Ethylene and propylene form the simplest chain-growth polymers. Since their polymer chains contain no polar groups, these polyolefins must rely on close contact between the molecular chains for strength. Thus, the physical characteristics of polyethylene are very sensitive to even a small number of chain branches. Very straight chains of polyethylene can form strong crystalline structures which exhibit exceptional strength. Protective fabrics made from this type of highly structured polyethylene are virtually impossible to penetrate or cut.
Polypropylene is more complicated.
Even without chain branching, each monomer unit adds one methyl group pendant to the chain. Polypropylene molecules can only pack closely in an isotactic arrangement. Synthesis of these polymers was a major challenge, but several stereoselective catalysts are now available, and high-density polypropylene has become a commodity product. Fibers made from it are lightweight, hydrophobic and highly crystalline. Their resistance to wetting gives them good moisture wicking and anti-staining properties.