Recent Developments in Radical Photoinitiator Chemistry

Radiation curing is an established technology used in many industrial manufacturing processes. New applications and technical specifications stimulate the continuous development of tailor-made photoinitiators which can efficiently meet specific requirements. A new class of radical photoinitiators, bisacylphosphine oxides (BAPO), give four initiating radicals per photoinitiator molecule and undergo photo-bleaching of the low-energy absorption band. These features make the compounds highly efficient for radiation curing of highly opaque white pigmented systems, thick coatings, or fiber-reinforced formulations

Light-based 3D printing of functional polydimethylsiloxane- based microfluidic chips

The objective of the work is to fabricate and functionalize 3D printed PDMS-based microfluidic chips through digital light processing DLP-3D printing

Photochemisch induzierte Reaktionen von 3-Amino-2H-azirinen

Photochemically Induced Reactions of 3-Amino-2H-azirines. Irradiation of 3-(N-methylanilino)-2H-azirines with a mercury low pressure lamp induces the cleavage of the C(2),C (3)-ring bond thus affording nitrilio- methanide dipols, substituted by an amino group at C(1). Depending on the substitution pattern at C(3), these intermediates can be trapped by dipolarophiles to yield five-membered heterocycles with high regioselectivity, or they undergo a 1,4-H-shift forming 2-azabutadiene derivatives. Further, the dipol is protonated at C (1) even by weak CH-acids

Photochemically Induced 1,3-Dipolar Cycloadditions of 3-Amino-2H-azirines

Irradiation of 3-(N-methylanilino)-2H-azirines in dimethoxyethane solution in the presence of dipolarophiles leads to five-membered heterocycles in 40-60% yield. A reaction mechanism via splitting of the C(2),C(3)-bond and formation of a nitrilio methanide which then undergoes a thermal 1,3-dipolar cycloaddition is reasonable

Photolatent Tertiary Amines – A New Technology Platform for Radiation Curing

The lack of efficient photolatent amine catalysts has so far prevented the use of base-catalyzed formulations in radiation curing. The development of two new photocatalysts which release 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or tertiary amines upon irradiation opens new opportunities for this technology. Photolatent DBN is used for crosslinking processes where strong bases are required as catalyst, such as coatings crosslinked via the Michael addition reaction, while a tertiary amine efficiently catalyzes reactions involving more acidic compounds. A UV-A curable car refinish clear lacquer has been developed as a first example of a commercial formulation using a photolatent amine as catalyst

Photochemie von in 4-Stellung substituierten 5-Methyl-3-phenyl-isoxazole

4-Trideuterioacetyl-5-methyl-3-phenyl-isoxazole ([CD3CO]-27), upon irradiation with 254 nm light, was converted into a 1:1 mixture of oxazoles [CD3CO]-35 and [CD3]-35 (Scheme 13). This isomerization is accompagnied by a slower transformation of [CD3CO]-27 into [CD3]-27. Irradiation of the isoxazole derivatives 28, 29, 30 and (E)-31 yielded only oxazoles 36, 37, 38 and (E),(Z)-39; no 4-acetyl-5-alkoxy-2-phenyl-oxazole, 2-acetyl-3-methyl-5-phenyl-pyrrole or 2-acetyl-4-methoxycarbonyl-3-methyl-5-phenyl-pyrrole, respectively, were formed (Scheme 9 and 10). Similarly (E)-32gave a mixture of (E),(Z)-40 only (Scheme17). Upon shorter irradiation, the intermediate 2H-azirines (E),(Z)-41 could be isolated (Scheme 11). Photochemical (E)/(Z)-isomerization of the 2-(trifluoro-ethoxycarbonyl)-1-methyl-vinyl side chain in all the compounds 32, 40 and 41 is fast. At 230° the isoxazoles (E)- and (Z)-32 are converted into oxazoles (E)(Z)-40. The same compounds are also obtained by thermal isomerization of the 2H-azirines (E),(Z)-41. The most probable mechanism for the photochemical transformations of the isoxazoles, as exemplified in the case of the isoxazole 27, is shown in Scheme 13. A benzonitrile-methylide intermediate is postulated for the photochemical conversion of the 2H-azirines into oxazoles. 2H-Azirines are also intermediates in the thermal isoxazole-oxazole rearrangement. It is however not yet clear, if the thermal 2H-azirine-oxazole transformation involves the same transient species as the photochemical reaction. A mechanism for the photochemical isomerization of the 2H-azirine 11 to the oxazole 15 is proposed (Scheme 3

Streamlined concept towards spatially resolved photoactivation of dynamic transesterification in vitrimeric polymers by applying thermally stable photolatent bases

Through a well-targeted design, vitrimers are able to reorganise their three-dimensional covalently crosslinked network structure by associative exchange reactions when the so-called topology freezing transition temperature (T-v) is exceeded. Although in the past decade a vast number of vitrimers have been developed, there is only a very limited number of elaborate strategies for enabling a controlled and spatially resolved onset of the exchange reactions above T-v. Herein, we describe a convenient approach for a locally controllable photoactivation of vitrimeric properties in a covalently crosslinked thiol-epoxy network by the UV-mediated release of a strong amidine base acting as an efficient transesterification catalyst. In contrast to previous work, the applied photolatent catalysts benefit from superior solubility properties, adequate curing characteristics in the non-activated state (the addition of a supplementary catalyst is not required) and an excellent thermal stability. The spatially controlled activation of vitrimeric properties in terms of dynamic transesterification, macroscopically visible as a viscoelastic flow, is demonstrated by stress relaxation studies and a reshaping experiment. Moreover, we confirm that an undesired purely thermally induced catalyst release can be entirely excluded

Stabile Zink-Komplexe von 3-Amino-2H-azirinen

3-Dimethylamino-2,2-dimethyl-2H-azirine (4a), which is known to react easily with Brönsted acids and electrophiles, forms a stable complex 5a with ZnBr2. In contrast to all other reactions of 4a, the three-membered ring in this complex is preserved. The spectral data are discussed in comparison with the recently reported Pd(II)-complexes of 2H-azirines

Delayed Thiol-Epoxy Photopolymerization: A General and Effective Strategy to Prepare Thick Composites

Photoinduced thiol-epoxy click polymerization possesses both the characteristics and advantages of photopolymerization and click reactions. However, the photopolymerization of pigmented or highly filled thiol-epoxy thick composites still remains a great challenge due to the light screening effect derived from the competitive absorption, reflection, and scattering of the pigments or functional fillers. In this article, we present a simple and versatile strategy to prepare thick composites via delayed thiol-epoxy photopolymerization. The irradiation of a small area with a light-emitting diode (LED) point light source at room temperature leads to the decomposition of a photobase generator and the released active basic species can uniformly disperse throughout the whole system, including unirradiated areas, via mechanical stirring. No polymerization was observed at room temperature and therefore the liquid formulations can be further processed with molds of arbitrary size and desired shapes. It is only by increasing the temperature that base-catalyzed thiol-epoxy polymerization occurs and controllable preparation of thick thiol-epoxy materials can be achieved. The formed networks display excellent uniformity in different radii and depths with comparable functionality conversions, similar Tg values, and thermal decomposition temperatures. The presented strategy can be applied to prepare thick composites with glass fibers possessing improved mechanical properties and dark composites containing 2 wt % carbon nanotubes