POLYMERIC PHOTOINITIATOR

However, MM A polymerization photoinitiated by Re2 (CO)io (A = 365 nm) shows a long-lived aftereffect, persisting for several hours at 25°C after irradiation has... 

Klos et at.2is6 described a range of polymerizable benzoin derivatives as photoinitiators (e.g. 83, 84). These and other polymeric photoinitiators have advantages as initiators over low molecular weight analogs in circumstances where migratory stability is a problem.287"2 ... 

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>. 

Kinetics and Mechanism of Methyl Methacrylate Polymerization Photoinitiated by Benzophenones in Tetrahydrofuran... 

Some of the most important benefits connected with the macromolecular architecture of polymeric photoinitiators can be summarized as follows. 

The present overview is divided in three main sections where the polymeric photoinitiators are presented according to their mechanism of action, i.e. pho-toinduced hydrogen abstraction, electron transfer and cleav e reactions. In each section, particular attention is devoted to the synthesis and relationship between molecular structure and photoinitiation properties of the polymeric systems. A further section deals with some special applications having particular interest from the point of view of fundamental research and technological development. 

On the basis of the above described results, tailor-made benzophenone-containing polymeric photoinitiators have been studied in order to establish the effect of the relative distance from the polymer chain of the side-chain benzophenone groups. Thus, the homopolymer of Uvecryl P36 [poly(UP36)] and its copolymers with MtA [poly(UP36-co-MtA)s], bearing the benzophenone moiety at quite a large distance from the backbone, have been checked as photoinitiators in the polymerization of the HDDA/BA equimolar mixture against poly(ABP), poly(VBP) and poly(VBP-co-MtA)s [20-22]. 

More detailed information on the activation pathway, which distinguishes the polymeric photoinitiators from the corresponding low-molecular-weight structural models, has been obtained by photophysical measurements [22,27,28] in terms of quantum yield and average lifetime of the triplet excited state of benzophenone moieties in the above systems. However, in order to get a better comprehension of this point, it is necessary to introduce some basic concepts about the kinetic treatment of a photoinitiated chain polymerization. 

In conclusion, on the basis of the collected results, polymeric photoinitiators bearing side-chain benzophenone moieties show, at least under nitrogen atmosphere, a large improvement of activity as comjMred with low-molecular-weight structural models. Pol)rmeric systems having even higher activity can easily be prepared by introducing in the macromolecules, via a copolymerization route. 

Thioxanthone compounds, having an intense absorption band around 380 nm, have been industrially developed as initiators of polymerization in UV curing of heavily pigmented (TiOa) resins [3]. However, conventional thioxanthone photoinitiators show problems associated with their poor solubility producing migration which may result in loss of adhesion and, possibly, gloss. To overcome these problems, polymeric photoinitiators with pendant thioxanthone moieties have been... 

Table 12. UV curing in film matrix of HDDA/BA equimolar mixture, under nitrogen, by polymeric photoinitiators bearing both benzophenone and tertiary amine moieties in the side chain [18,22, 53]...

The kinetic data (Table 12), determined in the presence of the above polymeric photoinitiators, clearly indicate the following order of activity poly (UP36-co-DAPA) = poly(ABP-co-DEEA)>poly(ABP-co-DAPA)>poly(UP36-c i-DEEA) > poly( VBP-co-DEEA) poly( ABP-co-DMAS) > poly(VBP-co-DMAS). 

These data suggested to the authors that the excited states of the polymeric photoinitiators are more difficult to be quenched by the above monomers than in the low-molecular-weight model, due to the steric hindrance of the polymer chain. However, this explanation does not fully justify the higher polymerization rate displayed by MBK with respect to the polymeric photoinitiators in the polymerization of MMA and VAc (Table 13). 

PCMS) 1 8 mol/mol mixture has also been checked (Table 16). Indeed, the above mixture is found to show a slightly lower activity with respect to PABOK, thus suggesting that polymeric photoinitiators based on photosensitive moieties undergoing homolytic fragmentation may also display an activity synergism, provided that different photoreactive groups are anchored to the same macromolecular chain. 

A novel class of low-odouring and non-yellowing polymeric photoinitiators, based on side-chain hydroxyalkylphenone moieties, is reported [89-91] to be very active for UV curable clear acrylic coatings. Due to low volatility and affinity for the organic phase, these photoinitiators are also claimed to be very effective in the curing of water-based acrylic emulsions in which water must be evaporated before the exposure to UV light. These commercial polymeric systems (KIP) are prepared by functionalization of a-methylstyrene prepolymers. 

The reaction of glycidyl acrylate with a-(2-carboxyethyl)benzoin methyl ether has allowed one to obtain [101] the corresponding acrylic monomer which, upon copolymerization with different amounts of MMA, butyl methacrylate and 2-(V, V-dimethylamino)ethyl methacrylate, gives rise to polymeric photoinitiators, containing side-chain benzoin methylether moieties, for photocurable coatings ... 

Three kinds of reactive BAE derivatives such a-(2-carboxyethyl) benzoin alkyl ethers (BAE-CA), a-methylol benzoin alkyl ethers (BAE-OH) and newly synthesized a-(2-cyanato ethyl)benzoin alkyl ethers (BAE-NCO) are employed for preparing the polymeric photoinitiators. 

The improved activity of the above polymeric photoinitiators has been explained [108] assuming that macroradicals generated by photofragmentation... 

Indeed, photophysical studies by laser flash photolysis combined with the determination of MMA polymerization rate (Rp) in toluene solution, have allowed evaluation of the triplet state lifetime of the above systems, as well as their relative quantum yields of initiation ( j) and a-cleavage ( ). As reported in Table 22, 3, values for poly(MBA) and poly(MBVE) are appreciably lower than those for MBI and MBEE, respectively. On this basis, the polymeric photoinitiators would be expected to display lower activity than the models in the polymerization of acrylic monomers. On the contrary, poly(MBA) and poly(MBVE), together with the related copolymers, show higher values of Rp and hence i, in the UV initiated polymerization of MMA in toluene solution. These findings, therefore, confirm the previously obtained results in film matrix, where a HDDA/BA equimolar mixture was used as curing formulation (Table 21). 

As with the previous reported conclusion concerning the polymeric photoinitiators based on hydroxyalkyl acetophenone moiety (KIP), in this case rm (Eq. 4) also is much higher in the polymeric systems than in the models (Table 22), indicating that the B radicals anchored to the polymer backbone are less prone to give radical-radical combination, thus favouring flieir reaction with the acrylic monomers. 

Further investigations on polymeric systems containing side-chain benzoin methyl ether and benzoin moieties confirm the enhanced reactivity of the polymeric photoinitiators [111,112]. Indeed, copolymers of MBA with methyl acrylate and methyl methacrylate [poly(MBA-co-MA) and poly(MBA-co-MMA), respectively] and the corresponding copolymers based on a-methylolbenzoin acry-... 

Table 23. Photoinitiated polymerization of methyl methacrylate (MMA) in benzene solution, using polymeric photoinitiators based on benzoin methyl ether and benzoin moieties (BE) as well as their low-molecular-weight analogues [111]...

In this context, polymeric photoinitiators bearing side-chain benzoin methyl ether and tertiary amine moieties in the same macromolecule, have been prepared and the effect of amine on the photoinitiation activity investigated [116-118]. 

All the MBA/A(,iV-dialkylamino acrylate copolymers behave similarly to poly(MBA-co-MtA)s (Tables 21 and 25), thus suggesting that the replacement of MtA by A,A(-dialkylamino acrylate co-units does not markedly affect the photoinitiation activity of the system. Accordingly, BMI/A,A(-dialkylamino isobutyrates mixtures exhibit substantially the same activity as MBI alone [118]. Similar results have previously been obtained for 2,2-dimethoxy-2-phenyl acetophenone (DMPA), when additioned with diethylmethylamine, in the UV initiated polymerization of -butyl methacrylate [113]. However, a remarkable shortening of the induction period (to) of UV curing is observed for all the polymeric photoinitiators in the presence of tertiary amines as compared with the low-molecular-weight MBl/A,A(-diatkylamino isobutyrates systems, the maximum effect resulting in the case of MBA/A(,iV-dialkylamino acrylate copolymers (Table 25). 

It is noteworthy that the final thickness of the coating is not much affected by the depth of the coating resin layer, but depends essentially on the thickness of the polymeric photoinitiator layer, which in turn is affected by the concentration of the solution of the photoreactive polymer used for the spin crating process (Table 30). Interdiffiision between the solid photoinitiator layer and the liquid resin on the top nnay play a crucial role besides the photochemical and radical processes. A further advantage of the two-layer system process is the need for a much smaller amormt of the polymeric photoinitiator with respect to the conventional UV curing, where a low-molecular-weight system in employed [96]. 

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... 

Another special application of polymeric photoinitiators is represented by the synthesis of block copolymers which can be realized by several routes with different efficiency. 

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