where there are present 0 to 60 mole percent [R2 Si] units and 40 to 100 mole percent [RSi] units and where each R is independently selected from the group consisting of hydrogen, alkyl radicals containing 1 to 20 carbon atoms, phenyl radicals, vinyl radicals, and radicals of the formula Ay X'.sub.(3-y) Si(CH2)z -- where A is a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms, y is