Improving the efficiency of chain initiation is highly important and also highly challenging in the development of olefin polymerization catalysts. A series of 2-methylallyl-based nickel complexes supported by aryl-N-bridged diphosphazane monoxide (PNPO) ligands containing different electronic and steric substituents were prepared and characterized. These nickel complexes are highly active single-component catalysts for ethylene polymerization and copolymerization with methyl acrylate (MA). 2-Methylallyl substituent on the mu-allyl catalysts led to an increase in the efficiency of chain initiation compared with the corresponding allyl-based analogues, improving the catalytic performances with high activity (up to 4.02 X 10(6) g PE (mol Ni)(-1)h(-1)). Linear polyethylenes with high molecular weight, narrow PDI values, and high melting temperatures were generated. Most importantly, these 2-methylallyl nickel catalysts can promote ethylene-MA copolymerization to afford functionalized polyethylenes with MA incorporation of up to 7.0 mol %. The current work demonstrates that the change of initiating units can lead to enhancement in catalyst performances. This provides an alternative, simple, and potentially general strategy to improve the properties of different catalyst systems.