microcellular, having a void volume of at most 25%, at most 10%, at most 5%, at most 2%, at most 1% or at most 0.6%. In some embodiments, the final curing step is performed in the absence of water and a physical blowing agent. In especially preferred embodiments, the final curing step is also performed in the absence of entrained gas. For example, a step of degassing the reactants used in the final curing step can be performed to remove entrained gasses therefrom prior to the final curing step. If desired, a small amount of physical blowing agent and/or entrained gas may be present during the final curing step to reduce density, as when a microcellular polyurethane is to be produced. Air, carbon dioxide, nitrogen or other gas can be whipped into the reaction mixture of the final curing step to form a froth, which is cured to form a microcellular elastomer. The various reaction steps can be performed within a temperature range of somewhat below room temperature (such as 0°Cor more but preferably at least 10° C or at least 15°C)to as much as 120°C.If a prepolymer-forming step is performed before the final curing step, it is generally advantageous to use an elevated temperature such as at least 50° C and, for example, up to 90°C,up to 80°Cor up to 70°C during the prepolymer manufacturing step. A final curing step is often performed at ambient or a slightly elevated temperature (such as 10°Cto 45°C,especially 15°Cto 35°C),particularly in the case in which the final polyurethane product is a cured adhesive, sealant or coating. A final curing step may be performed at elevated temperatures such as described for the prepolymer- forming step; it is often advantageous to use such a somewhat higher curing temperature, especially when making molded or coated products, to speed the cure and reduce cycle time on production equipment. When a prepolymer-forming step is performed, the reaction is typically continued until a constant isocyanate content is achieved, indicating the consumption of all of the isocyanate-reactive compounds. The final curing step is generally continued until the polyurethane has achieved constant mechanical properties. The final curing step may be continued, for example, for at least 6 minutes, or at least 30 minutes, and up to 7 days or even longer, although shorter curing times such as up to 3 hours or up to 2.5 hours are generally preferred, especially when making molded parts or coating, from the standpoint of achieving commercially reasonable reaction rates and increasing equipment utilization. The final curing step may be performed under superatmospheric