This Thesis is concerned with the synthesis of post-metallocene group 4 amidinate complexes as pre-catalysts for the polymerisation and oligomerisation of α-olefins. Activation of half-sandwich group 4 amidinate complexes with 1,3-conjugated dienes is also investigated and their underlying chemistry is explored. <strong>Chapter 1</strong> introduces homogeneous Ziegler-Natta catalysis, with a specific focus on group 4 compounds. A review of metallocene and post-metallocene complexes of group 4 will also be presented and their behaviour in olefin polymerisation will be discussed. <strong>Chapter 2</strong> describes the synthesis and characterisation of _K1-amidinate supported titanium complexes. Base-free activation of the titanium cyclopentadienyl-amidinate dialkyl complexes will be investigated and the resulting cationic species will be further explored with commonly applied trapping agents. The synthesis and characterisation of new titanium cyclopentadienyl-amidinate diene complexes and their corresponding tri(pentafluorophenyl)borane-activated zwitterionic compounds will also be described and their performance in the co-polymerisation of ethylene and propylene will be discussed. <strong>Chapter 3</strong> explores the addition of 1,3-conjugated diene reagents to titanium cyclopentadienyl-amidinate and guanidinate alkyl cations. Mechanistic and computational studies of allyl and diene formation will also be described. The polymerisation capabilities of the newly formed complex will be discussed and compared to previously reported dialkyl complexes. <strong>Chapter 4</strong> describes the synthesis and characterisation of aminopyridinato titanium complexes and _K1-_K2-bis(amidinate) titanium complexes. The activation chemistry of the newly formed dialkyl complexes will be investigated. Furthermore, the synthesis of a new bis(aminopyridinato) titanium complex will also be discussed alongside their activity in EPM and EPDM polymerisation. <strong>Chapter 5</strong> describes the performance of the aminopyridinato titanium complexes (synthesised in Chapter 4) in EPDM polymerisation. The sensitivity of the aforementioned complexes towards hindered phenols will also be discussed and their behaviour in ethylene oligomerisation will be explored. <strong>Chapter 6</strong> presents full experimental procedures and characterising data for the new complexes reported in this Thesis.