Benzoin-containing polymers

Similarly, side chain benzoin-containing polymers were also used as promoters to yield graft copolymers [76]. 

Cationic Systems Photoinduced cationic polymerization can be initiated directly or indirectly upon exposure to U V light in the presence of onium or pyridinium salts with suitable nucleophilic counterions [33], These initiating systems have been employed in synthesizing block and graft copolymers [34], For example, side-chain benzoin-containing polymers were used as promoters to yield poly(benzoin acrylate-g-cyclohexene oxide) copolymers [35] by using iV-ethoxy-2-methyl-pyridinium... 

Polymers containing benzoin terminal groups can act as photochemical macroinitiators and are effective in photogenerating polymeric electron donor radicals. The initiation of polymerization by means of azo-benzoin initiators yields polymers with one or two benzoin end-groups according to the termination mode of the particular monomer involved [72-74], The general synthetic procedure is depicted below as illustrated for the case styrene polymerization (Scheme 18). 

An inverted sequence of the same procedure has also been used [139] to prepare the same three-block copolymers. Indeed, thermal polymerization of MMA by ABME gives rise to a polymer mainly containing only one benzoin methyl ether moiety per macromolecule, since growing MMA radicals terminate mostly by disproportionation. Thus, terminally photoactive poly(MMA) is used to obtain the photoinitiated block copolymerization of styrene. In this case, a 90% yield of block copolymers is obtained, appreciably higher than in the preceding method, fully consistent with the usual assumption that the termination in styrene polymers occurs by combination. In fact, coupling of the growing styryl radicals with the less reactive poly(MMA)-bound methoxy benzyl radicals also contributes to the formation of block copolymers. 

The Ink formulations usually contain an unsaturated monomer or prepolymer, a radical precursor and/ or an actinic sensitizer, a polymer vehicle base, and pigmentation. Recent formulations Include polyunsaturated polyenes, and polyacrylates with thiol or benzoin radical sources.. The acid or hydroxyl content of the resin base is usually varied to adjust the water or oleophillic wetting action of the composition. 

The photosensitive layers of early plates were composed of acrylate/methacrylate and acrylated cellulose acetate mixtures. Other printing plates contained polyamides or nylon derivatives as binders. Generally, printing plates contain a mixture of reactive monomers and multifunctional oligomers (pre-polymers), polymeric binders, and photoinitiators with exceptional cure depth. The original photoinitiators were benzoin derivatives. Later, anthraquinone and other systems were used. 

Polymers containing terminal photoactive benzoin groups have been synthesized using azo-benzoin initiators [210,211]. The thermal treatment of these initiators in the presence of styrene leads to benzoin groups at both ends of the polystyrene chain, as polystyryl radicals tend to terminate via recombination. Upon irradiation of the styrene-based photoinitiators, benzoyl and alkoxy-benzyl radicals are produced, both capable of initiating polymerizations to give mixtures of homopolymers and block copolymers [212]. 

Macroinitiators containing groups of benzoin type in the side chain were synthesized by several authors [13,218-233]. Some examples are summarized in Table 15. Notably, a spacer between the polymer backbone and the benzoin moiety enhances the activity of the initiator in vinyl polymerization considerably [36]. This phenomenon is attributed to an easier accessibility of reactive benzoin sites. 

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