Small repeats. Big differences.

HDPE has relatively little branching, so chains pack closely. LDPE has more branching and packs less efficiently, giving lower density and greater flexibility. In polypropene, the arrangement of side groups (tacticity) affects how regularly chains pack.

Polyethene, polypropene and PTFE have carbon-chain backbones. Polyesters, polyamides and PLA contain characteristic ester or amide linkages. Some of these bonds can be hydrolysed under suitable conditions.

Biodegradability depends on chemistry and conditions, not simply whether a material is made from plants. Temperature, water, microorganisms and accessible bonds matter. Industrially compostable does not mean rapid breakdown in the ocean.