Polyolefin plastics—such as polyethylene (PE) and isotactic polypropylene(iPP)—account for roughly 50% of the annual global plastic production. Their widespread use is due to their favorable properties including toughness and durability; however, these properties also contribute to their pitfall. The chemical stability of the hydrocarbon backbone severely limits its recycling by chemical means. Furthermore, mechanical recycling is limited due to the difficulty of separating mixed polyolefin waste streams and by the inherent immiscibility of these blends. As a result, recent studies have sought to address this issues by introducing functionalities, or cleavage sites, along polyolefins’ hydrocarbon backbones. Chapter one summarizes various functionalities that have been incorporated into polyolefins through the use of partially unsaturated polyolefin intermediates accessed from top-down post-polymerization or bottom-up copolymerization routes. A top-down approach installs cleavable functionalities into a preexisting polymer backbone, and a bottom-up approach introduces functionality during the copolymerization of ethylene or propylene gas with a suitable comonomer. Chapter two describes examples of a top-down and bottom-up approach for introducing cleavable ester functionalities along the (-CH2-) backbone of high-density polyethylene (HDPE) to produce chemically recyclable HDPE. Lastly, chapter three discusses an architecture modification approach that aims to improve the effectiveness of HDPE mechanical recycling by using preexisting vinyl chain ends on post-consumer or recycled waste HDPE to synthesize star-like polyethylene graft copolymers. The vinyl chain ends are grafted onto a polymethylhydrosiloxane (PMHS) backbone to synthesize PMHS-g-HDPE copolymers that exhibit enhanced strength and toughness. Overall, chemically recyclable and star-like polyethylene materials provide promising circular solutions for advancing the sustainability of a very useful class of commodity polymers.