Real-Time Polymer Viscosity-Catalytic Activity Relationships on the Microscale

Polymer growth induces physical changes to catalyst microenvironments. Here, these physical changes are quantified in real time and are found to influence microscale chemical catalysis and the polymerization rate. By developing a method to "peer into" optically transparent living-polymer particles, simultaneous imaging of both viscosity changes and chemical activity was achieved for the first time with high spatiotemporal resolution through a combination of fluorescence intensity microscopy and fluorescence lifetime imaging microscopy techniques. Specifically, an increase in microenvironment viscosity led to a corresponding local decrease in the catalytic molecular ruthenium ring-opening metathesis polymerization rate, plausibly by restricting diffusional access to active catalytic centers. Consistent with this diffusional-access model, these viscosity changes were found to be monomer-dependent, showing larger changes in microenvironment viscosity in cross-linked polydicyclopentadiene compared to non-crosslinked polynorbornene. The sensitivity and high spatial resolution of the imaging technique revealed significant variations in microviscosities between different particles and subparticle regions. These revealed spatial heterogeneities would not be observable through alternative ensemble analytical techniques that provide sample-averaged measurements. The observed spatial heterogeneities provide a physical mechanism for variation in catalytic chemical activity on the microscale that may accumulate and lead to nonhomogeneous polymer properties on the bulk scale.