Steric Control in Polymerization of Conjugated Dienes

The subject has also received considerable attention. Nevertheless, the mechanism is still not fully understood. It is reasonable to assume that the form and structure of the catalyst and the valence of the transition metals must play a role. The conformation of the monomer (s-cis or s-trans) is probably also important. It is known that CoCl₂/Al(C₂H₅)Cl can polymerize trans-1,3-pentadiene but not the cis isomer. This suggests that a two-point coordination is required. Several reaction schemes that provide for an attack at either C₁ or at C₄ positions were proposed over the years . One mechanism for polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may be bound by either a p or as linkage, and it is suspected that a continuous σ →π isomerism is possible [264]:

where, MT means metal.

Soluble catalysts from transition metal acetone acetonates (from nickel to titanium) combined with triethylaluminum or triethylaluminum chloride yield cis polymers of the dienes. The cis placement decrease when bases are added [265]. This decrease is proportional to the base strength. The addition is believed to decrease electron densities of the orbitals of the transition metals . This suggests that electrostatic interactions between the nearly non-bonding electrons of transition metals with the dienes or with growing chain ends must play an important role. Such interactions must affect placements of the incoming monomers. Hirai et al. studied ESR signals obtained during polymerizations of butadiene, 1,3-pentadiene, and isoprene  with catalyst based on n-butyltitanate / triethylaluminum. They concluded that the catalyst must possess two substituted p-allyl groups and one alkoxy group during the chain growing process [267]:

The insertions of the monomers are believed to occur in two steps . In the first one, the incoming monomer coordinates with the transition metal. This results in formation of a short-lived o-allylic species. In second one, the metal-carbon bond is transferred to the coordinated monomer with formation of a л-butenyl bond. Coordination of the diene can take place through both double bonds, depending upon the transition metal  and the structure of the diene. When the monomer coordinates as a monodentate ligand, then a syn complex forms. If however, it coordinates as a bidentate ligand, then an anti complex results . In the syn complex, carbons one and four have the same chirality while in the anti complex they have opposite chiralities . Due to lower thermodynamic stability the anti complex isomerizes to a syn complex . If the allylic system does not have a substituent at the second carbon, then the isomerization of anti to syn usually occurs spontaneously even at room temperature [268].

Transition metal alkyls probably cannot be classified as typical Ziegler-Natta catalysts. Some of them, however, exhibit strong catalytic activity and were, therefore, investigated . This also led to the conclusion that the polymerization mechanisms involve formations of л-allylic complexes as intermediates. It is similar to the mechanism visualized for the Ziegler-Natta catalysis . The initial formation of the л-allylic ligands and the solvents used in catalyst preparations strongly influence the catalytic activity and stereospecificity of the product . NMR studies of polymerizations of conjugated dienes with π-crotуl-nickel iodide  showed that the monomers are incorporated at the metal-carbon bonds with formations of syn-π-crotуl ligands. The syn-ligands transform to trans-1,4-segments next to the crotyl group and the trans-1,4-segments become trans-1,4-units in the polymers [282]. In summary, the general mechanisms of cis and trans placement by coordinated catalysts were pictured as follows [280]:

where, L means ligand and MT means transition metal. It is possible to apply the homogeneous rate equation to some Ziegler–Natta catalysts. This can be expressed as: Rp = kp(C ) (M) where C* represents the concentration of active sites in moles per liter. Heterogeneous kinetics, however, are needed because the adsorption phenomena are important in most coordinated anionic polymerizations. In addition, because there is likely to be an excess of components of Groups I–III metals in solution, it is assumed that they compete with the monomer for the active sites. The fractions, therefore, of the active sites and the fractions of sites covered by these metal compounds are expressed as follows : Fraction of the active site coordinated with the monomer:

Fraction of active sites coordinated with components of metal Groups I–III:

where M represents the amount of monomer in solution and MT the concentration of metal components of Group I–III. K1 and K2 are the equilibrium constants for the adsorption. The rate of polymerization that takes place trough the reaction of the adsorbed monomer at the active sites can be expressed as:

If there is no competition with components of metal Group I–III, then

The degree of polymerization can be derived by dividing the propagation rate by the sum of the rates of terminations (by transfer) as follows:

The above equation is written with the assumption that there is no adsorption of hydrogen at the active sites.

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مواضيع ذات صلة

Polymerization of Aldehydes

Isomerization Polymerizations with Coordination Catalysts

Reduced Transition Metal Catalysts on Support

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization