Propylene-A
Basis for Creative Chemistry Harold Wittcoff Koor Chemical Ltd. P.O. Box 60 Beer Sheva, Israel
edited by: W. Conard Fernelius, Harold Wiltcofl, and Robed E. Varnerin
A previous article1describes ethylene as the most important source of organic chemicals. The second most important source is propylene. Thus we use almost 30 billion poundsper year of ethylene as a base for chemicals and about 15 billion pounds of propylene. The article on ethylene' tells us that all the ethylene we use comes from the thermal or steam cracking of hydrocarbons, either from natural gas or from petroleum fractions. Cracking to produce ethylene also produces propylene. Depending on what is cracked and a t what temperatures, anywhere from 40 to 500 lbof propylene results for every 1,000 Ib of ethylene. There is, however, still another source for propylene-the catalytic cracking of high molecular weight petroleum fractions to obtain lower molecular weight molecules useful for gasoline. Catalytic cracking, like thermal cracking, produces propylene whether we want it or not. This means that there has always been a large amount of propylene available, and accordingly, its price has always been lower than the price of ethylene. Excesses can be highly motivating. There is no better proof of this than in the creative chemistry that is based on propylene-chemistry that evolved because propylene was cheap. Does this chemistry duplicate that of ethylene? Only slightly. Most important, as we shall see, is the chemistry that depends on propylene's allylic hydrogen atoms. First let us see what the important chemicals are that come from propylene. Then we shall look at the chemistry associated with these. For a guide, we can use the information in the table. Polymerlzatlon Since we cannot examine each of these chemicals in detail, let us look at those whose chemistry is most interesting. Polypropylene involves the most dramatic chemistrv. Indeed it is safe tu say that the excitement which the discovery of polypn,pylene generated has seldom, if ever, been duplicated in chemical science. Propylene cannot be polyme&ed to polypropylene with peroxide initiators because propylene's three allylic hydrogen atoms cause low molecular weight, crosslinked gummy materials to form. What a pity since propylene is cheaper t h i n ethylene. I t was Karl Ziegler working at the Kaiser Wilhelm Institute in Germany who discovered a coordination c a t a l v s t a combination of titanium tri- or tetrachloride wirh a" aluminum trinlkyl such a i aluminum triethsl- - which made oossible the polymerization of ethylene to a high molecular weight. Further, the Ziegler catalyst, unlike the peroxide catalyst, gave Chemlcals and Polymers from Propylene Chemical or Polymer Polypropylene Acrylonitrile Propylene Oxide lkopropanol Cumene (for phenol and acetone) n-Bmanoi and lsobmanol Acrylic Acid Other chemicals
% of
Total Propylene Used 25 17 15 12 10
8
6
ISOTACTIC
SYNDIOTACTIC
ATACTIC Figure 1. Polypropylene structures.
a product with higher density, greater linearity, and greater strength and made possible operation a t lower temperatures and pressures. The Italian chemist, Gulio Natta, extended Ziegler's discovery to propylene. And because he did, hoth were rewarded with a Nobel prize. The reason for all the excitement: The Ziegler catalyst not only caused propylene to polymerize to provide the hitherto unknown ~olvoroovlene. . ,. ., hut more important chemtcally the pulymerization 01 this non-symmetrical m(decu!e ~ 3 stereospe~:itir s and the onducls that resulted were stereoregular. ~ i g u r e1 shows ihe three forms of polypropylene. The last is atactic, that is, it has no stereoregularity. Indeed all polymers that had been synthesized prior to this time such as poly(viny1 chloride) or polystyrene did not have stereoregularity. But the polymers made by Natta were either isotactic or syndiotactic. We see in each z r o u, ~ around s tho of these structures in Fizure 1 that the ~, -~~ . . rarhon atoma are arranged reg~~larly in each succeeding unit. The symmetry in the isotact~coolvmer is diiierrnr from the symmetry in
