The Stereoregulation

Considering a crystallographic model of the a-TiCl3 surface, Cossee noticed that the layers basic plane was not parallel to it but formed an angle of about 55° that situation creates a non-equivalence between the 2 vicinal coordination positions located in a coordination cavity and very strong steric compression around one of them (Fig. 11). As a consequence  

Since the stereocontrol is essentially due to steric interactions with the site environments, it seems logical that -TiCl3, a much less regular type of crystal, will give lower performance in fact, a typical value of 50% isoactivity might seem still quite high (see below). 

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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] ... 

Busico, V., R. Cipullo, F. Cutillo, G. Talarico, and A. Razavi, Macromol. Chem. Phys., 204, 1269 (2003a). Busico, V., R. Cipullo, F. Cutillo, M. Vacatello, and V. V. A. Castelli, Macromolecules, 36,4258 (2003b). Busico, V., R. Cipullo, N. Friederichs, S. Ronca, and M. Togru, Macrolmolecules, 36, 3806 (2003c). Bywater, S., Carbanionic Polymerization Polymer Configuration and the Stereoregulation Process, Chap. 28 in Comprehensive Polymer Science, Vol. 3, G. C. Eastmond, A. Ledwith, S. Russo, and... 

Our study concerns the polymerization of 2-VP, Initiated by organomagneslum derivatives of the type R Mg R, In hydrocarbon solvents. In order to obtain a detailed knowledge of the stereoregulating mechanism, we focused this study on the determination of the nature and of the structure of active centers. 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

Analysis of multads distribution by 13c 11 , propagation kinetics and study of electron density on active centers model molecules, suggest the stereoregulating mechanism. 

Each of the fluorinated catalysts has an optimum temperature-range for stereoselective polymerization. At the lower temperatures, the rate of propagation and yield of polymer decrease dramatically. At higher temperatures, the molecular weight of the polymer produced becomes lower, presumably because chain transfer or termination processes increase in importance. At still higher temperatures, the stereoregulation is lost, and the low-d.p. polymer produced has a mixed, anomeric configuration. 

Results from polymerizations of various types of homologs of monomers indicated that, in general, the requirement imposed on the catalyst to realize the stereoregulation was severest for the monomer having the smallest side group, i.e., the lowest homologs. Therefore, acetaldehyde and propylene oxide were most extensively investigated. 

In this mechanism, the stereoregulation in the propagation reaction is realized faithfully by the steric effect acting between the incoming monomer and the growing end monomeric unit, both of which have interactions with an aluminum atom. The penta-coordinate aluminum atom of the catalyst, which is similar to that shown in the dimeric acetaldehyde-catalyst complex, plays an important role as an intermediate compound or as a transition state. In principle, an identical mechanism may be applied to the polymerization by the dialkyl-aluminum monoalkoxide catalyst. 

Poly[o-phthalaldehyde] was reported to be soluble in organic solvents, in contrast with isotactic polyacetaldehyde. This solubility behavior afforded us a good chance to test the stereoregulating capacity of our catalyst in the polymerization process. 

The strategy we adopted for attacking our problem, i.e., the complete understanding of the stereoregulation mechanism in the stereospecific polymerization reaction, has been successfully applied to the stereospecific polymerization of acetaldehyde and propylene oxide. The same strategy should be applicable also to other types of catalysts and monomers, even if the difficulty encountered in the experimental performance is greater. The fruitful harvest must await future investigation. 

Investigation of the stereoregulation and polymerization mechanism using optically active monomers and polymers... 

The highly enantioselective reduction of benzils was achieved by the use of the chiral Ru complex (S,S)-28 with an S/C of 1,000 in a formic acid-triethylamine mixture to give the R,R diol in >99% ee (Scheme 34) [108]. The sense of enan-tioselection was the same as that of the reduction of simple aromatic ketones, suggesting that the adjacent oxygen atom does not participate in the stereoregulation. Introduction of electron-accepting functions at the 4 and 4 positions increased the reaction rate, while the enantioselectivity was not affected by the electronic properties of the substituents. Use of 2-propanol as a hydride source caused both the rate and enantioselectivity to decrease. An unsymmetrical 1,2-... 

As already mentioned, nearly all experiments and theories agree that polymerization occurs by addition of an olefin to a catalyst center, followed by insertion of the (stereoregulated, sometimes stereospecific) complexed olefin into a metal-carbon bond at the catalyst center. Figure 9 shows how such an active center can be situated at the edge of a crystal-lattice. It will be seen that the environment of the coordinatively unsaturated, but alkylated, Ti atom demands the stereospecific coordination of the propylene (81). 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

It has been suggested recently [410] that, also in the case of heterogeneous isospecific Ziegler-Natta catalysts, the stereoregulation mechanism assuming a growing chain orientation is more reasonable than that predicted by the enantiomorphic site model [411,412],... 

The influence of electronic factors on the stereospecific behaviour of metallocene-based catalysts in a-olefin polymerisation is an important aspect of the stereoregulation mechanism. Two phenomena, lanthanide contraction and relativistic effects, that appear in group 4 transition metal series result in... 

Which are the essential factors determining the stereoregulation mechanism in the polymerisation of ot-olefins with coordination catalysts ... 

Let us recall that the origin of stereospecificity in the syndiospecific polymerisation of styrene lies in a chain end stereochemical control mechanism [52,70]. Key features of the stereoregulation mechanism are stereorigid rf coordination of the growing chain end and diastereoselective rj2 coordination of the styrene... 

Considering the above stereochemical model for syndiospecific styrene polymerisation, one may conclude reasonably that tf coordination of the monomer at the active site could hardly be possible, and r 2 coordination would always be involved in the syndiospecific polymerisation of this monomer [87]. One should note that preliminary concepts concerning the stereoregulation mechanism of syndiospecific styrene polymerisation assumed the styrene monomer to undergo only t]4 coordination at the titanium centre, the propagating chain being anchored via a benzylic bond as an t]3 ligand at the titanium [44,55,70]. 

The origin of the stereoregulation in conjugated diene coordination polymerisation was recognised by Porri et al. [7,41] who explained the different modes of formation of the isotactic and the syndiotactic polymers in the presence of various transition metal-based catalysts. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

Since coordination of the ether oxygen is involved in the stereoregulating step, any factor which weakens this will decrease stereospecificity. This explains why the more hindered, higher alkyl vinyl ethers give less stereoregular polymerization than vinyl methyl ether. 

On the basis of the experimental finding that the stereoregulating power of the base persists even after a prolonged pre-aging of the Al-alkyl/base mixture, Kissin and Sivak 68) have recently suggested that the true poisons are the byproducts of the Al-alkyl/ester interaction rather than the ester itself. 

The explanation given by Keii119), according to which the stereoregulating effect of the base is due to the fact that it lowers the effective Al-alkyl concentration, though consistent with the results he observed, does not appear to be applicable in those cases, such as those reported by Gardner48 and Galli81 , where the alkyl increases rather than decreases the stereospecificity, as previously mentioned in Section 6.1.2. 

A similar scheme was reported by Pino, based on the stereoregulating effect of electron donors and on the stereoelectivity obtained in the polymerization of racemic a-olefins in the presence of chiral bases 109). 

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