Transfers in coordination polymerization

Briefly, these can be characterized as being of low intensity. This is probably caused by the close connection between termination and transfer in coordination polymerizations. In recent years, the connection between Lewis acidity of centres and their ability to participate in reactions leading to transfer is becoming ever more evident [64], Only two types of transfer have been proved, and may be considered as roughly known to organometals and to hydrogen. In industrial production, the former must be taken into account and the latter is being exploited. 

The concept of degenerative transfer in coordination polymerization has been previously used for Fe-based systems [68, 69] and, commercially, to make block copolymers from simple olefins [70]. 

It was again Natta who discovered transfers to hydrogen in coordination polymerizations [71]. He assumed breakage of the metal-polymer bond in the active centre... 

Average molecular weight is the other property from which rate coefficients can be evaluated provided there is adequate information on the mechanism. In coordination polymerization it is usual for the molecular weight to rise to a limiting value (Fig. 6) indicative of transfer reaction. ... 

Among several chain terminating reactions that can occur in coordination polymerization, a common one is an elimination in which a P-hydrogen is transferred to the metal. 

As in the case of intramolecular hydrogen abstraction, branching by chain transfer is not a problem when alkenes are polymerized under Ziegler-Natta conditions because free radicals are not intermediates in coordination polymerization. 

The mechanism of branch formation in LDPE, as described in detail in Chapter 3, is different from that in coordination polymerization in LDPE, LCBs are formed by transfer... 

Several chain-transfer mechanisms are operative in coordination polymerization transfer by j8-hydride elimination transfer by yS-methyl elimination transfer to monomer transfer to cocatalyst and transfer to chain-transfer agent - commonly hydrogen - or other small molecules. The type of termination reaction determines the chemical group bound to the active site and the terminal chemical group in the polymer chain. The first three types produce unsaturated chain ends, while the last two types produce saturated chain ends. Figure 8.11 illustrates these five transfer mechanisms. 

In contrast, in coordination polymerization chain growth and termination take place by insertion of the monomer or chain-transfer agent into a metal-carhon bond, as proposed by the Cossee mechanism. Consequently, electrical and steric effects around the active site affect polymerization kinetics as much as does the monomer type. The mechanisms of free-radical and coordination polymerization are contrasted in Figure 8.19. 

The most important transfer reactions in coordination polymerization are (1) fl-hydride elimination (2) transfer to chain-transfer agent (3) transfer to monomer and (4) transfer to cocatalyst. 

The polymerization model most commonly adopted for olefin copolymerization is the terminal model, particularly for studies of polymerization kinetics. In the terminal model, only the last monomer molecule added to the chain end influences polymerization and transfer rates. Besides the fact that it is logically expected, there is also significant experimental evidence supporting the terminal model for olefin polymerization. Since monomer propagation and chain-transfer reactions take place by insertion between the chemical bond formed by the metal in the active site and the polymer chain end, it is certainly reasonable to assume that both the nature of the active site and the type of monomer last added to the chain will affect these reactions. On the other hand, higher-order models such as the penultimate and pen-penultimate models have not found widespread use in coordination polymerization. 

High-density polyethylene (HDPE) is made with Ziegler-Natta (Z-N) catalyst systems. It has a totally different structure from that obtained by radical polymerization in having a much lower degree of branching (0.5-3 vs. 15-30 side chains per 500 monomer units). Chain transfer to polymer is not possible in coordination polymerization. 

Anionic and anionic coordination polymerizations of epoxides are often slow processes that require long reaction times to achieve high monomer conversions. Moreover, as reported in previous sections, a majority of these polymerizations suffer from side reactions, illustrated by the chain transfer reaction to monomer in alkali metal anionic polymerization and by a very low initiation efficiency and the formation of several polyether populations in coordination polymerizations. 

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. 

In the polymerization reaction the monomer is coordinated at a vacant site of the metal, followed by insertion into the metal-carbon bond (Fig. 9.5-5). Finally, the chain-propagation is terminated by transfer of a hydrogen atom in a P-position to the metal or to the coordinated monomer. 

Reaction Pathway. The simplest pathway is illustrated by the /3-keto ester substrate in Scheme 50. As suggested by reaction with RuCl2[P(C6H5)3]3 as the catalyst precursor (40c, 96), this hydrogenation seems to occur by the monohydride mechanism. The catalyst precursor has a polymeric structure but perhaps is dissociated to the monomer by alcoholic solvents. Upon exposure to hydrogen, RuC12 loses chloride to form RuHCl species A, which, in turn, reversibly forms the keto ester complex B. The hydride transfer in B, from die Ru center to the coordinated ketone to form C, would be the stereochemistry-determining step. Liberation of the hydroxy ester is facilitated by the al-... 

---