Controlled radical polymerization 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 6H 4(CH 2NMe 2)-2; R=( C– N) ( 5), Me ( 6), CH 2SiMe 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 6H 4(CH 2NMe 2)-2; R=( C– N) ( 1), Me ( 2), CH 2SiMe 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 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 Hückel 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 (α-chloroethyl)benzene and (α-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.