Polymer microenvironment polarity

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... 

In those binary solvents in which selective sorption of the more polar component on the polymer chain may take place, an increased polarity of microenvironment could be observed, compared with the polarity of the binary mixture used. In aU the binary solvents studied here (alcohol/, dioxane/, and acetone/water) a qualitative eement was found between the polarity of the microenvironment of PHEMA in solution determined from the energy of the CT absorption band of pyridinium-betaine (SB) and values obtained from the energy of the emission transition of PHEMA-bound DNS fluorophore (Figure 11). 

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. 

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 ... 

R. Hayashi, S. Tazuke, and C. W. Frank, Twisted intramolecular charge-transfer phenomenon as a fluorescence probe of microenvironment. Effect of polymer concentration on local viscosity and microscopic polarity around a polymer chain of poly(methyl methacrylate), Macromolecules 20, 983 (1987). 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

The triphase hydrolysis of 1-bromoadamantane with catalysts 50 (n = 1-16) and 51 was studied kinetically 170). The enthalpies of activation (AH ) for the catalyzed reactions were 6-12 kcal/mol lower than for the uncatalyzed reaction. (The free energies of activation were 1-2 kcal/mol lower). This considerable variation in AH was attributed to a much different microenvironment in catalysts 50 and 51 from that in the absence of the catalyst. Thus Regen 170> pictures both the organic phase and the aqueous phase in the polymer matrix, as if homogeneous. Studies using spin-labeled compounds also indicated that catalyst 50 affects the polarity and the motional freedom of the microenvironment171). 

Z values have been widely used to correlate other solvent-sensitive processes with solvent polarity, e.g. the a absorption of haloalkanes [61], the n n and n n absorption of 4-methyl-3-penten-2-one [62], the n n absorption of phenol blue [62], the CT absorption of tropylium iodide [63], as well as many kinetic data (Menschutkin reactions, Finkelstein reactions, etc. [62]). Copol5mierized pyridinium iodides, embedded in the polymer chain, have also been used as solvatochromic reporter molecules for the determination of microenvironment polarities in synthetic polymers [173]. No correlation was observed between Z values and the relative permittivity e, or functions thereof [317]. Measurement of solvent polarities using empirical parameters such as Z values has already found favour in textbooks for practical courses in physical organic chemistry [64]. 

A nonpolar solvent favors conformation A, whereas conformation B is favored by more polar solvents (e.g. dimethylformamide, hexamethylphosphoric triamide) because the cation is more solvated (cf. Table 9, entries 1 and 2). However, this solvent effect is absent when BujP Cu" is used as counterion. Conformation A is more favored by relatively small counterions, such as the lithium and sodium ion, as compared to the larger potassium ion, due to the higher degree of association of the former. Steric strain between ASG and ASG is minimized in conformation B. Conformations A and B lead to trans- and c -substituted cyclopropanes, respectively. A study of cyclopropane esters, -in which the stereoselectivity of the reaction of polymer-supported reagents was compared with molecules of low-molecular weight, made clear that the steric and polar microenvironment of the polymer-supported reaction is not different enough in bulk to influence the selectivity substantially. Nevertheless, a specific influence of the solid phase can be observed at low temperatures. 

The concept of immobilized ionic liquids entrapped, for instance, on the surface and pores of various porous solid materials (supported ionic liquid phase, SILP) is rapidly become an attractive alternative. In addition, the SILPs can also answer other important issues, such as the difficult procedures for product purification or IL recycling, some toxicity concerns and the problems for application in fixed-bed reactors, which should be addressed for future industrial scale-up. This new class of advanced materials shares the properties of true ILs and the advantages of a solid support, in some cases with an enhanced performance for the solid material. Nevertheless, a central question for the further development of this class of materials is to understand how much the microenvironment provided by the functional surfaces is similar or not to that imparted by ILs. Recent studies carried out using the fluorescence of pyrene to evaluate the polarities of a series of SILPs based on polymeric polystyrene networks reveal an increase in polarity of polymers, whereas the polymer functional surfaces essentially maintain the same polarity as the bulk ILs. However, this is surely not a simple task, in particular if we consider that the basic knowledge of pure ILs is still in its infancy, and we are just starting to understand the fundamentals of pure ILs when used as solvents. 

Organic polymers are comparable to the above catalysts, having microenvironments different from those of the surrounding media and being swellable (see 14.2.4.1). Micelles, which are colloidal species produced by aggregation of ca. 20 to thousands of surfactant molecules or ions with both polar and nonpolar portions, also have these characteristics. In a typical aggregate, the hydrophobic ends of the molecules are clustered in the core of the micelle, and the polar ends are located at the interface to the aqueous phase. Micelles may be spherical or (in highly concentrated solutions) cylindrical or lamellar. Inverted micelles may form in a hydrocarbon solvent. 

The sp values of most perfumery ingredients fall between ca. 16MPa1/2 (non-polar materials such as terpene hydrocarbons) and ca. 25 MPa1/2 (polar materials such as alcohols). In general, we expect that materials will have lower activity coefficients in microenvironments characterised by similar values of sp. For instance, limonene (sp value of 16.5) is expected to be compatible with plastics such as polyethylene and polypropylene (sp range typically 16-18), and to exhibit good solubility and retention in these polymers. We would anticipate that... 

Sequential complexation was confirmed in reference 97, where it was reported that P-CD addition decreases the R value and increases the intensity of the two vibronic bands. Solutions of P-CD containing 13 always exhibited a biexponential decay the shortest, t, = 130 ns, has the same lifetime of 13 in water, the largest, Tj = 300 ns, indicates that 13 experiences a hydrophobic environment. The ratio of the preexponentials /42Mi grew monotonically with [/ -CD]. The data are consistent with a sequential complexation, as in the complexation of 13 with a-CD. In the 1 1 complex, the included pyrene has the same lifetime as 13 in water because a substantial portion of the molecule is still exposed to the solvent when 13 is encapsulated by two cyclodextrins, it experiences a low-polarity microenvironment and its lifetime consequently increases. In the same paper, the complexing ability of a polymer-supported P-CD of the general formula... 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

Use has also been nude of the fact that chain fragments within a crosslinked polymer have restricted mobility. Under certain conditions active species attached to die polymer can thus be effectively isolated from each other at relatively high concentrations, providing the advantages of high dilution and specificity along with rapid kinetics. In other cases, properties of die backbone itself such as polarity, pore size and chirality were utilized to achieve unique reactions, the polymer providing a specifrc microenvironment for the reaction. These aspects of PRs have been extensively reviewed (6.7). 

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