61st Annual Report on Research 2016

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 report, we have described our success in using magnetic tweezers to monitor the growth of individual polynorbornene molecules catalyzed by the G2 catalyst, in which norbornene was used as a model monomer. We observed, unexpectedly, the extension of the polymer does not increase continuously, but instead shows wait-and-jump behaviors, i.e., we observe that the extension of the polymer tether shows sudden increases, which we term jumps, at different time points. Each jump corresponds to the release of the newly grown polymer that was hidden in a hair-ball like structure during the preceding waiting time.

In the past year, we have been focusing on collecting statistics in single-polymer growth behaviors and under several reaction conditions. To summarize, we have made the following progresses: (1) Collected sufficient single-polymer growth data to increase the statistical significance at two different applied forces (18 pN vs. 5 pN). (2) Examined how the norbornene monomer concentration affects the single polymer growth behavior. (3) Examined how the identity of the monomer (i.e., norbornene vs. cyclooctene) affects the single polymer growth behavior. Cis-cyclooctene has lower ring string than norbornene. (4) Analyzed how the polymerization growth rates of individual polymers are determined considering their dispersion even under identical reaction conditions.

We have found that: (1) G2-catalyzed ROMP of norbornene has force-deactivated reaction rate – higher force leads to lower average polymerization rate of individual polymers. (2) At lower monomer concentrations, even though individual polymers grow slower, the appearance of the waiting periods are more pronounced, in which the hair-ball structure stays longer to untangle. (3) The wait-and-jump behavior of single polymer growth is independent of the monomer, using cyclooctene, which forms simple linear polymers without side branches, results in similar behaviors. (4) The overall ROMP rate of a single polymer chain is significantly determined by both jump length and waiting time, which are not directly correlated with each other and both of which seem to be determined by the microscopic configuration of the hair-ball like structure. These results demonstrate, for the first time, that polymer growth can be monitored at the single-molecule level in real time, leading to unexpected first-of-its-kind discoveries, from which insights can be learned into the dynamic behaviors of single-polymer growth in solution.