What is the primary reason for the variability in lengths of polymer chains formed during free radical polymerization?

In free radical polymerization each chain grows by adding monomers one at a time, but the moment it stops is random. A chain can terminate by combining with another radical, by disproportionation, or by transferring to a solvent or impurity; the chance of any of these events occurring at any step is small and unpredictable. Because every chain experiences a different random sequence of growth and termination, some chains finish after only a few monomers while others grow dozens or hundreds before stopping. For example, one polymer chain might stop after 10 additions because it met another radical, whereas another chain might continue for 200 additions before a termination event occurs, producing a wide spread of lengths. This randomness in termination timing is the primary cause of the variability in polymer chain lengths.