Such systems have been named immortal. In order to avoid confusion they should be named living with reversible transfer. We discussed macromolecular control in terms of topology, functionahties and composition but refrain from discussing stereochemical microstructure. Control of the stereochemistry means, e.g., control of tacticity of polypropylene or of the various isomers in isoprene polymerization. It is best reached in coordination polymerization, and to a lesser extent in anionic polymerization.
Most polymers obtained in other chain-growth processes exhibit none or poor stereocontrol.
Since this is not always the synthetic aim, polymerizations without sterocontrol may still be named controlled. In case of reversible transfer with an added transfer agent the degree of polymerization is given by the ratio of the concentration of reacted monomer to that of initiator and transfer agent. The limit depends on the difficulty of the particular synthetic task. In some systems relatively well-defined polymers can be prepared in spite of the presence of side reactions. Usually the effect of chain breaking reactions becomes more significant with the chain length.