Copolymerization of olefin with carbon
monoxide has received considerable
interest from both academia and industry, and the introduction of
polar carbonyl group renders the resultant polyketones with excellent
mechanical strength, crystallinity, photodegradability, hydrophilicity,
surface, and barrier properties. However, most of the reported polyketones
are difficult to be processed because of limited solubility in common
solvents and high melting temperature (T
m ∼ 260 °C) resulting from the strictly alternative structure.
Nonalternating copolymerization of ethylene with CO is a very promising
method to circumvent the problem of processability of traditional
perfectly alternating polyketone. In the contribution, the palladium
coordinated diphosphazane monoxide substituted by strong electron-donating
groups is discovered to be highly reactive for producing nonalternating
polyketones, and up to 24.2% extra ethylene incorporation has lowered T
m values to 147 and 165 °C and further
improved thermal stability (T
d ∼
339 °C) of the resultant materials. Our data demonstrates that
cationic palladium complexes can also exhibit excellent reactivity
and an unprecedented nonalternating degree in this copolymerization.
Herein we report an efficient strategy for preparing isotactic polyesters and chiral epoxides via enantioselective resolution copolymerization of racemic terminal epoxides with anhydrides, mediated by enantiopure bimetallic complexes in conjunction with a nucleophilic cocatalyst. The chirality of both the axial linker and the diamine backbones of the ligand are responsible for the chiral induction of this kinetic resolution copolymerization process. The catalyst systems exhibit exceptional levels of enantioselectivity with a kinetic resolution coefficient exceeding 300 for various racemic epoxides, affording highly isotactic copolymers (selectivity factors of more than 300) with a completely alternating structure and low polydispersity index. Most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures in the range from 77 to 160 °C.
The asymmetric alternating copolymerization
of meso-epoxides with cyclic anhydrides promoted
by chiral catalysts or
reagents is a powerful strategy for the synthesis of optically active
polyesters with main-chain chirality. Herein, we show that, in conjunction
with a nucleophilic cocatalyst, enantiopure dinuclear Al(III) complexes
efficiently catalyze this asymmetric copolymerization, exhibiting
high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and
catalytic activity are revealed to be strongly affected by the axial
linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that
the reactions corresponding to ring opening at (R)-C–O and (S)-C–O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly
different Gibbs free energies of activation (Δ‡
G). Enantiopure dinuclear Al(III) complex 3 bearing a hydrogenated binaphthol linker with the matched
configuration is demonstrated to be the most active and enantioselective
catalyst, featuring a broad substrate scope and allowing one to obtain
a wide range of isotactic polyesters with a completely alternating
structure and low polydispersity. Notably, most of the produced isotactic
polyesters are typical semicrystalline materials with melting temperatures
between 120 and 240 °C. Additionally, the mixing of selected
isotactic (R)- and (S)-polyesters
in a 1:1 mass ratio afforded two crystalline stereocomplexes that
exhibited enhanced thermal stability as well as new crystallization
behavior and therefore significantly differed from the parent enantiopure
polymers.
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