Reports: DNI754880-DNI7: Diamondoid Block Copolymers for Microporous Membranes
Mark Roll, PhD, University of Idaho
Petroleum based diamondoids are unique, highly symmetric molecules with great potential for self-organizing behavior at the nanometer-scale via crystallization. Adamantane, for example, has a melting point of 270˚C, despite the fact that decane, with the same number of carbons, has a melting point of -30˚C. This research project focuses on the living 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 fundamental stages of this research are:
1. Diamondoid Monomer synthesis
2. Polymerization of diamondoid-diene/styrene Block Copolymers
3. Thin-film and Microphase Segregation Characterization
Monomer synthesis is the first challenge in this endeavor undertaken by graduate student Mr. Connor Hill. This past year has followed our second approach, based on the use of a vinyl grignard reagents to produce the tertiary vinyl alcohol with subsequent dehydration giving the adamantyl-butadiene. Using a novel, room-temperature route, nearly complete conversion of the alcohol into diene in high yield was realized, using Amberlyst-15. A systematic exploration of this route is now being completed out as a potentially general route for substituted butadiene synthesis with facile work-up using the heterogeneous acid catalyst. High yield dehydration of a tertiary vinyl alcohols to yield butadienes has been reported previously, however, these reports often entailed reactive-distillation procedures best suited to products with low molecular weight. The diamondoid monomers pursued here possess boiling points too high to utilize this route.
Polymerization of these novel monomers was first proposed through "living" anionic means, and the instrumentation necessary to carry out such a synthesis has been put in place. However, current testing is underway using the "living" free radical approach demonstrated by Hawker and co-workers to be suitable for styrene and diene polymerization. In principle, this initiation system provides a less experimentally stringent route to the block copolymers proposed in this study. Therefore, with gram-quantities of 1-adamantyl-butadiene now available, this route is being investigated.
The goal of exploring higher diamondoids beyond adamantane requires a generalized route to the functionalization of bridgehead carbons in these molecules. Two such routes have been described: photoacetylation of diamondoids using diacetyl and dichloromethylation with dichlorocarbene via phase-transfer catalysis. The latter route has seen little exploration since initial reports, though the reaction itself is schematically simple. Undergraduate research assistant Gabriela Portillo has conducted careful re-investigations of the dichloromethylation of adamantane and diamantane reported by Yoshida and Takahashi, and she has succeeded in determining key variables for reaction success along with a direct work-up allowing for the recycling of unreacted adamantane. Currently, hydrolysis of the dichloromethyl group into the aldehyde is being explored, with reactions using diamantane next on the agenda.
Diamondoids are unique molecular clusters with a strong tendency to crystallize, and other studies ongoing in this research group likewise focus on clusters and self-assembly. Therefore, undergraduate researchers Sean Instasi and Neale Ellyson have studied two other self-assembling and cluster systems complementary to the purely hydrocarbon diamondoid clusters. Mr. Instasi has been working with polyhedral silsesquioxanes, which show high melting points and thermal stability with a silica core structure suitable to act as plasticizing modifiers for the diamondoid polymers. Ms. Ellyson has pursued pyrogallol-calixarenes with branched alkyl chains, bowl-shaped molecules with a rim of aromatic hydroxyl species that self assemble into nanocapsular forms. In particular, she has developed a novel reaction-crystallization that is suitable for sterically hindered 2-ethylhexyl chains. The combination of self assembly with strongly hydrophobic and hydrophilic domains will be used to investigate diamondoid-calixarene nanocomposites.
We are all grateful for the support of the Petroleum Research Fund administered by the American Chemical Society for making this work possible.