The study explores synthesis and characterization of ultrahigh molecular weight isotactic polypropylene (UHMWiPP) using a hafnium–pyridyl amido catalytic system. The observations are that by modifying the catalyst through initial insertion of 1-decene, UHMWiPP having extremely high molecular weights, up to 13.5 × 106 g/mol, can be achieved. The effect of ligand modification on polymer properties, including molecular weight, entanglement density, and crystallization are investigated. The impact of ligand modification on UHMWiPP is assessed by DFT calculations. The calculations, aligned with the experimental observations, demonstrate that the ultrahigh molecular weight is a result of the increase in energy barrier for βH transfer and elimination in the 1-decene modified catalyst. The controlled synthesis provides the uniqueness of the tailored entanglement state during polymerization. Surprisingly, the molar mass between entanglements (Me) increases with molar mass, reaching the unprecedented value above 22700 g/mol. Rheological analysis reveals that the nonequilibrium polymer melt with higher molecular weights exhibits the slowest equilibration process. The slow equilibration of the nonequilibrium polymer melt allows following changing chain dynamics with the entanglement’s formation and its influence on crystallization kinetics. It is apparent that the higher the molecular weight, the higher is the Me and so is the crystallization rate.