Molecular Migrations in the Absence of Severe Local Pressure

Another subject that cannot be handled by a minimal atomic and molecular movement hypothesis concerns the numerous E Z, Z— , and s-trans s-cis (32, 

below) isomerizations within crystals. Their direction and occurrence can only be predicted on the basis of molecular migrations along cleavage planes and channels or to voids (63, below). In many cases, the geometric double bond isomerizations are unidirectional if only one of the stereomers exhibits a crystal structure ready for molecular migrations. This has been exhaustively analyzed for all known systems with complete crystal structure data and already reported in Refs. [4, 6, 10, 

48] and when the space conserving hula-twist mechanism (simultaneous double bond and adjacent single bond isomerization within a plane while only one C-H unit undergoes out-of-plane translocation) [51] or bicycle pedal movements are the only thinkable choices for the isomerization [4, 6, 47, 49]. 

Molecular Migrations in the Absence of Severe Local Pressure 

Thermal polymerizations of vinylic crystalline monomers giving isotactic or syndiotactic crystalline polymers are rare. More frequent are photolytical and radiolytical studies. The crystallographic packing requirements are most stringent. Clearly, the polymer chains cannot exit the crystal. They must therefore be able to accommodate with the monomer structure. Despite enormous efforts only a few 

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