AM1 semiempirical computational analysis of the homopolymerization of spiroorthocarbonate

Spiroorthocarbonates (SOCs) are monomers that have been shown to expand whe n homopolymerized. SOCs are potential monomer systems that can be combined with other monomers such as epoxy resin to produce a non-shrinking dental m atrix for dental composites. The purpose of this study was to use a compute r model (AML) to study possible homopolymerization pathways for several SOC monomers. The gas phase transition states of three feasible reaction mecha nisms for the homopolymerization of four spiroorthocarbonate 1,5,7,11-tetra oxaspiro[5,5]undecane (TOSU) systems have been examined using the AM1 semie mpirical quantum mechanical model. In addition to the base TOSU noted above , the 2,8-dimethyl, 2,4,8,10-tetramethyl, and the 3,3,9,9-tetramethyl analo gs were used in this study. The results of these calculations produced the heats of reaction, activation enthalpies and transition state structures. O ur calculations indicate stabilization of the transition states by electron -donating and resonance-stabilizing substituent groups. The energies of act ivation of all of these systems were between 24 and 38 kcal/mol and all rea ctions were endothermic. Further, we found that there was a significant int ermolecular attraction between TOSU monomers (approximate to 3.5 kcal/mol). When compared with experimental studies of methylated TOSU by Sakai and co -workers, our calculations agree with the preferred site of nucleophilic at tack, but not with the experimental rate results. It was concluded that the homopolymerization of the unsubstituted TOSU and its derivatives studied w as endothermic and that the rare of homopolymerization of TOSU depends on a n intermolecular pre-association of TOSU monomer in the condensed phase. (C ) 2000 Elsevier Science B.V. All rights reserved.