Conformational interconversion steric factors

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

The (m) fraction is in the range 0.6-0.7 for polymerization at high monomer concentration in the temperature range —25 to 25°C [Resconi et al., 2000]. Substituents on the phenyl groups of the 2-phenylindene ligands, especially meta substituents, slow down the conformational interconversion, and this increases the isotacticity of the polymerization. Both steric and electronic factors operate to affect the interconversion of conformers. Bis(2-phenylin-dene)zirconium dichloride/MAO yields polypropene with (mmmm) = 0.33 for polymerization of liquid propene at 20°C. 3,5-Di-f-butyl and 3,5-difluoromethyl substituents increase (mmmm) above 0.70 [Lin and Waymouth, 2002 Wilmes et al., 2002a,b]. 

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