Controlled radical polymerization of styrene in the presence of lithium molybdate(V) complexes and benzylic halides

The new lithium molybdate(V) complexes [LiMo(NAr)(2)(C-N)R] (C-N = C6H4(CH2NMe2)-2; R = (C-N) (5), Me (6) CH2SiMe3 (7), p-tolyl (8)), have been generated in situ from reaction of the corresponding molybdenum(V)) complexes [Mo(NAr)(2)(C-N>R] (C-N= C6H4(CH2NMe2)-2; R = (C-N) (1), Me (2), CH2SiMe3 (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(iso)- and A(iso)-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(x2-y2) 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 (alpha-chloroethyl)benzene and (alpha-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. (C) 1999 Elsevier Science S.A. All rights reserved.