Anionic polymerization of acrylic monomers. 10. Carbon-13 and lithium-7 NMR studies on the monomeric model of living poly(methyl methacrylate)

DFT study of the effect of sigma-ligands on the structure of ester enolates in THF, as models of the active center in the anionic polymerization of methyl methacrylate

A Density Functional Theory (DFT) study was carried out on structures of the lithium ester enolate of methyl isobutyrate (MIB-Li) in THF solution, in the presence of TMEDA, dimethoxyethane (DME), crown ether 12-crown-4, and cryptand-2,1,1, as electron donor ligands (sigma-ligands). Both specific solvation with THF and/or ligand molecules and nonspecific solvation by the solvent continuum were taken into account. The possibility of ligand-separated ion pair formation was analyzed for each of the ligands, including THF alone. In most cases peripherally solvated dimers are the most stable species. Only in the presence of cryptand-2,1,1 was a ligand-separated triple ion pair, (MIB-Li-MIB)(-)(THF)(2),Li(2,1,1)(1)(+), shown to be comparable in stability to the THF-solvated dimer, (MIB-Li)(2)(THF)(4). These results are in agreement with experimental NMR data on the structure of MIB-Li in the presence of DME, 12-crown-4, and cryptand-2,1,1. An upfield shift of the (13)C NMR signal of the alpha-carbon of MIB-Li observed in the presence of cryptand-2,1,1, originally attributed to a ligand-separated monomer, MIB(-),Li(2,1,1)(+), was well reproduced by Hartree--Fock calculated NMR shifts for the predicted ligand-separated triple ion pair.

In-Situ Direct Analysis of the Growing Species by 119Sn NMR Spectroscopy: Living Cationic Polymerization of Isobutyl Vinyl Ether with HCl/SnCl4/nBu4NCl1

119Sn NMR spectroscopy was employed for analysis of the interaction and reaction of SnCl4 with the HCl-IBVE adduct [1; CH3CHCl(OiBu)] in the presence of nBu4NCl in CH2Cl2 solution at -78 degreesC, which are model reactions for the living cationic polymerization of isobutyl vinyl ether (IBVE). The addition of 1 to an SnCl4 solution led to upfield shifts of the tin nucleus as the 1/SnCl4 mole ratio (<1) increases, which indicates the formation of SnCl5-, via the interaction between SnCl4 and the chlorine atom in 1. On further addition of 1, the pentacoordinated anion is converted into the hexacoordinated SnCl62-. These tin species are in fast equilibrium among each other, and the 119Sn NMR analyses support the formation of a carbocation [2; CH3CH+(OiBu)] from 1 and the dynamic equilibrium between 1 and 2. More effective chloride-anion donors such as nBu4NCl and Ph3CCl can quantitatively convert SnCl4 into SnCl5-, and then into SnCl62-. Thus under the conditions where living cationic IBVE polymerization proceeds (1 < nBu4NCl/SnCl4 < 2), SnCl4 is fully converted into a weaker Lewis acid, SnCl5-, with the aid of added nBu4NCl. The suppression of the carbocationic species in the living system has thus been found due to the interaction between the added salt and SnCl4.