Part I. Measuring chain length dependent rate constants in activation and diffusion controlled polymer end-end reactions. Part II. Effects of electron exchange rates and structure on relaxivity in polynitroxides

In Part I, a new method for the measurement of polymer chain end reaction rate constants is developed which employs gel permeation chromatography (GPC) with fluorescence detection to monitor the reaction of a fluorescent labeled reactive end functionalized polymer with a second reactive polymer chain end.; Using this GPC method, the chain length dependence was measured for the activation controlled reaction of monodisperse polystyryl lithium with a monodisperse pyrene-labeled styrene terminated polystyrene. This study was performed in benzene at the dilute/semi-dilute threshold and extending into the semi-dilute regime, and revealed a concentration dependence on polystyryllithium that is close to first order (0.9 ± 0.1). Interestingly, the reaction rate constant at fixed concentration of polystyryllithium (c = 0.3g/mL) increases with increasing chain length. Possible reasons for this unexpected rate increase with chain length are discussed in the context of aggregation and supramolecular effects.; The diffusion controlled dimerization of fluorescent labeled photochemically generated macroradicals has been monitored using the GPC with fluorescent detection. Photochemically induced dynamic nuclear and electronic polarization has also been explored as a means of amplifying the signal of polymer chain ends.; In Part II, the 1H-NMR relaxivity of dinitroxide substituted phthalate esters and nitroxyl functionalized poly(propylene imine) dendrimers has been measured in acetonitrile and methanol. Studies of dinitroxide relaxivity indicate that electron exchange rate has only a small effect on relaxivity. Outer sphere relaxivity has been measured using benzene as a probe molecule. In studies on dendritic polynitroxides, the per-nitroxide based outer sphere relaxivity nearly doubles for the generation 5 nitroxyl functionalized dendrimer. This relaxivity enhancement may be due to crowding of dendrimer surface groups in higher generation dendrimers. Water relaxivity has been measured for these polynitroxides, and significant inner sphere contributions to relaxivity are found. Dendritic polynitroxides exhibit higher per nitroxide based water relaxivity as compared to a mononitroxide model. These relaxivity enhancements are attributed to an increase in rotational correlation time for the dendritic polynitroxides.