Hindered organoboron groups in organic chemistry. 29 The effect of hindered triorganoboranes on anionic butadiene polymerisations

Abstract Three new highly hindered triorganoboranes tris(4- t -buthyl-2,6-dimethylphenyl)borane (BDP) 3 B), ( 5 ) tris -(4- t -methoxy-2,6-dimethylphenyl)borane, (DMP) 3 B ( 6 ), and t -butyl bis (2,4,6-trimethylphenyl)borane (Mes 2 BBu t ) ( 7 ), were prepared and compared with trimesitylborane ( 4 ) for their influence on the 1,4-content of anionically produced polymers of butadiene. Each was design to overcome known reactions of ( 4 ). Unexpectedly, none was more effective overall than ( 4 ) in increasing the 1,4- content of polymer. Borane( 7 ) simply quenched LiBu n induced polymerisations, whilst ( 6 ) and ( 7 ) somewhat enhanced the 1,4-character and sharpened the polymer profile. However, ( 6 ) and ( 7 ) quenched polymerisations by sodium bases. Studies of the reactions of bases with a variety of hindered boranes suggest reasons for the results obtained, and help with the design of other, possibly more effective, catalyst.