Homogeneous Metallocene-Mediated Propylene Polymerization: Catalyst Symmetry, Ion-Pairing, and Counteranion Effects on Polymerization Kinetics, Selectivity, and SpecificityPublic Deposited
Counteranion effects on the rate and stereochemistry of propylene polymerization mediated by ion-pair complexes derived from dimethylzirconocene precatalysts activated with strongly Lewis-acidic perfluoroarylmetalloid cocatalyst/activators are shown to arise from the strength of the cation-anion interaction. This is quantified using CS-symmetric Me2C(Cp)(Flu)ZrMe2 (Cp = C5H4, &#951;5 cyclopentadienyl; Flu = C13H8, &#951;5 fluorenyl) and C1-symmetric Me2Si(OHF)(CpR*)ZrMe2 (OHF = C13H16, &#951;5 octahydrofluorenyl; CpR* = &#951;5 3-(-)-menthylcyclopentadienyl, R* = (1R,2S,5R)-trans-5-methyl-cis-2-(2-propyl)cyclohexyl; (( ) menthyl) precatalysts activated using a broad family of mononuclear and polynuclear perfluoroarylborate, aluminate, and gallate cocatalysts/activators, including B(C6F5)3, B(o C6F5C6F4)3, and Al(C6F5)3, trityl salts Ph3C+ B(C6F5)4 and Ph3C+ FAl(o C6F5C6F4)3 , in-situ generated Ga(C6F5)3, and new mono- and polymetallic trityl perfluoroarylhalometallate salts Ph3C+ FB(C6F5)3 , Ph3C+ FB(o C6F5C6F4)3 , (Ph3C+)x Fx[Al(C6F5)3]yx (x = y = 1; x = 1, y = 2; x = 2, y = 3), Ph3C+ (C6F5)3AlFAl(o C6F5C6F4)3 , Ph3C+ XAl(C6F5)3 (X = Cl; X = Br), and Ph3C+ F[Ga(C6F5)3]2 . Catalyst system cation-anion interaction strength is of central importance in determining monomer insertion rates as well as competing misinsertion, reorganization, and chain release reactions, thus determining polymer physical characteristics such as overall tacticity and molar mass. This is demonstrated by fixing the metallocene precatalyst and modulating the perfluoroarylmetalloid cocatalyst, giving a series of ion-pair species showing a wide range of cation-anion binding characteristics. Systematic cocatalyst/counteranion dependences of the individual rates of different monomer insertion, misinsertion, catalyst ion-pair reorganization, and chain release processes are first quantified in syndiospecific propylene polymerizations catalyzed by systems using CS-symmetric precatalyst Me2C(Cp)(Flu)ZrMe2. Generality is then shown to extend to isospecific polymerizations catalyzed by systems based on C1-symmetric analog Me2Si(OHF)(CpR*)ZrMe2. Structural and dynamic features of the present catalyst complexes are determined by single-crystal X-ray diffractometry and one- and multidimensional Nuclear Magnetic Resonance spectroscopy. Polymerization behavior is gauged by examination of the product polymer materials, including polymer end-group morphologies, polymer molar mass distributions, melt behavior, and, most importantly, polymer production rates and the distribution of stereosequences in the polymer backbone. Modeling of these stereosequence distributions constitutes the primary tool for determination of absolute rates for a collection of reaction pathways proposed to be available during polymerization, and establishes the relationships between these rates and strength of ion-pairing in the catalyst systems.