Generally speaking there are two ways to obtain polymers, i.e., by chain growth reaction, characterized by initiation, growth, and termination reactions, and by step-growth reactions. Whereas for both types of polyreactions a Schulz-Flory molecular weight distribution is observed, the (number) average molecular weight only becomes high for high conversions in the latter case; in the former it is not dependent on conversion except for the very first stages of the polymerization. Moreover, in special cases, it can be shown that the type of reaction followed-chain growth or step growth reaction-is determined by the ratio of the rate constants of the elementary steps.([1]) A unique case is the chain-growth mechanism with the rate constant of termination (and that of transfer) being zero, generally referred to as ''living'' polymerization. The ring-opening polymerization of cyclic monomers is usually considered to follow a chain-growth mechanism. This is due to the fact that, normally, an active species is formed which reacts with the monomer via addition or insertion and which stays active either until the monomer is consumed (living polymerization) or until some kind of transfer or termination reaction takes place. Ring-opening polymerizations, from a preparative point of view, bridge the gap between the two types of polyreactions mentioned above: polyesters, polyethers, polycarbonates etc, may be obtained by step-growth reactions, i.e., polycondensation reactions as well as by chain-growth reactions, i.e., the ring-opening polymerization of lactones, cyclic ethers, cyclic carbonates etc. The latter reactions may be initiated by low-molecular-weight active species and result in homopolymers or by ''living'' polymers to yield the respective block copolymers. This applies particularly to the anionic mechanism. The ring-opening polymerization of cyclic monomers is most suitable for the preparation of sequential copolymers of the type (AAB)(n), the sequence being determined by the structure of the monomer. The ring-opening polymerization of cyclic monomers is an equilibrium reaction; in the equilibrium state, besides the polymer the monomer and a homologous series of cyclic oligomers are observed. During polymerization back-biting and/or end-biting reactions may occur upon which the ring-chain equilibrium is established. Often, the ceiling temperature of the polymers is rather low; thus, polymers obtained by ring-opening polymerization should be easily degradable, the monomer being recovered by distillation ready for further polymerization.