Monomers coordination

Efforts have been made to determine the approach of monomer onto the catalysts by theoretical modelling. DFT calculations on the ion pair [(1,2- 

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The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

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... 

As a result the polymer chain growth seems to stop due to the impossibility of the monomer coordination on the titanium ion. The propagation centers are nonuniform and the adsorption of the aluminumorganic compound takes place mostly at sterically more accessible nonstereospecific active centers. 

Using a combination of techniques such as EPR, resonance Raman, and MCD spectroscopy, the conversion of [2Fe-2S] into [4Fe—4S] centers has been found to take place under reducing conditions in E. coli biotin synthase 281). The as-prepared form of this enzyme has been thought to contain one [2Fe-2S] center per monomer, coordinated by the three cysteine residues of the motif Cys-X3-Cys-X2-Cys and by a fourth, noncysteinyl ligand. Upon reduction, a [4Fe-4S] cluster bridging two monomers may be formed in the active enzyme. In the reduced state, the [4Fe-4S] center is characterized by a mixture of S = I and S = k spin states giving EPR features at g 5.6 and... 

Several examples have been described in which a chiral natural polymer, such as silk fibroin or chitosan, act as chiral ligand and support at the same time. In such cases, the chiral ligand (the monomer or monomers coordinating... 

Increasing steric bulk at one of the NHC side groups will cause interference with the active site of the complex and generate an amplifying effect on alternation control. A methyl substituted one-carbon spacer like in l-mesityl-3-((rR)-l-phenylethyl)-4,5-dihydroimidazol-2-yhdene is enough to dramatically improve the alternation rate [129]. In Fig. 3.31 the steric impact on monomer coordination for different situations is rationahsed. 

The quantity Q therefore is the sum of two quantities, the number of polymer chains attached to metal atoms which are active in polymerization (Si CDn]) and which have the structure (IX), and species with structure (XXV) which are inactive (Xa [Z ]). In addition there are those chains with structure (IX) with monomer coordinated to the metal (52 [ E ]) these are considered to be negligibly small compared to The... 

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

By the time the concentration of monomer is low, the back-skip of the polymer chain to the less-hindered site is faster than the formation of the high-energy alkene coordinated intermediate (IV). For this reason, at low propene concentrations and elevated temperatures isotactic sequences are formed. The probability of monomer coordination at the aspecific site (IV) is enhanced when the propene concentration increases. The consequence is that single stereoerrors [mrrm] are introduced in the isotactic polymer chain. 13C-NMR was able to prove the mechanism because a... 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

Outer Sphere Electron Transfer Monomer Coordination Disproportionation... 

The absolute-energy minima labeled as a in Figures 1.7a,b correspond to 0i —60° for the re monomer coordination and are sketched in Figures 1.4 and 1.6a, respectively. These models minimize the interactions between the growing chain (at 0i —60°) and the methyl of the propene monomer (re coordinated). Therefore, these are assumed to be preinsertion intermediates suitable for the re monomer primary insertions. 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

The energy minima between the energy barriers for the monomer coordination and insertion correspond to alkene-bound intermediates of the kind simulated by our molecular mechanics calculations (Figures 1.7 and 1.9). The possible dissociation of the monomer coordinated with the wrong enantioface can lead back to the alkene-free intermediate or, directly, to the alkene-bound intermediate with the right enantioface (through some isomerization mechanism, for which the monomer does not leave the coordination sphere of the metal). 

Scheme 7.5 Monomer coordinated and monomer free intermediates of a widely accepted mechanism for d.v-l, 4 and 1,2 polymerizations of conjugated dienes.

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 driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

C -symmetric initiators have a pair of diastereotopic (nonequivalent) sites one site is sterically crowded and enantioselective, and the other site is less crowded and nonselective. The propagating polymer chain always prefers the less crowded site, but monomer coordination and migratory insertion occur at the more crowded enantioselective site. The polymer chain then back-flips to the less crowded site. This model offers a rationale for the back-flip of the polymer chain—the polymer chain is less stable at the more crowded site. 

Polymerization of the complex monomer is also interesting as a method for controlling the reactivity of the vinyl monomer and the steric configuration of the resulting polymer. Osada35,36) has reported the radical polymerization of methacrylate (MA) monomers coordinated to Co(III) complexes [Co(NH3)(MA)](C104) 2 26 and crs-[Co(NH3)4(MA)2]C104 27. It was found that the complex monomers 26... 

Since monomer coordination is required for polymerization, the gel modification additive can also slow the polymerization reaction by competing with the monomer for coordination sites on the metal center. 

Here, the constants KM and ks denote the equilibrium constant for the propylene monomer coordination and the rate constant for the insertion of the coordinated monomer, and L represents a ligand bound to vanadium. Then, the energy diagram of chain propagation reaction with propylene may be visualized as in Fig. 13. The... 

A clear consensus47 156>166) has emerged which indicates that various extents of ether complexation with active centers can reduce their reactivity in the chain propagation event. If cation-monomer coordination is important, the presence of ether in the coordination sphere might be expected to lead to less monomer interaction with a subsequent reduction in polymerization reactivity. Clearly, there is a need for further work, experimental and theoretical, on this topic. 

M-C as propagating species, 4, 1008 monomer coordination and insertion reactions, 4, 1010 monomer insertion regio- and stereochemistry, 4, 1015 overview, 4, 1005-1166 regioirregular insertions, 4, 1023 stereocontrol mechanism, 4, 1018 stereocontrol symmetry rules, 4, 1020 stereoregular polymers, 4, 1016 in Ru-Os heterodinuclear compounds, 6, 1046 in Ru-Os mixed-metal clusters, 6, 1064 semiconductor growth, conventional precursors, 12, 2 with silicon, 3, 514... 

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