Polymers derived from hydrocarbon molecules that possess one or more alkenyl (or olefinic) groups. The term polyolefin typically is applied to polymers derived from ethylene, propylene, and other alpha-olefins, and from isobutylene, cyclic olefins, and butadiene and other diolefins. Polymers produced from other olefinic monomers, such as styrene, vinyl chloride, and tetrafluoroethylene, generally are considered separately.