Stereochemistry of First Monomer Insertion into Metal−Methyl Bond:  A Tool for Evaluating Ligand−Monomer Interactions in Propene Polymerization with Metallocene Catalysts

Abstract

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

Kinetic resolution of racemic α-olefins with ansa -zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

“…It was found to be essential that all Me 3 Al was removed from the MAO. MAO batches that were not dried sufficiently led to depletion in selectivity, presumably because of an increased number of less selective insertions into metal-hydrides and metal-methyls (45)(46)(47). 3,4-dimethyl-1-pentene, 3,4,4-trimethyl-1-pentene (48), and (S)-3 (27) were synthesized as reported.…”

Stereoregular Polymers

“…The two coordination sites of the complexes have same chirality, and the coordination of propylene to the sites can take place in a stereoselective manner ( Fig. 9) (22,23). When propylene undergoes coordination to the metal center via the same prochiral face, the polymerization proceeds in an isospecific manner.…”

Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes

“…This mechanism is supported by experimental work. Propylene inserts into a Zr-CH 3 bond with no stereoselectivity at all (re/si=1:1), while insertion of butene occurs with 1:2 selectivity [58]. On the other hand, the selectivity in propylene insertion into Zr-CH 2 CH 3 is excellent [59].…”