Abstract
This contribution reports a combined synthetic, kinetic, mechanistic, and theoretical/computational study of the recently discovered catalyst-site-controlled coordination polymerization of polar vinyl monomers (such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA)] into highly syndiotactic polymers. Among the 12 C-s-ligated ansa-cyclopentadienyl (Cp)-R2E(C,Si)-fluorenyl (Flu) group 4 metallocene catalyst systems examined-which varied in metal center, anion structure, bridging atom and substituents, and ligand substitution pattern-cationic ansa-metallocene ester enolate catalyst 6+[B(C6F5)(4)](-), derived from the activation of the precatalyst [Ph2C(CP)(2,7-'Bu-2-Flu)]Zr[OC((OPr)-Pr-i)=CMe2](2) with [Ph3C][B(C6F5)(4)], stood out as the best catalyst in all aspects of the MMA polymerization at room temperature, including the highest activity (1554 h(-1) TOF), efficiency (98% I*), syndiotacticity (94% rr), and control (predicted number-average molecular weight and 1.14 molecular weight distribution). Kinetic and mechanistic results are consistent with a catalyst-site-control led, monometallic coordination-addition mechanism, involving fast intramolecular addition within the catalyst-monomer complex leading to the resting eight-membered ester enolate chelate, followed by the rate-limiting ring-opening of the chelate to regenerate the active species. This work has also uncovered several unique features of this polymerization system that are in marked contrast to the propylene polymerization by analogous Cs-ligated cationic alkyl catalysts: a constant syndiotacticity of PMMA produced over a wide polymerization temperature range (i.e., from 0 degrees C, 94% rr to 25 degrees C, 94% rr to 50 degrees C, 93% rr); insensitivity of its high activity, degree of control, and stereoselectivity to solvent polarity and structure of weakly coordinating anions; and deviation from a pure site-control mechanism at high [MMA]/[catalyst] ratios. Computational results provide theoretical support for the proposed monomer-assisted, catalyst-site epimerization, after an enantiofacial mistake, to a thermodynamically more stable resting state, which accounts for the observed higher than expected [mr] contents based on a pure site-controlled mechanism. DFT calculations rationalize why the Ph2C< bridged catalyst 6 exhibits higher stereoselectivity than other catalysts with the Me2C< or Me2Si < bridge: the bridge rigidity pushes the eta(3)-bound Flu ligand closer to the growing chain and the monomer, thereby increasing Delta E-stereo between the competing transition states for the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing chain. The relative polymerization activity of this catalyst series is shown to correlate with the relative energetics of the back-biting of the penultimate unit and ion-pair formation.