Cyclopolymerization of 1,6-Heptadiyne Derivatives using Ru-based Grubbs Catalysts: Synthesis, Analysis, and Applications

Abstract

Cyclopolymerization of 1,6-heptadiyne based on the olefin metathesis reaction is one of the useful methods to prepare polyacetylene derivatives. The polyacetylene backbone from cyclopolymerization is stable enough under the ambient condition. The solubility of this conjugated polymer is easily regulated by the substituent manipulation. Therefore, it is expected as a versatile candidate for studies of conducting and conjugated polymers. However, the catalytic system to control the polymer structure and molecular weight was limited to air- and moisture-sensitive metal catalysts, which made a high entry barrier for applications in a wide range. This dissertation describes the development and applications of cyclopolymerization mediated by Ru-based Grubbs catalysts. Ru-alkylidenes have been known as less reactive toward the cyclopolymerization than Mo- or W-alkylidenes. In this study, however, it was disclosed that coordinating solvents and other sufficient reaction conditions notably enhanced the efficiency of CP using Grubbs catalysts. The discovery provided an important clue to understand the low efficiency of CP catalyzed by Grubbs catalysts

furthermore, it led in-depth studies on the ligand effect on the CP. First, weakly coordinating agents, such as THF of pyridines, showed a critical role in preventing the rapid decomposition of the propagating carbene during the reaction. Low temperature or steric effect near carbene also increased the lifetime of propagating carbene in a similar manner. Second, the ligand-free condition resulted in dimerization and trimerization of 1,6-heptadiynes, rather than CP. This side reaction was catalyzed by decomposed Ru-species due to the lack of weakly-coordinating agents, lowering the efficiency of CP. These observations supported the strategy of effective living CP, which was achieved by the fast-initiating Grubbs catalyst in the presence of weakly coordinating agents. The living CP by Grubbs catalyst widened the area of application, such as the construction of complex macromolecules or self-assembled structures based on the block copolymer synthesis. With the aim of single molecular wires, defect-free dendronized polymers and molecular brushes were synthesized in high yield by CP. An interesting conformational transition in those giant molecules led the further investigation on the polymer structure. Poly(cyclopentenylene-vinylene) (PCPV) synthesized by CP showed a spontaneous cis-to-trans isomerization of olefin. This local change of chemical structure induced straight change in the macromolecular structure, as coil-to-rod transition. Atomic force microscopy (AFM) visualized afforded polymers in extended and rod-like shape. Lastly, the in situ self-assmbly of block copolymers prepared by the combination of ROMP-CP was investigated, resulting in spherical micelles. The isomerization of PCPV was readily applicable to alter the micelle structure, followed by the structural evolution into higher dimensional nanostructures.