Syndiotactic polypropylene conformational energy minima

Figure 2.4 Line repetition symmetries, according to equivalence principle, and corresponding possible conformations, according to minimum conformational energy, for (a) isotactic and (b) syndiotactic polypropylene (T = trans, G = gauche).

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.)...

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. 

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