On electron structure of monomers of cation polymerization of 4, 6-dimethylindene and 5, 7-dimethylindene

The first studies on cationic polymerization of monomers 4,6-dimethylindene and 5,7-dimethylindene were carried out in the early 1970s. The highest viscosity had the products obtained by BF8 * ОМе2. In the presence of BF8 * OR2 temperature does not affect the inherent viscosity. Relatively high molecular weight characterizes the products obtained in the presence of TiCl4. With this Lewis acid, the greatest value of Mn (molecular weight of the polymer) 20 000 at -30 °C and 120,000 at -72 °C. The enthalpy of polymerization was 18,5 ± 0,9 kcal/mol. However, geometric and electronic structure of monomers of the cationic polymerization of 4,6-dimethylindene and 5,7-dimethylindene are better to study by quantum-chemical PM3 method specifically parameterized for the calculation of multivalent atoms and cyclic compounds, which the investigated monomers belong to. In this regard, the aim of the present work is quantum-chemical calculation study of the monomer 4,6-dimethylindene and 5,7- dimethylindene РМ3с optimization of geometry on all parameters gradient descent method embedded in the program, which is partly based on the source code GAMESS (US), in the approximation of isolated molecules in the gas phase and theoretical evaluation of their acid strength. For a visual representation of the model molecule we used the program MacMolPlt. In the present work we first performed quantum-chemical calculation of molecules of 4,6-dimethylindene and 5,7-dimethylindene based on quantum-chemical method PM3. We determined the optimized geometrical and electronic structure of these compounds and evaluated theoretically their acid strength (pKa = 23). Furthermore, it is shown that the studied monomers 4,6-dimethylindene and 5,7-dimethylindene belong to the class of very weak H-acids (because pKa > 14).