Linear and branched polyethene, isotactic polypropene and ethene-propene copolymers were synthesized. Solution, slurry, bulk and gasphase polymerizations were performed using homogeneous or supported metallocene and nickel-based catalysts. The thermal behavior of the nascent, as-polymerized polyolefins has been investigated in relation to the used polymerization conditions. The nascent polymer samples exhibit in general a notable thermal behavior: differential scanning calorimeter traces show in most first heating scans (nascent) higher melting temperatures and higher enthalpy of fusion than in the second heating scans of the same, but now melt-crystallized samples. A similar thermal behavior is well known for ultra-high molecular weight polyethene (UHMW-PE), but less or even not identified for polyethene with lower molar mass and other semi-crystalline homo- and copolymers. By varying systematically the polymerization conditions we could prove that mainly the local kinetics of polymer formation at the active catalysts, which is controlled by the polymerization conditions (e.g. synthesis process, polymerization temperature), and the subsequent crystallization determine the thermal properties and the morphology of the nascent state. This is in accordance with the kinetic principle of crystallization during polymerization introduced by Wunderlich in the late 70ties of the last century.