Phenacyl onium salt photoinitiators: synthesis, photolysis, and applications

Abstract

Onium salts, namely sulfonium, phosphonium, ammonium, and pyridinium salts containing phenacyl group are photoinitiators appropriate for the polymerization of monomers such as oxiranes and vinyl ethers, which are not polymerizable by a free-radical mechanism. The initiation is accomplished by direct or indirect (sensitized) photolysis of the salts. Depending on the type of the salt, the direct photoinitiation of cationic polymerization involves reversible or irreversible processes. The photolysis of phenacylsulfonium compounds proceeds by a reversible process, while the other types undergo irreversible photolysis leading to complete fragmentation of the photoinitiator. An additionally useful tool, namely photosensitized generation of initiating species enlarges the versatility of these salts as photoinitiators. Photoinitiated free-radical and zwitterionic polymerizations by using phenacyl-type salts are also addressed. Keto-enol tautomerization of phenacyl pyridinium salts is discussed. Moreover, an interesting application concerning in situ synthesis of clay-poly(methyl methacrylate) nanocomposites with the aid of the phenacyl anilinium salt-based photopolymerization technique is noted.

Diphenylphenacyl sulfonium salt as dual photoinitiator for free radical and cationic polymerizations

“…However, solubility, compatibility, migration, and cost make bi‐component systems less attractive for industrial applications. Phenacyl salts are a class of onium salts that can overcome this problem to some extent by incorporating a chromophore group in the structure and slightly shifting the absorption to longer wavelengths . They are easily synthesized by the reaction of phenacyl halides and the corresponding heteroatom nucleophiles and subsequent ion exchange with the sodium, potassium, or silver salt of a non‐nucleophilic counter anion .…”

Synthesis of photoactive single‐chain folded polymeric nanoparticles via combination of radical polymerization techniques and Menschutkin click chemistry

“…Photodecomposition of polymeric nanoparticle and photoinitiation of released benzophenone derivative are identical to decomposition mechanism of phenacyl‐based onium salts . Synthesized product exhibits polymeric nano‐structure and it releases Michler's ketone which is able to initiate the polymerization upon UV light.…”

Recent advances on carbazole-based photoinitiators of polymerization

“…Besides, by quaternization of the amine of carbazole, photoinitiators for FRP can also be obtained as exemplified with Cbz_41 (See Scheme 11) [168]. Chemical structure of Cbz_41 is directly inspired from the structures of phenacyl onium salts that are highly photosensitive compounds [169]. Notably, phenacyl onium salts are well-known to initiate a homolytic and/or a heterolytic bond cleavage upon irradiation so that radicals and cations can be both generated upon photolysis [170][171][172][173][174][175][176][177][178].…”