Polymeric palladium-mediated carbene polymerization

Abstract

Metalloenzyme-inspired polymeric palladium catalyst, which was prepared by self-assembly of polymeric imidazole ligand with PdCl 2 , promoted carbene polymerization with high catalytic activity and reusability. Besides the atactic polycarbethoxycarbene (PCEC), small amount of stereoregular PCEC was also obtained with poly(imidazole-Pd)-mediated polymerization of ethyl diazoacetate (EDA). This is the first example that stereoregular polycarbene in 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 palladium center exhibited that Pd II was reduced to Pd I , which suggests an intramolecular electron transfer process occurred during the interaction of palladium and EDA. Kinetic study 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 radical inhibitor, the reaction rate was found to be greatly decreased. In the presence of 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 occurrs through a stepwise carbene radical migration and insertion process to afford polycarbene.Scheme 1 Poly(imidazole-Pd)-mediated polymerization of EDA to produce atactic and stereoregular PCEC.

Polymerization of Alkyl Diazoacetates Initiated by Pd(Naphthoquinone)/Borate Systems: Dual Role of Naphthoquinones as Oxidant and Anionic Ligand for Generating Active Pd(II) Species

“…On the other hand, the polymer prepared with the Pd­(cod)­(Cl-nq)­Cl/NaBPh 4 system exhibits a dominant peak at the same position as for syndiotactic polymers obtained with the Rh­(diene)-based initiator in each 1 H and 13 C NMR spectrum (Figure B). Although the existence of some minor peaks on both sides of the dominant one suggests that the stereoregularity is not so high as that in Rh­(diene)-initiated polymerization, the appearance of signals in Figure B indicates that much higher stereoregulation is achieved with the Pd­(cod)­(Cl-nq)­Cl/NaBPh 4 system compared to the Pd­(nq) 2 /NaBPh 4 system as well as previously reported Pd-based initiating systems. ,− ,, Likewise, high stereoregulation is also observed in the spectra of the polymers obtained from BDA (Figure S3 in the Supporting Information). As described below, the high stereoregularity could be ascribed to the steric congestion around the propagating Pd center in the Pd­(cod)­(Cl-nq)­Cl/NaBPh 4 system.…”

Fabrication of polyamide 12T micro‐cellular foams with ultra‐high compressive strength via in situ polytetrafluoroethylene fibrillation using supercritical carbon dioxide foaming

“…After cooling, the higher viscosity of CEPA12T hindered the formation of molecular chains into crystal lattices, resulting in an inadequate assembly. 31 The 2,2 0 bis-(2-oxazoline)(ΔW) and enthalpy of melting(ΔH m ) of CEPA12T were lower than those of PA12T. The introduction of CE led to a decrease in the content of free amino and carboxyl groups, which reduced the hydrogen-bond density 32 and hindered the crystallization process to a certain extent.…”

Radiation effects on microstructure and EPR signal of yttrium oxide rods

“…A powerful technique for characterization of ionizing induced defects is Electron Paramagnetic Resonance (EPR). In addition, EPR is used to evaluate polymerization reactions [1] , catalysis [2] , laser [3] , free radicals in living tissues and fluids [4] and drug detection [5] . EPR is a useful technique for solid state dosimetry.…”