ansa-Metallocene derivatives XVII *. Racemic and meso diastereomers of group IV metallocene derivatives with symmetrically substituted, dimethylsilanediyl-bridged ligand frameworks. Crystal structure of R ,S-Me 2 Si(3-t.Bu-5-MeC s H 2) 2ZrCI2
Bis(2-N,N-dimethylamino-indenyJ) zirconium dichloride, (2-(CH 3)2 N-C 9 H 6)2 ZrC1 2 • and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride. (CH3)2Si(2-(CH3)2N-C9Hs)2ZrCI2' were prepared by reaction of the corresponding ligand lithium salts with ZrCl" in toluene. Diffractometric structure determinations reveal C 2-symmetrlc complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3-0.4 V morr negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution. (CH3)2Si(2-(CH3)2N-C9Hs)2ZrCI2 catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Mel Si-bridged bis(indenyJ)ZrCl 2 complexes. but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and AI-Me species.
Propene has been polymerized with three different metallocene complexes: Me2Si(Ind)2ZrCl2 (I), Me2Si(BenzInd)2ZrCl2 (II), and Me2Si(MeBenzInd)2ZrCl2 (III) by using 13 C-enriched cocatalyst MAO/ Al( 13 CH3)3. The stereochemical analysis of selectively 13 C-enriched methyl chain end groups has shown that, while a moderate first step enantioselectivity is present with catalyst I, the bulky benzannelated substituents (catalysts II and III) lead to an almost total lack of steric control on the first monomer insertion. The methyl substitution in the 2-position of cyclopentadienyl ring (catalyst III) does not seem to influence the choice of the face.Since the discovery of the first soluble zirconocene/ methylaluminoxane catalyst for the isotactic polymerization of propene, 1 much effort has been devoted to the question of how the mode and degree of stereospecificity could be explained. 2 The purpose of this paper is to report our contribution to understanding the mechanism of stereospecificity by focusing on the relationship between the position and the bulkiness of Cp-substituents and the steric control of monomer insertion. In the last few years the investigation, by 13 C NMR analysis, of the stereochemical structure of selectively 13 C-enriched chain end groups has been shown to be extremely informative on the contribution of the various ligands to steric control and also on the mutual interactions between the ligands, in heterogeneous ZieglerNatta catalysis. 3 This method has been also successfully applied to the study of homogeneous metallocenebased systems. 4 In the present paper a stereochemical study of the first monomer insertion into the 13 C-enriched Zr-13 CH 3 bond with three different metallocene complexes, Me 2 -Si(Ind) 2 ZrCl 2 , Me 2 Si(BenzInd) 2 ZrCl 2 , and Me 2 Si(MeBenzInd) 2 ZrCl 2 (Chart 1), is presented. Such an analysis is made possible by introducing 13 Cenriched CH 3 groups into the metallocene complex/MAO catalyst by alkyl exchange with Al( 13 CH 3 ) 3 . As has been extensively shown in previous papers, 3 the two possible resulting chain end groups are distinguishable and evaluable by 13 C NMR (Chart 2). 5 Erythro (or isotactic) is the stereoisomer in which the first two monomeric units have the same configuration and threo (or syndiotactic) that in which the first two monomeric units have the opposite configuration. The signals of the enriched methyl carbons of the erythro and threo stereoisomers are at 20.4 4 and 21.7 5 ppm, respectively, downfield from hexamethyldisiloxane (HMDS). If e and t are the integrated peak areas of the enriched methyl resonances assigned respectively to the erythro and to the threo placements of the first propene unit, the molar fraction [e] of the erythro X
The stereochemistry of first monomer insertion in the
polymerization of racemic 3-methyl-1-pentene ((RS)-3MP1) with three different metallocene
complexes,
Me2Si(Ind)2ZrCl2
(I), Me2Si(BenzInd)2ZrCl2 (II), and
Me2Si(MeBenzInd)2ZrCl2
(III), in the presence of
MAO/Al(13CH3)3
cocatalyst
has been studied and the results have been compared to those previously
obtained using the traditional
isospecific
TiCl3/Al(13CH3)3
heterogeneous catalyst. The highest enantioselectivity and
diastereoselectivity
are observed with the least hindered metallocene (catalyst
I). While enantioselectivity decreases due
to
benzannelation of the ligand with catalyst II, the methyl
substitution in the 2-position of the
cyclopentadienyl rings with catalyst III causes an almost
complete lack of diastereoselectivity.
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