Ultraviolet-induced polymerization

In a patent, a simple procedure for ultraviolet-induced polymerization of the monomer is indicated. This procedure is given here for reference. 

Decker, G., and Jenkins, A. D. "Kinetic Approach of O2 Inhibition in Ultraviolet-Induced and Liwer-Induced Polymerizations." Macromolecules 18 (1985) 1241-44. 

The previously discussed principles of grafting-to and grafting-from can also be applied for the modification of polymer surfaces with polymer brushes. However, the binding of linkers and polymerization initiators to polymer surfaces is not as straightforward as it is for oxidic inorganic materials. Thus, dedicated pretreatments are usually necessary. These may include rather harsh reaction conditions due to the chemical inertness of many polymers (see Chapter 3). Alternatively, radiation treatment of polymers (to form radicals) followed by exposure to air may be used to form peroxides and hydroperoxides, which can be directly used as initiators for thermally or ultraviolet-induced graft polymerizations [16,17] (see Chapter 2). 

Ultraviolet curing of epoxy resins has become important. Cycloaliphatic epoxy resins are combined with substances that induce polymerization under the influence of UV radiation or an electron beam. Industrially important products include tri-arylsulfonium salts (UVE 1014, 1016, General Electric) and arene-ferrocenium compounds (CG 24-061, Ciba-Geigy). To ensure good film flexibility and chemical resistance, polyols should be added. The films should be postcured for 30-120 s at ca. 100"C [2.126]. 

The polymerization of radicals within the core of the polymer particles is considered to be very slow since it is limited by the diffusion of monomer from the particle surface to the poorly swollen core. Bamford and Jenkins showed that the swelling of the polymer is so poor that the radicals within the particle core are highly restricted in mobility. These radicals react with available monomer faster than it can diffuse into the core. As a result, the core radicals become trapped within the particle matrix. This effect is best demonstrated with ultraviolet (UV) or gamma-ray-induced polymerization because the source of free radicals can be easily removed. The persistence of free radicals, can be demonstrated directly using electron spin... 

In the presence of maleic anhydride, the ultraviolet-induced terpolymer-ization of A/-vinylpyrrolidone, methyl methacrylate, and maleic anhydride exhibits charge-transfer polymerization characteristics without any zinc chloride being present [90]. The mixture of N-vinylpyrrolidone and maleic anhydride, on exposure to ultraviolet radiation, changes color from yellowish through pink to scarlet. The ultraviolet spectrum indicates the formation of a one-to-one complex. 

Other work on the ultraviolet radiation-induced polymerization in the solid state is also discussed in references [65-69]. 

It has been known that the irradiation of acetylene gas with ultraviolet rays or ionizing radiation produces cuprene and benzene. Radiation-induced polymerization of acetylene was first investigated using a-rays from radon as early as the 1920s. 

Peroxides form in a process called auto-oxidization, a reaction with oxygen in the air. Ultraviolet radiation, a normal component of light, provides enough energy for the reaction to occur. This is why peroxidizable compounds are packaged in dark glass or metal containers. Peroxides can induce polymerization, and the polymerization products are... 

With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides. 

Compounds capable of generating free radicals on exposure to light and thereby inducing free radical polymerization are known as photoinitiators. However, the utility of many of these compounds is limited to activation by ultraviolet light only. The addition of a compound that absorbs incident light more efficiently may activate the photoinitiator to produce free radicals using visible light. Such compounds are known as photosensitizers. 

One very interesting new application of Lewis acids in curing epoxy adhesives has appeared within the last 25 years. The Lewis acid initiator for the cationic polymerization is formed by the heat or ultraviolet light-induced decomposition of Lewis acid Lewis base salts. Several patents by Crivello and coworkers43 47 describe compounds containing... 

Free radical induced cationic polymerization may also be performed by irradiating monomer solutions containing polysilane and pyridinium ions [59]. Polysilanes have strong ultraviolet absorption in the 300-350 nm region and rapid photodegradation occurs upon irradiation at this band [60] ... 

Hydrogen cyanide condenses at 25.6°C to a liquid with a very high dielectric constant (107 at 25°C). Here, as in similar cases, such as water, the high dielectric constant is due to association of intrinsically very polar molecules by hydrogen bonding. Liquid HCN is unstable and can polymerize violently in the absence of stabilizers in aqueous solutions polymerization is induced by ultraviolet light. 

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