Anionic coordination polymerization

Fiq. 5. Hypothesis of addition of a monomer molecule on the bond between the catalytic complex and the growing chain, in the anionic coordinated polymerization. 

The data here related on the kinetics of the propylene polymerization and of the transfer processes and the studies of the catalysts carried out with C-labelled alkylaluminums, derive from a series of researches mostly carried out some time ago, when the knowledge of the mechanism of the considered catalytic processes was still rather limited. Nevertheless, it helped remarkably to know these new processes of anionic coordinated polymerization their true catalytic nature (which regard to a-TiCU) differentiates them from the more usual polymerization processes (radicalic) which, actually, are not catalytic. They substantially contributed to demonstrate that the anionic coordinated polymerization is a step-wise addition process in which each monomeric unit inserts itself into a metal carbon bond of the catalytic complex. 

Figure 9.1 Mechanism for anionic coordination polymerization with isotactic placement.

This propagation reaction can be visualized as involving the formation of an incipient alkoxide anion on cleavage of the oxygen-metal bond in the propagating chain — hence the name anionic coordination. Many anionic coordination polymerizations proceed with stereochemical consequences. 

Up till now, the predominant and, it should be mentioned, successfully solved problems have been related to the determination of the nature (cationic, free-radical or anionic) and the structure of the active center of the growing polymer chain represented by an asterisk in Scheme 1. However, the investigation of the process of the direct insertion of the monomer in the polymer chain, i.e. everything represented in Scheme 1 by an arrow - was considered to be of secondary importance, with the exception of anionic coordination polymerization. It is usually a priori assumed that this is an elementary single-stage activation transition in the literal sense without any peculiar features, and if these features even exist, they are completely predetermined by (Fig. 1). 

Attempts at a detailed determination of -> made in anionic coordination polymerization may be characterized as steric-geometric approaches. Their distinctive peculiarity is the introduction of an intermediate stage of monomer complexing with the active center. It is assumed to be the geometric features of the complex that determine the stereospecifidty of the entire process. The fact of complex formation is considered to be an indispensable and even almost sufficient reason for the existence of stereospecificity. This is assumed in spite of the fact that the process of monomer insertion in the polymer chain from the postulated complex of a certain structure is usually not considered at all. 

On the other hand, in the case of anionic-coordination polymerization on paramagnetic catalytic centers, the existence of free spins in the active center can lead to the stabilization of the triplet intermediate by involving unpaired electrons of the transition metal in the exchange interaction with those of the intermediate. This should increase the effect of spin exclusion. However, the final result will depend on the competition of two groups of factors stabilizing (pseudosymmetry and exchange interaction) and destabilizing (electric and quantum-chemical fields of the counterion) a locally symmetric triplet intermediate. 

According to the opinion of the author, there are no fundamental differences between anionic and anionic-coordination polymerization. Moreover, the former should be regarded as an adequate simplified model for the latter. From this viewpoint, the effect of the principles considered above should also be extended to the range of anionic-coordination processes and, possibly, to Ziegler-Natta heterogeneous catalysis. However, although these types of polymerization are similar, they naturally should exhibit great differences. 

Table 8. Catalysts of anionic-coordinative polymerization based on a-ally) complexes of transition metals [49, 68]...

Thirdly, the established correlation between the microstructure of the polymer and the paramagnetism of the active center in anionic-coordination polymerization is both unexpected and not trivial. The predictions of results of this kind cannot be fortuitous. The correct predictions suggest that these postulates actually anticipate the real physical bases of the elementary act of catalytic polymerization. 

Figure 9.1 Mechanism for anionic coordination polymerization with isotactic placement. (After Odian, 1991.)...

The difference between this principle and the four preceeding ones is that the range of its application is limited to anionic and anionic-coordination polymerization. Only in the anionic propagation mechanism, is the formation of triplet intermediates possible (Scheme 5). This is probably one of the main reasons for the high structure- and stereoregulating ability of anionic and anionic-coordination catalytic systems including Zie r-Natta catalysts and is one of their distinguishing features. 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

When the stereochemistry of addition is influenced by the metal ion, the reaction is described as an anionic coordinated polymerization. Reactions of this type occur when the E—Me bond possesses largely covalent character. The influence of metal arises because of complex formation. If complexing occurs between metal and the electron pair of the monomer double bond, -k bonding is involved. On the other hand, if the electron pair comes from a polar group attached to the chain or monomer, the coordination involves a bonding. 

In order to account for stereocontrol in anionic coordinated polymerizations, it is necessary to know what species are involved in addition of monomer. This is usually difficult owing to the tendency of organometallic compounds to associate. The problem is further complicated because many of the compounds are new or difficult to prepare and handle. Our knowledge of the solution chemistry is therefore very meager. One can perhaps appreciate these difficulties by remembering that it has taken about 60 years to elucidate the structures of the Grignard reagent (Dessy et al., 1964). 

Tsuruta et al. (1963) prepared optically active poly (propylene oxide) from one mole of diethylzinc and two moles of (- -)-bomeol or (—)-menthol. They interpreted the catalyst activity as due to monomeric or dimeric zinc dialkoxide and that anionic coordinated polymerization took place by a four-membered ring intermediate similar to (LXV). The proximity of the optically active alkoxide to the point of reaction was thought to cause the asymmetric synthesis. 

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