Water-soluble photoinitiators

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

Iwai and coworkers [56] have introduced a novel type of multicomponent photoinitiating system for water-soluble monomer (acrylamide, acrylic acid, acrylonitrile, etc). 

Recent concerns over the residual concentration of free thioxanthone photoinitiators and amine coinitiators in coatings and inks on food packaging have resulted in the evolution of macroinitiators <2002PLM4591, 2003JPH(159)103> or polymeric photoinitiators <1996MI379> wherein the thioxanthone unit and, in some instances, the amine coinitiator are covalently bound to a copolymer which is formulated with the ink or coating and then photocured in the normal way. Several copolymers, for example, 636, based upon the polymerization of acrylate esters derived from 4-hydroxythioxanthones with either acrylamide or 2-acryloxyethyltrimethylammonium iodide co-monomer have been explored as water-soluble photoinitiators <2005JPH(169)95>. 

On the basis of the PET of dye systems, water-soluble photoinitiators can be created, for which many application fields exist. 

Polymerization of a CD Complexed Photoinitiator and a Water Soluble Monomer 190... 

For evaluating the influence of CD on the photoinitiated polymerization process, the polymerization of a water soluble monomer, AMsopropylacrylamide (NIPAAM), using complex of Me-P-CD and 2-hydroxy-2-methyl-1-phenylpropan-l-one (13) as... 

Water-Soluble Photoinitiators Due to their polar structure, hydrophilic or water-soluble photoinitiators derived from basic structures (hydroxyl alkyl ketones [92] such as the one in (10.41), phosphine oxides [93,94], etc.) or modified compounds (e.g., by carbohydrates [95]) are usable in water-based matrices (water-soluble monomers and aqueous dispersions). Their reactivity has been studied [96,97]. 

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymerisation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. 

Photopolymerisation. The kinetics of photoinitiated polymerisation of the monoacrylate resin were monitored using a differential scanning photocalorimeter developed at the CSIC on the basis of a modified Perkin-Elmer DSC-4 calorimeter ( H). The monoacrylate resin was mixed with 20% v/w of water followed by the addition of 10% v/w of N,N-dimethylaniline. The latter amine was chosen for study here owing to practical limitations with this technique for the resin used. The water-soluble benzophenones were dissolved in the resin to give an absorbance of 0.3 at 365 nm using a cell path length of 1.5 mm. 

The results clearly show that the photoinitiating activity of the water soluble benzophenone in the presence of an amine is mainly associated with the ability of the lowest excited triplet - state to abstract an electron from the amine co-synergist via an intermediate exciplex shown in Scheme I. The radical anion will then induce hydrogen atom abstraction to give a ketyl radical and an alkylamino radical. The latter is mainly responsible for inducing polymerisation of the acrylic monomer and supports earlier work on the benzophenone-triethylamine tetramine-induced photopolymerisation of methylmethacrylate during which terminal amine groups were detected (2 ) ... 

Figure 1 Schematic representation of the synthesis of defined SPB In the first step, a PS latex is generated, which is then covered by a thin layer of the photoinitiator 2-[p-(2-hydroxy-2-methylpropiophenone)]-ethylene glycol methacrylate (HMEM) in the second step. Photoinitiation in the presence of water-soluble monomers, such as acrylic acid, leads to polyelectrolyte chains grafted onto the surface of the core particles in the third step. Taken with permission from Ballauff, M. Macromol Chem. Phys. 2003, 204, 220. ...

Still another group of water soluble initiators were prepared by Wang and coworkers, who converted the primary hydroxy group in a commercial photoinitiator, Diarocur 2959 (2- hydroxy-l-[4-(2-hydroxyeflioxy)phenyl]-2-methylpropan-l-one) to various amine and sugar moieties ... 

Liska reported preparation of water-soluble photoinitiators that contain carbohydrate residues as well as copolymerizable derivatives of the carbohydrate residues. These materials consist of alkylphenones, benzophenones and thioxanthones. To these compounds were attached carbohydrates like glucose, cellobiose, and 1-amino-1-deoxy-D-glucitol. In addition selected initiators were reacted with methacryloyl chloride to form polymerizable photoinitiators. The glucose modified photoinitiators were claimed to yield the best results with respect to compatibility with resins, reactivity and gel content. ... 

Some additional water soluble photoinitiators that were reported in the literature are shown in Table 2.8. 

Table 2.8. Some Examples of Water Soluble Photoinitiators...

In addition there is a report of a water soluble polymeric (copolymer) photoinitiators. These initiators were synthesized by copolymerizing 2-acryloxy and 2-acrylamido anthraquinone monomers with three water-soluble comonomers acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and 2-acryloxyethyl trimethylammonium iodide. The polymerization activity correlates well with their measured photo reduction quantum yields in water... 

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