Hendrik J. Hageman

Use of acylphosphine oxides and acylphosphonates as photoinitiators

Abstract Results are presented which show that acylphosphine oxides are more efficient photoinitiators than acylphosphonates for the curing of acrylates and unsaturated polyesters, and that their efficiency is further increased by adding amines. The performance of these initiators was compared with two commercial photoinitiators 2,2-dimethoxy-2-phenylacetophenone and benzoin methylether. From these studies it was concluded that the acylphosphonates cannot be considered as useful photoinitiators for the production of surface coatings. The value of laser nephelometry as a method for screening photoinitiators is underscored. Some of the limitations of this method and that of bulk polymerization are assessed.

Photoinitiators and photoinitiation Part 11. The photodecomposition of some O-acyl 2-oximinoketones

The photodecomposition of several O-acyl 2-oximinoketones was studied using electron spin resonance and proton nuclear magnetic resonance chemically induced dynamic nuclear polarization techniques. Primary NO bond cleavage into α-ketoiminyl and acyloxy radicals from the triplet excited state was observed in all cases. α-Ketoiminyl radicals observable at T < −70°C underwent rapid fragmentation into a benzoyl radical and a nitrile at temperatures exceeding −60°C. Chemical evidence was provided by radical trapping using a thiol and a stable nitroxyl respectively. These experiments also showed that, for most O-acyl 2-oximinoketones 1, [E] ⇌ [Z] photisomerization was the major process, whereas for O-acyl 2-oximinoketones 2 photodecomposition was almost exclusive.

A study of the photodecomposition products of an acylphosphine oxide and 2,2-dimethoxy-2-phenylacetophenone

Abstract The photodecomposition of 2,4,6-trimethylbenzoyldiphenylphosphine oxide and 2,2-dimethoxy-2-phenylacetophenone was investigated under normal ultra-violet curing conditions. The rate of formation and the identity of the many photodecomposition products produced by each photoinitiator were similar, whether amines (triethylamine or N-methyldiethanolamine) were present or not. Experimental evidence indicates that the role of amines is predominantly one of oxygen scavenger. It was also established that the shelf-life (stability) of the photoinitiator 2,4,6-trimethylbenzoyldiphenylphosphine oxide is short when present in an epoxydiacrylate N- methyldiethanolamine ultra-violet curable formulation. The decomposition products identified indicated that this is due to the occurrence of transesterification and hydrolysis reactions.

The application of some polymeric type-I photoinitiators based on α-hydroxymethylbenzoin and α-hydroxymethylbenzoin methyl ether

Abstract Some polymeric photoinitiators based on α-hydroxymethylbenzoin and its methyl ether are used to initiate the polymerization of methyl methacrylate in solution and the crosslinking of an epoxyacrylate formulation in thin films. In general the polymeric photoinitiators perform better than corresponding low molecular weight model compounds. Moreover, it is found that polymeric photoinitiators based on α-hydroxymethylbenzoin methyl ether perform better than those based on α-hydroxymethylbenzoin. Explanations for these observations are advanced.

Photoinitiators and photoinitiation - Part 15. The photodecomposition of some [Z] O-sulphonyl 2-oximinoketones and some [Z] O-sulphamoyl 2-oximinoketones

Abstract The photodecomposition of some [Z]- O -sulphonyl 2-oximinoketones is studied by ESR spectroscopy. At low temperatures ( T O -sulphonyl (and O -sulphamoyl) fragments are split off as sulphonyloxy (and sulphamoyloxy) radicals in much the same way as acyloxy radicals in the case of O -acyl 2-oximinoketones studied before. At room temperature in the presence of a spin-trap, the trapped benzoyl radical is observed, which must have been formed by fragmentation of the α-ketoiminyl radical. The title compounds are shown to photoinitiate the polymerization of acrylates and to photoinduce the acid-catalyzed crosslinking of a melamine resin.

Acylphosphine oxides as photoinitiators for a TiO2 pigmented acrylic resin

Abstract A comparison was made between the photoinitiating efficiencies of 2-chlorothioxanthone and the acylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, for the polymerization of a pigmented (TiO2), epoxydiacrylate resin in thin films containing an amine co-initiator. The photoinitiators appeared to initiate cure at similar speeds. Considering all the data, the use of acylphosphine oxides offers little advantage over more conventional photoinitiators for pigmented systems.

Photoinitiators and photoinitiation 10. The photodecomposition of some carboxylic esters of α-hydroxymethylbenzoin

Abstract The photodecomposition of some carboxylic esters of α-hydroxymethylbenzoin was studied using electron spin resonance (ESR) and proton nuclear magnetic resonance-chemically induced dynamic nuclear polarization ( 1 H NMR-CIDNP) techniques. Exclusive α-cleavage into benzoyl and α-hydroxy-α-acyloxymethylbenzyl radicals through the triplet excited state was observed. The α-hydroxy-α-acyloxymethylbenzyl radicals mainly undergo disproportionation with the other radicals present to give benzoylmethyl carboxylates. Elimination of carboxylic acids from the α,α-disubstituted benzyl radicals occurs to some extent, but at a much lower rate than the elimination of sulphonic acids from the corresponding α-hydroxy-α-sulphonyloxymethylbenzyl radicals generated from the sulphonic esters of α-hydroxymethylbenzoin.

The photodecomposition of α-nitratomethylbenzoin as studied by ESR

Irradiation of α-nitratomethylbenzoin 1 in aromatic solvents gives rise to one observable radical identified as benzoyl benzoylmethyl nitroxide 5. This nitroxide must have been formed through trapping of both a benzoyl and a benzoylmethyl radical by nitric oxide.

The photoinduced α-cleavage of α-bromomethylbenzoin: the generation and behaviour of β-bromoketyl radicals

Abstract The photodecomposition of α-bromomethylbenzoin was studied by electron spin resonance and by radical trapping using I-dodecanethiol and a stable nitroxyl (TMPO) respectively. Photoinduced α-cleavage into benzoyl and α-hydroxy-α-bromomethylbenzyl (β-bromoketyl) radicals is observed. The β-bromoketyl radicals are found to undergo an extremely fast elimination of hydrogen bromide to give benzoylmethyl radicals. These β-bromoketyl radicals are also generated by hydrogen-abstraction from 2-bromo-1-phenyl-ethanol using triplet excited benzophenone. The only products observed are ‘unchanged’ benzophenone, acetophenone and hydrogen bromide. The same result is obtained on irradiation of 2-bromoacetophenone in the presence of benzhydrol. Possible mechanisms and implications are discussed.

Photoinitiators and photoinitiation. Part 13. The photodecomposition of sulphonic esters of α-hydroxymethylbenzoin ethers

Abstract The photodecomposition of some sulphonic esters of α -hydroxymethylbenzoin ethers is examined using ESR and radical trappingtechniques. Photoinduced α -cleavage into benzoyl and α -alkoxy- α -sulphonyloxymethylbenzyl radicals from a short-lived triplet excited state is observed. The α , α -disubstituted benzyl radicals undergo subsequent fragmentation, generating benzoylmethylradicals and sulphonic acids at a rate k el 7 s −1 . The fate of the alkyl ether part of the α,α-disubstituted benzyl radicals after elimination of sulphonic acid is unknown at present.