Enol ethers are widely used as quenching reagents for
Grubbs catalysts.
However, we report the surprisingly effective ring-opening metathesis
polymerization (ROMP) of cyclic enol ethers, because the resulting
electron-rich ruthenium alkylidene complex remains active toward metathesis
of electron-rich olefins, despite its deactivation toward hydrocarbon
olefins. We demonstrate the first example of ROMP of cyclic enol ethers,
using 2,3-dihydrofuran as the monomer, producing a new type of degradable
and depolymerizable poly(enol ether). The polymers exhibited perfect
regioregularity, and their molecular weights can be regulated by the
loading of Grubbs initiators or by the use of a linear vinyl ether
as the chain transfer agent. We also developed protocols to deactivate
the catalyst following metathesis of enol ethers and cleave the catalyst
off the resulting polymers using H2O2 oxidation.
The resulting poly(dihydrofuran) can be recycled to monomer via depolymerization
with Grubbs catalyst or degraded to small molecules by hydrolysis
under acidic conditions. This work opens exciting opportunities for
a new class of ROMP monomers that lead to degradable polymers.
The locations and sequence of discrete monomers along a polymer chain can affect polymer properties and behaviors but are challenging to control even in living polymerizations. Xia and co-workers report selective single additions of a type of cyclopropene to precisely place various functional moieties at desired locations in a narrow-disperse homopolymer or block copolymer chain, opening the door to precise synthesis of polymer structures and architectures and thus control of polymer properties and self-assembly.
We have recently reported a polymechanophore system, polyladderene (PLDE), which dramatically transforms into polyacetylene (PA) upon mechanical stimulation. Herein, we optimized conditions to synthesize unprecedented block copolymers (BCPs) containing a force-responsive block by sequential ring-opening metathesis polymerization of different norbornenes and bromoladderene. Successful extension from PLDE to other blocks required careful timing and low temperatures to preserve the reactivity of the PLDE-appended catalyst. The PLDE-containing BCPs were sonochemically activated into visually soluble PA with a maximum absorption λ ≥ 600 nm and unique absorption patterns corresponding to noncontinuous activation of ladderene units. Access to polymechanophore BCPs paves the way for new stress-responsive materials with solution and solid state self-assembly behaviors and incorporation of polymechanophores into other materials.
Frontal
ring-opening metathesis polymerization (FROMP) is a rapid,
low-energy manufacturing reaction that is useful for curing thermosetting
materials. FROMP of dicyclopentadiene (DCPD) results in poly(dicyclopentadiene)
(p(DCPD)), a tough thermoset with excellent mechanical performance
and chemical stability. Like most thermosets, p(DCPD) cannot be reprocessed
and is therefore difficult to recycle. Previous work demonstrated
that the incorporation of a small quantity of cleavable units in the
strand segments of p(DCPD) networks enables their deconstruction.
Here, we report that a commercially available multifunctional comonomer,
2,3-dihydrofuran (DHF), both acts as a potent Grubbs catalyst inhibitor
during FROMP and introduces acid cleavable units. The resulting materials
retain high performance characteristics, including glass-transition
temperatures ranging from 115 to 165 °C and ultimate strength
ranging from 35 to 40 MPa. The addition of DHF above critical loading
levels enables deconstructable thermosets. We further demonstrate
freeform three-dimensional (3D) printing of deconstructable thermosets
via frontal polymerization.
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