Siloxy-Substituted Group IV Metallocene Catalysts - ACS Publications

Chapter 3

Siloxy-Substituted Group IV Metallocene Catalysts Reko Leino and Hendrik J. G . Luttikhedde

Downloaded by MONASH UNIV on July 26, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0749.ch003

Laboratory of Polymer Technology, Åbp Akademi University, Biskopsgatan 8, FIN-20500, Åbo, Finland

The synthesis, characterization and olefin polymerization behavior of siloxy functionalized group IV bis(indenyl)-type metallocene catalysts is reviewed. Ethylene bridged 2- and 1-siloxy-substituted bis(indenyl) ansa-metallocenes are commonly obtained in moderate yields and high excess of the desired racemic diastereomer. In combination with methylaluminoxane (MAO) or other activators, the 2-substituted complexes form highly active catalyst systems for isospecific polymerization of propylene, homopolymerization of ethylene and copolymerization of ethylene with higher α-olefins. The 1-siloxy substituted catalysts are aspecific and show low activities in polymerization of propylene but show high activities in homo- and copolymerization of ethylene. The active catalysts can be generated with unusually low [A1]:[M] ratios ( M = Zr, Hf), commonly ranging from (100-250):1 as compared with the conventional metallocene/MAO catalyst systems. The observations are attributed to the stabilizing and electron-donating effect from the sterically bulky Lewis-basic siloxysubstituent.

Metallocene catalysts for the polymerization of α-olefins are the focus of intense current interest. Fourteen-electron cations [L MR] (L = cyclopentadienyl, indenyl, fluorenyl or related ligands; M = Ti, Zr, Hf; R = alkyl) have been identified as the active species, whereas activity and stereoregularity are determined by the steric and electronic properties of the ancillary ligand framework and the ion-ion interactions between this highly electrophilic metallocene cation and its counterion. Functionalization of metallocene catalysts with Lewis basic substituents would be desirable in order to tune the electronic properties of the catalyst precursors and to stabilize the cationic active sites essential for α-olefin coordination and chain propagation reaction. The chemistry of bis(cyclopentadienyl) complexes with pendant donor functionalized side chains has been reviewed recently. Several disiloxane, 1

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© 2000 American Chemical Society

In Olefin Polymerization; Arjunan, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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phosphine, and pyridine bridged group IV metallocenes have also been reported. The presence of N-Zr interactions in the latter complexes appears to favor the formation of zirconocene cations. Halogen and methoxy substitution in the six-membered rings of the indenyl ligands decrease the catalytic activity and the polymer molecular weight. These observations have been attributed to coordination of the donor groups to the methylaluminoxane (MAO) cocatalyst, resulting in inductive electron withdrawal. Similar effects have been reported for halogen and alkoxy substituted (fluorenyl)(cyclopentadienyl) α/wa-zirconocene/MAO catalysts, and for alkoxy and methylthio substituted monoindenyltitanium trichloride/ΜΑΟ catalyst systems. Amino substituted bis(cyclopentadienyl) and bis(indenyl) complexes have been prepared in this and other laboratories. The bis(2-dimethylaminoindenyl) ansazirconocene/MAO catalyst systems show modest activities in polymerization of ethylene and propylene, despite of the unusually long induction periods. Other recent examples include phosphorus bridged and propylthio substituted bis(cyclopentadienyl) and bis(indenyl) complexes. 70

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We have recently reported the preparation of several siloxy substituted bis(indenyl) and bis(tetrahydroindenyl) metallocene complexes and their application in polymerization of α-olefins. ' E.g., rac-[ethylenebis(2-(^-butyldimethylsiloxy)indenyl)]zirconium dichloride (1) and its 1-substituted analogue (2) are active catalyst precursors for homo- and copolymerization of ethylene. Complex 1, in combination with M A O or other activators, is also highly active in isospecific polymerization of propylene. This paper reviews some of our earlier work on these complexes and summarizes the current status of the ongoing research. 16 17

Synthesis and Characterization of the Catalyst Precursors Metallocene Synthesis. Generalized synthesis of the siloxy substituted metallocene complexes is presented in Scheme 1. Reaction of 2- or 1-indanone with ter/-butyldimethylchlorosilane, thexyldimethylchlorosilane (thexyl = 1,1,2-trimethylpropyl) or triisopropylchlorosilane gives the corresponding 2- and 3-(trialkylsiloxy)indenes as distillable oils in 70-90% yield. Subsequent deprotonation with BuLi and the reaction of the lithium salt with 0.5 eq of dibromoethane gives the ethylene bridged ligand precursors as analytically pure crystalline solids in 45-70% yield, with the exception of



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Scheme 1


34 bis(l-(triisopropylsiloxy)indenyl)ethane which is obtained as a fairly pure oil in nearly quantitative yield. Double deprotonation of the bridged ligands with BuLi and the subsequent reaction with ZrCl gives the corresponding α/wa-metallocene complexes in moderate 20-35% yields. The 2-siloxy substituted complexes are formed in a high excess of the racemic diastereomer, e.g., for complex 1 a 14:1 raclmeso ratio is observed, whereas for the bulkier 2-triisopropylsiloxy substituted analogue no meso form was detected by *H N M R analysis of the crude product. The pure racemic complexes are easily isolated by crystallization from an appropriate solvent. The 1siloxy substituted complexes are formed as 5:1 mixtures of the rac and meso diastereomers. The racemic complex 2 has a better solubility in common organic solvents compared with its meso diastereomer and the pure rac form is obtained only after exhaustive recrystallizations. In the case of the 1-triisopropylsiloxy substituted analogue the meso diastereomer is more soluble and can be extracted from the crude mixture. The pure racemic complex can be isolated after crystallization. The corresponding 2- and 1-siloxy substituted bis(tetrahydroindenyl) complexes are obtained by catalytic hydrogénation of the parent metallocenes. A number of unbridged 1- and 2-siloxy substituted bis(indenyl) or bis(tetrahydroindenyl) complexes have also been prepared. Separation of the rac and meso diastereomers of bis(indenyl) α^α-metallocene complexes is non-trivial since only the chiral C2 symmetric racemic catalyst precursor produces isotactic polypropylene. In many cases, separation from the undesired achiral meso isomer can not be achieved. The bulky siloxy substituent in the a-position to the ethylene bridge directs the metallation step in favor of the desired racemic diastereomer, presumably by steric interactions (e.g., complex 1). In the β-position the effect is weaker but significant. The dimethylsilylene bridged analogue of 1 is, on the other hand, formed in an approximately 2:1 ratio of the rac and meso diastereomers and only small amounts of the fairly pure racemic complex have been obtained by fractional crystallization techniques.


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Characterization. The siloxy substituted bis(indenyl) ν)(Α) (;av)(A) (av)(A) (deg) (deg) (deg) (av) (deg) c 126.1 2.210 97.1 57.6 2.412 1.383 125.2 backward rac-Et(2-(/BuMe SiO)IndH4)2HfCl 125.1 2.231 1.376 98.2 125.2 58.6 16b 2.439 backward rac-Et(2-(/BuMe SiO)IndH4) ZrCl 94.3 125.1 57.8 2.229 1.361 130.2 16c 2.432 backward rac-Et(2-(iPr SiO)IndH4) ZrCl d d c 125.5 97.1 57.9 2.232 2.439 m^o-Et(2-(iBuMe SiO)IndH4) ZrCl staggered 125.9 2.254 1.363 99.3 61.0 126.0 16a 2.412 backward rac-Et(2-(ffiuMe SiO)Ind) ZrCl 120.3 129.0 16f 2.243 1.356 97.9 58.6 2.433 backward rac-Et(2-(thexMe SiO)Ind) ZrCl 125.5 2.414 94.3 57.7 2.237 1.366 129.2 16c rac-Et(2-(/Pr SiO)Ind) ZrCl forward 126.4 63.6 126.5 2.238 2.415 1.361 98.7 16e mc-Et(l-(/BuMe SiO)IndH4) ZrCl forward 126.4 64.6 1.341 96.2 133.0 16e 2.252 2.431 meso-Et( 1 -(iBuMe SiO)Ind) ZrCl staggered 2.253 96.9 130.3 51.5 127.2 16d 2.414 1.358 (2-(iBuMe SiO)-4,7-Me Ind) ZrCl gauche 2.426 1.364 97.1 127.3 54.9 125.9 2.242 16d syn (2-(iBuMe SiO)-4,7-Me Ind) ZrCl M = Zr, Hf; Cen refers to the centroids of the C rings; (av) = average; Cp-Cp = angle between the cyclopentadienyl planes. Conformation of the ligand backbone or the indenyl ligands. Unpublished. Disordered.


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38 ethylene and dimethylsilylene bridged catalysts exhibit similar polymerization activities compared with their unsubstituted congeners and produce polypropylenes with higher melting points but lower molecular weights. The higher melting points are consistent with higher strereoregularities. The hydrogenated analogue of complex 1 shows a very low activity but produces at T = 20 °C isotactic polypropylene with higher molecular weight than the high-performance rac-Me Si(2-MeBenz[e]Ind) ZrCl /MAO catalyst. Its steroselectivity and the molecular weight of the produced polypropylene decline, however, rapidly with increasing polymerization temperature. For 1/MAO the polypropylene isotacticity declines from [mmmm] = 94-95% at T = 20 °C to the level oï[mmmm] = 70% at T = 80 °C p

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Table V. Comparison of Different Racemic ansa-Zirconocene/MAO Catalyst Systems in Polymerization of Propylene Under Similar Conditions ref MyJM Metallocene M A T (kgPP/mol (°Q Zr/h) 150 16b 2.1 98 500 rac-Me Si(2-MeBenz[e]Ind) ZrCl 7 200 149 1.9 24 200 16f 5 500 rac-Me Si(2-(/BuMe SiO)Ind) ZrCl 131 16b 27 900 2.2 rac-Et(Ind) ZrCl 5 400 148 2.4 16a 19 100 5 300 rac-Et(2-(/BuMe SiO)Ind) ZrCl (1) 142 16b 1.8 54 200 rac-Me Si(Ind) ZrCl 4 400 16f 146 16 100 2.1 2 700 rac-Et(2-(thexMe SiO)Ind) ZrCl 145 16b 2.1 54 800 rac-C4H Si(Indrl4) ZrCl 2 400 146 2.0 16a 53 200 mc-Me Si(IndH4) ZrCl 2 300 2.0 138 16b 33 000 800 mc-Et(IndH4) ZrCl 16b 150 2.1 123 200 mc-Et(2-(rBuMe SiO)IndH4) ZrCl 20 16e n.d. n.d. n. d. 20 rae-Et(l-(*BuMe SiO)Ind) ZrCl (2) % = 20 °C; P(C H6) = 2.0 bar; [Al]:[Zr] = 3000:1; polymerization time = 60 min; [metallocene] = 11 μπιοΙ/200 mL of toluene. '"Not determined. 3

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The fairly low polypropylene molecular weights obtained with the siloxy substituted bis(indenyl)

Siloxy-Substituted Group IV Metallocene Catalysts - ACS Publications

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