Polypropylene isotactic/syndiotactic structures, conformational

Figure 3.1 The 25-MHz spectra of three preparations of polypropylene isotactic, atactic, and syndiotactic. (From Bovey, F.A., Chain Structure and Conformation (f Macromolecules, Academic Press, New York, 1982. With permission.)...

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

On the other hand, in the solid-state high resolution 13C NMR, elementary line shape of each phase could be plausibly determined using magnetic relaxation phenomenon generally for crystalline polymers. When the amorphous phase is in a glassy state, such as isotactic or syndiotactic polypropylene at room temperature, the determination of the elementary line shapes of the amorphous and crystalline-amorphous interphases was not so easy because of the very broad line width of both the elementary line shapes. However, the line-decomposition analysis could plausibly be carried out referring to that at higher temperatures where the amorphous phase is in the rubbery state. Thus, the component analysis of the spectrum could be performed and the information about each phase structure such as the mass fraction, molecular conformation and mobility could be obtained for various polymers, whose character differs widely. 

Being acquainted with the structure of poly(a-olefin)s, one may reasonably explain some of the differences in their physicochemical properties. For example, isotactic polypropylene, the chains of which in the helical conformation can be closely packed, has rather a high density (0.92-0.94 g/cm3) and melting point (175°C) and is insoluble in low-boiling aliphatic hydrocarbons at boiling point. Syndiotactic polypropylene, consisting of chains in the form of binary helices, which cannot be packed so closely as in the previous case, has a density of 0.89-0.91 g/cm3 and a melting point of 135°C, which is 40 k lower from that of isotactic polypropylene syndiotactic polypropylene is also moderately soluble in... 

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 ( ). 

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

Syndiotactic polypropylene also exists as two crystalline modifications, the more stable one possessing a helical conformation and the other a planar zig-zag, A detailed analysis of the Raman spectrum confirms these structures and there is an excellent correlation between observed and predicted vibrational modes of the helical structure. Painter et al. have made a Fourier transform infrared spectroscopic study of isotactic polypropylene in the crystalline and amorphous state. The spectrum of the amorphous regions is broadly similar to that of the melt, but there is evidence for short sequences of helical segments in the amorphous phase, while the crystalline phase has longer sequences. This puts a different interpretation on the distinction between amorphous and crystalline regions than previously considered. 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

FIG. 6 Conformational structure of crystalline polypropylene (a) isotactic and (b) syndiotactic. Solid circles, CH3 open circles, CH2 circles with dot, CH. 

Now we report the results of a conformational analysis which was performed on isotactic and syndiotactic polypropylene many years ago (more refined calculations on isotactic polymers performed by us lately, will be reported in the lecture The crystalline structure of addition polymers. Research problems ). 

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