Transition metals have long been used as both catalysts and mediators in the polymerisation of many monomers. This thesis explores the use of iron and titanium amino-phenolate complexes as mediators in the polymerisation of alkene monomers. Initial work focused on understanding the fundamental mechanisms behind the organometallic-mediated radical polymerisation (OMRP) of styrene, methyl methacrylate (MMA) and vinyl acetate using a novel and fully characterised tert-butyl substituted aminebis(phenolate) iron(II) complex. A range of temperatures and conditions were explored to elucidate the equilibrium between propagation and termination reactions in the polymerisation. It was found that in the polymerisation of MMA, propagation was favoured at low conversion with good control and reasonable dispersities. Mechanistic studies suggest that propagation proceeds through a reversible-termination OMRP mechanism. At higher conversions, irreversible termination reactions become dominant. The polymerisation temperature significantly affects the nature of termination, dictating whether termination is either bimolecular or via catalytic chain transfer (CCT). The polymerisation of styrene shows well-controlled behaviour with dispersities as low as 1.27, which is the first time this has been achieved for iron-mediated OMRP. The use of alternative initiation methods, such as macroinitiators and photoinitiators, is also discussed. A family of titanium(III) amino-phenolate complexes were used as mediators in the polymerisation of methacrylates. Well-controlled polymerisations were achieved, with linear first-order kinetics and dispersities as low as 1.09. The nature of the substituents on the ligand greatly affects the tacticity of the resultant polymer, with large bulky groups having a more significant effect in promoting isotactic polymer. Detailed experimental and computational studies suggest that the polymerisation mechanism is not radical or ionic, but instead proceeds through a coordinating-type mechanism. This mechanism is suggested to be bimetallic, involving a titanium(IV)-enolate complex and an MMA-coordinated titanium(III) species, with polymerisation propagating via a group transfer mechanism, which is rarely exhibited in transition metals. This also likely represents the first example of the initiation of a coordination polymerisation with a conventional azo initiator, without the need for pyrophoric or expensive activators.