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Single polymer growth dynamics

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Science  20 Oct 2017:
Vol. 358, Issue 6361, pp. 352-355
DOI: 10.1126/science.aan6837

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Watching growth, step by step

Polymers can grow through the stepwise addition of monomers to an active end site. One might think that this would happen in a continuous linear process. With a focus on ring-opening metathesis polymerization of norbornene catalyzed by a Grubbs' catalyst, Liu et al. describe the growth of a single polymer chain. By attaching one end of a growing polymer to a bead exerting a constant force on it, they measured the extension occurring during the growth of the polymer. Oddly, the extension of the growing polymer did not increase continuously. Instead, it exhibited consecutive wait-and-jump steps, owing to conformational entanglements formed by newly incorporated monomers.

Science, this issue p. 352

Abstract

In chain-growth polymerization, a chain grows continually to reach thousands of subunits. However, the real-time dynamics of chain growth remains unknown. Using magnetic tweezers, we visualized real-time polymer growth at the single-polymer level. Focusing on ring-opening metathesis polymerization, we found that the extension of a growing polymer under a pulling force does not increase continuously but exhibits wait-and-jump steps. These steps are attributable to the formation and unraveling of conformational entanglements from newly incorporated monomers, whose key features can be recapitulated with molecular dynamics simulations. The configurations of these entanglements appear to play a key role in determining the polymerization rates and the dispersion among individual polymers.

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