Vacant sites migration

Interstitial diffusion is rarely possible when two metals interdiffuse, since their atomic radii are usually of the same order. Several mechanisms have been proposed, but it is now generally accepted that interdiffusion is due to the motion of vacant sites within the lattice, solvent and solute atoms moving as the vacant sites migrate. The diffusion process is thus dependent upon the state of imperfection of the solvent metal and the alloy being formed. 

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. 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

Figure 4,6 Migration of a cation M from an occupied site to a vacant site in an ionic crystal MO. Trajectory passes through centers of equilateral triangles O1-O4-O3 (X ) and O2-O4-O3 (X2).

Figure 8-11 shows the proposed mechanism for isoselective propagation. Monomer coordinates at the vacant site of titanium, resulting in a four-center transition state and subsequent insertion of monomer into the polymer-transition metal bond. The insertion is referred to as migratory insertion since the polymer chain migrates from its original site to that occupied... 

The linear variation of the CEC with the Li concentration also supports the incorporation and migration of the Li cations in the octahedral vacant sites. 

Dissociation of DIBAH from (16) then creates a vacant site for CO coordination. Hydride migration from Zr or external attack by hydride from DIBAH leads to the formation of a formyl that is then reduced by the aluminum hydride species (81). The steps for chain propagation and termination are outlined below. Since the exact nature of the Zr and A1 species is unknown in these steps, they are represented by (Zr) and (Al), respectively. 

The stages of migration of adsorbed A and B particles are written as (5) jZf+YZg<-+YZf+jZg, where j — A, B / and g are adjacent sites, V is a vacant site (a vacancy). The index a corresponds to the indicated stage numbers. It is enough to consider the interactions of the first and second neighbors in the quasi-chemical approximation. There are two possibilities of the equation constructions for the distributed two-dimensional model, and for point models. In the last subsection the next question will be discussed - How the form of the systems of equations alters for a great difference in the mobilities of the reactants ... 

Remember that the polymerisation mechanism devised by Cossee [268] implies two main steps coordination of the monomer at the titanium vacant site with the double bond parallel to the Ti-C bond, and chain migratory insertion of the coordinating monomer molecule (with migration of the growing polymer chain to the position previously occupied by the coordinating monomer molecule) isospecificity of the active site is assumed only if the polymer chain skips back to the original position before further insertion [scheme (50)]. 

The migration reaction diminishes the total electron count of the complex by two, and creates a vacant site at the metal P-elimination does the opposite. P-Elimination requires a vacant site at the metal centre, and the electron count of the complex increases by two electrons during the process. The reaction resembles the P-elimination reaction occurring in many organic processes, but the difference lies in the intramolecular nature of the present process, as the eliminated alkene may be retained in the complex. In organic chemistry the reaction may well be a two-step process, e.g. proton elimination with a base followed by the leaving of the anion. In transition metal chemistry, however, it is the availability of d orbitals that greatly facilitates a concerted cis P-elimination. 

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