistry (1963) Prof. A. Fredga[23] concluded: Nature

Magnesium Chloride Supported Catalysts for Propylene PolymerizationThe remarkable rapid progress of polyolen technology is closely associated with breakthroughs in catalyst development and innovations in process technology. During the pioneering days poly(propylene) was far from being an attractive polymeric material. During the 1960s poor catalyst activity and lack of stereoselectivity required extensive purication by means of solvent extraction, thus removing colored and corrosive catalyst residues as well as tacky atactic poly(propylene). Slurry polymerization in hydrocarbon diluents required recycling and purication of the solvent. The rst generation of poly(propylene) was very difcult to process and exhibited poor thermooxidative stability. Today propylene is polymerized in solvent-free environmentally friendly gas phase and liquid pool processes to produce economically and ecologically attractive materials which nd many applications ranging from bers to lms and injection molded and extruded parts. As is apparent from the ow chart displayed in Figure 7, polyolen production has been simplied eliminating deactivation, solvents, polymer purication and even pelletizing extrusion steps. A comprehensive review on

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Figure 7.

Simplied industrial processes for polyolen production.

catalyst generations and propylene polymerization was published by Albizzati and coworkers.[24] Poly(propylene) production was revolutionized within surprisingly regular intervals of approximately fteen years. After the discovery of Nattas stereospecic propylene polymerization during the early 1950s, the development of highly active and stereoselective magnesium chloride supported catalysts started end of the 1960s and eliminated the need for removal of catalyst residues and atactic byproducts. It was recognized that magnesium chloride, which is isomorphous to g-TiCl3, is a very effective support for titanium-based active sites. It is able to substitute bulk titanium chloride which does not have vacant coordination sites. Supporting titanium complexes on high surface area, anhydrous magnesium chloride affords very high catalyst activities because all titanium is located at the surface and has vacant coordination sites. High activity means low contents of catalyst residues which can be left in the polymer without causing color and corrosion problems. The stereospecic olen polymerization using magnesium chloride based catalysts requires the addition of Lewis bases in order to selectively poison non-stereoselective sites and to promote deagglomeration of the supported catalyst during polymerization, thus improving catalyst activity. In the early generations of magnesium chloride supported catalysts Lewis bases such as alkyl benzoates and phthalates were added during preparation of the magnesium

chloride support and during activation with aluminium alkyls. Preferably silanes such as phenyltriethoxysilane or dialkyldialkoxysilanes were added together with the aluminium alkyls during catalyst activation. In the 1990s powerful new generations of magnesium chloride supported catalysts were discovered. When 1,3-diethers such as 2,2-disubstituted-1,3-dimethoxypropane are present during preparation of the supported catalyst, no additional Lewis base is required during aluminium alkyl addition. The interaction of magnesium chloride supported catalysts with diether is schematically presented in Figure 8. The basic principles of catalyst design, the selection of Lewis base systems and the inuence of Lewis base modication on poly(propylene) microstructures were reviewed by Chadwick[25,26] who investigated the inuence of Lewis bases and polymerization conditions on poly(propylene) microstructures.[2729] This Lewis base modication and morphology control of the catalyst particles afforded control of polymer morphology. In the Spheripol process, such catalyst are employed in liquid pool propylene polymerization, performed in liquid propylene and a loop-type reactor, to prepare spherical poly(propylene)s, thus eliminating pelletizing extrusion and offering new opportunities for polymer design. The spherical poly(propylene) with well-dened internal porosity can be used as reactors to produce multiphase polyolens. The fascinating history of magnesium chloride supported catalysts


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Figure 8. Magnesium chloride supported catalysts and effect of 1,3-diether donor modication (data from Basell).

was summarized by Moore in his book entitled The Rebirth of Polypropylene: Supported Catalysts How the People of the Montedison Laboratories revolutionized the PP industry.[30] The insight in polymer morphology development as a function of catalyst morphology led to the development

of the Reactor Granule Technology. Poly(propylene) morphology control and reactor granule technology is based upon scientic achievements by Paolo Galli and his coworkers at Basell.[31] The technical term reactor granule technology was dened by Galli as: controlled, reproducible polymerization of olenic monomers on an active magnesium chloride supported catalyst to give a growing spherical polymer granule that provides a porous reaction bed within which other monomers can be introduced to form a polyolen alloy.[32] The current state of the art of Basells Reactor Granule Technology was highlighted by Cecchin and coworkers.[33] As illustrated in Figure 9, the grains of Lewis-base modied spherical catalyst are composed of agglomerated microparticles consisting of magnesium chloride crystallites. The microporosity of the catalysts results from microparticle interspacings. Only when overheating and melt-down of the polymer is prevented during the carefully controlled prepolymerization, deagglomeration is achieved and catalyst particles serve as templates for the formation of poly(propylene) particles with controlled porosity. Globular and onion-like morphologies can be produced as a function of the catalyst morphology. This template effect and the porosity of the parent catalyst grains represent the keys to the formation of poly(propylene) particles with controlled porosity. Since the deagglomerated catalysts remain active, it is possible to polymerize other olens, e.g., ethene/propene, in the pores of spherical poly(propylene) grains to obtain impact modied poly (propylene) blends with ethene/propene rubber. This second

Figure 9. Formation of spherical poly(propylene) granules with controlled porosity using microporous catalysts as templates (data from Basell).

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Figure 10. Spheripol process: prepolymerization combined with liquid pool propylene polymerization and subsequent ethylene/propylene gas phase polymerization for impact modication.

stage of Basell Catalloy process is performed as a gas phase polymerization following liquid pool propene polymerization in propene bulk. A typical reactor cascade system of the Spheripol process from Basell is displayed in Figure 10. Moreover, the pores of poly(propylene) granules can be used as reactors to perform free radical polymerization of monomers such as styrene and acrylics. Today reactor granule technology and novel multizone reactor technology are able to produce versatile poly(propylene) products with properties ranging from superstiff, high uidity homopolymers to stiff/impact and clear/impact multiphase copolymers and supersoft alloys. These advanced polyolen products meet the demands of highly diversied industries. In 2005 polyolen production will exceed 80 million tons. The average size of the average polyethylene reactors is expected to increase signicantly and to approach 600 000 tons per year.[34] Large production growth in conjunction with declining prot margins and globalization forced well-established companies to stream-line their operations and to engage in strategic alliances with other companies. In future a rather small number of key players will share large scale polyolen production. Global changes of the industry are reected by changing names of companies. For example, in October 2000 the three companies Montell, Elenac and Targor were merged together to form the new company Basell which is a joint venture of BASF and Shell. Fifty years after the discovery of the stereospecic propylene polymerization the research cultures of Nattas and Zieglers competing groups are now joined together under the same corporate roof.

Single Site Catalyst TechnologyDuring the rst decades of polyolen development, most catalysts were developed by means of trial-and-error optimization and comprised a rather complex mixture of catalytically active and inactive organometallic compounds producing a blend of polyolens containing polymers with different molecular weights, endgroups, regio- and stereoregularities. Since the active center content was very low due to poor catalyst activities, it was impossible to identify the architectures of the catalytically active sites. Therefore, most information was gained using organometallic model systems.[35] During the rst two decades following his pioneering advances Ziegler rmly believed that polyinsertions of Ziegler catalysts occurred at the aluminium metal center. Other groups favored bimetallic sites involving both aluminium and titanium. Since Fink presented his very sound 13C-NMR spectroscopic evidence that polymerization of 13C-enriched ethylene on Cp2TiRCl (R Et, Me)/ EtnAlCl3-n (n 1, 3) occurred unambiguously at the TiC bond, catalytically active transition metal compounds were considered to be the catalytically active intermediates of Ziegler catalysts.[36] During the rst three decades, most research on stereospecic 1-olen polymerization was focused on heterogeneous catalysts. It was the pioneer Giulio Natta himself who stated in his Nobel Prize lecture in 1963: The rst highly stereoregular isotactic polymers were obtained in the presence of heterogeneous catalysts; however, it soon became clear that the heterogeneity of the catalyst system is


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an essential factor for the polymerization of aliphatic olens to isotactic polymers, but not for the polymerization of other types of monomers. Here the Nobel laureates vision has proven to be wrong. The discovery of new activators during the 1970s and novel asymmetric single site catalysts during the 1980s expanded the frontiers of polyolen technology well beyond traditional titanium based heterogeneous catalysts and changed the landscape of olen polymerization. During the mid 1970s several groups discovered accidentally that traces of water accounted for improved catalyst activity of metallocene-based homogeneous catalysts in the presence of AlMe3. It was Sinn and Kaminsky who rst identied the potential of AlMe3/H2O activators for metallocene-catalyzed ethylene polymerization. Their German patent DE 2608863, led jointly with BASF AG on March 4, 1976, claimed novel halogen-free catalysts comprising bis(cyclopentadienyl)titaniumdialkyl, aluminiumtrialkyl and water with Ti/Al molar ratios varying between 1:1 and 1:105 and Al/H2O between 6:1 and 6:(< 9).[37] Their second patent DE 2608933, led the same day,[38] disclosed zirconocene catalysts activated with AlMe3/H2O. The discovery of the role of the AlMe3/H2O molar ratio ultimately led Sinn and Kaminsky to the discovery of methylaluminoxanes (AlMeO)n (n 520), abbreviated as MAO, which boostet the activity of metallocene-based catalysts and produced uniform polyethylene with narrow molar mass distributions typical for single site catalysts.[39,40] The actual architectures of MAO activators has been subject of many heated debates and was a hot topic of a special conference organized by Sinn and Kaminsky in Hamburg in 1994.[41] Today it is well recognized that MAO forms cationic metallocene alkyl complexes containing a weakly or non-coordinating anion which is complexed by the cage-like MAO molecule. A schematic view of MAO-based catalytically active sites is displayed in Figure 11. Later MAO-free activators such as peruoroarylboranes and trityl and ammonium borates as well as new supported activators were developed. The current status of the activator chemistry was reviewed by Chen and Marks.[42] In 1980, for the rst time, MAO activation of metallocene by Sinn and Kaminsky afforded

Figure 11.

Single site metallocene catalyst activated with MAO.

polymerization of propylene, producing completely atactic poly(propylene). MAO and related activators have proved to be very versatile with respect to the activation of many different complexes and formation of highly active homogeneous catalysts. The discovery of MAO activators has led to a wealth of new homogeneous catalysts for olen polymerization. Since the 1980s it is an important challenge in catalyst development to nd new low cost activators as alternatives to the rather expensive MAO-based activators. The discovery of MAO activators paved the way for the development of many new families of highly active single site catalysts which have very well dened molecular architectures and give clear correlations between catalyst architectures and polymer properties. It was Brintzinger who rst proposed the use of chiral ansa-metallocenes for the stereospecic 1-olen polymerization against heavy opposition of some of his established German colleagues who were in accord with Nattas vision and criticized his research proposal because according to their feeling stereospecicity would remain the domain claimed exclusively by heterogeneous catalysts. Brintzingers synthesis of chiral bridged (ansa) metallocenes provided the solid scientic base for the discovery of the homogeneous stereospecic 1-olen polymerization and exciting progress with respect to novel stereoselective single site catalysts and the identication of correlations between catalyst structures and polymer architectures.[43] Ewen[44] and Kaminsky jointly with Brintzinger[45] demonstrated that MAO-activated homogeneous catalysts were indeed able to produce stereoregular poly(propylene). While meso ansa metallocenes gave atactic poly(propylene), racemic ansa metallocenes produced isotactic poly(propylene). Ewens sound spectroscopic analysis of the poly(propylene) microstructures provided strong experimental evidence for the presence of enantiomorphic site control as well as chain end control of the stereoselective propene polymerization. Since the 1980s the performance of metallocene-based catalyst systems was improved to produce isotactic, syndiotactic, and stereoblock poly (propylene)s on an industrial scale.[46] In the early 1990s supported single site metallocenes catalysts were introduced to enable gas phase polymerization.[47] Also ethylene/ 1-olen copolymers with high 1-olen content, cycloolen copolymers, ethylene/styrene interpolymers, syndiotactic polystyrene, and long-chain branched ethylene copolymers became availabe. Metallocene-based innovations with respect to both catalyst and polymer development were reviewed in the multi-authored book entitled MetalloceneBased Polyolens, edited by Scheirs and Kaminsky.[48] The concept of single- site catalysts has been expanded successfully to bridged half-sandwich titanium amide complexes, which became known as constrained geometry catalysts.[49] For many years the major focus of catalyst development was placed upon catalysts derived from Group 4 transition

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Figure 12. Single site catalysts based upon group 4 ansa-metallocenes and bridged half-sandwich complexes as well as late transition metal complexes with chelating ligands.

metals such as Ti, Zr, Hf. Today, as illustrated in Figure 12, also the potential of the late transition metal complexes of Ni, Pd, Co and Fe was recognized when other ligand frameworks were developed in order to evade the patent mine eld around catalysts based upon ligands derived from cyclopentadiene. Recent advances and new concepts of non-metallocene catalysts for olen polymerization were summarized in comprehensive reviews by Gibson.[50,51] Since the mid-1990s the range of highly active catalysts for ethylene polymerization was expanded substantially. The progress of Group 10 single site catalyst development is quite remarkable and was reviewed by Mecking.[52,53] During the rst decades nickel catalyst development was aimed almost exclusively at ethylene oligomerization and formation of 1-olens as attractive comonomers. This was inuenced by Zieglers nickel effect implying that only ethylene oligomers are formed on nickel based catalysts. Preferred catalysts systems of the Shell Higher Olen Process (SHOP) were Keims nickel ylides shown in Figure 13.[54] Klabunde[55] and Ostoja-Starzewski[56] modied ylide ligands in order to promote successfully chain propagation with respect to chain termination, thus achieving formation of high molecular weight polyethylenes. Basic correlation between ligand frameworks and polyethylene molecular weight were established by OstojaStarzewski. During the 1990s Brookharts diimine nickel catalysts, which are described in detail in 5.4, and Grubbs activator-free iminophenolate nickel catalysts[57] improved activity of nickel catalysts and molecular weights of the resulting polyethylenes. While many of these nickel cata-

lysts produce branched ethylene homopolymers, the pyridine-diimine catalysts of iron and cobalt, introduced by Brookhart[58] and Gibson,[59] produce highly linear high density polyethylene and require 1-olen addition to introduce short chain branching. Yasudas rare earth metal

Figure 13. The evolution of nickel catalysts: from Zieglers Nickel Effect and the Shell Higher Olen Process (SHOP) toward ethylene polymerization and the formation of linear as well as branched polyethylenes.


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complexes such as bulky organolanthanide(III) complexes were found to be effective catalysts for ethylene polymerization and the formation of polyethylene-block-poly (methyl methacrylate) diblock copolymers.[60] With the titanium catalysts supported on high surface area anhydrous magnesium compounds, the Brintzinger-type ansametallocenes, the Dow/Exxon constrained geometry half sandwich catalysts, Brookhart-type Ni- and Pd diimine catalysts, Brookhart/Gibson-type pyridine-diimine Fe and Co catalysts, and the Grubbs iminophenolate Ni catalysts powerful catalyst systems are at hand to tailor polyolen materials. The impact of these systems on the preparation of polyolen materials is described in section 5. Since the pioneering days of Ziegler and Natta the remarkable success of catalyst development is reected by a steady increase of catalyst activities well beyond 1 000 kg per gram of transition metal which is equivalent to less than 1 ppm transition metal left in the polymer. With further increases of catalyst activity the amount of residual transition metal is approaching 0 ppm. In fact several groups reported the successful development of transition metal free catalyst systems using only aluminium alkyl without transition metals as catalysts. Their back to the roots approach is aiming at the improvement of Zieglers aluminium alkyl mediated ethene polyinsertion process (Aufbaureaktion) which is restricted to the formation of rather small molecular weight oligoethylenes. In 1979 Columberg at Battelle in Geneva claimed transition metal alkyl free ethylene polymerization using Et2AlCl together with anhydrous magnesium diacetate modied by heating it with a mixture of acetic acid and acetic anhydride.[61] When MAO activators became available several groups noticed that in special cases ethylene polymerization was initiated before the transition metal was injected. Sens transition metal free catalyst system is based upon MAO/AlMe3 and MAO in conjunction with other Lewis acids such as B(C6F5)3. High molecular weight linear polyethylene, ethene/propene copolymers and atactic poly(propylene) were obtained.[62] Other transition metal free catalysts are based upon cationic aluminium alkyl catalysts such as Jordans amidinate complexes[63] and Gibsons cationic aluminium alkyl complexes with monoanionic N,N,Npyridyliminoamide[64] and salicylaldiminato[65] ligands. The proposed catalyst architectures are shown in Figure 14.

The theoretical study of Busico[66] on the balance of olen insertion and b-hydride transfer to monomer for such cationic aluminium alkyl catalysts arrives at the conclusion that this balance is not better than that of Me2AlEt. Therefore, olen polymerization at a single aluminium center appears to be rather unlikely. Although it is not possible to exclude the presence of other mechanisms relating to the possible presence of di- and polynuclear active sites, several groups agree with Busicos favored alternative explanation that the contamination of the catalysts with trace amounts of transition metals well below the detection level of conventional analytical tools is quite likely to account for some of these observations.

Advanced Polyolen Materials and Tailor-Made Property ProlesIsotactic Poly(propylene)The development of single site catalysts changed the way of doing catalyst development. Instead of empirical optimization of mixed catalysts, catalyst architectures were designed to produce polyolens with specic property proles. As is apparent in Figure 15, single site catalyst produce uniform polymers with a narrow molecular weight distribution (polydispersity M w/M n 2), welldened regio- and stereoregularity, and molar mass independent random or sequenced comonomer incorporation. In contrast, the conventional multi-site catalysts produce rather complex mixtures of polymers. Today single site catalysts and advanced processes for olen polymerization offer new opportunities for tailor-making polymers. Selected milestones are highlighted below. It should be noted that modern polyethylenes and poly(propylene)s are very different from polyethylenes and poly(propylene)s produced during the 1960s. Performance of polyolens was improved substantially as a result of successful catalyst and process improvements. The development of single site catalyst technology is paralleled by achieving unprecedented control of molar mass, molar mass distribution, short and long chain branching, as well as stereochemistry. As a function of catalyst structures and process conditions it is possible to tailor polyolen materials according to customers demands. Today the basic reaction mechanisms of stereoselective 1-olen polymerization are well understood. High resolution 13C NMR spectroscopy has proven to be an excellent analytical tool for evaluating polyolen microstructures which are ngerprints for catalyst performance, especially regio- and stereoselectivities of the catalyst systems.[67] Comprehensive reviews on stereoselective 1-olen polymerization, including catalyst design, tailormaking polyolens, and molecular modeling, were published by Kaminsky,[68] Brintzinger,[69] Resconi,[70] Fink[71] and Coates.[72] When these reviews published very

Figure 14. Cationic aluminium alkyl complexes as transition metal free catalysts.

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Figure 15. Single site (left) versus conventional multi-site catalysts (right): single site catalyst affords uniform molar-mass independent comonomer incorporation.

close to the 50th anniversary of Nattas discovery of the stereoselective polymerization are compared with the review published by Pino[73] on occasion of the 25th anniversary of stereospecic 1-olen polymerization, it is obvious that the knowledge of the basic correlations between well dened structures of single site catalysts, polymer architectures and polymer properties has progressed signicantly during the last two decades. While the actual architectures of the conventional heterogeneous Ziegler catalysts remain unknown, the well-dened architectures of metallocene complexes can be correlated with molecular architectures of poly(propylene)s, as displayed in Figure 16. Among thermoplastic polymers poly(propylene) is quite outstanding with respect to its versatility. The exciting progress of poly(propylene) development is the subject of several books.[7476] Today tunable catalyst systems are at

hand to prepare in a well controlled fashion most of the feasible poly(propylene) molecular architectures and to ne-tune poly(propylene) properties. Although isotactic poly(propylene) remains the polymer with predominant commercial attractiveness, the new opportunities of controlling polyolen microstructures have helped to achieve a better understanding of basic structure/property correlations of poly(propylene) and to expand the poly(propylene) property range. While many conventional multi-site isoselective catalysts produce a mixture of high molecular weight highly isotactic poly(propylene) and lower molecular poly(propylene)s containing regio- and stereoirregularities, metallocene catalysts produce uniform isotactic poly(propylene)s with random distribution of regio- and stereoirregularities along the isotactic poly(propylene) chain. Fischer discovered in 1994 that the isotactic segment length between steric errors in the isotactic poly(propylene)

Figure 16. Correlations between poly(propylene) architectures and metallocene catalyst structures.


301

and toughness with respect to isotactic poly(propylene). Other benets of syndiotactic poly(propylene) are reported to be enhanced stability against X-ray irradiation and lower heat sealing temperatures. The quite remarkable differences in processing, e.g. melt ow, of isotactic and syndiotactic poly(propylene) has been attributed to the differences of their entanglement molecular weights and conformations.[84,85] There exists a much higher content of entanglements with respect to isotactic poly(propylene).Figure 17. Tuning poly(propylene)s molecular architectures.

chain affected poly(propylene) crystallization.[77] With decreasing segment length (cf. Figure 17) the melting temperature decreases and the content of the g-modication increases. For the rst time it became possible to prepare high molecular weight isotactic poly(propylene) which crystallizes to form predominantly the g-modication which at that time was thought to be typical exclusively for very low molecular weight poly(propylene).[78] In the gmodication of isotactic poly(propylene) the orthorhombic unit cell is formed by bilayers composed of parallel helices with the adjacent bilayers being tilted by an angle of 808. In contrast to the morphology of conventional isotactic poly(propylene) spherulites were absent and the resulting nanostructures accounted for improved optical clarity.[79,80] At present industrial applications of metallocene-based isotactic poly(propylene) are still focusing on niche markets, e.g., high-speed spinning of poly(propylene) bers and biaxial orientation of poly(propylene) lms. The narrow molar mass distribution of metallocene-based isotactic poly(propylene) facilitates molecular orientation and accounts for enhanced tenacities of bers and improved strength of biaxially oriented lms.

Stereoblock Poly(propylene) and Elastomeric Poly(propylene)Since Natta discovered the stereoselective propene polymerization, most of the research effort in propylene polymerization was aimed at improving stereoselectivity in order to prevent removal of low molecular weight atactic by-product with wax-like properties. During the 1970s Wisseroth at BASF AG in Ludwigshafen realized the potential of stereoblock poly(propylene)s containing alternating exible atactic and crystalline isotactic segments. He designed a special glove-test, which is displayed in Figure 18 to demonstrate that poly(propylene)s with low stereoregularities are not tacky and viscous liquids but exible and soft non-tacky lms exhibiting elasticity. In a rather aggressive publication he attacked Nattas group and questioned the validity of their extraction method and their conclusions with respect to the limited usefulness of low stereoregular poly(propylene).[86] He stated that low stereoregular poly(propylene)s did not possess peanut butter like consistency, as he quoted Natta and his group. Today the origins for this apparent controversy are well understood. In Nattas slurry polymerization low stereoselective catalytically active sites always produced low stereoregular poly(propylene)s which had low molecular weights, sticky consistency, and were easy to remove by means of solvent extractions. In contrast, Wisseroths gas phase polymerization afforded low stereoregular poly-

Syndiotactic Poly(propylene)Syndiotactic poly(propylene) was rst isolated by Natta and coworkers as a small fraction which was separated from other poly(propylene) stereoisomers by means of tedious solvent extractions.[81] Later Zambelli introduced vanadium catalysts which gave low yields of syndiotactic poly(propylene) with acceptable regio- and stereoregularity only at 78 8C.[82] The breakthrough occurred in 1988 when Razavi and Ewen discovered that bridged cyclopentadienyl-uorenyl metallocene catalysts were highly syndiospecic at room temperature. The formation of syndiotactic poly(propylene) was attributed to changing the position of the poly(propylene) chain after each insertion, thus causing alternation of the site chirality and consequently strict alternation of the two possible congurations of adjacent repeat units in the poly(propylene) chain.[83] Typical metallocene based syndiotactic poly(propylene) has lower density, lower crystallinity, lower crystallization rate, lower exural modulus combined with higher clarity

Figure 18. Wisseroths gloce test designed to demonstrate the useful properties of high molecular weight stereoregular poly(propylene) as soft and exible materials in contrast to the rigid isotactic poly(propylene).

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(propylene) with much higher molecular weight. When the molecular weight is high enough more than two isotactic segments of the poly(propylene) are sufciently large to cause cocrystallization. This cocrystallization of isotactic segments in rather exible low stereoregular poly(propylene)s affords thermoplastic elastomers with thermally reversible crystallite links between the chains. As a function of the crosslink densities it is possible to vary properties from very soft to elastomeric and tough/rigid. This concept stimulated the development of elastomeric poly(propylene)s (ELPP). End of the 1970s Collette at Du Pont used ZrR4 (R CH2C6H5, C(CH3)2C6H5) supported on anhydrous Al2O3 with large pores to polymerize propylene and to produce an elastomeric reactor blend consisting of isotactic poly(propylene) together with high molecular weight stereoblock poly(propylene).[87,88] The cocrystallization was elucidated as a function of poly(propylene) microstructures and reactor blend compositions. With Chiens CH3CH(Me4Cp)(Ind)TiCl2/MAO,[89] Waymouths oszillating non-bridged (2-Ph-Ind)2ZrCl2/MAO,[90]and Riegers dual side metallocene catalysts[91,92] it became possible to produce high molecular weight stereoblock poly(propylene) with much narrower molecular weight distributions and without formation of isotactic poly(propylene) as by-products. Resconi used non-stereospecic Me2Si(Flu)2ZrCl2 based catalysts to produce completely atactic poly(propylene) with molecular weights exceeding 500 000 g/mol. Due to cocrystallization of isotactic segments his completely atactic poly(propylene)s are also elastomeric.[93] The high glass transition temperature around 0 8C is limiting the applications of elastomeric stereoblock poly(propylene)s and their blends with isotactic poly(propylene). The chemistry of thermoplastic elastomers based upon stereoblock poly(propylene)s and elastomeric alternating propene/CO copolymers was reviewed by Rieger.[94] Another approach to the formation of stereoblock poly (propylene)s and also other block copolymers is based upon the reversible transfer of polymer chains to the aluminium alkyls. While many activated ansa-metallocenes are believed to give only irreversible transfer from the transition metal to the aluminiumalkyl,[95] there is experimental evidence that reversible transfer of the polymer chain to main group metals can be achieved with single site catalysts and used to prepare linear 1-olens with Poisson distribution of the molecular weights, e.g., by using di(imino)pyridine iron together with zinc alkyls in ethylene oligomerization.[96] In Riegers oxygen containing, dual side zirconocenes the examination of poly(propylene) microstructures with respect to stereoerror formation as well as the role of the Al/Zr molar ratio implies that reversible chain transfer to aluminium takes place.[97] Chien used iso- and syndiospecic ansa-metallocenes, activated with AliBu3/Ph3C B(C6F5) to polymerize propylene 4

with this binary catalyst system where the activity proles of both iso- and syndiospecic catalysts were very similar. The resulting polymers were quite different from reactor blends of isotactic and syndiotactic poly(propylene)s. Although a bimodal molecular weight distribution was obtained, all fractions of the size exclusion chromatography (SEC) contained both isotactic and syndiotactic sequences, as proven by FT-IR measurements coupled with SEC. According to AFM investigation on toluene etched samples of the poly(propylene)s obtained and iPP/sPP blend reference samples, stereoblock homopolymers containing iso- and syndiotactic segments were formed.[98] Effective reversible transfer of polymer chain between transition metal and aluminium sites offers attractive opportunities for designing stereoblock homopolymers as well as new block copolymers. In view of the opposing views of different groups, more research is needed to elucidate the inuence of the catalyst architectures on reversible and irreversible chain transfer and the prospects of the formation of novel segmented polymers.

Syndiotactic PolystyreneAlthough Natta and Pino succeeded in preparing isotactic polystyrene already in 1954,[99] it took more than thirty years to nd a catalytic route leading to syndiotactic polystyrene. In 1985 Ishihara developed CpTiX3/MAO and Cp*TiX3/MAO catalysts (X Cl, F, Me, OMe) which form syndiotactic polystyrene in yields exceeding 200 kg PS/g Ti.[100] The polymerization is highly regioselective and proceeds via 2,1 insertion.[101] Progress in the development of syndiotactic polystyrene was reviewed by Schellenberg and Tomotsu.[102] The sPS melting temperature of about 270 8C is around 40 8C higher than that of isotactic polystyrene which crystallizes at much lower rates. Due to its high heat distortion temperature of 250 8C and the absence of hydrolytically instable groups in its backbone, syndiotactic polystyrene is a potentially strong competitor for conventional engineering thermoplastics based upon polycondensates. Its density and melt viscosity are much lower with respect to other engineering polymers prepared by polycondensation. However, syndiotactic polystyrene is inherently brittle and requires impact modication, e.g., by compounding it with rubbers or other impact modiers. The dielectric constant of 2,6 is attractive for electrical applications. The markets are being developed for syndiotactic polystyrene by Idemitsu Petrochemical Co. (trade name Xarec) and Dow (trade name Questra).[103]

Branched PolyethylenesAs schematically illustrated in Figure 19, short chain branches can be introduced into the polyethylene backbone either by means of copolymerization of 1-olens (pathway A, Figure 19) or by multibranching homopolymerization


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Figure 19. Branched polyethylene prepared by ethylene/1-olen copolymerization (pathway A) and branching ethylene homopolymerization (pathways B and C).

(pathway B and C, Figure 19), which does not require comonomer addition. The multibranching homopolymerization is of interest with respect to the incorporation of rather long chain alkyl branches because higher 1-olens and macromonomers are less reactive and difcult to remove from the polymer. The single site catalyst innovations had a major impact on polyethylene manufacturing and expanded the range of polyethylenes into very low density grades and novel thermoplastic elastomers, also referred to as plastomers or exomers. In the past most polyethylene and poly(propylene) catalysts failed to incorporate higher 1-olens, because incorporation of the less reactive 1-olens slowed down chain propagation with respect to chain termination. As a consequence, 1-olen comonomers were incorporated predominantly in wax-like low molecular weight fractions. The poor reactivity of many titanium catalysts with respect to the incorporation of higher 1-olens caused formation of blocky copolymers or mixtures of ethene copolymers with low 1-olen incorporation and ethylene homopolymers. As a consequence, special vanadium-based catalysts were developed during the 1960s to achieve random placement of 4060% of propene in the polyethylene chain to form amorphous ethene/propene (EPM) or ethene/propene/diene (EPDM) rubbers.[104] Today catalysts based upon ansa-metallocenes and halfsandwich titaniumamides are highly reactive with respect to the incorporation of higher 1-olens. Basic correlations between catalyst architectures and catalyst activities, stereoselectivities, short chain branching have been identied for ansa-metallocene and half sandwich titanium amide catalysts. It is possible to prepare the entire range of ethene/1-olen copolymers. In contrast to opposite statements in the literature, reactivities of Brintzingers ansa metallocenes[105] and halfsandwich titaniumamides[106] are very similar, provided that the appropriate metallocene ligands are selected. A bar chart presentation of catalyst performance, determined by means of an automated catalyst screening in ethylene/1-octene copolymerization, is displayed in Figure 20 for halfsandwich (CBT) and ansametallocene catalysts.[107]

While 2-methyl substitution of dimethylsilylene-bridged ansa-metallocenes enhances molecular weight, benzannelation and 4-substitution promotes 1-octene incorporation and enhances catalyst activity. This experimental result was conrmed by molecular modeling.[108] With 4-phenyl(MPI) or 4-napthyl- (MNI) substitution, respectively, the comonomer incoporation was very similar to that of the CBT half sandwich catalyst. The activity of the MNI and MPI catalysts is much higher and sustained for many hours, whereas the half sandwich titanium catalysts suffer deactivation within a few minutes. Therefore, ansa-metallocenes are preferred catalysts for gas phase and slurry polymerization with hold-up times of several hours, whereas many halfsandwich catalysts are used also in solution polymerization with much shorter hold-up times. Both catalyst systems incorporate very effectively higher 1-olens.[109114] and even up to 50 mol-% styrene.[115,116] In the past all attempts to incorporate large amounts of styrene had failed because styrene had poisoned many conventional catalysts and had formed homopolymer byproducts via free radical styrene polymerization. Cyclocopolymerization of the non-conjugated 1,5-dienes such as 1,5-hexadiene accounted for 1,3-incorporation of cyclopentane rings into the polyethylene backbone.[117] Shaffer and coworkers at Exxon reported the successful copolymerization of isobutylene and ethylene with half sandwich complexes.[118] Molecular architectures of the different ethylene copolymer families are displayed in Figure 21. In 1985 the branching homopolymerization of olens was discovered and led to the development of novel branched ethylene homopolymers and new branch distributions without requiring 1-olen copolymerization. Using his nickel aminobis(imino)phosphorane catalyst, introduced by Keim[119,120] for branching ethylene homopolymerization, system for 1-olen polymerization Fink identied by means of 13C NMR spectroscopic microstructure analysis that poly(1-olens) were not formed. The resulting polymers consisted of methyl branched polyethylene. For example, polymerization of 1-pentene did not form conventional poly(1-pentene) via 1,2 linkage but

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Figure 20. Ethene/1-octene copolymerization (75 mol-% 1-octene in feed) using various catalysts systems (data from Tuchbreiter[107]). Row R1 comonomer incorporation of 1-octen F0 [mol-%], row R2 Al/metal-ratio (103), row R3 molar mass M n [kg/mol], row R4 Am[(molinserted monomer units)/(mol/Lolen concentration*molZr*hpolymerization time)104]. Catalysts systems are meso/rac-MBI: rac-dimethylsilylenebis(2-methy-indenyl)zirconium dichloride, I: rac-dimethylsilylenebisindenyl zirconium dichloride, MI: rac-dimethylsilylenebis(2-methylindenyl) zirconium dichloride, BI: rac-dimethylsilylenebis(4,5-benz-indenyl) zirconium dichloride, MPI: rac-dimethylsilylenebis(2-methyl-4-phenylindenyl) zirconium dichloride, MNI: rac-dimethylsilylenebis(2-methyl-4-naphthylindenyl) zirconium dichloride, CBT: dimethylsilylene(tetramethylcyclopentandienyl)(tert-butylamido) titanium dichloride, Cp2ZrCl2: zirconocene dichloride, Ph-Lig: bis(2-phenylcyclopenta[I]phenantryl) zirconium dichloride, Ph-Ind: bis(2-phenylindenyl) zirconium dichloride, Me-Lig: bis(2-methylcyclopenta[I] phenantryl) zirconium dichloride, H-Lig: bis(cyclopenta[I]phenantryl) zirconium dichloride.


305

Figure 21. Molecular architectures of polyethylenes obtained with metallocene and halfsandwich catalysts. Displayed from above are: linear polyethylene (HDPE), short chain branched ethylene/1-butene copolymer, long chain branched HDPE, ethylene/norbornene copolymer, cyclocopolymer from ethylene and 1,5-hexadiene copolymerization, and ethylene/styrene copolymer.

formed a methyl-branched polyethylene equivalent to the strictly alternating ethylene/propylene copolymer. Fink proposed a new mechanism involving migration of the catalyst complex up and down the growing chain following the 2,1 insertion of the 1-olen in a nickel alkyl bond. Only at the chain end in o-position, the less sterically hindered Ni alkyl is able to insert another 1-olen and to continue propagation.[121124] Fink referred to this new mechanism as 2, o-polymerization, which ten years later was rediscovered and renamed chain walking. Also higher 1-olens, obtained by means of b-hydride elimination during ethylene homopolymerization, can polymerize according to the chain walking mechanism. As illustrated in Figure 22, the repeated sequence of b-hydride elimination followed by reinsertion accounts for migration of the transition metal alkyl up and down the polyethylene chain. Immediate ethylene insertion after 2,1 insertion produces methyl branched polyethylene, whereas insertion during migration affords the rather peculiar branching pattern comprising randomly distributed n-alkyl branches with odd and even number of carbon atoms as well as branched branches. This branch distribution is different from that of binary catalysts containing polymerization and oligomerization catalysts which form exclusively branches with even number of carbon atoms. The catalytically active transition metal alkyls also walk through tertiary carbon atoms on a growing polymer chain. In 1995 Brookhart introduced diimine-Ni and diimine-Pd complexes which are very effective chain walking catalysts producing branched ethylene homopolymers with branching controlled by catalyst structure and polymerization condi-

tions.[125128] In contrast to Ni-diimine catalysts with pressure dependent branching, the diimine-Pd catalysts gave pressure independent much higher branching and produced liquid hyperbranched polyethylenes with molecular weights exceeding 100 000 g/mol. Performing chain walking in the plant represents a challenge for the reaction engineering. At high ethylene pressures mainly methyl branches are formed with many of the diimine-Ni-based catalysts. Adequate time must be assured to allow migration of the transition metal alkyls and formation of higher branches. This research on chain walking led to led to the development of Du Ponts Versipol process.[129,130] The diimine-Ni(II) catalysts are quite exceptional with respect to their capability to polymerize trans-2-butene.[131] For more than three decades the dogma was accepted that internal double bonds would not copolymerize because of low reactivity of internal doubly bonds. Only when isomerization takes place the copolymerization of 1-buetene isachieved with conventional catalyst systems. Another attractive feature of diimine-Pd(II) catalysts is their potential to afford living polymerization of ethylene and 1-olens.[132] This is the key to formation of polyolen blockcopolymers with well dened sequence distributions. Branching homopolymerization was also detected when ethylene was polymerized with certain metallocene catalysts. Alt performed an extensive study on the inuence of Group 4 metallocene ligand frameworks on catalyst activity in ethylene polymerization and polyethylene molecular weights.[133] In the case of his self-immobilizing copolymerizable alkenyl-substituted metallocenes, he noticed the formation of ethyl branches with very regular distribution without adding 1-butene comonomer. Such ethyl branches were formed directly from the ethylene monomer and the

Figure 22. Branched ethylene homopolymers produced via chain walking (pathway A) and polyethylene containing isolated butyl branches resulting from b-hydrogen transfer to ethylene (pathway B).

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Figure 23. Polyethylene morphology changes from chain-folded lamella to fringed micelles with increasing degree of short chain branching produced by means of either ethylene/1-olen copolymerization or chain walking.

extent of their formation depended primarily upon the catalyst type.[134] A similar observation of ethyl branching in ethylene homopolymerization was made by Izzo and coworkers[135,136] who used meso ansa metallocenes of Group 4, e.g., ethylenebis(1-indenyl)ZrCl2, activated with different types of activators, to produce ethylene homopolymers containing various amounts of randomly distributed ethyl branches. With increasing substitution of the indenyl ligand ethyl branching is reduced. Most likely ethyl branches, as seen in Figure 22, result from b-hydride chain transfer to monomer followed by reinsertion of the vinylterminated polyethylene. As seen in Figure 23 morphology changes of polyethylenes from chain-folded lamella to fringed micelles occur with increasing degree of short chain branching produced by means of ethene/1-olen copolymerization and chain walking. The breakthroughs in ethylene homo- and copolymerization by means of single site catalysts stimulated the industrial development of high density, linear low density, and very low density polyethylenes and of new ethylene/ 1-octene thermoplastic elastomers with high 1-octene content (Engage from Dow). Moreover, the uniform branching distribution stimulated research on basic structure/ property correlations. According to transmission electronmicroscopic studies the increasing 1-octene content prevents chain folding of polyethylene chains and at 1-octene content exceeding 10 wt.-% fringed micelles are formed, whereas at 1-octene contents above 40 wt.-% the resulting copolymers are completely amorphous.[137] The chain conformations are schematically illustrated in Figure 23. For the rst time syndiotactic ethylene/1-olen copolymers with high 1-octene content are available and show similar inuence of alkyl branches on crystallization behavior.[138] The inuence of both stereoregularity of poly(1-olens)

and n-alkyl branching of polyethylenes on crystallization behavior as a function of catalyst type and polymerization conditions was compared by Kressler and coworkers.[139] The inuence of the degree of branching of polyethylenes prepared by means of ethylene/1-olen copolymerization, chain walking, and ethylene polymerization with hybrid catalysts producing 1-butene by means of ethene dimerization were examined with respect to polyethylenes thermal, mechanical and morphological properties. Figure 24 displays the thermal properties.[140] While the polyethylene segment length between two branches correlates with the melting temperature as proposed by Flory during the 1950s, the number of side chain carbon atoms must be taken into account to predict glass temperature as a function of n-alkyl branching without taking into account the mechanism responsible for the branches.[141] The inuence of copolymer molecular architectures on the compatibility of ethene/1-butene copolymers blended together with isotactic poly(propylene) was evaluated. As a function of the 1-butene content it is possible to prepare highly exible miscible blends at high 1-butene content exceeding 90% or rigid impact modied two-phase blends with improved low temperature impact resistance at 1-butene content around 40 wt.-%.[142144] Fine-tuning of branched polyolens is an important route to polyolens with tailor-made properties meeting the demands of very different applications ranging from exible packaging to automotive bumpers. In contrast to Group 4 catalysts and conventional aluminium alkyl activated systems, which are severely poisoned by trace amounts of polar groups, Brookharts palladium catalysts are capable of copolymerizing ethylene with various polar comonomers due to the lower oxophilicity of the late transition metal compounds. Up to 40 wt.-% methyl acrylate were incorporated, whereas the sterically


307

Figure 24. Inuence of polyethylene n-alkyl short chain branching on melting temperature (above), plotted according to Flory, and glass temperature (below), as proposed by Mader[141] (DB: degree of branching, BC number of branching carbon atoms, MC: number of carbons in the main chain, SC: number of carbons in the side chain).

more hindered methyl methacrylate is not copolymerized with ethylene.[145] According to NMR sequence analysis the methyl acrylate comonomer is not incorporated in the chain but at the end of branches. Although the late transition metal catalysts can tolerate polar functional groups, the catalyst activity is still far too low for industrial processes. The copolymerization of polar commodity monomers such as vinyl acetate, acrylonitrile, and methyl methacrylate is still an important challenge for the future with high potential for future breakthroughs in catalysis. The current state of the art in copolymerization of polar monomers with olens was reviewed by Novak[146] and Sivaram.[147]

Exxon and others employed thermal degradation of isotactic poly(propylene) to narrow down molecular weight distribution by preferentially cleaving high molecular weight chains due to b-scission induced by free radicals present in the extrusion process.[148] During the 1960s such modied poly(propylene)s became known as controlled rheology (CR) poly(propylene)s. As a consequence of the rapid progress in catalyst development, polyolens with much narrower molecular weight distributions became available during the 1980s when metallocene catalysts were employed. Melt ow and melt strength depended upon catalyst type and polymerization conditions. Poor shear thinning and melt strengthening of many metallocenebased homo- and copolymers with narrow molecular weight distributions caused severe processing problems which were encountered when the rst generations of metallocene-based polyolens were brought to the market place. This peculiar problem was circumvented when a better insight in the basic correlations between metallocene architectures, polyolen microstructures and rheological properties was achieved. Catalysts such as Dows halfsandwich titaniumamides give effective copolymerization of the higher 1-olenssuch asvinyl-terminated polyethylenes macromonomers resulting from b-hydride elimination. The copolymerization of ethylene with such ethylene-based macromonomers affords long chain branched polyethylenes which account for the shear thinning. This is in accord with observations by the groups of Soares[149] and Seppala.[150] The effect of long chain branches is apparent from Figure 25 for polyethylenes prepared by means of Dows Insite Technology.[151] Model studies on ethylene/1-eicosene copolymers prepared with MAO-activated with ansa-metallocenes revealed that at high 1-eicosene content the resulting copolymersarelinearanddonot contain longchainbranching because at high 1-olen content vinylidene endgroups are formed which do not copolymerize with ethylene using such catalysts.[152] Long chain branches were successfully introduced by copolymerization of trace amounts of non-conjugated dienes such as 1,7-octadiene in order to achieve melt

Controlled Rheology PolyolensDuring the pioneering days many processing problems were encountered due to the broad molecular weight distribution and the presence of considerable amounts of polyolens with very high molecular weight. Kowalski atFigure 25. Long chain branching on polyethylene melt rheology accounts for shear thinning (data from polyethylenes prepared by means of Dows Insite technology).[151]

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strengthening useful in blow molding.[153,154] The NMR microstructure analysis gives only a total content and does not take into account that the architectures of long chain branched polymers can vary signicantly during polymerization when the endgroup content increases. Also it should be noted that formation of compartments when immobilizing catalysts on supports can result in signicant changes of the content of long chain branches. As a consequence, the choice of supports and processes for supporting catalysts can affect processing properties. Mixtures of linear, starand comb-shaped long chain branched polymers are obtained with very low degree of long chain branching close to the limiting detection levels. In addition to long chain branching also short chain branching, comonomer type and content, stereoregularity and molecular weights and molecular weight distribution inuence rheology.[155] The balance of short and long chain branching of low density polyethylene LDPE, produced in free radical high pressure ethylene polymerization, is still superior to that of many linear low density polyethylenes LLDPE prepared with metallocene catalysis. More research on model compounds is needed to quantify long chain branching by rheological methods. Today multiple single-site catalysts, also referred to as binary catalysts, tandem catalysts, or hybrid catalysts, are being developed in many labs in order to combine catalytic macromonomer formation on one type of single site catalyst with copolymerization on the other single site catalyst present in the catalyst blend.[156] In this process the macromonomer content is much higher in the reactor and promotes the formation of long chain branches via copolymerization.[157] Also this concept of single site catalyst blends (multiple single site catalysts) is pushed forward to control short chain branching of polyethylene using in-situ formation of 1-olen comonomers.[158] All these approaches encounter difculties with the real life of the plant operation. Carefully matched polymerization rate/ time proles of the different single site catalysts are prime requirements to prevent formation of very complex, time and conversion dependent polymer mixtures. Their composition responds to minute variations of polymerization process parameters. Also it should be noted that the second catalyst addition is likely to inuence the performance of the rst catalyst. This interaction can affect molecular weights and branch distribution simultaneously, thus prompting new challenges for reaction engineering, polymer characterization and especially quality control of the resulting products.

Figure 26. Polyethylene with bimodal molecular weight distribution containing a small fraction of high molecular weight molecules with small degree of short chain branching for bonding together the crystallites (tie molecules).

reactors, respectively. Reactor blends comprising a small fraction of very high molecular weight polyethylenes (molecular weight larger than 500 000 g/mol) containing a small amount of comonomer, in conjunction with lower molecular weight polyethylenes affords polyethylene reactor blends exhibiting improved mechanical property, toughness and extraordinary fatigue life.[159] This signicant improvement is assigned to the bonding of polyethylene crystallites by means of the high molecular weight tie molecules. A typical example of such reactor blends with multimodal molecular weight distribution is displayed in Figure 26.[160] Such polyethylenes with bimodal molecular weight distributions exhibit the strength and stiffness of high density polyethylene while retaining the high stress crack resistance and processability of medium density grades. They are being applied as light weight water and gas pipes and qualify for the PE100 rating requiring that the polymer must withstand a minimum circumferential stress of 10 MPa for the duration of 50 years at 20 8C.[161] The discovery of new catalyst generations in conjunction with single site catalyst technology and innovations relating to the formation of supported catalysts offer attractive opportunities for reactor blend formation and tailoring of multimodal molecular weight and comonomer distributions. Sequenced polymerization of polyolen and olen copolymer rubber represents an important approach to produce polyolens with properties ranging from supersoft to rigid and impact resistant at low temperatures.

Reactor BlendsSince the multiple single site technology is still at its infancy, molecular weight distributions and reactor blends are being tailored preferably in cascade reactors, using parallel or sequenced reactor systems, or multizone loop

Cycloolen Polymers and Advanced Functional MaterialsThe discovery of single site catalysts also had a major impact on the development of novel cycloolen polymers for applications as engineering resins, medical packaging and also new functional applications as waveguides,


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adhesives, photoresists and encapsulants for various electronic applications. Several routes have been developed to prepare polymers containing cycloaliphatic groups in their backbones: catalytic cycloolen homo- and copolymerization, cyclopolymerization of non-conjugated 1,4and 1,5-dienes, and hydrogenation of ROMP polymers. Waymouths metallocene catalyzed cyclopolymerization of 1,5-hexadiene affords methylene linked cyclopentane rings with steroregularities varied as a function of the metallocene catalyst type.[162164] It was Kaminskys group who recognized the extraordinary potential of single site catalysts in homo- and copolymerization of cycloolens such as cyclobutene, cyclopentene and norbornene. Successful cyclopentene polymerization yielded hydrocarbon polymers with melting temperatures and thermal decomposition temperatures above 380 8C.[165] Due to isomerization occurring after each insertion of cyclopentene poly(1,3-cyclopentene) was obtained.[166] However, the major emphasis of the cycloolen polymer development is being placed upon the polymerization of bi- and polycyclic olen monomers, preferably derived from Diels-Alder reactions of cheap cyclopentadiene feed stocks. The rst successful norbornene polymerization dates back to the 1960s when Sartori and coworkers used TiCl4 based catalysts.[167] The incorporation of cycloolen monomers such as ethylidene norbornene into ethylene/propylene copolymers was an important research issue in the 1970s to improve performance of EPDM rubber. Also ethylene copolymers with dicyclic dienes were examined using conventional Ziegler catalysts.[168] Since the 1980s the single site catalyst innovations have revolutionized norbornene homo- and copolymerization. An excellent brief overview was published by Goodall.[169] As outlined in Figure 27, there exist two very different research and development directions: the development of thermoplastics based upon ethylene/cycloolen copolymers (COC) and the development of soluble, infusible homopolymers with very high glass temperatures and low dielectric constants. Thermoplastic ethylene/norbornene copolymers are obtained either by means of hydrogenation of norbornenebased ROMP polymers (JSR, Nippon Zeon) or metallocene catalyzed ethylene/norbornene copolymerization (Ticona and Mitsui). The dependence of the COC glass temperature on the cycloolen content is displayed in Figure 28 for ethylene/norbornene and ethylene/tetracyclododecene. The catalytic copolymerization yields amorphous or semicrystalline transparent cycloolen copolymers with very low water uptake.[170] Mechanistic features of metallocene catalyzed norbornene copolymerization and polymer architectures were examined by Fink,[171,172] Goodall and Rhodes,[173,174] Tritto,[175177] Kaminsky and ArndtRosenau.[178180] Optically transparent ethylene/norbornene copolymers are produced by Ticona Co. (former Hoechst AG) under the trade name of Topas.[181] As

Figure 27.

Norbornene polymers.

hydrocarbon polymers the COCs have very low moisture permeability and very low water up-take combined with hydrolytic stability and resistance to polar solvents. Moreover, hydrocarbon polymers are easy to recycle by remolding and thermal cleavage to recover oil and methane gas without residues. Nonpolar solvents such as toluene do attack COC. The COC properties depend upon the norbornene content. With increasing norbornene content COC polymers are stiffer, have higher glass temperatures, higher heat distortion temperatures but also higher brittleness. COC polymers with glass temperatures below 100 8C are more exible and give much higher elongation at break at the expense of lowering tensile strength and heat distortion temperatures. Such COCs combine moisture barrier performance with low thermoforming temperature and are attractive materials for physiologically inert medical and food packaging, e.g., blister packs for drugs in tablet form.

Figure 28. Glass temperatures of ethylene/norbornene and ethylene/tetracyclododecene as a function of the cycloolen incorporation.

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COCs with higher norbornene content and higher stiffness are the materials of choice to make modern microtiter plates used in genomics and high throughput drug discovery to substitute glass test tubes. In contrast to other polymers, COC is optically transparent in the near-UV region and resistant to polar solvents. Medical COC applications also include disposable syringes. As binder for color toners COC helps to improve both speed and quality of color copiers and color printers. The second family of polycyclic polymers is based upon norbornene homopolymers and also copolymers of different norbornene derivatives. At Promerus (former B. F. Goodrich Company) highly active cationic allyl palladium catalysts were developed for the addition polymerization of various norbornenes, following earlier advances by Sen[182] and Risse.[183] Their catalysts produce a variety of soluble polynorbornenes with glass temperatures > 350 8C and molecular weights variable between 1 000 and 2 000 000 g/ mol using 1-olens as chain transfer agents. The polycyclics are soluble and processed by solvent casting or spin coating. Functional groups such as carboxylic acid esters, ethers and trisalkoxysilanes of norbornene derivatives are readily incorporated into the polynorbornene backbone because the Pd and also to a less extent the nacked Ni catalysts are not poisoned by polar groups. In a addition to high glass temperature of the rigid polycyclic backbone, norbornene offer other very attractive properties such as low moisture uptake, low dielectric constant, low dielectric loss, high breakdown voltage, chemical resistance, and a wide spectral window. The molecular architectures of the polynorbornenes are easy to ne-tune via the functional groups, as illustrated in Figure 29. While copolymerization of around 90% alkyl-substituted norbornene improves toughness and solubility, peruoroalkyl substituents promote optical properties, and incorporation of 210 mol-% of alkoxysilane groups improve adhesion to various substrates. These polycyclics from Promerus Company[184] are used as optical polymers (AppearTM) for wave guides and at panel displays. The application of such substituted polynorbornenes in photolithography is possible because they are optically transparent in the deep UV region (193 nm).[185] As is seen in Figure 29, the substitution pattern of the polycyclic backbone is varied by means of copolymerization of substituted norbornenes in order to achieve the appropriate hydrophilicity/hydrophobicity balance needed for effective surface wetting, solubility switching, and high reactive ion etch resistance.[186] The photoinduced solubility switching employs photocatalytic cleavage of protective groups that renders the polymers soluble in aqueous bases, thus causing chemical amplication of the photoresist. The AvatrelTM polycyclics from Promerus are also used as dielectric polymers for electronic packaging.[187] Polycyclics (AprimaTM from Promerus) are designed as adhesives, encapsulants and covercoats. The chemistry of polycyclic polymers is an excellent illustration

Figure 29. Tuning molecular architectures of polynorbornene 193 nm photoresist polymer DUVCOR from Promerus via the substitution pattern of the norbornene monomer (image made available by courtesy of Promerus Company).

how polymerization catalysis enters new markets far away from those typical for commodity polyolens.

Catalytic Chain Transfer (CCT) and Atom Transfer Polymerization (ATRP)Catalytic chain transfer reactions such as b-hydride elimination in olen polyinsertion, hydrogen transfer to monomer in free radical methacrylate polymerization (CCT) and halogen atom transfer radical polymerization (ATRP) play an important role and offer new opportunities in polymer syntheses. As reported above, the ligand design of single site catalysts gave unprecedented control of molecular weights and end groups. In addition to Shells nickel ylide based higher olen process (SHOP), various other catalysts systems became available for olen oligomerization with tailor-made balance between chain propagation and chain transfer reactions. Now metallocene catalysts are at hand to prepare olen oligomers containing exclusively one olen endgroup via b-hydride elimination. These endgroups can be functionalized to prepare oligoolens with polar end groups such as hydroxy, carboxylate, thiol, amine, silane, boranes. Vinylidene-terminated oligopropenes were converted into such intermediates with polar end groups[188,189] including new macromonomers with


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polymerizable methacrylate and oxazoline[190] endgroups. Maleination of olen-terminated oligopropene gave succinic anhydride terminated oligo(propylene)s which were employed as compatibilizers in reactive blending of isotactic poly(propylene) with 30 wt.-% polyamide 6 in order to study the inuence of molecular weight and stereoregularity on blend performance. While the average particle size of the dispersed polyamide 6 was only dependent upon the compatibilizer volume fraction, increasing molecular weight and isotacticity improved interfacial adhesion, as reected by increasing yield stresses of the blend.[191] Olen oligomers and their derivatives are attractive additives used as processing aids and dispersing agents in polyolen compounding. Transition metal complexes having an unpaired electron, preferably chelates of cobalt(II) such as tetraphenylporphyrin and dioximate complexes of cobalt(II), interact with the polymer radicals in free radical polymerization. They catalyze the transfer of hydrogen to the monomer via the intermediate Co(III)-H and formation of macromonomers with polymerizable unsaturated end groups. A simplied scheme for the chain transfer catalysis (CCT) of free radical methyl methacrylate (MMA) and the formation of methacrylate-terminated oligoMMA macromonomers is presented in Figure 30. It was the groups of Smirnov and Marchenko who discovered in 1975 that catalysts control very effectively the molecular weights obtained in MMA polymerization by means of catalytic chain transfer to the monomer without sacricing high yields of free radical polymerization.[192] Methacrylate functional MMA dimers can be produced in high yields and used in radical addition fragmentation (RAFT) polymerizations. MMA is the most

commonly used monomer for CCT but also other methacrylates, acrylates, acrylontrile, methacrylonitrile, styrene, a-methyl styrene, dienes were applied in CCT homoand copolymerization in bulk, solution and emulsion. CCT is a very versatile method to control molecular weight and end group formation in free radical polymerization without requiring the addition of large excess of initiators or chain transfer agents required in conventional free radical polymerization and causing toxicity and high content of volatile organic compound (VOC). Therefore, CCT is very attractive for coating applications requiring low VOC. In addition to macromonomers CCT can produce a large variety of tailor-made block and graft copolymers as well as hyperbranched polymers. The CCT technology was developed and introduced in commercial scale by Du Pont. The exciting CCT history on occasion of its 25th anniversary[193] and the comprehensive state of the art, especially with respect to the complex reaction mechanisms, possible side reactions and the contributions of many research groups, was reviewed by Gridnev and Ittel.[194] Catalysis plays an important role in the development of the living radical polymerization, which is also known as controlled radical polymerization. For more than one hundred years the well-known irreversible chain termination via bimolecular recombination and disproportionation of two growing polymer chains has prevented living polymerization and formation of block copolymers. In controlled radical polymerization the bimolecular termination is suppressed and the concentration of the free radicals is reduced by means of reversible chain termination, which also has been referred to as inifer (initiation/transfer) process prior to the creation of the technical term controlled

Figure 30. Chain transfer catalysis (CCT) with methyl methacrylate produces methacrylate-terminated oligo(methyl methacrylate) as macromonomer useful in coating applications.

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Figure 31. Controlled radical polymerization involves a rapid equilibrium between active and dormant endgroups.

radical polymerization. As shown in Figure 31, the growing radical is trapped by forming covalent bonds which then are homolytically cleaved to form radicals and reinitiate free radical polymerization. A wide variety of such chain transfer agents were introduced. The basic principles of the living radical polymerization were reviewed by Sawamoto and Kamigaito.[195] A comprehensive review on the nitroxide mediated living polymerization, also being referred to as the TEMPO process (TEMPO stands for the preferred reagent 2,2,6,6-tetramethylpiperidine-N-oxyl radical), was presented by Hawker.[196] The development of catalytic living radical polymerization builds upon the well-known metal-catalyzed radical addition reactions, known in the world of small molecules as the Kharasch or atom-transfer radical addition. The controlled radical polymerization displayed in Figure 31 proceeds in a similar fashion involving a rapid equilibrium between polymer endgroups being composed of either inactive carbon halogen molecules or the active polymer radicals. In this atom transfer radical polymerization (ATRP) process halogen atom and electron transfer are also the important features.

2003

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