Photoinitiated systems, crosslinking

Commercially important photoinitiators for crosslinking by cationic polymerization are onium salts, particularly the thermally and hydrolytically stable diaryliodonium and triarylsulfonium salts,and mixed ligand arene cyclopentadienyl metal salts with complex metal halide anions. Direct and sensitized photolysis of these systems have been reviewed " and are discussed in detail in Volume 4, Chapter 20. 

The crosslinking of such types of silicones can be described by means of a polymerization reaction. The reaction rate (rP) of this process is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, the photoinitiator and also of the oxygen content of the system. A typical reaction rate/time profile is shown in the Fig. 1. 

In this work, the kinetics of these reactions are closely examined by monitoring photopolymerizations initiated by a two-component system consisting of a conventional photoinitiator, such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and TED. By examining the polymerization kinetics in detail, further understanding of the complex initiation and termination reactions can be achieved. The monomers discussed in this manuscript are 2-hydroxyethyl methacrylate (HEMA), which forms a linear polymer upon polymerization, and diethylene glycol dimethacrylate (DEGDMA), which forms a crosslinked network upon polymerization. 

Epoxy resins and other polymers (tetrahydrofuran, vinyl ethers, styrene, etc.) can be cured when exposed to an acid or cation intermediate species. The photoactive catalyst system commonly used to cure epoxy resins and multifunctional vinyl ether materials is composed of salts of aryldiazonium, triarylsulfonium, and diaryliodonium. These systems are commonly employed in coatings and adhesives for electronic products. The acid initiator generated from the photoinitiator continues to be active even after uv curing, and so conversion of reactants and crosslinking continue even in the absence of uv light. This phenomenon is typically referred to as dark cure. 

Formulations of epoxy coatings can be varied over a wide range by combining different epoxy resins and cationic photoinitiators discussed in this paper. The reactivity of a formulation depends upon the structure of the epoxies and the photoinitiators which can be further influenced by additives such as fillers, pigm its, colorants, stabilizers etc. Complete crosslinking of epoxide/initiator systems can be achieved by an irradiation and a heat treatment step, both of a very short duration... 

Moreover, the polymerization of MMA, photoinitiated by poly(AVBP-co-St), proceeds at a lower rate than that promoted by the low-molecular-weight analogue AIBP and is enhanced in solvents where the macromolecules are more expanded, thus reducing free radical recombinations [60]. The presence of cage recombinations in this type of systems is also substantiated by the occurrence of a very efficient crosslinking when films of poly(VBP-ca-DMAS) are subjected to UV irradiation [54]. 

Ester derivatives of benzoin are known to display usually quite a low photoinitiation activity in the polymerization of vinyl monomers [85,104]. By contrast, ben2 in alkyl ethers are claimed to generate [17], by a photofragmentation mechanism, benzoyl and a-alkoxy benzyl radicals resulting in a much more active polymerization and crosslinking initiating species (Scheme 25). Thus, polymeric systems having the above moieties in the side chains have been prepared and their photoreactivity studied in more detail [105-107],... 

Another interesting procedure for obtaining a two-layer system involves [97] the formation of a base layer of the polymeric photoinitiator, through a spin coated film onto different substrates, followed by their immersion, under UV irradiation, into multifunctional acrylic formulations dissolved in solvents unable to remove the base layer from the substrate. By this method, a top layer of crosslinked acrylic resin is obtained, having strong adhesion to the substrate. The replacement of the polymeric photoinitiator by a low-molecular-weight analogue... 

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

---