Stereoregular, conformation

V. STRUCTURE OF POLYPROPYLENE AND OF THE HIGHER POLYOLEFINS STEREOREGULARITY, CONFORMATION AND CRYSTALLINITY... 

Fig. 3 Energetically favorable stereoregular conformations of isotactic (/-PS) and syndiotactic (s-PS) polystyrene.

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

In concluding this discussion, it is important to point out that crystalline polymers can be polymorphic because of slight differences in the conformation of the helical disposition of stereoregular polymer chains the polymorphism is attributable to differences in the weak intermolecular bonds. This abstruse phenomenon (which does not have the same centrality in polymer science as it does in inorganic materials science) is treated by Lotz and Wittmann (1993). 

As an example we report in this paper the conformational energy maps of two already cited stereoregular polymers, which have been obtained very recently, syndiotactic polystyrene s-PS and syndiotactic polybutene s-PB (Fig. 4 and 5, respectively). In fact, the energy map calculated for s-PS shows... 

The difference in conformational energy between the two conformers of the trans-isomer is only 2.1 kJmol1, resulting in an equilibrium mixture of 30% of 29 and 70% of 30. Therefore, polymerization of 2 7 to a trans-1,4-tetrahydropyranoside polymer by direct displacement on the trialkyloxonium ion by monomer would be detected by -NMR as an axial acetal proton to equatorial acetal proton ratio (Hax)/(Heq) of 2.3. The most stereoregular polymer that obtained using 0.2 mol% of SiF4 at -78 °C possessed an (H /CHeq) ratio of 3.0. 

Chain Conformation, Crystal Structures, and Structural Disorder in Stereoregular Polymers... 

A particular kind of disorder, characterized by maintaining three-dimensional long-range periodicity only for some points of the structure, has been found in samples of syndiotactic polypropylene having a relatively low degree of stereoregularity.189 190 In these samples the chains present conformational disorder, which produces defects frozen in the crystals. 

Many studies, the first of which began shortly after the discovery of stereoregular polymerization of olefins, demonstrated that macromolecules could adopt stable helical conformations not only in the solid state but also in solution. These efforts have led to the realization that certain helical polymers reach a level of chiral recognition adequate for commercial development as an important aspect of chromatography. Researchers from the leading laboratory in this field, Okamoto, Yashima, and Yamamoto, have written Chapter 3 painting a detailed picture of the current status and future possibilities in this... 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

In the crystal state most stereoregular polymers have helical conformations. Group s(M/N) 1 comprises all the isotactic vinyl polymers [polypropylene, polybutene, polystyrene, etc., M/N = 3/1 poly-o-methylstyrene, etc., 4/1 ... 

A better knowledge of force constants, the use of more detailed physical models, and the availability of large computers and of new methods of calculation has permitted the prediction, within close approximation, of the frequencies and, to a lesser degree, the intensities of absorptions of stereoregular polymers in ordered conformations (helix, zigzag, etc.). In this way data on molec-... 

The presence of helical conformations in nascent vinyl polymers has often been advanced as an explanation for some of the characteristics of stereoregular... 

Munoz-Guerra also studied stereoregular polyamides fully based on d- and L-tartaric acid [73]. The bispentachlorophenyl esters of both 2,3-di-(9-methyl-tartaric acids (22 and 23) were condensed with (2S, 3S )-2,3-dimethoxy-l,4-butanediamine (41) to obtain optically active (PTA-LL) and racemic (PTA-LD) polytartaramides. Fiber-oriented and powder X-ray studies of these polyamides demonstrated that PTA-LL crystallized in an orthorhombic lattice, whereas PTA-LD seemed to adopt a tricUnic structure. In both cases, the polymeric chain appears to be in a folded conformation more contracted than in the common y form of conventional nylons. 

Figure 4.9 Stereoregular vinyl polymers in (a) the ideal zig-zag conformation, and (b) in the Fischer projection. If the termini of the chain are chemically different, all tertiary C atoms assume the same configuration and each isotactic chain becomes chiral. When the two chain termini are identical, each chain is superimposable with its mirror image. When R = methyl, we have polypropylene.

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