Radical–TripletPair Interactions as Probesof Long-Range Polymer Motion in Solution.

Radical–triplet pair interactionsare used to investigatethe dynamics of acrylic polymers in dilute solution. Methyl methacrylatewas randomly copolymerized with a small amount of an amine-containingmonomer to create the polymers. The amine subunits were then oxidizedto nitroxide moieties (stable free radicals). Using time-resolvedelectron paramagnetic resonance (TREPR) spectroscopy on the sub-microsecondtime scale, competition is observed between two deactivation processesof the ester side chain photoexcited triplet state: (1) Norrish Iα-cleavage, leading to a 21-line main chain polymeric free radicalTREPR spectrum with emissive triplet mechanism spin polarization,and (2) spin polarization transfer or quenching by a nearby stablenitroxide radical, which gives a 3-line spectrum exhibiting emissiveradical–triplet pair spin polarization. The main chain polymerradical signal dominates the TREPR spectrum if the probability ofradical–triplet pair encounters is low. These competing reactionsshow a strong dependence on nitroxide incorporation (mol %), temperature,solvent, and acrylic polymer ester side chain structure. A comparisonof steady state EPR and TREPR signals from inter- vs intramolecularprocesses clearly demonstrates the influence of polymer chain dynamicson the observed phenomena. [ABSTRACT FROM AUTHOR]