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1900020 (2 of 5)
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In this work we present the use of the readily available
1,3,5-triethanol hexahydro-1,3,5-triazine (TrAz) as an initiator
for the AROP of epoxides, leading to acid labile star polymers.
Kinetic studies on the pH dependent hydrolysis of this com-
pound were reported by Riha and coworkers. In moderately
acidic solution the hexahydro-triazine ring rapidly and com-
pletely degrades to the amino-ethanol and formaldehyde, but
it is stable under highly basic conditions.
[17]
This renders TrAz
an ideal initiator for a “sacrificial star polymer strategy.” In this
work, we demonstrate that polymerization of EO and propylene
oxide (PO), respectively, leads to well-defined three-arm star
polymers. We show that after the polymerization the amine-
terminus can be conveniently released under acidic conditions.
This represents a unique route to
α-amino-ω-hydroxyl PEGs and
PPOs with high atom economy, based on inexpensive reagents.
The synthesis is shown in Scheme 1. The initiator TrAz is
commercially available in 75% purity.
1
H and
13
C NMR spec-
troscopy identified water as only major contamination of the
pure TrAz (Figures S1 and S2, Supporting Information). For
the AROP, 1 eq of TrAz initiator was partly deprotonated by
adding at maximum 0.2 eq. of KOtBu with respect to the
hydroxyl groups. The limited degree of deprotonation was
chosen to reduce aggregation of the initiator salt. To drive the
formation of the alkoxide, water and butanol were removed
in high vacuum. 18-crown-6 was added to increase the rate of
the polymerization by complexation of the potassium counter
ion.
[18]
This step is essential for the polymerization of PO,
which was performed without a solvent at room tempera-
ture. The polymerization of the gaseous EO was conducted
in dimethyl sulfoxide (DMSO) as a solvent at room tempera-
ture, resulting in full conversion after 24 h reaction time. As
the TrAz initiator possesses three hydroxyl group, polyether
three-arm star polymers with a hexahydro-triazine cores were
formed, as confirmed by NMR and SEC data discussed below.
In the case of PO polymerization undesired chain transfer is
known to result in a certain fraction of linear PPO without pri-
mary amine functionality, in contrast to the main product.
[19]
However, this side reaction of PO can be suppressed at low
temperatures.
[20–22]
Consequently, the polymerization of PO
was performed at room temperature, using a reaction time
of five days. For samples TrAz-PPO
85
and TrAz-PPO
101
no
full conversion could be achieved within this time due to the
low polymerization rate. For this reason, the obtained molec-
ular weights for these samples are lower than the targeted
value based on the ratio of PO and TrAz initiator. When tar-
geting molecular weight exceeding 10 000 g mol
−1
for PPO,
broadened distributions, nonuniform end groups and lim-
ited molecular weights are obtained.
[18,22]
The polymeriza-
tion was terminated by the addition of water. The TrAz-PEG
star polymers were subsequently separated from DMSO and
crown ether by liquid-liquid extraction with dichloromethane,
followed by several washing steps with water. In the case of
TrAz-PPO star polymers, petroleum ether was used for extrac-
tion. Then the organic phase was washed with water, resulting
in full removal of salts and crown ether from the polymer.
The TrAz-initiated PEG and PPO stars were characterized in
depth via size exclusion chromatography (SEC), NMR, and
MALDI-TOF mass spectroscopy. Typical monomodal mole-
cular weight distributions determined by SEC of the TrAz-
PEG stars are shown in Figure 1A and for the TrAz-PPO stars
in Figure S7, Supporting Information. All TrAz polyether
stars were obtained with a dispersity (Đ
= M
w
/M
n
) below
1.1. Star polymers are known to have a lower hydrodynamic
radius compared to linear polymers with identical molec-
ular weight.
[23]
Therefore, molecular weights determined via
calibration with linear PEG standards are slightly underesti-
mated and differ from the targeted values (Table 1) as well as
from the values determined by
1
H NMR end group analysis.
Figure 1B displays the
1
H NMR spectrum with assigned sig-
nals of the TrAz-PEG
61
star polymer as a typical example. An
exemplary
1
H NMR spectrum of TrAz-PPO
85
star is shown in
Figure S9A, Supporting Information.
Integration of the separated methylene signal (labeled d)
next to the nitrogen atom of the TrAz end group allowed for the
determination of the degree of polymerization. The resulting
molecular weights are summarized in Table 1 and are in good
agreement with the targeted molecular weights for all TrAz-
PEG stars. As discussed before, M
n
of TrAz-PPO
85
and TrAz-
PPO
101
stars is below the theoretical values due to incomplete
conversion of the monomer PO at room temperature. The
13
C NMR spectrum and detailed characterization via 2D NMR
techniques of TrAz-PEG
61
can be found in Figures S3–S6, Sup-
porting Information. To ensure exclusive initiation by TrAz,
MALDI-TOF measurements of the polyether stars were per-
formed. These measurements show a single distribution that
can be assigned to the TrAz end group as described before.
[21,24]
Exemplary spectra with the assigned peaks are shown in
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