Cation−π Interactions Accelerate the Living Cationic Ring-Opening Polymerization of Unsaturated 2‑Alkyl-2-oxazolines

Cation–dipole
interactions were previously shown to have
a rate-enhancing effect on the cationic ring-opening polymerization
(CROP) of 2-oxazolines bearing a side-chain ester functionality. In
line with this, a similar rate enhancementvia intermolecular
cation−π interactionswas anticipated to occur
when π-bonds are introduced into the 2-oxazoline side-chains.
Moreover, the incorporation of π-bonds allows for facile postfunctionalization
of the resulting poly­(2-oxazoline)­s with double and triple bonds in
the side-chains via various click reactions. Herein, a combined molecular
modeling and experimental approach was used to study the CROP reaction
rates of 2-oxazolines with side-chains having varying degrees of unsaturation
and side-chain length. The presence of cation−π interactions
and the influence of the degree of unsaturation were initially confirmed
by means of regular molecular dynamics simulations on pentameric systems.
Furthermore, a combination of enhanced molecular dynamics simulations,
static calculations, and a thorough analysis of the noncovalent interactions
was performed to unravel to what extent cation−π interactions
alter the reaction kinetics. Additionally, the observed trends were
confirmed also in the presence of acetonitrile as solvent, in which
experimentally the polymerization is performed. Most intriguingly,
we found only a limited effect on the intrinsic reaction kinetics
of the CROP and a preorganization effect in the reactive complex region.
The latter effect was established by the unsaturated side-chains and
the cationic center through a complex interplay between cation−π,
π–π, π–induced dipole, and cation–dipole
interactions. These findings led us to propose a two-step mechanism
comprised of an equilibration step and a CROP reaction step. The influence
of the degree of unsaturation, through a preorganization effect, on
the equilibration step was determined with the following trend for
the polymerization rates: n-ButylOx < ButenOx
< ButynOx ≥ PentynOx. The trend was experimentally confirmed
by determining the polymerization rate constants.