Vanadium placement

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

X-ray examination showed these polymers to be highly crystalline with the polymer assuming a helical configuration (Fig. 2). A soluble catalyst consisting of a combination of dialkyl aluminum halide and vanadium tetrachloride gives a regular structure with alternating d and 1 placements (syndiotactic) (18). Certain other soluble catalysts or those based on non-crystalline... 

Syndiotactic placements originate from nonbonded interactions between the monomer molecule undergoing insertion and ligands on the vanadium atom. 

Four different stereoisomers are possible for polymer XLII, poly (cyclobutane-1,2-diyl) (Sec. 8-lf). Cis and trans isomers are possible for pol3mier XLin, poly (but-1-ene-1,4-diyl). (XLni is the same polymer obtained by the l,4-pol3fmerization of 1,3-butadiene— Sec. 8.10). Traditional Ziegler-Natta initiators based on vanadium and metallocene initiators yield polymerizations almost exclusively through the double bond. Titanium, tungsten, and ruthenium initiators yield predominantly ROMP with varying amounts of cis and trans placements. 

The 1,2-disubstituted olefmic monomers will usually not homopolymerize with the Ziegler-Natta catalysts. They can, however, be copolymerized with ethylene and some a-olefins. Due to poorer reactivity, the monomer feed must consist of higher ratios of the 1,2-disubstituted olefins than of the other comonomers. Copolymers of cw-2-butene with ethylene, where portions of the macromolecules are crystalline, form with vanadium-based catalysts. The products have alternating structures, with the pendant methyl groups in erythrodiisotactic arrangements. Similarly, vanadium-based catalysts yield alternating copolymers of ethylene and butadiene, where the butadiene placement is predominantly rm/w-1,4. ... 

Highly syndiotactic polypropylene was prepared by Natta and co-Workers with homogeneous catalysts formed from VCU or from vanadium triacetylacetonate, aluminum dialkyl halide, and anisole at -48 to -7 8 °C. No isotactic fractions formed. This led to the 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 must be maintained between 3 and 10. The organic portion of the organoaluminum compound can be either methyl, ethyl, isobutyl, neopentyl, phenyl, or methylstyryl. " In addition to VCI4 and to vanadium triacetylacetonate, various other vanadates can be used, like [ VO(OR)/ l3 c], where x = 1,2, or 3 The exact nature of the vanadium... 

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