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

Abstract

As evidenced by the many contributions to this symposium, siloxanes represent a class of semi-organic polymers of great importance to industry. Traditionally, polydimethylsiloxane can be prepared by base-catalyzed ring-opening of cyclic dimethyl siloxanes. Ab initio electronic calculations were conducted to examine a gas-phase reaction path for the KOH catalyzed ring-opening polymerization of octamethylcyclotetrasiloxane (D4) and hexamethylcyclotrisiloxane (D3). Two different stable KOH-D4 adduct structures were found for the initial step along the D4 reaction path: (1) a multidendate interaction between the K atom and the four O atoms in the D4 ring, and; (2) a side-on addition complex between KOH and a Si-O bond in the ring, where the Si atom is five-fold coordinated. Continuing from the addition complex, the D4 reaction path leads to five-fold coordinated Si atom transition state, then to a stable insertion product (KOH inserts into the ring). Retention of stereochemistry about the five-fold coordinated Si atom is maintained in the formation and cleavage of the Si-O bonds. The relative stability of a ring-opened product HO[Si(CH3)2O]4K is also considered. The KOH-D4 results are compared with previous calculations for the KOH-D3 reaction path. Adducts between D3 and other K-containing bases larger than KOH were studied to determine whether KOH is a realistic model for the chain propagation step. The energy along the D3 reaction path was also modeled both 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 solvation energy was studied.