Free radical photopolymerization initiation

The one- and two-photon excited fluorescence property and crystal structure of a substituted stilbene-type compound frans-4-diethylamino-4 -bromostilbene (DEARS) has been reported [17]. Results indicate that this compound has a strong two-photon-exdted blue fluorescence at 440 nm when the 700 nm laser is used as the pump source. The one- and two-photon absorption and fluorescence properties of a free radical photopolymerization initiator, ( , )-4- 2-[p -(]yf,N-di-w-butylamino)stil-ben-p-yl]vinyl pyridine (Figure 3.8), in various solvents have been investigated [18]. The dye has a moderate two-photon absorption cross section of = 0.91 x 10 cm s/photon at 532 nm. This compound showed a strong two-photon-induced blue fluorescence of432 nm when pumped with 800 nm laser irradiation. Quantum chemical calculation indicated that the new initiator possesses a large delocalized... 

DEB 99] De Boer B., Simon H.K., Werts M.P.L. et al., Living Free Radical Photopolymerization Initiated from Surface-Grafted Iniferter Monolayers , Macromolecules, vol. 33, pp. 349-356, 1999. 

A detailed study of mechanisms both of photodecomposition of triarylsul-fonium salts to yield Bronsted acids and of catalysis of cationic polymerization of representative monomers—styrene oxide, cyclohexene oxide, tetrahydrofuran (THF), and 2-chloroethyl vinyl ether—was reported in 1979 by Crivello and Lam [14]. Crivello [15] and Green et al. [16] provided further reviews shortly thereafter. The mechanisms of photodecomposition of a variety of initiators for free radical photopolymerization, including onium salts, were compared by Vesley [17] in 1986. A review, similar in scope, but providing more mechanistic detail was also published in 1986 by Timpe [10a]. An updated coverage of aspects of this chemistry has been provided by the same author in his review of photoinduced electron transfer polymerization [10b]. 

There is a large group of metal-based compounds capable of initiating the free radical photopolymerization of unsaturated compounds (see Table 10.4) [23, 24]. 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

The vast majority of photopolymerizations used in industry are free-radical polymerizations, which have been studied extensively (for reviews, see references 1-5 as well as Photopolymerization, Free Radical (qv)). By far the most widely used classes of monomers for UV-initiated free-radical photopolymerizations are multiftinctional acrylates and methacrylates. Several investigations have demonstrated that free-radical polymerizations of these monomers exhibit imusual kinetic behavior, including immediate onset of autoacceleration, the formation of heterogeneous polymers (2,6-11), and the attainment of a maximum conversion... 

In order to produce free radicals that initiate polymerization, photoinitiators absorb light of a certain frequency. Upon absorption, the photoinitiator molecule is promoted from the ground electronic state to either a singlet or triplet excited electronic state. This excited molecule then undergoes either cleavage or reaction with another molecule to produce initiating free radicals. Numerous photoinitiators have been developed to meet the needs of a variety of photopolymerization systems, as described in a number of recent papers and reviews (3-6,8-12). 

If the photoinitiator is bleached, then a front with a constant velocity can be aeated. Terrones and Pearlstein considered a model of free-radical photopolymerization with a photobleaching initiator. They derived an analytical expression for the front speed of the localized traveling wave... 

Ionic Polymerization Ionic chain polymerizations can also be initiated with the aid of Vis/UV light although, as in the case of free-radical photopolymerization, the light serves only as an initiating tool. Most studies on ionic photopolymerization have focused on cationic polymerization, which proceeds rapidly and is not inhibited by oxygen [18,21-24]. Moreover, cationic photopolymerization is apt to polymerize monomers such as vinyl ethers, oxiranes (epoxides), and other heterocyclic compounds that do not polymerize via a free-radical mechanism (Table 3.9). 

Vlayl fluoride undergoes free-radical polymerization. The first polymerization iavolved heating a saturated solutioa of VF ia tolueae at 67°C uader 600 MPa (87,000 psi) for 16 h (24). A wide variety of ioitiators and polymerization conditions have been explored (25—27). Examples of bulk (28,29) and solution (25,28,30,31) polymerizations exist however, aqueous suspension or emulsion methods are generally preferred (26,32—40). VF volatiflty dictates that moderately high pressures be used. Photopolymerizations, usually incorporating free-radical initiators, are also known (26,28,29,35). 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Polymers in Schemes 12 and 13 were the first examples of the preparation of pyridinium and iminopyridinium ylide polymers. One of the more recent contributions of Kondo and his colleagues [16] deals with the sensitization effect of l-ethoxycarbonyliminopyridinium ylide (IPYY) (Scheme 14) on the photopolymerization of vinyl monomers. Only acrylic monomers such as MMA and methyl acrylate (MA) were photoinitiated by IPYY, while vinylacetate (VA), acrylonitrile (AN), and styrene were unaffected by the initiator used. A free radical mechanism was confirmed by a kinetic study. The complex of IPYY and MMA was defined as an exciplex that served as a precursor of the initiating radical. This ylide is unique in being stabilized by the participation of a... 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

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... 

Finally, we should indicate that we have not ruled out the possibility that there is a contribution to initiating photopolymerization in maleimide/vinyl ether systems from an exciplex type complex between an excited state maleimide and ground state vinyl ether. A biradical formed from such a complex might initiate free radical polymerization in lieu of cyclization to form a 2 + 2 adduct. However, we note that at present we have no evidence for such a reactive exciplex. 

General Electric Co.) were added to the acrylic and epoxy monomers, respectively, to initiate photopolymerization. The acrylic monomers that polymerized by a free-radical mechanism were irradiated under nitrogen. The output of the lamp measured at the substrate was about 4 mW/cm2 at 260 nm and about 11 mW/cm2 at 310 nm. 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

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