Poly high optical purity polymers

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. 

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. 

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 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. 

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. 

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. 

Chiral separation is an analytical technique for evaluation of enantiomeric purity of chiral compounds. Such property also found in the optically active polymers. Polyelectrolyte multilayer (PEMU) is an example for such property and made up by polypeptides, such as L-and D-poly(lysine), poly(glutamic acid), poly(iV-(5)-2-methylbutyl-4-vinylpyridininum iodide), poly(styrene sulfonate) etc. PEMUs allow at very high enantiomer permeation rates for chiral membrane separations [139-141]. 

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). 

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