Ring-opening metathesis heterocyclic monomers

In the search of novel materials with low dielectric constants (e) and low water absorption for the electronics industry as well as transparent, high oxygen permeable and crosslinkable materials for the fast production of contact lenses, the Ring-Opening Metathesis Polymerization (ROMP) became an interesting opportunity for our company [1]. In 1988, it was shown, that strained (heterocyclic) monomers with or without pendant functionalities, especially 7-oxa-norbornene (7-oxabicyclo[2.2.1]hept-5-ene) type monomers, can be efficiently polymerized in aqueous solutions at moderate temperatures (Scheme 1) [2]. 

Ring-opening polymerization (ROP) encompasses polymerization of cyclic compounds (monomers) with at least one heteroatom or a double bond in the molecules. Polymerization of the latter proceeds by the metathesis mechanism and is called ring-opening metathesis polymerization (ROMP). It has become customary to use the expression ROP mostly for heterocyclic monomers. Both processes belong to the larger family of chain polymerizations. In this volume of Polymer Science A Comprehensive Reference there are a few chapters that do not entirely belong to ROP. 

Besides the heterocyclic monomers, mostly discussed in this chapter, this volume is also devoted to the ring-opening metathesis polymerization (ROMP). This polymerization is initiated and propagated by metal carbene complexes and metallacyclobutane complexes as shown in Scheme 2. 

Most cyclic monomers of interest in the field of composites happen to be heterocyclic in nature. Polymerizability of some monomers is summarized in Table 1.1. Ring opening polymerizations invariably follow ionic mechanisms, although a few are known to proceed via the free radical route and some via metathesis involving metallocarbene intermediates. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

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