| Popis: |
Cis-trans isomerization of butenes in free-anion systems proceeds through the α-butene intermediate. Direct cis - and trans -transformation of crotyl anions does not occur. Reactions of crotyl anion and its ion pairs with proton-donor compounds in hexamethylphosphortriamide (HMPTA) are accompanied by the formation of an equilibrium mixture of butenes containing 97 per cent β-butenes. The addition of these active species to alkylene oxides and sulphides, cyclohexadiene-1,3, styrene and 2,3-dimethylbutadiene occurs with the participation of the terminal carbon atom of the crotyl active centre, resulting in the predominant formation of 1,4-units. It has been shown that the reactivity ratios r 1 and r 2 for the pairs butadiene-styrene, butadiene-2,3-dimethylbutadiene and butadiene-cyclohexadiene-1,3 become very close in HMPTA. Studies of copolymer microstructure reveal an increase of butadiene 1,4-units content paralleling a rise of second monomer content. |
