Calculation of a reaction path for KOH catalyzed ring-opening polymerization of hexamethylcyclotrisiloxane

Polysiloxanes represent an important class of industrial polymers. Traditionally, poly (dimethylsiloxane) (PDMS) can be prepared by base-catalyzed ring-opening of cyclic dimethylsiloxanes. Ab initio electronic calculations were conducted to examine a reaction path for the KOH catalyzed ring-opening polymerization of hexamethylcyclotrisiloxane (D-3). The overall picture that emerges is initial side-on attack by KOH on a Si-O bond in the D-3 ring leading to a stable addition complex with a 5-fold coordinated Si atom. The reacth path leads to a five-coordinate transition state, then to a stable insertion product (KOH inserts into the ring). The relative stability of a ring-opened product HO[Si(CH3)(2)O]K-3 is also considered. The energy along the reaction path was modeled both in the gas phase and in a moderately polar solvent (tetrahydrofuran, THF). The solvation energy was calculated using a recent implementation of an electrostatic model, where the solute molecule is placed in a non-spherical cavity in a dielectric continuum. The effect of basis set and electron correlation on the gas-phase energy and the effect of basis set on the solvation energy was studied. Along the solvated reaction path calculated at the Hartree-Fock level (with a 6-31G* basis set for the Si and O atoms), the apparent transition state energy is nearly equal to the reactants energy and is 4 kcal/mol above the addition complex energy.