A new thermosetting matrix for second-order nonlinear optical (NLO) materials based on the. hydrosilation reaction of tetramethylcyclotetrasiloxane (TMCTS) with NLO chromophores and dicyclopentadiene is disclosed. Chromophore Synthesis, hydrosilation reaction kinetics, processing of prepolymers, and the thermal and second-order optical properties of fully cured films are reported. Thin polymer films (0.5-2.0 mu m thick) were spin-cast, precured, and corona-poled while continuing the cure cycle to provide noncentrosymmetric films. The thermal stability of the second-order nonlinearity was measured monitoring the SGH signal as a function of temperature. A formulation comprising dye 7 and TMCTS, which possessed the capability for locking-in the orientation of the chromophore by multisite covalent bonding, maintained a nonlinearity:of d(31) = 3 +/- 1 pm/V and demonstrated thermal stability in excess of 190 degrees C.