This paper presents a class of nonlinear optical (NLO) urethane-urea copolymers,where dimethylsiloxane linkages of different length are incorporated into the polymer main chain. For comparison, a urethane-urea polymer having virtually no siloxane units has also been prepared. Results show that for copolymers with siloxane linkages poled samples have exceptional orientational stability, consistent in part with strong hydrogen-bonding effects. By the introduction of siloxane linkages, low optical loss materials suitable for device-quality applications have been obtained. The low optical loss can be attributed to the relatively lower number density of C-H bonds in the polymer matrix. Simultaneous realization of enhanced temporal stability and lower optical loss has been achieved in a single polymer system.