Carbon-13 NMR determination of poly(propylene oxide) microstructure.
01 January 1986
The ring opening polymerization of propylene oxide can produce poly(propylene oxide) (PPO) with a broad range of microstructures. Depending on the optical purity of the cyclic propylene oxide monomer and the mode of ring opening produced by the catalyst, it is possible to obtain stereoregular, crystalline PPO with no Head-to-Head:Tail-to-Tail (H-H:T-T) additions, stereoirregular, amorphous PPO with substantial H-H:T-T content, and PPO's with intermediate degrees of microstructural complexity. In the present study we employ (13)C NMR spectroscopy to identify the various microstructures present in commercial PPO samples. Our microstructural identifications are materially aided by the multiple pulse (13)C NMR INEPT and DEPT techniques, which permit separate observation of methyl, methylene, and methine carbon resonances. In addition, our ability to predict the relative (13)C chemical shifts expected for the carbons in normal (H-T) and defect (H-H:T-T) PPO units via the gamma-gauche effect method, permits a detailed assignment of H-T, H-H:T- T, and end group resonances and microstructures in PPO.