How the reduction of macromolecular entanglements in two polypropylenes affects their rheology and mechanical properties, as well as their isothermal and non-isothermal crystallization has been investigated. In the partially disentangled polymers, lower values of viscosity were observed. The mechanical test in compression showed that strain hardening was slower in partially disentangled polymer. However, the crystallization was faster when the polypropylene had a reduced density of entanglements. In the second part of the experiments, it was investigated how quickly entanglements are re-established in the partially disentangled polymer subjected to annealing in the melt. The time of re-entangling was longer if the polypropylene was less entangled, there was a lower annealing temperature, or the polymer had a higher molecular weight. It turned out that the times of re-entangling measured in three different experiments varied significantly. This time was 60-150 min during crystallization studies, 120-180 min during the mechanical properties studies and well above 120 min in rheological researches. The reason for this is the different dependence of the individual properties of polypropylene on the degree of macromolecular entanglement.