The invention provides a process of polymerizing ethylene and at least one α-olefin, said process comprises polymerizing the ethylene and the at least one α-olefin in the presence of 5-ethylidene-2-norbornene, which comprises low levels of impurities, as determined by the absorbances at the following wavelengths: 320 nm and 343 nm. The invention also provides a method of increasing the catalysts efficiency, and a method of determining the expected catalyst efficiency, each in a polymerization of ethylene and at least one α-olefin in the presence of the 5-ethylidene-2-norbornene, which comprises low levels of impurities, as determined by the absorbances at the above wavelengths. The invention further provides a method of purifying 5-ethylidene-2-norbornene to form a purified 5-ethylidene-2-norbornene, which comprises low levels of impurities, as determined by the absorbances of the above wavelengths.