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Reports: ND754289-ND7: Mechano-Catalysis Coupling within a Single Growing Polymer

Peng Chen, Cornell University

Geoffrey Coates, Cornell University

 Mechanochemistry is at the core of mechano-activatable polymerizations that are crucial for making mechano-responsive polymers. To fully control the design and synthesis of mechanochemo-coupled systems that can efficiently capture a force load to drive useful chemistry, we need fundamental, quantitative understanding of how rates of reactions, including catalytic reactions, respond to mechanical load. The objective of our research is to develop a magnetic tweezers (MT) based approach to define the relation of catalytic kinetics vs. force at a single catalytic center within a single growing polymer. We have chosen to study ring opening metathesis polymerization (ROMP) catalyzed by the second-generation Grubbs catalyst (i.e., G2 catalyst) with norbornene as the monomer. In the past year, we have made the following progresses:

1) Synthesis of a new immobilizable G2 catalyst to allow for implementation of our magnetic tweezers approach. We added a silane group to the NHC ligand of the catalyst, which allowed linking the catalyst to a silica-coated magnetic particle via stable covalent bonds. The coordination environment of the Ru metal center stays identical to the original unmodified catalyst, thus providing least perturbation on the catalyst activity for the following ROMP reactions.

2) Visualization of real-time growth of single polynorbornene under polymerization catalysis with a fixed force load. Using our MT approach, we then monitored growth of the polymer during catalysis. Upon adding norbornene monomer solution to initiate the ROMP reaction without changing the pulling force, unexpectedly, we observe that the extension of the polymer shows sudden increases, which we call jumps, at different time points, and the extension remains almost unchanged between the jumps. By analyzing each growth trace, we extract the following basic parameters. (1) Jump Length, which is the sudden increase of polymer contour length. (2) Growth Length, which is the change of polymer contour length between the end of the previous jump and the beginning of the current jump. (3) Waiting Time, which is the time difference between the end of the previous jump and the beginning of the current jump. (4) Absolute Jump Time, which is time difference between monomer addition and each jump starts. It is interesting that all of them show single-exponential distributions. This means that for each parameter, the underlying process has only one rate-limiting step and the individual events are Poisson processes. These results demonstrate, for the first time, that polymer growth can be monitored at the single-molecule level in real time. And the polymer extension, under real-time growth, does not increase continually, and instead shows sudden jump behaviors with pausing periods in between, which is an unexpected first-of-its-kind discovery.