Polymerization gamma initiated

Since radicals arc neither formed nor destroyed during reversible chain transfer, RAFT polymerization must, like conventional radical polymerization, be initiated by a source of free radicals as shown in Scheme 9.38. RAFT polymerization is usually carried out with conventional radical initiators. Most often thermal initiators (e.g. AIBN, ACP, BPO, K2SnOs) are used. S polymerization may be initiated thermally between 100-130°C. Polymerizations initiated with UV irradiation, a gamma source " or a plasma tleld have been reported. In these polymerizations, radicals generated directly from the RAFT agent may be responsible for initiation. It was initially suggested by Pan and coworkers that the mechanism for molecular weight control in and y-... 

Bulk polymerization initiated via more novel methods have also been used to form bulk PS-MMT nanocomposites. Zhang et al. [19] used gamma irradiation to initiate the polymerization of PS-MMT nanocomposites with different surface modifications (3, 34) and successfully prepared exfohated morphologies when reactive clay modifications were used. Uthirakumar et al. [51-54] modified MMT with a cationic radical initiator which was used to initiate the bulk polymerization of styrene. Because the polymerization was initiated from the clay surface and the monomer and the clay were suitably compatible, exfoliated morphologies were formed. 

Radiation Polymerization A polymerization reaction initiated by exposure to radiation such as ultraviolet or gamma rays rather than by means of a chemical initiator (Odian GC (2004) Principles of polymerization. Wiley, New York Lenz RW (1967) Organic chemistry of synthetic high polymers. Interscience Publishers, New York). 

Crosslinked poly(VAEE) and poly(AANa-VAEE) were synthesized by gamma-initiated radical polymerization in the presence of divinyl ether of... 

A third variety is the graft copolymer. In this case, branches of one monomer are grown on a main stem of a previously formed polymer molecule. For example, polyethylene can be irradiated in air with gamma rays or accelerated electrons, which leave peroxides or free radicals trapped on the polymer backbone. Exposed to a reactive monomer such as acrylonitrile (CH2=CHCN), polymerization is initiated at the free-radical sites, and branches of polyacrylonitrile grow on the polyethylene crystal stem [57]. 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

Peggion E, Cosani A, Mattucci AM, Scoffone E (1964) Polymerization of gamma-ethyl-L-glutamate-N-carboxyanhydride initiated by Di-N-butyl and Di-isopropyl amine. Biopolymers 2 69-78... 

Precipitation polymerizations dominated the early work which aimed at preparing industrially important hydrocarbon polymers in C02. In 1968, Hagiwara and coworkers explored the polymerization of ethylene in C02 using both gamma radiation and AIBN as free radical initiators [79]. Reactions were conducted at pressures of 440 bar and over the temperature range of 20-45 °C. 

In dispersion polymerization, the monomer and initiator are dissolved in the continuous phase, which acts as a nonsolvent for the developing polymer. The continuous phase can be organic, aqueous, or a mixture of miscible phases. Two methods of initiation have been employed, including gamma radiation [75] and chemical initiation by potassium perox-odisulphate [76]. As the polymer is formed, it precipitates as nanoparticles. These particles are not polymeric precipitates as in precipitation polymerization. Rather, they are swollen by a mixture of the monomer and the continuous phase [39],... 

Kreuter and Speiser [77] developed a dispersion polymerization producing adjuvant nanospheres of polymethylmethacrylate) (PMMA). The monomer is dissolved in phosphate buffered saline and initiated by gamma radiation in the presence and absence of influenza virions. These systems showed enhanced adjuvant effect over aluminum hydroxide and prolonged antibody response. PMMA particles could be distinguished by TEM studies and the particle size was reported elsewhere to be 130 nm by photon correlation spectroscopy [75], The particle size could be reduced, producing monodisperse particles by inclusion of protective colloids, such as proteins or casein [40], Poly(methylmethacrylate) nanoparticles are also prepared... 

Figure 1. Conversion curves of the polymerization of acrylic acid in toluene solutions (4). Monomer concentrations (volume per cent) (1) 100% (2) 95% (3) 90% (4) 85% (5) 80% (6) 65% (7) 50% (8) 27%. The polymer precipitates as a fine powder at all concentrations. Initiation by gamma-rays at 20°C and...

Figure 8. Arrhenius diagram of the gamma-ray initiated polymerization of acrylonitrile in bulk 1161 Curve 1 Initial rates" curve 2 Pseudostationary rates. ...

There are numerous examples of solid state polymerizations. Here we will briefly describe examples based on addition polymers. Generally, the crystalline monomer is irradiated with electrons or some form of high-energy radiation, such as gamma or x-rays. Since many monomers are solids only below room temperature, it is customary to begin irradiation at lower temperatures with the temperature raised only after initial polymerization occurs. (Some reactions are carried to completion at the lower temperature.) After polymerization, the monomer is removed. Table 6.10 contains a list of some of the common monomers that undergo solid-state irradiation polymerization. 

Radioactive sources and particle accelerators are used to initiate polymerizations. Electrons, neutrons, and a-particles (He2+) are particulate radiations, while gamma and X rays are electromagnetic radiations. The interactions of these radiations with matter are complex [Chapiro, 1962 Wilson, 1974]. The chemical effects of the different types of radiation are qualitatively the same, although there are quantitative differences. Molecular excitation may occur with the subsequent formation of radicals in the same manner as in photolysis, but ionization of a compound C by ejection of an electron is more probable because of the higher... 

Bulk polymerization of acrylonitrile initiated by gamma radiation was described by Chapiro at The process is greatly complicated by other reactions. Occlusion of... 

On the other hand, a number of qualified investigations on the emulsion polymerization initiated by chemical catalysts and on the experimental verification of the quantitative theory of Smith and Ewart were performed by van der Hoff (14) and Gerrens et ah (4,6). The high experimental standard of these papers in determining the kinetic data is rarely met by papers on the gamma-induced emulsion polymerization. 

The penetrating nature of gamma and X rays suggest their use in initiating polymerizations within materials. Ballentine et al. have attempted to improve wood quality by irradiating wood-soaked monomers. 

Notwithstanding the physical presentation of the polymeric backbone—fabric, fiber or film—the immersion media should contain the monomer, with or without addition of a solvent, either in liquid or in vapor phase. The initiation proceeds by direct attack to the polyamide or by the preliminary introduction of labile groups that are then removed by monomer attack, usually by exposure to gamma-rays. 

Polymerization of monomer-impregnated concrete was initially carried out with 60Co gamma radiation. A principal advantage of using radiation is that free radical chain reactions can be induced at ambient temperature. This limits vaporization loss and may produce a better concrete-polymer bond. The disadvantages are the dose requirements and the relatively high initial investment required for the radiation source and facility. 

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. 

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