Helical conformation isotactic/syndiotactic polymers

Also, in studies with optically active vinyl ethers it was observed [104] that trimethyl vinyl silane, which is bulky and non-chiral forms highly syndiotactic polymers. Equally bulky, but chiral (—)-menthyl vinyl ether, however, produces isotactic polymers in polar solvents. This suggests that isotactic propagation is preferred in a polar medium because of helical conformation of the polymer... 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

Interesting geometrical differences between helical conformations in isotactic and syndiotactic polymers have recently been found.25 27... 

Isotactic and syndiotactic vinyl polymers cannot be chiral unless they adopt a single-handed helical conformation because they possess a mirror plane, and they do not show optical activity except for that due to the asymmetric centers in the vicinity of the chain ends. Several negative examples have been shown such as those for the polymers of styrene,propylene, methacrylates,and acrylonitrile. ... 

The protective effects of syndiotactic and isotactic poly-2-vinylpyridine-l-oxide remarkably differ in the spatial arrangement of the structural units (Holt et al. 1970). The isotactic polymer has probably a helical conformation. When the cultures... 

Higher values of the ratio r/R, close to V2 -1 (similar to the situation stabilizing NaCl), are obtained in the cases of many vinyl polymers with chains in complex helical conformation s M N) with a fractional ratio MIN, ranging between 3 and 4, such as form I of isotactic poly(4-methyl-l-pentene) (7/2 helix) [38], form II of isotactic poly(l-butene) (11/3 helix) [46], isotactic poly(m-methylstyrene) (11/3 helix) [86], and syndiotactic poly(4-methyl-l-pentene) (12/7 helix) [72]. In these cases, a tetragonal packing of enantiomorphous chains. 

A general example is the monosubstituted vinyl polymers (CH2—CHX) , which can exist in two possible stereoregular forms — isotactic (substitution on the alternate carbons on the same sides of the chain) and syndiotactic (substitution on the alternate carbons on the opposite sides of the chain). These ordered stereoregular forms can assume either planar or helical conformations. Each of these ordered structures has well-defined, unique selection rules for IR and Raman activity. These various structures can be distinguished on the basis of spectral properties without a detailed knowledge of the molecular motions or energies, that is, without normal coordinate analysis. 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

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