SOLID-STATE NMR INVESTIGATION OF POLYMORPHISM AND STEREOCOMPLEX FORMATION IN OPTICALLY-ACTIVE POLYESTERS

Solid-state C-13 NMR spectra are reported for samples of poly(alpha-methyl-alpha-ethyl-beta-propiolactone) (PMEPL) differing in tacticity and thermal history. Two types of spectra are obtained, distinguished primarily by the chemical shifts of the alpha-methyl and main-chain methylene carbons. These resonances appear at 23.4 and 66.1 ppm, respectively, in the spectrum of melt crystallized isotactic PMEPL, and at 18.1 and 74.1 ppm in a variety of other samples, including films of the isotactic polymer prepared by solution casting. The peak displacements are interpreted in terms of conformational changes, both in the alpha-ethyl group and the chain backbone. By comparison to the spectrum of oriented films, it is concluded that isotactic PMEPL crystallizes from the melt in the extended chain conformation. This conclusion is supported by infrared spectroscopy. Solid-state NMR spectra are also reported for atactic PMEPL and the stereocomplex that is formed in mixtures of the two isotactic polymers of opposite absolute configuration. These spectra resemble that obtained for solution cast films of the isotactic sample and are attributed to the 2(1) helical conformation. Although NMR spectroscopy indicates that atactic PMEPL and the stereocomplex share similar conformations, X-ray diffraction powder patterns reveal different crystal structures. NMR spectra of poly(alpha-methyl-alpha-n-propyl-beta-propiolactone) (PMPPL) suggest that both the atactic and isotactic polymer crystallize in a helical conformation.