Conspectus
Inspired
by the perfect helical structures and the resulting exquisite
functions of biomacromolecules, helical polymers have attracted increasing
attention in recent years. Polyisocyanide is well known for its distinctive
rodlike helical structure and various applications in chiral recognition,
enantiomer separation, circularly polarized luminescence, liquid crystallization,
and other fields. Although various methods and catalysts for isocyanide
polymerization have been reported, the precise synthesis of helical
polyisocyanides with controlled molecular weight, low dispersity,
and high tacticity remains a formidable challenge. Owing to a limited
synthesis strategy, the controlled synthesis of topological polyisocyanides
has barely been realized. This Accounts highlights our recent endeavors
to explore novel catalysts for the living polymerization of isocyanides.
Fortunately, we discovered that alkyne-Pd(II) catalysts could initiate
the living polymerization of isocyanides, resulting in helical polyisocyanides
with controlled structures, high tacticity, and tunable compositions.
These catalysts are applicable to various isocyanide monomers, including
alkyl isocyanides, aryl isocyanides, and diisocyanobenzene derivatives.
Incorporating chiral bidentate phosphine ligands onto alkyne-Pd(II)
complexes formed chiral Pd(II) catalysts, which promoted the asymmetric
living polymerization of achiral isocyanide, yielding single left-
and right-handed helices with highly optical activities.
Using
alkyne-Pd(II) catalysts, various topological polyisocyanides
have been facilely prepared, including hybrid block copolymers, bottlebrush
polymers, core cross-linked star polymers, and organic/inorganic nanoparticles.
For instance, various hybrid block polyisocyanides were easily produced
by coupling alkyne-Pd(II)-catalyzed living isocyanide polymerization
with controlled radical polymerization and ring-opening polymerization
(ROP). Combining the ring-opening metathesis polymerization (ROMP)
of norbornene with Pd(II)-catalyzed isocyanide polymerization, bottlebrush
polyisocyanides and core cross-linked star polymers were easily prepared.
Pd(II)-catalyzed living polymerization of poly(lactic acid)s with
isocyanide termini resulted in densely grafted bottlebrush polyisocyanides
with closely packed side chains. Moreover, the surface-initiated living
polymerization of isocyanides produced a family of polyisocyanide-grafted
organic/inorganic hybrid nanoparticles using nanoparticles with alkyne-Pd(II)
catalysts anchored on the surfaces. Surprisingly, the nanoparticles
and star polymers with helical polyisocyanide arms performed exceptionally
well in terms of chiral recognition and resolution. Incorporated organocatalysts
such as proline and prolinol units onto the pendants of optically
active helical polyisocyanides, a family of polymer-based chiral organocatalysts,
were generated, which showed significantly improved stereoselectivity
for the asymmetric Aldol reaction and Michael addition and can be
easily recycled.
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