Substrate concentration inside polymer

Remarkably, the use of a fluorous biphasic solvent system in combination with a [Rh(NBD)(DPPE)]+-type catalyst (NBD = norbornadiene) copolymerized into a porous nonfluorous ethylene dimethacrylate polymer, resulted in an increased activity of the catalyst relative to a situation when only toluene was used as solvent [30]. The results were explained by assuming that fluorophobicity of the substrate (methyl-trans-cinnamate) leads to a relatively higher local substrate concentration inside the cavities of the polymer when the fluorous solvent is used. That is, the polymer could be viewed as a better solvent than the fluorous solvent system. This interpretation was supported by the observations that (i) the increase in activity correlates linearly with the volume fraction of fluorous solvent (PFMCH) and (ii) the porous ethylene dimethacrylate polymer by itself lowers the concentration of decane in PFMCH from 75 mM to 50 mM, corresponding to a 600 mM local concentration of decane in the polymer. Gas to liquid mass transport limitation of dihydrogen could be mled out as a possible cause. 

Substrate-catalyst interaction is also essential for micellar catalysis, the principles of which have long been established and consistently described in detail [63-66]. The main feature of micellar catalysis is the ability of reacting species to concentrate inside micelles, which leads to a considerable acceleration of the reaction. The same principle may apply for polymer systems. An interesting way to concentrate the substrate inside polymer catalysts is the use of cross-linked amphiphilic polymer latexes [67-69]. Liu et al. [67] synthesized a histidine-containing resin which was active in hydrolysis of p-nitrophenyl acetate (NPA). The kinetics curve of NPA decomposition in the presence of the resin was of Michaelis-Menten type, indicating that the catalytic act was accompanied by sorption of the substrate. However, no discussion of the possible sorption mechanisms (i.e., sorption by the interfaces or by the core of the resin beads) was presented. 

The rate In a batch reactor must be based on the total amount of substrate In the system. There Is no way to measure the concentration (or moles) of substrate inside the polymer beads It must be computed. Substrate Is depleted from the bulk phase for two reasons Initially the polymer phase contained no substrate, and substrate reacts Inside the polymer beads. Total moles of substrate are easily determined In systems which Involve addition of a gas phase reactant at constant pressure. The reaction rate Is developed using the conversion of substrate. Equation 2 uses the total moles of substrate In the system. A material balance Is performed over the batch reactor at any time t to give the moles of substrate in the bulk and polymer phases. The derivative of this balance may be used to calculate reaction rates by making use of Equation 3. The derivative of the total mole balance with respect to dimensionless time is... 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

In this mode, a small SECM tip is used to penetrate a microstructure, for example, a submicrometer-thick polymer film containing fixed redox centers or loaded with a redox mediator, and extract spatially resolved information (i.e., a depth profile) about concentrations, kinetic- and mass-transport parameters [33, 34]. With a tip inside the film, relatively far from the underlying conductor or insulator, solid-state voltammetry, at the tip can be carried out similarly to conventional voltammetric experiments in solution. At smaller distances, the tip current either increases or decreases depending on the rate of the mediator regeneration at the substrate. If the film is homogeneous and not very resistive, the current-distance curves are similar to those obtained in solution. 

The same group also reported an amphiphiHc PS-PEG resin-supported mthe-nium complex, which could catalyze the Kharasch reaction in water under heterogeneous conditions without any radical initiators (Scheme 8.29) [65]. Owing to the self-concentration of hydrophobic organic substrates inside the polymer matrix in water, it was claimed that the catalytic efficiency of the PS-PEG Ru in water was comparable to the most efficient homogeneous Ru catalysis reported thus far. 

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