Photopolymers suffer from their polymer architecture which is typically formed by a radical chain growth reaction. The high number of crosslinks along the polymer backbone (branches every second carbon atom) leads to brittle materials with low impact strength. Furthermore, during photocuring, the gel point is reached already after ~ 20% conversion which leads to high shrinkage stress of the material. Although thiol-ene polymerization has demonstrated in the last decade that it is able to circumvent most of these drawbacks, new problems arise such as bad odor or poor storage stability.

With the use of Addition Fragmentation Chain Transfer Reagents one is able to delay the gel point to higher conversions and therefore reduce the shrinkage process significantly. Furthermore the toughness of the material can be increased by a factor of up to ten due to the more homogeneous network architecture.

C. Gorsche, M. Griesser, G. Gescheidt, N. Moszner, and R. Liska; "β‑Allyl Sulfones as Addition−Fragmentation Chain Transfer Reagents: A Tool for Adjusting Thermal and Mechanical Properties of Dimethacrylate Networks" Macromolecules 2014, 47 (21), pp 7327–7336

C. Gorsche, T. Koch, N. Moszner, R. Liska: "Exploring the benefits of β-allyl sulfones for more homogeneous dimethacrylate photopolymer networks"; Polymer Chemistry, 6 (2015), S. 2038 - 2047.

UV curing of photopolymerizable formulations has been used for more than a half century for protective and decorative coatings of paper, wood, metals or plastics. Advantages can for sure been found in the high curing speed that allows the conversion of typically (meth)acrylate-based monomers within the fraction of a second. Furthermore, a large variety of monomers is commercially available so that the mechanical properties and other polymer characteristics can be easily tuned.    

Thiol-Ene polymerization is also known since the 50’s of the last century and has gained tremendously increasing interest during the last decade, thanks to the rediscovery by Hoyle [1] and recent efforts by Bowman [2]. Advantages such as low oxygen inhibition and shrinkage, uniform networks with significantly improved mechanical properties are accompanied by up to now unsolved disadvantages such as unpleasant odour and poor storage stability.

Very recently significant progress in that field has been achieved. On the one hand we were able to introduce a new concept in thiol-ene stabilization which gives excellent storage stability with nearly no increase in viscosity with several months up to one year [3] By combining classical radical stabilizers with phosphonic acid derivates we were able to inhibit all mechanisms of thermal gelation.

We have also demonstrated that vinylesters and vinylcarbonates [4] are a promising class of new, low toxic monomers not only for biomedical applications [5] but also for classical coatings. The only limitation of moderate reactivity between those of methacrylates and acrylates has been circumvented by thiol-ene polymerization [6].  Degradation can be easily tuned giving non-toxic low molecular polyvinyl alcohol as degradation product and various non-toxic alcohols such as glycerol or polyethylene glycol. In vivo experiments demonstrated excellent biocompatibility.

This concept has also been transfered to the efficient curing of vinylester derivatives of hyaluronic acid and gelatin in the field of hydrogels for tissue engineering.

[1] Hoyle, C. E.; Hensel, R. D.; Grubb, M. B. J Polym Sci, Polym Chem (1984), 22(8), 1865-73

[2] Cramer, Neil B.; Bowman, Christopher N. J Polym Sci, Polym Chem (2001), 39(19), 3311-3319.

[3] (a) Cherkaoui Z.; Esfandiari P.; Frantz R.; Lagref J.-J.; Liska R. WO2012126695  (A1);  (b) Esfandiari P.; Ligon S. C.; Cherkaoui Z.; Frantz R.; Lagref J.-J.; Liska R. "Efficient stabilization of thiol‐ene formulations in radical photopolymerization" Macromol. Rapid Comm. 2013, 51, (20) 4261-4266.

[4] B. Husar, R. Liska; "Vinyl carbonates, vinyl carbamates, and related monomers: synthesis, polymerization, and application" Chem Soc Rev 2012;41(6):2395-2405

[5] C. Heller, M. Schwentenwein, G. Russmüller, T. Koch, D. Moser, C. Schopper, F. Varga, J. Stampfl, R. Liska: "Vinylcarbonates and vinylcarbamates: Biocompatible monomers for radical photopolymerization" J Polym Sci, Polym Chem 49 (2011), 3; 650 - 661.

[6] A. Mautner, X. Qin, B. Kapeller, G. Russmueller, T. Koch, J. Stampfl, R. Liska, "Efficient Curing of Vinyl Carbonates by Thiol-Ene Polymerization" Macromol. Rapid Comm., 2012, 33, 23, 2046 - 2052.

Polysiloxanes have found widespread application due to their unique properties, like good thermal stability and high hydrophobicity. Silicone release coatings are one of the numerous applications. High curing speed (up to 500 m/min) of this ultra-thin coatings is one of the major issues. Silicone acrylates are state of the art monomers but suffer from sufficient long term stability. Silicone acrylamides, especially with ß-hydroxy groups, show improved reactivity and better long term stability [1]

 [1] R. Liska, S. Kopeinig, S. Knaus, H. Gruber, E. Hummer-Koppendorfer: "Methacrylamide derivatives  of  Polysiloxanes“. EP 1956038 B1

Acrylates and methacrylates are current state-of-the-art monomers for UV curable formulations. A large variety of mono-, di-, and multifunctional (meth)acrylates of low and high molecular weight is present on the market today. They are generally used as protective and decorative coatings and have found applications as paints, surface coatings, printing inks, resists, etc. Some selected methacrylates are also used as biocompatible materials, such as bone cements or dental filling materials. These monomers possess many attractive properties, such as good storage stability, fast curing rates, tunable mechanical properties, and allow solvent free processing. However, higher price of these monomers in comparison with other common monomers (e.g. styrene) has to be accepted due to their preparation from (meth)acrylic acid. Furthermore, some methacrylates and especially acrylates and acrylamides exhibit some specific unattractive properties regarding their toxicology profile. Skin irritancy or toxicity of some monomers will be a serious reason to limit their use due to health and environmental legislation. These drawbacks can be mainly addressed to the reactivity of the acrylate double bond towards Michael Addition reactions with amino- or thiol- groups of proteins.

There are not many alternatives if one wants to sustain the excellent performance profile of (meth)acrylates and the polymers thereof along with lower irritancy or cytotoxicity of the monomers. Vinyl esters might be a good substitute. However, currently there are only few monofunctional, one difunctional, and no multifunctional vinyl esters commercially available. It has been shown recently that this class of monomers has a surprisingly high photoreactivity and low cytotoxicity and thus could be a suitable alternative to (meth)acrylates [1]. However, hydrolytic sensitivity of the less sterically demanding ester group is a serious disadvantage. Alternative monomers are based on Vinylcarbonates [2]. Current synthetic routes to vinyl carbonates are expensive and only suitable for laboratory scale preparation. Therefore these monomers have not been studied in detail by the scientific community and have hardly been used in industrial applications. Nevertheless, a recent patent from BASF describes a high-yield synthesis from cheap reagents applicable on industrial scale which may afford even cheaper monomers than (meth)acrylates [3]. By using a thiol-ene polymerization even higher reactivity than acrylates has been demonstrated recently.

[1] C. Heller, M. Schwentenwein, G. Russmüller, F. Varga, J. Stampfl, R. Liska: "Vinyl Esters: Low Cytotoxicity Monomers for the Fabrication of Biocompatible 3D Scaffolds by Lithography Based Additive Manufacturing"; Journal of Polymer Science Part A: Polymer Chemistry, 47 (2009), 6941 - 6954.

[2] C. Heller, M. Schwentenwein, G. Russmüller, T. Koch, D. Moser, C. Schopper, F. Varga, J. Stampfl, R. Liska, "Vinylcarbonates and vinylcarbamates: Biocompatible monomers for radical photopolymerization", J. Polym. Sci., Part A: Polym. Chem., 2010, 49, 650-661.

[3] W. Staffel, R. Kessinger, J. Henkelmann, "Method for the production of O-vinylcarbamates and vinylcarbonates" WO2008/084086 (2008).

[4] A. Mautner, X. Qin, H. Wutzel, S. C. Ligon,B. Kapeller, D. Moser, G. Russmueller, J. Stampfl, R. Liska „Thiol-Ene Photopolymerization for Efficient Curing of Vinyl Esters“ J Polym Sci, A, 2013, 51, 203-212.

Photopolymerizable coatings, suffer from a major disadvantage: the migration of unconsumed photoinitiator and photoproducts thereof. In the last 10 years, especially in the field of food packaging, serious concerns were addressed as isopropylthioxanthone was found in baby food and also 4-methyl benzophenone has been detected in significant amount. Therefore, in industry polymeric initiators are currently a hot topic, but diffusion limitation reduces activity significantly.

Our aim is to provide photoinitiating monomers that are able to start the radical polymerization and have polymerizable groups in their chromophore. Similar to maleimides investigated in detail by Hoyle and Jönsson, diacrylimides are able to absorb light and efficiently initiate radical polymerization similar to benzophenone/amine systems, although their absorption behaviour is significantly worse [1,2]

[1] F. Karasu, C. Dworak, S. Kopeinig, E. Hummer, N. Arsu, R. Liska: "Photoinitiating Monomers Based on Diacrylamides"; Macromolecules, 41 (2008), 21; S. 7953 - 7958.

[2] C. Dworak, S. Kopeinig, H. Hoffmann, R. Liska: "Photoinitiating monomers based on di- and triacryloylated hydroxylamine derivatives"; Journal of Polymer Science Part A: Polymer Chemistry, 47 (2009), 2; S. 392 - 403.