For polymers and other homologous mixtures that span a molecular weight (MW) range, many reaction mechanisms involve the formation, addition and decomposition of free radicals. In the chain reactions of polymerization and depolymerization, radicals and polymer molecules interact by a variety of primary reactions. We postulate that these radicals and molecules can be described by continuous molecular weight distributions (MWDs). Their chemical interactions are developed from generic, reversible addition or bond scission reactions. Distribution balance equations are proposed for initiation-termination and propagation-depropagation reactions, such as hydrogen abstraction, disproportionation, chain cleavage, and radical coupling. A comprehensive model including all these events conforms to the special mechanisms for chain-end depolymer-ization and for random-scission polymer degradation. The evolution of an MWD from initial state to final equilibrium is described by the time dependence of MW moments. Irreversible degradation or polymerization relationships are obtained as limiting cases. First-order rate constants observed in pyrolysis and thermolysis reactions are composed of algebraic groups of primary reaction rate constants.