Intermediates of tris(pentafluorophenyl)borane and dimethyl carbonate pave the way for deeper organosiloxane depolymerization reactions

Abstract

Cleavage of siloxane bonds (Si-O) in organosiloxanes (OSs) is one of the most fundamentally and practically important challenges in the modern chemistry of silicones. However, no benign approaches have been found to cleave these bonds under mild conditions. In this paper, siloxane bond cleavage in polydimethylsiloxane (PDMS) was studied using a mixture of the “green” reagent dimethyl carbonate (DMC) and an environmentally benign reagent, tris(pentafluorophenyl)borane (B(C₆F₅)₃). The obtained results indicate that the Si-O bonds are cleaved by reaction with DMC to form lower-weight polymer fragments with alkoxy end groups, while the addition of B(C₆F₅)₃ to the depolymerizing mixture significantly increases the degree of PDMS depolymerization at room temperature or with increasing temperature. Quantum chemical calculations show that the DMC molecules form stable complexes with B(C₆F₅)₃ by preferable formation of C=O····B bonds, which increases the solvation effects and kinetic energy upon collision with PDMS. This result provides an appropriate electronic structure and orientation for the complexes for more effective interaction with Si-O-Si via a concert mechanism. This proposed benign approach can be used for the production of new OS polymers, silicone recycling and functionalization of metal or metalloid oxides.