Copolymerization of ethylene and styrene by the INSITE(TM) technology from Dow presents a new polymer family identified as ethylene-styrene interpolymers (ESI). Based on the combined observations from melting behavior, density, dynamic mechanical response, and tensile deformation, a classification scheme with 3 distinct categories is proposed. Polymers with up to 50 wt % styrene are semicrystalline and are classified as type E. The stress-strain behavior of low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress-strain curve at higher strains, and large instantaneous recovery. The structural origin of the elastomeric behavior is probably a network of flexible chains with fringed micellar crystals serving as multifunctional junctions. Polymers with more than 50 wt % styrene are amorphous. Because the range of glass transition temperatures encompasses ambient temperature (nominally 25 degrees C), it is useful to differentiate ESIs that are above the glass transition as type M and those that are below the glass transition as type S. Type M polymers behave as rubber-like liquids. They have the lowest modulus and lowest stress levels. Some elastic characteristics are attributed to the entanglement network. Type S polymers exhibit large strain rate sensitivity with glassy behavior at short times and rubbery behavior at longer times. The term "glasstomer" is coined to describe these polymers. The division between type ICI : and type S is based on chain dynamics, rather than solid state structure, and thus depends on the temperature of interest. At ambient temperature, ESIs with 50 to 70 wt % styrene are classified as type M; polymers with more than 70 wt % styrene are classified as type S.