2i-helix

Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]...

Figure 2.10 (a) All-frflni (linear zigzag) conformation of syndiotactic polypropylene. (b) The 2i helix of syndiotactic polypropylene. (From Ref. 11)... 

Problem 2.4 What is the repeat distance between pendant methyl groups that form a row (a) in the 3i helix of isotactic polypropylene and (b) in the 2i helix of syndiotactic polypropylene. Assume each carbon-carbon bond length is 1.54 Aand each bond angle is 109.5°. 

Fig. 10.1 The orthorhombic unit cell of polyethylene (a) in the ab plane, (b) in the ac plane showing the all-trans planar zigzag conformation of crystalline polyethylene. The 2i helix (one complete turn for each two CH2 units) is also apparent.

Poly(3-hydroxybutyrate) is a linear polyester with helical macromolecules. The secondary stracture of PHB is specified as left-hand 2j helix in a g g tt conformation, while the structure of oligolides consists of right-hand 3i helices [99]. The surface of the 3i(-l-) helix is covered by methyl groups, leading to the lipophilic nature of the macromolecule. The carbonyl bonds in the 2i(—) helix are placed perpendicularly, while in the 3i(-l-) counterpart, they are parallel to the helix axis. The latter is the reason for ability to form ionic complexes. 

Polymers of high tacticity can show a helical cOTiformation, generated by regularly repeating rotations around the backbone bonds of a macromolecule. A helix with M helix residues in N turns can be denoted as Mn helix [971UP1]. Typical examples are it-PP (3i-helix), st-PP (2i-helix), POM (95-helix), PTFE (T < 292 K 136-helix T >292 K 157-helix). 

Figure 2.21 View along helical axis of POM (2i helix) carbon - shaded hydrogen — white oxygen — dotted.

As the sidechain length increases, the baseplane dimensions of the PHA becomes more rectangular as shown in Figure 5. At the same time the density of the material decreases in keeping with differences in inter-molecular packing forces (see Figure 6). Nevertheless, the dipolar interactions of the backbone atoms are retained since the 2i helix symmetry persists although the fiber repeat (c-axis of unit cell) itself drops... 

In this cis polymer, the reason for the formation of 2i helix is the minimization of the repulsion between n electrons of the double bonds. 

Syndiotactic polypropene crystallizes mainly in a conformation (TTGG)2 which corresponds to a 2i helix. 

Because the planar zigzag conformation of polyethylene is a 2i helix, the observed IR and Raman lines for the various modes are active at values of = 0 and n, respectively, depending on the nature of the motion involved. For the infinite chain, no other modes are observable because for all > 0, there is a corresponding

[Pg.72]

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