Scope and mechanism of chain transfer catalysis with metalloradicals and metal hydrides
The H/D exchange rate constants kreinit were determined by an approximate treatment from the reaction between MMA-d 5 and (C5R5)Cr(CO)3H (R = Ph, Me, H). Similarly kreinitD was derived from the reaction between (C5Ph5)Cr(CO)3D and the MMA dimer. These rate constants indicate that the H/D exchange rate decreases as the steric hindrance on the carbon-carbon double bond or the hydride increases.
The Mayo method was used to determine the chain transfer constant CS for chain transfer polymerization of MMA catalyzed by (C5Ph5)Cr(CO)3• under various conditions. It was proved that the operation of fast chain transfer and slow reinitiation converted the metalloradical into the stable corresponding metal hydride, which suggests the resulting chain transfer constant for chromium catalyst is an apparent value that would be smaller than the true chain transfer constant. Simulations based on a simplified kinetic model reveals that the free radical species, metalloradical and metal hydride each reaches a steady state during the polymerization. The simulation results agree with the observations during the chain transfer polymerization catalyzed by chromium metalloradicals.
The rate constant of chain transfer from the methyl isobutyryl radical to (C5Ph5)Cr(CO)3•, k tr(monomer), was estimated by either treating (C5Ph 5)Cr(CO)3H with MMA or heating (C5Ph5)Cr(CO) 3• with a radical source AIBMe. The consistent results of these attempts show that the chain transfer rate constant decreases significantly with chain length. The ktr(monomer) and kreinit value combined imply a ΔG (50 °C) of +12 kcal/mol for H• transfer from (C5Ph5)Cr(CO) 3H to MMA, which in turn estimates a β C-H bond dissociation energy of 45 kcal/mol for the methyl isobutyryl radical.
The Mayo method was used to determine the apparent chain transfer constant for various metalloradicals and metal hydrides. The chain transfer activity decreased with the steric congestion on the metal center, while it was negligible when the decrease of steric hindrance resulted in stable dimers. These results, combined with the mechanistic knowledge, established the rational criteria for designing better chain transfer catalysts.
Using metalloradicals and metal hydrides as chain transfer catalysts, the investigation in this thesis provides not only an alternative catalysts but an opportunity of a better understanding of catalytic chain transfer.