Size effects in catalysis by supported metal nanoparticles

This paper is concerned with the study of size effects in reactions of low-temperature CO oxidation on the catalysts Au/γ-Al₂O₃ and Au/δ-Al₂O₃ and complete oxidation of methane on the catalysts Pt/γ-Al₂O₃. For the synthesis of gold catalysts, four techniques have been applied: ionic adsorption, deposition-precipitation, chemical liquid-phase grafting, and decomposition of volatile gold complexes. Platinum catalysts have been prepared by aluminum oxide impregnation with aqueous solutions of H₂[Pt(OH)₆] that, depending on preparation conditions, contained mono- or oligonuclear hydroxocomplexes of platinum. Series of catalyst samples with a narrow size distribution of particles and a mean size variation from 0.5–1 to 20–25 nm have been prepared. The study of the catalytic properties of the prepared catalysts has shown that a decrease in mean size of supported metal particles leads to a sharp increase in specific catalytic activity in both systems. The activity maximum has been achieved for active component particles of 2–3 nm. A conclusion has been made that the application of nanosize catalysts is promising for the cleaning of air in closed rooms and vehicle exhaust gases from CO, for the utilization of methane, and for the obtaining of energy by the combustion of natural gas.     

RAFT聚合方法合成自由基聚合物PTMA

以苄基三硫代碳酸酯基丙酸(BSPA)为链转移试剂,采用RAFT聚合方法合成了分子量分布较窄的聚(4-甲基丙烯酰氧基-2,2,6,6-四甲基哌啶醇酯)(PTMPM-RAFT),并用氧化剂间氯过氧苯甲酸(m CPBA)氧化PTMPM-RAFT为聚(4-甲基丙烯酰氧基-2,2,6,6-四甲基哌啶氮氧自由基) PTMA。1H NMR、傅里叶红外光谱仪、紫外-可见分光光度计、凝胶渗透色谱以及X光电子能谱等证实了PTMPM-RAFT的结构及其分子量,并得到了氧化产物PTMA。     

Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization

A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120 °C in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88 wt%. All ‘synthesized’ composites gave stable suspensions in toluene and tetrahydrofuran.     

Solution of free radical polymerization

The steady state approximation in free radical polymerization is known to break down during the operation of commercial reactors, in which case the unsteady mole balance of polymer radicals have to be solved numerically. We observed that the differential equations governing the zeroth, first, and second moments of polymer radicals in free radical polymerization are nonlinear in nature. The numerical solution of these is “stiff” because the concentration of polymer radicals is very small. Assuming an isothermal reactor, in the absence of the gel effect, these differential equations can be transformed in a suitable new domain where they are linear and can be solved analytically. We have subsequently developed a computer program for nonisothermal reactors in the presence of the gel effect using the analytical solution between any time increment. The results have been compared with those obtained from the fourth order Runge-Kutta technique. We show that when the temperature changes are fast, the Runge-Kutta technique cannot handle the stiffness, and the results begin to deviate from the exact solution. We find that our computer program is computationally efficient in handling this stiffness, takes considerably less time, and can be adapted to any personal computer.     

AN/MA自由基共聚合可控过程研究

控制单体配比,采用丙烯腈(AN)与丙烯酸甲酯(MA)自由基溶液共聚合,以偶氮二异丁腈为引发剂在溶剂二甲基亚砜中合成了丙烯腈共聚物.分析了单体浓度、引发剂、反应温度、反应时间等因素对反应体系的影响.结果表明,增加单体浓度和引发剂的用量、升高反应温度等条件,都可以使聚合体系的转化率提高,但是会使共聚物的分子量降低,分子量分布变宽.通过TGA的研究,发现共聚物的失重可以明显分成3个阶段,这对后期研究原丝的预氧化、碳化过程都有积极的影响.     

Multiblock copolymers by polymeric initiators via free radical mechanism

Polymeric peroxycarbamate initiators containing polyazoester or polyethylene adipate blocks were synthesized. They can be used to prepare multiblock copolymers by radical polymerization in a procedure involving several steps. Bulk and solution polymerizations are suitable techniques in these steps. The polymerization of styrene with these initiators was investigated over a wide range of conversion. Longer polymerization times led to an expected decrease in peroxygen contents of the active polystyrenes. Styrene/methyl methacrylate/n-butyl methacrylate, polyethylene adipate/styrene/methyl methacrylate, polyethylene adipate/styrene/methyl methacrylate/n-butyl methacrylate and polyethylene adipate/styrene/methyl methacrylate/acrylonitrile multiblock copolymers were obtained in this way.     

Structural heterogeneities and mechanical properties of vinyl/dimethacrylate networks synthesized by thermal free radical polymerisation

Dimethacrylate based networks usually have a poor impact resistance. In this work, we have tried to understand the origin of this brittleness. Two systems based on a same dimethacrylate monomer polymerised with styrene and divinylbenzene comonomers, respectively, have been chosen to correlate structural parameters with mechanical properties. The increase of structural heterogeneity, characterized by the width of relaxation time distribution, was measured as a function of double bonds conversion using dynamic mechanical tests. Atomic force microscopy observations of network structure after laser ablation show that the heterogeneity of networks is spacially organised due to the formation of microgels and their agglomeration. The presence of microgels strongly affects the polymerisation kinetics and controls the mechanical behaviour. Results show that the more densely crosslinked the network, the more heterogeneous is its structure. The impact resistance is shown to be related to the level of networks heterogeneities.     

Design of silica-tethered metal complexes for polymerization catalysis

Metal complex catalysts that are covalently tethered to oxide supports have been utilized for many years as hybrid molecular/heterogeneous catalysts. In this work, recent results from our laboratory in the area of silica-tethered metal complex polymerization catalysts are reviewed. The critical parameters for catalyst design when catalyst recovery and recycle are important are discussed in the context of silica-immobilized CuBr complexes for atom transfer radical polymerization (ATRP) as well as silica-tethered Zn--diiminate (BDI) complexes for lactide or epoxide/CO₂ polymerization. Additionally, a new strategy for the preparation of site-isolated metal complex catalysts on a silica surface is illustrated through the design of tethered constrained geometry catalysts (CGCs) for ethylene polymerization. The novel synthetic protocol is demonstrated to result in catalysts that appear to be more well-defined and that have improved catalytic properties. Major challenges and future directions in the field of oxide-tethered metal complex catalysis are discussed. Invited paper for a special issue on The Interface between Heterogeneous and Homogeneous Catalysis in Topics in Catalysis     

Precision control of radical polymerization via transition metal catalysis: from dormant species to designed catalysts for precision functional polymers

In the past decade, living radical polymerization has provided one of the most versatile methods to precisely construct designed polymer architectures with complexity and polar functionality. This process takes advantage of carbon-radical intermediates, which tolerate a variety of functional groups in monomers and reaction media. "Transition metal-catalyzed living radical polymerization", one of these living systems, has widely been employed for precision polymer synthesis. Not only can this process produce well-defined functional polymers, but it can also generate hybrids or conjugates with other (often biological) materials. Metal-catalyzed systems retain the advantages of conventional radical polymerization but distinguish themselves through a catalytic reversible halogen exchange equilibrium: the growing radical exists alongside a dormant speciesa covalent precursor capped with a terminal halogen from an initiator. The catalyst dictates the selectivity, exchange rate, and control over the polymerization. This Account provides an updated overview of our group's efforts in transition metal-catalyzed living radical polymerization with specific emphasis on the design of metal catalysts and the resulting precision polymer syntheses. With increasing use of the living processes as convenient tools for materials synthesis, researchers are currently seeking more active and versatile metal catalysts that are tolerant to functional groups. Such catalysts would enable a wider range of applications and target products, would have low metal content, could be readily removed from products, and would allow recycling. Since we first developed the "transition metal-catalyzed living radical polymerization" with RuCl 2(PPh 3) 3, FeCl 2(PPh 3) 2, and NiBr 2(PPh 3) 2, we have strived to systematically design metal catalysts to meet these new demands. For example, we have enhanced catalytic activity and control through several modifications: electron-donating or resonance-enhancing groups, moderate bulkiness, heterochelation via a ligand, and halogen-donor additives. For some catalysts, the use of amphiphilic and polymeric ligands allow efficient recovery of catalysts and convenient use in aqueous media. We have also used ligand design (phosphines) and other methods to improve the thermal stability of iron- and nickel-based catalysts and their tolerance to functional groups.     

Synthesis and characterization of star-like microgels by one-pot free radical polymerization

A one-pot free radical polymerization process was used to prepare methyl acrylate/ethylene glycol dimethacrylate (MA/EGDMA) and methyl methacrylate/ethylene glycol dimethacrylate (MMA/EGDMA) polymers. The role of monomer and crosslinker reactivity ratios in producing different network structures was demonstrated. While both systems produced branched polymers that exhibited low intrinsic viscosities with little variation across a wide range of molecular weights, the star-like microgels formed between a less reactive monomer (MA) with a more reactive crosslinker (EGDMA) gave lower bulk solution viscosities than the more statistical polymers formed between similarly reactive monomers and crosslinkers (MMA and EGDMA). This paper presented a simple and cost-effective synthetic route for the production of polymers with high molecular weight and low viscosity with considerable potential for industrial-scale processing.     

Tacticity control by conformational isomerization in free radical polymerization of acrylate

Contribution of conformational isomerization to polymer tacticity has been studied in free radical polymerization of (−)-menthyl 2-acetamidoacrylate with azo initiators. It has been demonstrated that the chain end of the growing polymer radical, which is generated from the s-trans and s-cis conformers of monomer, produces stereoselectively new R and S configurational quaternary carbons, respectively, for the attack of monomer. In addition, polymerization reactivity of both conformers is indistinguishable under the present conditions, and the polymerization is considered to proceed through a chain-end controlled mechanism, which excludes a penultimate unit effect on tacticity in the polymerization. The results obtained would give a clue to understand an origin of tacticity in conventional free radical polymerization of acrylates.     

微波辐射无溶剂催化合成季戊四醇双缩苯甲醛

微波辐射下采用硫酸氢钾作催化剂,无溶剂合成季戊四醇双缩苯甲醛.考察了微波辐射时间、辐射功率、原料配比、催化剂用量对产率的影响.实验表明,当微波辐射功率为465 W、辐射时间为5 min、苯甲醛用量为30 mmol、季戊四醇用量为15 mmol、硫酸氢钾用量为0.3 mmol时,产率可达84.5%.     

Controlled radical polymerization of styrene with an oxazolidinyl-N-oxyl stable free radical

Summary The bulk polymerization of styrene at 110 °C in the presence of 7-oxo-15-azadispiro[5.2.5.1]pentadecanyl-N-oxyl was investigated. The M _n values of poly(St) formed were increased linearly with conversion, and the M _w/M _n values were 1.48–1.54 at 90% conversion. On the basis of the results it was concluded that 1 controlled radical polymerization of St. Received: 19 October 1998/Revised version: 21 November 1998/Accepted: 26 November 1998     

Interfacial grafting and crosslinking by free radical reactions in polymer blends

Relevant developments in polymer blends based on immiscible products have shown how efficient the amphiphilic species are in improving mechanical properties and morphological stabilization throughout successive processing steps. However, it appears that the amount of compatilizer that has to be introduced for reaching the expected level of global properties is largely higher than the calculated concentration based on the interfacial area. In order to obtain the required concentration of amphiphilic copolymer at the interface, it seems of interest to test the possibility of creating these species by in situ reactivity at the interface. As most of the polymers are hydrocarbon compounds, they are subjected to free radical reactivity by hydrogen abstraction on the different hydrocarbon sites, which may lead to crosslinking or grafting. Therefore, this work deals with reciprocal free radical reactivity of two immiscible semicrystalline homopolymers, namely, low density polyethylene (LDPE) and polyamide-11 (PA-11). The reaction has to occur mainly at the interface, where the resulting grafted copolymer has to be anchored for final stabilization of the biphasic system. Different analytical techniques help in characterizing the reacted blend and determining the level of interfacial grafting.     

Photochemically masked benzophenone: Photoinitiated free radical polymerization by using benzodioxinone

A new photoinitiating system for free radical polymerization of methyl methacrylate (MMA) is reported. This system consists of benzodioxinone and hydrogen donors such as triethylamine (TEA), N,N-dimethyl ethanol amine (DMEA) and tetrahydrofuran (THF). A feasible mechanism involves photoinduced formation of benzophenone from benzodioxinone and subsequent hydrogen abstraction of photoexcited benzophenone from hydrogen donors to yield radicals capable of initiating polymerization of MMA.     

The free radical oxidation of polyunsaturated lecithins

Two unsymmetric polyunsaturated lecithins were allowed to air oxidize and the primary products of autoxidation were isolated and characterized. 1-Palmitic-2-linoleic-phosphatidylcholine undergoes significant oxidation after 16 hr at room temperature under air. A new phospholipid product may be isolated by reverse phase high pressure liquid chromatography (HPLC) and this HPLC fraction is shown to be made up of lipid hydroperoxides formed by free radical oxidation of the homoconjugated diene of the linoleate component of the lecithin. 1-Stearic-2-arachidonic-phosphatidylcholine undergoes a similar oxidation with the arachidonate polyunsaturated functionality being oxidized. The structure of the oxidation products was established by reduction of hydroperoxide with triphenylphosphine, snake venom hydrolysis of the C-2 ester, and HPLC analysis of the resulting hydroxy fatty acids or their methyl esters.     

Lithographic results of electron beam photoresists prepared by living free radical polymerization

Summary A new family of random copolymers composed of chloromethylstyrene and a silicon based styrenic monomer was prepared using living radical polymerization. The lithographic efficiency of the resulting electron beam resists was examined. A pronounced improvement on the lithographic resolution and image quality of resists with a narrow molecular mass distribution was observed and is described. Received: 20 April 1999/Revised version: 11 June 1999/Accepted: 17 June 1999