Polymer racemic

Optical compensation for polymers with chiral monomeric units may also occur when the racemic polymer consists of crystallites, each composed only of the rectus chains or only of the sinister polymer chains, and a same amount of optical antipode crystallites is present. This intercrystallite optical compensation211 has been found, for instance, in isotactic poly(propylene sulfide),212 poly ((3 -methy lpropiol ac (one),213 and poly(isopropylethylene oxide),214 where isochiral 2/1 helical chains are included in orthorhombic unit cells according to the space group P2 2 2. ... 

Pu reported the synthesis of axially chiral-conjugated polymer 82 bearing a chiral binaphthyl moiety in the main chain by the cross-coupling polymerization of chiral bifunctional boronic acid 80 with dibromide 81 (Equation (39)). The polymer is soluble in common organic solvents, such as THE, benzene, toluene, pyridine, chlorobenzene, dichloromethane, chloroform, and 1,2-dichloroethane. The polymer composed of racemic 80 was also synthesized, and the difference of characteristics was examined. Optically active polymer 82 was shown to enhance fluorescence quantum yield up to = 0.8 compared with the racemic 82 ( = 0.5). Morphologies of the optically active and racemic polymers were also compared with a systematic atomic-force microscopy (AEM). 

In this structural approach, and as already detailed in the paper referred to earlier [8], two major criteria may be used to characterize the building blocks of the lamellar crystals, namely the stems. They are the stem length and, for helical chiral but racemic polymers, the helical hand. 

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. 

The helical hand of the individual stems in chiral but racemic polymers is a very severe and therefore critical criterion in the crystallization process. The constraints apply for each stem and are dictated by the symmetry of the unit-cell. Contrary to the stem length (which, if incorrect, can be healed or adjusted by later structural reorganization), helix chirality involves a flip of a coin type of decision - right- or left-handedness. 

Signals are split into two closely located singlets and comparison of the C-NMR spectrum of the racemic polymer, obtained from the racemic monomer, with that of the optically active polymer (prepared from (+)-( R, 5S)-6,8-dioxabicyclo[3,2,l]oc-tane indicate, that the lower field, smaller peak of each signal pair comes from the D-L (syndiotactic) dyad. The hi er field signal can thus be ascribed to the dyad structures of D-D and L-L consecutive units (isotactic dyads). 

In the presence of catalysts such as ZnEt2/H20 or AlEt3 20, optically active 2-substituted -propio-lactones readily polymerize to give optically active stereoregidar polyesters exhibiting quite unique properties compared with the corresponding racemic polymers (Scheme 4). 

FIG. S.3 Optical appearance of polymer films (A) racemic polymer, P8 M (B) chiral polymer, P8 M. 

From the crystal structure it is concluded that, on irradiation, two different double bonds in the crystal react to give a linear hetero-adduct racemic polymer, in which a cyclobutane ring is in the 1,3-trans configuration (see Scheme II). 

This observation is of particular value when racemic polymers have physical... 

In addition, the crystal structures of both the racemic copolyamidc 11.7 and the cquimolccular mixture of the two configurationally homogeneous d- and i.-polyamides were studied and compared with that of optically pure 110.93 This study combined X-ray. electron microscopy, and 13C CP-MAS NMR measurements with computational methods. The two optically compensated and the optically pure polymers were shown to be highly crystalline systems the melting point of the racemic mixture was 250°C, considerably higher than those of the homopolymer (232°C) and the racemic polymer (226°C). The crystal structure of the racemic mixture could be... 

Lastly, the lactone was formed from the amino acid by Reaction (4). The resolution of the amino ester was done via the formation of diastereo-isomeric salts with (+) or (-)-dibenzoyltartaric acid, as described by Testa et al. The amino ester chiral precursor was shown to have an enantiomeric excess of 80%. It was then assumed that racemizatlon did not occur during the following steps of the monomer synthesis or during the polymerization. This assumption was indirectly verified by the measurements of a melting point of 260 C for the optically active polymer as compared to 11 0 C for the racemic polymer. 

DSC analysis of the optically active poly-L-malolactone benzyl ester also showed multiple endotherms in the temperature range of 140-160regardless of the catalyst system used to prepare the polymer. The difference in the melting points of the optically active and racemic polymers could be due to differences in either the degree of stereoregularity or the molecular weights of the polymers. 

One example of such a polymer-encoded self-assembly process was demonstrated for BaCOs crystallization in presence of a stiff phosphonated DHBC [353]. Here, the selective adsorption of the stiff DHBC onto the (110) witherite surfaces leads to the tectonic arrangement of the elongated orthorhombic BaCOs by programmed self-assembly resiflting in remarkable helical structures from a non-chiral mineral crystal system and a racemic polymer (Fig. 28a). The amount of left- and right-handed helices was foimd to be similar [353]. 

In the as-prepared form, after precipitation from solution, both types of polymers appeared to be highly crystalline by this method of analysis, and both were comparable in this property to polypivalolactone, which is known to be a very highly crystalline polyester. In addition, both the optically-active and racemic polymers had considerably higher degrees of crystallinity than those previously observed for racemic poly-o-methyl-o-propyl-B-propiolactone (1). Also of importance, in addition to the different x-ray diffraction patterns of the racemic and optically-active polymers, was that the racemic polymer did not readily crystallize from the melt in the DSC characterization while the optically-active polymer of high optical purity did. Hence, the higher stereoregularity also imparts a more favorable rate of crystallization to the polymer as would be expected. 

Tsuruta (2) had proposed that the presence of chains of different tacticities is due to active sites of various stereospecificity. There is probably a full spectra of sites in the initiator with a more or less pronounced R and S character. The sites with pure R and S character produce isotactic fraction. This hypothesis which satisfactoraly explains most of the results was substantiated by optical resolution of racemic polymers. Thus, fractions of one sign were isolated from polymethyl-oxirane by preferential complexation (28) and fractions of both sign of very low optical activity were obtained using sucrose (29). 

When compared, optically pure and racemic polymers reveal some significant differences in their properties such as crystallization, solubility or crystalline structure. The way of racemiza-tion of a polymer could be realized either by intercrystallite compensation, as in the case of polymethylthiirane (56) or by formation of a racemic lattice, as observed for monomers with bulky substituents such as t-butylthiirane (57). The properties of the racemic polymer are then very different of those of the optically pure one, for example, the melting points could differ of more than 50 C. A similar behaviour was recently observed in the case of substituted 6 propiolactones (58-60). Therefore the preparation of pure optically active polymers remains of inte-... 

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