Polymerization of Lactones by Coordination Mechanism

The mechanism of coordination polymerization was pictured by Yong, Malzner, and Pilato [90] as being an intermediate between the above two modes of polymerization (a cationic and anionic one)  

The above shown mechanism, however, is incorrect when caprolactone is polymerized with tin compounds [95]. Yet, it appears to be correct for polymerizations of propiolactones with an ethylzinc monoxide catalyst [95]. 

Polymerization of E-caprolactone with a catalyst system consisting of tris(2,6-di-tert-butylphenoxy) yttrium and 2-propanol is first order with respect to the monomer and initiator [105]. This led to the conclusion that the reaction proceeds via a three-step mechanism that can be illustrated as follows [105]  

It was also reported that living s-caprolactone polymerization can be carried with bis(acryloxy-) lanthanide (II) complexes based on samarium [110]. Thus, (ArO)2Sm(THF)4, (where ArO = 2,6-di-ferf-butyl-4-methyl-phenoxy) yielded 98% conversion in toluene at 60°C in 1 h. The central ions and ligands appear to have an effect on the activity of the catalyst [110]. 

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