Free radical condensation

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

In view of the possibility of random free-radical scission of some covalent bonds, despite a great deal of dissipation of energy in the same sample, this repolymerization may be the likely interpretation, as the effect could result from recombination (or rather free-radical condensation) in the compressed material. 

Diethynylbenzene and 1,2-diethynyl-tetrafluorobenzene were used as the precursors, the synthesis of which can be found elsewhere. The precursors were vaporized in vacuum, and the vapor was transported into the hot chamber where the substrates were placed. Polymer film formation requires the substrates to be maintained at 350°C. Unlike the parylenes where bond dissociation occurs, in this case, the high temperature surface of the substrate causes chemical bond rearrangement leading to the formation of free radicals. Condensation of these free radicals is immediately followed by polymerization. The proposed polymerization scheme is shown in Figure 15. 

ESR Studies of Free Radicals Condensed from Vapors of Acetone, Ethylene, Methyl Alcohol, Ethyl Alcohol, and /er/-Butyl Alcohol after Irradiation by 1 Mev. He+ Ions... 

R. N. Haward, ed., Dev. Polym., Vol. 2, Free Radical, Condensation, Transition Metal, and Template Polymerization, Appl. Sci. Publ., London, 1979. 

Proanthocyanidins have different physiological effects. Recently acknowledged are their anti-inflammatory and anti-aUergic effects, and beneficial effects in the development of atherosclerosis (the effect is related to their reaction with free radicals). Condensed tannins have also found use as food additives. A diet that is high in tannins may, however, also show negative effects, such as lower utihsation of proteins. 

Crosslinked silicone rubber can be manufactured by free radical, condensation and addition cure mechanisms and is described as follows ... 

Fig. 15. The naphtholate ion is oxidized by Cu++ to a free radical ( ). Free radicals condense to dinaphthol. The course of the reaction is influenced by the position of a substituent group -R. Fara-substituted naphthol radicals condense in ortho position, o-substituted radicals condense in para position. The dinaphtols are oxidized to quinones (bis-naphthalene indigo).

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

Both vapor-phase and Hquid-phase processes are employed to nitrate paraffins, using either HNO or NO2. The nitrations occur by means of free-radical steps, and sufftciendy high temperatures are required to produce free radicals to initiate the reaction steps. For Hquid-phase nitrations, temperatures of about 150—200°C are usually required, whereas gas-phase nitrations fall in the 200—440°C range. Sufficient pressures are needed for the Hquid-phase processes to maintain the reactants and products as Hquids. Residence times of several minutes are commonly required to obtain acceptable conversions. Gas-phase nitrations occur at atmospheric pressure, but pressures of 0.8—1.2 MPa (8—12 atm) are frequentiy employed in industrial units. The higher pressures expedite the condensation and recovery of the nitroparaffin products when cooling water is employed to cool the product gas stream leaving the reactor (see Nitroparaffins). 

Uses. About 35% of the isophthahc acid is used to prepare unsaturated polyester resins. These are condensation products of isophthahc acid, an unsaturated dibasic acid, most likely maleic anhydride, and a glycol such as propylene glycol. The polymer is dissolved in an inhibited vinyl monomer, usually styrene with a quinone inhibitor. When this viscous hquid is treated with a catalyst, heat or free-radical initiation causes cross-linking and sohdification. A range of properties is possible depending on the reactants used and their ratios (97). 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

Since the thermal dehydrocondensation proceeds by a free-radical mechanism (37), various radical-forrning promoters like acetone, ethanol, or methanol have been found useful in improving conversion of ben2ene to condensed polyphenyls. In the commercial dehydrocondensation process, ben2ene and some biphenyl are separated by distillation and recycled back to the dehydrocondensation step. Pure biphenyl is then collected leaving a polyphenyl residue consisting of approximately 4% o-terphenyl, 44% y -terphenyl, 25% -terphenyl, 1.5% triphenylene, and 22—27% higher polyphenyl and tars. Distillation of this residue at reduced pressure affords the mixed terphenyl isomers accompanied by a portion of the quaterphenyls present. 

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

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