Polymer microenvironment

S. Tazuke and R. K. Guo, Effects of polymer microenvironment on the thermodynamics of the twisted intramolecular charge transfer fluorescence, Macromolecules 23, 719 (1990). 

Secondly, it should be noted that MV" Cl homo-dimer was formed more readily than PXV Cl homo-dimer. This may be because the polyxylyl group of PXV prevents PXV from dimerization in the polymer microenvironment. The V Cl homo-dimer can transfer its electron to NAD only when the relative configuration of the reactants is suitable for the transfer, whereas V Cl monomer can easily change its orientation to fit the suitable configuration. This may explain the observation that the PXV mediated reaction is more efficient than the case of MV. The calculation result is also in good agreement with the experimental proposal. ... 

Pospisil J. Nespurek S. Pilar J. Effect of environmental stress and polymer microenvironment on efficiency trials and fate of stabilizers. In Service Life Prediction of Polymeric Materials Martin, J. W. Ryntz R. A. Chin J. Dickie R. A. Eds. Springer, New York, USA, 2009, pp 493-520. 

The properties of the microenvironment of soluble synthetic polymers such as polymethacrylamide (PMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(2-vinylpyridine) (P-2VP), poly(4-vinylpyridine) (P-4VP), poly(methyl methacrylate) (PMMA), poly(butyl methacrylate) (PBMA), polystyrene (PS), poly(4[5]-vinylimidazole) (PVIm), and poly(N-2-hydroxypropyl methacrylamide) (PHPMA) and cross-lined polymers were studied by the shift and shape of the band in electronic spectra of a solvatochromic "reporter" molecule embedded in polymer chains. Preferential interaction of parts of the polymer molecule with a reporter and the shielding of interactions between solvent molecules and a reporter molecule of a polymer causes a shift and broadening of its solvatochromic band. This shift is mechanistically interpreted as a change in the polarity of the microenvironment of a polymer in solution in comparison with polarity of the solvent used. 4-(4-Hydroxystyryl)-N-alkylpyridinium-betaine, spiropyran-merocyanine, and l-dimethylamino-5-sulfonamidonaphthalene (Dansyl) reporters were used. In almost all cases the polarity of the polymer microenvironment was lower than that of the solvent. At the same time, the dependence of the nature of the environment on the distance of the reporters from the polymer chain was studied. 

The monomer MPI was not found to be suitable for measuring the polarity of the polymer microenvironment. In the region most interesting for measuring either the solvent polarity or the polarity of the polymer microenvironment, the solvatochromic band of compounds MPI and EPI is not separated from a much more intense adjoining band. 

In Table I, the semiempirical parameter of the solvent polarity and the polymer microenvironment polarity in the same solvents are compared. In all the cases, the microenvironment polarity of a polymer in solution was lower than that of the solvent. In polymers with a partially nonpolar character, such as poly(4-vinylpyridine), poIy(2-vinyl-pyridine), poly(methyl methacrylate), as well as poly(2-hydroxyethyl methacrylate), part of the interactions (dipole-dipole, dipole-induced dipole, multipole, charge-dipole, specific association such as hydrogen bonding, etc.)38 are shielded by the nonpolar backbone of the polymer chain and by the side chains. Solvation of the polymer polar group differs from the solvation of the low-molecular analogue also in other respects. In spite of a relative polarity of the polymer units, the orientation of their dipoles to a bound polar reporter or reactive residues is not as free as for a solvent molecule so that a much wider dispersion of orienting electric dipoles and energy interactions may be encountered (see p. 21h. 

The smallest difference between the polarity of the bulk solution and that of the polymer microenvironment was observed for PHEMA, a larger difference was found for P-2VP, and the largest difference was found for P-4VP. The difference for PBMA was also larger than that for PMMA. This sequence corresponds to that of the expected polarity of the polymers. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

For all the polymers, and the model compound SB, the half-width of th CT absorption band was measured in solvents of different polarities. The half-width of the CT absorption band of the compound SB increases with increasing solvent polarity (Figure 7). At the same polarity of the polymer microenvironments the half-width of the absorption band of SB embedded in the polymers decreases in the order... 

The applicability of a semiempirical polarity scale, based on comparison with a solvent of the same polarity, to processes occuring in the region of the polymer chain microenvironment represents only a first approximation. Since the polarity of the microenvironment is determined using a given solvatosensitive process, the applicability of the result to any other process depends on similarities in the various solvation interactions. Nevertheless, characterization of the polymer microenvironment by the method used in this study has been found to be suitable for a semiquantitative interpretation of the reaction rate of polymer sustituents and of the rate of reactions catalyzed by polymer catalysts. 

The polyelectrolyte covalently functionalized with reactive groups may be viewed as an enzyme-like functional polymer or as a molecular reaction system in the sense that it has both reactive centers and reaction rate-controlling microenvironments bound together on the same macromolecule. 

This finding may be rationalized for the following reasons. The total length of the Me pendant moiety from the cylinder axis is approximately 15 A when the dye moiety is stretched out from the polymer main chain. Since the dye moiety is linked to the main chain via a flexible chemical bond, it may be able to reside at any distance between 3 and 15 A from the cylinder axis. Thus, on average the Me residues would experience the polyanion microenvironment at a distance of about 10 A. 

The dielectric constant of the solvent in the microenvironment of the polymer chain has been shown to be different from that in the bulk solvent (19). This change in dielectric constant might enhance the nucleophilicity of the pyridine ring and therefore increase the rate of quaternization. The kinetic results are consistent with the observations of Overberger et al., (20), who showed that increased hydrophobic nature of the substrate led to faster reaction rates in nucleophilic catalysis. In the present case one would expect the butadiene copolymer to be more hydrophobic than the methylvinylether copolymer. An alternative synthesis of supernucleophilic polymers has been achieved using the following reaction sequence. 

Mechanical and chemical methods for qualitative and quantitative measurement of polymer structure, properties, and their respective processes during interrelation with their environment on a microscopic scale exist. Bosch et al. [83] briefly discuss these techniques and point out that most conventional techniques are destructive because they require sampling, may lack accuracy, and are generally not suited for in situ testing. However, the process of polymerization, that is, the creation of a rigid structure from the initial viscous fluid, is associated with changes in the microenvironment on a molecular scale and can be observed with free-volume probes [83, 84]. 

It was in article [52] where the main reason responsible for the above-mentioned peculiarities was explicitly formulated and substantiated. Its authors related these peculiarities with partitioning of monomer molecules between the bulk of a reaction mixture and the domain of a growing polymer radical. This phenomenon induced by preferential sorption of one of the monomers in such a domain is known as the bootstrap effect. This term was introduced by Harwood [53], because when growing a polymer radical can control under certain conditions its own microenvironment. This original concept enabled him to interpret many interesting features peculiar to this phenomenon. Particularly, he managed to qualitatively explain the similarity of the sequence distribution in copolymerization products of the same composition prepared in different solvents under noticeable discrepancies in composition of monomer mixtures. 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

In many biological systems the biological membrane is a type of surface on which hydrophilic molecules can be attached. Then a microenvironment is created in which the ionic composition can be tuned in a controlled way. Such a fluffy polymer layer is sometimes called a slimy layer. Here we report on the first attempt to generate a realistic slimy layer around the bilayer. This is done by grafting a polyelectrolyte chain on the end of a PC lipid molecule. When doing so, it was found that the density in which one can pack such a polyelectrolyte layer depends on the size of the hydrophobic anchor. For this reason, we used stearoyl Ci8 tails. The results of such a calculation are given in Figure 26. 

The microenvironment of polysoaps estimated by the use of probes reflects the microenvironment where probes are bound. Strop et al. (1976) synthesized the copolymers involving the probe units [14] and [15] as comonomer, and directly estimated the microenvironments along the polymer chain. In all the... 

A criticism often aimed at the use of extrinsic fluorescent probes is the possible local perturbation induced by the probe itself on the microenvironment to be probed. There are indeed several cases of systems perturbed by fluorescent probes. However, it should be emphasized that many examples of results consistent with those obtained by other techniques can be found in the literature (transition temperature in lipid bilayer, flexibility of polymer chains, etc.). To minimize the perturbation, attention must be paid to the size and shape of the probe with respect to the probed region. 

We should first emphasize that viscosity is a macroscopic parameter which loses its physical meaning on a molecular scale. Therefore, the term microviscosity should be used with caution, and the term fluidity can be alternatively used to characterize, in a very general way, the effects of viscous drag and cohesion of the probed microenvironment (polymers, micelles, gels, lipid bilayers of vesicles or biological membranes, etc.). 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

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