sentences:  After natural rubber from rubber trees was made unavailable by the Japanese conquest of Southeast Asia, SBR provided a new material for tires for the Allies' military vehicles.Shell's Charles Wilcoxen later demonstrated the kinetics of the polymerization process, amending the assumptions of the Ziegler-Natta polymerization equations for which a Nobel prize was awarded in 1963.Shell Development also pioneered a class of materials called block copolymers, useful for medical applications such as artificial heart valves.