Three coordinate steric effects

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

An alternative SN2-Si mechanism may be considered, without a penta-coordinate intermediate. In this alternative, the silicon is rehybridized from spy to sp2. If bond order is conserved, then it is reasonable to ascribe a bond order of 0.5 to the Si—O bond of both the entering nucleophile and the leaving group [52], It has been shown that the bond order is related to the bond distance [45]. Even though there are five substituents in the vicinity of the silicon in an SN2-Si type mechanism, two of the substituents are significantly further away from the central atom (silicon) than the other three substituents. A looser transition state structure than that for an SN2 -Si or an SN2 -Si process results. The steric effects of the alkyl group bonded to silicon should, therefore, be considerably less in an SN2-Si type mechanism. 

It is interesting that the stability, urdike the geometry, of these radicals MX3 is so sensitive to steric effects. Bulky groups X thus hinder the formation of a four-coordinate X3M—MX3 or X3M—H and favor the three-coordinate MX ) (44). A similar effect has been noted for transition metal... 

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