Polypropylene

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring [36]. They are, therefore, polymerized by coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. Also, there is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, is still mainly a laboratory curiosity. The polypropylene that was originally described by Natta contained less than 50% of isotactic fractions. The remainder was atactic material. Some stereoblocks composed of isotactic and atactic polypropylenes were also formed. This type of product forms when a-olefins are polymerized in inert hydrocarbons with catalysts prepared by reducing high valence metal compounds, like TiCl₄, with organometallic compounds like Al(C₂H₅) prepared by reducing high valence metal compounds, like TiCl₄, with organometallic compounds like Al(C₂H₅)3. Later heterogeneous highly crystalline catalysts based on transition metals (valence 3 or less) like TiCl₂, TiCl₃, ZrCl₃, and VCl₃ were developed that yielded stereospecific polypropylene. The metal halides were combined with selected metal alkyls. Only those alkyls were picked that would not destroy the crystalline lattice of the transition metal salts in the process of the reaction. The resultant catalysts yielded crystalline polypropylenes with high fractions of the isotactic material. The products, however, also contained some low molecular weight fractions, some amorphous and stereo block materials, that still required costly purification and separations to obtain relatively pure isotactic polypropylene. The atactic polymer is a wax-like substance that lacks toughness. Also, presence of amorphous materials, or very low molecular weight compounds, causes tackiness

and impedes processing. Table 6.3 lists some of the catalysts and the amounts of crystallinity in polymers that were reported by Natta et al. [37]. To avoid costly purification of isotactic polypropylene, three-component catalyst systems were developed. Some of the original ones appear to have been reported by Natta, himself, who found that addition of Lewis bases enhances the quantity of the crystalline material. Table 6.4 shows the effects of addition of Lewis bases on the amount of crystallinity, reported by Natta et al. [38].

Many other three-component systems were developed since [39-43]. Also, development of more active catalysts [44, 45] eliminates a need to remove them from the finished product [15]. The first improvement in catalyst productivity came from treating TiCl₃ (formed from TiCl4 and Al(C₂H₅) Cl₂) with aliphatic ethers resulting in yields of 520 g of polymer for each gram of Ti [46]. Further improvement was achieved by supporting TiCl, on MgCl, or by producing a supported catalyst by reacting TiCl4 with Mg (OC₂H₅) or with other magnesium compounds. This raised the productivity to over 3,000 g of polymer for every gram of Ti [46]. The products, however, contained low percentages of the isotactic isomer (20-40%). Addition of a Lewis base like N,N,N',N'-tetramethyl ethylenediamine in solid component and ethyl benzoate in solution raised the isotactic content to 93% with a productivity of 2,500 g of polymer per gram of Ti [41]. Claims are made today for much greater catalyst activity. It was reported, for instance, that catalyst efficiencies of 40 kg of polymer per 1 g of Ti can be achieved. Such yields require proper choice of catalysts and control over polymeri- zation conditions. The isotactic fractions in the products are reported to range from 95 to 97% [47-49].

In a catalyst system TiCla/MgCl₂/C₆H₅COOC₂H₅/AI(C₂H₅)₃, the high activity was initially attributed to higher propagation rates rather than to an increase in the concentration of the active sites [50]. The higher activity of these catalysts, however, was shown instead to be due to higher numbers of active centers and only slightly higher values of Kp [51]. Subsequent trends in modifications of supported Ziegler-Natta catalysts consisted of using sterically hindered amines [52-54]. For instance, 2,2,6,6-tetramethylpiperidine might be used together with different trialkylaluminum compounds as modifier-cocatalyst systems for the supported catalysts:

where X represents a halogen. Other analogous amines, like 1,2,4-trimethylpiperazine and 2,3,4,5-tetraethylpiperidine, are also used in preparations of titanium halide catalysts supported on MgCl₂. The amine remains as a built-in modifier in the catalyst system [53]. Subsequent research efforts concentrated on soluble catalytic systems, like di-Z5-cyclopentadie nyldiphenyltitanium and tetrabenzylzirconium complexed with methylaluminoxane, (CH₃)₂Al [–O–Al(CH₃)–]n–Al(CH₃)₂. Such catalysts, however, yield products that contain only about 85% isotactic polypropylene [55–61], and only if the reactions are conducted at low temperatures, 45C or lower. A major breakthrough occurred when rigid chiral metallocene initiators were developed, like 1,1-ethylene-di-Z5-indenylzirconium dichloride, complexed with methylaluminoxane. In place of zirconium, titanium and hafnium analogs can also be used. These catalysts are highly isospecific [62–64] when used at low temperatures. The compounds are illustrated in Chap. 4. Typical catalysts consist of aluminum to transition metal ratios of 103 or 104:1. Many of them yield 98–99% isotactic fractions of the polymer. In addition, these are very active catalysts, yielding large quantities of polymer per gram of zirconium. It was also reported that elastomeric polypropylenes can be formed from the monomer with the aid of some metallocene catalysts [62–64]. Because rigid, chiral metallocene catalysts produce isotactic polypropylene, while the achiral ones produce the atactic form, Waymouth and Coates [62] prepared a bridged metallocene catalyst with indenyl ligands that rotate about the metal–ligand bond axis. The rotation causes the catalyst to isomerize between chivalric and nonchiralic geometries:

Indenyl ligands, however, were found to rotate faster that the polymerization reaction. This prevents formation of stereoregular polymer blocks [62]. To overcome that, phenyl substituents were added to the ligands to slow down the rotation below the speed of monomer insertion, yet rotate faster than the time required for formation of the whole polymeric chain. The product, a catalytic system of bis(2-phenylindenyl)zirconium dichloride plus methylaluminoxane, was found to yield elastomeric block copolymers of isotactic and atactic polypropylene [62]. Vincenzo et al. [63] reported that 13C NMR microstructural analysis of polypropylene samples produced with two representative “oscillating” metallocene catalysts was found to be largely different in steric hindrance. The original mechanistic proposal of an “oscillation” between the two enantio morphous, a racemic-like (isotactic-selective) and a meso-like (non-stereoselective) conformation, according to them, cannot explain the observed polymer configuration. They further feel that isotactic-stereo block nature of the polymers obtained with this catalyst proves unambiguously that the active cation “oscillates” between the two enantiomorphous racemic-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less hindered catalyst gives instead a largely stereoirregular polypropylene, which is the logical consequence of afaster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and point rather to a conformationally “locked” racemic-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counter anion, which would play the role of the inter-annular bridge in the racemic-bis(indenyl) ansa-metallocenes, was tested by computer modeling and found viable. Preparation of elastomeric polypropylenes was also reported by Chien et al. [64]. Two metallocene catalysts of different stereospecificities were used. The isospecific catalyst precursors were either rac-ethylene bis-(1-Z5-indenyl) zirconium dichloride or rac-dimethylsilylene bis(1-Z5-indenyl) zirconium dichloride. The unspecific one was ethylene bis(9-Z5-fluorenyl) zirconium dichloride. The precursors were activated with triphenyl carbenium tetrakis  (pentafluorophenyl) borate and triisobutylaluminum. The resultant catalysts exhibit very high activity, yielding products that range from tough plastomers to weak elastomers [64].

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مواضيع ذات صلة

Manufacturing Techniques

Materials Similar to Polyethylene

Commercial High-Density Polyethylene, Properties, and Manufacture

Preparation of Polyethylene by Coordination Mechanism

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