Single-site catalysts metallocene polyethylene

Two recent developments in non-metallocene single site catalysts for polyethylene are noteworthy ... 

What differences would be observed between linear low density polyethylenes manufactured with a Ziegler-Natta catalyst versus a single site catalyst, such as a metallocene ... 

High-Density Polyethylene (HDPE). Polymerization of ethylene to polyethylenes is most often carried out at low temperature and pressure, using either the Ziegler aluminum triethyl plus titanium tetrachloride catalyst system, the Phillips chromic oxide plus silica plus alumina system, or more recently the newer metallocene single-site catalyst systems. 

In the 1990s, polyethylenes produced with metallocene single site catalysts were commercialized and non-metallocene single-site catalysts were discovered by Brookhart and coworkers. 

Table 1.2 provides a summary of commonly used classifications in the polyethylene industry. A brief note is warranted here to conclude the survey of polyethylene classifications and nomenclature. In the early 1990s, several types of polyethylene manufactured with metallcxiene catalysts (a type of single site catalyst, see Chapter 6) were introduced to the market. To differentiate polyethylene produced with metallocenes from polyethylene manufactured using older conventional catalysts, metallocene grades are sometimes abbreviated mVLDPE, mLLDPE, etc. 

These developments and non-metallocene single site catalysts in general represent the next wave of innovation in polyolefin catalysis which should permit production of polyethylenes with unique properties at lower cost. They will complement, and perhaps even supplant, many of the metallocene single site catalysts commercialized in the 1990s. 

Polyethylenes synthesized by metallocene/MAO catalysts have a molecular weight distribution of M /M = 2. The molecular weight can easily be lowered by increasing the temperature, increasing the metallocene concentration, or decreasing the ethene concentration. The narrow molecular weight distribution is characteristic for a single site catalyst. Nearly every zirconocene forms an active site of a cationic metallocene - and an anionic MAO compound or a complex of both. The nature of the active site would be clearer if more details were known about the structure of the alumoxane. 

Ziegler-Natta catalysts are widely used in the production of high-density and linear low-density polyethylene (HDPE and LLDPE). More than half the world production of HDPE and over 90%o of LLDPE is based on Ziegler-Natta catalysts, although increased use of metallocene and other single-site catalysts is expected throughout the next decade. 

General Description The fundamental attribute of ExxonMobil Exceeds metallocene-catalyzed linear low density polyethylene (mEEDPE) is a consistent, uniform distribution of polymer molecules based on single-site catalysts. This uniformity eliminates molecular extremes (narrowing the molecular weight and composition distribution), resulting in a range of property improvements. 

Metallocene catalysts (Figure 2.14(c)), on the other hand, are single-site catalysts, that is, they produce polyolefins with unimodal and narrow CCD (Figure 2.6) and narrow MWD with PDIs close to 2.0, although, under some conditions, usually when supported, they may make polymer with broader distributions. Metallocenes had a very large impact in the polyolefin industry when they were discovered in the 1980s because, for the first time, polyethylene and polypropylene could be produced under conventional industrial conditions with uniform and well controlled microstructures, without the complex MWD and CCD correlations observed with Ziegler-Natta and Phillips catalysts. 

Metallocene plastomers are plastomers made using metallocene single-site catalysts. Other polyolefins exist in this composition range made with conventional catalysts Dow ATTANE (10), Union Carbide FLEXOMER (11), and Mitsui TAFMER products (12). These are generally called very-low-density poly-ethylenes (VLDPEs) and ultra-low-density polyethylenes (ULDPEs), and are not discussed in this chapter. 

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