Diene polymerization by mixed sodium‐ and lithiumalkyl initiators in hydrocarbon medium

Abstract

Polymerization of butadiene and isoprene initiated by the hydrocarbon-soluble mixture 2-ethylhexyllithium/2-ethylhexylsodium in toluene was studied. The components of the initiator when taken separately give rise to polymers greatly different in both their molecular weight and microstructure: living polydienes with high 1 ,4-unit content are formed when alkyllithium is used while organosodium initiators produce polymers of high 1,2(3,4)-structure and of low molecular weight due to strong chain transfer to toluene. Polymers obtained with mixed initiators were found to have the same structure as the "sodium" ones, while their molecular weights are either the same as those of the polymers obtained with alkyllithium alone (in the case of butadiene) or decrease gradually with increase of sodium content in the initiator (in the case of isoprene). The calculations performed show that the simple scheme assuming chain propagation through individual lithium and sodium active centres and fast exchange of counter-ions between them cannot explain the results obtained. It is suggested that the true active centres in mixed systems are bimetallic complexes containing both sodium and lithium atoms simultaneously.

Dienes polymerization in the presence of metal hydrides and triethylaluminum

“…The use of sodium and potassium-based initiators remained so far extremely limited for several reasons including the insolubility of most metal alkyl derivatives in hydrocarbon media and their low selectivity towards 1,4 insertion. 2-Ethylhexylsodium [4,5] and bimetallic systems such as Bu 3 SnNa [6] that are reported to be soluble in hydrocarbons can initiate isoprene polymerization but yield polyisoprene with a low amount of 1,4 units. Patterson and Halasa have investigated the polymerization of butadiene with a series of potassium-based catalysts obtained by the association of potassium alkoxides and alkyl-magnesium [7] or alkyl-lithium [8].…”

Study on the mechanism of anionic polymerization with mixed RLi-R′OK initiators, 1. Polymerization of butadiene

“…This mechanism is, implicitly or explicitly, accepted by most authors 1,5,15,25,26) . However, it seems that nobody tried so far to quantitatively verify its validity for any particular polymerization, while some recent qualitative observations evidently contradict this mechanism 15,20,27) . The alternative "complexation" scheme assumes that the active centres are not single-metal species but multicomponent complexes.…”

“…All polymerizations were carried out in toluene at 30 8 C. The reagents were purified according to standard procedures, finally being treated with n-BuLi 27) . Potassium tert-amylate (t-AmOK) and lithium tert-amylate (t-AmOLi) were obtained via the reaction of corresponding metal with t-AmOH solution in cyclohexane.…”

“…Molecular weights and molecular weight distributions (MWD) were measured by GPC 27) . The relative constant of chain transfer to solvent, (C s = k tr /k p (k tr and k p are the rate constants of chain transfer and propagation, respectively), was calculated using the wellknown expression 30) :…”

Copolymerization of N,N‐ Dimethylacrylamide with Styrene and Butadiene: The First Example of Polar Growing Chain End/Nonpolar Monomer Cross‐Initiation

“…[12,15] The polymerization of DMAA was performed at -30 8C, and the polymerization of nonpolar monomers at ambient temperature. After completion, polymerization was quenched with a drop of methanol, the polymer was precipitated by pouring the reaction mixture into a large excess of hexane and then dried in vacuo to constant weight.…”