Yttrium Catalyzed Dialkyl Vinyl Phosphonate Polymerization: Mechanistic Insights on the Precision Polymerization from DFT

Single-site yttrium catalysts have the ability to produce poly­(dialkyl vinyl phosphonate) with a very narrow molecular weight distribution. Following the report of Rieger and co-workers (J. Am. Chem. Soc. 2013, 135, 13030), the mechanistic features of yttrium­(III) catalyzed polymerization of vinyl phosphonate are explored using DFT calculations, which reveal that the living nature of the polymerization arises from the thermodynamically and the kinetically favorable monomer addition steps (propagation). The precise polymer synthesis, therefore, would require fast initiation, and the rate-limiting propagation steps. The fast initiation can be achieved with various types of initiators; however, their initiation mechanism may differ. The alkyl initiators, which prefer initiation by deprotonation, yield the polymer with an olefinic tail. The deprotonation pathway is slower than the initiation by addition; therefore, the experimental observation of deprotonation selectivity is most likely due to high-temperature barrier tunneling. From a set of examined initiator ligands, the appealing are the ones that feature π-conjugated systems with the ability to mimic the propagation transition state