Initiation in cellulose

There are many chemical methods for generating radicals reported in the hterature that do not involve conventional initiators. Specific examples are included in References 64—79. Most of these radical-generating systems carmot broadly compete with the use of conventional initiators in industrial polymer apphcations owing to cost or efficiency considerations. However, some systems may be weU-suited for initiating specific radical reactions or polymerizations, eg, grafting of monomers to cellulose using ceric ion (80). 

Grafting of methylmethacrylate onto cellulose using ceric ammonium nitrate (Ce ) as the initiator in a benzene-water system is also demonstrated. The grafting yield in a benzene-water system is much lower than in the case of the methanol-water system and decreases by increasing the ratio of benzene to water. This can be due to the lower polarity and wetting power of benzene, which leads to poor swelling of the cellulose. 

Freudenberg, Kuhn and their co-workers showed that both the velocity constants and the courses followed by the hydrolyses of these various polymers can be accounted for by postulating that one or the other or both of the terminal linkages, a and h of Table VIII, in these various species hydrolyze more rapidly than the internal c linkages. All of the latter can be assumed to hydrolyze at the same rate. If, for example, one of the two terminal linkages, a or 6, in an x-mer reacts at a rate equal to cellobiose, 1.07 X10, and the rate for each of the other X —2 linkages corresponds to the initial average rate, 0.305 X10, of hydrolysis of the bonds in cellulose then the calculated... 

Larger Scale Testing. The standard card gap test (2) is test No. 1 of a series of larger scale tests designed to determine the sensitivity of liquid propellants to hydrodynamic shock. In this test, relative sensitivities of various propellants are determined in terms of the number of 0.01-inch thick cellulose acetate cards required to attenuate a standard shock sufficiently just to prevent initiation in the test sample. When performed according to the exacting conditions of apparatus and procedure, the results are very reproducible from one laboratory to another. However, small variations in the apparatus or procedure can cause major variations in the resulting data, and therefore the test can be considered only relative. A major drawback of the standard test is that it cannot accommodate materials that are volatile under the test condition. At TCC-RMD some special equipment has been developed that allows tests to be made on confined samples at elevated temperature and pressure. 

Soxhlet Extraction Analysis. Powdered samples of GMC were degassed and sealed under vacuum into pyrex tubes, and irradiated at room temperature (25-30 °C) in a Cobalt-60 Gammacell. The dose rate determined by Fricke dosimetry was 0.24 Mrad/hr. Doses up to 20 Mrad were utilized. After irradiation, the tubes were opened and the samples placed in cellulose thimbles. A Soxhlet extraction was performed with methylethyl ketone at 76 1°C for 18-20 hrs. The insoluble material was dried to constant weight and the gel fraction calculated from initial weight. 

Since the cell wall structure of the wood is not swollen by the vinyl monomer, there is little opportunity for the monomer to reach the free radical sites generated by the gamma radiation on the cellulose to form a vinyl polymer branch. From this short discussion, it is reasonable to conjecture that there should be little if any difference in the physical properties of catalyst-heat initiated or gamma radiation initiated in situ polymerization of vinyl monomers in wood. 

An Interpretative review of the reactions initiated by macrocellulosic free radicals with vinyl monomers to yield block and graft copolymers of fibrous cellulose was made. Macrocellulosic radicals are usually formed by interactions with radiation or chemical redox systems. Important factors in these heterogeneous reactions are lifetimes and accessibilities of the radicals and interactions of solutions of monomer with fibrous cellulose. Changes in organochemical, macromolecular, and morphological structures in cellulosic fibers through formation of copolymers are made. 

In the photosensitized experiments, the cellulose sample was treated with a solution of the initiator in acetone for 1 h at room temperature and the solvent evaporated in air. When this kind of adsorption was not possible, the initiator was dissolved in THF and added to the reactant medium. 

Various free radicals are generated in cellulose and cellulose derivatives by ultraviolet light, which may be capable of initiating graft copolymerization reactions with vinyl monomers. The graftability of these photoinduced free radicals in homogeneous and heterogeneous media was studied. 

Cellulose esters (e.g., cellulose triacetate, cellulose diacetate, cellulose propionate, and cellulose butyrate) are prepared by initially treating cellulose with glacial acetic acid (or propionic acid and butyric acid) followed by the corresponding acid anhydride with a trace of strong acid as a catalyst in chlorinated hydrocarbon. Complete esterification reactions result in the formation of a triester, which undergoes water hydrolysis to form a diester. Cellulose acetate alone or in combination with cellulose triacetate or cellulose butyrate is used as a semipermeable membrane for osmotic pumping tablets, primarily in controlled release systems. The permeability of the membrane can be further modulated by adding water-soluble excipients to the cellulose esters. 

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