Synthetic and natural silica-aluminates as inorganic acidic catalysts in ring opening polymerization of cyclosiloxanes

摘要

Ring opening polymerization (ROP) of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) was promoted by acid-treated synthetic and natural silica-aluminates. Silica-alumina (1:3 Si/AI molar ratio) was obtained using a simple and economic route from precipitation of aluminum sulfate solutions. The material was treated in an acidic medium to improve the content of acid sites and successfully tested as inorganic acidic catalyst for ROP of D3 or D4 cyclosiloxanes. Natural bentonite was treated and used in a similar manner. Once the ROP reaction completed, the catalyst was easily removed and it was found that the recovered synthetic silica-alumina was active in a second ROP reaction. The effect of the concentration and type of catalyst in respect to the molecular weight and polydispersity of polydimethylsiloxanes was analyzed: increasing the amount of silica-alumina in ROP of D4 from 0.05 to 0.1 g decreased the average molecular weight (M? = 13-1.8 kDa) associated with an increase in the polydispersity (2.95 vs. 1.81). Analogous results were found with bentonite. These values suggest that an increase in the catalyst concentration led to a lower M_n, with a more homogeneous molecular chain dimension.