Radiation-initiated free radical polymerization

Radiation Initiated Free Radical Polymerization of Styrene in n Dibutyl Disulfide at 25 C. 

Monomers and Resins. Radiations such as 7 rays, ultraviolet light, and electron beams can initiate free-radical polymerization of vinyl monomers or unsaturated resins. Styrene, vinyl acetate, acrylonitrile, acrylates. 

Different radioactive sources can initiate free-radical polymerizations of vinyl monomers. They can be emitters of gamma rays, beta rays, or alpha particles. Most useful are strong gamma emitters, like to or Sr. Electron beams from electrostatic accelerators are also efficient initiators. The products from irradiation by radioactive sources or by electron beams are similar to, but not identical to, the products of irradiation by ultraviolet light. Irradiation by ionizing radiation causes the excited monomer molecules to decompose into free radicals. Ionic species also form from initial electron... 

Both electron beams and ultraviolet light initiate free-radical polymerizations with very low activation energy. This allows high polymerization rates at room temperature because the rates are not temperature dependent. Once initiated, free-radical polymerizations follow typical paths. It is, however, peculiar to radiation curing of coating materials that the gel states form at very early stages of the reactions. This is due to extensive use of polyfunctional monomers and prepolymers. In fact, it was demonstrated that the gel points occur at around 5% conversion of the prepolymers in typical commercial formulation, yet conversions to about 63% of the prepolymers to polymers are required to obtain dry films. ... 

Photo-initiation has been recognized as an excellent method for initiating free radical polymerization however, due to limited penetration depth of the UV radiation, its commercial application is limited. The film thickness of an SDR is generally less than 500 microns, so radiation-induced polymerization can be performed effectively, even on the large scale. 

Ionizing radiations (P, y) can also initiate free radical polymerizations through two mechanisms involving any molecule (AB) present in the reaction medium (monomer, solvent...) ... 

Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic monomers is first-order with respect to monomer concentration, and one-half order with respect to the initiator concentration. Rate data for polymerization of several common methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

Fig. 3. Polymerization initiation and propagation by radiation-generated free radicals. A is the initiating radical produced by irradiating the Hquid coating. (1) represents the Hquid monomer—unsaturated polymer reactive coating system. R is functional. (2) is the growing polymer chain (free radical). The cured...

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

The architecture of macromolecules is another important synthetic variable. New materials with controlled branching sequences or stereoregularity provide tremendous opportunity for development. New polymerization catalysts and initiators for controlled free-radical polymerization are driving many new materials design, synthesis, and production capabilities. Combined with state-of-the-art characterization by probe microscopy, radiation scattering, and spectroscopy, the field of polymer science is poised for explosive development of novel and important materials. New classes of nonlinear structured polymeric materials have been invented, such as dendrimers. These structures have regularly spaced branch points beginning from a central point—like branches from a tree trunk. New struc-... 

As with other chain reactions, free radical polymerization is a rapid reaction which consists of the characteristic steps of initiation, propagation, and termination. Free radical initiators are produced by the homolytic cleavage of covalent bonds as well as numerous radiation-associated methods. 

The free radical polymerization methods use either initiation by chemically formed radicals or those created by the influence of radiation. A few cases are known where grafting is initiated by radicals formed by mechanical treatments, such as milling. 

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. 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

A more detailed study has been made of the effect of temperature on the free radical polymerization of methyl methacrylate (33). Polymers were prepared at —78° and at 0° using Co80 gamma radiation and at 50° and 100° using benzoyl peroxide initiator. In Table 6 are summarized the structures of these polymers. A trend toward increasing syndiotactic character with decreasing temperature is clearly evident. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

Polymerization of the vinyl monomer in the wood may be done with either radiation or free radical catalysts. The polymerization of the vinyl monomers in both processes depends upon the same mechanism, that is, initiation by free radicals. In the radiation process, the gamma rays passing through the monomer and the woody tissue create a large number of excited and ionized molecules, many of which break into fragments,... 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

It is important to pay attention to the potential role of peroxides created on the surface of plasma-treated, including plasma polymer-coated, TPOs in the formation of durable bonds between the substrate and primer. It has been known for decades that the peroxides formed on the irradiated polymers (by y-ray. X-ray, electron beams, etc.) can be utilized in graft copolymerization of various monomers. This method is known as the peroxide method of radiation copolymerization [27]. The trunk polymer is first irradiated by ionizing radiation in a vacuum or in an inert gas environment. The irradiated polymer is exposed to air or oxygen to convert free radicals to peroxides. Thus created peroxides-containing polymers were used as the initiator of the free radical polymerization of the second monomer. The polymer peroxides are decomposed by heat or by the use of reduction/oxidation accelerator, i.e., peroxides are converted to free radicals. 

One final, though extremely important, method of generating reactive cations is by the use of ionizing radiation. Though irradiation with 7-rays from a °Co source was known to initiate free radical, liquid state, polymerizations [36], it was not until 1957 that the polymerization of isobutene at —78°C was shown unequivocally to be a cationic process [37]. Presumably on irradiation an electron is ejected from a suitable liquid monomer with the generation of a radical cation [38] which can then propagate. 

Regarding the initiation process of polymerization, it can be started by y-radiation. It is a method that has been used for the synthesis of hydrogels of PEO as well as hydrogels based on vinyl monomers " in this latter case, azo-compounds such as 2,2-azo-isobutyroni-trile (AIBN)f or 2,2 -azobis (2-amidine-propane) dihydrochloride or V-SO, and aqueous salt solutions such as aqueous ammonium peroxodisulfate are also used. Among the monomers most used in the preparation of hydrogels through free-radical polymerization are 2-hydroxyethyl methacrylate (HEMA) and A-vinyl-2-pyrrolidone (VP). ... 

After the template-monomer complexes have been formed, an azo initiator (usually azo-V,M-bis-isobutyro-nitrile, AIBN) is added to the polymerization mixture. Free-radical polymerization is initiated by heating at 40-60°C or by photochemical homolysis by ultraviolet (UV) radiation (0-15°C). MIPs prepared at lower temperatures (0°C) by photopolymerization have been found to exhibit better molecular recognition. It is theorized that the template-monomer complexes are more stable at lower temperatures thus, the imprints are more homogeneous and better defined in the resulting MIPs. 

The starting point for the Ifee-radical polymerization is to choose a monomer (M) that will react with a free radical R to add to the radical and also create another radical centre R M- of high reactivity that may add another monomer molecule. The source of the free radical to initiate the process may be the monomer itself or an added initiator (1) chosen to produce two free radicals, R -, per molecule cleanly and efficiently at a temperature suitable for the addition of monomer to occur. In the case of self-initiation, heat or radiation must be supplied in order to fragment the monomer or otherwise create the radical to initiate the chain polymerization. Table 1.5 gives examples of typical vinyl monomers that undergo free-radical polymerization, and Table 1.6 shows some initiators. Provided that certain thermodynamic... 

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