KAUST scientists have developed a new catalyst that can transform a mixture of three monomers into diblock dialternating terpolymers in a single step. Image: 2022 KAUST; Veronica Moraru.
KAUST scientists have developed a new catalyst that can transform a mixture of three monomers into diblock dialternating terpolymers in a single step. Image: 2022 KAUST; Veronica Moraru.

A catalyst developed by researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia could be key to achieving structural diversity in polymer materials and industrial-scale polymerizations involving multiple monomers. As the researchers report in a paper in Nature Communications, the catalyst can transform a mixture of three monomers into well-defined, ordered diblock terpolymers in one step.

Block copolymers comprise at least two polymer segments, each made up of different monomer species, combining the properties of the two polymers in the same molecule. This makes block copolymers appealing for a wide range of applications, from thermoplastics to biomaterials for drug delivery.

Up to now, chemists have relied on two-step polymerizations in one or two separate vessels to synthesize block copolymers. A ‘two-pot/two-step’ polymerization requires isolating the first polymer segment before proceeding with the next reaction, which is cumbersome and costly.

A ‘one-pot/two-step’ approach can bypass this isolation step, but demands a so-called living polymerization, where, once the first monomer is consumed, the growing chain remains reactive to accept another monomer and both monomers are fully converted. However, this can lead to unwanted side reactions if the second monomer is not added in time.

Nikos Hadjichristidis and co-workers at KAUST have now devised a ‘one-pot/one-step’ approach for producing diblock terpolymers, in which three monomers form two different alternating copolymer segments. “This is the simplest method,” says postdoc fellow Jiaxi Xu, who co-led the study.

Polymerizations involving three monomers usually produce random terpolymers. Therefore, it was crucial to find a smart catalyst to regulate the monomer sequence during the polymerization.

The researchers discovered an auto-switchable phosphazene-based catalyst that can stimulate the polymerization of one monomer while inhibiting that of others. The switchability of the catalyst depends on its ability to exchange protons with the growing chain.

“When the first monomer is consumed completely in the alternating copolymerization with the second monomer, the catalyst turns on to promote the alternating copolymerization of the excess of the second monomer with the third one,” explains Xu.

Selecting cyclic monomers from the nitrogen-containing N-sulfonyl aziridine family, and from the oxygen-containing epoxide and anhydride families, was essential for the terpolymerization. “We had previously synthesized block copolymers consisting of one alternating copolymer and a homopolymer using aziridines and anhydrides,” Xu says. This suggested that three monomers with different reactivities could effectively be used for the ‘one-pot/one-step’ preparation of diblock dialternating terpolymers.

The researchers found that the catalyst promoted the aziridine/anhydride alternating copolymerization before switching to the epoxide/anhydride alternating copolymerization, demonstrating its high monomer selectivity and kinetic control.

The team plans to further tap into aziridine, epoxide and anhydride terpolymers. “These families contain hundreds of monomers, which will provide extremely diverse diblock dialternating terpolymers for many potential industrial applications,” Hadjichristidis says. “This study also opens new horizons for the terpolymerization of other monomer families.”

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.