Polyethylene, metallocene-based

Metallocene-based polyethylene does not offer the lower production costs associated with LLDPE. Hence there will be a price premium for the new materials but this is felt to be justified in view of their improved property profile. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

Table 6 Metallocene-Based Linear Low Density Polyethylene...

With the details associated with ADMET chemistry reasonably well understood, we have embarked on a study of the synthesis of well-controlled polymer structures via metathesis polycondensation chemistry [37]. A series of well-defined polyolefins have been designed to model the crystallization behavior of polyethylene and its related copolymers, including new materials synthesized by metallocene-based catalysts. This synthesis concept has been reduced to practice, and polymers that will aid in the understanding of branching within polyethylene itself have been produced. 

Fig. 9.5-1, right, compares the co-monomer incorporated into the polymer, on a molecular basis. With the Ziegler-Natta, multi-site catalysts less monomer is incorporated in the high-molecular-weight fraction. The low-molecular-weight fraction is rich in co-monomer content as shown by the negative slope of curve a. In a metallocene-based polyethylene the comonomer is uniformly distributed (curve b). 

These monocyclopentadienyl amidotitanium complexes, which are classified as constrained-geometry catalysts, are capable of producing low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) that also contain long-chain branches, in contrast to strictly linear low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) produced by bent metallocene-based catalysts [30,105,148,149]. 

Also worthy of mention are the new very-low-density polyethylenes (VLDPEs), which are produced using metallocene-based catalysts [9]. These materials, which remain largely experimental at the moment, have a density of less than 0.9g/cm. ... 

A narrow molecular weight distribution" (MWD) is a unique feature of polymer products of homogeneous metallocene-based catalysts. Typical values of Myj/Mn are 1.6-2.4 for polyethylenes and 1.9-2.6 for polypropy-... 

A unique feature of polymer products obtained with homogeneous metallocene-based catalysts is their narrow molecular weight distribution (MWD). Thus the values of for polyethylenes and polypropylenes are typically near 2 which... 

New types of LLDPEs based on the metallocene catalyst technology have been introduced recently in the market place. Such LLDPEs are characterized by narrower molecular weight and homogeneous short-chain branching distribution. Some of the metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to have LCB. For the properties of metallocene LLDPE see the entry Polyethylene, metallocene linear low density, in this handbook. 

INTRODUCTION New types of linear low-density polyethylenes (LLDPE) based on the metallocene catalyst technology have been introduced recently in the market place. Metallocene-based Ziegler-Natta catalysts utilize a new synthetic approach for the polymerization of poly(a-olefins).( 5) Metallocene precatalysts are based primarily on group IV transition metals (primarily titcuiium and zirconium straddled by a pair of cyclic alkyl molecules) and require a coactivator, which is typically methylalumoxane but certain acids containing noncoordinating anions as bases also work well. 

The recent proliferation of metallocene-based polyolefins and polyolefin elastomers have gained their popularity owing to their density, cost, and ease of processabUity. PVC/POE blends have therefore been investigated as flexible PVC compounds. However, these blends are thermodynamically immiscible and needed suitable compatibiUzers such as the chlorinated polyethylenes (Eastman and Dadmun 2002). Since they are not miscible, POEs do not lower the PVC modulus sufficiently unless some plasticizer or a compatible elastomer such as EPE is also added. Commercially, some PVC/POE alloys are offered by TeknorApex under Flexalloy trade name with a shore A hardness 40-60 and brittle points down to —50 °C. They are claimed to have excellent low-temperature toughness, flexibility, compression set-resistance, and oil resistance. Suitable applications include automotive hoses, seals, gaskets, wire jacketing, etc. 

A unique feature of polymer products obtained with homogeneous metallocene-based catalysts is their narrow molecular weight distribution (MWD). Thus the values of My,jMn for polyethylenes and polypropylenes are typically near 2 which would indicate the presence of uniform active species in catalysts. Some researchers have thus suggested that the catalysts could be single site, while others have explained that two or more kinds of species can have very similar values for the kpjktr ratio, giving rise to narrow MWD products. 

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