Photo-initiated Polymerizations

UV dryers are intended for curing of coatings. UV-curable coatings consist of a blend of reactive monomers or oligomers capable of free-radical-initiated polymerization. Photo initiators are used with UV-curable coatings and they are the source of free radicals produced on irradiation. 

Stabilized radicals, and so on [1]. An unprecedented example is the in situ observation of a light-induced radical pair produced in a crystal of 2,2 -di (p-chlorophenyl)-A,A, 5,5 -tetraphenyl-biimidazole (o-Cl-HABI) by X-ray diffraction, as shown in Scheme 7.1 [2], HABl derivatives are well known as the photo- and thermochromic compounds, the polymerization photo-initiators in imaging materials, and holographic photopolymers since it was prepared in 1960 [3-5], It has been proposed that the photolysis of the H ABI derivatives in benzene solution instantaneously produces a pair of lophyl (triarylimidazoyl) radicals. The lophyl radical has been proposed to be an initiator for the radical polymerization. Despite the intriguing properties of HABI derivatives, there is very few report [6]. 

As an example stereolithography is a 3-D rapid process that produces automatically simple to very complex shaped models in plastic. Basically it is a method of building successive layers across sections of pho-topolymerized plastics on top of each other until all the thin printed layers can be joined together to form a whole product. The chemical key to the process, photopolymerization, is a well established technology in which a photo initiator absorbs UV energy to form free radicals that then initiate the polymerization of the liquid monomers. The degree... 

Since the dithiocarbatnyl end groups 8 are thermally stable but pholochemically labile at usual polymerization temperatures, only photo-initiated polymerizations have the potential to show living characteristics. However, various disulfides, for example, 9 and 10, have been used to prepare end-functional polymers37 and block copolymers38 by irreversible chain transfer in non-living thermally-initiated polymerization (Section 7.5.1). 

The processes described in this section should be contrasted with RAFT polymerization (Section 9.5.3), which can involve the use of similar thioearbonylthio compounds. A. A -dialkyl dithiocarbamates have very low transfer constants in polymerizations of S and (mctb)acrylatcs and arc not effective in RAFT polymerization of these monomers. However, /V,A -dialkyl dithiocarbamates have been successfully used in RAFT polymerization of VAc. Certain O-alkyl xanthates have been successfully used to control RAFT polymerizations of VAc, acrylates and S. The failure of the earlier experiments using these reagents and monomers to provide narrow molecular weight distributions by a RAFT mechanism can he attributed to the use of non-ideal reaction conditions and reagent choice. A two part photo-initiator system comprising a mixture of a benzyl dithiocarhamate and a dithiuram disulfide has also been described and provides better control (narrower molecular weight distributions).43... 

Fig. 3 Photo-initiators releasing phosphonyl radicals for photo-initiated free radical polymerizations...

Dalglish, J., Jachuck, R.J., and Ramshaw, C. (1999) Photo-initiated polymerization using spinning disc reactor, in Process Intensification for the Chemical Industry (ed. A. Green), Professional Engineering Publishing, Ltd, Bury St. Edmunds, pp. 209-215. 

For SCVP of styrenic inimers, the mechanism includes cationic (14 [18], 19 [29]), atom transfer radical (15 [22, 27]), nitroxide-mediated radical (16 [21]), anionic (20 [19]), photo-initiated radical (17 [2], 18 [52-55]), and ruthenium-catalyzed coordinative (21 [56]) polymerization systems. Another example in-... 

Hyperbranched poly(ethyl methacrylate)s prepared by the photo-initiated radical polymerization of the inimer 13 were characterized by GPC with a lightscattering detector [51]. The hydrodynamic volume and radius of gyration (i g) of the resulting hyperbranched polymers were determined by DLS and SAXS, respectively. The ratios of Rg/R are in the range of 0.75-0.84, which are comparable to the value of hard spheres (0.775) and significantly lower than that of the linear unperturbed polymer coils (1.25-1.37). The compact nature of the hyperbranched poly(ethyl methacrylate)s is demonstrated by solution properties which are different from those of the linear analogs. 

A variety of photo-initiators and water-soluble diluents were used to study the photopolymerization of 2-hydroxyethyl methacrylate (HEMA). The rate of polymerization can be correlated to the structures as well as the concentration of initiators and diluents. It was found that, in terms of rate of polymerization,... 

The photopolymerization of mixtures of maleimides and vinyl ethers is shown to be an efficient, rapid process in the absence of external photo initiators. Polymerization proceeds both in the presence and absence of oxygen. Films produced by the photopolymerization of maleimide/vinyl ether systems exhibit little absorbance at wavelengths greater than 300 nm. The thermal stability of these films are also excellent. 

The various morphological variants available in bead form can be repHcated in thin films ( 2 cmx8cmx50-100 pm) produced simply by photo-initiated free radical polymerization of comonomer mixtures introduced by capillary action into an appropriate mold formed with microscope sHdes [48]. With appropriate choice of comonomers, and porogen in the case of macroporous films, reasonably mechanically robust self-supporting films can be removed from the mold for further exploitation (Fig. 1.9). 

Polymerization of styrene and p-isopropylstyrene could be photo-initiated with radiation from the ruby laser in the absence of photosensitizers and oxygen Since ordinarily no unsensitized photoinitiation of styrene is detected for wavelengths longer than 4000 A, the results of this experiments must be due to two-photon processes. 

The term photosensitizer was originally used to refer to the second pathway, especially when it involved energy transfer, but that distinction has become blurred. The mechanism for photoinitiation in a reaction system is not always clear-cut and may involve both pathways. Photosensitizer is now used to refer to any substance that either increases the rate of photo-initiated polymerization or shifts the wavelength at which polymerization occurs. 

Lee and coworkers report on a photo-initiated polymerization for the preparation of poly(butyl methylacrylate-co-ethylene dimethylacrylate) within fused silica capillaries. UV light-initiated polymerization is well suited to monolith formation in restricted spaces since polymer forms only in those areas that are exposed to irradiation. This leads to far greater control over the length and size of the monolithic column formed. The resulting columns are robnst since their separation ability does not deteriorate with time or number of injections and their reprodncibility is excellent. ... 

Common thermosets are cured by a free radical addition mechanism. These types of composites are cured by heat initiators, such as peroxides, or by photo initiators, such as a-diketones. A characteristic of cured acrylates is large shrinkage in the course of polymerization, which is undesirable for many uses. Another undesirable characteristic of acrylates is the formation of an oxygen-inhibited layer on the surface upon curing. 

Another type of thermoset polymers is based on epoxy monomers. These thermosets are cured by use of a two-component system or by photo initiators. Disadvantages of epoxies are high water uptake in service and polymerization shrinkage (22). 

The rotating-sector method was applied to determine the individual rate constants of chain propagation and chain termination of the radical polymerization of ethylene [23,24]. The photo-initiator was diphenyldisulfide. First, the overall rate of polymerization was measured under steady illumination at pressures of 50 - 175 MPa and 132 - 199°C (Fig. 3.3-9). It increases first steeply and then less steeply with increasing pressure. At 175 MPa the rate of polymerization is ten times higher than at the low pressure of 50 MPa. 

The chemistry involved in both systems involves the photo-initiated, free-radical polymerization of polyfunctional ethylenically unsaturated oligomers and monomers. 

Of these, the most commonly used in letterpress printing is photo-initiated free-radical polymerization of acrylates. 

Photo-initiators for cationic polymerization may be classified in four groups the diazonium salts, the diaryliodonium salts, the triarylsulphonium salts, and the mixed ligand arene cyclopentadienyl Fe11 salts. 

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

Photo-initiated cationic polymerization combines two distinct chemical reactions. In the first, the cation for the initiator of polymerization is formed in the second, the cation initiates polymerization of the cationic monomer. The first reaction is subject to the laws of photophysics and photochemistry, whilst the second is governed by the laws of thermochemistry. 

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