Controlled Radical Polymerisation of Styrene in the Presence of Lithium Molybdate(V) Complexes and Benzylic Halides

Abstract

The new lithium molybdate(V) complexes [LiMo(NAr){2}(C-N)R] (C-N=C{6}H{4}(CH{2}NMe{2})-2; R=(C-N) (5), Me (6), CH{2}SiMe{3} (7), p-tolyl (8)), have been generated in situ from reaction of the corresponding molybdenum(VI) complexes [Mo(NAr){2}(C-N)R] (C-N=C{6}H{4}(CH{2}NMe{2})-2; R=(C-N) (1), Me (2), CH{2}SiMe{3} (3), p-tolyl (4)) with n-BuLi. The nature of these radical anions was studied by EPR spectroscopy. The spectra of toluene solutions of in situ prepared complexes 5-8 revealed the presence of two different paramagnetic species, i.e. a molybdenum compound with distinct g{i}{s}{o}- and A{i}{s}{o}-values and an unidentified radical with a sometimes strong signal at g=1.986+/-0.001, lacking any hyperfine coupling. Extended Huckel calculations on the crystal structure of 5 showed that the single electron occupies a molybdenum centered orbital, merely d{x}{2}{-}{y}{2} in character. In situ prepared complexes 5-8 were successfully applied in the atom transfer radical polymerization (ATRP) of styrene using benzyl chloride as the initiator. The efficiency of the benzyl chloride initiator is rather poor (6-18%). Reaction of the lithium molybdate(V) complex 5 with (@a-chloroethyl)benzene and (@a-bromoethyl)benzene resulted in the formation of 1, LiCl and LiBr, respectively. The molecular weights as well as the molecular weight distributions show that the catalytic system, BzCl/5-8, catalyses styrene polymerization successfully but does not exercise much control over the polymerization reaction due to the poor initiator efficiency of benzyl chloride and probably the extreme air-sensitivity of the lithium molybdate(V) compounds. The unidentified radical (g=1.986+/-0.001) is unable to initiate radical polymerization but possibly influences the ATRP activity.