Solid-state sup 13 C NMR study of poly(3,3-diethyloxetane).

Poly(3,3-diethyloxetane) (PDEO) crystallizes in two different forms depending on the temperature of crystallization. In the higher melting form I crystals the PDEO chains adopt the all trans T sub 4 conformation, while the T sub 2 G sub 2 conformation is found in the lower melting form II crystals. High resolution sup 13 C NMR spectra of both crystal forms have been obtained by employing cross polarization with high power proton decoupling and magic angle sample spinning. Chemical shifts observed for both the backbone and side-chain methylene carbon resonances differ by 4-6 ppm between the two crystal forms, while the quaternary and methyl carbon resonances are not as significantly affected. The sup 13 C NMR spectra of PDEO forms I and II are discussed in terms of the differences in their polymer chain conformations (T sub 4 vs. T sub 2 G sub 2), with primary emphasis placed upon gamma-gauche shielding effects on the sup 13 C resonances.