Synthesis of Poly(Methylsilsesquioxane‐ <i>co</i> ‐Dimethylsiloxane) Liquids With a Long Pot Life by Cosolvent‐Free Water‐Rich Dilute Acid‐Catalyzed Hydrolytic Polycondensation With Aging

Cosolvent‐free hydrolytic polycondensation followed by aging is utilized to synthesize poly(methylsilsesquioxane‐ co ‐dimethylsiloxane) liquids for the precursors of the so‐called “DT‐type” methyl silicone thermosetting resins. High water to alkoxide molar ratios of 20–30 and relatively low acid catalyst to alkoxide molar ratios of 0.0001–0.0002 are optimal for reducing the average molar mass of oligomers and viscosity of liquids. This reduction of viscosity is accompanied by an increase in glass transition temperature, and this correlation originates from an increase in the kinetic fragility parameter up to ∼0.86. The lowest viscosities of the resulting liquids at 40°C are ∼6 × 10 3 and ∼3 × 10 4 mPa s at the mole fractions of methylsilsesquioxane units of 0.7 (methyl group to Si molar ratio of 1.3) and 0.8 (1.2), respectively. These liquids contain many residual SiOH groups, whereas their viscosity stability during storage at room temperature is good over a month.