2-Methylene-1-dioxepane, free radical

Ring-opening polymerization of 2-methylene-l,3-dioxepane (Fig. 6) represents the single example of a free radical polymerization route to PCL (51). Initiation with AIBN at SO C afforded PCL with a of 42,000 in 59% yield. While this monomer is not commercially available, the advantage of this method is that it may be used to obtain otherwise inaccessible copolymers. As an example, copolymerization with vinyl monomers has afforded copolymers of e-caprolactone with styrene, 4-vinylanisole, methyl methacrylate, and vinyl acetate. 

FIGURE 6 Synthesis of PCL by the free radical polymerization of 2-methylene-l,3-dioxepane. (From Ref. 51.)... 

Bailey, W, J., Ni, Z., and Wu, S.-R., Synthesis of poly-e-capralactone via a free radical mechanism. Free radical ringopening polymerization of 2-methylene-l, 3-dioxepane, J. Polym. Sci., Polym. Chem. Ed.. 3021-3030, 1982. 

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. 

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... 

Jin, S., Gonsalves, R E. A study of the mechanism of the free-radical ring-opening potymerization of 2-methylene-l,3-dioxepane. Macromolecules. 1997,30, 3104-3106. 

An alternative modification method that has a similar impact on PDMAEMA chains for in vivo applications is the introduction of ester bonds into the polymer backbone by free radical polymerization of a cyclic ketene acetal [5,6-benzo-2-methylene-l,3-dioxepane (BMDO)] and DMAEMA using a... 

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] ... 

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-l-piperidinyloxy free radical (TEMPO) [232] The reaction was initiated with di-ferf-butyl peroxide at 125°C... 

Z., and Wu, S.R. (1982) Free radical ring opening polymerization of 4,7-dimethyl-2-methylene- 1,3-dioxepane and 5,6-benzo-2 methylene-l,3-dioxepane. Macromolecules, 15, 711 (c) Liu,... 

PCL can also be obtained by free radical polymerization of 2-methylene-1,3-dioxepane initiated with AIBN. However, this reaction pathway is not of great importance except in the case of the synthesis of some copolymers. 

Radical ring-opening polymerization has been essentially developed by Bailey et al. in the mid-80s. ° With appropriate monomers such as cyclic ketene acetals, the method leads to polyesters, while keeping all the advantages of a free-radical polymerization process. With NMP, however, results are very scarce and only TEMPO was used so far in the polymerization of 2-methylene-1,3-dioxepane initiated by di-tert-butyl peroxide at 125... 

---