Free radicals, photopolymerization

Photocrosslinking. The second class of photopolymer chemistry that is used in some commercial products is based on the reaction of unsaturated moieties attached to an organic polymer. These photopolymer materials include the [2+2] cycloaddition of the ethylenic groups in poly(vinyl cinnamate) polymers and in the newer styryl pyridinium (10) and thiazolium (77) derivatives of poly(vinyl alcohol). The main advantage of this chemistry is that, unlike free-radical photopolymerization, they are insensitive to the presence of oxygen. This photopolymer mechanism is principally used in applications employing a washout development process (e.g. resists). 

Kinetics of Photoinitiated Reactions 4.3.1 Kinetics of Free Radical Photopolymerization... 

The heteroaromatic thioles, in particular 2-mercapto-6-nitrobenzothiazole, were studied in regard to their abilities to function as coinitiators in free-radical photopolymerizations induced by camphorquinone and isopropylthioxanthone [598],... 

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]. 

In this chapter, we will focus on photosensitive systems that are used in free radical photopolymerization reactions. We will give the most exhaustive presentation of the commercially used or potentially interesting systems developed on a laboratory scale together with the characteristics of their excited-state properties. We will also show how modem time resolved laser spectroscopy techniques and quantum mechanical calculations allow to probe the photophysical/photochemical properties as well as the chemical reactivity of a given photoinitiating system. 

Hybrid Sol-Gel Photopolymerization Photosensitive organic-inorganic sol-gel glasses are interesting alternative materials that combine the properties of glasses and polymers [316]. A typical hybrid sol-gel material consists (10.88) in an acrylated silane derivative (which in a first step, leads to a glass-like material via hydrolysis and condensation) and a photoinitiator that allows the free radical photopolymerization of the acrylate. The requirements for the choice of the photoinitiator in such a matrix are crucial solubility, thermal stability, PH compatibility, polarity, and so on. 

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]. 

To understand this new class of materials requires an interdisciplinary approach free-radical photopolymerization chemistry, low-molar-mass liquid crystals physics, materials science of thermosets and display technology. This short review will touch on aspects of understanding of the morphology of these polymer networks formed in liquid crystal media. 

Free-radical photopolymerizations (see Chap. 10) of multifunctional acrylic monomers result in cross-linked polymeric networks. The kinetic picture of such polymerizations varies from ordinary linear polymerization because the diffusion of free radicals and functional groups becomes severely restricted. This causes growing polymer chains to rapidly cyclize and cross-link into clusters (microgels). The clusters become linked up into networks. Many free radicals become trapped, but terminations take place by combinations and by chain transferring. The cumulative chain length in such polymerizations can be calculated from the following equation [125] ... 

ESOYO was then modified by reactions with AA or acryloyl chloride and the acrylated derivatives characterized by spectroscopic techniques before being submitted to free radical photopolymerization. The NMR spectrum of the acrylated ESOYO (AESOYO) is shown in Fig. 3.4. Table 3.6 summarizes the peak assignments and their... 

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... 

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... 

M6th)acrylatG Systems. Acrylate and methacrlate monomers are by far most widely used in free-radical photopolymerization processes. The generalized structure of these monomers and their corresponding polymer is shown in Figure 4. 

Other Monomer Systems. The above sections are by no means an exhaustive list of monomers that are used in free radical photopolymerizations. Diallyldiglycolcarbonate (18) has been used for many years in optical components such as lenses (51). Acrylamide (19) is used in stereolithography and to prepare holographic materials (52-54). A(-vinylpyrrolidinone (20) is copolymerized with acrylates and methacrylates for cosmetic and biomedical applications (55). Norbornene (21) is copolymerized with thiols for optical fiber coatings (56). Liquid crystal polymers (22) based on acrylates and thiol-enes are being developed to produce mirror coatings, polarizing films, and liqiud crystal displays (57,58). 

Complex Photopolymerization Systems. Kinetic modeling of free-radical photopolymerizations becomes more complicated as comonomers are added to the reaction system and as different polymerization methods are used to tailor the pol5uner properties. Although free-radical reaction mechanisms still hold true, rates of propagation and termination must be reconsidered to account for variables such as differences in double bond reactivities, reaction diffusion, and chain transfer. 

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