ma8b00321_si_001.pdf (2.3 MB)
Urea and Thiourea H‑Bond Donating Catalysts for Ring-Opening Polymerization: Mechanistic Insights via (Non)linear Free Energy Relationships
journal contribution
posted on 2018-04-13, 00:00 authored by Jinal
U. Pothupitiya, Rukshika S. Hewawasam, Matthew K. KiesewetterHammett-style
free energy studies of (thio)urea/MTBD mediated ring-opening
polymerization (ROP) of δ-valerolactone reveal the complicated
interplay of reagents that give rise to catalysis through one of two
mechanisms. The operative mechanism depends most greatly on the solvent,
where polar solvents favor a (thio)imidate mechanism and nonpolar
solvents favor a classic H-bond mediated ROP. Data suggest that the
transition state is only adequately modeled with ground state thiourea–monomer
interactions in the H-bonding pathway, and elusive urea/reagent ground
state binding interactions may be irrelevant and, hence, not worth
pursuing. However, neither relationship is robust enough to be predictive
in the absence of other data. Isotope effects suggest that the base/alcohol
binding event is directly observable in the ROP kinetics. New opportunities
for catalysis emerge, and a reason for the observed mechanism change
is proposed.