We are investigating copolymerizations of ethylene with polar monomers and in water using single-site catalysts. We have used three different types of such catalysts, whose syntheses have been mentioned in the literature, to compare their polymerization behavior: (1) 2,6-diacetyl pyridine bis(2,4,6-trimethylanil) iron(II) chloride with methylaluminoxane, (2) rac dimethylsilyl bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dimethyl with tris(pentafluorenyl)borane, and (3) a neutral P,O chelated nickel catalyst bearing a trifluoromethyl substitutent. All three catalysts enable the incorporation of polar monomers at reasonable polymerization activities. Besides the syntheses of HDPE, methyl acrylates were used primarily as polar comonomers with ethylene, whereby the iron based catalyst results in relatively low comonomer incorporation. Based on the polymerization rate of ethylene measured by a mass flow meter, we were able to observe the catalyst activity also during individual polymerization runs. The activities of the iron based catalyst and the metallocene stay relatively constant, whereas the activity of the nickel based catalyst does slightly decrease with time. Only the latter catalyst can be used in water to polymerize ethylene copolymers, for example effectively in mini-emulsion polymerizations. With the nickel based catalyst it is also possible to copolymerize ethylene with allyl alcohol at relatively high activities. Data from DSC, TGA, GPC, and IR and NMR spectra of the copolymers will also be discussed.