ABSTRACT
SCHEME 7.1 Stereochemical and regiochemical conformational alternatives for a representative ansametallocene catalyst shown with growing polypropylene chain.
This chapter presents a review of theoretical studies of stereo-and regiocontrol in propylene polymerization. The working hypothesis of researchers in this field is that an enhanced understanding of the molecular sources of stereo-and regiocontrol will lead to the development of improved polymerization catalysts. The intellectual challenge of this and other theoretical catalytic studies is that understanding selectivity is an exercise in transition state energetic differentiation. For site control in propylene polymerization, this means computing the energetic difference between transition states that are merely conformeric alternatives for the methyl group of the approaching propylene (Scheme 7.1). In this scheme, the upper left conformation illustrates stereonormal 1,2-insertion, whereas the upper right illustrates stereoerror 1,2-insertion. The two lower conformations illustrate 2,1-regioerror conformations. Given that heroic efforts are needed to compute bond energies or excitation energies to “chemical accuracy” for even the smallest of organic molecules,1 one must ask the question of how meaningful stereo-and regiodifferentiations can be computed for the large complexes of olefin polymerization. In this chapter, the limitations of alternative theoretical methodologies are described, and a roadmap for computing the necessary small energy differences are provided along with validating examples.
