Thieo E. Hogen-Esch

Ion-Pairing Effects in Carbanion Reactions

Publisher Summary This chapter explores the correlation of ion pairs with chemical behavior in a variety of carbanion systems. An understanding of the various factors determining the extent and type of ionic association is indispensible in describing ion pair effects. The chapter discusses these factors, focusing on the effect of carbanion structure. It reflects that ion pairing effects are often similar from one reaction to another, provided that the reactions are like one another in terms of the degree of charge dispersion in going from reactants to products. An understanding of the various factors determining the extent and type of ionic association is indispensible in describing ion pair effects. The chapter illustrates that the role of the counter-ion in many carbanion systems is important, sometimes even dominant, and should be studied carefully before the reactive behavior of carbanions may be evaluated. Moreover, such effects, when properly identified, may serve to probe changes in charge delocalization in carbanion reactions and may lead to a better understanding of carbanion chemistry.

Percutaneous fiber-optic sensor for chronic glucose monitoring in vivo

We are developing a family of fiber-optic sensors called Sencils (sensory cilia), which are disposable, minimally invasive, and can provide in vivo monitoring of various analytes for several weeks. The key element is a percutaneous optical fiber that permits reliable spectroscopic measurement of chemical reactions in a nano-engineered polymeric matrix attached to the implanted end of the fiber. This paper describes its first application to measure interstitial glucose based on changes in fluorescence resonance energy transfer (FRET) between fluorophores bound to betacyclodextrin and Concanavalin A (Con A) in a polyethylene glycol (PEG) matrix. In vitro experiments demonstrate a rapid and precise relationship between the ratio of the two fluorescent emissions and concentration of glucose in saline for the physiological range of concentrations (0-500mg/dl) over seven weeks. Chronic animal implantation studies have demonstrated good biocompatibility and durability for clinical applications.

Hydrophobically Modified Polyelectrolytes, 4. Synthesis and Solution Properties of Fluorocarbon-Containing Poly(acrylic acid)

Hydrophobically modified (HM) poly(acrylic acid) (PAA) was prepared by radical copolymerization of acrylic acid (AA) and a small amount of 2-(N-ethylperfluorooctanesulfoamido)ethyl acrylate (FA) or 2-(N-ethylperfluorooctanesulfoamido)ethyl methacrylate (FMA) in benzene. In aqueous solutions, above a critical concentration, the modified polymer in contrast to PAA exhibits a sharp increase in viscosity with increasing polymer concentration. This behavior, typical of hydrophobically modified water-soluble polymers, is due to the formation of strong intermolecular hydrophobic association of pendent fluorocarbon groups. As shown by viscosity measurement factors such as NaCl, surfactants, temperature and pH play a very important role in the formation of hydrophobic association. For instance the behavior of viscosity as a function of pH and NaCl indicates the occurrence of competition of polyelectrolyte effects and hydrophobic association. Thus, hydrophobic association is favored under conditions where the chains carry lighter charge or where the charges are screened by the presence of salts. Studies related to the dilute solution property of the HMPAA provide information of competition between electrostatic repulsion and intramolecular hydrophobic association, which is reflected by the variation of intrinsic viscosity as a function of NaCl concentration. The extremely high value of Huggins coefficient (kH) of modified polymers at high NaCl concentration also gives us evidence of intermolecular hydrophobic association between polymer coils. Fluorescence studies of HMPAA show that the pH effects obviously on the FC hydrophobic association, and also that the pyrene fails to detect aggregation of fluorocarbon groups at pH > 7 by combining with the results of viscosity properties, due to its poor affinity to the fluorocarbon microdomains.

Block Copolymer-like Self-Assembly of Fluorocarbon End-Functionalized Polystyrene and Polybutylmethacrylate

We demonstrate the self-assembly through fluorophilic interactions of a blend of perfluorocarbon (RF) end-functionalized polystyrene and the corresponding RF-polybutylmethacrylate into optically transparent materials that retain domain characteristics typical of the component polymers but show well-defined lamellar nanostructured morphologies that qualitatively resemble that of the corresponding block copolymers.

Investigation of macrocyclic polymers as artificial light harvesters: subpicosecond energy transfer in poly(9,9-dimethyl-2-vinylfluorene).

The spectroscopy and dynamics of a novel molecular architecture that mimics natural light harvesting have been characterized. The deployment of 9,9-dimethyl-2-fluorenyl (DMF) chromophores in atactic macrocyclic poly(9,9-dimethyl-2-vinylfluorene) is similar to that in the light harvesting antenna LH2 of the purple photosynthetic bacteria. A variety of spectroscopic probes are used to study the dynamics in these novel polymer systems. The number of chromophores is tuned from 12-142 identical chromophore units. Steady-state absorption and emission measurements, time-resolved fluorescence, and ultrafast transient absorption anisotropy techniques provide evidence for distinct differences in the photophysics of matching molecular weight linear and cyclic polymers and of the occurrence of energy transfer in these polymers. There is direct evidence of energy transfer in these macrocycles manifested in the depolarization decay components, which are characterized by two exponentials and are substantially faster than observed for reorientation of the free DMF chromophore. The time constants for the macrocycles are 700-900 fs and 7-8 ps and are size dependent; the biexponential decay arises from conformational and stereochemical disorder and can be well described by a master equation simulation assuming Förster incoherent hopping on model polymer structures. The results suggest energy hopping between adjacent chromophores on a 1 ps time scale. The pathway for energy migration is shown to be primarily between nearest neighbors along the cyclic backbone, but there is a considerable spread in the site-to-site hopping rates. Small cycles adopt a pseudoplanar ring type arrangement of the chromophore transition dipoles as observed in bacterial light harvesting antenna, and it is found that the linear polymers also show similar short-range planarity of transition dipoles. Overall, it is found that such small macrocyclic polymers possess excellent characteristics for light harvesting among identical chromophores and behave as a circular photonic wire.

Graft Copolymerizations of Polysaccharides. II. Acrylamide Grafting to Yellow Dextrin

Abstract Graft copolymers of acrylamide and yellow dextrin were prepared using cerium(IV) as initiator. The yellow dextrin had a very broad molecular weight distribution but was fractionated utilizing dialysis and ultrafiltration membranes. Initiator efficiencies were determined using size exclusion chromatography and were found to be between 2.4 and 34%. Initiator efficiency increased with acrylamide concentration at constant cerium (IV) and yellow dextrin concentrations, and decreased with increasing cerium(IV) concentration at constant acrylamide and yellow dextrin concentrations. Plots of acrylamide conversion and intrinsic viscosity vs initial acrylamide concentration at constant yellow dextrin and ceric ion concentrations showed a maximum at about 2.0 M.

Stereochemical effects of ion pairing in anionic vinyl oligomerization and related carbanion reactions

Abstract The oligomerization stereochemistry of the products of equation 1 was studied by 1H and 13C NMR. The stereochemistry of methylation and 2-vinylpyridine addition was found to be identical. With Li and Na as counterion both reactions lead to the formation of meso dyads so that the products are predominantly isotactic. The alpha carbon stereochemistry is consistent with a cation side attack of electrophile on carbanion and on the basis of unequal proportions of diastereomeric ion pairs [3] and [4] in which the Li or Na counterion is coordinated with the penultimate 2-pyridine group leading to specific conformations in which [3] is preferred because of steric reasons. With larger or more extensively solvated counterions the interaction of metal ion with penultimate 2-pyridine group is decreased and [3] and [4] become more nearly equal in energy leading to a decrease in stereoselectivity. Beta carbon stereoselectivity of products [2] prepared with beta deuterated vinylpyridines was also investigated. It was shown that this stereoselectivity may be related not only to the mode of presentation of monomer but also to anion geometry. Equilibrium 2 demonstrated by 1H and 13C NMR was shown to be highly cation and solvent dependent the (E)/(Z) ratio decreasing with increasing cation size and cation coordination. It was shown that for the Li and K systems the beta carbon selectivity of the trans addition correlates well with equilibrium 2. Providing that metal ion - 2-vinylpyridine coordination occurs prior to addition this correlation is explained by equation 3. Since alpha carbon stereochemistry of the carbanion and beta carbon stereochemistry of the incoming monomer are simultaneously determined the lower (E)/(Z) ratio found for the K system is expected and found to lead to less selective placement of deuterium.

Design and fabrication of disposable, percutaneous chemical sensors

Sencils (sensory cilia) are chemical sensors that are minimally invasive, disposable and easily readable to make frequent measurements of various analytes in vivo over a period of 1-3 months. A percutaneous optical fiber permits precise, reliable photonic measurement of chemical reactions in a nano-engineered polymer matrix attached to the internal end of the fiber. The first Sencils sense interstitial glucose based on measurement of fluorescence resonance energy transfer (FRET) between fluorophors bound to dextran and Concanavalin-A in a polyethylene glycol (PEG) matrix. In vitro experiments demonstrate a rapid and precise relationship between the ratio of the two fluorescent emissions and concentration of glucose in saline over the physiological range of 0-500mg/dl. Chronic implantation in pigs has demonstrated biocompatibility. The Sencil platform can be adapted to detect other analytes in interstitial fluids.

Synthesis and characterization of fluorocarbon-modified poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide) copolymers (PNIPAMs) containing pendent perfluoroalkyl (R F ) or dodecyl groups have been synthesized by copolymerization of NIPAM with small amounts of R R -acrylates or -methacrylates containing a sulfonamido moiety between the acrylate and R F groups or with dodecyl acrylate. Evidence for strong intermolecular hydrophobic association of the fluorocarbon groups is provided by large viscosity increases with copolymer concentration and upon addition of NaCl and surfactants. These interactions appear to be much stronger than that of the corresponding copolymers of poly(N,N-dimethylacrylamide) with similar comonomer contents. Hydrophobic association between the R F groups is found to be much stronger than that of the corresponding dodecyl groups. The viscosity of some of the copolymer solutions, particularly in the presence of perfluorocarbon surfactants, was unusually temperature sensitive, decreasing by a factor of at least 1000 upon increasing the temperature from 10 to 20 °C. This large decrease is most probably related to the collapse of the copolymer coils near the lower critical solution temperature. This is in sharp contrast to the corresponding polyacrylamide or poly(N,N-dimethylacrylamide) R F -acrylate copolymers that show viscosity increases with increasing temperature in the 40-60 °C range. The NIPIAM copolymers were also found to be different from the acrylamide or N,N-dimethylacrylamide perfluorocarbon acrylate copolymers in that they were found to be Newtonian at a low RF content but dilatant at a higher comonomer content. # 2000 Society of Chemical Industry

Synthesis of PMMA and PMMA block copolymers at elevated temperatures by phosphor ylide-mediated polymerizations

The MMA polymerization initiated by (1-naphthyl) triphenylphosphonium triphenylmethyl anion(TPM,NTPP) or the NTPP salt of the methylisobutyrate anion (MIB,NTPP) in THF at temperatures varying from 25 to 70°C appears to be a living polymerization as indicatred by a linear Mn vs. MMA conversion plot and by the narrow MW distributions. As indicated by the proton spectrum of MIB, NTPP in THF d8 the predominant polymerization intermediate appears to be the ylide formed exclusively by addition of the MIB anion to the 4-carbon of the 1-naphthyl group. This ylide shows upfield shifts for the 3-proton of the cyclohexadienyl ring and of the 6- and 8-protons of the remaining aromatic ring. The rates of the NTPP mediated polymerizations are reduced by factors of around 104-105 compared to that mediated by the corresponding ylides formed by addition of MIB to the phenyl ring of tetraphenylphosphonium ion. The reductions in rate in the presence of NTPP make it possible to carry out MMA polymerizations under conditions not normally accessible.