Hyperbranched polyethers obtained from glycerol carbonate
Paweł Rakoczy, Paweł Parzuchowski, Marcin Sobiecki, Gabriel Rokicki*
Warsaw University of Technology, Faculty of Chemistry, ul. Noakowskiego 3, 00-664
Warsaw, Poland
e-mail address: [email protected]
Abstract: Perfectly branched dendrimers can be constructed in a tedious, stepwise approach. Potencial
alternatives are hyperbranched macromolecules. Sunder et al. used glycidol to synthesized hyperbranched
aliphatic polyethers (HAP), which was named by them a latent cyclic AB2-type monomer. We have found
that in the synthesis of the HAP easily available monomer (glycerol carbonate) can be used instead of
glycidol. This cyclic carbonate was obtained from glycerol and dimethyl carbonate under mild conditions.
1,1,1-Tris(hydroxymethyl)propane was used as a trifunctional starter and central core for the anionic
polymerization. We controlled narrow molecular weight distribution by slow monomer addition rate. The
chemical structure and share of branching was monitored by means of DEPT 13C NMR, and GPC was used
to characterize molecular weight of the polymer. A plausible reaction mechanism was proposed and
discussed.
Introduction
Macromolecules like dendrimers and hyperbranched polymers with branch-on-branch
structure have unique molecular features and properties, which are not displayed by small
molecules [1, 2]. In contrast to linear polymers dendrimers are highly branched entities
with repeating units emanating from a central core and regular three-dimensional
architecture [2]. They are currently being synthesized and investigated for application
mainly in catalysis [3], molecular encapsulation [4], and drug delivery [5]. However, the
synthesis of dendrimers often involves multiple steps of protection/deprotection and
complicated purification. For this reasons they are not commonly used.
Potencial alternatives are hyperbranched macromolecules. These types of polymers are
typically prepared in a one-step reaction of polymerization of ABm-type monomers [6].
Hyperbranched polymers have a lot of common features with dendrimers. A promising
class of these highly branched molecules are aliphatic polyether polyols. Due to their
multiple, reactive chain ends they possess promising potencial as supports for catalysts and
functional organic molecules. In addition, their biocompatibility and excellent water
solubility renders them as valuable compounds for polymer therapeutics [7].
Sunder et al. used glycidol, a latent cyclic AB2-type monomer, to synthesize
hyperbranched aliphatic polyether (HAP) [8]. Hyperbranched polyglycerols were obtained
via the ring-opening multibranching anionic polymerization (ROMBP). The HAP showed
high molecular weights and narrow molecular weight distribution. This can be explained
by the method of polymerization and reaction conditions (slow addition of glycidol and
high dilution). Furthermore, glycidol has been polymerized cationically leading to
branched polymers in work by Penczek and Dworak [9].
We have found that in the synthesis of the HAP glycerol carbonate can be used instead of
glycidol. This latent cyclic AB2-type monomer was obtained from glycerol and dimethyl
carbonate under mild conditions.
1
In this paper we describe anionic ring-opening polymerization of glycerol carbonate used
instead of glycidol. 1,1,1-Tris(hydroxymethyl)propane was used as a trifunctional starter
and central core for the polymerization. The hyperbranched polyether structures were
obtained by slow addition of the monomer.
Experimental
Glycerol carbonate. Reaction was carried out under mild conditions. We used molar
excess of dimethyl carbonate. Dimethyl carbonate was added to glycerol in one ration. The
reaction was run over 4 hours at 40°C yielding almost quantitatively glycerol carbonate.
Hyperbranched polyethers. Polymerizations were carried out in a reactor equipped with
a stirrer and dosing pomp under nitrogen atmosphere. 25 per cent potassium methylate
solution in methanol was used to partially deprotonated 1,1,1-tris(hydroxymethyl)propane.
An excess of methanol was removed under reduced pressure. Glycerol carbonate was
slowly added at 175 ºC over 12 hours. The reaction was carried out until no absorption
band corresponding to carbonyl group was present in the IR spectrum. The product was
dissolved in methanol and neutralized by filtration over cation-exchange resin.
NMR. 13C NMR spectra were recorded in d6-methanol on a Varian Mercury 400
spectrometer, operating at 400 MHz.
IR. All measurements were performed with Biorad FTS-165 FTIR spectrometer as KBr
pellets.
GPC. Measurements of the molecular weight were performed with GPC LabAlliance
apparatus using water as an eluent at 35°C. Poly(oxyethylene) glycol was used for
calibration.
MALDI-TOF-MS. All Spectra were recorded on a Kratos Kompact MALDI 4 V5.2.1
mass spectrometer equipped with a 337 nm nitrogen laser with a 3 ns pulse duration. The
measurements were carried out in the linear mode of the instrument at an acceleration
voltage of +20 kV. For each sample, spectra were averaged over 200 laser shots. The
samples were dissolved in methanol (5 mg/ml) and mixed with a solution of the MALDITOF matrix (2,5-dihydroxybenzoic acid, 0.2 M in THF).
Results and discussion
We have developed the convenient method of the synthesis of glycerol carbonate (4hydroxymethyl-1,3-dioxolan-2-one) using cheap and easy available raw materials: glycerol
and dimethyl carbonate. Dimethyl carbonate was used in molar excess to shift the reaction
equilibrium towards products. According this method glycerol carbonate can be obtained
in almost quantitative yield under mild conditions (40°C) in the presence of potassium
carbonate as a catalyst.
It was found that glycerol carbonate can be used instead of glycidol in the process of
formation of hyperbranched glycerol polyether. As a central core and initiator for anionic
polymerization partially deprotonated 1,1,1-tris(hydroxymethyl)propane (ROH) was used.
The deprotonation was carried out with potassium methylate. Only 10 % of the hydroxyl
groups were converted into alkoxide ones. According this procedure the concentration of
active sites (alkoxides) and simultaneous intramolecular ion transfer can be controlled.
Depending on temperature, the reaction can proceed according to the mechanism
2
comprising alkoxide attack on dioxolane ring (path a) or formation of glycidol (path b)
followed by the reaction with the alkoxide. To reduce possibility of the formation of
macrocyclic polyethers as well as linear carbonate units glycerol carbonate should be
added very slowly in a dropwise manner to the anionic initiator. Moreover, a slow addition
of cyclic carbonate allow to achieve complete decarboxylation (Scheme 1, eq. b). When
glycerol carbonate was added to fast, at temperature below 160°C, in the structure of
polyglycerol linear carbonate units were observed. In the IR spectrum of the product the
absorption band at 1745 cm-1 was present. (Fig. 1).
ROH + H3CO- K+
Initiation
ROH + RO- K+
-CH3OH
90%
10%
-
O
a)
R
O
OH
O
O
O
Propagation
a')
O
-O
a) O
RO-
a')
+
O
R
OH
O
O
OH
-
-
O
O
b)
b)
O
RO
RO
OH
O2
-C
O
OH
O2
-C
O
c)
OH
OH
RO-
RO
-
O
- CO2
Scheme 1.
a)
b)
1745
Fig. 1. The IR spectrum of the hyperbranched polyethers obtained with slow (a), and (b)
fast glycerol carbonate addition.
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When the reaction was carried out at 175°C for 12 h no carbonate linkages were present in
the product (Fig. 1a).
During the growth of this polyether four structural units can be identified. If the secondary
alkoxy group has propagated linear 1,2 unit (L12) is formed. If the primary alkoxy group
has propagated 1,3 unit (L13) is formed. If both alkoxy groups have reacted with glycerol
carbonate, the results is branched, dendritic unit (D).
O
C
O- K +
O
O
O-R
OH
OH
O
O
2
1
OH
(L )
O-R
(L )
O-R
(D )
12
3
O
C
OH
O K+
O
O
O
OH
OH
O
2
1
O K+
C
O
O
O-R
OH
O
2
1
3
O K+
O
T
13
O
O-R
O
3
2 L13
O-R
OH
O K+
O
OH
ROH
-R O -K +
O
OH
1
2
(T )
T, 2D
3
L12
L12
CH2
O- K +
O
OH
L12
D
T
L13
CH
Figure 2. The DEPT 13C NMR spectrum (400 MHz, CD3OD) of the hyperbranched
aliphatic polyether obtained from glycerol carbonate and trimethylolopropane at 175°C.
It is possible to deactivate polymer chain growth by proton exchange or the acid addition.
According these manner terminal units (T) with two hydroxyl groups are formed.
In the DEPT 13C NMR spectrum of the product the signals corresponding to carbon atoms
of dendritic as well as linear and terminal units can be observed (Fig. 2).
The chemical shift of the carbon atoms of 61.3 ppm can be assigned, as given in Figure 2,
to the CH2 carbon atoms (3), 69.5 ppm to the CH2 carbon atoms (1) and 79.9 ppm to the
CH carbon atoms (2) of the linear 1,2 units (L12); 72.5 ppm (both signals) (1 and 3) to the
CH2 carbon atoms and 69.2 ppm to the CH carbon atoms (2) of the linear 1,3 units (L13);
63.0 ppm to the CH2 carbon atoms (3), 70.90 ppm to the CH carbon atoms (2) and 71.2
ppm to the carbon atoms CH2 (1) of the terminal units (T); 71.0 ppm (both signals) to the
carbon atoms CH2 (1 and 3) and 78.2 ppm to the carbon atoms CH (2) of the dendritic
units (D). The DEPT 13C NMR spectrum of the product obtained from glycerol carbonate
as one can notice is very similar to that obtained from glycidol reported by Sunder et al.
[8].
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The molecular weight of the polyglycerol obtained from glycerol carbonate measured
using GPC was much higher than that from MALDI-TOF measurement.
1266
1340
Fig. 3. The MALDI-TOF mass spectrum of hyperbranched polyglycerol obtained from
glycerol carbonate (GC) and trimethylolopropane (TMP) at 175°C for the molar ratio of
GC:TMP = 15:1.
Figure 3 shows the MALDI-TOF mass spectrum of polyglycerol obtained by anionic
polymerization of glycerol carbonate initiated with 1,1,1-tris(hydroxymethyl)propane trifunctional starter and central core. This series of peaks characterized by mass increment
of 74 Da from one peak to the next is equal to the mass of the repeating unit in the in the
hyperbranched polyglycerol 1,1,1-tris(hydroxymethyl)propane (residual mass 134 Da).
Conclusion
Easy to obtain glycerol carbonate can be used for preparation of hyperbranched aliphatic
polyether instead of glycidol. The structure of this polymer is very similar to that obtained
using oxirane monomer.
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