Polymer microenvironment cross-linked polymers

Properties of the microenvironment of soluble and cross-linked polymers were studied by the shift of bands in the electron spectra of solvatochromic reporter molecules embedded in polymer chains. Generally, the charge-transfer (CT) absorption spectra and emission spectra of a number of compounds were used to correlate solute-solvent interactions with physical and chemical properties of interest. The energy of the band maxima of these chromophores is quite solvent sensitive and is linearly correlated with empirical solvent polarity parameters. The observed shift of the maximum of the solvatochromic reporter embedded in the polymer chains, compared with a low-molecular weight analog in the same solvent, was interpreted in terms of a change in the polarity of the microenvironment of the polymer in solution. 

To determine the polarity of the microenvironment of polymers, polymer labelled with solvatochromic reporters were prepared, either by (1) copolymerization with solvatochromic monomers 2 >3 or (2) polymer analogous reactions (e.g., the reaction of copolymer acUve esters with primary amino groups of the solvatochromic molecule or alkylation reaction of PVIm and cross-linked polymers with solvatochromic mole-cule.34i37 The properties of the microenvironment of polymers were studied by the shift and shape of band in electronic spectra of a solvatochromic reporter molecule embedded in polymers. 

For practical reasons, polystyrene is used as polymer support in many cases. In most cases of the polymer-supported Lewis acids polystyrene-based cross-linked polymers have still been utilized as support materials mainly due to their easy synthesis and chemical modiflcations. Some other examples of polyethylene (PE) fibers, PE glycols and silica-supported Lewis acids were also developed. Microenvironments within network structme are different in individual support materials. 

In another approach, a fluorescent conjugated polymer was used as the material for the preparation of a chemosensor to detect 2,4,6-trinitrotoluene (TNT) and its related nitroaromatic compounds. To this end, microparticles, made of three-dimensionally cross-linked poly(l,4-phenylene vinylene) (PPV) via emulsion polymerization, were synthesized [61]. This material was chosen due to its high fluorescence intensity and sensitivity to changes in its microenvironment. The chemosensor was exposed to vapour containing different amounts of TNT and quenching of the polymer luminescence at 560 nm was observed after excitation at 430 nm. The dependence of the fluorescence signal in response to the analyte was described by a modified Stem-Volmer equation that assumes the existence of two different cavity types. The authors proposed the modified Stem-Volmer equation as follows ... 

A typical example is the conversion of octanol to actyl chloride in the presence of a carbon tetrachloride-triphenylphosphine reagent. A large increase in rate is measured when polymeric triphenylphosphines replace the momomeric reagent. The effect is particularly noticeable when a cross-linked rather than a linear polymer is used (rate increases 25-fold)157. This phenomenon is attributed to the specific microenvironment of the poly-styrene-divinylbenzene based polymer. Polvmeric phosphines are also used in the conversion of carboxylic acids to acid halides158-165. 

The hydrophobic microenvironments created on the cross-linked polystyrene may raise the intrinsic peptide-hydrolyzing activity of the metal center, as noted above. The microdomains created on the synthetic polymer may facilitate complexation of the protein substrate with the catalytic center. Possible inactivation of the metal center by formation of hydroxo- or oxo-bridged dimers or oligomers can be prevented upon immobilization of the metal center to a sohd support. Higher pH values inaccessible by a soluble metal complex can be attained by the corresponding immobilized metal complex. The enhancement in the proteolytic activity of the Cu(II) complex of cyclen upon attachment to the polystyrene may be attributed to some of these effects. 

Two different catalysts for hydrogen peroxide decomposition, the enzyme peroxidase (isolated from the horseradish root, HRP), and polymer-supported catalyst (acid form of poly-4-vinylpyri-dine functionalized by ferric sulfate, apFe) [99,100], are examined with an aim to compare their activity. The active center in the peroxidases is the ferric ion in protoporphyrin IX. Besides the complex made of ferric ion and protoporphyrin IX, that is ferricprotoporphyrin IX, also known as ferric heme or hemin, peroxidase possesses a long chain of proteins [101,102]. On the other hand, the macroporous acid form of polyvinyl pyridine functionalized by ferricsulfate is obtained from cross-linked polyvinyl pyridine in macroporous bead form [103]. Pyridine enables it to form coordination complexes or quaternary salts with different metal ions such as iron (111) [104]. An active center on the polymeric matrix functionalized by iron, as metallic catalyst immobilized on polymer by pyridine, has similar microenvironment conditions as active center in an enzyme [105]. 

The properties of the microenvironment of soluble and cross-linked synthetic polymers were studied using solvatochromic reporters bound to polymers. The polarity of the domain of polymer chains was estimated in one-component and binary solvents and compared with the polarity of solvents used. The polarity was expressed semiempirically by the absorption or emission band energy of a solvatochromic compound. The polarity of the microenvironment of soluble polymers and also the polarity in the vicinity of matrix of cross-linked polymers suspended in aqueous buffer was almost in all cases lower than that of the solvent. 

The polarity of the microenvironment of synthetic polymers in solution and cross-linked polymers was studied in order to characterize semiquantitatively, from one point of view, the influence of the polymer on the reactions of bound functional groups and on interactions of the macromolecules with low- or high-molecular weight compounds. 

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