Radical photopolymerization

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

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). 

Reaction Behavior and Kinetic Modeling Studies of Living Radical Photopolymerizations... 

Once the reaction of DTC radicals with monomer is neglected, propagation of radicals follows the mechanism seen in other radical photopolymerizations ... 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

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. 

Photoinitiators as Iniferters The availability of iniferters that behave as an initiator, a transfer agent and a terminator is rather limited [98,99]. A renewal of interest is noted for the development of efficient compounds usable in controlled radical photopolymerization reactions [100-104]. Examples of cleavable photoiniferters are shown in (10.42) the particular tetrazole derivative shown here is noticeably attractive due the generation of a tetrazoyl radical that presents a low selectivity and a high efficiency for the addition to acrylate double bonds [105],... 

Few things are known in the control of photopolymerization reactions. In a light-induced reaction, a photoiniferter can be used. Both the initiation and the reversible termination are photoinduced. The mechanism of a classical living radical photopolymerization process is recalled in e21. 

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. 

Several papers have reported on the charge-transfer cationic and radical photopolymerization of N-vinylcarbazole (VCZ).33 In the study by Asai et al., the initiator was a sodium chloroaurate solution. In the work by Tada et al. and Crellin and Ledwith, formation of the photodimer (4) was shown to be in... 

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]. 

Luo N, Hutchinson JB, Anseth KS et al (2002) Integrated surface modification of frilly polymeric microfluidic devices using living radical photopolymerization chemistry. J Polym Sci A Polym Chem 40 1885-1891... 

Simms HM, Brotherton CM, Good BT et al (2005) In situ fabrication of macroporous polymer networks within microfluidic devices by living radical photopolymerization and leaching. Lab Chip 5 151-157... 

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. 

Gratings could not be formed with MM-TMS. Dramatic enhancing in the diffraction efficiency to about 86% (induction period of 144 s) was observed in the case of MM-TMOS, even with only 10 wt% TMPTA. Figure 4.13 shows the realtime diffraction efficiency of holographic gratings formed with various < >-metha-cryloxyalkyltrialkoxysilanes capable of radical photopolymerization and hydrolysis condensation. 

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