Heat-initiated free-radical

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). 

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa. 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

Another possibility might be to use solutions of acrylic polymer dissolved in acrylic monomer and to harden the glue in situ by heat-activated, free-radical-initiated polymerization. Such a glue should have acceptable shrinkage properties and, if the components are properly chosen, could remain fairly tough. Further, if non-cross-linking components are used, it should be a reversible system. 

The process represented in Figure 5.82(d) is similar to that in Fignre 5.82(c), except that polymerization is initiated in the water-immiscible solvent phase. A vinyl monomer, e.g., styrene, methyl methacrylate, or vinyl acetate, is dissolved in a water-immiscible solvent together with an initiator. The solution is emulsified in water using an emulsifier and heated to initiate free-radical polymerization. The resulting polymer deposits at the solvent-water interface forming the capsule wall. 

Ultraviolet light initiates free radical oxidation at the exposed surface of an elastomeric product to generate a layer of oxidized rubber. Heat, moisture, or high humidity can then initiate crazing of the surface, which subsequently can be abraded off. Such degradation of the surface is more severe with nonblack stocks than with black compounds. Nonblack compounds such as white tire sidewalls thus require higher levels of nonstaining antioxidants than carbon black-loaded formulations. 

The stability of charge-transfer complexes depends upon internal resonance stabilization. This degree of stabilization determines how easily the diradical opens up. Consequently, this stability also determines how the copolymerization occurs. It can occur spontaneously, or under the influence of light or heat, or because of an attack by an initiating free radical. 

Precaution Combustible rapid polymerization in presence of moisture incompat. with reducing agents, water avoid heat, moisture, free radical initiators may be shock-sensitive... 

By far, the most diverse use of biomaterials exists within the polymer class in these organic materials, ionizing radiation and plasma discharges have a unique ability to initiate free radical and ionic reactions without the need to add catalysts or to heat. 

Free-Radical Polymerization. Autopol5mierization of chloroprene monomer occurs readily imder free-radical and photochemical conditions (see Radical Polymerization). The electron-rich and electronegative chlorine atom facilitates the high reactivity of this monomer. Over the temperature range 20-80° C, the initiation depends on the formation of di-radicals or the added free radical that initiates polymerization (see Initiators, Free-Radical). Polymerization proceeds at a rate that follows first-order kinetics with an activation energy of 82 kJ mol (19.6 kcal mol ) and a heat of pol5unerization of 68-75 kJ mol (16-18kcalmol-i)(l,3,7,8). 

Azo Initiators, in this section we report the preparation of telechelics involving thermally labile azo compounds. Upon heating, aliphatic azo compounds evolve nitrogen, thus forming two carbon-centered free radicals (see Initiators, Free-Radical). 

Basically a latex is synthesized by mixing monomer, surface active agent (surfactant), and an initiator (free radical source) into the water see Rgure 4.22 (88). The initiator is usually activated by heat. A widely used initiator is potassium persulfate. Surfactants are molecules that have a hydrophilic end and a hydrophobic end. These form micelles in the water, with the hydrophilic end facing outward, shielding the hydrophobic portion. The surfactant can be a soap or detergent, sodium lauryl sulfate being a typical example. The turbid or hazy appearance of soap in water is due to the presence of micelles. The surfactant dissolves the oil-soluble monomer in much the same way as the soap micelles dissolve skin oil in ordinary toiletry. Some of the surfactant remains... 

Methacrylated dopa ° or dopamine (Figure 10.4K) can be copolymerized into acrylate-based polymers through photo-initiated or heat-activated free-radical polymerization (Figure 10.6A). A variety of co-monomers with... 

Initiating systems which are effective at room temperature normally consist of mixtures of a peroxy compound and an activator ( accelerator ). In the presence of the accelerator, the peroxy compound rapidly decomposes without the application of heat into free radicals. The two most important peroxy materials now used for the cold curing of polyester resins are methyl ethyl ketone peroxide (MEKP) and cyclohexanone peroxide. These names are rather misleading in that neither of these materials is a single compound and both have a variable composition depending on their method of manufacture. The main... 

Decomposes upon irradiation with UV, initiate free radical polymerisations and other radical reactions. A classic example of a radical reaction that can be initiated by AIBN is the an/i-Markovnikov hydrohalogenation of alkenes. 

Some commercially available protein-inert polymers commonly used in microfluidic applications, all of which require permanent surface modification, are polyacrylamide, poly(N-hydroxyethylacrylamide), poly(NJl -di-methylacrylamide) (PDMA), polyvinylpyrrolidone (PVP), poly(vinyl alcohol) (PVA), hydroxyethylceUulose (HEC), and hydroxypropylmethylcellulose (HPMC). To permanently attach protein-resistant materials to the channel surface, high-energy sources, special chemistries, or even strong physical adsorption have been employed to introduce reactive functionalities. After activation, protein-resistant polymers can be anchored via UV-initiated free-radical polymerization. Polymeric materials usually do not have good solvent and heat resistance compared with inorganic materials, and hence it is necessary to take precautions during surface treatment to avoid serious damage to the microstructure or alteration of the physical properties of the bulk material. 

With the exception of nylon 6, all the polymers mentioned in the above paragraph are made from a monomer containing a double bond. The principle of polymerisation is quite simple, all we need to do is open up the double bond and then join the pieces together, but the actual mechanism of polymerisation is more complex than this. Polymerisation only takes place when an initiator or catalyst is added to the monomer. The initiator provides free radicals, anions or cations, which are the active centres for polymerisation. Benzoyl peroxide is a commonly used free radical initiator that decomposes upon heating into free radicals which then react with monomer (here vinyl chloride is our example). 

Maleic anhydride may be grafted to polyolefins, such as polyethylene, by mechanochemical, mechanochemical with free-radical initiators, free radical,ionic,and radiation-initiation techniques. The solid-phase mechanochemical modification studies were run by a low-temperature mechanical dispersion, extruder,die extruder,and grindingtechniques. Modification of polyethylene with <20% MA leads to crosslinking during extrusion of the polymer-anhydride blend in the extruders. This crosslinked material generally retains its form during heating to >300°C. 

The reaction rate at the initial temperature must be vanishingly small but rapid at the front temperature. The front temperature is determined by the enthalpy of the reaction, heat capacity of the product and the amount of heat loss. Free-radical polymerization is ideal because for most peroxide and nitrile initiators the rate of polymerization at ambient temperature is low but high at elevated temperatures. Amine-cured epoxies suffer from the problem of short pot life but cationic cured systems are very similar to free-radical systems (10). 

Heat of Polymerization = -729 Btu / lb Normally stable at ambient conditions polymerizes in presence of air, sunlight, moisture, heat, or free radical initiators unless stabilized by inhibitors. 

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