Enantioface-selective polymerization

Properties of polymeric systems with high enough molecular weight strongly depend on the molecular weight distribution. Living anionic polymerization, established by M. Szwarc in 1956 has been utilized to control these factors in many carbon-based polymers. Stereochemical structure is another very important factor to control the polymer property. Such control has usually been achieved by enantioface-selective polymerization of unsaturated carbon-carbon bonds for carbon-based polymers, typically seen in stereospecific olefin polymerization. 

Use of the optically resolved complex leads to the optically active polymer, but this property, which arises from the helical chain structure, is found only in the swollen polymer and is easily lost in toluene or dichloroacetic acid solution 144). The polymerization occurs with a high degree of enantioface selection, and the model for the product backbone is indeed chiral. However, because of the presence of a mirror, plane in the polymer chain (effects of chain termini neglected), the product does not have chiral properties in solution. 

A few reports have highlighted the increase in enantioface selectivity on going from propylene to 1-butene, in both chain-end-controlled and site-controlled polymerizations. Basell researchers have recently... 

Whereas poly(a-olefins) have only two microstructures of maximum order (isotactic, syndiotactic). cyclopolymers ° have /bur microstructures due to the rings present in their main chain which can be either cis or trans in configuration (Scheme 17). While the key issues concerning selectivity in the polymerization of a-olefins are regioselectivity (head-to-tail monomer incorporation) and enantioface selectivity (tacticity). cyclopolymerization of a.co-diolefins has added concerns. First, since the monomer has two olefins, either cyclization or cross-linking of the... 

Because of the mechanism of enantioface selectivity and the two-site, chain migratory insertion mechanism, the microstructure of a poly(l-olefin) made with a given metallocene is, to a large extent, predictable. In a series of landmark papers, Ewen and co-workers and Kaminsky and co-work-ers described a series of stereoselective metallocene catalysts which define what are now referred to as Ewen s symmetry rules . These are summarized in Chart 2. When the metallocene molecule is C v, meso Cs-symmetric, or highly fluxional, an aspecific polymerization has to be expected. 

A series of chiral (3-R-Ind)2ZrCl2 (e.g., R = neoisomenthyl, 5 a-cholestan-3 a-yl) was shown to be appreciably stereoselective, albeit always at low polymerization temperature. In these cases, the mode of enantioface selectivity is predominantly site control. For example, one of the two rac-like diaste-reoisomers of (3-neoisomenthylindenyl)2ZrCl2 (C2-II-... 

The polymerization mechanism is not immediately obvious by looking at the pentad region of the C NMR, due to the presence of all 10 pentads. Application of the statistical triad tests allows one to identify the source of the weak enantioface selectivity in C2-1-31/MAO as being enantiomorphic site control, as it is the case of all other C2-symmetric systems. This is possible by looking at the Tp dependence of the E and B triad tests (see section II.G) only the correct mechanism shows invariance with Tp of the corresponding triad test. 

The isospecificity of a catalyst is defined by the statistical parameter b, which represents the probability of a correct monomer insertion in the enantiomorphic site, at a given polymerization temperature. Assuming epimerization to be negligible in liquid monomer (hobs = h). the Arrhenius plot of In-[h/(l — h)] versus 1/Tp yields straight lines of slope AAi /R, from which the values of enantioface selectivity AAi enant = I AE jj — AE rel is estimated (Scheme 32). 

As noted above, even if monomer addition to the growing species is highly enantioface-selective in an isotactic-spedfic polymerization, the resulting isotaaic polymer is configurationally achiral except for the chirality of the stereocenters in the vicinity of the chain terminal. Related examples have been reported for coordination oligomerization and polymerization of propylene, f-pentene, and 4-methyl-f-pentene with optically active zirconocene catalysts and for radical... 

Polymerization conditions can affect the synthesis of iPPby C2-symmetric catalysts. A temperature dependence of polymer tacticity has been observed, with higher temperatures resulting in a decrease in the mmmm content of the polymer. This observation has been rationalized in terms of the relative energy of enantioface selectivity of propylene insertion. ... 

Resconi, L. Abis, L. Franciscono, G. 1-Olefin polymerization at bis(pentamethylcyclopentadienyl) zirconium and -hafnium centers Enantioface selectivity. Macwmolecules 1992, 25, 6814—6817. 

Efficient enantioface selection during insertion is caused by monosubstitution at the C4-carbon atom of the dihydroxazole ring (R or = H). The enantioface prevailingly inserted (and, therefore, the absolute configuration of the asymmetric carbon atoms in the polymeric chain) depends on the geometry of the C4-carbon atom. The presence of an R substituent is not essential for the discrimination process. 

The NMR spectrum for polypropylene produced at 50°C with this catalyst reveals a high degree of stereocontrol (81% rrrr pentads). Moreover, analysis of the stereochemical defects (predominantly rmmr pentads) were indicative of a site control mechanism. For a site control mechanism to operate in syndiospecific polymerization, the olefin must alternately bind to coordination sites with opposite enantioface selectivity. The model for this polymerization is shown in Scheme HI. 

The enantioface selectivity of the polymerization is unknown. It was arbitrarily assumed that non bonded sterie forces promote coordination of the Re face of the monomer at R configuration sites in the derivations that follow. However, reversed enantioface selectivity leads to the same conclusions. 

It is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer enantioface) are operative in stereoselective a-olefm polymerizations. In the simpler cases, the discrimination between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin polymerization has been reviewed extensively.162,172-178... 

In conclusion, in site-controlled stereoselective polymerizations, it is accepted and proved that the site chirality is unable to select directly between the two enantiofaces of the inserting monomer. Instead, it is accepted and proved that the site chirality can force a chiral orientation of the growing chain, which in turn is able to select between the two enantiofaces of the inserting monomer. Thus, the growing chain acts as a messenger to transfer the chiral information from the catalytic site to the monomer.172... 

The selection of the enantioface of an olehn will he discussed later in association with the stereospecihc olehn polymerization. 

One of the key pieces of information was provided by a brilliant experiment by Pino to establish the absolute stereochemistry of propylene polymerization.44 Brintzinger had demonstrated that the chiral metallocenes could be resolved into their respective enantiomers, 0 and thus for the first time it was possible to correlate the absolute stereochemistry of the catalyst with the prevailing enantioface of the olefin selected for... 

On the basis of this experiment, Pino and coworkers were able to determine that catalysts derived from the (/ )-ethylenebis(tetrahydroindenyl)zirconium binaptholate preferentially selected the Re enantioface of propylene. These results led to a model for the transition state where the polymer chain is forced into an open region of the metallocene, thereby relaying the chirality of the metallocene to the incoming monomer through the orientation of the p-carbon of the aUcyl chain (Scheme IIA).43 Here, the role of the C2-symmetry of the catalyst site can be readily appreciated since as the polymer chain migrates to the coordinated olefin, the coordination site available for binding of the olefin alternates between two coordination sites (A -> B -> C). Because these two sites are related by a C2-symmetry axis, they are homotopic and therefore selective for the same olefin enantioface. The result is polymerization to yield an isotactic polyolefin. 

Using a Cp2Ti(CgH5)2/MAO catalytic system, Ewen polymerized propylene at -45 C to obtain ani - PP containing 85% of meso dlads. It was concluded that the last monomeric unit chains can select the particular enantioface of propylene. At higher Tp this type of stereoregularity is lost and a substantially atactic PP is formed. Consequently, this nonchiral catalyst is incapable of selecting enantioface in the synthesis of i-PP above Tp = O C. 

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