Evaluating polymerization kinetics using microrheology

Monitoring the kinetic evolution of molecular weight for growing polymers is critical to understand and optimize polymerization reactions for materials development and discovery. In this work, we propose the use of passive probe microrheology as a facile and low-cost method to monitor polymer growth kinetics by indirectly tracking the molecular weight evolution of a polymerizing reaction mixture using time-resolved measurements of sample viscosity. To do so, a recently developed Brownian probe microrheology method based on differential dynamic microscopy (DDM) was applied to a model system of dimethylacrylamide undergoing reversible addition-fragmentation chain-transfer (RAFT) polymerization. The polymerization rate constants extracted from microrheology were within 20% of those obtained from conventional nuclear magnetic resonance (NMR) spectroscopy and size-exclusion chromatography (SEC) measurements. A simple and intuitive workflow based on a single-point Mark-Houwink analysis was then used to estimate an apparent viscosity from NMR and SEC data and, equivalently, an apparent molecular weight from microrheology data. Over the expected range of validity of the analysis, the results are in reasonable quantitative agreement with the corresponding independently measured values. The results demonstrate the ease and reliability of inferring the molecular weight from viscosity data and highlight the capability of DDM microrheology to monitor polymerization of polymer systems. High-throughput microrheology and simple viscosity modeling can be used to continuously monitor the kinetic evolution of polymer molecular weight during controlled polymerizations.