Cossee-type polymerization mechanism

Cossee type. The mechanism comprised of olefin metathesis type elementary processes involving a metal carhene complex and a metallacyclobutane was established only recently after the development of the chemistry of metal-carbene complexes [129], Understanding of the novel kind of reaction mechanism opened a new horizon in polymerization. Various new types of polymerization of cyclic monomers have been realized by ROMP. An advantage of the process is that the processes are tolerant to polar reactants and solvents enabling ready incorporation of polar substituents into polymers. Another advantage is that the ROMP is living in nature and polymers of narrow molecular weight distributions are available by the method. 

Following that year, many work groups studied the theoretical concepts concerning this type of catalytic system. Griffiths et al. [27] presented the first theoretical studies on this catalyst system, having assumed the generally accepted Cossee-Arlman polymerization mechanism [25-27] (Scheme 3.13). 

Figure 4. Schematic representation of Cossee-Arlman type reaction mechanism for homogeneous polymerization with metallocene-based catalysts.(Reprinted with permission from ref 51. Copyright 1994.)...

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. 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

However, the possibility that the type of conformation of the monomer which can be coordinated on the catalytic complex in a cisoid or transoid conformation plays a role, cannot be excluded. This role is decisive in Cossee s mechanism (70), according to which a monodentate-transoid or bidentate-cisoid coordination of the diolefin is responsible for the formation of either trans or cis 1,4-units respectively in the polymer (Scheme 14). As for the mode of the addition of metal and growing chain to the entering unit, the results obtained by Porri and Aglietto (71) in the study of the stereospecific polymerization of cis,cis-l,4-di-... 

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