The incorporation of step-growth polymerization in additive manufacturing techniques is a viable strategy for obtaining homogeneous and consequently tough photopolymers. On this front, the thiol-ene reaction has attracted great interest in the last years owing to its high reactivity and efficiency. Thiols, however, suffer from a set of shortcomings ranging from high toxicity, strong odor and poor storage stability.1 A possible way to mitigate these challenges is their replacement with alcohols, as they are free of these drawbacks and even offer a wider range of available monomers.2 As is the case for most Michael addition chemistries, the oxa-ene subtype requires a base catalyst. In literature, most oxa-Michael addition reactions involve a Brønsted base catalysis, while examples utilizing Lewis bases remain relatively scarce.3
In this work, we introduce new Lewis photobase generators (PBG) as catalysts for light triggered oxa-Michael addition reactions. Their preparation and characterization by means of UV/Vis spectroscopy is described. A mechanistic understanding is derived from a monofunctional model study followed by testing the applicability of the PBGs in polymeric formulations, which render their 3D printing by Hot Lithography possible.4
(1) Hoyle, C. E.; Lowe, A. B.; Bowman, C. N. Thiol-click chemistry: a multifaceted toolbox for small molecule and polymer synthesis. Chem. Soc. Rev. 2010, 39 (4), 1355-1387.
(2) Ratzenböck, K.; Fischer, S. M.; Slugovc, C. Poly (ether) s derived from oxa-Michael polymerization: a comprehensive review. Monatsh. Chem. 2023, 154 (5), 443-458.
(3) Fischer, S. M.; Kaschnitz, P.; Slugovc, C. Tris (2, 4, 6-trimethoxyphenyl) phosphine–a Lewis base able to compete with phosphazene bases in catalysing oxa-Michael reactions. Cata. Sci. Technol. 2022, 12 (20), 6204-6212.
(4) Ligon, S. C.; Liska, R.; Stampfl, J.; Gurr, M.; Mülhaupt, R. Polymers for 3D printing and customized additive manufacturing. Chemical reviews 2017, 117 (15), 10212-10290.