Coordination polymerization basic mechanism

The basic mechanism of metallocene-based polymerization involves a catalytic cycle very similar to that of Fig. 6.5. The precatalysts 6.22 and 6.23, in combination with MAO, produce polypropylene of high isotacticity and syndiotac-ticity, respectively. As shown in Fig. 6.7, 6.22 has C2 symmetry and is chiral, while the symmetry of 6.23 is Cs and is therefore achiral. Two points need to be noted before we discuss the mechanism of stereospecific insertion of propylene. First, propylene is a planar molecule that has two potentially nonequivalent, prochiral faces (see Section 9.3.1). Second, the symmetry around the metal atom determines whether or not coordinations by the two faces of propylene are equivalent. 

A serious problem in the control of silica polymerization is that, in general, initial particle formation is a result of random bond formation between a range of polysilicate species in solution. Moreover, the aggregation process is uncontrolled under conventional preparation conditions. It is important to understand the basic mechanisms of particle formation and aggregation in aqueous solution, as the physical properties of such silicas are determined primarily by the coordination number and nature and strength of interactions between essentially spherical primary particles. 

Rh138 was almost the same (almost atactic, slightly syndiotactic) as the tacticity of those obtained with conventional radical initiators such as AIBN under similar conditions. The triad ratio of rr.mr.mm as determined by 13C NMR is usually 58 38 4 and does not change even with the use of chiral and/or bulky ligands.103116 These results may exclude a coordination mechanism and suggest a radical nature. However, the stereochemical structure alone is not strong evidence for the radical polymerization because, for example, group-transfer polymerization, basically via an anionic mechanism, results in a stereo structure of PMMA similar to those for free radical processes.263... 

Two basic mechanisms of fixation of a homogeneous catalyst onto a polymeric support have been cited. The first involves different means of catalyst component precipitation, impregnation, inclusion in gel, microencapsulation, etc. The second concerns fixation of catalytic sites by valence, coordination and ionic bonds, immobilization, ion exchange, etc. 

In fact, it beccroes obvious that we have underestimated the refinements of these controls, and in particuler the determinant role of rather small modifications in the overall gecroetry of the oonplexes in the reaction mixture itself. The following chapters will be devoted to the analysis of these controls at different levels, cifter a critical discussion of the main basic mechanisms v ch have been proposed to account for the mciin features of olefins and diolefins polymerization by coordination oatiDlexes. 

Vandenberg, following a particularly penetrating line of research, used the polymerization of cis- and trans-2,3-epoxybutane to distinguish the polymerization mechanisms of oxirane coordination polymerization and then to generalize this basic mechanism to monosubstituted oxiranes (95). The mechanism... 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

Zinc and cadmium alkyls have not been successful as stereospecific catalysts in the absence of co-catalysts, presumably because they do not complex strongly enough with the monomer and the metal-carbon bonds are too covalent. Cadmium alkyls were first reported by Furukawa and coworkers (260) to induce vinyl polymerization, but it was shown later (267, 262) that oxygen was a co-catalyst and the reactions were free radical in nature. Similar free radical results were obtained with zinc alkyls (261—263) and vinyl monomers. However, with more basic and more easily polarized monomers, such as olefin oxides and aldehydes, the zinc catalysts operate by a coordinated anionic mechanism (250). 

Titanium compounds with MAO or borate as co-catalysts effectively produce syndiotactic polystyrene from styrene monomer. The design of high-performance catalyst systems is now well demonstrated. The basic structure of the active site, the mechanism of coordination and insertion and the kinetics are also now well understood for this new polymerization. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

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). 

Other challenges not only in metal alkoxide catalysis but in catalytic processes generally are development of catalytic protocols which on one hand could work in solvent free condition or in green solvent such as water or liquid CO, and on the other, could recover without loss of its activity. Supporting metal alkoxide onto the inorganic solids [47,49, 50] especially magnetic ones [38] can effectively solve the later one. Reported results (Tables 7.1-7.4) are also showed that in most cases solvent free conditions made products with better properties especially in the catalytic polymerization processes. In these processes, coordinative solvents drastically reduced the quality of product because they competed with monomer to coordinate to the metal core. This step is the basic process in coordination-insertion mechanism in ROP reactions [4, 11, 31, 54]. 

There appears to be a basic difference in the polymerization mechanism below and above pH 2. (See also foregoing section on the isoelectric point.) Above pH 2, the rate of disappearance of monomer is a second order reaction below 2 it is third order. The order of the reaction has been explained by Okkerse (91) on the basis that silicon increases its coordination number to 6, as a three-silicon intermediate is formed below pH 2, shown at B in Figure 3.13. 

Polymerization Mechanism. The basic mechanistic steps of olefin coordination to the metal center and the subsequent insertion into the metal-carbon bond are not entirely understood. Several plausible hypotheses have been... 

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