Photoinitiators efficiency

Local excitation was also studied for primary and secondary amines under irradiation at 313 nm. The results are summarized in Table 11. In order to estimate the photoinitiating efficiency of the amines, the measurement was performed at a chosen constant absorbance (0.40) of the reaction mixture. The rates of polymerization were found to be in the following order ... 

The photoinitiating efficiency is pH dependent, a feature which has been capitalized upon with the tethered thioxanthone-amine co-initiator system 635, which at pH < 6 fails to generate free radicals as a result of protonation of the tertiary amino group, but at higher pH regains its normal initiator activity (Scheme 244) <1998PSA2563, B-2002MI1>. 

The efficiencies of benzoin derivatives in promoting the light induced hardening of styrene-containing unsaturated polyester resins have been found to vary considerably (7), benzoin alkyl ethers and a-alkylated benzoins being more reactive than benzoin itself. However, as photoinitiators for the polymerization of bulk methyl methacrylate, benzoin and benzoin methyl ether exhibit comparable efficiencies (20), and this is also indicated for polymerisation of methyl acrylate in tert-butanol (21). The relative photoinitiating efficiencies of benzoin derivatives may be influenced by several factors, some of which will depend on the nature of the monomer system and the environment in which the polymerizations are carried out. 

Heine (7) has given an account of studies trf the photoinitiating efficiencies of a series of -ketosulphides and those p-substituted bmzo-phenone sulphides which exhibit phenylogous /S-cleavage ... 

Thus the relative photoinitiating efficiencies of benzophenone derivatives may be influenced by whether or not their photoexcited states are quenched by monomer, and the fraction of in-cage combination of the radicals formed by hydrogen abstraction, as wdl as the stents to which the respective semi-pinacol radicals terminate polymer chains. At elevated tmiperatures, differences in the initiating abilities of the semi-pinacol radicals must also be considered. 

Such a sequence allows one to conclude that the insertion of a flexible spacer between the tertiary amine function and the main chain causes a sharp increase of photoinitiation efficiency. In addition, the relative distance of tertiary amine and benzophenone groups from the polymer chain appears to be quite an important structural requirement. Indeed, the above findings suggest that, provided the spacer is flexible, an improvement of activity is observed when both amine and ketone moieties are located at similar distances from the backbone. 

Incorporation of 4,4 -dihydroxybenzil into the backbone of a polyurethane produces a significant improvement of photoinitiation activity, whereas the insertion of benzil-4,4 -diglycidylether into a polymer chain fails to affect the photoinitiation efficiency to any notable degree. The photoactivity enhancement afforded in... 

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). 

The cure rates of cationic formulations based on epoxy resins are usually much slower than the acrylates. Studies of photoinitiation efficiency have shown that polymerization rates may be enhanced by the appropriate choice of sensitizers (3,21,24). ... 

Valderas et al., also studied the photopolymerization of methyl methacrylate initiated by 2-chlorothioxanthone in the presence of various amines of different structures. Here too, the photoinitiation efficiency of these systems was found to be highly dependent on the structure of the amine. The polymerization rate increases with the amine concentration and reaches a constant value at an amine concentration range of 10—30 mm. At these amine concentrations, aliphatic hydroxyaUcyl amines are more efficient photoinitiators than the corresponding trialkyl-substituted compounds. Dimethylanilines with electron acceptor substituents in the 4-position give higher polymerization rates than electron donor substituted anilines. Their data also show that the singlet and triplet excited states of thioxanthones are efficiently deactivated by the amines. Rate constants correlate well with the oxidation potentials of the amines. The effects of the chemical structure of the amine on the polymerization rates of 2-... 

Bradley and Davidson pointed out that for any particular aromatic ketone, the efficiency of photoinitiation is determined by the structure of the ketone and by the molecular geometry of the a-aminoalkyl radical produced. On the other hand, Paczkowski et al, suggested that the initial electron-transfer reaction from aromatic amines to the excited state of xanthene dyes, used in their study, is responsible for the variation of the photoinitiation efficiency. 

There are also several reports in the literature on use of poly(arylsilanes) as initiators in photopolymerization of acrylic monomers. The photoinitiating efficiency of silane polymers with thioxanthone side groups was found to depend not upon exiplex formation with tertiary amines but rather... 

Here, la is the amoimt of photons absorbed by the photoinitiator molecules expressed in mol photon/L s or Einstein/L s f is the photoinitiator efficiency calculated by determining the moles of propagating chains (R—M ) started per mole of free-radical active centers (R ) generated and (p is the photoinitiator quantum yield which relates the moles of active center pairs (R ) produced per moles photons absorbed ( is defined such that it is <1). The integer 2 is included in the expression when both radical species initiate poljunerization. If only one of the radical species is active, it is omitted. 

Garton A, Carlsson DJ, Wiles DM. Polypropylene oxidation the apparent rate constant for per-oxy radical termination and the photoinitiation efficiency. Macromolecules 1979 12 1071-3. 

Photoinitiator efficiency depends upon several factors ... 

The overall photoinitiation efficiency is expressed by the quantum yield of photoinitiation (p) according to... 

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