Solid-State NMR Study of Poly(phenylacetylene) Synthesized with a Rhodium Complex Initiator

The polymerization mechanism and the structure of poly(phenylacetylene) synthesized using the Rh(C⋮CC 6 H 5 )(2,5-norbornadiene)[P(C 6 H 5 ) 3 ] 2 initiator were investigated by solid-state NMR. The 13 C Pake doublet of the poly(phenylacetylene) containing 5% [1,2- 13 C]phenylacetylene was observed by effectively removing the chemical shift anisotropies using a composite inversion pulse sequence. The 13 C 13 C bond distance was determined to be 1.386 Å from the spectrum, which corresponds to a double bond length. This result clearly shows that the polymerization by the Rh initiator progresses by the cis-insertion mechanism. The 13 C isotropic chemical shifts of the protonated and nonprotonated olefin carbons were deduced to be 131.8 and 141.4 ppm, respectively, from differences between some overlapped CPMAS spectra. Two-dimensional 13 C CPMAS exchange experiments revealed conformational inhomogeneity of the main chain and the presence of slow π-flip motion of the phenyl ring.