Activation of 12 group IV metallocene bis(ester enolate) complexes with B(C(6)F(5))(3) at room temperature (RT) affords quantitatively the corresponding isolable cationic eight-membered ester enolate metallacycles. This rapid two-step reaction consists of vinylogous hydride abstraction to form the anion [HB(C(6)F(5))(3)](-), and nucleophilic addition of the second enolate ligand to the methacrylate resulted from loss of a hydride in the first enolate ligand to form the chelating cation. This activation methodology for generating the active species (structural models for resting intermediates involved in methacrylate polymerization) is rather general, as demonstrated by a broad substrate scope examined in this study, including group IV metallocene bis(ester enolate) complexes that varied metals (Ti, Zr, Hf), bridging atoms (Ph(2)C<, Ph(2)Si<, Me(2)C<, -CH(2)CH(2)-), substituents ((t)Bu, Et(3)Si), substitution patterns (on 3-Cp and 2,7-Flu ring positions), and ligand symmetries (C(2), C(2v), C(1), and C(s)), all of which lead to the clean formation of their corresponding cationic metallacycles. Comparative methyl methacrylate (MMA) polymerization studies have identified metallacycle 4, {[Ph(2)C(Cp)(2,7-(t)Bu(2)-Flu)]Zr[OC(O(i)Pr)═CMeCH(2)C(Me(2))C(O(i)Pr)═O]}(+)[HB(C(6)F(5))(3)](-), as being the most active, efficient, and syndiospecific catalyst within the C(s)-ligated catalysts. Kinetic experiments at room temperature show that the MMA polymerization by 4 follows first-order kinetics in both [MMA] and [Zr], consistent with a monometallic, intramolecular coordination-addition mechanism that involves the eight-membered ester enolate chelate resting state. Thermodynamic experiments at varied temperatures yield activation parameters of ΔH(double dagger) = 6.23 kcal/mol, ΔS(double dagger) = -41.7 eu, and ΔG(double dagger) = 17.6 kcal/mol (273 K). As compared to ansa-Flu-Cp ligated chelating cations paired with more commonly used weakly coordinating anions such as [MeB(C(6)F(5))(3)](-) and [B(C(6)F(5))(4)](-), the same cations paired with the anion [HB(C(6)F(5))(3)](-) behave differently in MMA polymerization in terms of activity, stereospecificity, and sensitivity to solvent polarity. Most uniquely, [HB(C(6)F(5))(3)](-)-based catalysts effect substantial internal chain-transfer reactions, especially for polymerizations carried out in toluene and in the presence of excess B(C(6)F(5))(3), thus releasing polymer chains with a terminal double bond and achieving a catalytic polymerization. Computational results show the thermodynamics feasibility of the activation steps and the reversibility of the hydride abstraction step during activation, thus indicating that [HB(C(6)F(5))(3)](-) can uniquely act as a weak hydride donor. The picture emerging from the combined experimental and theoretical study has led to a new hydride-shuttling chain-transfer mechanism promoted by the hydridoborate anion, involving a hydride addition and abstraction sequence through the borane center.