Chemistry

Polymer chains grow one ‘hairball’ at a time

A magnetic-tracking technique revealed that molecules added to a polymer exhibit ‘wait-and-jump’ behaviour.

Chain-growth polymerization is widely used for synthesizing long molecules, through the addition of building blocks called monomers to the end of a molecular chain.

A team led by Peng Chen, Geoffrey Coates and Fernando Escobedo of Cornell University in Ithaca, New York, has now observed the growth of an individual polymer as it gains monomers one at a time.

To follow this process, the researchers tethered magnetic particles to one end of polymers. They then used magnetic tweezers, which deploy magnetic fields to manipulate and image the particles, to track single monomers as they were inserted into the polymers. Chains, the team found, did not extend steadily, but instead jumped every few minutes by hundreds of nanometres — equivalent to the addition of thousands of monomers.

Simulations indicate that this ‘wait-and-jump’ behaviour is due to newly appended monomers becoming tangled and held together by weak interactions, forming an ‘entangled hairball’ that suddenly unravels. The researchers used a gentle magnetic force to keep the polymer extended and aligned as it grew. In the absence of this influence, such hairballs might be even more prevalent in polymer growth, they say.