Low molecular weight semicrystalline propylene-alpha olefin copolymers containing propylene crystallinity are used as a nucleating agent in crystalline polypropylene and polypropylene copolymers. The nucleating copolymers are propylene-alpha olefin copolymers having a percent crystallinity of 5-75%, a melting point of 45° C.-105° C., and an MFR between 300 and 5000 g/10 min. Nucleated polypropylene compositions comprise a nucleating amount of the copolymer blended in a matrix of polypropylene homopolymer, or 0.01 to 9 wt % α-olefin copolymer, having a melting point greater than 110° C. The nucleating copolymer improves processing time with little or no effect on the crystallinity-associated characteristics of the matrix. The invention also provides methods of crystallizing polypropylene with the nucleating agent and of forming articles with the composition, and also provides articles formed from the composition and/or by the method.