Garry L. Rempel

Bimetallic dendrimer-encapsulated nanoparticles as catalysts: a review of the research advances

The tutorial review summarizes the recent research advances in the field of bimetallic dendrimer-encapsulated nanoparticles (DENs) with a focus on the application in homogeneous and heterogeneous catalysis.

Chemical modification of polypropylene with peroxide/pentaerythritol triacrylate by reactive extrusion

To explore the possibility of producing branched polypropylene (PP) by a reactive extrusion (REX) process, isotactic PP was reacted with a polyfunctional monomer, pentaerythritol triacrylate (PETA), in the presence of 2,5-dimethyl-2,5(t-butylperoxy) hexane peroxide (Lupersol 101). Experiments were carried out in an intermeshing, corotating twin-screw extruder at 200°C using three concentrations of peroxide (200, 600, and 1000 ppm) and four concentrations of PETA (0.64, 1.8, 2.8, and 5.0%, by weight). Shear viscosity and MFI of the whole polymers was found to increase with PETA concentration and decrease with increasing the peroxide concentration at a given PETA concentration. The macrogel amount in the materials produced was determined in refluxing xylene using Soxhlet extraction and at PETA concentrations higher than 1.8wt % the macrogel content increased with increasing peroxide concentration. No macrogel was detected at low PETA concentrations (<0.64%) at all three peroxide levels, suggesting that low concentrations of PETA and peroxide should be used in order to minimize the formation of macrogels. The xylene soluble portions (sols) of the modified materials were characterized by FTIR and DSC. Generally, the relative intensities A1740/A841 in the FTIR spectra increased with increasing PETA incorporated into PP. Two melting peaks (Tm1 and Tm2) were observed in the DSC traces of some of the sols, and the crystallization temperatures (Tc) were higher than those of the virgin and degraded polypropylenes. The DSC behavior of the sols suggests that the modified PPs contain branched and/or lightly crosslinked chain structures.

Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers

High water-absorbent copolymers comprising acrylic acid (AA) and acrylamide (AM) were prepared in the presence of a crosslinking agent, monofunctional aldehyde, by a solution polymerization technique using a redox initiation system. Such copolymers have very high water absorbency and absorbing kinetics to the distilled water. The copolymer formed which absorbed about 900 g water/g dry copolymer was used to study the influence of sodium chloride on the absorption capacity at 24°C. The swelling of this copolymer was studied in alcohol/water mixtures of increasing alcohol content at 294, 304, and 314 K. The main transition for ethanol/water and methanol/water mixtures is a rapid decrease of the retention capacity of the copolymer at 50–60 vol % ethanol and 55–65 vol % methanol, respectively. Swelling in distilled water at different temperatures (T) and the effect of solvent composition were also studied. Among the variables examined were initiator concentration, polymerization temperature, and amount of AM in the copolymer.

Kinetics and mechanism of the oxidation of thiosulphate ions by copper(II) ions in aqueous ammonia solution

Copper(II) ions in aqueous ammonia solution oxidize thiosulphate ions initially to tetrathionate ions; the latter then undergo a subsequent disproportionation reaction to yield trithionate and thiosulphate ions. The detailed kinetics of the reaction suggest a mechanism which involves substitution of thiosulphate ion into the co-ordination sphere of a triamminecopper(II) complex in the rate-determining step. A one-equivalent electron transfer from the thiosulphate to the copper(II) ion, occurring in the intermediate triamminecopper(II)–thiosulphate complex, gives rise to copper(I) and S2O3– ions, the latter dimerizing to tetrathionate ions.

A kinetic study of the aldol condensation of acetone using an anion exchange resin catalyst

The rate of the reversible aldol condensation of acetone (Ac) to diacetone alcohol (DAA) catalysed by Amberlite IRA-900 anion exchange resin in the hydroxide form is measured in the liquid phase, without solvent, using batch and continuous reactors. The desired reaction product is DAA, but its dehydration leads to the formation of mesityl oxide (MO) as a byproduct. Although the rate of DAA production is pore diffusion controlled, it may be modelled as second order with respect to [Ac] in the forward direction, and first order with respect to [DAA] in the reverse direction. Water in the reaction mixture plays a very important role in determining the reaction rate, the product selectivity, and the catalyst lifetime. Adding water to the reaction increases the catalysts lifetime and selectivity toward DAA, but slows down the rate of reaction.

Synthesis of tailor-made polyethylene through the control of polymerization conditions using selectively combined metallocene catalysts in a supported system

Tailoring of the molecular weight distribution (MWD) in ethylene polymerization was attempted by selectively combining different types of metallocene catalysts onto a single support. The catalyst produced by supporting Et[Ind]2ZrCl2 and Cp2HfCl2 onto a single MAO pretreated silica support was able to produce polymers with unimodal or bimodal MWDs. This approach permits the synthesis of polyethylene with different MWDs using the same catalyst as a function of the polymerization conditions.

Use of hydrogen for the tailoring of the molecular weight distribution of polyethylene in a bimetallic supported metallocene catalyst system

The shape of the molecular weight distribution (MWD) of polyethylene produced with a bimetallic supported metallocene catalyst was controlled with hydrogen in a semi-batch slurry reactor. Polymers produced at different hydrogen pressures have MWDs varying from unimodal to bimodal, with high or low molecular weight shoulders.

Chromium(III) removal by poly(ethyleneimine) granular sorbents made by a new process of templated gel filling

By the new process of templated gel-filling (TGF), a gel-type granular sorbent has been prepared using poly(ethyleneimine) as the chelating resin, Cr3+ as the template ion and glutaraldehyde as the crosslinking agent. The templating is found to increase the Cr(III) selectivity of the base resin PEI, in binary mixtures of Cr(III) with Cu(II), Ni(II) and Fe(III). The new TGF sorbent has, however, significant Cr(III) selectivity only relative to Ni(II), being much superior to the commercial resin Chelex-100 in this respect. The Cr(III) sorption on both these sorbents is enhanced significantly by NaCl and Na2SO4, the effect being more marked on the TGF sorbent which shows greater than 100% increase in Cr(III) sorption capacity in 0.5 M NaCl and 0.25 M Na2SO4 solution containing Cr(III) in low concentrations. The sorbent is thus suitable for removing low level chromium from strong salt solutions and has fast kinetics with a t12 of 1.5 min in 2 mM Cr(III) solution.

Terminal functionalization of polypropylene via the Alder Ene reaction

Abstract A terminally anhydride functionalized polypropylene was synthesized via the Alder Ene reaction from a low molecular weight amorphous polypropylene. A Lewis acid, SnCl2·2H2O, was found to catalyse the reaction, thereby improving anhydride content in the polymer for reaction conditions comparable to those in an extrusion environment, i.e. elevated temperature and short residence time. The reaction was carried out at 230°C for 5 min in the presence of TEMPO, which acted as a free radical trap, preventing maleic anhydride from being grafted onto the backbone of the polypropylene. The product was characterized by several techniques, including FTi.r. and n.m.r., to provide evidence of the anhydride location on the chain. Several runs of varying catalyst concentration have shown that an optimum in anhydride incorporation onto the polypropylene exists. Examination of the system kinetics has shown that the overall order of the reaction remains second-order despite the presence of a catalyst.

Ligand exchange sorption of arsenate and arsenite anions by chelating resins in ferric ion form: II. Iminodiacetic chelating resin Chelex 100

Arsenate and arsenite are selectively removed by ligand exchange sorption on iminodiacetic chelating resin Chelex 100 used in ferric ion form, the saturation sorption capacities being 45 and 70 mg As / g wet resin with respect to As(V) and As(III) anions, respectively. The liquid sorption of As(V) anions is maximum at pH ∼ 2 and that of As(III) anions at pH ∼ 10. Monovalent anionic species H2AsO−3 are mostly involved in ligand sorption on Chelex (Fe3+), and on average approximately one and two anionic species of As(V) and As(III), respectively, are coordinated to each resin-bound Fe3+ ion at saturation. The sorbed anionic species are readily stripped with dilute sodium hydroxide into a concentrated form for recovery. An enrichment of more than 40-fold relative to influent concentration (< 100 ppm As) is obtained in ligand exchange column operation for arsenate, as compared to less than 20-fold enrichment for arsenite. Reactivation of the stripped resin is performed in one step by treatment with a mildly acidic solution of ferric chloride (pH ∼ 2), which reconverts the resin to the chelated-Fe3+ form. The regenerated ligand exchanger exhibits a higher sorption capacity due to additional ion-exchange sorption on the accumulated ferric hydroxide gel. The ligand sorption capacity of the polymer-chelated Fe3+ ion is, however, several-fold higher than the anion exchange capacity of the ferric hydroxide gel, with respect to both As(V) and As(III) anions.