New photoacid generator for cationic polymerization
Cationic photopolymerization offers not only wide range of polymerizeable monomers (e.g. epoxides, oxetanes, vinyl ether) but also a clear advantage compared to the classical free radical photopolymerization, which is the independence of the process from oxygen.1
The state of the art cationic photoinitiators (so called photo acids) are usually iodonium or sulfonium salts of fluorometallate anions (e.g. BF4-; PF6-; AsF6-; SbF6-) or [BAr4F]--anions. These anions suffer from different drawbacks like toxicity, hydrolysis or tough synthesis.
Very recently we have developed a new class of photoinitiators (sulphonium as well as iodonium salt)2 based on a fluorinated alkoxyaluminate,3 that
• is expected to have a low toxicity,
• has a high stability against hydrolysis,
• is easy to synthesize and
• shows a high reactivity.
Especially the high reactivity towards the usually rather inactive glycidylethers (e.g. bisphenol A diglycidyl ether BADGE) is remarkable.
Photo-DSC Plot of bisphenol-A-diglycidyl ether with different iodonium-based initiators
1. Dietliker, K.; Crivello, J. V. In Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, Bradley, G., ed., SITA Technology: London, 1991, Vol. 3: Photoinitiators for Free Radical and Cationic Polymerisation
2. Liska, R., Knaack, P. Bomze, D. Krossing, I. „Cationic Photoinitiator” AT Patent Application 2015
3. Krossing, I. (2013). 1.23 - Weakly Coordinating Anions: Fluorinated Alkoxyaluminates. Comprehensive Inorganic Chemistry II (Second Edition). J. R. Poeppelmeier. Amsterdam, Elsevier: 681-705.
Photoinitiators for Two Photon Polymerization
Generally, in Two Photon induced Photopolymerization (TPIP) a resin, that contains mainly a multifunctional monomer (typically acrylate-based) and a photoinitiator, is cured inside the focal point of a fs pulsed near infrared laser beam to produce a desired 3D shape with a resolution down to 100 nm.
As usual single photon initiators have only limited two photon absorption properties specialized molecules have to be desinged containing a long planar pi-system and strong electron donor and or acceptor groups are required.
Initiators known from literature have the disadvantages of deactivation of the excited singlet state by fluoreszenz and cis-trans isomerization of the double bonds.
Therefore our group has synthesized the triple bond containing 1,5-bis(4-(dibutylamino)phenyl)penta-1,4-diyn-3-one (B3K). Investigation of the photophysical properties showed high extinction coefficients at the desired wavelength and vanishingly small emission quantum yields. In spite of a modest TPA cross-section of 238 GM in 800 nm, the evaluated initiator showed at least similar activity, and in some cases slightly broader processing windows as well-known highly-active initiators R1 from literature and by far better results than commercially available one photon initiators in terms of two-photon induced microfabrication. As mentioned before, long conjugation bridges and good coplanarity are critical to enhanced TPA. Therefore, based on the molecular structure of B3K containing triple bonds and cross-conjugated D-</span>π<span lang="EN-US">-A-</span>π<span lang="EN-US">-D lead structures, we continue to investigated several aromatic ketone-based (benzophenone, fluorenone and anthraquinone) TPA PIs. All aromatic ketone-based TPA PIs exhibit large two-photon cross sections ranging from 250 GM to 440 GM. The 2, 7-substituted fluorenone-based PI B3FL, with strong TPA and diminishing fluorescence, exhibits broader processing window in TPP structuring tests than that of B3K and R1. While high reactivity could be obtained in TPIP with PI concentrations as low as 0.1% wt, B3FL is surprisingly stable under one photon condition and nearly no photoinitiation activity was found in classical photo DSC experiments.
New Chromophores for photoinitiators:
During the last two decades research in photoinitiators concentrated on new cleavable structures, but only few systems were established on the market [1]. Modification of well known PIs gave improved performance properties in some cases, but the overall efficiency was not drastically changed. This could be explained by the limited reactivity of the formed radical towards polymerizable double bonds. New chromophores should not only increase this reactivity, also light absorption and performance properties should be adjusted.
By investigating pyridin analogous hydroxyalkylphenones, it has been found that the benzoyl moiety is replaceable by suitable chromophores [2].
Recent work focuses on cross-conjugated photoinitiators. Beside high reactivity and low initiator concentrations (0.1 wt%), amine based coinitiators can be avoided [3]. Unusually high activity has been found in Two Photon induced Photopolymerization [4]. The concept of triple bond containing initiators has also been realized in the field of cationic initiators based on sulfonium and iodonium salts [5]
In the field of dental filling materials long wavelength photoinitiators are of interest. As bimolecular systems like camphorquinone suffer from low reactivity especially in aqueous formulations, a hydrophilic bisacylphosphine oxide as been developed [6].
The long wavelength absorption behaviour of a-cleavable initiators was up to recently limited to bisacyl phosphine oxides with ~450 nm. We have now introduced germanium-based compounds that are successfully applied in dental filling materials under the tradename Ivocerin [7].
[1] Robert Liska: 'New chromophores for photoinitiators' (bookchapter) in "Photochemistry and UV Curing", Research Signpost (2006)
Photoinitiators for water based formulations
Due to ecological reasons water based resin formulation were introduced during the last decades and are nowadays state of the art. Nevertheless, several inadequate characteristics of radiation curable formulations have to be considered. Beside insufficient mechanical and chemical characteristics of the cured film, photoinitiators are often poorly soluble or they are evaporated during the pre-drying step. Improved solubility of PIs is often achieved by the introduction of ionic groups, as found in various commercially available products. Application of these initiators can lead to decreased stability of emulsion type resins, as well as poor compatibility of the PI after evaporation of the water. Recent developments are dealing with polyethylene oxide residues. Also renewable resources such as glucose or glucamine are not only environmentally friendly, they also give high reactivity and good compatibility [1].
Especially in the field of acidic dental primer formulations special requirements are given. Bimolecular initiators suffer from low reactivity due to the solvent cage effect, which can be avoided by phenyl glycine based coinitiators that undergo spontaneous decarboxylation. [2] Monomolecular acylphosphine oxides with hydrophilic residues have been introduced recently as highly reactive alternative. [3] Very recently, Li and Na-salts of mono- and bis-acylphosphine oxides have been introduced that show high water-solubility and reactivity in water-based formulations. Low cytotoxicity makes them suitable for biomedical applications [4]
