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=CC6H5)(2,5-norbornadiene)[P(C6H5)(3)](2) initiator were investigated by solid-state NMR. The C-13 Pake doublet of the poly(phenylacetylene) containing 5% [1,2-C-13]phenylacetylene was observed by effectively removing the chemical shift anisotropies using a composite inversion pulse sequence. The C-13-C-13 bond distance was determined to be 1.386 Angstrom 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 C-13 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 C-13 CPMAS exchange experiments revealed conformational inhomogeneity of the main chain and the presence of slow pi-flip motion of the phenyl ring.