CYCLIC ANHYDRIDE RING-OPENING REACTIONS - THEORY AND APPLICATION

The development of a zero net shrinkage dental restorative material ba sed upon a polymer-bioactive-glass composite requires a second-phase m aterial that expands. This study details the mechanisms of organic cyc lic anhydride ring expansion via hydrolysis. Six cyclic anhydrides wer e used to represent potential side groups, each of which could be an e xpanding phase or component. Maleic, 4META, tetrahydrophthalic, norbor nene, itaconic, and succinic anhydrides were modeled using the Austin method (AM1), a semi-empirical molecular orbital method. The reaction pathways were determined for the anhydride ring opening reaction to fo rm an acid for each case. The activation barriers (Ea) for the ring op enings were found from the transition state geometries wherein only on e imaginary eigen value in the vibration spectrum existed (a true sadd le point). In each case the reaction pathway included the hydrogen bon ding of a H2O molecule to the ring, weakening of the C-O bridging bond s of the ring, and, finally, the dissociation of the H2O, forming two carboxyl groups and opening the ring. The activation for the ring open ings are +34.3, +36.9, +40.6, +43.1, +45.9, and +47.7 kcal/mol, respec tively. The volumetric expansion of the anhydrides was estimated based upon the dilation of C-O-C atomic distances. The dimensional change w as found to be 24.0%, 24.0%, 19.1%, 20.3%, 20.8%, and 17.9% for the an hydride rings, respectively. Finally, it was found that a Linear corre lation exists between the cyclic anhydride C-O asymmetric rocking (as- v) vibration and the activation energy (Ea) for hydrolysis to an acid. This may be used as an experimental indicator of a cyclic anhydride's activity. (C) 1998 John Wiley & Sons, Inc.