Photonics photoinitiators

K.D. Belfield and KJ. Schafer, Two-photon photoinitiated polymerization, in K.D. Belfield and J. V. Crivello (Eds.), Photoinitiated Polymerization, ACS Symposium Series 847, pp. 464-481, American Chemical Society, 2003. 

K.D. Belfield, et al., Near-IR two-photon photoinitiated polymerization using a fluorone/ amine initiating system. J. Am. Chem. Soc. 2000, 722(6), 1217-1218. 

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. 

If the reactive species in the chemical activation step initiates a radical chain with a chain length CL, then the overall quantum yield based on the ultimate product is X CL, and can be greater than 1. Photons are rather expensive reagents, and are only used when the product is of substantial value or when the overall quantum yield is large. Examples are the use of photoinitiators for the curing of coatings (a radical-polymerization process (Section 7.3.1)), and the transformation of complex molecules as medications. 

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 nonuniformity of polymerization with depth of penetration is less for photohleaching photoinitiators compared to photoinitiators that are regenerated. Photohleaching allows photon penetration deeper into the reaction system. 

While a more detailed review of two-photon 3D microfabrication can be found in another chapter of this issue, this chapter will concentrate on our work on characterization and apphcation of commercial photoinitiators. The development of better 2PA photoinitiators would be expected to facilitate 3D photopolymerization technologies. Efficient 2PA compoimds based on phenylethenyl constructs bearing electron-donating and/or electron-withdrawing moieties have been reported [105-107]. Among these are electron-rich derivatives that have been found to undergo a presumed two-... 

In addition to these examples, an electron-transfer free radical photoinitiator H-Nu 470 (5,7-diiodo-3-butoxy-6-fluorone) has been also successfully used for 3D microfabrication by near-IR two-photon induced polymerization... 

In the UV curing process, photons from the UV source are absorbed by a chromophoric site of a molecule in a single event. The chromophore is a part of the photoinitiator. The light absorption by the photoinitiator requires that an emission light from the light source overlap with an absorption band of the photoinitiator. 

In general, upon exposure to UV radiant energy, a photoinitiator can generate free radicals or ions, as pointed out earlier. These are generated at a rapid rate, and their depth profile corresponds to the inverse photon penetration profile. Similar to electron penetration, the final cure profile often deviates from the initial radical or ion distribution, since they can live much longer than the exposure time. The mechanisms of the processes for the generation of reactive species are discussed in detail in Davidson. ... 

Where [PI] is the concentration of photoinitiator. The quantity 1 is also termed the photon penetration path. 

Laser-initiated Radical Production. Although there are different physical mechanisms involved in laser chemistry, we are concerned here with the photodissociation, i.e., the breaking of molecular bonds directly by UV photons. The laser emission is used to produce electronically excited molecules which split into reactive radicals, with the highest possible quantum yield. Since the substrate usually behaves as a poor photoinitiator, an additional molecule must be introduced in order to enhance the radical production, much in the same way as in conventional photoinitiated reactions. In this work,... 

S.-K. Lee and D. C. Neckers, Two-photon radical-photoinitiator system based on iodinated... 

A special instance of photoinitiation was already di us in Sect. VI-B, within the context of bare cation formation. In those sterns in fact, h -energy photons were used to produce the ejection of an electron from the moiK>mer molecule. In this chapter we will briefly review other photochemical tediniques involving the framation of electronically excited intermediates, which in turn generate suitable species for the initiation of cationic polymerisation. Crivello has recently reviewed this topic and other authors have published more specialised monographs on some specific aspects of cationic photo-initiation. We will therefore reduce our coverage to the basic premises on whidi the various methods are founded and to a few comments on the more recent contributions and medianistic interpretations. 

The effect of pulsed discharge on plasma polymerization may be viewed as the analogue of the rotating sector in photoinitiated free-radical chain growth polymerization. The ratio r of off time I2 to on time li, r = (t2/h), is expected to influence the polymerization rate depending on the relative time scale of I2 to the lifetime of free radicals in free-radical addition polymerization of a monomer. This method was used to estimate the lifetime of free radicals in conventional photon-initiated free radical polymerization. 

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