Ring-Opening Polymerizations by a Free Radical Mechanism
There are some reports in the literature of ring-opening polymerizations 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:
Ahigh molecular weight polymer forms. In place of nitrile groups ester groups can be utilized as well. The polymerizations of vinyl cylopropanes proceed by cationic and coordination mechanisms exclusively through the double bonds. Free radical polymerizations of these substituted vinyl cyclopropanes, however, take place only through ring-opening polymerizations of the propane rings. 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-1,3-dioxepane [230]:
The product is an almost pure poly(e-caprolactone). 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]:
It was also reported recently that a controlled free-radical ring-opening polymerization and chain extension of the “living” polymer was achieved in a polymerization of 2-methylene-1,3-dioxepane in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) [232] The reaction was initiated with di-tert-butyl peroxide at 125C At high concentrations of the piperidinyloxy radical, the polydispersity of the product was 1.2 [232]
