Syndiotactic polypropylene 2 helix

As an example chosen in the macromolecular field the C NMR spectrum of syndiotactic polypropylene might be mentioned In solution (averaged random coil conformation, molecular model corresponding to 7) it presents three signals in the crystal state, where a chiral rigid conformation exists [(2/1)2 helix], it shows four signals (Figure 17). 

Syndiotactic polypropylene exists in various crystal modifications. The most stable chain conformation is helical and involves a succession of the type TTGG or TTG G . The helix conformation is very nearly a rectangular stair-... 

Fig.6 (a) AFM image of the be contact face of sPP in its antichiral Form I, epitaxially crystallized on p-terphenyl. Note the strict alternation of right- and left-handed helices (helix axis at one o clock). Left-handed helices are better resolved than right-handed helices. Reproduced from [33] with permission (b) Fourier-filtered image of the upper left area of part (a) showing more clearly the right-handed helices that alternate with the left-handed ones. Reproduced from [33] with permission (c) The surface structure of p-terphenyl used as a substrate for the epitaxial crystallization of syndiotactic polypropylene (parts... 

Fig. 6. Polypropylene isotactic arrangement of methyl groups along the helix (described by a tape) the structure of syndiotactic polypropylene (drawing in the middle) is also regular the chain in atactic polypropylene (below) is statistically irregular ... 

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°. 

For syndiotactic polypropylene, the band at 11.54 ja has been very useful for the determination of the helix content and, indirectly, of the steric structure. The ratio Is between the absorbance of this band and the mean value of the two bands at 2.32 and 2.35 p has been called the index of syndiotacticity (97). The proportionality between Is and the content of crystalline form in the samples was checked by density measurements these, combined with a knowledge of the volume of the unit cell, allowed a direct, though approximate, evaluation of the crystallinity of the samples. The concept of syndiotacticity index was extended to C3— butene 1 copolymers (96) and can be applied to C2-C3 copolymers as well (98). Its value is strongly dependent upon the preliminary thermal treatment of the copolymer (spectra are recorded on samples annealed at 80° C for 12 hours) this is why standard values are not usually given. 

In the previous section the helical jumping of the molecules in the crystallites of POM was discussed. Similar motions have been observed in a number of other crystalline polymers, including poly(ethylene oxide) (PEO), isotactic polypropylene (IPP) and polyethylene. At 240 K each helical chain in PEO makes about three jumps per second. In the crystalline regions of IPP helical jumps along the 3i helix are observed at temperatures in the range 340 380 K, whereas in syndiotactic polypropylene, which has a more complicated 4j helix, no jumps are observed. 

Figure 6 Different helical conformations adopted by macromolecules in the crystalline state. The molecular helices are shown for PE, isotactic polypropylene, syndiotactic polypropylene, and polyethylene oxide from left to right, respectively. The type of the helix is denoted by the number of monomers per integer number of turns shown in subscript.

Syndiotactic polypropylene, sPP, exists in three different conformations a stable two-fold helix, a planar zigzag both with orthorhombic lattice and an intermediate conformation with a triclinic lattice. The crystallographic data of sPP are reported in Table 3. The stable two-fold helix structure is called the high-tem-perature orthorhombic form. Three different unit cells for this form are proposed [78-85]. 

The fig. 10 shows, for syndiotactic polypropylene, three minima in the energy map, two of which corresponding to a right-handed helix and a left-handed helix, while the third corresponding to a trans planar confor-mat ion. 

This polymorphism is different from the case of isotactic polypropylene, which is also polymorphous, but in which, in the different crystalline forms, always a threefold helix is observed or from the case of polybute-ne-1 which is also polymorphous, but the different helices observed correspond, however, to the same region of minimum of the conformational energy map. Instead in the case of syndiotactic polypropylene, as shown before, the two different crystalline forms correspond to chain conformations which are widely separated in the conformational energy nap. 

Fig. 3.29 IR spectra of (a) atactic, (b) syndiotactic, and (c) isotactic polypropylene (reproduced with permission from Klopffer, Introduction to Polymer Spectroscopy ). The absorptions at 970 and 1460cm do not depend upon tacticity, whereas the absorptions at 840, 1000, and 1170cm are characteristic of the 2 3H helix of isotactic polypropylene, and the absorption at 870 cm is characteristic of the 4 2 1 helix of syndiotactic polypropylene.

The CP-MAS C-NMR spectrum of syndiotactic polypropylene (sPP) is compared with the spectrum of iPP in Fig. 9.6 [31]. The solid state NMR spectra are substantially different in spite of the common chemical structure because of the differences in the conformations in the solid state. The sPP has a 2 helix with a (gg)(tt)(gg)(tt) structure. This structure involves external (outside the helix) and in-... 

The steric configuration is extremely important in the polymer. Only isotactic polypropylene (iPP) has the properties necessary for forming fibers. The molecules are cross-linked only by Van der Waals forces, so it is important that they pack as closely as possible. The isotactic molecules form a 3, helix, as shown in Fig. 12.21,16 and exhibit a high crystallization rate. The atactic molecules, shown in the figure, do not pack well, and although the syndiotactic molecules can pack better and crystallize, this configuration is not a normal product of commonly used catalyst systems. 

Isotactic polypropylene does not exist in the two zig-zag and helix conformations, but syndiotactic PP is a good candidate for this search. Indeed, according to the preparation procedure of the polymer ( ), it can exist in a helix or zig-zag planar conformation. Syndio-PP films were prepared following both ways, their conformation checked by IR, and studied by XPS. Their valence band spectra again show distinct differences in the C2s band (Figure 16) for zig-zag PP that is probably highly amorphous, the C-C band width increased by about leV, whereas the bonding subband increased in intensity and became more structured ( ). 

Fig. 4.3 The polarised infrared spectra of various forms of oriented polypropylene (a) syndiotactic helix, (b) syndiotactic planar zigzag...

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