Materials based on semi-crystalline polymers are ubiquitous in today’s material science due to their mechanical properties, low densities and affordable prices. A thorough understanding of the influence of temperature, flow and additives on the molecular dynamics of polymer chains with implication on processes such as polymer crystallization is necessary for the determination of structure-property relationships and the development of theories for the prediction of material properties. In order to clarify these relations, model systems based on different isotactic polypropylenes were prepared and characterized using a new low-field RheoNMR technique [1-3] which relies on the implementation of a portable low-field 30 MHz TD-NMR unit in a commercial rheometer. By studying the isothermal crystallization simultaneously with TD-NMR and rheometry, a correlation of microscopic molecular dynamics and the macroscopic mechanical response during polymer crystallization under various conditions was achieved. The systematic investigation of different temperatures, flow profiles and fillers and a comparison with other methods such as benchtop low-field NMR, differential scanning calorimetry (DSC) and microscopy led to new insights in the role of gelation-like solidification processes during the crystallization of polymers. [1] S. Kahle, W. Nussbaum, M. Hehn, H. P. Raich, M. Wilhelm, and P. Blümler, Kaut. Gummi Kunstst. (2008), 61, 92–94. [2] V. Räntzsch, K.-F. Ratzsch, G. Guthausen, S. Schlabach, and M. Wilhelm, Soft Mater. (2014), 12, S4–S13. [3] V. Räntzsch, M. Wilhelm, and G. Guthausen, Magn. Reson. Chem. (2015), in print. doi:10.1002/mrc.4219