We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes: Cp 2 ZrMe 2 (Cp), rac-SiMe 2 -bis(1-(2-Me-(4-PhInd))-ZrMe 2 (4-PhInd), and rac-SiMe 2 -bis(1-(2-Me-(4,5-BenzInd))ZrMe 2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically active ion pair, anion displacement, chain propagation, and chain termination steps, are reported for ethene homopolymerization, alongside with investigation of ethene−propene copolymerization reactions. Initially, all catalysts form inner-sphere ion pairs ([L 2 ZrMe] + −[B(C 6 F 5 ) 4 ] − ) with a direct Zr−F interaction, which is weak enough to be displaced by the incoming monomer. In comparison to Cp, the bulky and electron-rich 4-PhInd and 4,5-BenzInd show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to Cp. The borate counterion is shown to have a minor influence after the catalyst activation step. ■ INTRODUCTION Single-site α-olefin polymerization catalysis is an industrially important application of Group 4 organometallic complexes, 1 particularly of metallocenes. The metallocene complexes need an activator to form a catalytically active [L 2 MMe] + [A] − ion pair. 2 The catalytic properties of the resulting ion pair are highly dependent on both the ligand framework of the metallocene cation [L 2 MMe] + and the structure of the counterion [A] − . 2−4 Typical activators used in the process i n c l u d e m e t h y l a l u m i n o x a n e ( M A O ) , 5 , 6 t r i s -(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ), 7 and organoborates such as [CPh 3 ] + [B(C 6 F 5 ) 4 ] − , the last giving rise to the weakly coordinating tetrakis(perfluoroaryl)borate counterion [B-(C 6 F 5 ) 4 ] − . 7,8Regarding catalyst properties, the stability of the ion pair plays a key role. 9,10 Experimental data suggest that weak coordination of the counterion is usually beneficial for catalytic activity. 11 On the other hand, the counterion needs to stay close to the metallocene cation in order to compensate for its positive charge. 12 Optimally, the counterion provides the needed stabilization for the electron-deficient metallocene cation but is easily displaced by the incoming monomer.The overall mechanisms of catalyst activation are not precisely understood. In the case of MAO, two mechanisms have been proposed: (1) abstraction of a leaving group from the precatalyst by a Lewis acidic site of MAO 2 and (2) abstraction of an AlMe 2 + end group from the MAO by the precatalyst followed by dissociation of AlMe 3 by the incoming monomer. 13,14 Recent computational studies have suggested mechanism 2 to dominate by thermodynamic considerations. 15 In any case, all computational studies dealing with the MAO activator suffer from its elusive structure, thereby requiring the use of model systems. In that respect boron activators, formin...

Herein, we introduce an approach for the computational screening of stoichiometric reactions between trimethylaluminum (TMA) and water. The thermodynamic products of these reactions are methylaluminoxanes (MAOs) with different compositions, which have the general formula (AlOMe)n(AlMe3 )m, in which n describes the degree of oligomerization and m is the number of associated TMA molecules. These reaction products were thoroughly explored up to n=4, thus demonstrating the thermodynamically preferable association of up to four AlMe3 molecules, that is, TMA molecules in their monomeric form. The relative Lewis acidities of the Al sites in these MAOs were systematically explored and we found that the associated TMA molecules were a key ingredient for co-catalytic activity in olefin-polymerization catalysis. This conclusion was supported by computational studies on catalyst activation, which revealed an exergonic insertion of ethene into the metallocene/MAO complex.

A comparative quantum chemical study has been performed to shed light on the fundamental differences between hafnocenes and zirconocenes concerning reactions between the metals and R-olefin monomers, namely, ethene, propene, 1-butene, and 1-hexene. Analogous species along the R-olefin polymerization pathways were studied for bis(cyclopentadienyl) zirconocene and hafnocene, taking into account the structural variations of the first two monomer insertion steps and of the competitive chain-termination reactions. The results were analyzed as a function of both the metal and the monomer, the metal showing more distinct differences. The most notable difference in the reactions of the zirconocene and hafnocene can be seen in β-hydrogen transfer to metal, activation energies for which are significantly higher for the hafnocene.

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The effect of the metal on elementary chain propagation and termination reactions in hafnocene- and zirconocene-catalyzed olefin copolymerization processes has been systematically studied by quantum chemical methods. Two consecutive monomer insertions, parallel with the competing chain termination reactions, were studied for copolymerization of ethene with propene, 1-butene, and 1-hexene. For the purpose of a comparative study, analogous species along the reaction pathway were studied for each metallocene/monomer combination. Effects due to incorporation of a comonomer were analyzed as a function of both the central metal and the comonomer size. In accordance with the general experimental observations of zirconocenes producing lower molecular weight polymer than the hafnocenes, differences in activation energies for chain propagation and termination are smaller for the zirconocene. For the zirconocene, the activation energy is particularly low for β-hydrogen elimination after secondary comonomer insertion.