Radiation-catalyzed polymerization

Only syntheses which involve the formation of new glycosidic linkages will be considered in this article. This restriction excludes many interesting examples of copolymerization in which only one of the monomers is a carbohydrate (or carbohydrate derivative), the polymerization of carbohydrate derivatives which contain a polymerizable group4 (such as acrylate), and the polymerization of sugar lactones.1 Many of these topics have already been discussed in reviews.1-8 Also outside the scope of this article is the chemical modification of naturally occurring polysaccharides thus, we have excluded the industrially important process of dextrini-zation,10 except as it may pertain to acid condensation processes. The radiation-catalyzed polymerization and modification of carbohydrate poly-... 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

IMPREGNATION OF SOFT SOUND WOOD WITH MONOMERS, which are then polymerized in situ by 7 radiation, was a method used in many laboratories during the 1960s in an effort to obtain wood-plastic composites. The process was attractive in two respects there was a large choice of consolidants, and radiation-induced polymerization had many advantages. Various vinyl monomers are cured by 7 rays. By proper selection of the polymer or copolymer, materials can be tailor made for specific applications. The radiation process presents several advantages over the chemically catalyzed polymerization of monomers in wood. 

Because of acid-catalyzed hydrolysis of N-vinylpyrrolidone in water, polymerization was carried out in organic solvent - DMF. Three types of samples of poly(methacrylic acid) were used syndiotactic - obtained by radiation polymerization, atactic - obtained by radical polymerization, and isotactic - obtained by hydrolysis of isotactic poly(methyl methacrylate). It was found that in all cases the rate enhancement appeared in comparison with the blank polymerization (without template). The rate enhancement became more pronounced with increasing chain length and syndiotacticity of the template. According to the authors, the rate enhancement is connected with the stronger complex formation between poly(vinyl pyrrolidone) and syndiotactic poly(methacrylic acid) then with isotactic template. This conclusion was supported by turbimetric titration in DMF/DMSO system and by model considerations. It is worth noting, however, that... 

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

There exist communications on chemical reactions catalyzed by magnetic fields in connection with electromagnetic radiation. A photoresist was cross--linked by Panico under such conditions [10]. Teffal and Gourdenne polymerized (without initiator) 2-hydroxyethyl methacrylate by means of microwaves in a waveguide [11]. Actually this should be regarded as thermal polymerization, with the heat generated by dipole vibrations of the polar groups the monomer. 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

Additives Effect on the Catalyzed Monomer Solution. Soluble dyes can be added to the catalyzed monomer solution to color the final wood-polymer composite. Any color of the visible spectrum can be added, browns to simulate black walnut, red and blues for national colors. The color emphasizes the grain structure of the particular species and combines with the polymer to add a three-dimensional depth not present in surface-finished wood. A dense black wood-polymer, so desirable for musical instruments, is difficult to obtain because of wood s light color and the tendency of the microstructure to chromatographically separate a dye of several components into its separate colors. Dyes have an inhibiting effect on the polymerization of wood-monomer composites, some more so than others. Additional catalyst can be added to overcome this inhibition, but in the radiation process of a given geometry additional time must be allowed for complete curing. 

These results imply the production or modification of acid sites by irradiation, both with y-rays and with heavy particles. The site can hardly be acid, because the reactions (except in one instance) were studied between 149° and 400°, and acid was observed to have a half-life of 1 to 1.5 hour at 100°. A radiation-produced site similar to acid might be more stable in silica-alumina, but some of the above experiments were on pure silica. At —78°, irradiated silica gel catalyzed the polymerization of isobutylene (7d). Since the loss of this catalytic activity on annealing paralleled the loss in acidity, the activity is probably attributable to acid. 

A system that has recently been receiving attention Is radiation induced cationic curing. The interest in cationic curing has been Inspired by the development of onlum salt catalysts. Strong acids are liberated when the coatings are Irradiated In the presence of certain onlum salts. These acids are capable of catalyzing cationic polymerization reactions (7). Onlum salts have been used mostly In UV curable systems. However, It has been shown recently that they may also be used for electron beam Induced curing (8,9). 

Ethylene, propylene Ziegler-Natta-catalyzed chain polymerization Automotive parts, radiator and heater hoses, seals... 

Isocyanates polymerize through the carbon-to-nitrogen double bonds by anionic mechanism. Reactions can be catalyzed by sodium or potassium cyanide at-58 °C. N,N -dimethylformamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of various carboxylic acids to sodium-naphthalene, are also effective. In addition, polymerizations can be carried out by cationic, thermal, and radiation-induced methods. 

The involvement of the M°P species in the mechanism was also confirmed by van Behar et Co P species formed from reduction of Co porphyrin by electrochemical, photochemical, and radiation methods was found to be unreac-tive towards reduction of C02. But one electron reduction of the Co P resulted in a species which bound CO2 and reduced it with the formation of CO and formic acid as products However, Riqelme et al. reported that the Co°/Co couple (rather than the M°/M couple) was responsible for the catalytic reduction of CO2 to CO and formic acid by polymeric CoTAPP (on GCE). The monomeric form of the catalyst did not catalyze the reductionGrodkowski et al. found the Co and Fe complexes to be the active species when Co and Fe corroles electrocat-alyzed the reduction of C02 . The Fe corroles showed better catalytic activity than the Co corroles. The catalytic behavior of the corroles towards the reduction of CO2 is different from that of MP complexes in that the latter do not react with CO2 until they are reduced to beyond state . 

The failure to initiate the homopolymerization of maleic anhydride (MAH) in the presence of a free radical catalyst under normal conditions was long considered evidence of the steric hindrance imposed by 1,2-disubstitution of the double bond. However, in recent years, the radical homopolymerization of MAH has been carried out under the influence of V-radiation, ultraviolet radiation in the presence of a sensitizer, and shock waves, as well as in the presence of free radical catalysts at high concentrations and/or at temperatures where they have a short half-life. These and other aspects of the polymerization of MAH have been reviewed by Gaylord, who has proposed the participation of n excited charge transfer complex and of cationic intermediates in the radical catalyzed homopolymeri zation. 

Short-chain monomers, in particular ethyl acrylate and methyl methacrylate in a volume ratio of 5 1, are introduced in liquid form without solvents in a dosed system. Low-intensity y-radiation polymerizes the monomers under ambient conditions. Additionally, the liquid monomer mixture has been modified to indude an amine-substituted alkyl methacrylate to neutralize acidity and establish a buffer against future acid catalyzed hydrolysis [34]. The presence of a small amount of a diacrylate leads to cross-linking and hence strengthening of the paper. 

VinyUdene fluoride can be polymerized in saturated fluorinated or fluorochlorinated solvents. These solvents dissolve fluoroalkenes like vinylidene fluoride and organic peroxide catalysts. Polymerization thus takes place in a homogeneous phase and the resulting poly-vinylidene fluoride is insoluble in the solvent, making the product readily separable from the solvent. In addition to organic peroxides, the reaction can be initiated or induced by radiation. This is helpful in avoiding contamination of the product with other reaction components such as the initiator, surfactant, and the others. Alkyl boron activated by oxygen has also been reported to catalyze vinylidene fluoride and vinyl fluoride polymerization in water and solvents. ... 

Frequently, three preparation methods are widely used in polymer/clay nanocomposite technology. The first one is in situ polymerization. This method consists of the use of a monomer as a medium to the clay dispersion while favorable conditions are imposed to perform the polymerization between the clay layers. These layers present high surface energy and the monomer units are thus attracted to the inside of the galleries until equilibrium is reached. Polymerization can be initiated by heat or radiation, by the diffusion of an adequate initiator or a fixed catalyzer inside the layers before the filling step by the monomer. After that, polymerization reactions occur between the layers with lower polarity, dislocating the equilibrium and then aiming at the diffusion of new polar species between the layers. 

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