Cationic Photopolymerization of Epoxy Monomers Promoted by Epoxy Functionalized Silsesquioxanes

WALTER F. SCHROEDER; IGNACIO DELL' ERBA; GUSTAVO ARENAS; SILVANA ASMUSSEN; CLAUDIA I. VALLO

Sitges, Barcelona

Simposio; Third International Symposium Frontiers in polymer science; 2013

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Silsesquioxanes (SSO) are usually synthesized by the hydrolytic condensation of  organotrialkoxysilanes RSi(OR´)3, performed in the presence of an acid or base as catalysts. SSO structures may vary from fully condensed perfect polyhedra, incompletely condensed polyhedra, open structures, linear polymers, and all their possible combinations. The introduction of appropriate functionalities in the organic branch (R) has been used to obtain a large variety of organic-inorganic hybrid materials.Photoinitiated ring opening polymerization of epoxy monomers has gained renewed interest during the last decade. Results reported in the literature show that these monomers display different behaviors depending on their molecular structures. Epoxy monomers that bear no means of stabilizing the intermediate oxonium ions proceed rapidly and exothermically on irradiation. These monomers are commonly termed "class I" monomers. In contrast, epoxy monomers (class II) that bear neighboring oxygen atoms in the structure able to stabilize the oxonium ion intermediates undergo slow photoinitiated cationic polymerization. Included in this group is diglycidyl ether of bisphenol A (DGEBA). In this work we show that it is possible to increase the cationic photopolymerization rate of DGEBA monomers by conducting the reaction in the presence of SSO functionalized with class I epoxy branches. The silsesquioxane functionalized with epoxy cycloaliphatic groups (EPOX-SSO) was synthesized by hydrolytic condensation of trimethoxy[2-(7-oxabicyclo[4.1.0]hept-3-yl)ethyl]silane (TOHES) using HCOOH (0.1N) as catalyst. The synthesis was carried out in a first stage in tetrahydrofuran (THF) at 50 ºC, followed by a second stage in DGEBA monomer, where temperature was increased in steps up to 140 ºC. Homogeneous solutions containing different amounts of EPOX-SSO in DGEBA were prepared. Experimental results showed that EPOX-SSO can be successfully used as a co-monomer to generate a polymer network by reaction with DGEBA via a visible-light cationic photopolymerization mechanism. A detailed characterization of the final materials was performed.