A beaker left in the sunlight has brought the world an unusual class of polymer crystals. A research team based at the University of California at Los Angeles and at Santa Barbara reports in the January 14 issue of Science (DOI: 10.1126/science.1245875, p. 272) the quantitative conversion of a class of monomers into large single-crystal linear polymers after exposure to visible light. “This is fascinating because they have a monomer that can undergo a topochemical polymerization to generate enormous [1.5 cm long] single crystals with exceedingly long polymers that are highly oriented,” said Paula Hammond of the Massachusetts Institute of Technology. In a topochemical poly-merization, monomers are pre-assembled into their approximate end positions, with reaction initiated by heat or light.
“Growth of a polymer crystal is really a pain. What [first author] Letian [Dou] found is a way to trigger the reaction, make everything very simple, and it takes just an hour,” said Yang Yang of UCLA, one of the corresponding authors. Fred Wudl (UCSB), the other corresponding author, said, “In general, when anyone does an organic photochemical reaction in the solid state, the product absorbs more than the starting material, and in the same region [of the spectrum, blocking further reaction]....This is probably the first case where a quantitative solid–solid reaction has been observed.”
The researchers found that orange-colored crystals of alkylcarboxylate-substituted bis(indene)dione monomers paled and became insoluble when exposed to sunlight or to light from a sodium lamp. X-ray diffraction analysis determined the structure of the resulting polymeric single crystals. R-factors similar to those of the monomer crystals demonstrated a surprising absence of amorphous regions within the polymer. The identity of the alkylcarboxylate side chains is important to the reaction, with 6- and 8-carbon linear alkyl groups maintaining the monomers at the appropriate distance and orientation to permit topochemical polymerization, while smaller or branched alkyl-containing monomers failed to polymerize. The yellow polymer crystals revert at 195°C to orange monomer crystals, which are re-polymerized with light.
“The reversibility is interesting, and is certainly not the case with the diacetylenes,” said Hammond. “This could be compelling if you are able to manipulate them into new orientations and arrangements and re-polymerize.”
If the polymerization is photochemically reversible it could lead to microlithography applications, Wudl said. Photopolymerization of concentrated monomer solution and of spin-cast thin films led to tiny, disordered crystals.
Kirk Fields (UCSB) investigated the tensile stress–strain properties of the single crystal of the highly oriented linear polymer, observing individual strands sliding relative to each other. Dou was able to isolate single unentangled polymer chains by mechanical exfoliation and examine them microscopically. “From the fundamental study point of view it is also interesting to have a single 1D [one-dimensional] polymer chain. It’s the synthesis of a new compound and a new chemical reaction under visible light,” said Dou.
(a) Conventional polymer by mechanical exfoliation versus (b) single-crystal polymer by mechanical exfoliation. Note the entanglement of the conventional polymer. (c) Reversibility: under heating the polymer (yellow) reverts to monomer (orange).
The research team is looking into mechanical applications for the polymers, such as reinforcement for lightweight armor, and expanding the crystal-forming polymer family by functionalizing the alkyl chains for several potential applications. “The beauty [of this work] is that since we published this, other scientists can now work on these materials and this reaction,” said Yang.
