Polymeric palladium-mediated carbene polymerization

Abstract

A metalloenzyme-inspired polymeric palladium catalyst, which was prepared by self-assembly of a polymeric imidazole ligand with PdCl₂, promoted carbene polymerization with high catalytic activity and reusability. Besides the atactic polycarbethoxycarbene (PCEC), a small amount of stereoregular PCEC was also obtained with poly(imidazole-Pd)-mediated polymerization of ethyl diazoacetate (EDA). This is the first example that a stereoregular polycarbene in an isolated form is obtained with a palladium-initiating system, suggesting that the steric polymeric imidazole ligand could affect the carbene insertion process. Detailed end group analyses on the obtained atactic and stereoregular PCEC revealed the chain initiation and chain termination mechanisms. XPS analyses on the oxidation state changes of the palladium center exhibited that Pd^II was reduced to Pd^I, which suggests that an intramolecular electron transfer process occurred during the interaction of palladium and EDA. Kinetic studies showed a zero order kinetic dependent on the concentration of EDA, which indicates a strong binding of EDA on the catalyst preceding carbene formation. With the addition of a radical inhibitor, the reaction rate was found to be greatly decreased. In the presence of a spin trap, a striking electron paramagnetic resonance (EPR) spectrum could be detected, which suggests the involvement of radical intermediates. DFT calculations on the proposed Pd-carbene radical species supported the EPR analyses, which assigned a significant amount of spin density on the carbene-carbon atom. Based on these results, we proposed an unprecedented metal-carbene radical polymerization mechanism, which occurs through a stepwise carbene radical migration and insertion process to afford polycarbenes.