E. A. Litmanovich

The effect of Pluronics on the photocatalytic activity of water-soluble porphyrins

Solubilization of hydrophilic porphyrins, 2,7,12,18-tetramethyl-3,8-di(1-methoxyethyl)-13,17-di(2-oxycarbonylethyl) porphyrin disodium salt (dimegin) and N-methyl-di-D-glucose amine salt of chlorine e6 (photoditazine), as well water-insoluble meso-tetraphenylorphyrin in the micelles of Pluronics (triblock copolymers and propylene oxide and ethylene oxide), increases the photocatalytic activity of porphyrins in the course of the oxidation of L-tryptophan in aqueous salt-containing solutions. The maximum photocatalytic activity is attained for the photocatalysts based on the Pluronics P85 and F127 containing 50–70% ethylene oxide units. As a result of solubilization, the activity of tetraphenylporphyrin increases by a factor of 50, while the activities of hydrophilic dimegin and photoditazine increase by factors of 1.5 and 6, respectively. The increased activity of tetraphenylporphyrin is due to the dissolution and dissociation of aggregates in the presence of Pluronics. The increased activity of dimegin, which is known to aggregate in water, is primarily provided by disaggregation. In the case of photoditazine, which does not form aggregates in aqueous solutions and is likely to be localized in the polar micellar “crown,” the effect of a polymer is due to the local concentration of both a substrate and a catalyst in the micellar pseudophase.

Controlled synthesis of multiblock copolymers by pseudoliving radical polymerization via the reversible addition-fragmentation chain-transfer mechanism

With the use of two classes of reversible addition-fragmentation chain-transfer agents—dithiobenzoates and trithiocarbonates—multiblock copolymers based on styrene and n-butyl acrylate, which are the best-studied monomers in these processes, are synthesized. It is shown that the polymers containing dithiobenzoate and trithiocarbonate groups are highly efficient for the synthesis of block copolymers, which is independent of the number of stages at which the polymeric RAFT agents are used in polymerization: In all cases, the polymeric RAFT agent is fully consumed in the polymerization of the “alien” monomer. The mechanism governing chain formation during the synthesis of multiblock copolymers, that is, the character of monomer insertion into the polymer chain, via one or both ends, is studied. It is found that the order of monomer loading determines the ratio of chains growing through one or two ends. The thermal stability of amphiphilic multiblock copolymers, their solubility in various solvents, and self-organizing ability are investigated.

The problem of bimodal distributions in dynamic light scattering: Theory and experiment

A new method is proposed for analyzing the results of dynamic light scattering measurements of polymer solutions containing macromolecules with substantially different molecular masses. The processing algorithm makes it possible to correctly allow for effects of the quality of solvent and the conformation and polarizability of macromolecules on the contributions of components of a system to the intensity of scattering. The developed method is tested for a model mixture of two polystyrene samples dissolved in a good solvent (tetrahydrofurane) and a θ solvent (cyclohexane) and is applied to study a system containing a linear polymer and an interpolymer complex.

Phase separation in a poly(acrylic acid)-polycation system in acidic solutions

Phase separation in aqueous solutions of mixtures of poly(acrylic acid) with polycations [poly(diallyldimethylammonium chloride), poly(1,2-dimethyl-5-vinylpyridinium methylsulfate)] in the presence of decimolar hydrochloric acid has been studied by static and dynamic light scattering. The systems are characterized by the upper critical solution temperature. The temperature-dependent association of macromolecules is observed in the single-phase region. A decrease in temperature leads to phase separation; the composition of the diluted phase is determined by the temperature and composition of the initial mixture; and the composition of the concentrated phase remains almost invariable. The occurrence of association in the mixtures is probably the formation of interpolymer complex due to ion-dipole interactions between the carboxyl groups of poly(acrylic acid) and the functional groups of the polycation.

Cross-metathesis of polynorbornene with polyoctenamer: a kinetic study

Summary The cross-metathesis of polynorbornene and polyoctenamer in d-chloroform mediated by the 1st generation Grubbs’ catalyst Cl2(PCy3)2Ru=CHPh is studied by monitoring the kinetics of carbene transformation and evolution of the dyad composition of polymer chains with in situ 1H and ex situ 13C NMR spectroscopy. The results are interpreted in terms of a simple kinetic two-stage model. At the first stage of the reaction all Ru-benzylidene carbenes are transformed into Ru-polyoctenamers within an hour, while the polymer molar mass is considerably decreased. The second stage actually including interpolymeric reactions proceeds much slower and takes one day or more to achieve a random copolymer of norbornene and cyclooctene. Its rate is limited by the interaction of polyoctenamer-bound carbenes with polynorbornene units, which is hampered, presumably due to steric reasons. Polynorbornene-bound carbenes are detected in very low concentrations throughout the whole process thus indicating their higher reactivity, as compared with the polyoctenamer-bound ones. Macroscopic homogeneity of the reacting media is proved by dynamic light scattering from solutions containing the polymer mixture and its components. In general, the studied process can be considered as a new way to unsaturated multiblock statistical copolymers. Their structure can be controlled by the amount of catalyst, mixture composition, and reaction time. It is remarkable that this goal can be achieved with a catalyst that is not suitable for ring-opening metathesis copolymerization of norbornene and cis-cyclooctene because of their substantially different monomer reactivities.

Synthesis of poly(N,N-dimethylaminoethyl methacrylate) nanogels in reverse micelles for delivery of plasmid DNA and small interfering RNAs into living cells

Hydrogel nanoparticles of poly(N,N-dimethylaminoethyl methacrylate) with hydrodynamic radii of 40–50 nm are synthesized via the copolymerization of a water-soluble monomer and N,N′-methylenebis(acrylamide) in a solution of Brij-97 reverse micelles in cyclohexane. All amino groups of nanoparticles are protonated in a weakly acidic solution; however, almost one-third of them remain inaccessible to a flexible polystyrenesulfonate polyanion, while almost two-thirds of them remain inaccessible to rigid double-helical DNA. Complexes of nanogels with plasmid DNA carrying the firefly luciferase gene transfect eukaryotic cells in a cultural medium, and the products of interaction of nanogels with small interfering RNAs suppress expression of the marker enzyme. In both systems, the replacement of linear polyamine with nanogel significantly increases the efficiency of delivery. The activity of nanogels in transfection experiments depends in an extremum pattern on the crosslink degree and achieves a maximum value at a crosslinking-agent concentration of 5 mol %. The results of this study suggest that the developed procedure offers promise for the synthesis of cationic nanogels as vectors for delivery of genetic material into living cells.

Rheological properties of concentrated aqueous solutions of anionic and cationic polyelectrolyte mixtures

The rheological properties of aqueous solutions of poly(acrylic acid), poly(diallyldimethyldiammonium chloride), and their mixtures at 25°C have been studied. The concentrated solutions of the mixtures contain 18 wt % of both polymers taken at different ratios. The ratio of cationogenic and anionogenic groups φ varied from 0 to 0.4 is taken as a criterion for selection of mixture composition. An increase in φ, reflecting a more intense formation of polyelectrolyte complexes in solution, is accompanied by a significant rise in the low-frequency loss modulus and, especially, in the storage modulus, as well as by an increase in viscosity over the entire studied range of shear rates. This behavior may be explained by the presence of an additional spatial structure with junctions formed by interacting complementary charged groups. In the general case, the formation of poly(acrylic acid)-poly(diallyldimethyldiammonium chloride) polyelectrolyte complexes is said to take place in solution. The excess of rheological characteristics of mixture solutions over the corresponding characteristics of poly(acrylic acid) solutions is found to be the power function of parameter φ. The additional spatial network derived from polyelectrolyte complexes and occurring in solution is destroyed at lower shear stresses than is the network of intermolecular entanglements. At high shear stresses, orientational effects may cause phase separation of the systems owing to a change in the hydrophilic-hydrophobic balance between complexes of poly(acrylic acid) with poly(diallyldimethyldiammonium chloride) and water.

Effect of pH on complexation in mixed dilute aqueous solutions of poly(acrylic acid), poly(ethylene glycol), and Cu2+ ions

The phase states of mixed dilute solutions of PAA, PEG, and Cu2+ ions largely determines the mechanism governing the growth of metal nanoparticles during the subsequent reduction of copper ions. Mixtures with PAA: PEG > 1 base-mol/base-mol and PAA: Cu2+ ≥ 5 base-mol/mol are studied. It is shown that the simultaneous complexation of PAA with PEG and Cu2+ ions in these mixtures at pH values below the intrinsic pH of a solution is accompanied by phase separation related to insolubility of PAA-PEG interpolymer complexes. A decrease in the pH of the ternary mixture is caused by the release of a strong low-molecular-mass acid due to complexation with Cu2+ ions. The minimum pH value, above which the PAA-PEG-Cu2+ system becomes single-phase (a transparent solution), depends on the concentration ratio between PAA and PEG chains (the mean degree of polymerization). This value is either 6.8–7.0 (if all macromolecules are incorporated in the insoluble interpolymer complex with PEG) or 4.0 (if chains occur in excess). Methods of preparing single-phase systems in the pH range 4.0–7.0 via exchange reactions of the PAA-Cu2+ complex with PEG or the nonstoichiometric soluble interpolymer complex PAA-PEG are developed. Viscometry, electron microscopy, and dynamic light scattering are used to investigate the compositions and structures of soluble complexes, in which either each chain (if the chain is long) may be linked with both PEG and Cu2+ ions or PAA chains are redistributed between two complexes (at comparable lengths of PAA and PEG chains).

Chain statistics in vinyl acetate-vinyl alcohol multiblock copolymers

The kinetics and structure of products of the alkaline hydrolysis of poly(vinyl acetate) in an acetone-water mixture are studied via 1H NMR spectroscopy. The reaction is considerably accelerated and, according to the dynamic light-scattering data, is accompanied by polymer-coil expansion. The model of the effect of neighboring units is shown to be inapplicable to description of the general kinetics of the process; however, it may be successfully used for the quantitative interpretation of the experimental dependences of the triad composition of macromolecules on conversion. Thus, the conformational factors affect the reactivity of vinyl acetate groups, regardless of the nature of their nearest chain neighbors. This circumstance makes it possible to use the neighbor-effect model to describe the unit distribution over chains of the reaction product, vinyl acetate-vinyl alcohol copolymer, and thus to obtain information necessary for studying the relationship between the chain structure and properties of statistical multiblock copolymers.

The Effect of Modification with Fluorescent Groups on the Physicochemical Characteristics of Poly(alkylene oxides)

Covalent modification of poly(alkylene oxides), block copolymers of ethylene oxide and propylene oxide with M ∼ 2000, with fluororescein isothiocyanate or 7-nitrobenzo-2-oxadiazole taken in a ratio of 0.6–1.0 mol of label/mole of polymer leads to changes in the physical state of polymers and their solubility in water decreases, as well as the partition coefficient in the hexane-water system and the critical concentration of aggregation and dimensions of the formed particles.