Reaction mechanisms, polymers metal coordination

This chapter does not intend to provide a complete collection of newly synthesized organometallic or coordination complexes for alkene polymerization, but rather aims to review a cross-section of transition metal catalysts from the viewpoint of polymers and polymerization reactions. We focus particularly on polymers that are difficult or virtually impossible to prepare using conventional catalysts. In this light, we narrow our attention to well-defined molecular catalysts, including a study of progress in the understanding of active species, reactive intermediates, and reaction mechanisms that are indispensable for the synthesis of such polymers. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

The first linear polymers containing [2]catenanes in their backbones (mechanically linked main chain polymers, architecture A) were based on the Sauvage transition metal-coordinated catenanes. Polycondensation reactions were carried out between a diacid and a catenane diol to give a poly[2]catenane with Mn (600000) and Mw (4200000) based on polystyrene standards (Figure 4). 

This chapter will walk through the various forms these catalytic resins take. The catalysts covered in this review fall into three classes, (i) transition metals covalently bonded to the polymer support through an organometallic bond, (ii) transition metals coordinated to the polymer support, typically in ionic form and (iii) transition metal clusters that are formed by precipitating metals into nanoparticles within the polymeric framework. Additionally, this chapter covers the synthetically useful and industrially practiced reactions catalyzed by transition metals loaded onto organic supports and comments on the mechanisms and reusability aspects of the processes [1]. 

Most addition polymerizations involve vinyl or diene monomers. The opening of a double bond can be catalyzed in several ways. Free-radical polymerization is the most common method for styrenic monomers, whereas coordination metal catalysis (Zigler-Natta and metallocene catalysis) is important for olefin polymerizations. The specitic reaction mechanism may generate some catalyst residues, but there are no true coproducts. There are no stoichiometry requirements, and equilibrium limitations are usually unimportant so that quite long chains are formed 7iv > 500 is typical of addition polymers. 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

---