Discovery of Methylaluminoxane as Cocatalyst for Olefin

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Discovery of Methylaluminoxane as Cocatalyst for Olefin Polymerization Walter Kaminsky Institute for Technical, Macromolecular Chemistry, University of Hamburg, Bundesstr. 45, D-20146 Hamburg, Germany ABSTRACT: The discovery of methylaluminoxane (MAO) was the start for investigations and innovations of new classes of highly active olefin polymerization catalysts. Different transition metal complexes together with MAO as cocatalyst allow the synthesis of polymers with a highly defined microstructure, tacticity, and stereoregularity as well as new cycloolefin, long chain branched, or blocky copolymers with excellent properties. These new polyolefins could not be obtaind with such a purity before by Ziegler−Natta catalysts. The single site catalyst character of metallocene and other transition metal complexes activated by MAO leads to a better understanding of the mechanism of the olefin polymerization.

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INTRODUCTION There are few inventions that have pushed the polyolefin industry in such a way that they produce in 2010 with about 145 million tons more than 50% of all polymers. The first invention was the high-pressure polymerization by ICI,1 followed by the catalytic olefin polymerization by Ziegler,2 Natta,3 and Phillips.4 The supporting of Ziegler−Natta catalysts on magnesium dichloride and deactivation of the atactic working polymerization active sites by internal and external donors were the start for a fast growing polypropylene production.5 The latest invention, carried out at the Chemical Department of the University of Hamburg, is the use of methylaluminoxane (MAO) containing catalysts for the tailored synthesis of polyolefins.6,7 Catalysts formed by MAO and metallocenes or other transition metal containing complexes have widened up the possibility to design the microstructure of polyolefins in a way which was not possible in years before. These “single site” catalysts show only one kind of active sites and are used today mainly for the rapidly increasing production of linear low density polyethylene (LLDPE) and some special copolymers.8 Before the invention of methylaluminoxane as cocatalyst, homogeneous Ziegler−Natta catalysts have been preferentially studied in order to understand the elementary steps of the polymerization, which is simpler in soluble systems than in heterogeneous systems. Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used in Ziegler systems, are indeed capable of polymerizing ethylene, but at a very low activity. Only with the discovery and application of methylaluminoxane (MAO) was it possible to enhance the activity, surprisingly, by a factor of 10 0009 or more. Therefore, MAO plays a crucial © 2012 American Chemical Society

part in the olefin polymerization by metallocenes and other complexes and has motivated research groups to thousands of publications in the past 20 years.10 Impressive is the industrial use of MAO-based catalysts, too. In 2010, over 5 million tons of polyolefins, especially different kinds of polyethylene, are produced in commercial processes using MAO as cocatalyst. Nearly every second new plant for EP or EPDM elastomers based on MAO activated catalysts. For a use in new or excisting plants (drop-in technology) MAO is heterogenized by absorption on silica, aluminia, or other supports.11 By this procedure, gas phase polymerization is possible, too. Therefore, the amount of industrial used MAO increased rapidly, mainly produced by Albermale, Akzo, Chemtura, and Tosoh. Kinetic studies and the application of various methods have helped to define the nature of the active centers, to explain the aging effects of Ziegler catalysts, to establish the mechanism of interaction with olefins, and to obtain quantitative evidence of some elementary steps.12−15 It is necessary to differentiate between the soluble catalyst system itself and the polymerization system. The homogeneous systems are soluble, but they become heterogeneous when polyethylene is formed up to polymerization temperatures of 50 °C. By higher temperatures (Dow solution process) they stay homogeneous. Before 1980, the most investigated homogeneous catalyst systems are based on bis(cyclopentadienyl)titanium(IV), bis(cyclopentadienyl)zirconium(IV), tetrabenzyltitanium, vanadium chloride, with trialkylaluminum, or alkylaluminum halides. Received: November 7, 2011 Revised: February 13, 2012 Published: March 8, 2012 3289

dx.doi.org/10.1021/ma202453u | Macromolecules 2012, 45, 3289−3297

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Breslow16 discovered that bis(cyclopentadienyl)titanium(IV) compounds, which are readily soluble in aromatic hydrocarbons, could be used instead of titanium tetrachloride as the transition metal compound together with aluminum alkyls for ethylene polymerization. Subsequent research on this and other systems with various alkyl groups has been conducted by Natta,17 Patat and Sinn,18 Shilov,19 Henrici-Olivé and Olivé,20 Reichert and Schoetter,21 and Fink et al.22 With respect to the kinetics of polymerization and side reactions, this soluble system is probably the one that is best understood. It is found that the polymerization takes place primarily into a titanium carbon bond were the titanium exists as titanium(IV) alkyl cation23 formed by alkylation and dissociation (eqs 1 and 2). (C5H5)2 TiCl2 + AlR2Cl → (C5H5)2 TiRCl + AlRCl2 (1)

(C5H5)2 TiRCl + AlRCl2 → [(C5H5)2 TiR]+ + [AlRCl3]−

Figure 1. Used glass equipment to fill and seal a NMR tube with a solution of metallocene, trimethylaluminum, and ethylene.

(2)

The polymerization of olefins, promoted by homogeneous Ziegler catalysts based on biscyclopentadienyltitanium(IV) or analogous compounds and aluminum alkyls, is accompanied by a series of other reactions such as alkylation, hydrogen transfer, and reduction that greatly complicate the kinetic interpretation of the polymerization. In the case of β-hydrogen transfer by ethyl alkylated titanium complexes, Ti−CH2−CH2−Ti structure units are formed (eq 3), which are unstable and decompose within some seconds into titanium(III) species and ethylene (eq 4).12

torch by a glass blower, introduced into the NMR equipment at −40 °C, and slowly warmed to room temperature. This procedure was very time-consuming, and the PhD student involved with these measurements in 1975 simplified the preparation method by filling the reaction mixture directly into a simple NMR tube and putting a plastic lid on.26 Comparing the C NMR spectra obtained with the simple method with those obtained with the sealed NMR tubes, he discovered after 5 h reaction time a small peak by 8.59 ppm in the area of CH2 bonds in the NMR spectrum which he had never seen before (Figure 2). Fortunately, the student did not ignore these results using the safe procedure again but discussed the effect with me. To recognize such effects and details, a deep knowledge of the investigated subject is necessary. We decided to scale up the experiment and repeat it in a 1 L glass autoclave which was cooled down to −40 °C and subsequently warmed to −14 °C. After pressing on ethylene, the pressure decreased very slowly. The autoclave was opened to take a sample of the solution for NMR control. After the addition of new ethylene to a pressure of 8 bar, the pressure decreased much faster than in the beginning. This procedure was repeated several times, and the reactivity increased each time; for details see ref 27. What was the reason for this increasing polymerization rate of ethylene by a system known before to be polymerization inactive? We discussed the results with our supervisor Hansjörg Sinn and came to the conclusion that traces of chloride, oxygen, or water, introduced by opening the autoclave, could be responsible for the activity. In the following experiments, using titanium complexes (Cp2Ti(CH3)2) containing traces of chloride or oxygen, no polymerization of ethylene was observed. Finally, we added small amounts of water, as a last possibility since water has been discussed to be a strong poison for olefin polymerization catalysts. Surprisingly, a high polymerization rate was observed depending on the amount of water.28 The polymerization rate reached a maximum when the molar ratio of water to trimethylaluminum was 1:1. Some years before Reichert29 had found an increase of the activity of about a factor 2 by the addition of small amounts of water to the halogen-rich homogeneous system Cp2TiEtCl/EtAlCl2, and Breslow30 obtained similar results by the system Cp2TiCl2/Me2AlCl. Obviously, there are great differences in the halogen-free and halogen-containing catalysts.

2Cp2Ti(Cl)CH2−CH3 → Cp2Ti(Cl)−CH2−CH2−Ti(Cl)Cp2 + CH3−CH3 (3)

Cp2Ti(Cl)−CH2−CH2−Ti(Cl)Cp2 → 2Cp2TiCl + CH2CH2

(4)

The bridged titanium complexes as well as the reduced titanium(III) species are polymerization inactive. Analogous zirconium complexes were used to isolate intermediates and to study alkyl exchange and β-hydrogen transfer since zirconium is less easily reduced compared to titanium.24

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METHYLALUMINOXANE The research group of Hansjörg Sinn at the University of Hamburg investigate these side reactions, especially the αhydrogen transfer reaction of biscyclopentadienyltitanium dimethyl and trimethylaluminum and the formation of new CH2-bridged titanium aluminum complexes (eq 5).25This

reaction is very slow, and no reduction of titanium(IV) takes place so that NMR measurements are possible of the complexation with ethylene by NMR analysis at low temperature. To start the reaction, a toluene solution of the titanium complex and a solution of trimethylaluminum were mixed together at −78 °C by the Schlenk technique and filled into a NMR tube (Figure 1). This NMR tube was then sealed with a 3290


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Figure 2. Original NMR spectrum of the first measurements of Cp2Ti(CH3)2, Al(CH3)3, and ethylene in toluene.

It was clear that trimethyaluminum reacts rapidly with water in a toluene solution. The next step was therefore to isolate the product formed in the 1:1 mixture of water and trimethylaluminum. The general reaction is shown by eq 6. At higher water

levels, explosions could happen; therefore, inorganic salts were used containing bonded water, such as CuSO4·5H2O or Al2(SO4)3·14H2O in toluene suspension.31After 20 h, the reaction mixture was filtrated and the solvent evaporated. The white powder obtained was dried and analyzed. The compound was named methylaluminoxane (MAO). MAO was investigated by elementary analysis, cryoscopy, NMR measurements, and decomposition with HCl. It was found that MAO is a mixture of different oligomers, including some ring structures (Figure 3). Even today the exact structure is not known because there are equilibria between the oligomers and complexation of the oligomers with each other and with unreacted TMA. MAO is a compound in which aluminum and oxygen atoms are arranged alternately and free valences are saturated by methyl substituents. According to investigations by Sinn32 and Barron,33 it consists mainly of units of the basic structure [Al4O3Me6], which contains four aluminum, three oxygen atoms, and six methyl groups. As the aluminum atoms in the unit structures are coordinatively unsaturated, the units join together forming clusters and cages. These have molecular weights from 1200 to 1600 measured by cryoscopy in benzene and are soluble in hydrocarbons especially in aromatic solvents. The use of this separately produced MAO together with Cp2Ti(CH3)2 further increased the activity by a factor of 100

Figure 3. Unit cyclic (a), linear (b), and associate (c) structures of methylaluminoxane (MAO); red balls: oxygen; gray balls: aluminum and methyl groups. 3291


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Table 1. Polymerization Activities of Metallocene/Aluminoxane Catalysts in 330 mL of Toluene by 8 bar Ethylene Pressurea metallocene

aluminoxane

temp (°C)

activity (g PE/(g Zr h bar))

mol wt (g/mol)

Cp2Ti(CH3)Cl Cp2TiCl2 Cp2Zr(CH3)2 Cp2ZrCl2 Cp2ZrCl2 Cp2ZrCl2 Cp2Hf(CH3)2 Cp2ZrCl2 Cp2Zr(CH3)2

MAO MAO MAO MAO MAO MAO MAO ethylaluminoxane tertaisobutyldialuminoxane

20 20 70 20 70 90 70 60 70

50000 90000 700000 620000 1000000 3100000 60000 23000 175000

350000 n.d. 190000 730000 190000 150000 300000 n.d. 400000

Polymerization conditions: 330 mL of toluene, ethylene, pressure: 8 bar, metallocene amount: 10−6−10−8 mol, aluminoxane amount: 5 × 10−3 mol Al units. a

with a narrow molecular weight distribution of ∼2. Only traces (0.1%) of low molecular weight oligomers are formed compared to Ziegler catalysts (2−5%).38 In the following years, we continued polymerizations of ethylene using methylaluminoxane in combination with different transition metal complexes such as VOCl3 and the acethylacetonate (acac) complexes V(acac)3 VO(acac)2, Cr(acac)3, Co(acac)2, Ni(acac)2, and Fe(acac)3.39 Surprisingly all these complexes were able to polymerize ethylene. Vanadium-based catalysts were most active, but less than zirconocenes. Copolymers were carried out with ethylene, 1-butene, 1-hexene, 1,7-octadiene, and butadiene.40 There was a question which became stronger by every polymerization success with our homogeneous metallocene/ MAO catalyst. Is it possible to synthesize pure isotactic polypropylene by a homogeneous catalyst? There was no positive example in the past. The first idea was to use (cyclopentadienyl)(pentamethylcyclopentadienyl)zirconium (methyl) chloride (CpCp*Zr(CH3)Cl), a zirconocene with four different substituents, which could have a chiral structure, which was necessary by a hypothesis of Giulio Natta, that the formation of isotactic polypropylene depends on chiral titanium centers on the surface of TiCl3 crystals. We were disappointed that this complex together with MAO produced only atactic polypropylene. Discussion with colleagues from the inorganic institute indicated that the movement of the four ligands around the metal center is much faster than insertion of an olefin. Such a complex does not have one stable enantiomer and therefore yields atactic polypropylene. At that time Hans-Herbert Brintzinger (University of Konstanz, Germany) had just published a paper of a chiral ansa ethylidenebis(4,5,6,7-tetrahydroindenyl)titanium dichloride.41 I contacted him and asked for some milligrams of such an ansa metallocene, if possible with zirconium as metal, since Cp2ZrCl2 had been much more active than Cp2TiCl2. In spring 1984, my PhD student Klaus Kuelper traveled from Hamburg to Konstanz to pick up the zirconocene. Back in Hamburg, we carried out the first polymerization runs using this complex together with MAO. After some minutes isotactic polypropylene was formed. This could clearly be seen by the formation of a suspension of insoluble polymer in the autoclave while all our previous experiments had given a clear solution of atactic polypropylene. The ansa zirconocene [En(THind)2]ZrCl2 exists in three structures (Figure 4). The rotation of the indenyl rings is hindered by the CH2−CH2−bridge. Beside the racemic mixture of the R and the S form, a meso form is possible. In the case of

compared to the system of Cp2Ti(CH3)2/Al(CH3)3/H2O.31 For the first time it was possible to polymerize propylene with a soluble biscyclopentadienyltitanium/MAO catalyst to obtain atactic polypropylene and to generate ethylene−propylene copolymers. Pino and Mülhaupt,34 who analyzed a sample, found that the polypropylene synthesized with this catalyst was the purest atactic PP they had ever seen. In 1978, Hansjörg Sinn became Minister of Science and Research of the City State of Hamburg, and 1 year later I received the position of a full professor at the University of Hamburg. At this time my co-workers and I used bis(cyclopentadienyl)zirconium dimethyl and MAO for ethylene and propylene polymerization and obtained extremely high activities, higher than for the titanium system. Up to this date, biscyclopentadienylzirconium complexes, activated by aluminum alkyls, were described to be totally inactive for olefin polymerization. A patent application was written covering these exciting results.6 It was discovered, too, that the Cp2ZrCl2, which can easier be synthesized and is more stable than Cp2Zr(CH3)2, is an active catalyst precursor in combination with MAO. In 1980 and 1981, I reported for the first time on zirconocene/MAO catalysts on the IUPAC Polymer Congress in Firenze35 and the Macromolecular Meeting in Midland.36 In those years we synthesized the analogue ethylaluminoxane and a tetraisobutyldialuminoxane. The results for ethylene polymerization are compared for different metallocenes and aluminoxanes in Table 1.37 It can be seen that titanocenes are less active than zirconocene catalysts. They cannot be used at higher temperatures and for longer polymerization times because the titanium(IV) is then reduced into the inactive titanium(III). Hafnocenes are about 10 times less active than titanocenes but produce polyethylene with a higher molecular weight. Under the condition that every zirconocene complex formed a polymerization active site, shown by Peter Tait30 and Jimmy Chien,31 the most active zirconocene produces about 15 000 polymer chains per hour at a polymerization temperature of 90 °C. The insertion time of one ethylene unit is only 3 × 10−5 s. The use of ethylaluminoxane instead of MAO decreases the activity by a factor of 40. Tetraisobutylaluminoxane is more active, but still 6 times less than MAO. Compared with classical Ziegler catalyst, zirconocene catalysts are 10−100 times more active and show an activity which is maintained at nearly the same level for several days. For the first time, we could show that a soluble catalyst and highly active Cp2ZrCl2/MAO catalyst is able to produce polyethylene with molecular weights up to 1 million g/mol and 3292


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Figure 4. Structures of the R, S, and meso form of [C2H4(THInd)2]ZrCl2. Figure 5. Microstructures of polypropylenes produced by different metallocene/MAO catalysts. (The hydrogen atoms of the backbone are not shown.)

[En(THind)2]ZrCl2 only traces of the meso form are obtained, which can be eliminated by recrystallization of the complexes. The meso form has no asymmetric symmetry and produces therefore atactic polypropylene similar to the unbridged Cp2ZrCl2/MAO catalyst. We investigated the influence of temperature and catalyst concentration on molecular weight and isotacticity and wrote together with the research group of Hans Brintzinger a publication.42 At the same time John Ewen,43 working for Exxon, had carried out similar experiments with the ansa-[En(Ind)2TiCl2]/MAO catalyst and had obtained a mixture of isotactic and atactic polypropylene due to the fact that the complex had not been clean and had contained a high amount of meso-complex. These results and the separation of the racemic mixture of the ansa-zirconocene into the optically pure enantiomers confirm the hypothesis that the formation of isotactic polypropylene needs an asymmetric (chiral) active site. Optically active tri- and tetramers of propylene were obtained using the S-enantiomer (Figure 3) together with MAO by high catalyst concentration.44 The discovery of the production of isotactic polypropylene by a highly active homogeneous metallocene/MAO catalyst increased the interest of research groups working worldwide in the field of olefin polymerization. In the following years, the number of patents increased from 1 in 1980 up to more than 10 000 until 2010. Similarly high is the amount of publications. An overview can be found in refs 45−55. The new catalyst provided the opportunity to tailor not only the polymer structure but also the tacticity of polyolefin resins in combination with an extremely high activity.

(molZr h), a molecular weight of 700 000 g/mol, an isotacticity of 99%, and a melting point of 160 °C. In 1987, Ewen, Jones, and Razavi57 obtained pure syndiotactic polypropylene using a Cs-symmetric zirconocene complex with a bridged cyclopentadienyl and a fluorenyl ring. This metallocene offers two different bonding positions for the inserted propylene in an alternating structure. We discovered that unbridged biscyclopentadienyl zirconocenes with bulky substitutions such as neomentyl produces syndioblock polypropylenes with elastic properties.58 Such a bulky ligand could stabilize an asymmetric (chiral) active site for a short time. During this time, some propylene units could be inserted with the same stereospecificity forming an isotactic block, followed by a change in conformation. Table 2 summarizes the polymerization of propylene by different metallocene/MAO catalysts. Polypropylenes with molecular weights up to 700 000 g/mol and by activities of 15 000 kg/(g Zr h cP) could be obtained. Using different ligand substituents, it was now possible to obtain isotactic, isoblock, stereoblock, syndiotactic, and atactic polypropylenes in high purity. Encouraged by this success, we used metallocene/MAO catalysts for the first time for different copolymerizations, such as ethylene/propylene (EP),31 ethylene/propylene/diene (EPDM),59 ethylene/1-butene, ethylene/1-hexene (LLDPE)59 ethylene/1,3-butadiene or isoprene, and ethylene/cyclopentene or norbornene (COC).60 The comonomers in these copolymers are distributed randomly in the polymer chain. The low amount of oligomers compared to copolymers produced with Ziegler−Natta catalysts is responsible for a high tensile strength. Surprisingly, it was also possible to homopolymerize cyclic olefins such as cyclopentene and norbornene to partially crystalline materials and to copolymerize them with ethylene.43 We observed only double bond opening by the homo- and copolymerization of cyclic olefins in contrast to Ziegler−Natta catalysts, where ring and double bond opening occur simultaneously. Poly(cyclopentene), synthesized by isotactic working metallocene catalysts, are crystalline and have a low solubility and a melting temperature of 395 °C. The unusual formation of 100% of 1,3-enchainments for poly(cyclopentene) was later shown by Collins.61

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NEW POLYMERS Using zirconocene/MAO catalysts, it is possible to produce polypropylenes with different kinds of microstructures in a purity, which cannot be obtained by Ziegler−Natta catalysts (Figure 5). Stereo errors in polypropylenes produced by [En(Ind)2]HfCl2/MAO are randomly distributed along the polymer chain while in polypropylene made with Ziegler−Natta catalysts the errors are concentrated at chain ends and in oligomers. This isoblock-polypropylene is characterized by a higher film transparency because the crystal size is in the nano scale. Researchers of the Hoechst Co. optimized later the ansa zirconocene complexes by using different bridges and substituents at the indenyl rings.56 They were able to obtain isotactic polypropylene with an activity of 900 000 kg PP/ 3293


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Table 2. Propylene Polymerizationa with Metallocene/Methylaluminoxane Catalysts metalloceneb

activityc

mol wt (g/mol)

isotacticity mmmm (%)

microstructured

mp (°C)

Cp2ZrCl2 (NmCp)2ZrCl2 [En(Ind)2]ZrCl2 [En(Ind)2]HfCl2 [En(2,4,7Me3Ind)2]ZrCl2 [Me2Si(Ind)2]ZrCl2 [Ph2Si(Ind)2]ZrCl2 [Me2Si(2,4,7Me3Ind)2]ZrCl2 [Me2Si(2Me-4PhInd)2]ZrCl2 [Me2Si(2Me-4,5BenzInd)2]ZrCl2 [Ph2C(Fluo)(Cp)]ZrCl2 [Me2C(Fluo)(Cp)]ZrCl2 [Me2C(Fluo)(Cp)]HfCl2 [Me2C(Fluo)(3-t-BuCp)]ZrCl2

140 170 1690 610 750 1940 2160 3800 15000 6100 1980 1550 130 1045

2000 3000 32000 446000 418000 79000 90000 192000 650000 380000 729000 159000 750000 52000

7 23 91 85 >99 96 96 95 99 98 0.4 0.6 0.7 89

a sb i ib i i i i i i s s s ib

118 136 126 162 148 136 155 160 157 141 138 138 130

a Propylene pressure = 2 bar, temperature = 30 °C, [metallocene] = 6.25 × 10−6 M, metallocene/MAO = 250, solvent = toluene. bCp = cyclopentadienyl; Nm = neomenthyl; Ind = indenyl; En = C2H4; BenzInd = benzoindenyl; Flu = fluorenyl. cIn kg/(mol Zr or Hf h) concentration of propylene . da = atactic; i = isotactic; s = syndiotactic; sb = stereoblock; ib = isoblock.

Table 3. Copolymerizationa of Norbornene (N) and Ethylene by Different Metallocene/MAO Catalysts

For polynorbornenes it was shown by hydro-oligomerization that norbornene is inserted into the metal−carbon bonds of the growing polymer chain by 1,2-insertion in a cis−exo fashion.62 While the tactic homopolymers of cycloolefins cannot be processed because of their high melting points there is much interest in the copolymers of norbornenes or cyclopentene with ethylene or propylene, respectively (Figure 6). These copolymers (COC) are amorphous with an useful glass transition temperature between 80 and 200 °C.63

metalloceneb Cp2ZrCl2 [En(Ind)2]ZrCl2 [Me2Si(Ind)2ZrCl2 [En(IndH4)2]ZrCl2 [Me2C(Flu)(Cp)] ZrCl2 [Ph2C(Flu)(Cp)] ZrCl2 [Ph2C(Ind)(Cp)] ZrCl2

t [min]

activity (kg/(mol h))

incorp of norbornene (wt %)

30 10 15 40 10

1200 9120 2320 480 7200

21.4 26.1 28.4 28.1 28.9

10

6000

27.3

15

2950

33.3

Ethylene pressure = 2 bar, c(N) = 0.05 mol/L, temperature = 30 °C, [metallocene] = 5 × 10−6 mol/L, metallocene/MAO = 200, solvent = toluene. bCp = cyclopentadienyl, Me = methyl, Ind = indenyl, En = C2H4, Flu = fluorenyl.

a

Figure 6. Reaction of ethylene and norbornene.

Cyclopentene, norbornene, or other cyclic olefins are incorporated exclusively by 1,2-insertion into the growing copolymer chain, and no ring-opening occurs. The insertion of the huge norbornene monomer is very fast by metallocene/ MAO catalysts. Table 3 compares activities and incorporation of norbornene by different catalysts. Under special conditions the polymerization rate of a 1:1 molar mixture of ethylene is higher than the homopolymerization of ethylene (comonomer effect).63 The [Ph2C(Ind)(Cp)]ZrCl2/MAO catalyst not only shows high activities for the copolymerization of ethylene with norbornene but also gives an alternating structure, too. Most metallocenes produce polymers with a statistical structure. The melting point of the alternating copolymer depends on the molar ratio of norbornene units in the polymer while the glass transition temperature is nearly independent of this. A maximum melting point of 320 °C was reached.64,65 COC materials characteristically have an excellent transparency and low density (1.02 g/mL) and show low adsorption of water and a very high continuous service temperature. From cycloolefin insertion rates of 10 mol % upward, these cycloolefin copolymers (COC) are no longer crystalline but amorphous. They are very resistant toward solvents and chemicals; they exhibit high softening temperatures (glass temperatures of up to 200 °C) and can be processed on a thermoplastic basis.66 A further peculiarity of these materials is

their tendency to absorb little light, which makes them suitable for optoelectronic applications. Norbornene−ethylene copolymers are most interesting for technical uses because of their readily available monomers. COC polymers are already being used for the production of compact discs, lenses, optical fibers, and films.

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FUTURE TRENDS

Mechanistic studies of metallocene/methylaluminoxane catalysts have strongly increased the knowledge of the olefin polymerization catalysis. This knowledge have made it possible to find new bulky and weakly coordinating cocatalysts such as perfluorophenylborate anions and boranes. The discovery is not completed of new transition metal complexes activated by MAO or other cocatalysts with special polymerization properties. It would be another task to decrease the amount of MAO, needed for the activation. In recent years, there is an increasing interest to synthesize polyolefin nanocomposites because of their high potential as materials with novel properties.67 The properties of the nanocomposites are influenced not only by the kind of fillers but also by the microstructure of the polyolefins and the preparation process. As nanofillers are used metal oxides, 3294


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sulfides, silica, layered silica as well as fibers such as carbon nanotubes, carbon nanofibers, and polymer fibers.68−71 Most composites have been prepared by mechanical blending of the particles or fibers above the melting temperature of the desired matrix. Melt compounding of polyolefins with nanoparticles often leads to an insufficient filler dispersion, especially at a high filler content, what leads to aggregation and intercalation, which in turn deteriorates the mechanical properties. Such disadvantages can be solved by in situ polymerization with metallocene/MAO catalysts. Methylaluminoxane is soluble in hydrocarbons, and therefore it can be perfectly adsorbed or anchored on the surface of the nanofillers such as particles, fibers, carbon nanofiber (CNF), and multiwalled carbon nanotubes (MWCNT), changing the surface to a hydrophobic one.72 Excess MAO is washed out. In a second step, the metallocene is added forming catalytically active polymerization sites on the nanosurface. The thickness of the polymer films, formed by addition of ethylene or propylene, depends on the polymerization time and the pressure of the monomer. The in situ polymerization leads to composite materials, where the particles or fibers are intensively covered with the polymer. The composite materials show, for example, an improved stiffness with a negligible loss of impact strength, high gas barrier properties, significant flame retardance, better clarity, and gloss as well as high crystallization rates. Multiwalled carbon nanotubes (MWCNT) are specially attractive class of fillers for polymers because of their intriguing mechanical and thermal properties.73 High molecular weight isotactic polypropylene filled with MWCNT is an exceptionally strong composite material (Figure 7).

The main advantage of CNT or MWCNT filled PP is the change of mechanical properties. The tensile strength of a composite film increases by 20% if only 1 wt % of MWCNT is incorporated, but also the form stability and the crystallization rate from a melt increase strongly and make this composite material suitable for new applications such as in the automotive plastic industries. Metallocene and other single site catalysts allow the synthesis of tailored polyolefin structures in a way that was impossible in the years before. The known dependence of the kind of metallocene-based polymers from the catalyst structure allows the modeling of their reaction kinetics.74 Reactor models could be developed using mass and energy balances, and they describe the polymer composition as well as reactor operating conditions, required for a given polymer design. It would be possible to design polyolefins with tailored molecular weight comonomer content, long chain branching,75 and comonomer distribution independent and controlled. Late transition metal complexes, which are more stable in water, can be used for emulsion polymerization.76,77 A lot can be done to tailor the microstructure of copolymers. Today it is only possible to create statistically ordered or alternating copolymers of polyolefins.78 It is not possible to design polyolefins with a wanted sequence order of two or more monomers, which will be essential to establish selforganization of the polymer chain. To design three-dimensional crystallizing polyolefins for materials with special properties such as cages for catalysts or membranes, the controlled selforganization will be necessary. Important for this and for blends is the easier synthesis of polyolefins with polar comonomers.79 There is first success to form block copolymers by single site catalysts which shows no or only a slow chain transfer reaction like living systems.80 In a first step only propylene is polymerized forming a hard polypropylene block; then, by addition of ethylene a soft ethylene/propylene copolymer or a polyethylene block follows because ethylene is much faster inserted. Another method is described to use a chain-shuttling agent to form block copolymers.81 Two different single site catalysts and monomers are used in one reactor. Cocatalyst is a fluorinated phenylborate. One catalyst is able to homopolymerize from a mixture of ethylene and 1-octene only or mainly ethylene forming a hard polyethylene segment; the other catalyst copolymerizes ethylene and 1-octene to a soft copolymer segment. By the addition of zinc diethyl as a chain transfer agent, the growing polymer chain is shuttling between the two catalysts. A blocky copolymer results with soft and hard segments with elastomeric properties. It could become possible to use MAO activated catalysts for chain-shuttling, too. The development and commercialization of metallocene/ MAO and other single site catalysts have just started and have already expanded the product ranges of polyolefins. New designed catalysts will enlarge the polyolefin industries and the applications of polymers.

Figure 7. TEM micrograph of an iPP/MWCNT composite material containing 14 wt % of nanotubes.

As expected for in situ polymerization, the polymer grew directly on the fiber surface and covers them with a thin PP layer. Nearly every tube is coated. The diameter of the carbon nanotubes and the polypropylene covering is about 36 nm (20 nm for the MWCNT and 8 nm for the polymer coat). There is a good adhesion on the tubes. The morphology of nanocomposites prepared by in situ polymerization is, in comparison to melt compounded ones, generally characterized by a homogeneous tube distribution in the matrix and a good adhesion of the polymer on the MWCNT surface.72

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 3295


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Perspective

Biography

(13) Kaminsky, W.; Vollmer, H.-J.; Heins, E.; Sinn, H. Makromol. Chem. 1974, 175, 443. (14) Bochmann, M. Angew. Chem. 1992, 104, 1206. (15) Keii, T. Kinetics of Ziegler-Natta Polymerization; Kodansha Ltd.: Tokyo, 1972. (16) Breslow, D. S.; Newburg, N. R. J. Am. Chem. Soc. 1959, 81, 81. (17) Natta, G.; Pino, P.; Mazzanti, G.; Longi, P. Gazz. Chim. Ital. 1957, 87, 549. (18) Patat, F.; Sinn, H. Angew. Chem. 1958, 70, 496. (19) Shilov, A. E. Dokl. Akad. Nauk SSSR 1960, 132, 599. (20) Henrici-Olivé, G.; Olivé, S. J. Organomet. Chem. 1969, 16, 339. (21) Reichert, K. H.; Schoetter, E. Z. Phys. Chem. 1968, 57, 74. (22) Fink, G.; Rottler, R. Angew. Makromol. Chem. 1981, 94, 2. (23) Eisch, J. J.; Pombrick, S. I.; Zheng, G. X. Organometallics 1993, 12, 3856. (24) Kaminsky, W.; Sinn, H. Justus Liebigs Ann. Chem. 1975, 424. (25) Sinn, H.; Hinck, H.; Bandermann, F.; Grützmacher, H. F. Angew. Chem. 1968, 80, 190. (26) Mottweiler, R. Thesis, University of Hamburg, 1975. (27) Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3911. (28) Andresen, A.; Cordes, H. G.; Herwig, J.; Kaminsky, W.; Merck, A.; Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H. J. Angew. Chem., Int. Ed. Engl. 1976, 15, 630. (29) Reichert, K. H.; Meyer, K. R. Makromol. Chem. 1973, 169, 163. (30) Long, W. P.; Breslow, D. S. Justus Liebigs Ann. Chem. 1975, 463. (31) Herwig, J.; Kaminsky, W. Polym. Bull. 1983, 9, 464. (32) Sinn, H. Macromol. Symp. 1995, 97, 27. (33) Koide, Y.; Bott, S. G.; Barron, A. R. Organometallics 1996, 15, 2213. (34) Pino, P.; Muelhaupt, R. Angew. Chem. 1980, 92, 869. (35) Kaminsky, W.; Sinn, H.; Woldt, R. IUPAC International Symposium on Macromolecules, 7−12 Sept 1980, Firenze, Abstract Vol. 2, p 59. (36) Kaminsky, W. 11th Midland Macromolecular Meeting, 17−21 Aug 1981, Midland, Proceedings, Transition Metal Catalyzed Polymerizations; Quirk, R. P., Ed.; MMI Press: New York, 1983; Part A, p 225. (37) Kaminsky, W.; Sinn, H. IUPAC 28th Macromolecular Symposium, 12 July 1982, Amherst, Proceedings, p 247. (38) Kaminsky, W.; Miri, M.; Sinn, H.; Woldt, R. Makromol. Chem., Rapid Commun. 2000, 21, 1333. (39) Herwig, J. Thesis, University of Hamburg, 1979. (40) Kaminsky, W. Nachr. Chem. Tech. Lab. 1981, 29, 373. (41) Wild, F. R.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem. 1982, 232, 233. (42) Kaminsky, W.; Kuelper, K.; Brintzinger, H. H.; Wild, F. R. Angew. Chem., Int. Ed. Engl. 1985, 24, 507. (43) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355. (44) Kaminsky, W.; Ahlers, A.; Moeller-Lindenhof, N. Angew. Chem. 1989, 101, 1304. (45) Coates, G. W. Chem. Rev. 2000, 100, 1223. (46) Nomura, K.; Liu, K. J. Dalton Trans. 2011, 40, 7666. (47) Keii, T. Heterogeneous Kinetics: Theory of Ziegler-Natta-Kaminsky Polymerization; Springer Series in Chemical Physics Vol. 77; Springer: Berlin, 2004. (48) Rieger, B., Baugh, L. S., Kacker, S., Striegler, S., Eds. Late Transition Metal Polymerization Catalysis; Wiley-VCH: Weinheim, 2003. (49) Heurtefen, B.; Bouilhac, C.; Cloutet, E.; Taton, D.; Deffieux, A.; Cramail, H. Prog. Polym. Sci. 2011, 36, 89. (50) Baugh, L. S., Canich, J.A.M., Eds. Stereoselective Polymerization with Single-Site Catalysts; CRC: Boca Raton, FL, 2008. (51) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. 1995, 107, 1255; Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (52) Scheirs, J., Kaminsky, W., Eds. Metallocene-Based Polyolefins: Preparation, Properties, and Technology; Wiley: Chichester, UK, 2000; Vols. 1 and 2.

Walter Kaminsky is Emeritus Professor of Technical and Macromolecular Chemistry at the University of Hamburg. He obtained his Diploma degree and PhD (1971) in Chemistry under the supervision of Dr. Hansjörg Sinn from the University of Hamburg where he was promoted to Full Professor in 1979. His research interest includes aspects of olefin polymerization by metallocene/methylaluminoxane catalysts and feedstock recycling of plastics by a fluidized bed process. He has published more than 400 papers. Among others, Kaminsky has received the Carothers Award (1997), the Walter Ahlström Prize (1998), the Benjamin Franklin Medal in Chemistry (1999), and the Hermann Staudinger Prize (2003), and he was awarded Honorary Fellowship from RSC (1996) and Honorary Professor of the Zhejiang University (1998) and of the East China University (2008).

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REFERENCES

(1) Fawcett, E. W.; Gibson, R. U.; Perrin, M. W.; Paton, J. G.; Williams, E. G. Brit. Pat. 471,590 to ICI 1936, C.A. 1938, 32, 1362. (2) Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. 1955, 67, 541. (3) Natta, G. J. Am. Chem. Soc. 1955, 77, 1708. (4) Clark, A.; Hogan, J. P.; Banks, R. L.; Lanning, W. C. Ind. Eng. Chem. 1956, 48, 1152. (5) Kashiwa, N.; Tsutsui, T. Makromol. Chem., Rapid Commun. 1983, 4, 491. (6) Sinn, H.; Kaminsky, W.; Vollmer, H. J.; Woldt, R. DE Patent 3007725, Feb 29, 1980. (7) Sinn, H.; Kaminsky, W.; Vollmer, H. J. Angew. Chem., Int. Ed. Engl. 1980, 19, 390. (8) Chum, P. S.; Swogger, K. W. Prog. Polym. Sci. 2008, 33, 797. (9) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99. (10) Busico, V. Dalton Trans. 2009, 41, 8794. (11) Severn, J.; Robert, L. In Handbook of Transition Metal Polymerization Catalysts; Hoff, R., Mathers, R. T., Eds.; John Wiley and Sons: New York, 2010; p 157. (12) Waters, J. A.; Mortimer, G. A. J. Polym. Sci., Part A1 1972, 10, 905. Chien, J. C. W. J. Am. Chem. Soc. 1959, 81, 86. 3296


Macromolecules

Perspective

(53) Razavi, A.; Thewalt, U. Coord. Chem. Rev. 2006, 250, 155. (54) Kaminsky, W., Ed.; Olefin Polymerization. Macromol. Symp. 2005, 236, 1−258. (55) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111, 2450. (56) Spaleck, W.; Aulbach, M.; Bachmann, B.; Kueber, F.; Winter, A. Macromol. Symp. 1995, 89, 237. (57) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. P. J. Am. Chem. Soc. 1988, 110, 6255. (58) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85. (59) Kaminsky, W.; Miri, M. J. Polym. Sci. 1985, 23, 2151. (60) Kaminsky, W.; Spiehl, R. Makromol. Chem. 1989, 190, 515. (61) Collins, S.; Kelly, W. M. Macromolecules 1992, 25, 233. (62) Kaminsky, W.; Bark., A.; Arndt, M. Makromol. Chem., Macromol. Symp. 1991, 47, 83. (63) Cherdron, H.; Brekner, M.-J.; Osan, F. Angew. Makromol. Chem. 1994, 223, 121. (64) Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3911. (65) Coates, G. W. Chem. Rev. 2000, 100, 1223. (66) M. Arndt, M.; Kaminsky, W.; Schauwienold, A.-M.; Weingarten, U. Macromol. Chem. Phys. 1998, 199, 1135. (67) Dong, M.; Wang, L.; Jiang, G.; Sun, T.; Zhao, Y.; Yu, H.; Chen, T. J. Appl. Polym. Sci. 2006, 101, 1291. (68) Alexandre, M.; Martin, E.; Dubois, P.; Garcia-Marti, M.; Jerome, R. Macromol. Rapid Commun. 2000, 21, 931. (69) Xalter, R.; Halbach, T. S.; Muelhaupt, R. Macromol. Symp. 2006, 236, 145. (70) Funck, A.; Kaminsky, W. Composites, Part A: Technol. 2007, 67, 906. (71) Kaminsky, W. Macromol. Chem. Phys. 2008, 209, 459. (72) Kaminsky, W.; Funck, A.; Klinke, C. Top. Catal. 2008, 48, 84. (73) Kaminsky, W. J. Chem. Soc., Dalton Trans. 2009, 8803. (74) Busico, V.; Cipullo, R.; Corradini, P. Macromol. Chem., Rapid Commun. 1993, 117, 195. (75) Stadler, F. J.; Arikan, B.; Kaschta, J.; Kaminsky, W. Macromol. Chem. Phys. 2010, 211, 1472. (76) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (77) Mecking, S. Colloid Polym. Sci. 2007, 285, 605. (78) Kaminsky, W. Macromol. Chem. Phys. 2008, 209, 459. (79) Heinemann, J.; Muelhaupt, R.; Brinkmann, P.; Luinstra, G. A. Macromol. Chem. Phys. 1999, 200, 384. (80) Saito, J.; Mitani, M.; Matsui, S.; Kashiwa, N.; Fujita, T. Macromol. Rapid Commun. 2000, 21, 1333. (81) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714.

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