Sequential polymerizations of first propylene and then ethylene, propylene mixtures with the same Ziegler-Natta catalyst system produce in situ blends known as high-impact polypropylenes. Over 100 high-impact polypropylenes are characterized in terms of weight fractions and sequence distributions for isotactic polypropylene, atactic polypropylene, an amorphous ethylene propylene copolymer, and a crystalline ethylene propylene copolymer. The apparent r(1)r(2) behaviors of the E/P copolymers suggest that the amorphous and crystalline E/P copolymers principally arise from different types of catalyst sites as opposed to originating strictly from compositional heterogeneities. The amorphous copolymers consistently have r(1)r(2) values close to unity over a broad range of compositions, while the corresponding crystalline copolymers have apparent r(1)r(2) values that range from 2 to over 20. An apparent r(1)r(2) close to unity not only reflects random sequencing but also indicates a narrow compositional distribution. This r(1)r(2) result indicates that the amorphous E/P copolymers are produced from a singular type of catalyst site. The higher r(1)r(2) values shown by the crystalline E/P copolymers indicate broad compositional distributions that are produced by a different type or types of catalyst sites. The ratio of amorphous to crystalline ethylene, propylene copolymers is nominally around 80/20 over a broad range of impact copolymer compositions. The consistency of this result suggests that the two basic types of catalyst sites producing E/P copolymers are also in an approximate 80/20 ratio.