Mechanistic Studies of Palladium(II)-α-Diimine-Catalyzed Polymerizations of cis- and trans-2-Butenes

Complexes [(N∧N)Pd(CH 3 )(L)]BAr‘ 4 ( 3, 4: L = NCAr‘; 5: L = OEt 2 ) (N∧N ≡ ArN C(R)−C(R) NAr, 3, 5: Ar = 2,6-C 6 H 3 (Me) 2; 4: Ar = 2,6-C 6 H 3 ( i Pr) 2; Ar‘ ≡ 3,5-C 6 H 3 (CF 3 ) 2 ) catalyze the polymerization of trans - and cis -2-butene. Both the productivity and the molecular weight of poly( ci s-2-butene) are much lower than those of poly( tran s-2-butene). The polymers exhibit atactic regioregular microstructure with a methyl group on every third backbone carbon. Low-temperature NMR studies show that migratory insertion in the η 2 -butene complexes [(N∧N)Pd(CH 3 )(CH 3 CH CHCH 3 )] + ( 6 and 11 ) occurs to give isomerized alkyl olefin complexes [(N∧N)Pd(CH 2 CH 2 CH(CH 3 ) 2 )(CH 3 CH CHCH 3 )] + ( 8 and 12 ). The first insertion barrier of tran s-2-butene (19.1 kcal/mol) is slightly lower than that of ci s-2-butene (19.3 kcal/mol), while the subsequent insertion barrier of ci s-2-butene (20.2 kcal/mol) is 0.6 kcal/mol higher than that for tran s-2-butene (19.6 kcal/mol). The isopentyl palladium complexes 14a and 14b, which most closely model the propagating species, exhibit nearly equal binding affinities for cis- and trans -2-butene.