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Monodentate sulfonamide titanium and zirconium complexes, (RNSO$_2$R')$_2$ M(NR$_2$)$_2$ (M=Ti, Zr), which were synthesized {it in situ} from M(NR$_2$)$_4$ and 2 equivalents of sulfonamide ligands, RNHSO$_2$R', and which were characterized by $^1$H NMR, were used to catalytic polymerization of ethylene with either methylaluminoxane (MAO) or a mixture of i-Bu$_3$Al and (PhNHMe$_2$)$^+${B(C$_6$F$_5$)$_4$}$^-$ as cocatalyst. Catalytic activities depended on the reaction temperature and steric bulkiness of the sulfonamide ligands. The highest activity ($sim$0.505 KgPE/mmol h) was available at 80 $^circ$C using the complex bearing bulky PhMe$_2$CNTs ligands. Polyethylenes with higher molecular weight was obtainable when i-Bu$_3$Al and (PhNHMe$_2$)$^+${B(C$_6$F$_5$)$_4$}$^-$ were used as the cocatalyst. Broad molecular weight distribution ($M_w/M_n=4.0$ - 38) of the polymer indicates the existence of dual catalytically active species. Effects of the sulfonamide ligand structure and possible active species are discussed.