Radical ring-opening cyclic ketene acetal

Free radical polymerization of cyclic ketene acetals has been used for the synthesis of polyfy-butyrolactone), which cannot be prepared by the usual lactone route due to the stability of the five-membered ring. The polymerization of 2-methylene-l,3-dioxalane at high temperatures (above 120 °C) gave a high molecular mass polyester [59,79]. Only 50% of the rings opened when the polymerization was carried out at 60 °C, and this led to the formation of a random copolymer. The presence of methyl substituents at the 4- or 5-position facilitated the reaction. The free radical initiators generally used in such polymerizations are ferf-butyl hydroperoxide, ferf-butyl peroxide, or cumene hydroperoxide. The various steps involved are described in Scheme 5 [59]. 

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... 

Copolymer Structure and Sequence Distribution. The free radical polymerization of cyclic ketene acetals had two possibilities forming two different structures. One was the ring opening producing a polyester, and the other was ring retention producing polyacetals. The free radicd polymerizations of monomer MDO... 

Figure 21.8 Radical polymerisation of cyclic ketene acetals showing both direct vinyl potymerisation and indirect ring-opening...

Scheme 2 depicts another example, that is, the radical ROP of cyclic ketene acetals. In this case, the co-methylene group is the radical acceptor. The ring-opening reaction is accompanied by the formation of an acyclic ester linkage, which is thermodynamically much more stable than the original cyclic acetal. In addition, the resulting radical is stabilized by the adjacent... 

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 first route relies on the ROP of cyclic ketene acetals [1-3]. The electron-rich double bond is prone to react with radicals and electrophiles. Therefore, this class of monomers undergoes cationic and radical polymerization. For example, radical initiators react with the double bond to provide a new tertiary radical (Fig. 2). Two distinct mechanisms of polymerization can then take place direct vinyl polymerization or indirect ring opening of the cycle accompanied by the formation of a new radical, which is the propagating species (Fig. 2). The ester function is formed... 

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). 

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.,... 

Reviews on radical ring-opening polymerization include those by Sanda and Endo, Klemm and Schultz, Cho, Moszner et Endo and Yokozawa, Stansbury, ° and Bailey. A review by Colombani and Moad et al. on addition-fragmentation processes is also relevant. Monomers used in ring opening are typically vinyl- (e.g., vinylcyclopropane -Scheme 37) or methylene-substituted cyclic compounds (e.g., ketene acetals) where addition to the double bond is followed by P-sdssion. 

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... 

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