13C NMR Analysis of Carbon-13 Enriched Ethylene Copolymers with Propylene

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Abstract
Carbon-13 nuclear magnetic resonance (13C NMR) has been used rather extensively in the characterization of ethylene-propylene copolymers and terpolymers. Chemical shift assignments on these copolymers were first made by Carman and revised only slightly by him recently. Despite all the work that has been done to assign the lines in the 13C NMR spectrum to the various sequence structures in ethylene-propylene copolymers, ambiguity still remains in some of the assignments, such as the assignment of the Sαβ line. It still is not known whether it is attributable entirely to two propylenes with one of them inverted, to an ethylene sandwiched between two propylenes with one of them inverted, or to an equal population distribution of these two structures. Carman and coworkers state that inversion of propylene in their copolymers is certain, but because of the large error in the calculation, the degree of inversion can be between 10 and 40% of the propylene content. His suggestion is that the Sαβ line is attributable to a combination of the two structure forms indicated above. Tanaka6 also makes this same suggestion in his 13C NMR work. He based his conclusion on some earlier infrared work. Most recently Randall describes a sequence length distribution method in which he estimates that the total amount of propylene inversions is the order of 20–30% for a concentration range of about 50–60 wt% propylene in copolymers with ethylene. Ray does not have to deal with the problem, as his 13C NMR spectra show no evidence of propylene inversions. His copolymers were prepared with an aluminum-titanium catalyst. Ours as well as Carman's were prepared with an aluminum-vanadium catalyst. Our reason for undertaking this program was to aid in the assignment of the Sαβ line. For this effort we prepared a series of ethylene-propylene copolymers that are carbon-13 enriched in both carbons of the ethylene, and we analyzed them by 13C NMR.

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