The role of molecular weight of a chain in the chain breakdown in diluted solution at a constant concentration of chain molecules and constant temperature is discussed theoretically using the methods of the theory of the activated complex and kinetics of cooperative processes. The reaction starts at the certain molecular weight of the chain. The chain breakdown is the flat increasing function of the chain molecular weight. This function has a limit when chain molecular weight approaches infinity. Such dependence is explained by the increase of flexibility (following two theories) and increase of extra entropy of molecules of different sizes (following kinetics of cooperative processes. Unequal reactivity of chemically identical active centers links to the dependence of chain flexibility on the location of the more flexible part (place of reaction) of the chain.