Polymer Made in a Nano Test Tube

MATERIALS CHEMISTRY

Using a single-walled carbon nanotube as a highly confining reaction vessel, researchers have polymerized a fullerene derivative to form a linear, unbranched polymer that has never before been observed. The technique may allow other unprecedented linear polymers to be synthesized.

With supercritical carbon dioxide as the solvent, the researchers inserted molecules of the known fullerene epoxide C₆₀O into nanotubes. The CO₂ was then allowed to escape from the nanotubes. The resulting peapod structure consists of epoxide molecules lined up in a single row inside the nanotube, which is only wide enough (about 1.4 nm) to accommodate one row of fullerenes.

The nanotubes were then heated to 260 C in a vacuum, causing the strained epoxide rings to open. Each epoxide oxygen atom attacks one of the C=C bonds of its neighbor, forming a C–O–C bridge. The two carbon atoms--one on each buckyball--that have been left with unsatisfied valences then join "hands," completing the formation of a rigid furan-type ring between the two cages. The result of this head-to-tail polymerization is a linear chain, (C₆₀O)_n, that, in principle, can be as long as the length of the nanotubes [Chem. Commun., published online Nov. 18, http://xlink.rsc.org/?DOI=b414247k].

This is a different outcome from that observed when C₆₀O is heated in the solid state, according to the research team--materials graduate student David A. Britz of the University of Oxford, chemistry research fellow Andrei N. Khlobystov of the University of Nottingham, and their colleagues. When not confined inside nanotubes, C₆₀O reacts "to form a tangled, branched, three-dimensional polymer where the single oxygen atom bonds to any of its 12 nearest neighbors," they write.

Generally speaking, Khlobystov and Britz tell C&EN, the selectivity and yield of organic reactions carried out inside nanotubes are expected to be greater than they are in solution because of the nanotube's confining environment. "The nanotube can be thought of as a shield around the molecules, lowering the activation barrier for some chemical reactions and/or favoring the formation of one isomer out of many possible in solution."

Khlobystov and Britz haven't tried to remove the polymer from the nanotube; they believe it would be very difficult because of the strong van der Waals interactions between the fullerene chains and the nanotubes.

However, they note that if nonfullerene molecules were to be polymerized inside the nanotube cavity, it might be possible to extract the polymeric product or to selectively oxidize the nanotube to release the polymer.

According to Lorna Jack, an assistant editor of Chemical Communications, both referees who reviewed this paper cited it as one of unusually high interest