ANIONIC-POLYMERIZATION OF ISOPRENE, BUTADIENE AND STYRENE WITH 3-DIMETHYLAMINOPROPYLLITHIUM

The synthesis of dimethylaminopropyllithium (DMAPLi), from the corresp onding chloride and lithium metal, by using high-vacuum techniques, is described. DMAPLi was purified by crystallization under vacuum. Gas c hromatography-mass spectrometry analysis of the purified product, afte r neutralization with degassed methanol, showed the existence of simil ar to 2% impurities. The most important of these impurities (similar t o 1.5%) were tetramethylhexamethylenediamine and dimethylaminohex-1-en e. By using DMAPLi as initiator, several polymerizations of isoprene, butadiene and styrene were carried out in benzene, under high-vacuum c onditions. The molecular-weight range of the polymers obtained was fro m about 3 x 10(3) to 4 x 10(4). 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, th e 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 unpurif ied initiator, which contains polar impurities, the polydispersity ind ex recovers the usual values, for molecular weights higher than simila r to 1 x 10(4).