Ultraviolet radiation polymerization reactions

Eq. 2-248) [Braun and Wegner, 1983 Hasegawa et al., 1988, 1998]. This polymerization is a solid-state reaction involving irradiation of crystalline monomer with ultraviolet or ionizing radiation. The reaction is a topochemical or lattice-controlled polymerization in which reaction proceeds either inside the monomer crystal or at defect sites where the product structure and symmetry are controlled by the packing of monomer in the lattice or at defect sites, respectively. 

But vitrification may be a problem when the cure is started at room temperature and no external heat source is provided (the only source of heat generation is the polymerization reaction). This is the case of UV (ultraviolet radiation), EB (electron beam), or X-ray curing processes. 

The free radical photoinitiator may be any compound that produces a free radical on exposure to radiation, such as ultraviolet or visible radiation, and thereby initiates a polymerization reaction. 

Certain diacetylene derivatives undergo a remarkable polymerization reaction in the solid state under the influence of heat, ultraviolet light. X-rays or y radiation - . The reaction involves 1,4-addition of the conjugated triple bonds and produces a... 

Our earlier structural studies (15) have shown that in the case of plasma-polymerized HMCTSN and HMCTSO other crosslinking reactions may take place. Ultraviolet radiation emitted by the plasma may cause a hemolytic cleavage of Si-C and C-H bonds in SiCH groups, fo-lowed by crosslinking in the polymer via formation of methylene and ethylene linkages between silicon atoms. 

The polymerization of MAH does not occur under normal conditions but is readily initiated under gamma or ultraviolet radiation and by the use of radical catalysts at high concentrations or having a short half life at the reaction temperature. The radical initiated homopolymerization is promoted by the presence of photosensitizers in the absence of light 2, 2 ). It has been proposed that under these conditions MAH undergoes excitation and the excited monomer, actually an excited dimer or charge transfer complex, polymerizes. The participation of the excimer or excited complex and the cationic character of the propagating chain has been confirmed by the total inhibition of MAH polymerization in the presence of small amounts of dimethyIformamide which has no effect on the polymerization of "reactive acrylic monomers ( ). 

Polymerization occurs when some outside agent provides the energy to break the double bond in ethylene to get the reaction started. Heat, light, ultraviolet radiation, a beam of electrons, and gamma rays have all been used to initiate polymerization. Polymerization occurs at lower energy levels, and it has been the search for catalysts to achieve this objective... 

Various types of radiation such as ultraviolet (UV) radiation, and electron beams can be used for initiation of polymerization reactions. Curing of polymer matrices by UV irradiation can be applied to a variety of processes in the production of composite components, as long... 

Radiation polymerization n. A polymerization reaction initiated by exposure to radiation such as ultraviolet or garmna rays rather than by means of a chemical initiator. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New... 

The polymerization of crystalline acrylic acid or methacrylic acid at a temperature only slightly below their respective melting points with ultraviolet radiation gave rise to noncrystalline and disoriented polymers. While under these reaction conditions the solid methacrylic acid contained measurable concentrations of free radicals, acrylic acid contained no detectable free radicals. The formation of free radicals in methacrylic acid was found to be very temperature-dependent. As the temperature was lowered, the concentration of free radicals decreased. 

The preferred polymerization medium was a saturated fluorocarbon or chlorofluorocarbon solvent, although an aqueous medium could also be used. The solvent system was used to control the reaction conditions and increase the polymerization reaction rate. The reaction medium could also improve melt processability and increase the thermal stability and chemical resistance of the polymer. The reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization regimes. Azo compounds, peroxy compounds, ultraviolet radiation, or high-energy ionizing... 

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... 

Such studies did, however, suggest that the initial step in the polymerization reaction is a diradical (26), see Figure 5, apparently contrary to the evidence of the existence of carbene radicals in thermally polymerizing crystals (27), This apparent conflict has been resolved by an extensive series of studies of the photo- and thermal polymerization of diacetylenes at low temperatures conducted at the Universities of Stuttgart and Bayreuth- A review of this worK is in the course of publication and references to the extensive literature will be found there (20 - 30)- The most thorough studies have been made of the monomer TS, similar but less detailed results have been obtained for a number of other diacetylene monomers (31). In most of the experiments monomer crystals at 4 K are irradiated with ultraviolet radiation and studied by optical, EPR and ENDOR spectroscopy. At 4 K broad band irradiation produces stable oligomeric intermediates since polymerization cannot proceed by thermal activation. 

In order to formulate an answer to the obviously important question of the length of this interval of acceleration and to ascertain under what conditions it may be long enough to observe experimentally, we shall examine the non-steady-state interval from the point of view of reaction kinetics. Let us suppose, however, that the polymerization is photoinitiated, with or without the aid of a sensitizer. It is then possible to commence the generation of radicals abruptly by exposure of the polymerization cell to the active radiation (usually in the near ultraviolet), and the considerable period required for temperature equilibration in an otherwise initiated polymerization can be avoided. Then the rate of generation of radicals (see p. 114) will be 2//a s, and the rate of their destruction 2kt [M ]. Hence... 

---