Linear isotactic polypropylene (PP) was chemically modified by solvent-free, peroxide-initiated grafting of trimethylolpropane trimethacrylate and triallyl trimesate. The resulting coagent-modified PP materials possessed bimodal molecular weight and branching distributions, whose populations varied with coagent structure. In addition to changing polymer chain architecture, these chemical modifications generated a small amount of well-dispersed, coagent-derived nanoparticles. When compared with the parent material and PP samples treated with peroxide alone, coagent-modified materials demonstrated significantly higher crystallization temperatures and crystallization rates, as well as a finer spherulitic structure. The importance of a particle-induced, heterogeneous nucleation mechanism is confirmed by comparative analysis of a long-chain branched PP derivative synthesized by homogeneous silane grafting and moisture curing chemistry. Crystallization studies show that whereas branching has a moderate effect on crystallization kinetics, heterogeneous nucleation effects dominate the crystallization of coagent-modified PP materials.