Four titanium complexes derived from 2-(2-ethylanilino)-, 2-(3,5-dimethylanilino)pyridine, 2-(4-n-butylanilino)- and 2-(2-t-butylanilino) pyridine were synthesized and characterized by spectroscopic methods. These complexes were used to catalyze the polymerization of ethylene in the presence of MAO as cocatalyst. The effect of the complex structures on the polymerization behavior was studied. All the alkylphenylaminopyridinato titanium complexes used in this study yielded higher molar masses than the unsubstituted bis(phenylaminopyridinato) titanium dichloride complex. Correspondingly, activities were lower and molar mass distributions were broader than those in the case of the unsubstituted bis(phenylaminopyridinato) titanium catalyst. The fluxional behavior of alkylphenylaminopyridinato titanium catalysts is probably the reason for the broad molar mass distributions. This might be due to the electron-donating effect from the alkyl substituent because the alkyl substituent enhances the active site isomerization rate. (C) 2007 Elsevier B.V. All rights reserved.