In this work, we present a versatile approach to achieve well-defined conjugated polymers and oligomers from a single standard synthesis route via preparative size-exclusion chromatography. Using a special recycle mode and well-defined endcappers, we are able to prepare pure oligomers with three to eight repeat units and high-molecular-weight polymers with dispersity ĐM as low as 1.06 in just one preparation step with reasonable yield. To demonstrate the capabilities of our method, we conducted a fundamental spectroscopic study on the influence of chain length and molecular weight distribution on electronic properties and aggregation behavior of a series of fluorene based oligo- and polymers. Due to the well-defined nature of the investigated compounds, we consistently find a first-order phase transition and an increase of the critical transition temperature from the amorphous phase to the ordered β-phase as a function of the number of repeat units in accordance with the Sanchez model on coil–globule transitions.