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

Abstract

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 13C Pake doublet of the poly(phenylacetylene) containing 5% [1,2-13C]phenylacetylene was observed by effectively removing the chemical shift anisotropies using a composite inversion pulse sequence. The 13C13C 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 13C 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 13C CPMAS exchange experiments revealed conformational inhomogeneity of the main chain and the presence of slow π-flip motion of the phenyl ring.