Free Ethylene Radical Polymerization under Mild Conditions: The Impact of the Solvent

Ethylene polymerization is performed industrially either by radical polymerization under severe conditions (1000-4000 bar, 200-300°C) or by catalytic mechanism at lower temperatures (usually less than 100°C) and pressures (below 50 bar). Standard radical polymerization conditions are too severe to permit a fine control of the macromolecular architecture. Under milder conditions radical ethylene polymerization is assumed to be ineffective, which has been confirmed using toluene as solvent. The efficiency of free radical polymerization under mild conditions (up to 250 bar of ethylene and a polymerization temperature between 50°C to 90°C) has been investigated in THF which is a more polar solvent than toluene. In this solvent, polyethylene has been obtained with relatively good yields highlighting an unexpected high solvent effect in the free radical ethylene polymerization. This solvent effect has been rationalized using theoretical consideration

Structural, spectroscopic, electrochemical and computational studies of C,C '-diaryl-ortho-carboranes, 1-(4-XC6H4)-2-Ph-1,2-C2B10H10 (X = H, F, OMe, NMe2, NH2, OH and O-)

The influence of aryl ring substituents X (F, OMe, NMe2, NH2, OH and O−) on the physical and electronic structure of the ortho-carborane cage in a series of C,C′-diaryl-ortho-carboranes, 1-(4-XC6H4)-2-Ph-1,2-C2B10H10 has been investigated by crystallographic, spectroscopic [nuclear magnetic resonance (NMR), UV–vis], electrochemical and computational methods. The cage C1–C2 bond lengths in this carborane series show small variations with the electron-donating strength of the substituent X, but there is no evidence of a fully evolved quinoid form within the aryl substituents in the ground state. In the 11B and 13C NMR spectra, the ‘antipodal’ shift at B12, and the C1 shift correlates with the Hammett σ p value of the substituent X. The UV–visible absorption spectra of the cluster compounds show marked differences when compared with the spectra of the analogous substituted benzenes. These spectroscopic differences are attributed to variation in contributions from the cage orbitals to the unoccupied/virtual orbitals involved in the transitions responsible for the observed absorption bands. Electrochemical studies (cyclic and square-wave voltammetry) carried out on the diarylcarborane series reveal that one-electron reduction takes place at the cage in every case with the voltage required for reduction of the cage influenced by the electron-donating strength of the substituent X, affording a series of carborane radicals with formal [2n + 3] electron counts