Quantum Dots Catalyze Polymer Creation

Using semiconductor quantum dots (QDs) as a catalyst for light-mediated radical polymerization, materials scientists are exploring a stable and economical method for making polymers. Such a method could be used in place of molecular catalysts or expensive transition metals.

Rice University graduate student Yifan Zhu holds a vial of photosensitive, semiconducting quantum dots the lab uses as a catalyst to make functional synthetic polymers powered by light. Courtesy of Jeff Fitlow/Rice University.

The materials scientists, from Rice University, used various light sources, including natural sunlight and a household lamp, to illuminate a solution of dispersed cadmium selenide QDs. This process introduced the generation of free radical atoms from a bromide-based initiator, triggering acrylate monomers in the solution to link. This form of linkage, which cannot be terminated until all the monomers in the chain are consumed or the researcher chooses to terminate it, is an example of a living polymerization process.

The team demonstrated polymerizations of methacrylates and styrene and the construction of block copolymers. Temporal control of the polymerization by the light source was also demonstrated.

Photoluminescence experiments revealed that the key to the light-mediated radical polymerization was the reduction of alkyl bromide initiator by photoexcited QDs.

Rice University’s Eilaf Egap, an assistant professor of materials science and nanoengineering and chemical and biomolecular engineering, holds a vial of quantum dots her lab is using to catalyze the creation of functional polymers. Courtesy of Jeff Fitlow/Rice University.

Professor Eilaf Egap said that QD polymerization shows promise as a method for highly controlled growth of sophisticated polymers.

“The beauty of this is, if you have monomer A and you want to add monomers B and C in a specific sequence, you can do that,” she said. “In a random polymerization, they would be randomly dispersed along the polymer backbone.”

Egap added, “The implication here — and part of our broader goal — is that we can synthesize organic-inorganic hybrid structures in a controlled and periodic way for many applications.”

Rice University researchers, from left, Yiming Huang, Eilaf Egap and Yifan Zhu are employing the power of the sun to build functional synthetic polymers using photosensitive, semiconducting quantum dots as a catalyst. They said the living polymerization process could lead to the creation of novel polymers. Courtesy of Jeff Fitlow/Rice University.

Egap says the process could also be used for the discovery of novel polymers, like a QD photocatalyst with an attached semiconducting polymer to simplify the manufacture of solar cells and other devices.

“These could also be relevant to light-emitting diodes, magnetoelectronics and bioimaging,” she said. “We could grow them all at once. That’s the dream, and I think we are within reach.”

The research was published in ACS Macro Letters (doi: 10.1021/acsmacrolett.7b00968).