Kinetik der oxidativen Dimerisation von ?thynylbenzol mit den Cu-Komplexen von 2-Phenylpyridin und von einem polymeren Pyridin-Derivat als Katalysatoren

Ohne Zusammenfassung     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl methacrylate). VIII. Influence of template concentration

The influence of template concentration on the radical polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) template was studied. The polymerizations were carried out on three template polymers with different molar masses in dimethylformamide at ?5°C. The initial polymerization rate increased linearly with template concentration until the distribution of template chain segments became homogeneous. At that critical concentration a strong increase in the polymerization rate was observed, whereas still higher template concentrations had only a slight effect on the polymerization rate. The polymerizations were stopped when the weight ratio of formed polymer and template was equal to one. The viscometrically determined molar mass of the formed polymers showed a remarkable behavior in the low template concentration region. It was obviously related to the molar mass of the template polymer and was lower than the molar mass found for blank polymerization. This decrease in molar mass was most pronounced in the case of the lowest template molar mass. It is suggested that nondegradative chain transfer occurring near a template chain end is responsible for this decrease. An increase in the molar mass occurred at the critical concentration, similarly to the change of polymerization rate. However, at still higher template concentrations, where template coils started to overlap each other, the molar mass of the formed polymers increased further. The growing chains could leap from one template chain to another and attain a greater chain length than the blank polymerizate.     

A regenerable immobilized NADH model. Reduction of α,α,α-trifluoroacetophenone by 1, 4-dihydronicotinamide anchored to macroreticular polystyrene

A regenerable dihydronicotinamide immobilized on a macroreticular polystyrene carrier was developed and used in an aqueous medium for the reduction of trifluoroacetophenone to the corresponding alcohol. It was shown that the second-order rate constant for the polymer-bound reagent is significantly lower than that of the low-molecular-weight analog 1-benzyldihydronicotinamide. The observed reaction rate, however, was of the same order of magnitude as that due to enrichment of substrate in the polymeric pores. It was possible to depress the undesired side reactions to 10–15% per reaction cycle, thereby allowing reutilization of the functionalized polymer.     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl metacrylate). VII. Determination of the rate constants for template polymerization

The separate rate constants k_p and k_t for propagation and termination of radical template polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) as a polymer template have been determined. The polymerizations were carried out in the strongly complexing solvent dimethylformamide at 5°C. For the evaluation of k/k_t from stationary kinetic experiments, the rates of initiation were determined by employing a scavenger method. The nonstationary experiments yielding k_p/k_t were performed by means of the rotating sector technique. As the template rate effects increased with decreasing initiator concentration, the rotating sector curves were corrected for variation in light intensity. It appeared that the radical lifetime increases from 8.4 sec for normal or blank polymerization to 64 sec for template polymerization. The calculated values of k_p are 26.6 and 5.9 l./mole-sec and of k_t 140 × 10⁴ and 1.7 × 10⁴ l./mole-sec for blank and template polymerization, respectively. The changes in k_p and k_t, due to the presence of template polymer, are explained in terms of an extra loss of activation entropy in the stereoselective propagation step and a strong hindrance of segmental diffusion for the termination reaction of the chains growing along the polymer template.     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). II. Syndiotactic PMMA as matrix

Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C in DMF in the presence of preformed syndiotactic PMMA (sMA) with about 90% syndiotactic triads and of different M?_v, viz., sMA-1, 1.6 × 10⁵; sMA-2, 3.0 × 10⁵; and sMA-3, 8.7 × 10⁵. The MMA:sMA ratio was 6:1. The collected polymers were separated into two fractions by extraction with boiling acetone and characterized by 60 MHz NMR. In all cases isotactic PMMA (i-PMMA) was produced, especially in the initial reaction stages, which associated with the syndiotactic substrate to form acetone-insoluble 1:2 i/s-stereocomplexes. The isotacticity decreased with conversion and was highest in the presence of sMA-3. Characterization of the acetone-soluble fractions indicated that i,s-stereoblock polymers were also produced. From these results it is concluded that this reaction can be considered a stereospecific replica polymerization, the driving force being the strong tendency of i-PMMA and s-PMMA to associate. With sMA of M?_v below about 1.2 × 10⁵, no i-PMMA is formed; in other words, no replica polymerization occurs. For polymerizations in the presence of sMA-2, the critical M?_v of propagating chains, with has to be exceeded before stereoassociation is strong enough to effectuate replica polymerization, has been estimated to be 0.6 × 10⁵.     

Contributions of a long side chain to the binding affinity of an anthracene derivative to DNA

Systematic studies on the DNA binding of a new anthracene derivative, carrying a 1,8-octyldiamine side chain, were carried out. Calorimetric, spectroscopic, and helix melting studies show that the side chain, consisting of eight methylene groups, enhances the binding constant by a factor of approximately 35 when compared to the binding of a probe lacking the long side chain. Furthermore, the enthalpy of binding of the long-chain derivative to calf thymus DNA (Delta H = 4.1 +/- 0.1 kcal/mol) is far greater than the sum of the enthalpy changes associated with the binding of the probe lacking the long side chain, and the enthalpy for the binding of 1,8-octyldiamine.2HCl. Strong synergistic effects, therefore, are seen with the long-chain derivative. Spectroscopic data indicate bathochromism, strong hypochromism, and quenching of anthryl fluorescence when the above ligand binds to calf thymus DNA. Fluorescence energy transfer studies and circular dichroism data strongly suggest intercalation of the anthryl ring system. The binding stabilizes the double helix, and the helix melting temperature is increased from 78 degrees C to >90 degrees C. The binding to DNA is reversible, depended on the ionic strength, and the major binding mode was suppressed at high ionic strengths and a new mode begins to dominate binding. Substitution of the anthracene ring with 1,8-octyldiamine chain provided a simple method to enhance the binding constant by nearly a factor of 35.     

One-step multi-electron transfer mechanism of polynuclear copper complexes for molecular conversions

The oxidative coupling reaction of 2,6-dimethylphenol may result in either a desired polymeric substance (i.e. the polyphenylene ether, PPE) or the undesired “dimeric” species diphenoquinone, DPQ. The relative amounts of each product depend on the experimental conditions and the used catalytic system. Usually copper amine compounds are used as a catalyst for the oxidative coupling reactions. They have the advantage of easy access and produce high yields of high molecular PPE; however, other metal coordination compounds, like those of Mn, may also be used as catalysts. The present paper focuses on mechanistic studies with various copper (aliphatic and aromatic) amine compounds as catalysts. Owing to the steric constraints of the amine ligands, dinuclear Cu(II) compounds, with small bridging anionic ligands, are easily formed. Such species are believed to be the catalyst precursors. Upon addition of a base (1:1 on copper) and excess phenol, phenolate ligands coordinate as bridging ligands to copper. After a two-electron transfer reaction, the resulting phenoxonium ligand, which is a rather poor ligand, remains attached to the Cu(I), probably coordinating via its aromatic ring. Nucleophilic attack by a phenol to the phenoxonium ion at the 4-position is likey to be most important to the coupling reaction. In the beginning of the reaction the undesired side product DPQ is also formed via a C–C coupling reaction. With copper(II) compounds containing imidazole-type chelating ligands, good activity was obtained; in the case of pyrazole-based and bridging S-donor chelating ligands, that no or very weak activity was found. In a study of the mechanism of the propagation reaction the rate-determining reaction was thought to be probably a one-step, two-electron transfer, during which the two Cu(II) ions in the dinuclear complex oxidize the phenolate to phenoxonium. After the phenoxium ion is formed the bonding with the (then) Cu(I) species is weakened and the reactions with phenolic end groups can take place. The effect of the amine ligands appears to be both steric and electronic. With certain ligands the reoxidationof the reduced catalyst is not possible.     

Selective oxidative para C-C dimerization of 2,6-dimethylphenol

Mechanistic investigations on the oxidative coupling of 2,6-dimethylphenol have led to the development of a selective and efficient procedure to prepare 3,5,3',5'-tetramethyl-biphenyl-4,4'-diol, via a C-C coupling, mediated by a hypervalent form of iodine, i.e. (diacetoxyiodo)benzene and for which a mechanism is proposed.