Radical ring-opening acetal

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. 

Agarwal S (2010) Chemistry, chances and limitations of the radical ring-opening polymerization of cyclic ketene acetals for the synthesis of degradable polyesters. Polym Chem 1 953-954... 

Free Radical Ring-Opening of Cyclic Ketene Acetals... 

Figure 13. Radical ring-opening polymerization of cyclic ketene acetals.

The living R-ROP of cyclic ketene acetals was achieved with nitroxy-mediated polymerization (NMP) (29), ATRP (30), and RAFT (31) methods to afford the polyesters with low polydispersities. Recently, it has been reported that the block and random copolymers with vinyl monomers showing low polydispersities could also be obtained by living radical ring-opening copolymerizations (32, 33). 

Reviews on radical ring-opening polymerization include those by Sanda and Endo, " Klemm and Schultz, Cho, " Moszner e( Endo and Yokozawa Stansbury and Bailey. A review by Colombani on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. kctenc acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scis.sion. 

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... 

Radical Ring-Opening Polymerization (RROP) of Cyclic Ketene Acetals 27... 

Chart 2.1 A wide variety of cyclic ketene acetals are available in the literature undergoing radical ring-opening polymerization for the formation of polyesters. CKAs studied in details in literature are presented here with their names and structures [12]. 

Scheme 2.4 H-transfer reactions during radical ring-opening polymerization of cyclic ketene acetals lead to branched polymers. Unstable primary growing radicals at the chain ends cause back-biting reactions.

Yuan, J.Y. and Pan, C.Y. (2001) The effects of monomer structure of cyclic ketene acetals on the behavior of controlled radical ring-opening polymerization. Chiru J. Polym. Sci.,... 

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... 

Monomers of this type are expected to polymerize via their vinyl groups. However, their acetal groups may also participate in the free radical polymerization, since it is known that cyclic acetals such as shown below undergo a free radical ring opening rearrangement to the corresponding ester (24). 

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. 

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