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Sequence-regulated radical polymerization with a metal-templated monomer: repetitive ABA sequence by double cyclopolymerization.

In nature, macromolecules are generated by elaborate mechanisms, often involving a template, in which the sequence of repeating units (i.e., monomers) along a main chain is perfectly programmed or regulated. In peptides, for example, the repeat unit sequence is well-defined, although as many as 20 amino acid comonomers are involved in the polymerization process. Thanks to these well-defined sequences, even a single molecule of a biologically formed polypeptide (protein) forms a specific structure through hydrogenbond-directed folding to efficiently function as, for example, an enzyme. From this viewpoint, the “sequence” should be the most essential structural factor for polymers, but no one thus far can regulate the sequence of synthetic polymers. An exception is the solid-phase synthesis of “artificial” peptides; however, this strategy of circumvention based on repetition of stepwise reactions of monomers requires complicated procedures such as protection, deprotection, and purification. Therefore, manipulation of the polymerization through templates, as observed for natural polymers, is promising for regulating the sequences of artificial polymers. Herein, we provide a strategy for such a template-based sequence-regulated artificial polymer synthesis by radical polymerization. Radical polymerization, a chain-growth polymerization of vinyl monomers, is important for industrial polymer production. The majority of products from radical polymerization is in fact copolymers in which the composition of the repeat units statistically depends on the reactivity ratio of the monomers or on an “average” within molecules of varying compositions. Accordingly the “absolute” sequence is illdefined, not precisely controllable, and often statistically distributed. An exception is the so-called alternating copolymerization, giving an ABABAB... regular sequence, by modulating the reactivity ratio of the monomers by, for example, the addition of a Lewis acid. Now that such primary structural factors as, for example, the chain length, finally can be controlled in recently discovered living radical polymerizations, the sequence of polymers is recognized as the next structural factor that can be controlled precisely in artificial polymerizations to express advanced functions of polymers as observed in nature. We have thus focused on template-based sequence regulation in living polymerization by designing initiators and monomers. A recent example which is in line with the design of monomers is to employ a naphthalene framework as a template that anchors two monomeric units (e.g., methacrylate and acrylate) spatially close to each other in the periposition through a “cleavable” ester linkage (Scheme 1A). Such an AB-templated divinyl monomer can be polymerized by a metal-catalyzed living radical polymerization under diluted conditions to give soluble polymers without the effect of gelation. Upon hydrolysis of the ester linkers and subsequent methylation, the resultant copolymers proved to possess highly regulated methyl methacrylate–acrylate ABalternating sequences. Because the conventional radical copolymerization of these two monomers gives totally random sequences, this result is significant, because it suggests the possibility of template-assisted sequence regulation beyond the inherent reactivity ratio. Though effective the naphthalene-based design is by definition confined to ABalternating sequences alone and, equally serious, would be cumbersome and laborious to realize regulated sequences of hydroxyl, amino, and other functional groups, unless far more versatile and perhaps orthogonal linkers other than ester groups are designed.