Stereoregular plastic

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Even more important is the stereoregular catalytic polymerization of ethene and other alkenes to give high-density polyethene ( polythene ) and other plastics. A typical Ziegler-Natta catalyst can be made by mixing TiCU and Al2Eti in heptane partial reduction to Ti " and alkyl transfer occur, and a brown suspension forms which rapidly absorbs and polymerizes ethene even at room temperature and atmospheric pressure. Typical industrial conditions are 50- 150°C and 10 atm. Polyethene... 

These additives usually enhance specific properties of polymers. Thus solid, pure stereoregular polystyrene (PS) is brittle yet, as a result of the addition of the proper impact modifiers and other additives, the modified PS exhibits the properties of a good plastic and rubber. 

The importance of ZN catalysts in industrial production is enormous. They enable us to produce polymers with stereoregular or tactic struc-tures-isotactic (di-isotactic) or syndiotactic (Chap. 5, Sect. 4). They are used in the synthesis both of plastics and of elastomers. 

Many polymers are now manufactured on a, commercial scale using Ziegler-Natta catalysts. Indeed, stereoregular (isotactic) polypropylene of high molecular weight, which cannot be made by free-radical or ionic polymerization, has already achieved the status of a commodity plastic. The scientific and practical significance of Ziegler and Natta s work earned them the joint award of the Nobel Prize in Chemistry in 1963. 

A good reason to look at history is to select those parts we should like to repeat. One part of science history that both polymer chemists and businessmen would surely like to see emulated is the unprecedented, explosive burst of creativity, invention, and successful development that occurred in the 1950s and gave the world a new class of polymers (stereoregular), a family of new plastics (linear and stereoregular polyolefins), a family of new synthetic rubbers, including the first duplication of a natural high polymer... 

Purists may have been disappointed to learn that the theoretical ideal of a monodisperse, "chemically pure" stereoregular polymer is not the practical ideal. But we are all beneficiaries of the modifications and even "adulterations" that alleviated the deficiencies that would otherwise have kept the pure polymers from becoming useful plastics. 

---