I. Epoxy functionalized H-poly(DCPD): an effective synthetic approach for cyclic olefin polymers with enhanced thermal properties II. Ligand-assisted nitrene group transfer reaction of a cobalt complex supported by a SiP2 ligand

Abstract

Transparent polymers with good optical properties have attracted significant attention due to their potential in a wide range of applications, including optical storage devices, optical fiber, sheet products, camera lenses, projection displays, and diffractive optical lenses. Among many transparent polymers, cyclic olefin polymers (COPs) have excellent transparency, high thermal stability, good chemical resistance, and low birefringence. However, for automotive applications, high glass transition temperature is necessary because it is directly related to the thermal stability required for end-use. COPs are mainly synthesized through ring-opening metathesis polymerization (ROMP) of cycloolefin followed by subsequent hydrogenation of remaining double bonds in the polymer backbone. The hydrogenation enhances the chemical resistance and heat resistance of COPs obtained by ROMP but decreases the glass transition temperature due to the increased backbone flexibility.In Chapter 2, epoxy functionalized dicyclopentadiene (DCPD) was synthesized and polymerized efficiently through ring-opening metathesis polymerization (ROMP) using ruthenium-based Grubbs catalyst (G1). The hydrogenated polymer was achieved through the subsequent hydrogenation using Pd/C. The corresponding polymer exhibited improved thermal and optical properties as well as high thermal stability due to the presence of compact polar functional groups.In Chapter 3, the post-polymerization modification of the given COP was carried out using the versatile epoxy functional groups remaining after hydrogenation. With a polar group in its side chain, we expect that the resulting polymers show improved adhesion and thermal properties. To our best knowledge, post-polymerization modification of COPs has not been reported, this effective and facile approach could be used to synthesize a variety of functionalized COPs with improved properties.Transition metal complexes play an important role in the catalytic reaction, and transition metal complexes having a metal-nitrogen multiple bond are important reactive intermediate species of the nitrene group transfer. However, only limited catalytic nitrene transfer reactions mediated by metal-nitrene species have been reported due to the high reactivity of late transition metal-nitrene species. In order to overcome such limitations, metal-ligand cooperation (MLC) can be one possible strategy. Such cooperation is at the heart of highly efficient catalytic cycles and valuable stoichiometric reaction sequences. Because several examples of group transfer to silyl groups have been reported, our approach is to introduce a silyl moiety to assist the metal center in stabilizing a nitrene group. Recently, we reported unusual metal-ligand cooperativity of nickel pincer complexes supported by a silyl containing diphosphine ligand, demonstrating a silyl moiety can actively participate in a group transfer reaction as a selective MLC site.In Chapter 4, we present cobalt complexes supported by an anionic diphosphinosilyl ligand MeSiP2 (MeSiP2– = MeSi[2-PiPr2-C6H4]2–). We found that a silyl anchor allows reversible metal-ligand bond formation, resulting in group transfer reactivity. The reaction of the cobalt dinitrogen species with adamantyl azide generates a Co–silylamido complex {(MeSiNAdP2)Co}2(μ-N2) via the nitrene group insertion into the Co–Si bond. Interestingly, such a compound can produce the adamantyl isocyanate (AdNCO) from the reaction with CO by forming the bis-carbonyl cobalt species (MeSiP2)Co(CO)2.In Chapter 5, catalytic isocyanate generation reaction of (MeSiP2)Co(CO)2 with an excess amount of CO and AdN3 was investigated under photolysis. The detailed characterizations of cobalt complexes and the proposed catalytic cycle are discussed. This is a unique method to stabilize a reactive nitrene intermediate without metal-ligand multiple bond (MLMB) stabilization and selectively deliver it to a substrate. Besides, intermediates generated during the catalytic cycle were explored to investigate the details of the reaction mechanism.