Photoinitiators-difunctional

Difunctional vinvl ether/difunctional N-maleimide. Up until this point, our results have centered on the reactivity of monofunctional maleimide divinyl ether mixtures. From Kloosterboer s26 work for acrylate polymerization, it is known that the rate of polymerization of a free-radical process is increased dramatically as the functionality of the acrylate is increased. In order to enhance the polymerization rates of maleimide divinyl ether systems, it was decided to synthesize difimctional maleimides for copolymerization with difunctional vinyl ethers. The results in Table V indicate that the photoinitiated TTDBM [bismaleimide made from maleic anhydride and 4,7,10-... 

Real-Time FTIR. For our IR studies, we utilized a stoichiometrically equivalent amount of a trifunctional thiol, trimethylolpropane tris(2-mercaptoacetate), with a difunctional allyl, trimethylolpropane diallyl ether. The thiols were protected from oxidative polymerization by the addition of hydroquinone. The monomers and hydroquinone were purchased from Aldrich Chemicals and were used as received. This formulation was mixed for five minutes and then a commercial photoinitiator, Esacure TZT (Sartomer Inc.), which contained a blend of methyl benzophenones, was added at a level of 1.0% by weight of monomers to the formulation. Stirring was maintained for a further five minutes following the addition of the photoinitiator. The final formulation contained 2.0% by weight of hydroquinone. The samples were prepared prior to each experiment in order to ensure reproducibility of sample history. 

The photoinitiated addition of a multifunctional thiol to a difunctional monomer or oligomer gives a crosslinked polymer. The most widely used multifunctional thiol is pentaerythritol tetramercaptopropionate. Ene-monomers can be styrene, acrylates, vinyl ethers, allylic oligomers, etc. For example ... 

Quantum yields are found to follow the order BPX > BMX > difunctional poly(MMA)>monofunctional poly(MMA). Moreover, the photoinitiation efficiency increases with increasing the molecular weight of the functional poly(MMA). 

As reported in a recent review [88], difunctional photoinitiators containing two cleavable moieties exhibit a practical efficiency sometimes better than that of the mono derivatives. In the case of the compound shown in (10.40) (where X = CPU or O), the photochemical reactivity is almost not affected [89]. A better light absorption is noted this is due to a large change of the molecular orbitals. 

FIGURE 10.6 Calculated main n, ji, ji molecular orbitals of two difunctional photoinitiators - dfll (left) and dfI2 (right) - at a B3LYP/6-31G level. See text. 

These salts are demonstrated to have dual function that can initiate polymerization through either cationic and radicallic pathways. Their photoinitiation activity has been demonstrated by curing of a difunctional epoxy monomer. The following types of pathways were proposed for the initiation (Scheme 11.17). 

S. Minegishi, et al., Novel photocurable monomers the synthesis of difunctional vinyl ethers with a phosphonate group and difunctional 1-propenyl ethers with a phosphonate group and their photoinitiated cationic polymerization. J. Polym. Sci. A Polym. Chem. 2005, 43(14), 3105-3115. 

The photoinitiated cationic polymerization of liquid epoxidized polyisoprene is an efficient method to generate rapidly crosslinked elastomers. In the presence of a triarylsulfonium salt, the reaction develops readily upon UV exposure, with formation of both inter and intramolecular ether linkages. The formulation reactivity can be substantially enhanced by the addition of a difunctional vinyl ether or acrylate monomer, which acts as a reactive diluent and leads to the formation of an... 

Figure 1. Differential scanning photocalorimeter UV cure response for various difunctional epoxy-silicone monomers using 0.5 mole % (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate as photoinitiator.

The compositions of materials photocrosslinkable by cationic mechanism consist of mixtures of various vinyl ethers, or epoxides, or both. Difunctional cycloaliphatic epoxides have been used extensively in some UV curable systems, often as diluents for the various epoxy resins described in Chapter 6. Use of various divinyl ethers is also extensive. Because some cationic photoinitiators also generate free radicals, some compositions may contain mixtures of both types of materials, those that cure by cationic and those that cure by free-radical mechanisms. 

Oliva et al. [23] describe the EPR characterization of radicals trapped during the photoinitiated polymerization of trimethacrylate monomer (TMA), which leads to a highly cross-linked polymer structure also, vinylmethacrylate monomer (VMA), a difunctional monomer carrying different reactive groups, has been studied. The EPR spectra obtained at room temperature from photoinitiated samples of both trimethacrylate monomer TMA and vinylmethacrylate monomer VMA consist of a nine-line EPR pattern, typical of methacrylate propagating radicals (Structure I). 

Figure 4.2. Termination mechanisms in the photoinitiated polym zation of difunctional methacrylic monomers in an SBS matrix as a function of monomer concentration and double-bond conversion [2].

Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

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