Preparation of Polyethylene by Coordination Mechanism

Low-density polyethylene can be prepared by coordination polymerization through copolymerization of ethylene with a-olefins. This is discussed in the section on copoly mers of ethylene. Finding catalytic systems that would allow formation of amorphous, low-density polyethylene from the monomer alone by low-pressure polymerization, however, is an economically worthwhile goal. To this end, consider able research is being carried out to develop such catalytic systems. Particular attention is given to metallocenes and other single-site catalysts for olefin polymerization. Originally, the metallocene catalysts were typical metal complexes with two cyclopentadienyl or substituted cyclopentadienyl groups. Many variations were developed since. These materials are used in combination with methyl aluminoxane and have the potential of forming the polymers with high precision. Nevertheless, at this time it is probably still safe to say that low-density polyethylene is prepared by many but perhaps not by all of the processes and catalytic systems mentioned in this book. This is because the material is manufactured all over the world and different considerations govern the decisions on the processes and catalytic systems. The same is probably true of high-density polyethylene. New catalysts based on palladium and nickel complexes with bulky a-diimine ligands were developed [17–19]. They can yield highly branched or moderately branched polymers of ethylene, as well as propylene and 1-hexene. The polyolefins produced by such catalysts can contain a considerable amount of branches along the backbone that are randomly distributed throughout the molecules. In these catalysts, the molecular weight-limiting b-hydrogen elimination process that is common to palladium and nickel catalysts has been suppressed through use of bulky a-diamine ligands [19]. This allows formation of high molecular weight polymers from ethylene and a-olefins. Anickel-based catalyst can be illustrated as follows:

It is claimed that the branching of polyethylene can be controlled to the extent that the product can even bemorebranched than conventional low-density polyethylene (1,2—300 branches/1,000 atoms) [18, 19]. The cationic Ni-diimine catalyst shown above (R ¼ H, CH3), with the methylaluminoxane analog, has been found to polymerize ethylene in toluene at room temperature at the rate of 110,000 kg/Ni/h. This is comparable to the metallocene rates. The Pd-based catalysts are less active than their Ni analogs [19]. When nickel catalysts are used, the extent of branching is a function of the temperature, ethylene pressure, and catalyst structure. Branching increases as the temperature rises. At higher ethylene pressure less branching occurs. Brookhart et al. illustrate the mechanism of polymerization as follows [18]:

where X = CH₃, Br; M =Pd, Ni; R = (CH₂CH₂) nCH₃; Ar= 2,6-dialkylphenyl. A patent for the polymerization process of olefins (especially ethylene, a-olefins, cyclopentene, and some fluorinated olefins) describes the above catalytic systems [20]. The hindered diimines stabilize alkyl Ni(II) or Pd(II) with cationic complexes. After preparation, the complexes are reduced with methylaluminoxane and then activated with Lewis acids capable of forming non-coordinating counterions [20]. In addition, preparation of catalysts based on iron and cobalt [21] was also reported. These are complexes of bulky pyridine bis-imine ligands with iron or cobalt that are also activated by methylaluminoxane:

The iron-based catalysts are reported to be considerably more active than the cobalt analogs [21]. The yield of linear, narrow molecular weight distribution polyethylene per gram is reported to be very high [21]. Baugh et al. [22] synthesized and characterized a series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M ¼ Ni) and tridentate (for M ¼ Fe) heterocycle imine ligands. Activation of these pre-catalysts with methyl aluminoxane yields active catalyst systems for the oligomerization/polymerization of ethylene. Compared to a-diimine nickel and bis (imino)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities are significantly lower. Lower activities were attributed to reduced stability of the active species under polymerization conditions. The lower molecular weights of their products were explained to be the result of increased hydrogen transfer rates. Variations within the heterocyclic components of the ligand showed that both steric and electronic factors influence polymerization behavior of such catalysts. Hanaoka, Oda, and coworkers report [23] that single-site polymerization catalysts are of considerable interest industrially today, because they afford highly controllable polymerization performances based on precise design of catalyst architecture and their industrial applications. Among them, they point to constrained geometry catalyst and phenoxy-induced complex, they call phenics–Ti, that are used together with methyl aluminoxane

These are half-metallocene catalysts with an anionic armed-pendant that have now been well developed for industrial production of copolymers of ethylene with 1-olefins. Modification at the cyclopentadienyl ring system has been mainly tuned to finely control polymerization behaviors such as activity, molecular weight, and regiochemistry. In general, minimizing 2,1-insertion is essential to obtain high molecular weight polyolefins; otherwise, facile 6-elimination occurs, leading to termination of chain growth. Thus, the largely open coordination sites of half-metallocene catalyst systems possess an indispensable problem of irregularity in propagation. Through tuning bulkiness of substituents on the bridged-silicon unit of phenics–Ti, is claimed to have demonstrated that 2,1-insertion of propylene can also controlled by the bridging substituents to produce high molecular weight polypropylene [23]. Hong and coworkers [24] concluded that it is generally desirable to immobilize the single-site metallocene catalysts on a suitable carrier to obtain ideal product morphology. Ultrahigh molecular weight polyethylenes were successfully prepared by them through titanium complexes bearing phenoxy-imine chelate ligands

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مواضيع ذات صلة

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene

Commercial High-Density Polyethylene, Properties, and Manufacture

Preparation of Polyethylene by a Free-Radical Mechanism

Polyethylene and Related Polymers

Thermodynamics of Ring-Opening Polymerization

Ring-Opening Polymerizations by a Free Radical Mechanism

Polymerization of Trioxane

Ring-Opening Polymerizations of Cyclic Acetals

Polymerization of Oxepanes

The Propagation Reaction