Using Dewar Benzene and Its Derivatives as Monomers for Contemporary Polymeric Materials

Abstract

Department of ChemistryConjugated polymers have been and will continue to attract significant attention because of their potential usage in a wide range of contemporary areas (e.g., electronic devices, chemical sensors, photovoltaics, etc.). The representative example for these class of conjugated polymers is poly(acetylene). Poly(acetylene) is known to feature outstanding electric conductivity upon doping. However, contemporary methodologies to prepare poly(acetylene) cannot overcome issues such as reaching high molecular weights, which have, in turn, hampered its application in electronic devices. In this dissertation, Dewar benzene and its derivatives were explored as monomers for accessing both conjugated and other types of polymeric materials, expected to address aforementioned issues. Initially, we hypothesized that the high ring strain of the cyclobutene moiety would allow for polymerization via ring opening metathesis polymerization (ROMP) and isomerization sequence would facilitate the formation of poly(acetylene). However, crosslinking issue arose during polymerization, obstructed the formation of poly(acetylene). As a result, our efforts were re-directed toward (rac)-trans-5,6-dibromobicyclo[2.2.0]hex-1-en because the monomer featured a strain ring and disubstituted bromines which could be reverted back to an olefin when exposed to a reductant. As such, ROMP of the monomer proceeded in a controlled fashion and subsequent addition of strong reductant efficiently facilitated a cascade elimination isomerization reaction to result in regioselective trans-poly(acetylene). While the aforementioned challenges were addressed by this methodology, the poor solubility that is intrinsic to poly(acetylene) still remained an issue. Therefore, the synthesis of new monomer was required. 3,4-disubstuted cyclobutenes derived from Dewar lactone were employed due to sufficient ring strain for ROMP and installed pendant groups which can facilitate elimination and enhance solubility. As such, the preparation of soluble poly(acetylene) derivatives proceeded via ROMP of cyclobutene derivatives followed by elimination with organic base and accessing substituted poly(acetylene)s that exhibited high solubility in common organic solvents was achieved through the installation of ester functional groups. Fully saturated hydrocarbon materials (e.g., poly(ethylene) and poly(propylene)) have been also explored in detail due to their wide applicability (e.g., common household plastics, medical products, electrical insulators, etc.). Furthermore, among a series of hydrocarbon polymers, studies on hydrogenated cyclic olefin polymers (H-COP)s derived from with norbornene and its derivatives have been continuous in academy and industry as those exhibit outstanding thermal stability. To synthesize a new class of H-COP polymer exhibiting more improved properties than contemporary polymers, it was hypothesized that embedding certain carbocycles into the backbones of linear polymers would significantly improve their physical properties and smaller ring size may more efficiently contribute to increasing the properties. In addition, we also hypothesized subsequent hydrogenation on C=C double bonds may play key role in influencing inherent properties of material in an improved manner because saturated C-C bonds can have more freedom of C-C bond rotation than that of C=C bonds, which enabled the increase of the mobility of polymer chain and aid to organizing polymer chains into certain shape. In an effort to test the hypothesis, dihydro Dewar benzene was prepared because its hydrogenated polymer structure prepared via ROMP followed by hydrogenation would feature cyclobutane that can be conveniently compared with the polymers featuring cyclopentane and/or saturated linear hydrocarbon chain respectively. The comparison results would be expected to aid to understand how the size of carbocycles contributes to the enhanced thermal properties. The preparation of polymer material proceeded via ROMP with dihydro Dewar benzene and hydrogenation using conventional method. The structure of resulting polymer was characterized by a variety of spectroscopic techniques and was found to exhibit significantly enhanced thermal and/or mechanical properties compared to hydrogenated poly(norbornene) and various types of poly(ethylene)s.clos