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. By contrast, the pendant nitrile functions in polyacrylonitrile are sufficiently polar to produce very strong inter-chain forces. Pure homopolymers from acrylonitrile are non-thermoplastic and difficult to dissolve or dye. Thus, for most commercial acrylonitrile polymers, small amounts of other monomers with bulky side chains are introduced to force the chains apart, to reduce the inter-chain forces. Common co-monomers for these fiber applications include vinyl chloride, vinyl acetate, acrylic acid, and methyl acrylate.