I. Characterization of zeolite external surface area using electron paramagnetic resonance. II. End labeling efficiency in the atom transfer radical polymerization of styrene.

摘要

EPR methods have been applied to a series of monodisperse silicalite crystals as sensitive and structurally specific methods for investigating silicalite external surface areas.; The absorption of ortho-methyldibenzyl ketone (oMeDBK) on the external surface of silicalite crystals was studied using a coadsorbed nitroxide probe. The displacement of the adsorbed nitroxide probe by coadsorbed oMeDBK leads to the conclusion that the adsorbate molecules first adsorb on stronger binding sites (pore openings) characterized by slow motion of the probe (broad EPR lines) and then the adsorbate molecules adsorb on weaker binding sites (external surface framework between pore openings) characterized by fast motion of the probe (narrow EPR lines). The transition point from slow to fast motion provides a quantitative measurement of silicalite external surface area.; Using nitroxide as the sole adsorbate with increasing surface coverage, the EPR line shape shows a progressive change as a result of increasing dipolar interaction and electron spin exchange interaction at decreasing inter-spin distance. The minimum inter-spin distance as measured from the dipolar interaction corresponds to the complete coverage of the pore openings and the corresponding nitroxide loading is a measure of the external surface area of the silicalite crystals. The transition point from nearly zero to high spin-spin interaction is also surface coverage related and provides a highly sensitive means of characterization of the silicalite external surface area.; The adsorption strength on pore openings is molecular structure dependent and polar groups provide more significant contribution than the possible intercalation of phenyl/tolyl groups. Sequential adsorption of 14N and 15N spin labeled nitroxides shows the dynamic exchange between the adsorbates in the pore openings and those in solution or on weak binding sites.; The end labeling efficiency of polystyrenes prepared using Atom Transfer Radical Polymerization method was studied using spectroscopic and chromatographic methods. The labeling efficiency was found to be dependent on the monomer/initiator ratio of the starting material, with high ratios leading to low labeling efficiency.