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.