Ring-opening polymerization classifications

Initiation. The most recent classification of initiators for cationic ring-opening polymerization was presented and discussed by Penczek et al. (IJ. Only a few classes of initiators that are very useful both for mechanistic studies as well as for synthesis of well-defined polymers will be presented here. 

The three classifications just described are essentially based on monomer structure. Condensation polymerizations arise when the two monomers are stable but have functionalities that can react with each other. Addition polymerizations require unsaturation in the monomer that is vulnerable to attack by radicals or ions, and ring-opening polymerizations require cyclic monomers. An alternative and more modern classification emphasizes the differing mechanisms of polymerization, producing two classes step-growth reactions and chain-growth reactions. Figure 13.13 contrasts the two mechanisms. 

As disciissed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition or condensation polymers depending on the type of polymerization reaction involved in their synthesis. This classification scheme, however, does not permit a complete difierentiation between the two classes of polymers. A more complete but still oversimplified scheme that is still based on the dilTerent polymerization processes places polymers into three classes condensation, addition, and ring-opening polymers. This scheme reflects the stractures of the starting monomers. Probably the most general classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction (addition) polymerization. In this chapter, we will discuss the different types of polymers based on the different polymerization mechanisms. 

Although these earlier definitions were based on the chain structure of the polymers, they were closely related, as just described, to the mode of formation as well. It soon became apparent that such a classification has serious shortcomings, as so-called polycondensates could result from addition polymerization reactions. For example, although Nylon 6 can be prepared by the polycondensation reaction of e-aminocaproic acid (Braun et al., 1984), it is now synthesized by the ring-opening addition polymerization of e-caprolactam (Sandler and Karo, 1992), and this process has a profound effect on the... 

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