Tracking polymerization with laser focus

Researchers at Cornell University have devised a new single-molecule microscopy method for studying polymers as they grow (Nat Chem. 2023 DOI: 10.1038/s41557-023-01363-2). “We see every monomer,” says Peng Chen, who led the work.

Chen and his team accomplish this by tethering every monomer to a masked fluorescent dye that can be unquenched using a laser, then imaged and bleached with a different laser. By timing the laser pulses correctly, they are able to see monomers being added to surface-tethered polymer chains, one by one. They call the method CREATS, short for “coupled reaction approach toward super-resolution imaging.”

CREATS can track individual polymers as they grow through hundreds of sequential monomer additions. “In principle, we can follow it for as long as we want,” says Chen, though realistically the experiments are limited by time and data storage capacity.

The researchers used their method to study the kinetics of ring-opening metathesis polymerization (ROMP). They found that interactions with the surface slow down the reaction at the beginning of the polymerization, but monomer addition reaches a steady rate once the chain is about 20 nm long. When they looked at a copolymerization with 2 different monomers, each tagged with a distinct fluorescent label, they found that the sequence isn’t completely random. It’s slightly biased toward the next monomer added to the chain being the same as the one before it, leading to repeated segments.

In an email, Hao Shen of Kent State University calls the method “ingenious” and says he anticipates that many other researchers will want to try CREATS. Chen says his group has only looked at ROMP processes so far, but the method should work for any chain-growth polymerization mechanism.