PhD - Biochemistry There are 9 files which have been withheld at the author's request / A systematic investigation into the effect that alkyl and aryl substituents have on the ethene polymerisation behaviour of a series of MAO co-catalysed mono-substituted bis-(cyclopentadienyl)zirconium dichloride and bis-(indenyl)zirconium dichloride metallocenes was carried out. The electronic attributes of the substituents were related to the Hammett substituent functions. The sizes of the ligands were measured from the perspective of the metal using the Tolman cone angle, numerical solid angle and analytical solid angle methodologies. For the mono-substituted cyclopentadienyl metallocenes of formula (CpR)2ZrCl2 (R = Me, Et, iPr, tBu, SiMe3, CMe2Ph), it was found that an optimum ligand size (R = Et, iPr) was required for high activity. These substituents protected the active polymerisation site from deactivation processes. However, if the substituent was too bulky (R = tBu, CMe2Ph) activity dropped drastically. Electronic factors possibly also play a role as demonstrated by the much higher activity of the SiMe3 substituted catalyst over its similarly sized tBu analogue. A series of new bis-(indenyl)zirconium dichloride metallocenes of formula [2–R-Ind]2ZrCl2 (R = H, Me, SiMe3, Ph, Bz and 1-Naphthyl) and [1–R-Ind]2ZrCl2 (R = Me, Et, iPr, tBu, SiMe3, Ph, Bz and 1-Naphthyl) were synthesised. Ethene polymerisation reactions with these metallocenes revealed the same electronic and steric trends observed for the (CpR)2ZrCl2 catalysts. Steric effects were found to play an important role with polymerisation activity decreasing as the size of the substituent increases. Also, the 1-substituted metallocenes had higher activities than their 2-substituted analogues, the differences in their activity increasing with the size of the substituent. By studying the conformations adopted by the indenyl ligands in the solid state, we have rationalised this phenomenon to be a result of the greater steric protection afforded to the active polymerisation site by the 1-substituted metallocenes.