Addition polymerizations of both acyclic and cyclic monomers show the characteristic features of any aggregation process, in particular, a ceiling temperature above which the formation of the aggregate (long-chain polymer), under given conditions of monomer concentration, solvent, and pressure, would involve a positive change of free energy and is therefore impossible. Conversely, for a given temperature, pressure, and solvent, there is an equilibrium monomer concentration below which long-chain polymer cannot be formed. Monomers must therefore have a negative free energy of polymerization, under given conditions, if they are to be capable of undergoing a significant extent of polymerization. The effect of monomer structure, including ring size, on thermodynamic polymerizability is discussed, as well as the possibility of copolymeriting monomers that cannot themselves give homopolymers. Examples are given for various possible mechanisms of polymerization.