Motofumi Osaki

Ring-opening polymerization of cyclic esters by cyclodextrins.

Synthetic polymers, typically prepared by addition polymerization or stepwise polymerization, are used constantly in our daily lives. In recent years, polymer scientists have focused on more environmentally friendly synthetic methods such as mild reaction conditions and biodegradable condensation polymers, including polyesters and polyamides. However, challenges remain in finding greener methods for the synthesis of polymers. Although reactions carried out in water are more environmentally friendly than those in organic solvents, aqueous media can lead to the hydrolysis of condensation polymers. Furthermore, bulk polymerizations are difficult to control. In biological systems, enzymes synthesize most polymers (proteins, DNAs, RNAs, and polysaccharides) in aqueous environments or in condensed phases (membranes). Most enzymes, such as DNA polymerases, RNA polymerases, and ribosomes, form doughnutlike shapes, which encircle the growing polymer chain. As biopolymers form, the active sites and the substrate-combining sites are located at the end of the growing polymer chain and carefully control the polymerization. Therefore, a synthetic catalyst that could insert the monomers between the active site and binding site would create an ideal biomimetic polymerization system. In this Account, we describe cyclodextrins (CDs) as catalysts that can polymerize cyclic esters (lactones and lactides). CDs can initiate polymerizations of cyclic esters in bulk without solvents (even water) to give products in high yields. During our studies on the polymerization of lactones by CDs in bulk, we found that CDs function not only as initiators (catalysts) but also as supporting architectures similar to chaperone proteins. CDs encircle a linear polymer chain so that the chain assumes the proper conformation and avoids coagulation. The CDs can mimic the strategy that living systems use to prepare polymers. Thus, we can obtain polyesters tethered to CDs without employing additional solvents or cocatalysts. Although CD has many hydroxyl groups, only one secondary hydroxyl group attaches to the polyester chain. In addition, the polymerization is highly specific for monomer substrates. We believe that this is the first system in which the catalyst includes monomers initially and subsequently activates the included monomers. The catalyst then inserts the monomers between the binding site and the growing chain. Therefore, this system should provide a new environmentally friendly route to produce biodegradable functional polymers.

Artificial Molecular Clamp: A Novel Device for Synthetic Polymerases

Renewable materials have attracted much attention from the viewpoints of environmental protection and efficient utilization of natural resources. Certain polyesters, polyamides, and polylactides, which are synthesized by either biological methods or chemical processes using a metal catalyst, have been extensively investigated as biodegradable and renewable polymers. However, biological methods are inefficient, and chemical processes involve harmful metals and organic solvents. Thus, more efficient and environmentally benign processes are necessary. In our studies, we hypothesized that innovative syntheses are best developed using chemical processes that take advantages of biological systems. Herein, we successfully obtained synthetic polymerases including an artificial molecular clamp to yield high-molecular-weight polymers without solvents or co-catalysts. This system is reminiscent of highly efficient DNA polymerases including a sliding clamp where the ring-shaped protein assembly of DNA polymerases plays an important role in the replication of polynucleotides. Although the clamp does not have an active site, polymerization does not proceed well without the clamp. Similarly, cyclodextrins (CDs) are ringshaped host molecules, which include various guests to form supramolecular complexes such as rotaxanes. An early example of supramolecular catalysis is the hydrolysis of activated phenyl esters using CDs. These catalysts have also been utilized as enzyme models. Moreover, modern supramolecular catalysts using host– guest interactions have achieved highly efficient and selective reactions, including hydrolysis reactions, 20] C H bond activation, epoxidation of olefins, Diels–Alder reactions, and 1,3-dipolar cycloadditions. 33] The bases of supramolecular catalysts are selective molecular recognition and substrate activation. One limitation of these catalysts is product inhibition because of their complex design, but introduction of an artificial molecular clamp into supramolecular catalysts can resolve the problems. Herein, we show that cyclodextrins play an important role as an artificial molecular clamp in polymerization reactions. We selected b-CD as a supramolecular polymerization catalyst because it does not require a highly reactive catalytic center (metal complexes, cationic or anionic groups). CDs can include and activate lactones, yielding an oligomer tethered to a single CD at the end of the polymer chain. However, the produced oligo(lactone)s bearing a b-CD unit did not initiate the polymerization reaction. We hypothesized that an artificial molecular CD clamp attached to the active site of the b-CD plays an important role in the polymerization by holding the polymer chain and consequently securing the active site. First, we studied the polymerization activity of the a,bTPA-dimer linked with terephthalamide between the aand b-CDs for d-valerolactone (d-VL; Scheme 1). Polymerizations of d-VL initiated by CD dimers were carried out by stirring and heating a bulk mixture of the CD dimers and dVL ([d-VL]/[CD unit] = 50) at 100 8C.

Nanospheres with Polymerization Ability Coated by Polyrotaxane

BETA-cyclodextrin (beta-CD)-based nanosphere 1 initiated the oligomerization of delta-valerolactone (delta-VL) on the surface of 1 to give oligo(delta-VL)-tethered beta-CD nanosphere 2 in bulk. Atomic force microscopy indicated that the molecular size of 2 is twice that of 1. The addition of alpha-CD to 2 leads to the formation of poly-pseudo-rotaxane on the surface of 2 to give a nanosphere with poly-pseudo-rotaxane (alpha-CD[symbol: see text]2). 2D-NOESY NMR experiments showed correlation peaks between the inner protons of alpha-CD and the oligo(delta-VL) chains in an aqueous solution, indicating that the oligo(delta-VL) chains are included in the alpha-CD cavity. Alpha-CD[symbol: see text]2 has a core of beta-CDs with poly-pseudo-rotaxanes on the surface. It should be noted that 2 did not show polymerization ability for delta-VL, but after the formation of poly-pseudo-rotaxanes, oligo(delta-VL) of alpha-CD[symbol: see text]2 repropagated upon the addition of delta-VL. Alpha-CD[symbol: see text]2 is significantly larger than nanospheres 1 and 2. Additionally, postpolymerization increases the size of alpha-CD[symbol: see text]2. These behaviors are reminiscent of the function of a spherical virus, which forms an ordered spherical structure and releases RNA chains from the capsid surface.

Functional Supramolecular Materials Formed by Non-covalent Bonds

Molecular recognition is essential for realizing functional supramolecular materials. Non-covalent host-guest interactions are effective tools to introduce various functions and properties into materials. This review focuses on the functions such as selective molecular adhesions, self-healing, toughness, and actuation properties of the supramolecular polymeric materials. These functions have been achieved by using reversible bond formations between cyclodextrins (CDs) and guest molecules. The host-guest interactions involving CDs can be used to achieve efficient stimuli-responsive behaviors and self-healing properties. Furthermore, the supramolecular materials have been found to exhibit macroscopic rapid expanding and contracting driven by external stimuli under semidry conditions. Supramolecular actuator using host-guest complexations can be prepared by two approaches. The first is the functionalization of a supramolecular gel, which changes the cross-linking density between the polymers. The second is the utilization of a topological gel to change length of the polymer chain between cross-linked points. Both types of the actuators exhibit bending behaviors by external stimuli. This review summarizes the advancements within the past 10 years in supramolecular materials that utilize the host-guest interactions and the sliding motion of ring molecules functionalized by chemical or physical processes.