Polymerization Mechanism of Cyclosiloxanes by Acidic Pigments

Abstract

The polymerization of cyclic dimethylsiloxanes in the presence of pigments was studied by an ampoule method at 50°C. The products were polydimethylsiloxanes (PDMS), whose molecular weight were 200,000 and 8,700 in the reaction with kaolinite and prussian blue, respectively.
The results of GC/MS measurement indicated that cyclic oligomers formed in the first step from D₄ by kaolinite were not D₈, D₁₂, and D₁₆, which were kinetically controlled species, but D₅, D₆, and D₇, which were thermodynamically controlled ones. The results of the reaction between cyclic siloxanes and kaolinite in the presence of hexane as a solvent indicated that the lower the concentration of cyclic siloxane was, the lower the molecular weight of product became. In, the case of 0.1% solution of D₃, D₄, and D₅, the final products had an identical composition, D₃ : D₄ : D₅ : D₆=4 : 80 : 15 : 1. These results show that the meeting of the two ends of a linear species is the main factor determining the distribution of cycle siloxanes, where a low monomer concentration leads to self-end-to-end ring closure, while polymerization among different molecules is encouraged at a high monomer concentration.