Polymerization, free-radical ring-opening

The chemistry of polymerization of the oxetanes is much the same as for THE polymerization. The ring-opening polymerization of oxetanes is primarily accompHshed by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, iasertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

Since it was shown that free radical ring-opening polymerization (2) made it possible to introduce functional groups, such as esters C3j, carbonates (4), thioesters (5), and amides (6), into the backbone of an addition polymer, it was reasoned that simple hydrolysis would produce the desired oligomers that could be terminated with various combinations of hydroxyl, amino, thiol, and carboxylic acid groups. 

BAILEY ET AL. Free-Radical Ring-Opening Polymerization... 

Treatment of this monomer with benzoyl peroxide gave a high molecular weight polyester by a free radical ring-opening polymerization which can be rationalized by the accompanying scheme. The structure of the polyester IV was established by analysis and hydrolysis as well as infrared and NMR spectroscopy. 

This indicates the possibility of making addition polymers biodegradable by the introduction of ester linkages in to the backbone. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-1,3-dioxepane (1, Scheme I), made possible the introduction of ester groups into the backbone of addition polymers, this appeared to be an attractive method for the synthesis of biodegradable addition polymers. 

Bailey WJ (1985) Free-radical ring-opening polymerization. Polym J 17 85-95... 

Bailey WJ, Ni Z, Wu S-R (1982) Synthesis of poly-e-caprolactone via a free radical mechanism. Free radical ring opening polymerization of 2-methylene-l,3-dioxepane. J Polym Sci A Polym Chem 20 3021-3030... 

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). 

The above examples of free-radical ring-opening polymerization, which have been explored by Bailey and Endo, produce polymers containing ketonic carbonyl and/or ester groups in the main chain. In addition, these cyclic monomers can be copolymerized with vinyl monomers by free-radical mechanism. Thus, the variety of the polymers produced by radical polymerization has been enlarged. 

Errede (4) showed that the dimer of o-i Iylene would undergo free radical ring-opening polymerization to give the corresponding poly-o-xylylene. 

The free radical polymerizations of cyclic ketene acetals have recently evoked a lot of interest (10-13). TTie oly(e-caprolactone) (PCL) can be synthesized by free radical ring opening polymerization (10). The copolymerization of its monomer, 2-methylene-l,3-dioxepane (MDO), with some vinyl monomers resulted in an aliphatic ester backbone, as well as the pendant functional groups from the vinyl monomers (10). By free radical polymerization of MDO and the vinyl monomers vinylphosphonic acid (VPA), dimethylvinylphosphonate (VPE) and acrylic acid (AA), we synthesized a series of biodegradable copolymers including PCL... 

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

---