Cyclic ketene acetals copolymerization

Related work had shown that the nitrogen analogs of the cyclic ketene acetals were readily synthesized and would polymerize with essentially 100% ring opening. For this reason their copolymerization with a variety of monomers was undertaken (6). 

The radical copolymerization of cyclic ketene acetal with various vinyl monomers such as styrene, MMA, vinyl acetate, and methyl vinyl ketone could afford the copolymers with ester groups in the main chain (26-28). These copolymers showed enzymatic degradability and photodegradability and are applicable as environmentally degradable materials. 

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). 

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... 

Fig. 31. Preparation of biodegradable PS by incorporating ester linkages into the backbone via ring-opening copolymerization of styrene with a cyclic ketene acetal.

Since Potts had shown that low molecular weight aliphatic polyesters were the only general class of synthetic polymers that were biodegradable, it appeared the copolymers with the cyclic ketene acetals with various monomers should lead to copolymers that were really polyesters and therefore should be biodegradable. Thus, the copolymerization of the cyclic ketene acetal 1 produced a series of copolymers that were in fact polyesters, but with physical properties very similar to polyethylene. 

Formaldehyde is a strong electrophile, allowing acetal to polymerize by nucleophilic, anionic, or cationic addition of an alcohol to ketene carbonyl groups. Relatively weak bases such as pyridine initiate anionic addition polymerization cationic addition polymerization is catalyzed by strong acids. When the cyclic trimer trioxane is used as a copolymer to polymerize acetal copolymers, Lewis acids such as boron tiifluoride promote copolymerization. A more fundamental description is polymerization of an aldehyde or ketone -l- alcohol -i- an acid or base catalyst to form hemiac-etal, which further converts to acetal. The hemiacetal reaction is reversible to aldehyde and alcohol. 

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