A series of mono- or dialkyl/lphenyl o-substituted phenoxy ligands in half-metallocene titanium(IV) complexes wits examined to determine the structure-catalytic activity relationship in high temperature olefin polymerization. Five different types of polymerization catalysts with different Cp/Cp* and mono- or disubstituted symmetric/asymmetric alkyl/phenyl phenoxide ancillary ligands were compared. This series wits examined for ethylene homopolymerization after activation with Ph3CB(C6F5)(4) and mMAO-7 at high temperatures (140 degrees C). Type 4 complexes of compounds 15-18 [33.0-39.0 kg/(mmol of Ti.h)] showed much higher catalytic activity than those found in types 1-3 and 5 [3.6-27.6 kg/(mmol of Ti.h)], and among the type 4 complexes, the Cp*/2-phenylphenoxy combination of compound 18 [39 kg/(mmol of Ti.h)] Surpassed the Cp*/2-alkyl ligand systems of compounds 15-17 [33.0-36.0 kg/(mmol of Ti.h)]. The revolving nature of the phenoxy ligand around the Ti-O-C axis was identified by the NOSEY correlation peaks between the methyl protons of Cp* and protons of ancillary phenyl phenoxy ligand in compound 18. The conformational flexibility of the phenyl phenoxy ligand was further confirmed by a series of temperature-dependent ROSEY experiments based oil the optimization of two conformational structures related by this rotation. Rotational barriers of 4.3 and 6.6 kcal/mol were estimated from theoretical DFT studies. DFT calculations of the transition states for ethylene insertion and termination were carried out for representative examples of types 4 (15,16, 18), 3 (10,12), and 1 (3) catalysts as well as the constrained geometry catalyst (CGC) its a reference. The preference for back-side insertion was a unique feature of the monosubstituted type 4 catalysts. The type 4 catalysts showed significant activities for ethylene/1-octene copolymerization affording high molecular weight poly(ethylene-co-1-octene)s (M-w = 107 000-164 000) with unimodal molecular weight distributions (M-w/M-n = 2.08-4.15). The activity increased in the order of type 3 [90-132 kg/(mmol of Ti.h), in toluene, ethylene 30 atm, 1-octene 8 mL, 140 degrees C, 10 min.] < CGC (222) < type 4 (228-354). Among the type 4 series, compound 18 showed the best performance, reaching an activity of 354 kg/(mmol of Ti.h). The polymer density of 0.9148 g/mL for compound 18 was lower than that found in CGC (0.9154 g/mL), indicating higher I-octene incorporation, which was further confirmed by an analysis of the C-13 NMR spectra of the polymers (18, 2.73 mol % and CGC, 2.55 mol %).