Racemic polymer, crystalline

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. 

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... 

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. 

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-... 

Palade et al, 2001). The stereo-isomeric l/d ratio of the lactate unit association influenced PLA properties (Tsuji and Ikata, 1992). There are three types of PLAs because there are two stereoisomeric forms of lactic acid, poly (levo-lactic acid) and poly (dextro-lactic acid), which are both semi-crystalline and have identical chemical and physical properties. Poly (D,L-lactic acid) or poly (meso-lactic acid), a racemic polymer obtained from an equimolar mixture of D- and L- lactic acid, is amorphous, with weak mechanical properties. The... 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

The racemic poly(DL-lactide) DL-PLA is less crystalline and lower uelting than the two stereoregular polymers, D-PLA and L-PLA. Further, the copolymers of lactide and glycolide are less crystalline than the two homopolymers of the two monomers. In addition, the lactic acid polymer, because of the methyl group, is more hydrophobic than the glycolide polymer. 

Similarly the term isotactic was applied by Price and Osgan (78) to the crystalline polymer obtained from optically active and racemic propylene oxide. The zigzag and Fischer representations of an isotactic poly(propylene oxide) are shown in 36 and 37 (Scheme 7). Their different appearance is due, as already explained in a similar case, to the odd number of chain bonds existing in each monomer unit. Formula 36 presents alternately substituents on both sides of the chain and is very similar to the actual structure observed in the crystal state (79). 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The polymer of high molecular weight in the solid stage exhibited high crystallinity under a polarized microscope and insoluble in common organic solvents. When the polymer with high optical rotation was used as stationary phase or sorbent for the chromatographic resolution of racemic compounds, it showed the ability of resolution for many kinds of compounds, such as alcohols, amines, esters, and even hydrocarbons (28). 

In the next year, Price and his coworkers (6,7) found that the crystalline polymer obtained by Pruitt and Baggett was isotactie. The fact that the crystalline polymer obtained from racemic monomer with the iron catalyst had the same X-ray pattern as the optically active crystalline polymer obtained from the optically active monomer under the same condition showed that these polymers were isotactic, and that the asymmetric carbon atoms in this polymer had the same configuration as in the monomer from which it was derived, i.e., propylene oxide polymerized with retention of configuration of its asymmetric carbon atom. 

Table 6. Melting point and crystalline structure of the most stereoregular fractions of polymers of some optically active and racemic a-olefins... 

I. R. spectra of polymers of optically active and racemic monomers (12) having similar stereoregularity are identical in the case of poly-5-methyl-l-heptene, but slightly different in the case of poly-3-methyl-l-pentene and poly-4-methyl- 1-hexene. A very characteristic crystallinity band has been found in the I. R. spectrum of poly-5-methyl-l-heptene at 12.06 fi bands which seem connected with stereoregularity have been found in the I. R, spectrum of poly-4-methyl-l-hexene at 10.06 fi the nature of these bands has been proved when preparing a practically atactic sample by hydrogenation of poly-4-methyl-l-hexyne (24). 

The polymers have isotactic structure with helix conformations in the solid state (78) polymers of optically active and racemic (1-methyl-propyl)-vinyl-ether seem to have the same crystalline structure (Table 12). 

Investigations were made on the I. R. spectra of poly-[(S)-l-methyl-propylj-vinyl-ether and poly-[(S)-2-methyl-butyl]-vinyl-ether (65) a band at 911 cm-1 related to stereoregularity was detected in the spectrum of the former while a crystallinity band at 827 cm-1 was found in the latter. The spectra of polymers obtained from an optically active or a racemic monomer did not reveal any remarkable difference. 

The discovery that certain catalysts are capable of producing crystalline polymer from optically inactive (racemic) monosubstituted epoxides has caused a good deal of excitement in recent years because it seems virtually certain that such crystallinity must originate from an asymmetric synthesis of the polymer chain (24). 

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. 

The polymerisation of racemic propylene oxide with coordination catalysts leads to a polymer that can be fractionated into crystalline and amorphous polypropylene oxide)s ... 

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