Monomer Sequence Distribution in Ethylene Propylene Elastomers. I. Measurement by Carbon-13 Nuclear Magnetic Resonance Spectroscopy

The carbon-13 chemical shifts of ethylene propylene copolymers were found to be very sensitive to monomer sequence distribution. Methylene resonances were interpreted in terms of methylene sequence length and tertiary carbon resonances were interpreted in terms of propylene centered pentad sequences. Propylene inversion was detected and measured quantitatively in the spectra. A formula was derived for calculating r1·r2, which is independent of monomer feed, and which is based on measuring contiguous and isolated propylene sequences in the copolymer. The interpretations are shown to be consistent for copolymers containing 26, 34, and 62 mole% propylene. The r1·r2 products were determined for each of these polymers. Calculation of copolymer composition based on the 13C chemical shift assignments gave excellent agreement with the compositions as determined by infrared and 1H NMR. A formula was derived, based on copolymerization theory, for calculating the reactivity ratio product, r1·r2, directly from the copolymer composition (% ethylene) and the ratio of contiguous to isolated propylene sequences. Using this formula an average value for r1·r2=0.42±0.02 was determined for the copolymers made with vanadium acetylacetonate-diethylaluminum chloride catalyst system.