New polymeric UV photoinitiators, bearing the benzoin methyl ether moiety linked to the side chain through the 4′ position, were synthetized by radical homopolymerization of the corresponding monomers, 4′-methacryloyloxy benzoin methyl ether and 4′-methacryloyloxy α-methyl benzoin methyl ether, prepared in turn after a thorough investigation of synthetic methods. For comparison, low molecular weight structural models of the repeating co-units of the polymers having the pivaloyloxy group linked to the 4′ position of the benzoin methyl ether moiety were also prepared. All polymeric and model compounds were fully characterized and employed in the photoinitiated polymerization and crosslinking of a standard acrylic formulation for clear UV curable coatings. The photoinitiating activity of these systems, measured by microwave dielectrometry, indicates a higher cure fastness for the model compounds with respect to the corresponding polymeric derivatives, and such a behaviour is tentatively interpreted in terms of the fragmentation mechanism of the benzyl methyl ether radicals formed on α-photocleavage of the benzoin moiety.

New polymeric photoinitiators with pendant α-aminoacetophenone moieties, such as the homopolymers of 1-(4-morpholinophenyl)-2-benzyl-2-[N-methyl-N-(3-methacryloyloxypropyl)]aminopropan-1-one and of 1-(4-morpholinophenyl)-2-benzyl-2-[N-methyl-N-(3-methacryloyloxypropyl)]aminobutan-1-one have been prepared and fully characterized. Their photoinitiation activity has been also checked in the ultraviolet cure of a standard acrylic mixture, under irradiation over 380 nm, thus simulating the absorption conditions of a TiO2-pigmented coating formulation. The results have been compared with those found by using the corresponding low-molecular-weight structural models, purposely synthesized. The activity data obtained are discussed and related to the structural requirements of the above systems. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2237–2246, 1997

This overview deals with recent developments on structure-activity relationships in polymeric photoinitiators for UV curable coatings. In particular, polymeric systems bearing side-chain benzoin methyl ether moieties are examined and the effect on the photoinitiation activity discussed in connection with the different anchorage positions of the photosensitive group to the polymer backbone. In addition, novel polymeric photoinitiators based on acyldiphenylphosphinoxide moieties are described and their photoinitiation activity related to the nature of the acyl group in the photoreactive molecule. Finally, polymeric systems having pendant thioxanthone and alpha-morpholino acetophenone moieties in the same macromolecule that are able to behave as photosensitizing and photoinitiating species, respectively, are reported. The much higher photoinitiation activity found for the above copolymers, compared with the mixtures of the corresponding low molecular weight analogs, is interpreted in terms of excitation energy transfer from the side-chain thioxanthone to alpha-morpholino acetophenone moieties, favored try their forced proximity along the polymer chain.

Polymeric photoinitiators, based on side-chain benzoin methyl ether moieties differently anchored to the backbone, have been designed and applied to the UV cure of acrylic formulations for clarifying the role played by benzoyl and alpha-alkoxybenzyl primary radicals in the polymerization process. Kinetic data on the photocuring processes, in the presence of the above polymeric systems, as compared with those obtained by using the corresponding low-molecular-weight structural models, are presented and interpreted in terms of different reactivity of the two types of primary radicals towards initiation and termination steps of the polymerization process.

Laser irradiation at 355 nm of copolymers obtained from 1-[(2-acryloyloxy)ethoxycarbonyl]thioxanthone with 1-[4-(2-acryloyloxyethylthio)phenyl]-2-methyl-2-morpholino-propan-1-one in toluene or 1,6-hexanediol diacrylate/n-butyl acrylate (1:1) solution, clearly indicates that the rate constant of excitation transfer from side-chain thioxanthone to alpha-morpholinoketone moieties is two orders of magnitude higher than that observed for the corresponding low-molecular-weight structural models mixture, such as 1-[2-isobutyroyloxy)ethoxycarbonyl]thioxanthone and 1-[4-(2-isobutyroyloxyethylthio)phenyl]-2-methyl-2-morpholino-propan-1-one. Evaluation of the efficiency of the above photosensitization process confirms that it is much higher in the copolymer systems. Photophysical and photoinitiation polymerization data suggest that the sensitization process mainly occurs through an energy transfer rather than an electron transfer mechanism, favoured by the forced close vicinity of the two photosensitive moieties attached to the same macromolecule.

The synthesis and structural characterization of copolymers of 1-[(2-acryloyloxy) ethoxycarbonyl] thioxanthone (ATX) with 1-[4-(2-acryloyloxyethylthio)phenyl]-2-methyl-2-morpholino-propan-1-one (AMMP), as well as ATX, AMMP and n-butyl acrylate (BA) terpolymers, is reported. These copolymeric systems have been checked in the photoinitiated polymerization in film matrix of the 1,6-hexanediol diacrylate (HDDA)-BA equimolar mixture under UV irradiation over 380 nm, i.e., under conditions simulating a TiO2-pigmented acrylic coating formulation, and compared with the corresponding mixture of low-molecular-weight structural. models 1-[(2-isobutyroyloxy)ethoxycarbonyl]thio (ITX) and 1-[4-(2-isobutyroyloxyethylthio)phenyl]-2-methyl-2-morpholino-propan-1-one (IMMP). The much higher photoinitiation activity shown by the copolymers is discussed in terms of close vicinity of thioxanthone and alpha-morpholinoacetophenone moieties along the backbone, which favors the excitation energy transfer from the former to the latter photosensitive group. (C) 1995 John Wiley & Sons, Inc.

Homopolymers of 4-acryloyloxybenzoin methyl ether (ABME) and 4-acryloyloxy-alpha-methylbenzoin methyl ether (AMBE) [poly(ABME) and poly(AMBE)] as well as copolymers of AMBE with (-)-menthyl acrylate (MtA) and with different N,N-dialkylaminoethyl acrylates have been employed in the photoinitiated polymerization and crosslinking of a standard acrylic formulation for u.v. clear curable coatings. The photoinitiation activity of the above systems, detected by microwave dielectrometry, has been compared with that found for the corresponding low molecular weight structural models. The results clearly indicate that high and low molecular weight photoinitiators promote u.v. curing at a similar rate; however, a significant decrease of the induction period and hence an improved overall activity is found for the polymeric systems. An interpretation of the experimental findings, related to the behaviour of previously reported polymeric photoinitiators based on the benzoin methyl ether moiety, is also reported.

Optically active copolymers of racemic 4-acryloyloxy-alpha-methylbenzoin methyl ether (AMBE) with (-)-menthyl acrylate (MtA) have been radically prepared. The reactivity of the comonomers suggests that the copolymers display a certain tendency to an alternating distribution of the co-units, Optical activity at 589 nm of the copolymers seems to exclude any stereoselectivity and stereoelectivity during the copolymerization process. Chiroptical properties indicate that an induced optical activity on benzoin methyl ether chromophores occurs. Circular dichroism (c.d.) data, connected with n-->pi(*) and pi-->pi(*) electronic transitions of the benzoin methyl ether moiety, suggest that the observed ellipticity is substantially due to isolated AMBE units. C.d. features indicate that the macromolecules, due to the presence of optically active MtA co-units, assume conformations with a prevailing handedness for sections not sufficiently long to provide cooperative interactions between side-chain benzoin methyl ether chromophores disposed along the backbone with a mutual chiral geometry suitable for exciton couplings.