Syndiotactic Polypropylene

Isotactic polypropylene received most attention because it is commercially more desirable. Nevertheless, syndiotactic polypropylene, though less crystalline, has greater clarity, elasticity, and impact resistance. It melts, however, at lower temperature. This isomer was originally prepared with both, heterogeneous, titanium-based catalysts and soluble, vanadium-based ones. The heterogeneous catalysts gave very low yields of the syndiotactic fractions. In fact, original samples contained only a few percent of the desired material, almost an impurity. The yield of syndiotactic polypropylene increased with a decrease in polymerization temperature, but still remained low [65]. Highly syndiotactic polypropylene was prepared by Nattaetal.[38] with homogeneous catalysts formed from VCl₄ or from vanadium tri-acetylacetonate, aluminum dialkyl halide, and anisole at 48 to 78C.

No isotactic fractions formed. This led to development of many effective soluble catalysts. The catalyst components and the conditions for their preparation are quite important in maintaining control over syndiotactic placement. For the most effective soluble catalyst the ratio of AIR2X to the vanadium compound has to be maintained between 3 and 10 [66]. The organic portion of the organoaluminum compound can beeither methyl, ethyl, isobutyl, neopentyl, phenyl, or methyl styryl [9, 67]. In addition to VCl4 and to vanadium tri-acetylacetonate [66], various other vanadates can be used, like [VO (OR)xCl₃ x], where x = 1,2, or 3 [65]. The exact nature of the vanadium compound, however, is very important to the resultant steric arrangement of the product. For instance, VCl4 combined with Al (C2H5)2F forms a heterogeneous catalyst that yields the isotactic isomer [65]. Vanadium tri acetylacetonate, on the other hand, upon reacting with Al(C₂H₅)2F forms a soluble catalyst that yields the syndiotactic isomer [66]. Addition of certain electron donors increases the amount of syndiotactic placement. These are anisole, furan, diethyl ether, cycloheptanone, ethyl acetate, and thiophene [67]. Theoptimumresultsareobtainedwhenananisoletovanadiumratiois1:1. Also, the highest amount of syndiotactic polymer is obtained when the soluble catalysts are prepared and used atlow temperatures. Even at low temperatures, however, like 78C, the amount of syndiotacticity that can be obtained with a specific catalyst decreases with time [65, 66, 68]. This indicates a deterioration of the syndiotactic placing sites. On the other hand, polymerization of propylene with soluble vanadium tri-acetylacetonate–Al(C₂C₅)2Cl system was reported to be a “living” type polymerization [69]. The product has a narrow molecular weight distribution (Mw/Mn=1.05–1.20). A kinetic study indicates an absence of chain transferring and termination at temperatures below 65C. More recent catalysts for syndiotactic polypropylene are complexes, like i-propyl (cyclopen-tadienyl-1-fluorenyl) hafnium dichloride with methyl aluminoxane [70]. Another, similar catalyst is i-propyl(Z5-cyclopentadienyl-Z3-fluorenyl) zirconium dichloride with methyl aluminoxane. These catalysts yield polymers that are high in syndiotactic material (the zirconium-based compound yields 86% of racemic pentads) [70, 71]. Commercial production of syndiotactic polypropylene is in the early stages. What catalytic system is used, however, is not disclosed at this time. Some of the properties of the two isomers, isotactic and syndiotactic polypropylenes, are compared in Table 6.5. The molecular weights of syndiotactic polypropylenes can vary from a number average molecular weight of 25,000–60,000, depending upon reaction conditions [70]. Also, in isotactic polypropylene there is less than one double bond per 1,000 carbon atoms [72]. A typical Mw/Mn = 5–12.

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المزيد

مواضيع ذات صلة

Synthetic Polyisoprenes

High Molecular Weight Polybutadiene

Liquid Polybutadiene

Polybutadiene

Ionomers

Copolymers of Ethylene with a-Olefins and Ethylene with Carbon Monoxide

Ethylene and Propylene Elastomers

Poly (4-methyl pentene-1)

Polyisobutylene

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene