Poly . frustrated structures

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

Similar frustration has been evidenced for the crystal structure of isotactic poly(2-vinylpyridine)150 161 (Figure 2.28). Also in this case three independent threefold helices are included in the trigonal unit cell and the frustration is related to a different azimuthal orientation of the chains due to the different interactions between the chains.160 Two chains (chains A in Figure 2.28) maximize their interactions at the expense of the third one (chain B). 

Finally, we have attempted to evaluate the possible impact of an intermediate liquid crystalline phase and the possibility of transfer of helical hand information from the melt to the crystal throughout this process. Assuming that the melt is structured, the melt of chiral but racemic polyolefins would be made of stretches of helical stems that are equally partitioned between left- and right-handed helices. Formation of antichiral structures (such as in a iPP) could be interpreted as indicating a possible transfer of information (but the problem of the sequence of helical hands would still remain). This analysis is, however, ruined by the observation that many of these polymers also form chiral structures (frustrated p phase of iPP, Form III of iPBul). For the achiral poly(5-methyl-pentene-l), the chiral, frustrated phase is actually the more stable one, and can be obtained by melting and recrystallization of a less stable antichiral phase. 

Stocker, W. et al. (1998). Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure Isotactic Poly(propylene), p Phase. Macromolecules, Vol.31, Issue 3, pp. 807-814. 

In addition to the side chain conformation, chemically different side chains attached to the same main chain may also have a pronounced impact on the main chain conformation, particularly if the respective side chains are incompatible. Demixing of the side chain is hampered by the fact that the chemically different side chains are bound to the same main chain, leading to highly frustrated single chain structures. In order to experimentally address this point, the phase separation within statistical cylindrical brush copolymers comprising PMMA and poly-2-vinylpyridinium (PVP) side chains was investigated [93, 94]. The samples were prepared by radical copolymerization of methacryloyl end-functionalized PMMA M = 3,700 g/mol) and PVP M = 5,100 g/mol) macromonomers. Copolymer brushes with two different compositions were synthesized and characterized as shown in Table 1. Subsequent quatemization of the PVP side chains with ethylbromide was conducted in order to enhance incompatibiUty. 

Stocker, W Schumacher, M. Graff, S. Thierry, A. Wittmann, J.-C. Lotz, B. Epitaxial crystallization and AFM investigation of a frustrated polymer structure Isotactic poly(propylene), y phase. Macromolecules 1998, 31, 807-814. 

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