Syndiospecific Propylene Polymerization Using C₁-Symmetric ansa-Metallocene Catalysts:  Substituent and Bridge Effects

Abstract

A series of syndiospecific C1-symmetric ansa-metallocenes ([Me2X(Cp)(2-R1-3-R2-Ind)]ZrCl2, X ) C, Si; R1 ) H, Me; R2 ) Me, Et, CH2SiMe3) have been synthesized, and their catalytic behavior in the polymerization of propylene has been studied. Upon activation with MAO, these carbon-or siliconbridged cyclopentadienyl/indenyl systems afford polypropylene with various degrees of syndiotacticity ([rrrr] ) 28-66%), as a function of the substituent size, substitution pattern, and bridging moiety. The silicon-bridged systems produced PP samples of higher molecular weight, lower syndiotacticities, and comparable productivities with respect to their carbon-bridged analogues. Statistical modeling of the experimental pentad distributions suggests that site epimerization is the major cause of stereoerrors within both sets, while a decreased site stereospecificity can be implied for the silicon-bridged systems.

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

“…Finally, to close this section, other examples of C 1 symmetric syndiospecific metallocene catalysts system that were reported by Waymouth et al [186] are presented here briefly. Systematic investigations of the stereoselectivity of metallocene catalysts by these authors revealed that ansa-(3-R-indenyl)(Cp) zirconocene dichloride complexes whose representative example is depicted in Fig.…”

“…28 Generic structure of syndiotactic-specific C 1 symmetric metallocene reported by Waymouth [186] 96…”

“…On the basis of the propylene concentration dependency of the stereochemistry and an investigation of statistical modeling they implicate site epimerization or "back-skip" as the predominant contributor to the low stereospecificity of these metallocene catalysts. They claim that the indenyl's 2-substituent (the bridge-head position being position 1) in these catalyst systems helps to decrease their tendency to undergo site epimerization [186].…”

Stereochemistry of Polymerization

“…8-5c-1). C 1 metallocenes show a wider range of behaviors-from hemiisotactic to moderate syndiotactic to moderate isotactic to highly isotactic polymer-which offers the possibility of obtaining a range of polymer products from elastomeric amorphous or low crystallinity polypropene to crystalline thermoplastics Busico et al, 1997;Coates, 2000;Gomez and Waymouth, 2002;Kukral and Rieger, 2002;Kukral et al, 2000;Leino et al, 2001;Miller and Bercaw, 2002;Resconi et al, 2000;Rieger et al, 2002]. C 1 metallocenes are often referred to as dual-side metallocenes because of the range of stereoselectivities of the two coordination sites.…”

Novel monocyclopentadienyl zirconium and hafnium trialkoxide complexes: Syntheses and catalytic properties for olefin polymerization

“…[1][2][3] Utilization of various structures of Group 4 metal catalysts to provide effective polymer architectural control (e.g., molecular weight, 4,5 molecular weight distribution, 6,7 regiochemistry, [8][9][10] stereochemistry, [11][12][13][14] branching, [15][16][17] and chain end functionalization [18][19][20][21][22][23] ) has led to significant advancement in polymer chemistry. Studies have demonstrated that employing different structures of Group 4 metal catalysts in olefin polymerizations drastically affects the chain reaction pathways; 24,25 thus, specific polymer architectures can be obtained by the proper selection of Group 4 metal catalysts.…”