For more than two decades, the Ihara research group has been focusing on the development of new methods for polymer syntheses using diazocarbonyl compounds as monomers.
They found that diazoacetate can be polymerized by a palladium(Pd)-based initiator to afford carbon-carbon (C-C) main chain polymers bearing an alkoxycarbonyl group (ester) on each main chain carbon atom. In contrast to the well-known vinyl polymerization, where the main chain is constructed from two carbon units of vinyl monomers such as ethylene and styrene, the polymerization of diazoacetate forms the C-C main chain from one carbon unit, which is the remarkable characteristic of this polymer synthesis (C1 polymerization). The products of C1 polymerization are expected to possess enhanced physical properties and functionalities. Following the first publication on this C1 polymerization project (E. Ihara, et al., Macromolecules, 36, 36 (2003)), intensive efforts have been devoted to this C1 polymerization by many groups around the world.
A structure of the Pd-based initiators was devised leading to success in improving their activity for C1 polymerization. For example, it is now possible to prepare high molecular weight polymers (molecular weight > 50000), to control the stereo-structure (tacticity) of the C-C main chain, and to quantitatively incorporate a functional group at one end of the polymer chain.
In addition, utilizing the unique structures of the C1 polymers, success was made in preparing C1 polymers with enhanced acidity and much higher melting points, up to 130 °C, compared to their vinyl polymer counterparts with the same substituents. Also, it was found that the C1 polymers can be effectively degraded into low molecular weight compounds under mildly basic conditions, which is very important considering their potential as environmentally friendly polymeric materials.
Meanwhile, it was demonstrated that bis(diazocarbonyl) compounds bearing two diazocarbonyl groups in a molecule, can be used as a monomer for polycondensation, yielding a variety of new polymers with an unprecedented chemical structure. For the polycondensation, unique reactivities of diazocarbonyl groups were employed, such as O-H and N-H insertion and C=C bond formation. On the basis of this new polycondensation, it was possible to prepare polymers with facile degradability under mildly acidic conditions and electrochromic properties (reversible color change with the application of voltage in a film state). These results have stimulated research around the world in a similar project following the first publication on this polycondensation project (E. Ihara, et al., Macromolecules, 43, 4589 (2010)).