In this article, we investigate the effect of a new polymerized compounding method on the nucleation behavior of a sorbitol derivative nucleating agent for isotactic polypropylene (iPP). This method uses a sorbitol derivative nucleating agent, for example, 1,3: 2,4-dimethylbenzylidene sorbitol (DMBS), which displays itself as aggregated fibrillar crystals, as a support for C2-symmetric ansa-metallocenes best represented by rac-Me2Si[2-Me-4-Naph-Ind]2ZrCl2. Activating this DMBS-supported metallocene catalyst with methylaluminoxane, a following in situ propylene polymerization results in the formation of iPP/DMBS composition polymers having controllable DMBS concentrations (by virtue of polymerization productivity control), with the DMBS dispersion driven by catalyst fragmentation. Three iPP/DMBS composition polymers having DMBS concentrations of 0.22, 031, and 0.52 wt % were prepared by this means, which, along with their analog melt-mixed counterparts, were subjected to melt crystallization kinetics and crystal morphology studies using differential scanning calorimetry, polarized optical microscopy, and UVvis spectroscopy. With systematically higher nucleation efficiencies and lower crystallization activation energies, the polymerized iPP/DMBS composition polymers exhibited generally stronger DMBS nucleation effects when compared with their melt-mixed counterparts. Such a benefit was accounted for by a more homogeneous dispersion and better wettability by iPP matrix of DMBS that led to the formation of more nucleation sites and faster crystallization. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013