Syndiotactic polypropylene polymorphism

A case of polymorphism which is strictly analogous to that of Kevlar seems to be present in syndiotactic polypropylene (s-PP). In fact, depending on the... 

In syndiotactic polypropylene, the methyl groups are well separated and the TT form is favoured, but there are other energy minima among the gauche conformations and TT/G G and TT/G G sequences can generate left- and right-handed helices, respectively, where the repulsions are minimized (Boyd and Phillips, 1993). The chains may crystallize both in the TT and in the TTG G form, so syndiotactic polypropylene is polymorphic. 

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]. 

Keywords epitaxy, isotactic polypropylene, syndiotactic polypropylene, crystal structure, crystal polymorphism, helical hand, nucleating agents, AFM, a-phase, 3-phase, lamellae. 

Sozzani, R, Simonutti, R. and Comotti, A. (1994) Phase structure and polymorphism of highly syndiotactic polypropylene. Magn. Res. Chem., 32, S45-S52. 

Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior (the first part of this section on s-PP modifications is reported in the publication [203-212]). s-PP chains crystallize to various polymorphic crystalline phases and,... 

As an example, syndiotactic polystyrene (s-PS) [76] can be crystallized in two conformationally distinct polymorphs, form I, with all-trans, planar zig-zag chains, and form II, where the chains adopt the helical... ttggttgg... conformation, very similar to that observed [77] in syndiotactic polypropylene (s-PP) crystals. CPMAS/DD NMR spectra of both s-PS polymorphs are shown in... 

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. 

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

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... 

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