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 ΔH‡ and entropies ΔS‡ according to the Eyring equation. For (C5Me4H)3Ln complexes (Ln = Sm, Tb, Y), a change in the reaction
enthalpy ΔH‡ 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.