Syndiotactic structures helical conformation

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

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... 

Several works have shown that the aggregation of isotactic and syndiotactic chains leads to the formation of stereoeomplexes for which the iso/syndio stoichiometry is found equal to 1/2, probably with a structure composed of a double-stranded helix of a 30/4 helicoidal isotactic chain surroxmded by a 60/4 helicoidal syndiotactic chain. Syndiotactic PMMA self-aggregates exhibit similar structures, with conformations close to extended chains. Experimental data indicate that, in self-aggregated syndiotactic PMMA in solution, some of the ester groups are close in contact, probably in a double helix slructure with solvent molecules included in the cavities of inner- and inter-helices. Isotactic PMMA self-aggregates also exhibit conformational helix structures. 

The protective effects of syndiotactic and isotactic poly-2-vinylpyridine-l-oxide remarkably differ in the spatial arrangement of the structural units (Holt et al. 1970). The isotactic polymer has probably a helical conformation. When the cultures... 

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. 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

In the case of syndiotactic polymers with chains in helical conformation, as for sPP, the symbol s(2/l)2 indicates that neighboring structural units are repeated through the operation of twofold axes perpendicular to the chain axis and each pair of units is repeated according to a helix containing two pairs of monomeric units in one pitch. A line repetition group for sPP in the fully extended trans-planar conformation is tcm. In this case the repeating unit comprises two monomeric units [20]. 

A general example is the monosubstituted vinyl polymers (CH2—CHX) , which can exist in two possible stereoregular forms — isotactic (substitution on the alternate carbons on the same sides of the chain) and syndiotactic (substitution on the alternate carbons on the opposite sides of the chain). These ordered stereoregular forms can assume either planar or helical conformations. Each of these ordered structures has well-defined, unique selection rules for IR and Raman activity. These various structures can be distinguished on the basis of spectral properties without a detailed knowledge of the molecular motions or energies, that is, without normal coordinate analysis. 

The highly stereoregular syndiotactic polystyrene (s-PS) has received considerable interest recently. Depending on the thermal history, s-PS possesses several polymorphie erystalline structures. Helical (TTGG) conformation is formed in solution reeovered s-PS, whereas the thermodynamically favored all-trans (TTTT) eonformations are formed from either melt or annealing at an elevated temperature. The extent to which thermal history affects the crystalline strueture and erystallization kinetics for s-PS has been extensively studied. ... 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

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