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13C NMR spectra have been measured for some comblike polymers obtained by coordination polymerization of higher epoxides such as octadecyloxirane and decyloxirane. Backbone methine and methylene resonances of the high-melting polymer fractions appear as single unsplit peaks, thus indicating a high degree of stereoregularity (isotactic structure). In contrast, strong tacticity effects have been generally observed for the backbone resonances of the low-melting polymer fractions. These effects have been interpreted in terms of different stereosequences. The complete assignment to the triads mm, mr (rm), and rr and comparison with the spectra of poly(methyloxirane) and anionically prepared poly(octadecyloxirane) allowed us to conclude that the low-melting polymers are atactic and contain substantial amounts of abnormal (head-to-head and tail-to-tail) linkages. Measurement of Tx of the main-chain carbons and NOE showed their independence of the tacticity. Moreover, the length of the hydrocarbon side chain does not substantially affect the backbone relaxation. As for the side chains, only the T! values of the interior atoms showed a marked tacticity effect. Experimental dynamic parameters have been interpreted either on the basis of a single correlation time or with a model based on restricted motion on a diamond lattice.

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Previous papers from this laboratory') have shown that poly(4-biphenylyl acrylate)" (2a) is characterized by a smectic-like structure in the solid state and displays properties usually considered typical of highly crystalline polymers. It has also been demonstrated that the long-range smectic organization of the macromolecules of 2a takes place independent of the stereoregularity, although the smecticisotropic transition (which, for this polymer, presents all the features of a melting phenomenon) is observed at lower temperatures for isotactic samples (ca. 220 "C) than for atactic ones (ca. 270°C).Since it is known that biphenyl derivatives containing only one, reasonably small, para-substituent are not mesogenic, it is understandable that 4-biphenylyl acrylate