Propylene intrinsic activity

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

With vinyl chloride, the decrease in intrinsic viscosity of the homopolymer that forms during the reaction with an increase in the initial amount of rubber was far more marked with EPDM elastomers than with saturated ethylene-propylene copolymer (Figure 1). This suggests that EPDM elastomers have a high radical activity, even with small amounts of ENB. 

Different independent approaches were used to investigate the mechanism of polymerization of a-olefms with heterogeneous catalysts. As a result, it was shown that isotactic polymerization of racemic mixtures of a-olefms are stereoselective. Also, optically active polymers form with optically active catalysts.Stereoelectivity and stereoselectivity are due to the intrinsic asymmetry of the catalytic centers. This conclusion comes in part from knowledge that propylene coordinates... 

The approach, based on dilution of catalytically active components with inert or low active material, has been employed to improve the performance of VSb-based oxide catalysts in reactions of the selective oxidation of propane. In particular, the incorporation of low active A1 into the body of NiVSb, FeVSb, BiVSb and other mixed-metal oxides significantly improved their selectivity in the oxidative dehydrogenation of propane to propylene. As is seen in Fig. 11.5, the addition of A1 to the Feo,3 Vi.oSbo.eOx catalyst increased selectivity to propylene by 2-3 times. The selectivity of an Al-diluted catalyst was further improved by adding small amounts of alkali and alkaline earth metals. Figure 11.5 illustrates the effect of potassium which, along with increasing catalyst intrinsic selectivity, also decreased propylene over-oxidation. This improvement in selectivity has been related to the lower acidity... 

PP (mol-Ti-which is about one-fifth of the activity of 1/MAO. Radiolabeling determination found that about one-fourth of the Ti in the supported catalyst is active. Therefore, the difference in the activities of the heterogeneous and homogeneous catalysts may be attributed to lower utilization of Ti in the former, and that the two catalysts have comparable intrinsic propylene polymerization rates for their active species. 

It is evident from the previous section that in the multiple-site model the copolymer is a mixture of copolymers of different composition this fact alone accounts to a some extent for the experimentally determined copolymer blockiness. However, a study of the copolymer fractions demonstrates that it cannot explain all the results. The blockiness of the copolymer fractions remains high therefore the tendency to block formation is an intrinsic property of active sites rather than merely the result of copolymer mixing. For example, when the pro-pylene-butene-1 copolymer was Ihictionated (156), the resulting narrow fractions of composition Cp, 30-50% exhibit X-ray diffraction curves containing crystallinity peaks of both polypropylene and polybutene-1 type. An analogous result was demonstrated us (66) the blockiness of the fractions of propylene-4-methylpentene-l copolymers insoluble... 

However, acetalization of larger aldehydes such as 2-acetonaphthone with propylene glycol in tixe presence of microporous zeolites did not go well then compoxmd 6, with blossom orange scent, was selectively obtained in the presence of ITQ-2, a delaminated zeolite witix a very large and structured external surface (Scheme 2). The hydrophobic characteristics of the catalysts are as important as the concentration of their catalytically active acid sites. Zeolites were intrinsically more active than the delaminated and mesoporous materials in die acetalization of small size carbonyl compoxmds however, the use of substrates within larger sizes decreased the rate of diffusion of reactants due to geometrical constraints. 

Some very recent first-principle calculations together with kinetic Monte Carlo simulations have shown that the MBs with five or six methyl groups are not more active than those with fewer methyl groups [103], Propylene is intrinsically more favorable than ethylene when the reaction is not diffusion limited based on a side-chain hydrocarbon pool mechanism. The theoretical results are consistent with some experimental observations and can be rafionafized based on the shape selectivity of key reaction intermediates and transition states in the pore of catalyst [61,103],... 

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