catalysts and process technology led to a new material that had most of the attributes of LDPE but was produced by a more economical low-pressure process similar to that used for HDPE. It is a copolymer of ethylene and an alpha-olefin (like butene-1, hexene-1). Thus, short-chain branches of controlled length and number are introduced into the chain without any long-chain branches, and the material is called linear low-density polyethylene (LLDPE; see Figure 3.3). Production of this material grew at a rate of about 20 percent per year during the 1980s to current usage of about 5 × 109 pounds per year. As a result, the production of LDPE initially declined, but its production has been growing again since 1986. Construction of new high-pressure production facilities may be required in the next decade to meet demands. Currently this is the only process by which copolymers can be made with polar monomers such as vinyl acetate or acrylic acid. HDPE is fabricated primarily by molding. Blow-molded food bottles and auto gasoline tanks constitute major markets. Very large containers made by rotational molding represent a specialized growth area. A process known as "gel spinning" has been commercialized, which produces fibers of ultrahigh-molecular-weight polyethylene. The less crystalline LDPE and LLDPE are primarily extruded into film products, with each having specialized uses. New technology based on single-site metallocenes holds promise for the production of a new range of products.

This brief review of the history and future prospects for olefin polymers illustrates the need for research of all types (e.g., catalysis, process, and structural characterization) in order to capitalize on economic opportunities. These materials are complex in terms of molecular structure, and so there are many ways to tailor their behavior provided the basic knowledge and tools for structural determination are available and are integrated with innovative process technology. Much of the present research is directed toward the design of catalysts that yield materials that are easier to process. Rapid progress has resulted from an integration of catalyst synthesis and reactor and process design. As a recent example, a new polyolefin alloy product has been developed by exposing a designed catalyst to a series of different olefin monomer feeds to produce a polymer particle that is composed of polymers with different properties. Extrusion of those particles results directly in a polymer alloy.

Structural thermoplastics are a vital part of the national economy, and considerable opportunity remains for economic growth and scientific inquiry. New specialized materials will continue to offer rewards in the marketplace. At the high-performance end, several entirely new polymer structures are likely to emerge over the next decade. A major part of the growth in "new" materials will be in the area of blends or alloys. The vitality of thermoplastics cannot be judged only on the basis of the introduction of what might be called "new materials." Continuous improvement and diversification of existing polymers constitute another measure. One source estimates that the number of "grades'' of existing polymers tripled during the 1980s (Chemical & Engineering News,

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