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We present a novel method of preparing highly efficient and stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking induced vitrification under electric field. In this method a soluble prepolymer is first prepared that contains cross‐linking sites attached to the NLO‐active groups. Upon preparing samples of desired thicknesses, the prepolymer is heated (precured) to enable some chemical cross‐linking and thus to increase the glass transition temperature (T_g) to an optimum for poling. The precured polymer is then heated above its T_g and subjected to a high electric field to obtain the desired alignment of NLO moieties. Subsequent chemical cross‐linking (curing) under electric field continues to advance the T_g and hence leads to in situ vitrification of the polymer that stabilizes the electric‐field‐induced orientation of the NLO moieties. Detailed results of thermal, linear optical, poling kinetics …