Poly polymer optical purity

The third method (106), which has been so far adopted only to produce optically active polymers from poly-a-olefins obtained from racemic monomers, is of great interest, owing to its simplicity and to the relatively high optical purity attained in one step. 

An independent proof of this finding was recently obtained by Ciardelli, Benedetti, Pieroni and Pino (23, 24) who prepared an atactic poly-(S)-4-xnethyl-1-hexene having [M] >5 = +190 from a poly-(S)-4-methyl-l-hexyne having an optical purity of about 89.5% as demonstrated by the optical purity of (S)-3-methyl-pentanoic acid obtained by ozonization of the unsaturated polymer. The rotatory power of the polymer thus prepared is very near to the rotatory power of the non crystallizable poly-(S)-4-methyl-l-hexene obtained from a monomer having a 93% optical purity (see Table 8). 

These experimental findings suggest that the poly-a-olefins obtained in the presence of the conventional stereospecific catalysts should have the same optical purity as the monomers. Therefore, in the polymer of a (S) monomer having high optical purity, the remarkable differences in optical activity observed in the fractions having different stereoregularity cannot be attributed to the presence of different amounts of (R) asymmetric carbon atoms in the lateral chain, formed by racemization of the monomer during the polymerization. 

In these isotactic polymers, the optical purity of the monomer affected the optical activity via the relationship to the excess helical sense of the polymer (Figure l).47 In the case of isotactic poly[(5)-4-methyl-1-hexene] (2) and poly[(A)-3,7-dimethyTl-octene] (3), an increase in the optical purity of the monomers resulted in an increase in the optical activity of the polymers in a nonlinear fashion the optical activity of the polymers leveled off when the optical purity of the monomer reached ca. 80%. In contrast, in the case of isotactic poly[(5)-5-methyl-l-heptene] (4), the... 

The molar optical rotation of optically active it-poly(a-olefins) depends not only on the wavelength and temperature, but also on the optical purity of the monomers (and thus, also, of the polymers) (Figure 4-23). The molar optical rotation of these polymers remains constant when the optical purity of the monomer is high. 

The molar optical rotation of configurational copolymers of (S) and (R) isomers of the same monomer is generally, in the case of poly(a-olefins), a hyperbolic and not a linear function of the optical purity of the monomers. Thus, the molar optical rotation of the copolymers is always greater than that obtained by additivity rules. Whether this is caused by tactic blocks in the polymers or by mixtures of (S) and (R) unipolymers has not been established yet. 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

Di Lorenzo, M.L., Rubino, P., Luijkx, R., and Helou, M. (2014) Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 1 effect of optical purity of the monomer. Colloid Polym. Sci., 292, 399 -409. 

It is the purpose of this article to review studies which have been carried out along these lines in the laboratories of the authors, at Universities Pierre et Marie Curie (France) and Laval (Canada). For the sake of completion, studies carried out in other laboratories will also be mentioned. More specifically, the methods of synthesis used for a- and 3-substituted poly(3-propiolactones) will be discussed. It will be seen that the synthesis of the corresponding high optical purity monomers is particularly difficult, and tedious. The principal thermal properties of the polymers obtained will also be discussed, along with the properties of racemic mixtures obtained from two isotactic polymers having the same chemical structure but different chiralities. 

In contrast to di-a-substituted poly(B-propiolactones), racemic (atactic) b-substituted poly(3-propiolactones do not crystallize. However, crystallization is found with the isotactic polymer chains, and those prepared from monomer of optical purities larger than 70-80%, depending upon the monomer considered. 

Kricheldorf, H.R., Serra, A., 1985. Polylactones. 6. Influence of various metal-salts on the optical purity of poly(L-lactide). Polymer Bulletin 14, 497—502. 

Pure poly lactic acid (PLA) is a semi cry.stalline polymer whereas polymers prepared from meso and racemic lactides are amorphous. The melting temperature (T ,), degree of crystallinity and solubility are dependent on the MW, thermal history, optical purity and the copolymer ratio. 

F. Matsuoka, K. Hashimoto, Poly(lactic acid)-type conjugate staple fibers with good biodegradabihty and heat-bonding properties consisting of two types of lactic acid polymers with different optical purity and nonwoven fabrics therefrom and manufacture thereof, JP 2001049533,2001, CAN 134 179858. 

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. 

The commercial forms of PLA are the homopolymer poly(L-Lactide) (L-PLA or PLLA) and the copolymer poly(D,L-Lactide) (D,L-PLA or PDLLA), which are produced from L-lactide and D,L-lactide, respectively. The L-isomer constitutes the main fraction of PLA derived from renewable sources, since the majority of lactic acid from biological sources exists in this form [43]. Polylactides (PLAs) exhibit different properties depending on the D/L unit radio and sequence distribution. Generally, the crystallinity of PLLA and PDLA decreases with increasing racemic content. PLA polymers with an L-content >90% tend to be semicrystalline, while those with a lower optical purity are generally amorphous [43-45]. 

Other examples of stereoregular polymers obtained by chiral monomers having high optical purity are chiral poly-l-alkynes (XII) (181) and polyisocyanides (XIII (182), XIV (183)), although the configuration of the double bonds present in the macromolecules has not been thoroughly investigated up to now. 

L-4 Isopropyl 2-oxazoline was synthesized from L-valinol by the slight modification of the reported method.17 xhe polymers with different optical purity were synthesized by copolymerizing the L-monomer with the racemic monomer, followed by the reduction with LAH(PDL series). The copolirmers with N-methylethylenimine unit were also prepared by the copolymerization with 2-oxazoline (PUL series). For comparison, poly(L-isopropylethylenimine) and poly(N-methylethylenimine) were also prepared by the polymerizations of L-isopropylethylenimine and 2-oxazoline, respectively. 

Poly-/-lactic acid has been extensively studied. By cationic polymerization of Mactide, a highly crystalline isotactic polymer can be obtained with Fridel-Craft initiators, or better with zinc or lead oxide ones [130]. The basicity of the catalyst seems to have a substantial effect on the optical purity of the polymer obtained. A stereochemical study is reported by Schultz and Schwaad [131] on a poly-5-lactic acid (LII). 

The former method has been used in the case of poly-a-olefins which, due to their paraffinic structure, can be cleaved only under very drastic conditions. As a result of many experiments with monomers having different starting optical purity and with different conversions to polymer, it was shown that the unreacted monomer has the same optical purity as the starting one [24] (Table III). Analogous results were obtained in the case of vinylethers derived from both primary and secondary optically active alcohols (Table IV)... 

Fig. 1. Molar optical rotation ([0] vs polymerized monomer optical purity of isotactic vinyl polymers having the asymmetric carbon atom in the oc and 3 position to the main chain. -O poly-(5)-4-methyl-l-hexene - t- poly-(5)-3,7-dimethyl-l-octene poly- (5)-l-methylpropyl]-vinyl ether.

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