Mechanism monometallic

While a limited amount of experimental evidence does lend support to the bimetallic concept, majority opinion favors the second and simpler alternative, the monometallic mechanism. 

It is generally accepted that the J-orbitals in the transition element are the main source of catalytic activity and that it is the Ti-alkyl bond that acts as the polymerization center where chain growth occurs. The function of the aluminum alkyl is thus only to alkylate TiCls. The monometalhc mechanism described below is the one based mainly on the ideas of Cossee and Arlman (1964). The quantum 

According to the mechanism, the active center is formed by the interaction of aluminum alkyl with an octahedral vacancy around Ti. For or-TiCls catalyst the formation of active center can be represented as shown in Fig. 9.3. To elaborate, the five-coordinated Ti on the surface has a vacant J-orbital, represented by -Q, which facilitates chemisorption of the aluminum alkyl followed by alkylation of the Ti ion by an exchange mechanism to form the active center TiRCU-Q. The vacant site at the active center can accommodate the incoming monomer unit, which forms a r-complex with the titanium at the vacant i-orbital and is then inserted into the Ti-alkyl bond. The sequence of steps is shown in Fig. 9.4 using propylene as the monomer. 

After the monomer is inserted into the Ti- Ukyl bond, the polymer chain migrates back to its initial position, while the vacant site migrates to its original position to accept another monomer molecule. This migration is necessary, as otherwise an alternating position would be offered to the monomer leading to the formation of a syndiotactic polymer instead of an isotactic polymer. 

Problem 9.1 From the accurate kinetic data that have been obtained (Natta and Pasquon, 1959) for the polymerization of CsHg with or-TiCls and A1(C2H5)3 it appears that in the steady state the rate is strictly proportional to the pressure of CsHg. The polymerization rate is also proportional to the amount of or-TiCls and independent of the concentration of A1(C2H5)3. Suggest a kinetic scheme in conformity with these observations. A qualitative use may be made of the fact that an activation energy of 11-14 kcal/mol has been observed for this polymerization and that no stable complex between a-olefins and Ti has been found. 

S + M SM and SM — S, respectively, where S is the vacant site, SM is the complex between vacant 2 

Although limited experimental evidence does lend support to this concept, major objections have been voiced by Ziegler, who is of the opinion that as dimeric aluminum alkyls are inefficient catalysts in the Aufbau reaction, the Ti-Al complex is not likely to be the effective catalytic agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

The first stage is the formation of the active center, illustrated here using a-TiClj as the catalyst. The suggestion is that alkylation of the 5-coordinated TP+ ion takes place by an exchange mechanism after chemisorption of the aluminum alkyl on the surface of the TiClj crystal. The four chloride ions remaining are the ones firmly embedded on the lattice, and the vacant site is now ready to accommodate the incoming monomer unit. The reaction is confined to the crystal surface, and the active complex is purely a surface phenomenon in heterogeneous systems. 

The attacking monomer is essentially nonpolar bnt forms a ji-complex with the titaninm at the vacant d-orbital. A diagram of a section of the complex shows that the propylene molecnle is not mnch bigger than a chloride ion and, conseqnently, the donble bond can be placed adjacent to the Ti ion and practically as close as the halide. After insertion of the monomer betweai the Ti-C bond, the polymer chain then migrates back into its original position ready for a farther complexing reaction. 

New active center Active center (d) Transition state (c) 

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The monometallic mechanism is illustrated in Fig. 7.13a. It involves the monomer coordinating with an alkylated titanium atom. The insertion of the monomer into the titanium-carbon bond propagates the chain. As shown in... 

Figure 7.13 (a) The monometallic mechanism. The square indicates a vacant... 

Fig. 3. Monometallic mechanism of formation for polymer chain growth on transitionmetal catalyst, where (D) represents a coordination vacancy.

The actual mechanism by which the propagation occurs and the factors governing the formation of stereoregular polymers is state debatable. Among the several mechanisms proposed, the bimetallic mechanism of Natta and the monometallic mechanism of Cossee have received much attention. Cossee s... 

The monometallic mechanism proposed by Cossee assumes that the active centre is at the Ti-R part of the catalyst, while the aluminium alkyl acts only as an alkylating agent for the TiCl3. When the catalyst... 

Strong support for the monometallic mechanism was furnished by crystal structure studies303 and labeling experiments.311-315 It was observed, for instance, that ethylated titanium reacting with 13C-labeled ethylene yielded polymers with unlabeled chain end.314 315 In another experiment TiCl3, when methylated with... 

Variations in monometallic and bimetallic mechanisms have been proposed. The monometallic mechanisms seem to be inherently simpler than bimetallic mechanisms except for requiring a migration step. On the other hand, bimetallic mechanisms that consider the presence of the activator in the polymerisation system can be more convincing in some instances. 

In 1952, it was discovered by Schiller that rhodium salts generated highly active hydroformylation catalysts. It was from these early studies that rhodium was estimated to be 1000 to 10 000 times more active than cobalt. Rhodium was also found to be very selective to aldehydes, with httle hydrogenation to alcohols observed under normal catalysis conditions. It was suggested early on that HRh(CO)4 was the active catalyst species, analogous to HCo(CO)4, and the same monometallic mechanism was proposed (Scheme 6). 

The above discussion is not intended to conclude that a center with the same stereospecificity should exhibit the same kp value for various catalyst systems (other conditions being the same). It has been reported that the value depends on the type of organometal (e.g., 10 2S 61,8J) and catalyst support (e.g.3.98 117-119). These findings are not consistent with a simple monometallic mechanism concept, but this topic is out of the present paper scope. 

Figure 9.3 Interaction of aluminum alkyl with an octahedral vacancy around Ti in the first stage of monometallic mechanism. (After Ref. 11.)...

Arlraan [10], The monometallic mechanism was put on a sound theoretical basis by the quantum theory developed by Cossee [11). 

The main features of the monometallic mechanism are (1) an octahedral vacancy on the Ti " " is available to complex the olefin (2) the presence of an alkyl to transition metal bond at this site is required and (3) the growing polymer chain is always attached to the transition metal. 

The most important characteristic of Ziegler-Natta catalysts is their ability to produce stereoregular polymers. On the basis of the monometallic mechanism of Cossee and Arlman, described above, stereoregulation of propylene polymerization can be explained as follows. 

The monometallic mechanism of Cossee and Arlman [10] for Ziegler-Natta polymerization has found favor in the literature because it is based upon quantum mechanical considerations rather than on agreement with the kinetic data [5]. According to this mechanism, as described earlier and shown in Figs. 9.3 and 9.4, the initiation process involves interaction of aluminum alkyl with an octahedral ligand vacancy around Ti which results... 

The basic feature of proposals for the monometallic mechanism is that propagation occurs entirely at one metal center. A monometallic mechanism involving titanium in a lower valence state, for example, RTiCl, has been proposed (63) to be an active site for ethylene polymerization with propagation occurring by coordination and insertion into the titanium-carbon bond (Reaction 12). 

Figure 9.4 Monometallic mechanism for stereospecific polymerization. (After Cossee, 1967.)...

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