ABSTRACT
Acknowledgment................................................................................................................ 254
References .......................................................................................................................... 254
When propylene was first polymerized in the mid-1930s, the product was a low-melting
solid of no possible value as a resin from which synthetic fibers could be prepared. By the
mid-1950s, research had led to the preparation of a new kind of polypropylene that had all
or most of the pendant methyl groups in a regular position with respect to the polymer
backbone. Commercial production of these stereospecific polymers utilized catalysts that
were generally based on a combination of TiCl3 and an aluminum alkyl. The stereospecific
resin with all the methyl groups in the same position, rather than regular alternating
positions, was called isotactic polypropylene. This resin was crystallizable with melt
temperatures in the range of 160-1748C, and fibers capable of retaining molecular orientation at normal-use temperatures could be prepared. Since then, catalyst and process
improvements have resulted in approximately a tenfold increase in polymer yield per
simplified purification The percent
noncrystalline resin has been reduced, and clean, gel-free resin required for modern fiber
processes is now readily available. Isotactic polypropylene, as originally produced, had a
very broad molecular weight distribution compared to most synthetic fiber resins. Poly-
propylene resin with a narrower molecular weight distribution, which is beneficial to the
same fiber processes, is now manufactured.