Photoinitiator chemical structures

A wide variety of photoinitiators have been investigated for polymerization of different monomers, such as acrylates, epoxides, vinyl ethers, and thiol-ene monomers. From this point of view, the D-rc-D or A-rc-A chromophores are favored sensitizers if they are combined with a coinitiator [269, 563], The sensitizer excited by TPA can be either oxidized (route A) or reduced (route B) by the coinitiator, depending on the chemical structure of the coinitiator (Fig. 3.64). 

Fig. 9.12 Chemical structures of photoinitiators used in the CROP of oxetanes.

In this part, we will present an overview of the radical photoinitiating systems mentioned in the literature. They are classified as one-component (or type I system), two-component (type II systems), and multicomponent photoinitiating systems. According to the chemical structures and the composition of the system, they exhibit a photosensitivity under polychromatic UV lights or UV/visible lights or under laser exposure. Typical absorption spectra of relevant compounds are given in Fig. 10.3. 

In this case the surface polymerization an photoinitiator modified particles occured without agglomeration and the polymerization seems to )3e restricted mainly on the particle surface (immobilized polymer) Ixit the formation of a soluble polymer (dissolved in the monomer excess) cannot be excluded. This behaviour depends also chemical structure of the monomer (compare ctgglomeration with HE A) and it is an objective of further work to investigate various monomers. 

For photocured polymer materials, the Persoz hardness was shown to depend mainly on the chemical structure of the prepolymer chain, on the functionality of the reactive diluent, on the photoinitiator system and on the duration of the UV exposure. The hardness of tack-free coatings thus continues to increase with further irradiation, as shown by Figure 6 for polyurethane-acrylates. This slow hardening process is likely to result from some further polymerization of the unreacted acrylic functions since their concentration was found to decrease concomittantly. 

Chart 7.4 Chemical structure of 4 -(methylthio)-2-morpholino-propiophenone used as a photoinitiator in the cross-linking of the copolymer of Chart 7.3. 

Table 10.1 Chemical structures of typical type 1 free radical photoinitiators. ...

Table 10.3 Chemical structures of amines functioning as co-initiators for type II free radical photoinitiators.

Chart 10.2 Chemical structures of photosensitive moieties contained in typical macromolecular photoinitiators. 

W.3 Photoinitiation of ionic polymerizations 289 Table 10.8 Chemical structures of typical cationic photoinitiators [2, 27, 52, 53]. 

Carbonylbis(7-diethylaminocoumarin)(KC) Diphenyliodonium hexafluorophosphate(DPI) Figure 4.12. Chemical structures of photosensitizer (PS) and photoinitiator (PI) for this study. 

Valderas et al., also studied the photopolymerization of methyl methacrylate initiated by 2-chlorothioxanthone in the presence of various amines of different structures. Here too, the photoinitiation efficiency of these systems was found to be highly dependent on the structure of the amine. The polymerization rate increases with the amine concentration and reaches a constant value at an amine concentration range of 10—30 mm. At these amine concentrations, aliphatic hydroxyaUcyl amines are more efficient photoinitiators than the corresponding trialkyl-substituted compounds. Dimethylanilines with electron acceptor substituents in the 4-position give higher polymerization rates than electron donor substituted anilines. Their data also show that the singlet and triplet excited states of thioxanthones are efficiently deactivated by the amines. Rate constants correlate well with the oxidation potentials of the amines. The effects of the chemical structure of the amine on the polymerization rates of 2-... 

The absorption properties of Pis (ground state spectra and molar extinction coefficients, a) play a decisive role in the photoinitiation step as (i) the PI absorption spectrum has to match the emission spectrum of the light source and (ii) the polymerization rate Rp is directly connected with the amount of light absorbed. The development of Pis lies on the design of new chemical structures and the synthesis of new derivatives of... 

Fig. 11.2 Chemical structure of 2-isopropylthioxanthone (ITX) that is used as a photoinitiator in UV-cured printing inks...

LCEs forming nematic, cholesteric, and smectic phases have been synthesized, with various chemical structures comprising of both side-chain and main-chain types. Because of the multitudes of preparation techniques, I will here focus on our synthetic methods [7]. Reactive cyanobiphenyl monoacrylate mesogens were mixed with diacrylate cross-linkers to produce a photoinitiated side-chain acrylate NE (Fig. 10.2). As it will be described later, the LCE fixes the liquid crystal orientation at the time of cross-linking, and it can be said that this orientation... 

Despite the numerous papers devoted to photooxidation of hydrocarbon polymers [21], the initiation step has not been clearly established yet even for polyethylene or polystyrene which were the most studied [22,23]. Difficulties which follow from solution of this problem consist in the necessity of analysis of small amounts of decomposing unstable structures and products which are thereby formed. Moreover, photoinitiation does not include one reaction only but the overall complex of many chemical and physical processes, which importance depends on experimental conditions. 

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