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The promoting role of polycations in the cobalt(II) phthalocyanine tetrasodium sulfonate catalysed oxidation of thiols

Abstract The oxidation of 2-mercaptoethanol with polymer-bound cobalt(II) phthalocyanine catalysts has been investigated. Poly(vinyl amine) (PVAm), poly(ethylenimine) (PEI), poly (L-ly sine) (PLL) and 2,4-, 2,6- and 2,10-ionene hydroxide have been studied as polymeric ligands. All these ligands appear to enhance the reaction rate considerably in comparison with their monomeric analogues and all exhibit a similar behaviour towards the effect of salt and pH. The reaction rate appears to increase virtually linearly with the linear charge density on the ionenes, which is consistent with recent findings for copolymers of vinyl amine and vinyl alcohol. Three main parameters seem to provide the enhanced polycationic catalytic effect: a high linear charge density on the polymer, a large amount of base groups in the polymer domain and a low ionic strength for the reaction solution. Furthermore, a comparison of the linear charge density, catalytic activity and activation parameters for the ionene catalysts with those obtained earlier for the vinyl amine-vinyl alcohol copolymers shows that the presence of hydrophilic side-groups has a promoting effect on the oxidation of the hydrophilic thiol. From catalyst re-use experiments, it appears that the polyamine catalysts (PVAm, PEI and PLL) have a poor stability, but that the ionene hydroxides can be re-used many times, probably as a result of the chemical inertness of the quaternary ammonium groups in the ionenes.

SYNTHESIS, PURIFICATION AND CHARACTERIZATION OF CATIONIC LATICES PRODUCED BY THE EMULSION COPOLYMERIZATION OF STYRENE WITH 3-(METHACRYLAMIDINOPROPYL)TRIMETHYLAMMONIUM CHLORIDE

Abstract A study was made of the effect of varying the amount of 3-(methacryl-amidinopropyl)trimethylammonium chloride (MAD) on the emulsion copolymerization of styrene and MAD with azo-bis(isobutyramidine hydrochloride) as initiator. The addition of MAD accelerated the polymerization and decreased the particle size. Cleaning of these latices in order to remove polyelectrolyte not incorporated in the latex particles was found to be tedious. Serum replacement, diafiltration and centrifugation appeared to be unsatisfactory since desorption of polyelectrolyte was still observed after standing or stirring. A combination of centrifugation and treatment with silica appeared to clean the latices to a satisfactory level (expressed as [ N + ] −7 mol dm 3 ). Surface charge density, as determined by conductometric acid-base titrations, rose slightly on increasing the initial MAD concentration. Shot-growth (two-step) emulsion polymerization or the use of a cross-linking agent (divinylbenzene) hardly affected the surface charge density.

Autoxidation of thiols with cobalt(II) phthalocyanine-tetra(sodium sulfonate) attached to poly(vinylamine). Part 4. Influence of base density within the polymeric ligand

Abstract Water-soluble polymeric catalysts, consisting of cobalt(II) phthalo-cyanine-tetra(sodium sulfonate) coordinatively bound to poly(vinylamine-co-vinylalcohol), have been used in the oxidation by oxygen of 2-mercaptoethanol to its corresponding disulfide. An increase in the intrinsic activity of the catalytic sites was observed with increasing ethylamine (base group) content in the copolymers, while constant activation enthalpies indicated that the reaction mechanism remained unchanged. From potentiometric titrations of the basic copolymers with hydrochloride and from catalytic activity measurements, it could be deduced that reaction rate varied linearly with the mole fraction of charged monomeric units in these copolymers. A reaction scheme has been proposed, suggesting that the thiol anion is involved in the rate-limiting step. The present polymeric catalyst and vitamin B 12 were found to show similarities in their oxidation behaviour toward thiols.

Preparation and co-catalytic properties of functionalized latices

Abstract Stable cationic latices were prepared by emulsifier-free emulsion polymerization of styrene with 1-methyl-4-vinylpyridinium bromide or 1-methyl-4-vinylpyridinium iodide as comonomers using azobis(isobutyramidine hydrochloride) as initiator. Comonomers were prepared by a new and simple synthetic procedure. Addition of methanol and especially comonomer increased the surface charge density of cleaned latices and enhanced the polymerization rate. In the presence of iodide, the polymerization rate was lowered. The co-catalytic properties of silica-cleaned latices on the oxidative coupling of 2-mercaptoethanol were tested after cobalt(II)phthalocyanine-tetrasodiumsulphonate (CoTSPc) immobilization onto the functionalized latex particles. A significant increase in oxidation rate was found as compared with the polymer-free CoTSPc system.

Autoxidation of thiols with cobalt(II) phthalocyanine-tetra(sodium sulfonate) attached to poly(vinylamine). Part 5. Effect of surfactant and thiol variation

Abstract The autoxidation of various thiols, viz. 2-mercaptoethanol (ME), 3-mercaptopropane-1,2-diol (MP), mercaptoacetic acid (MA) and dodecylmercaptan (DM) with cobalt(II) phthalocyanine-tetra(sodium sulfonate) (CoPc(NaSO3)4) as a catalyst in the absence and presence of the basic poly-(vinylamine) has been studied. Differences among the oxidation rates of the various thiols are discussed qualitatively in terms of pKSH, charge, water solubility and size of the substrates. The effect of anionic, nonionic and cationic surfactant on the oxidation rate of these thiols has been investigated. In the absence of polymer the effect of various types of surfactant was small for the hydrophilic ME, MP and MA. However, for DM a micellar catalytic effect of cationic surfactant was observed, whereas anionic surfactant retarded the reaction. When CoPc(NaSO3)4 was used in the presence of poly(vinylamine), large differences were observed between the effects of various types of surfactant on the oxidation rate of ME. Nonionic surfactant did not affect the high reaction rate, whereas cationic and anionic surfactants appeared to diminish the rate considerably, even at far below the critical micelle concentration. These effects are discussed in terms of interactions between the polymer catalyst and surfactant. Furthermore, similarities and differences between polymeric catalysis and micellar catalysis in thiol oxidation are shown.

Effects of 2,4-ionenes of different molar masses on the oxidative coupling of thiol catalyzed by cobaltphthalocyanine

Abstract A photometric method is developed for the determination of the molar mass of 2,4-ionene by attaching a chromophoric moiety to its end-groups. Molar mass determination of these polyelectrolytes by acid-base titration of the amine end-groups is also possible, and confirms the photometric method. The effects of molar mass of 2,4-ionenes on the oxidative coupling of 2-mercaptoethanol (ME) was studied in the presence of cobalt(II)phthalocyanine-tetra-sodium-sulphonate (CoTSPc). Monomeric analogues of 2,4-ionene showed no increase of reaction rate as compared with the polymer-free system. However, relatively low molar mass ionene ( M n = 1740 ) showed a dramatic increase of reaction rate. The reactivity showed an optimum around pH = 8. Saturation kinetics (Michaelis-Menten kinetics) were observed. The calculated turnover number was 3200 sec−1 and 3600 sec−1 for M n = 6600 and M n = 1740 , respectively. The stoichiometry of the reaction was disturbed at low thiol concentration.

Cationic latices as cocatalyst in the cobaltphthalocyanine-catalyzed oxidation of 2-mercaptoethanol

Abstract Cationic latices, products of emulsion copolymerization of styrene with 3-(methacrylam-idinopropyl)trimethylammoniumchloride (MAD), and its homopolymer (PMAD) have been tested as cocatalyst in the autoxidation of 2-mercaptoethanol in the presence of co-balt(II)phthalocyanine tetrasodiumsulphonate (CoTSPc). It was found that all systems studied enhanced the catalytic activity compared with the polymer-free CoTSPc-containing system. The systems examined invariably showed an enzyme-like behaviour. Unfortunately, the stability after successive runs is still not quite satisfactory. From the observed catalytic activities and spectrophotometric measurements, it can be concluded that, in addition to a high surface charge density, flexible polymeric cationic chains of sufficient length are also necessary to enhance the activity of CoTSPc-bound latex systems in thiol oxidation.

Autoxidation of thiols with cobalt (II) phthalocyanine-tetra-sodium sulfonate, attached to poly(vinylamine)

SummaryThe oxidation of 2-mercaptoethanol with molecular oxygen in water with cobalt(II)phthalocyanine-tetra-sodium sulfonate attached to poly(vinylamine) has been investigated.Specially attention was paid to the effect of pH and chain dimensions on polymer activity. The polymer catalyst possesses a large conformational freedom at neutral pH, but at higher pH a shrinkage of the polymer coil occurs and diffusion limitations cannot be excluded. The catalyst shows an enzyme-like behaviour in the autoxidation of thiol. Overall activation energies appear to decrease with increasing pH. At pH = 7.4, Ea = 61 KJ mole−1; at pH = 9.5, Ea = 3 KJ mole−1.Electrostatic effects are of major importance in the chemical reactivity since they affect the local thiol-anion concentration in the close vicinity of the polymer attached oxidation sites.Summary The oxidation of 2-mercaptoethanol molecular oxygen in water with eobalt(II)phthalocyanine-tetra-sodium sulfonate attached to poly(vinylamine) has been investigated. Specially attention was paid to the effect of pH and chain dimensions on polymer activity. The polymer catalyst possesses a large conformational freedom at neutral pH, but at higher pH a shrinkage of the polymer coil occurs and diffusion limitations cannot be excluded. The catalyst shows an enzyme-like behaviour in the autoxidation of thiol. Overall activation energies appear to decrease with increasing pH. At pH = 7.4, E a = 61 KJ mole-I; at pH = 9.5, E a = 3 KJ mole -I. Electrostatic effects are of major importance in the chemical reactivity since they affect the local thiol-anion concentration in the close vicinity of the polymer attached oxidation sites.

Effects of complexation of oppositely charged water-soluble cobaltphthalocyanines on the catalytic mercaptoethanol autoxidation

Abstract In order to elucidate the different promoting effects polycations have on the cobalt(II) phthalocyanine catalyzed autoxidation of 2-mercaptoethanol, the properties of mixtures of oppositely charged water-soluble cobalt(II)phthalocyanines were studied. The contribution of polycation induced dimerization of the catalyst was investigated by means of combinations of cobalt(II)phthalocyanine-tetra(trimethylammonium) iodide (CoPc[N(CH 3 ) 3 I] 4 ) and cobalt(II)phthalocyanine-tetrasodium sulphonate (CoPc(NaSO 3 ) 4 ). A mixture of equimolar amounts of both phthalocyanines shows an increase in reaction rate for the 2-mercaptoethanol autoxidation as compared with an equal amount of one of the catalyst species separately. The highest activities are achieved when the positive charges of the positive phthalocyanine just match the charges of the negative CoPc(NaSO 3 ) 4 . A mixture of cobalt(II)phthalocyanine-octacarboxylic acid (CoPc (COOH) 8 ) and CoPc [N(CH 3 ) 3 I] 4 exhibits its maximum activity at a ratio of 1:2, indicating the formation of a trimeric catalyst species. Visible light spectroscopy showed that these effects can be ascribed to the formation of aggregates of the phthalocyanines. Addition of a poly(quaternary ammonium) salt, a so-called ionene, to a stoichiometric complex of oppositely charged phthalocyanines results in an increase in the catalytic activity due to substrate enrichment. The activities of an ionene containing equimolar CoPc[N(CH 3 ) 3 I] 4 /CoPc(NaSO 3 ) 4 system were never as high as those achieved for a conventional CoPc(NaSO 3 ) 4 /2,4-ionene system, probably as a result of the strong bonding between the two oppositely charged molecules, which prevents a break-up of the dimeric species. Conclusively, in order to achieve a high catalytic activity it is favourable to enhance the formation of aggregates of CoPc(NaSO 3 ) 4 , which probably will break up after the first reaction step.

Molecular mechanics calculations on cobalt phthalocyanine dimers

Abstract In order to obtain insight into the structure of cobalt phthalocyanine dimers, molecular mechanics calculations were performed on dimeric cobalt phthalocyanine species. Molecular mechanics calculations are first presented on monomeric cobalt(II) phthalocyanine. Using the Tripos force field for the organic part of the molecule and parameters derived from the literature and subsequently optimized to describe the CoII force field resulted in a geometry that is in very good agreement with experimental data from the literature. Optimization of the dimeric structure leads to a geometry in which both phthalocyanines are separated by 3.2 A and one of the molecules is shifted 2.38 A in both the X- and Y-directions with respect to the other. This geometry is in excellent agreement with literature data on β-Co(pc) crystals and with other calculated and experimental data on similar systems. All calculations were performed with three possible charge distributions in the phthalocyanine molecule and it was shown that varying the charge distribution had no significant effect on the final dimeric structure. This method provides valuable insight into the most important energetic interactions leading to dimer formation.