Models of the Active Sites, Epoxide

4 Models of the Active Sites, Epoxide Polymerisation Mechanism and Stereochemistry 

Although any coordination polymerisation involves a monomer coordination in each step, literature data that might concern complex formation between the monomer heteroatom and the metal atom at the active site are rather scant. This is due to the lability of such complexes of monomers with active sites if they were stable enough, they would not undergo any further rearrangement, which might lead to polymer chain growth. Thus, models have been studied that have consisted of a metal complex, of no effectiveness as a catalyst, and a monomer, or of a catalyst and a non-polymerisable heterocycle. 

The first isolated and characterised species that could be envisioned as intermediates in the initiation step for the coordination polymerisation of epoxides when using metal carboxylate catalysts were complexes formed between cadmium carboxylates, solubilised in organic solvents by the tris-3-phenylpyrazole hydroborate ligand, and epoxides such as propylene oxide and cyclohexene oxide [68]. Other epoxide complexes with various metal derivatives have also been reported in the literature [69-72], 

The inversion of the configuration at the carbon atom of the epoxide ring where cleaved in the course of the ring opening during polymerisation indicates that the monomer complexed with the metal atom is attacked from the back side by the nucleophilic substituent X [scheme (1)]. If front side nucleophilic attack of this substituent occurred on the coordinated monomer, i.e. via the four-membered transition state as in scheme (2), no inversion but rather the retention of the configuration at the epoxide ring carbon atom where cleaved should be observed however, this is not the case. 

Taking into account the associated structures of catalysts for the coordination polymerisation of epoxides and considering that the coordinated epoxide is attacked by the nucleophile from the back side [scheme (1)], the catalyst must engage its two metal atoms in order to make the polymerisation possible. In fact, zinc-based catalysts containing associated multinuclear species (— Zn-O-Zn-O— ), including those with condensed zinc atoms ( Zn O Zn O ), are all characterised by the appearance of active sites with two zinc atoms bridged via a nucleophilic oxygen atom in which the O atom, bound covalently to the octahedral zinc atom, is coordinated to the adjacent tetrahedral Zn atom (Zn O -Zn). 

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