Ring-Opening Polymerizations by a Free Radical Mechanism

16 Ring-Opening Polymerizations by a Free Radical Mechanism 

There are some reports in the literature of ring-opening polymerizatiMis by free radical mechanism. One is a polymerization of substituted vinyl cyclopropanes [227]. The substituents are radical stabilizing structures that help free radical ring-opening polymerizations of the cyclopropane rings. This can be illustrated as follows  

In a similar manner, ring-opening polymerizations of five-membered acetals are helped by free-radical stabilizing substituents [228]. Complete ring-opening polymerizations take place with phenyl substituted compounds  

Some other heterocyclic monomers, like acetals, also polymerize by free-radical mechanism [229]. Particularly interesting is an almost quantitative ring-opening polymerization of a seven-membered acetal, 2-methylene-l,3-dioxepane [230]  

Cyclic allylic sulfides were shown to polymerize by a free-radical ring-opening mechanism [231]. The key structural unit that appears to be responsible for the facile ring-opening is the allylic sulfide fragment. In it the carbon-sulfur bond is cleaved [231]  

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Explain ring-opening polymerizations by a free-radical mechanism, giving two examples. 

Very recently, Bailey and Endo have enlarged the scope of the free-radical ring-opening polymerization.13 15> Several examples of their studies are described below. All the polymerizations are represented by the following general pattern of fundamental reaction. It is seen that the free-radical ring-opening by a P-scission mechanism is coupled with the addition of a free radical to the carbon-carbon double bond 13). 

Polymers are macromolecules which are composed of smaller molecules linked by covalent bonds. In terms of the reaction kinetics, polymerizations are traditionally classified into several categories stepwise polymerization, free-radical polymerization, ionic polymerization, ring-open polymerization, and coordination polymerization or polyinsertion. Each polymerization method has a combination of requirements for reaction conditions, and they exhibit certain types of product and process features (Caneba, 1992a, 1992b Odian, 1991). Even though in principle, the FRRPP process can be implemented with a wide variety of polymerization mechanisms, its discovery and immediate implementation has occurred in conjunction with free-radical kinetics. 

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