Polypropylene optical activity

Are syndiotactic or isotactic forms of polypropylene optically active ... 

Should be distd under reduced pressure under nitrogen and stored in the dark. Purified via the nitrosochloride [Waterman et al. Reel Trav Chim Pays-Bas 48 1191 7929]. For purification of optically active forms see Lynn [J Am Chem Sac 91 361 1919]. Small quantities (0.5mL) have been purified by GLC using helium as carrier gas and a column at 90 packed with 20 wt% of polypropylene sebacate on a Chromosorb support. Larger quantities were fractionally distd under reduced pressure in a column packed with stainless steel gauze spirals. Material could be dried with CaH2 or sodium, and stored in a refrigerator CaS04 and silica gel were not satisfactory because they induced spontaneous isomerisation. [Bates, Best and Williams 7 C/iem Soc 1521 7962.]... 

Poly(phenylenethylene), dendronized, 522 Poly(phenylenevinylene) optically active, 510-511 synthesis of, 495-496 Poly(/ ara-phenylenevinylene)s, 472 Polypheny lquinoxaline (PPQ) hyperbranched, 312-314 synthesis of, 309-313 Polyphosphoric acid, 333 Polypropylene oxide) polyol, 223 Polypropylene polyols, 220 Poly (pyridine), synthesis of, 503-505 Polyquinoxaline (PQ), synthesis of, 309-313... 

Do you see any way to make a stereoregular polypropylene displaying optical activity ... 

Fukada,E. Piezoelectric effect and its temperature variation in optically active polypropylene oxide. Nature 221, 1235 (1969). 

Another example is illustrated in the relationship between the specific rotation and the microstructure of polypropylene oxide reported by Price. Optically active propylene oxide and racemic propylene oxide-a-d were polymerized under otherwise identical conditions by the freeze-dried ZnEt2-H20 (1 0.7) catalyst system containing varying amounts of ZnEt2. A linear relationship was observed between specific rotation of the former polymer and the tail-to-tail dyad content of the latter (Fig. 14). This result proves quantitatively that the decrease in the specific rotation of polymer prepared by several catalysts is due to the presence of head-to-head and tail-to-tail linkages, and also provides supporting evidence for our microstructure analysis. 

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

Olefin Polymers Isotactic polymers of propylene and 1-butene obtained by optically active metallocene catalyst (145) have been reported to show large specific rotation in suspension ([a]D-123°, -250° for polypropylene [a]D+130° for polybutene), which was lost when the polymers were completely dissolved or heated [176,177]. The optical activity was ascribed to a helical conformation of the polymer chain with preferential screw sense. 

The chiral complex EBTHI—Ti is an excellent chiral catalyst [40]. This complex is a derivative of titanocene and used as the Kaminsky catalyst, which has brought epoch-making progress in polypropylene production. The chiral bridged titanocene complex is used for the production of optically active polypropylene arising from the helical structure of the polymer chain. The chiral complex also behaves as an excellent... 

It is important to note that high molecular weight trans-isotactic poly(methy-lene-1,3-cyclopentane) contains no mirror or mirror glide planes of symmetry and is thus chiral by virtue of its main chain stereochemistry (it exhibits optical activity) this is in contrast to high molecular weight polypropylene and other poly(a-olefin)s, which contain an effective mirror plane perpendicular to the molecular axis in the middle of the molecule and are thus achiral [30,497],... 

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

Normal-mode analyses for polymers other than polyethylene (and the n-paraffins), for which neutron scattering data are available, have not been carried out in sufficient detail to yield complete phase-frequency relations. Calculations of the optically active phases of isotactic polypropylene have been completed by Miyazawa and co-workers 19) and by ScHACHTSCHNEiDER and Snyder 31), but these treatments neglected intermolecular forces, which could have a significant effect on the low-frequency modes observed by neutron scattering. The situation is similar for polytetrafluoroethylene, for which calculations are available for isolated chains in a planar zig-zag, rather than a helical, conformation (/5). 

Table 1. Comparison of the calculated optically active frequencies (in cmr -) of the fundamental vibrational modes for an isotactic helical polypropylene chain with the observed hands in inf raved and neutron spectra...

Not only polyethylene can be synthesized, but also many kinds of copolymers and elastomers, new structures of polypropylenes, polymers and copolymers of cyclic olefins. In addition, polymerization can be performed in the presence of fillers and oligomerization to optically active hydrocarbons is possible. For recent reviews and books see [17-20]. 

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

Problem 2.10 Explain why polypropylene of relatively high molecular weight is optically inactive despite having an asymmetric center at every other carbon, while, on the other hand, polyfpropylene oxide) is optically active. 

Answer Optical activity is influenced only by the first few atoms around an asymmetric carbon (C ). For the two sections of the main chain, these will be identical regardless of the length of the whole polymer chain. The carbons marked C in (Xlll) are thus not truly asymmetric and are termed pseudoasymmetric or pseudochiral carbons. Only those C centers near the ends of a polymer molecule will be truly asymmetric, but since there are too few chain ends in a high molecular-weight polymer such centers do not confer any significant optical activity on the molecule as a whole. Polypropylene is thus optically inactive. 

Much of the work on the conformation of polymers concerns itself with reactions of the lower olefins. By the use of certain catalytic systems, crystalline syndiotactic polypropylene may be formed exclusively, rather than as a co-product in the formation of the isotactic macromolecule. To accomplish this, rather specific reaction conditions are required. For example, a catalytic system of approximately five molar equivalents of aluminum dialkylmonochloride to one of vanadium triacetylaceonate at less than 0°C initiates the stereospecific polymerization of propylene to a syndiotactic resin. Other catalytic systems have also been described [108]. As discussed above, these polymers are not expected to be optically active. 

Of the three stereoisomers, atactic polypropylene was the first to be synthesized. This is because it is the atactic structure that is obtained by conventional polymerization of the monomers when no optically active catalyst is used. It was only when the Ziegler-Natta catalysts were introduced in the 1950s that the production of stereoregular polymers became possible. It is interesting to note that polypropylene did not find any commercial use until the Ziegler-Natta catalysts became available. In fact, the atactic structure that was originally produced leads to a viscous liquid at room temperature of limited use. Isotactic polypropylene instead is one of the most important commercial polymers. 

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