and Macromolecular Systems based on Interlocked Structure
Professor Toshikazu Takata has long studied on the fundamentals and applications of interlocked molecules. Throughout his research works, he has markedly contributed to the advancement of supramolecular, organic, and polymer chemistries and related scientific fields, by the great achievements on the creation of the novel dynamic molecular and macromolecular systems. His major achievements are summarized as follows.
1. Syntheses of Interlocked Molecules and Their Application to Chiral Chemistry
Interlocked molecules such as catenanes and rotaxanes are characterized by the high mobility due to their mechanically or topologically linked components, in which no attractive interaction is required. Dynamic molecular systems based on their structural characteristics are very attractive from viewpoint of unique functions, most of which cannot be attained with the covalently bonded molecules. Meanwhile, the structural diversity of polymer interlocked systems much greater than low molecular weight one can highly expand their applicability.
Prof. Takata has developed a variety of synthetic methods of simple and functionalized rotaxanes mainly possessing crown ethers as the wheel components. Among them the effective synthetic method of sec-ammonium-type rotaxane involves the highly efficient end-cap reaction of an axle terminal OH group of pseudorotaxane with a bulky acid anhydride in the presence of tributylphosphane as the key step. This method has been widely used for the preparation of various rotaxanes due to its high generality and usefulness. The synthetic methods developed by him were applied to the synthesis of various main chain- and side chain-type polyrotaxanes besides the first polycatenane-like polymer "bridged polycatenane", under both solution and solvent-free conditions. He also achieved his pioneering works on the modifications of rotaxane structures. For example, although no one could obtain the neutral sec-amine rotaxane from the corresponding ammonium one for 15 years, he first succeeded in synthesizing sec-amine one in 2010, which opened the certain way to the versatile applications of rotaxanes including rotaxane molecular switch. Meanwhile, first asymmetric synthesis of rotaxane and first catalytic reaction using rotaxane catalyst have been performed by Prof. Takata. He has emphasized the importance of the cooperative effect of the rotaxane components which directly causes the enhancement of enantioselectivity in the asymmetric benzoin condensation and O-acylation of meso-1,2-diol.
2. Development of Rotaxane Switch and Its Application to Dynamic Polymer Systems
From the freedom and controllable mobility of components, many chemists have already developed a variety of molecular switch systems based on the interlocked molecules so far. Directed toward the development of useful rotaxane switch, Prof. Takata has prepared very fascinating pH-sensitive rotaxane switches such as tert-ammonium/tert-amine rotaxane switch, a highly simple but very effective one. Prof. Takata's switch works in solid state, which consists of a tert-ammonium rotaxane having trichloroacetate counteranion. The switch functions only by adding trichloroacetic acid (TCA) to acidify and heating to neutralize which causes the efficient decomposition of TCA to carbon dioxide and chloroform. These rotaxane switches were actually introduced into side chains of polymers like polyphenylacetylene to control the helical structures even in solid state.
In addition to the rotaxane molecular switch, the macromolecular switch was also synthesized by Prof. Takata, to develop novel dynamic functionalized polymers such as stimuli-responsive polymers which switch the topology to control the morphology and property. Such structure-transformable polymers can be obtained by linking plural polymer chains with rotaxane units which have two or more "stations" on the polymer axle component as the attractive interaction points to the crown ether wheel. By the deactivation of the ammonium station, the crown ether wheel moves to the next stable station which is usually placed on the polymer axle terminal. Thus, the polymer topology can be changed by the macromolecular switch e.g. from branched to linear or vise versa. If the end of the axle component is connected to the wheel component (i.e. so called rotaxane), the movement of the wheel component from one terminal to the other results in the formation of cyclic polymer. Prof. Takata has first performed such type of linear/cyclic polymer topology transformation using macromolecular rotaxane as a linear polymer by applying the macromolecular rotaxane switch. Meanwhile, Prof. Takata has found the method to introduce only one wheel component into a long polymer chain by the rotaxane-from method. This synthetic protocol was applied to the syntheses of diblock and triblock copolymers bearing the polymer chains on the wheel and axle components which were connected by the rotaxane linkages. The smart topology transformation of these block copolymers in linear/branched fashion actually induced the big property changes such as hydrodynamic volume and viscosity.
Prof. Takata has prepared cross-linked polymers with excellent properties by introducing the rotaxane structures into the cross-link points, that is so-called "rotaxane cross-link". Unique polymer prepared along this concept in 2003 was a novel chemically recyclable cross-linked polymer, of which dynamic nature undertaking the reversible cross-linking and decross-linking is facilitated by both the rotaxane cross-link and dynamic covalent bond. Meanwhile, Prof. Takata carried out the efficient introduction of rotaxane cross-link into common vinyl polymers via vinyl polymerization, in particular by radical polymerization of vinyl monomer, from viewpoint of high generality of both vinyl polymer and radical polymerization. As a result, many rotaxane cross-linked polymers (RCP) were synthesized by using vinyl group-tethering rotaxane cross-linkers, most of which showed much greater toughness than cross-linked polymers obtained with conventional covalent cross-linkers. Namely, Prof. Takata proved the importance of the rotaxane cross-link in toughening polymer.
As mentioned above, Prof. Takata has performed many excellent original research works since the dawn of chemistry of interlocked molecules, through which he has largely contributed to the advancement of chemistry and related scientific fields. Therefore, the Chemical Society of Japan has recognized that Prof. Takata's achievements are enough worthy to award the CSJ Award.