Forces of Stereoregulation in Alkene Polymerizations

The value of kr/k is determined by the difference AAC 1 in the free energies of activation between the syndiotactic AG and isotactic AGj, placements 

There have been efforts to enhance stereoselectivity in radical polymerization by using fluoroalcohols or Lewis acids that complex with monomers such as MMA and vinyl acetate [Isobe et al., 2000, 2001a Okamoto et al., 2002]. In almost all instances the effects are nil or very small. For example, the use of perfluoro-f-butyl alcohol as solvent instead of toluene changes (rr) from 0.89 to 0.91 in the polymerization of MMA at —78°C. An exception is in the polymerization of acrylamide in the presence of some rare-earth Lewis acids such as ytterbium triflate. The polymer is atactic at 0 C, (m) = 0.46, in the absence of the Lewis acid, but significantly isotactic, (m) = 0.80, in the presence of the Lewis acid. The reason for this effect is unclear. More highly isoselective polymerization occurs in some radical polymerizations of MMA (Sec. 8-14h). 

Since radical polymerizations are generally carried out at moderately high temperatures, most of the resulting polymers are highly atactic. This does not mean that there is a complete absence of syndiotacticity. There is a considerable difference in the extent of syndiotacticity from one polymer to another. Thus, methyl methacrylate has a much greater tendency toward syndiotactic placement than vinyl chloride. Whereas the poly(vinyl chloride) produced at the usual commerical polymerization temperature ( 60°C) is essential completely atactic, that is, (r) (m) 0.5, this is not the case for poly(methyl methacrylate). The polymerization of MMA, usually carried out at temperatures up to 100°C, yields polymers with appreciable syndiotacticity—(r) is 0.73 at 100°C. The difference is a consequence of the fact that MMA is a 1,1-disubstituted ethylene, leading to greater repulsions between substituents in adjacent monomer units. 

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