Controlled preparation of amphiphilic triblock-copolyether in a metal- and solvent-free approach for tailored structure-directing agents

Article abstract of DOI:10.1039/c7cc09031e

Under mild conditions, PPO-PEO-PPO ("reverse Pluronics") and PBO-PEO-PBO copolyether were generated by way of N-heterocyclic olefin-based organocatalysis. Reverse Pluronics with molar masses > 20 000 g mol^-1 could be synthesized with excellent control (D_M ≤ 1.03) and were converted into (ordered) mesoporous carbons via organic self-assembly to showcase the need for tailor-made copolymer as structure-directing agent.

Full text of DOI:10.1039/c7cc09031e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Controlled Preparation of Amphiphilic Triblock-Copolyether in a
Metal- and Solvent-Free Approach for Tailored Structure-Directing
Agents
Received 00th January 20xx,
Accepted 00th January 20xx
Alexander Balinta, Marius Papendicka, Manuel Clauss b, Carsten Müller c, Frank Giesselmannc and
Stefan Naumanna*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Under mild conditions, PPO-PEO-PPO (“reverse Pluronics”) and In this context, we recently described the homopolymerization
PBO-PEO-PBO copolyether were generated by way of N- of PO using N-heterocyclic olefins (NHOs) as organocatalysts.13
heterocyclic olefin-based organocatalysis. Reverse Pluronics with NHOs, which increasingly occupy
molar masses > 20 000 g/mol could be synthesized with excellent polymerization catalysis14-17 and other fields,18-19 where shown
control (ÐM 1.03) and were converted into (ordered) to be excellently suited to synthesize PPO in high yields (up to
mesoporous carbons via organic self-assembly to showcase the 96%) in a setup without solvent or any other additives except
a prominent role in
≤
need for tailor-made copolymer as structure-directing agent.
an alcohol-based initiator. The received polyether was fully
controlled with respect to molar masses, polydispersity and
end groups. Importantly, transfer-to-monomer was also largely
absent. Together, these observations recommend the use of
NHO-catalysis for the construction of more complex, block-like
copolyethers in a metal- and solvent-free, highly controlled
process. Amid this versatile and promising class of polymers,20-
Organopolymerization of cyclic ethers is a challenging matter.
In contrast to cyclic esters (lactones) or carbonates, the range
of suitable catalysts is still very limited.1 Metal-free (anionic)
polymerization of typical representatives such as propylene
oxide (PO) or butylene oxide (BO) is usually hindered by slow
polymerization rates, low conversion and turnover as well as
by transfer-to-monomer, a side reaction which limits the molar
masses and complicates the construction of more block-like
polyether architectures (Scheme 1b).2
21
we considered this approach to be especially useful for
amphiphilic triblock copolyether of the type PEO-b-PPO-b-PEO.
Among the most widely employed polyether materials, used as
lubricants, surfactants or in medical applications,21 and known
as poloxamers (sometimes marketed as “Pluronics”), these
polymers also act as structure-directing agents (SDAs) for the
Early efforts focused on the application of Schwesinger’s
phosphazene bases for polymerization of ethylene oxide (EO)
and PO.3-7 While the considerable size of phosphazenium
cations improved polymerization rates, this was also found to
formation of mesoporous materials.22-26
For these
applications, finely tailored properties (micelle formation,
balance of lipophilic to hydrophilic blocks) are obviously
indispensable. However, this requirement is at odds with the
commercially available product, where only a certain number
of block variations is available. For so-called reverse Pluronics
(PPO-b-PEO-b-PPO) this situation is even more severe; the
formation of micellar networks27 complicates the generation
of ordered phases while at the same time the commercial
offers are even more limited. Consequently, tailor-made
triblock copolyether as presented in this work was considered
to facilitate access to defined mesoporous materials.
increase
transfer-to-monomer.6
Nonetheless,
these
investigations highlighted the potential of organocatalysis in
this area; especially for phosphazene-mediated (alternating)
copolymerizations of epoxides with other monomers a number
of impressive advances have been achieved lately.8-10
The application of N-heterocyclic carbenes (NHCs) further
improved the situation for EO as shown by Raynaud and Taton,
resulting in metal-free polymerizations with excellent control
over molar masses (Mn) with low polydispersity.11 Even for the
more challenging PO NHCs were found to be suitable catalysts,
however, some limitations were also observed with regard to
rather low conversion (30-40%) and slow reactions.12
For a first set of experiments, the NHOs
1 and 2 were
investigated (Scheme 1a). It had been shown earlier that
imidazole-derived NHOs are powerful catalysts for PO
polymerization, while the saturated analogues are inactive
under identical conditions.13
Catalyst
2 was initially expected to be the better choice, as it
cleanly starts an anionic polymerization; in contrast,
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Scheme 1. (a) employed NHO organocatalysts; (b) transfer-to-monomer as side reaction; (c) preparative scheme for amphiphilic block-copolyethers.
kg/mol). As determined from GPC peak areas, the
corresponding integral ratio of impurity/main peak was 8:92
(Figure S3). The high molecular weight suggested that a
condensation reaction takes place, base-catalyzed by the NHO;
and since no similar impurity had been observed for PO
homopolymerization,13 it was assumed that the reaction
directly involves two chain ends of macroinitiators (primary
alcohols, while a growing PPO-chain is terminated by a
secondary alcohol). Hence, after addition of a PO unit,
condensation might not take place readily anymore, explaining
why the desired product forms to a large majority. Indirect
proof for this was obtained from control reactions: a melt of
PEG 8000 at 80°C showed no formation of high molecular
weight fractions in the absence of catalyst, while in its
presence the side product occurred prominently (Figure S4). It
should be noted that after condensation involving one
terminus, the polymer can still grow, so its molecular weight
will always be larger than the twofold Mn of the initiator itself
Table 1. Polymerization results using NHO 1.
#
Initi-
ator
T
t
[NHO]/
Mn (ÐM)a)
Repeating
units PO b)
[°C]
[h]
[-OH]/[M]
[kg/mol]
1
2
PEG 8k
PEG 8k
PEG 8k
PEG 8k
PEG 8k
PEG 8k
PEG 4k
PEG 4k
PEG 4k
BnOH
80°C
80°C
80°C
80°C
80°C
80°C
80°C
80°C
80°C
50°C
50°C
50°C
50°C
1.0
3.5
5.0
14.5
25
1:5:1000
1:5:1000
1:5:1000
1:5:1000
1:5:1500
1:5:1500
1:5:1000
1:5:1000
1:5:1000
1:10:1000
2:5:1000
2:5:1000
2:5:1000
11.4 (1.03)
13.9 (1.02)
15.4 (1.02)
17.3 (1.03)
26.4 (1.02)
27.9 (1.03)
5.5 (1.03)
3
26
3
50
4
92
5
132
177
2
6
45.5
1.0
6.0
9.0
3d
7
8
8.9 (1.02)
34
9
13.2 (1.02)
1.2 (1.07)
89
10
11
12
13
20 c)
31 c)
53 c)
64 c)
PEG 8k
PEG 8k
PEG 8k
3d
19.9 (1.02)
21.3 (1.02)
22.1 (1.02)
5d
9d
a) according to GPC (CHCl3); b) determined via 1H NMR; c) monomer = BO
homopolymerization of PO with 1 delivers a very small, but (Figure S5 summarizes all (side-)reactions). Interestingly, mild
detectable “impurity” originating from zwitterionic introduction periods have been found for the polymerization
polymerization.13 The triblock copolymer was constructed (Figure S6), underlining that it is sensible to assume a certain
from
α,ω-dihydroxylated PEO (PEG 4000 and PEG 8000) as life time of the pristine macroinitiator without any PO
macroinitiator, on which blocks of PPO were grown (Scheme attached, in spite of the large excess of the monomer.
1c). Catalyst loadings of 0.1 mol.-% and a ratio of [-OH Obviously, suppression of this side reaction would be
termini]/[PO] = 1:200 resulted in a mixture that was fully rewarded by gaining access to very well defined polymer.
homogeneous at typical reaction temperatures (T = 50-80°C), While a screening of different reaction conditions clearly
containing 500-700 mg of PEO and about 1.5 g of PO. Since no influenced the proportion of condensation product in the
solvent was applied, PO acted not only as monomer substrate, polymerization mixture (down to 2:98, Table S1), it was the
but it also solubilized the initiator. For higher conversion the application of NHO
viscosity consequently rose considerably to honey-like in this regard, reliably delivering material with a purity >99.5%.
appearance with a pale yellow to orange discoloration, This behaviour was attributed to the somewhat decreased
1 which delivered constantly better results
basicity of
donating exocyclic methyl groups. With this setup and
elevated temperatures (80°C) series of analogous
1, a consequence of the absence of the electron
depending on NHO concentration and reaction times.
When PEG 8000 was reacted with PO at 50°C using NHO
2
for
a
17 h, a clean shift to higher molar masses was observed while
a very good control over polydispersity was achieved (ÐM
polymerizations was conducted with the aim to synthesize
PPO-PEO-PPO triblock copolyether with systematic variations
of the lipophilic PPO moieties.
=
1
1.02). H NMR analysis showed that 46 repeating units of PO
had been added, resulting in a structure with the average
formula PPO23-PEO180-PPO23. However, also a high-Mn impurity
was reproducibly observed in GPC, with its Mn slightly higher
than twice the value found for the main peak (42 kg/mol vs. 18
This succeeded at a loading of only 0.1 mol.-% of the
organocatalyst, delivering polymer with very narrow molecular
weight distribution (Table 1, entries 1-4, see Figure S7 for
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albeit considerably slower than found for PO (Table 1, entry
10). The resultant oligomeric PBO was found to be well-
defined (ÐM
= 1.07), with the end groups exclusively
originating from the employed initiator as found by MALDI-ToF
MS analysis (Figure S8). Based on these findings, the synthesis
of PBO-PEO-PBO triblock copolyether was investigated,
employing
1
in mildly increased concentration (0.2 mol.-%
-dihydroxylated PEG 8000. Allowing for
loading) and the
α,ω
the depressed polymerization rates, this fortunately resulted
in block copolyether, analogously to the PO-based system
(Table 1, entries 11-13). Again, polydispersity was nicely
controlled (ÐM = 1.02) and the number of incorporated BO
units linearly correlated with the Mn of the copolymer (Figure
S9). Hence, the different degrees of polymerization can be
addressed exactly and copolymer structures up to PBO32-
PEO180-PBO32 (entry 13) were prepared in this manner. NMR
analysis can be found in the SI (Figure S10-S12).
Figure 1. Correlation of PO repeating units and molar masses (Mn).
chromatograms). Closer inspection of polymerization time/Mn
reveals an induction period of slow monomer conversion
(Figure S6): after 2h, only about four repeating units have been
added, while after 3.5 h this has expanded to 26 PO units.
Potentially, this period of slow growth is caused by the
transition of a primary (oxyanionic) chain end to a secondary
one with increased basicity when the first PO is added to the
PEO. With progressing reaction the conversion runs into
saturation (after ca. 100 incorporated PO repeating units per
chain, 50% conversion) as the polymerization mixture gets
noticeably viscous. However, the preparation of higher
molecular weight polymer is readily achieved by an increase of
the initial monomer concentration; indeed, with a ratio of
1:5:1500 ([NHO]/[-OH]/[PO], = 0.067 mol.-% catalyst loading)
viscosity increases at a later stage and the PPO block length is
suitably increased to 130-180 repeating units (43-60%
conversion, Table 1, entries 5-6). Hence, a polyether like
PPO90-PEO180-PPO90, with equal molarity of hydrophilic and
lipophilic repeating units, is readily available and its
homologues with systematic block length variation are
accessible by simple adaption of reaction time and PO
concentration. The same system can also be employed with
another PEO-based macroinitiator (PEG 4000), displaying the
same well-behaved properties (ÐM ≤ 1.03) and suggesting that
the presented organocatalytic system can be employed in a
general manner for a wide range of amphiphilic copolyether
(Table 1, entries 7-9). Importantly, as documented by
monomodal chromatograms, the excellent correlation of
molecular weight and PPO repeating units (Figure 1) as well as
NMR analysis, no PPO homopolymer is present in the final
product. Traces of PPO, originating from zwitterionic initiation
As a proof-of-principle, reverse Pluronics were subsequently
applied as SDA, following the organic-organic self-assembly
approach developed by Dai and Zhao for the preparation of
mesoporous carbons (MCs),28-29 using the EISA (Evaporation
Induced Self-Assembly) process. This methodology is based on
the use of polar and H-bonding crosslinking agents
(oligomerized phenolic resins, formaldehyde), which stabilize –
and influence – the formation of ordered mesophases in
combination with Pluronic micelles by interaction with the
hydrophilic PEO-blocks. Thermopolymerization at 100°C then
forms a stable carbon precursor matrix, from which the non-
crosslinked parts can finally be burned off (T = 700°C) under
protective gas, engendering porosity and carbonization.
Experimental details and a scheme of the self-organisation
process are provided in Figure S2. Usually, to tailor the desired
morphology and pore size, indirect, process-related measures
are taken, such as variation of oven temperatures or a change
in the ratio of SDA to formaldehyde.29 The ability to prepare
tailor-made polyether SDAs with systematically varied block
lengths enabled us to work from the opposite direction,
keeping reaction conditions strictly constant to prepare MCs,
with only the polyether being changed (PPOn-PEO180-PPOn
including n = 20, 25, 40, 50, 66 and 90). All samples showed 25-
27% mass retention after carbonization. SAXS measurements
were applied to investigate the regularity of the carbon
material. Excitingly, this immediately highlighted the need for
tailored SDAs: under the applied conditions for self-assembly,
only the sample based on PPO66-PEO180-PPO66 delivered an
ordered MC (Figure S13, middle). In this case the SAXS profile
shows two broad scattering maxima at q1 = 0.91 nm-1 and q2 =
1.36 nm-1, the q-ratio of which however does not match the
ratio of 1/√3 required for the (10) and (11) peaks of a 2D
hexagonal lattice. We thus propose a short-range ordered
arrangement of pores on a 2D rectangular or distorted
hexagonal lattice. This is clearly supported by scanning
electron microscopy (Figure S14). N2 gas sorption data (Figure
2) also fit to a well-ordered material. A specific surface area of
790 m2/g was found, alongside a total pore volume of 0.65
cm3/g and an average pore size of 6.2 nm (adsorption, 4.4 nm
from desorption). Importantly, for longer and shorter PPO-
by NHO 1, are quantitatively lost during work-up (precipitation
form diethyl ether). No further purification is required.
Next, BO was investigated as monomer source for the
lipophilic blocks. Precedent for organocatalytic polymerization
of BO is scarce,8-9 and also for NHOs this had not been
reported to date, so homopolymerization experiments were
first studied. In comparison to PO, BO is somewhat more
electron-rich in its epoxide moiety and also sterically more
hindered. This seems to also affect NHO-mediated catalysis.
NHO
including reactions at 50-80°C over several days (BnOH as
initiator). In contrast, with NHO polymerization occurred,
2 was unable to generate meaningful conversion,
1
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block analogues (n = 50, 90) no ordered structure was
observed, which clearly localizes the window for the
preparation of ordered MCs under the described conditions.
Clearly, only in the case of PPO66-PEO180-PPO66 all polymer
parameters (balance of hydrophilic to lipophilic units, molar
mass) are suitable for the generation of an ordered structure
in the given self-assembly system. Nonetheless, in all cases
mesoporous carbons were formed. With respect to specific
surface area and pore size (distribution), also the disordered
materials displayed characteristic properties, as found by N2
sorption (Figure S15, for PPO40-PEO180-PPO40). This setup might
also be of interest to other types of self-assembly.30-31
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There are no conflicts to declare.
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