Ziegler Natta reactivity

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

AH higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerise both with other a-olefins and with ethylene (51,59). In these reactions, higher a-olefins are all less reactive than ethylene and propylene (41). Their reactivities in the copolymerisation reactions depend on the sise and the branching degree of their alkyl groups (51) (see Olefin polya rs, linear low density polyethylene). 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Low molecular weight liquid nitrile rubbers with vinyl, carboxyl or mercaptan reactive end groups have been used with acrylic adhesives, epoxide resins and polyesters. Japanese workers have produced interesting butadiene-acrylonitrile alternating copolymers using Ziegler-Natta-type catalysts that are capable of some degree of ciystallisation. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

Thus, in the presence of methylaluminoxane (MAO) at 23°C, (C5H5B-N(/-Pr)2)2 ZrCl2 polymerizes ethylene with an activity of 105 kg ofpolyethylene/(h [Zr] mol), similar to that observed with well-studied Cp2ZrCl2 as the catalyst. It is believed that MAO is functioning in its usual role in these Ziegler-Natta polymerizations (methylation of Zr and abstraction of methyl to form a highly reactive Zr cation).41... 

Propagation proceeds in a manner similar to that described for the traditional Ziegler-Natta initiators. The transition metal has two active sites—the polymer chain is held at one site (the one occupied by a methyl group in XXVII) and monomer at the other site (shown as the vacancy ). The reactivity of the active sites is high because the counteranion, which is either (ClMAO) or (OR MAO) or a mixture of the two, is a weakly coordinating anion. Reactivity is decreased when the counterion is strongly coordinating. 

It might be expected that the bond angle involving the centroids of the cyclopentadienyl rings and the metal center would anticipate the amount of space available to an incoming olefin and, therefore, reasonably expected to correlate with reactivity and/or selectivity of Ziegler-Natta processes. 

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

All higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerize both with other a-olefins and with ethylene. In these reactions, higher a-olcfins arc all less reactive than ethylene and propylene. 

While this review discloses the kinetic and stereochemical features of soluble Ziegler-Natta catalysts, we have little information on the structure of the active center. The steric environments of active centers must be very important in determining the monomer reactivity, regiospecificity and stereospecificity of soluble catalyst. The influence of ligands such as the aluminum components on the rates of chain propagation and chain-terminating steps should be correlated to the electronic structure of... 

The interest in the dimers is double. Dimerisation is a first model for the surface of the solids, increasing the coordination of the atoms without changing the oxidation states. Dimers placed on support (epitactically on the lateral surfaces of MgCl2 crystals or on the comers or edges of crystallites) have also supposed to be the reactive sites for the Ziegler-Natta reaction [40, 41]. 

They are used as a component in the Ziegler-Natta catalytic system (AIR TiCh, etc.) in the polymerisation and copolymerisation of olefines. Besides, the high reactivity of these compounds makes them practical for many interesting syntheses (Fig. 87). 

Supports used for obtaining Ziegler-Natta catalysts can differ essentially from one another. Some of the supports may contain reactive surface groups (such as hydroxyl groups present in specially prepared metal oxides) while others do not contain such reactive functional groups (such as pure anhydrous metal chlorides). Therefore, the term supported catalyst is used in a very wide sense. Supported catalysts comprise not only systems in which the transition metal compound is linked to the support by means of a chemical covalent bond but also systems in which the transition metal atom may occupy a position in a lattice structure, or where complexation, absorption or even occlusion may take place [28]. The transition metal may also be anchored to the support via a Lewis base in such a case the metal complexes the base, which is coordinatively fixed on the support surface [53,54]. 

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