2 research outputs found
H‑Bonding Organocatalysts for the Living, Solvent-Free Ring-Opening Polymerization of Lactones: Toward an All-Lactones, All-Conditions Approach
The developing urea class of H-bond
donors facilitates the solvent-free
ROP of lactones at ambient and elevated temperatures, displaying enhanced
rates and control versus other known organocatalysts for ROP under
solvent-free conditions. The ROPs retain the characteristics of living
polymerizations despite solidifying prior to full conversion, and
copolymers can be accessed in a variety of architectures. One-pot
block copolymerizations of lactide and valerolactone, which had previously
been inaccessible in solution phase organocatalytic ROP, can be achieved
under these reaction conditions, and one-pot triblock copolymers are
also synthesized. For the ROP of lactide, however, thioureas remain
the more effective H-bond donating class. For all (thio)Âurea catalysts
under solvent-free conditions and in solution, the more active catalysts
are generally more controlled. A rationale for these observations
is proposed. The triclocarban (TCC) plus base systems are particularly
attractive in the context of solvent-free ROP due to their commercial
availability which could facilitate the adoption of these catalysts
Bis- and Tris-Urea H‑Bond Donors for Ring-Opening Polymerization: Unprecedented Activity and Control from an Organocatalyst
A new
class of H-bond donating ureas was developed for the ring-opening
polymerization (ROP) of lactone monomers, and they exhibit dramatic
rate acceleration versus previous H-bond mediated polymerization catalysts.
The most active of these new catalysts, a tris-urea H-bond donor,
is among the most active organocatalysts known for ROP, yet it retains
the high selectivity of H-bond mediated organocatalysts. The urea
cocatalyst, along with an H-bond accepting base, exhibits the characteristics
of a “living” ROP, is highly active, in one case, accelerating
a reaction from days to minutes, and remains active at low catalyst
loadings. The rate acceleration exhibited by this H-bond donor occurs
for all base cocatalysts examined. A mechanism of action is proposed,
and the new catalysts are shown to accelerate small molecule transesterifications
versus currently known monothiourea catalysts. It is no longer necessary
to choose between a highly active or highly selective organocatalyst
for ROP