Ligand Induced Steric Crowding in Rare Earth Metal-Mediated Group Transfer Polymerization of Vinylphosphonates: Does Enthalpy Matter?

This contribution presents the first rare earth metal-mediated group transfer polymerization of vinylphosphonates from substituted cyclopentadienyl rare earth catalysts. Several initiators with increased steric demand of the coordinated ligand sphere and decreased size of the catalytically active metal center are prepared using salt metathesis. The effects of the modifications on molecular mass, propagation rate, initiation delay, and molecular mass distribution are monitored via activity measurements using a normalization method for living polymerizations, and the correlation between steric crowding and activity is presented. Temperature-dependent kinetic analyses are performed for several methyl-, trimethylsilyl-, and tetramethyl­cyclo­pentadienyl-substituted complexes to determine the activation enthalpies Δ<i>H</i>^‡ and entropies Δ<i>S</i>^‡ according to the Eyring equation. For (C₅Me₄H)₃Ln complexes (Ln = Sm, Tb, Y), a change in the reaction enthalpy Δ<i>H</i>^‡ is observed compared to un- and monosubstituted compounds. The metal–monomer and metal–poly­(phosphonate ester) bond distances are found to be prolonged for the pentacoordinated intermediate