T. Chen, et al.
MolecularCatalysis474(2019)110422
oil (Scheme 2). The degree of polymerization was determined by the
crude 1H NMR spectroscopy. Molecular weight and dispersity were
determined by GPC with THF as an eluent and using poly(methylme-
tacrylate) (PMMA) as standards.
In recent years, core-shell type polymeric nanoreactors through self-
assembly of amphiphilic copolymers have attracted more and more
attention. In these nanoreactors, the active catalyst was encapsulated
inside the hydrophobic core, which greatly increased its concentration,
and accordingly the catalytic activity. Besides, the hydrophilic shell
made the nanoreacters evenly dispersed in water, which enabled the
catalysis in water [27–33]. In most cases, the active catalyst was in-
troduced to nanoreactors by functionalization of hydrophobic part of
amphiphilic block copolymers by chemical reactions with high speci-
ficity, or copolymerization of active catalyst-containing monomer into
the hydrophobic block. For example, Weck et al. reported a class of
shell cross-linked micelles-supported Co(III)-salen complexes, which
were prepared by the esterification reaction of hydroxyl group from
salen ligand and carboxyl groups from the hydrophobic block, then
metalated with Co(II) and oxidized to Co(III)-salen complexes. These
supported Co(III)-salen complexes displayed not only high catalytic
efficiencies but also an unique substrate selectivity [34]. In O’reilly
et al.’s study, copolymerization of monomer with a L-proline moiety in
the hydrophobic block of amphiphilic block copolymers, then self-as-
sembly to form nanoreactors for asymmetric Aldol reaction in aqueous
2.3. Preparation of TEMPO-P(MMA25-b-OEGMA75
)
To a solution of NHS-P(MMA25-b-OEGMA75) (0.5 g, 0.02 mmol) in
THF (15 mL), 4-amino-TEMPO (6.85 mg, 0.04 mmol) and TEA (10 μL,
0.072 mmol) was added under nitrogen atmosphere and the resulting
mixture was stirred at room temperature for 24 h. The product was
precipitated from cold n-hexane to yield TEMPO-P(MMA25-b-
OEGMA75) as an orange-red viscous oil (Scheme 3).
2.4. Fabrication of polymer-supported nanoreactors
20 mg of TEMPO-P(MMA25-b-OEGMA75) was added to 2 mL of de-
ionized water. The mixture was oscillated continuously to fully dis-
solve. Nanoparticles formed through self-assembly of the amphiphilic
block polymers in water. The size and morphology of the nanoreactors
were investigated by dynamic light scattering (DLS) and scanning
electron microscope (SEM).
Herein we wish to report the facile synthesis of an amphiphilic block
copolyer with a TEMPO organocatalyst in the terminal of the hydro-
phobic block, the fabrication of core-shell TEMPO nanoreactors by self-
assembly of the amphiphilic block copolymers in water, and the in-
vestigation of their catalytic activities and recyclability in the selective
oxidation of alcohols. The results demonstrate high efficiency, easy
recycling and environmental friendliness of the TEMPO-based nanor-
eactors, which highlight the potential utility of this catalysis system in
organic synthesis and industrial production.
2.5. General procedure of the catalytic oxidation of alcohols
Alcohol (0.8 mmol), NaBr (1.25 mol%), and nanoreactors (20 mg,
0.1 mol%) were added to the mixed solution of NaClO (1.0 mmol,
1.25 mol equil., 2 mL) at 0 °C. The pH value of the aqueous solution was
adjusted to 9.1 using NaHCO3. The resulting mixture was vigorously
stirred at 0 °C. After completion of the reaction, ether was added to
extract unreacted alcohol and product. The conversion and selectivity
were analyzed by GC-MS. The catalyst was left in aqueous phase and
recovered by heating and centrifugation. And the recycled catalyst was
reused for next run after careful washing with cold ether.
2. Experimental section
2.1. Preparation of NHS-PMMA25 (Macro-CTA)
3. Results and discussion
NHS-CEPA was prepared followed by our previous report [36].
Macro-CTA was prepared as below. NHS-CEPA (144.0 mg, 0.4 mmol),
methyl methacrylate (MMA) (1.00 g, 10.0 mmol), and AIBN (6.57 mg,
0.04 mmol) were added to dioxane (4 mL) in a 50 mL ampule. The re-
sulting solution was degassed by 3 freeze–pump–thaw cycles and the
ampule was refilled with nitrogen and tightly sealed. The RAFT poly-
merization was carried out at 65 °C for 24 h, after that the reaction
mixture was opened to air and cooled down. The polymer was pre-
cipitated in cold n-hexane (4˜5 °C) to yield Macro-CTA as a yellow oil
(Scheme 1). The degree of polymerization was determined by the crude
1H NMR spectroscopy. Molecular weight and dispersity were de-
termined by GPC with THF as an eluent using poly(methylmetacrylate)
(PMMA) as standards.
3.1. Synthesis and structural characterization of TEMPO-P(MMA25-b-
OEGMA75
)
In our study, macro-chain transfer agent NHS-P(MMA25) (NHS-CTA)
(Mn =3.16 kg mol−1, PDI = 1.21) (Table 1) with a terminal activated
ester functional group was synthesized by RAFT polymerization em-
ploying NHS-CEPA as the chain transfer agent. Then, an amphiphilic
diblock copolymer incorporating a NHS moiety on the hydrophobic
block side was synthesized affording a low dispersity NHS-P(MMA25-b-
OEGMA75) (Mn =23.0 kg mol−1, PDI = 1.22) (Table 1), determined by
gel permeation chromatography (GPC) (Fig. S1).
Next, TEMPO was then brought into the end of the amphiphilic
diblock copolymer through activated ester functionalization strategy,
reaction of the NHS-P(MMA25-b-OEGMA75) with 4-amino-TEMPO to
provide TEMPO-P(MMA25-b-OEGMA75) in nearly quantitative yield
2.2. Preparation of NHS-P (MMA25-b-OEGMA75
)
Macro-CTA (286 mg, 0.10 mmol), OEGMA (1.50 g, 7.5 mmol) and
AIBN (1.64 mg, 0.01 mmol) were added to 1,4-dioxane (4 mL) in a
50 mL ampule. Similar procedure with the preparation of NHS-PMMA25
was adopted for the polymerization. The polymer was precipitated in
cold n-hexane (4˜5 °C) to yield NHS-P (MMA25-b-OEGMA75) as a yellow
The successfully introduction of TEMPO moiety into the amphi-
philic diblock copolymer was confirmed by the 1H NMR spectrum and
Electron Spin Resonance (ESR) spectrum. It can be found from 1H NMR
spectrum (Fig. S8) that the proton resonance at 2.85 ppm, which is
Scheme 1. Synthesis of Macro-CTA.
2