Heterofunctional condensation of alkoxysilanes and silanols. II. Synthesis of linear poly(aminoalkylsiloxane)s

Heterofunctional condensation of dialkoxysilanes with dihydroxysiloxanes has been used for the preparation of polysiloxanes of definite structure. The model reaction between N-β-aminoethyl-γ-aminopropylmethyl-dimethoxysilane and a monosilanol (PhMe2SiOH) has been studied by GC and 1H-NMR. The reaction proceeds without catalyst and the reactivities of the two alkoxy groups are very different. The first group reacts easily at room temperature whereas higher temperatures are necessary for the substitution of the second. When methanol is continuously eliminated, secondary reactions are maintained at an acceptable level. When an α,ω-dihydroxypolysiloxane is used, the same features are observed. The reaction is second-order and the optimal temperature range is about 100-110° in order to limit the homocondensation of silanol groups. Polysiloxanes with 20 or more silicon atoms have been prepared by this route. The specific structures which can be obtained include block copolymers of dimethylsiloxanes and methylphenylsiloxanes, containing a controlled number of amino functions.