61st Annual Report on Research 2016

Petroleum based diamondoids are unique, highly symmetric molecules with great potential for self-organizing behavior at the nanometer-scale due to their tendency to crystallize. Adamantane, for example, has a melting point of 270 degrees Celsius, despite the fact that decane, with the same number of carbons, has a melting point of -30 degrees Celsius. This research project focuses on the living anionic block-copolymerization of diamondoid-substituted butadiene monomers and styrene. Based on preliminary analyses of empirical interaction parameter data, these systems are predicted to possess unusually large chi parameters. The primary objectives of this research project are: Synthesis Diamondoid Monomer; "Living" Anionic Polymerization of Block Copolymers, and Thin-film and Microphase Characterization.

During this first year of work, the first objective was achieved by graduate student Connor Hill. Two different approaches were pursued, based on two published syntheses of the desired 2-adamantyl-butadiene monomer from a methyl-adamantyl-ketone precursor. The primary synthesis involved sequential addition of two methylene units using a sulfoxonium precursor route, but did not prove fruitful in our hands. Proton NMR indicated significant amounts of alkyl side products were produced in our synthetic replication, and a substantial excess of the trimethylsulfoxonium reagent was required to achieve high yield.

Scheme 1. Synthesis of targeted 2-adamantyl-butadiene.

            The second approach based on the use of a vinyl Grignard reagents to produce the tertiary vinyl alcohol with subsequent dehydration giving the 2-adamantyl-butadiene did prove successful (Scheme 1). Using a novel, room-temperature dehydration route, nearly complete conversion of the alcohol in high yield was realized, in comparison to the reported ca. 34% found with refluxing benzene. A systematic exploration of this new route is now being carried out as a potentially general route for substituted butadiene synthesis. High yield dehydration of a tertiary vinyl alcohols to yield butadienes has been reported previously, however, these reports often entailed distillation of the final product as part of the synthesis. Such reactive-distillation procedures are well suited to products with low molecular weight; however, the diamondoid monomers pursued here possess boiling points too high for straightforward replication.

            Simultaneously, Mr. Hill worked to master the fundamental instrumentation required for the next stage of the project, living anionic polymerization. Two major requirements are the use of a high vacuum line and high purity argon. Mr. Hill first worked with Departmental machinist Charles Cornwall to fabricate a stainless steel high vacuum line and rebuilt a vacuum pump capable of operation at a vacuum of ~1 micron. These elements have been coupled to the existing high vacuum glass manifold system in a dedicated hood. The requirement for high purity argon will be met by using the home-built atmosphere regeneration system coupled to the laboratory glovebox, which also provides a route to conduct polymerization reactions and other air sensitive manipulations.

            The next key steps towards block copolymer synthesis are multi-gram synthesis of the first targeted monomer, 2-adamantyl-butadiene, and mastering the manipulations of living anionic polymerization. In particular, monomer purification will be a critical aspect of this project, and we are currently mastering this skill. Once the initial butadiene and styrene monomers have been purified, we will proceed with polymerization studies.

            Beyond the substantial monetary cost of methyl-adamantyl-ketone for these initial studies, the goal of exploring higher diamondoids beyond adamantane requires a generalized route to the ketone-functionalized diamondoids. One potential route begins with dichloromethylation using dichlorocarbene via phase-transfer catalysis as reported by Iwao Tabushi and co-workers, which has seen little exploration since the initial reports, though the reaction itself is schematically simple. Dichloromethyl-adamantane is seen to be a precursor for two useful products: the adamantyl-ketone via hydrolysis and the adamantyl-carboxylic acid via oxidation. The addition of these useful functional groups to the diamondoid skeleton allows for substantial explorations in the areas of organic, polymer, and materials chemistry. Therefore, undergraduate research assistants Lorraine Mottishaw and Gabriela Portillo have conducted careful re-investigations of the dichloromethylation of adamantane. Currently, the phase-transfer dichloromethylation of adamantane cyclohexane is being scaled to the multi-gram level, to be followed by reactions with diamantane.

            As described, diamondoids are unique molecular clusters with a strong tendency to crystallize. Other studies ongoing in this research group likewise focus on clusters and self-assembly, so undergraduate researchers Jessica Lake, Sean Instasi, Margaret Fitzgerald, and Neale Ellyson studied three other self-assembling and cluster systems complementary to purely hydrocarbon diamondoid clusters at the center of this project.

            Jessica and Sean have been working with polyhedral silsesquioxanes, which show high melting points and thermal stability with a silica core structure. These clusters might be incorporated as cores for diamondoid based star polymers, or co-monomers capable of being selectively removed by fluoride etching. Margaret has been pursuing styrene-functionalized polyoxometalate imide clusters to be used as ionic co-monomers for diamondoid block copolymers in membrane applications. Neale has pursued pyrogallol based calix[4]arenes with branched alkyl chains, bowl-shaped molecules with a rim of aromatic hydroxyl species that self assemble into nano-capsular forms. The branched alkyl chains and their interaction with the local solvent environment are of great interest, so embedding these systems into the diamondoid polymers produced here is a major goal.

            The students in this project have been working well, and we are all grateful to the Donors of the American Chemical Society Petroleum Research Fund for making these projects possible.