Anionic polymerization of isoprene, butadiene and styrene with 3-dimethylaminopropyllithium

The synthesis of dimethylaminopropyllithium (DMAPLi), from the corresponding chloride and lithium metal, by using high-vacuum techniques, is described. DMAPLi was purified by crystallization under vacuum. Gas chromatography-mass spectrometry analysis of the purified product, after neutralization with degassed methanol, showed the existence of ∼ 2% impurities. The most important of these impurities (∼ 1.5%) were tetramethylhexamethylenediamine and dimethylaminohex-1-ene. By using DMAPLi as initiator, several polymerizations of isoprene, butadiene and styrene were carried out in benzene, under high-vacuum conditions. The molecular-weight range of the polymers obtained was from about 3 × 10³ to 4 × 10⁴. The polydispersity index (I = M_w/M_n) of the polydienes was similar (I < 1.1) to those obtained with the classical initiator s-butyllithium, but the vinyl content was higher. It was found that the lower the molecular weight of the polydienes, the higher the vinyl content. In the case of styrene, the polydispersity index of the polystyrenes produced was relatively high (I = 1.2 to 1.5), but by adding small amounts of tetrahydrofuran or by using unpurified initiator, which contains polar impurities, the polydispersity index recovers the usual values, for molecular weights higher than ∼ 1 × 10⁴.