Some evidence of a dual stereodifferentiation mechanism in the polymerization of propene by a-diimine nickel catalysts

Nickel diimine compounds of general formula [{(ArNdC(X)C(X)dNAr)}NiBr2] (X2 ) 1,8- naphthdiyl; Ar ) 2,6-diisopropylphenyl, 1; 2,6-dimethylphenyl, 2; 2-isopropyl-6-methylphenyl, 3; 2- tertbutyl- 6-methylphenyl, 4; 2,4-di-tert-butyl-6-methylphenyl, 5) were synthesized and tested in the polymerization of propene to address the effects of the coordination environment at the nickel center on the polypropylene microstructure. Compounds 1 and 2, having four isopropyl and four methyl ortho substituents, respectively, afford prevailingly syndiotactic polypropylenes at -45 °C (rr ) 75% for 1 and rr ) 61% for 2) through a “chain-end” control mechanism. Compounds such as 3, 4, and 5, having two different ortho substituents on the same aromatic ring, can exist as either rac or meso isomers. Rac-3 and rac-4 afford much less stereoregular polypropylenes with respect to 1 and 2 (e.g., mm ) 41% and rr ) 25% for rac-3). These C2-symmetric catalysts could provide an “enantiomorphic-site” type isotacticspecific steric control opposing the syndiospecific “chain-end” steric control. The hypothesis of a dual mechanism of steric control being operative has been supported by the synthesis of both the rac and the meso isomers of 5, with the former still affording a polypropylene with a low stereoregularity (rr ) 33% and mm ) 23%) and the latter yielding a prevailingly syndiotactic polymer (rr ) 66%).