THE ROLE OF BULKY SUBSTITUENTS IN BROOKHART-TYPE NI(II) DIIMINE CATALYZED OLEFIN POLYMERIZATION - A COMBINED DENSITY-FUNCTIONAL THEORY AND MOLECULAR MECHANICS STUDY

The role of the bulky ligands in Ni(II) diimine catalyzed ethylene pol ymerization has been examined with a combined density functional theor y quantum mechanics and molecular mechanics (QM/MM) model. Specificall y, we have examined the catalytic center of the type (ArN=C(R)-C(R)=NA r)Ni-II-R'(+), where R = Me and Ar = 2,6-C6H3(i-Pr)(2). The Ar and R g roups were treated by a molecular mechanics force field while density functional theory was applied to the remainder of the system. The chai n propagation, chain branching, and chain termination processes have b een investigated with the hybrid method and found to have barriers of Delta H-double dagger = 11.8, 15.3, and 18.4 kcal/mol, respectively, w hich is in excellent agreement with experiment in both absolute and re lative terms. This is in stark contrast to the pure QM model in which the influence of the bulky Ar and R groups was neglected and the estab lished order of the barriers is not even reproduced. The role played b y the bulky substituents is dual in nature. First, the Ar and R groups act to sterically hinder the axial coordination sites of the Ni cente r. This has the most dramatic destabilizing effect on the resting stat e and termination transition states, in which both axial positions are occupied. In addition to the steric factor, we find that the electron ic preference for the aryl rings to orient themselves in a coplanar fa shion with the diimine ring results in a stabilization of the insertio n transition state relative to the resting state. These two factors ac t to both lower the propagation barrier and increase the termination b arrier compared to the ''naked'' pure QM model system.