Heterogeneous titanium

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

As an example of the use of MIXCO.TRIAD, an analysis of comonomer triad distribution of several ethylene-propylene copolymer samples will be delineated. The theoretical triad Intensities corresponding to the 2-state B/B and 3-state B/B/B mixture models are given In Table VI. Abls, et al (19) had earlier published the HMR triad data on ethylene-propylene samples made through continuous polymerization with heterogeneous titanium catalysts. The data can be readily fitted to the two-state B/B model. The results for samples 2 and 5 are shown In Table VII. The mean deviation (R) between the observed and the calculated Intensities Is less than 1% absolute, and certainly less than the experimental error In the HMR Intensity determination. 

Note that the catalyst of Figure 10.6 contains the same titanium part as that of Figure 10.5, but that they differ in the aluminium Lewis acid and anions formed. The use of diethylaluminium chloride (DEAC, the common initiator for heterogeneous titanium catalysts) gave propene dimerisation only. This... 

Ziegler-Natta Catalysts. All isotactic polymers of higher a-olefins are produced with the same type of heterogeneous, titanium-based Ziegler-Natta catalyst systems as that used for the manufacture of isotactic PP. 

Rrijnen, S., Mojet, B. L., Abbenhuis, H. C. L., Van Hooff, J. H. C. and Van Santen, R. A. MCM-41 heterogenized titanium silsesquioxane epoxidation catalysts a spectroscopic investigation ofthe adsorption characteristics, Phys. Chem. Chem. Phys., 1999,1,361-365. 

Table 4.3 Polymerisation of styrene using various heterogeneous titanium-based catalysts activated with methylaluminoxane 1...

Two variations of the process are used, the only essential difference being the catalyst employed in the epoxidation step. In the Arco (Atlantic Richfield) process a homogeneous molybdenum catalyst is used. The Shell process employs a heterogeneous titanium/silica catalyst. 

In this context, the development of the heterogeneous titanium silicalite (TS-1) catalyst, by Enichem in the mid-1980s was an important milestone in oxidation catalysis. TS-1 is an extremely effective and versatile catalyst for a variety of synthe-... 

Values in the literature relate to olefin polymerization with heterogeneous titanium based catalysts where the necessary information has been acquired, and the data are given in Table 9. It would not appear to... 

The Oxirane process is a mature technology that has stood the test of time. Both ARCO and Shell have been successfully operating for more than two decades. More recently a heterogeneous titanium-substituted silicalite (TS-1) catalyst was developed by Enichem [43, 44]. In contrast to the Shell Ti /Si02 catalyst, TS-1 has a hydrophobic surface and is a remarkably effective catalyst for a variety of liquid-phase oxidations with 30 % aqueous hydrogen peroxide, including epoxidation [44]. It has been commercialized for the hydroxylation of phenol to... 

T. Maschmeyer, M. C. Klunduk, C. M. Martin, D. S. Shephard, J. M. Thomas, B. F. G. Jolmson, Modelling the active sites of heterogeneous titanium-centred epoxidation catalysts with soluble silsesquioxane analogues, Chem. Commun. (1997) 1847. 

Fig. 1. Illustrative molecular weight distributions of polyethylene prepared by soluble vanadium and heterogeneous titanium catalysts...

All these measurements are internally consistent and lead irrevocably to the conclusion that the heterogeneous titanium halide catalysts are... 

HCL Abbenhuis, S Krijnen, RA van Santen. Modelling the active sites of heterogeneous titanium epoxidation catalysts using titanium silasequioxanes Insight into specific factors that determine leaching in liquid-phase processes. Chem Common... 

Transition Metai-Catalyzed Epoxidation with Alkyl Hydroperoxides. Alkyl hydroperoxides are attractive oxidants on a technical scale because they can be produced by autoxidation of branched alkanes with oxygen. This concept has been realized on the largest scale in the so-called Halcon process, i.e., the transition metal-catalyzed epoxidation of propylene to propylene oxide (35) (Fig. 9). Homogeneous and heterogeneous titanium, vanadium, and molybdenum catalysts are capable of catalyzing the C=C-epoxidation by alkyl hydroperoxide (for a review see Ref. 36). 

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 a 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. ... 

Since the development of titanium silicate (TS-1) materials as catalysts in the 1980s, heterogeneous titanium-catalysed oxidation reactions utilising aqueous hydrogen peroxide have been used in many effective and versatile reactions such as olefin epoxidation, " alcohol oxidation and phenol hydro)q7lation, which adhere more closely to the principles of green chemistry. 

Rather than resorting to heterogeneous supported ligands, Choudaiy et al. used heterogeneous titanium-pillared montmorillonite instead of titanium tetraisopropoxide. Additional molecular sieves was not required and excellent enantioselectivities and yields were obtained (86% yield and 94% enantiomeric excess for 5). The titanium catalyst can be simply removed by filtration, however, no recycling experiment was reported. 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

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