Thermosensitive Water-soluble Transition Polymers Brandon Spradlin Zhao Group University of Tennessee Dept. of Chemistry, Knoxville Abstract. Three random copolymers were synthesized by atom transfer radical polymerization with varying proportions of DEGMMA and DMAEMA. The DMAEMA moieties were then modified with 1,3 –propylsultone to form zwitterionic pendant groups. Polymeric sulfobetaines such as these typically exhibit an upper critical solution temperature, in contrast to PDEGMMA, a known lower critical solution temperature-type polymer. These monomer units were combined in order to tune the solubility of the random copolymers in water. Molecular weight, composition, and degree of quarternization were determined by 1 H NMR spectroscopy.
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Thermosensitive Water-soluble Transition Polymers Brandon Spradlin Zhao Group University of Tennessee Dept. of Chemistry, Knoxville
Abstract. Three random copolymers were synthesized by atom transfer radical
polymerization with varying proportions of DEGMMA and DMAEMA. The DMAEMA
moieties were then modified with 1,3 –propylsultone to form zwitterionic pendant
groups. Polymeric sulfobetaines such as these typically exhibit an upper critical solution
temperature, in contrast to PDEGMMA, a known lower critical solution temperature-type
polymer. These monomer units were combined in order to tune the solubility of the
random copolymers in water. Molecular weight, composition, and degree of
quarternization were determined by 1H NMR spectroscopy.
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Introduction
Thermosensitive polymers are an important subset of environmentally responsive polymer
systems and have been intensively studied in recent years. Temperature is unique as an external
stimulus due to its relative ease of manipulation, as compared to that of other external stimuli,
such as pH. Thermosensitive polymers are typically divided into two types: those exhibiting a
decrease in solubility leading to a phase transition above a lower critical solution temperature
(LCST) or below an upper critical solution temperature (UCST). A variety of applications of
materials with such properties have been reported, ranging from catalysis to drug delivery and
smart surfaces.
It has been established that incorporation of hydrophobic/philic moieties into a
thermosensitive polymer allow for relatively precise tuning of LCST or UCST values;
hydrophobic groups raise UCST’s and lower LCST’s while hydrophilic groups lower UCST’s
and raise LCST’s. Little attention, however, has been given to the combination of UCST and
LCST monomer units in a single random copolymer. Such a combination could lead to
interesting phase diagrams exhibiting both UCST and LCST transitions, as shown in Figure 1 on
the next page. In this work, methoxydi(ethylene glycol) methacrylate (DEGMMA), and 2-
(dimethylamino)ethyl methacrylate (DMAEMA) were used to synthesize such random
copolymers (figure 2). PDEGMMA typically exhibits an LCST of 25 °C, while PDMAEMA is
easily converted to 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate
(PADPS), a classic polysulfabetaine with a UCST of 8.5 °C at moderate molecular weights.1
This was accomplished using atom transfer radical polymerization (ATRP), an example of a
“living” polymerization technique, meaning that it employs an equilibrium between “dormant”
and “active” states to achieve uniform chain growth.2
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Figure 1: Possible UCST and LCST Phase Diagrams3
Experimental
Materials: Methoxydi(ethylene glycol) methacrylate (DEGMMA, or di(ethylene glycol)
monomethyl ether methacrylate) and ethyl 2-bromoisobutyrate (EBiB, Aldrich) were dried over
calcium hydride and distilled under reduced pressure. CuBr (98 %, Aldrich) was purified with
glacial acetic acid, washed with ether and ethanol, and stored in a desiccator 1,1,4,7,7-
pentamethyldiiethylenetriamine (PMDETA, 97 %, Aldrich), dichloromethane (DCM), and
anisole (99 %, Acros) were used without further treatment; DCM, when referred to as dry, was
processed via the Grubbs system. N,N-Dimethylaminoethyl methacrylate (DMAEMA) (99 %,
Aldrich) was passed through a basic aluminum oxide column prior to use. Poly(ethylene glycol)
(HO-PEO-OH) with a molecular weight of 20,000 g/mol was obtained from Aldrich. All other
chemicals were purchased from either Aldrich or Fisher and used without further purification.
Characterization: 1H NMR (300MHz) spectra were recorded on a Varian Mercury 300 NMR
spectrometer and the residual solvent proton signal was used as the internal standard.
Synthesis of P(DEGMMA-co-DMAEMA) Random Statistical Copolymer: Below is a typical
procedure for the synthesis of P(DEGMMA-co-DMAEMA). First, DEGMMA (2.008g, 10.68
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mmol) and EBiB initiator (15.6 mg, 8.0 x 10-5 mol) were combined in a three neck round bottom
flask. Anisole (3.018 g) and CuBr (12.2 mg, 8.5 x 10-5 mol) were added next. Then, DMAEMA
(1.699 g, 10.82 mmol) and PMDETA (20.1 mg, 11.6 x 10-5 mol) were added to the flask. The
solution was quickly stirred after the addition of the last two reagents, then immediately frozen to
ensure no radicals had formed before the remaining oxygen had been displaced. Once completely
frozen, ‘freeze, pump, thaw 3x3’ (FPT 3x3) was performed. During this process, the solution is
completely frozen, placed under high vacuum, then nitrogen gas is pumped in, and the solution is
allowed to melt. This is done 3 times to make sure no oxygen is left in the reaction flask. After
FPT 3x3 had been performed, the solution was added to a 55°C oil bath. The polymerization was
monitored by 1H NMR spectroscopy and was stopped after 150 minutes by opening the flask to
air. The final monomer conversion was found to be 50.25% by integration of the monomeric and
polymeric ester peaks at 4.20 – 4.35 ppm and 4.03 – 4.20 ppm, respectively. This conversion
corresponds to a degree of polymerization (DP) of 135. A similar procedure was carried out with
monomeric feed ratios of 80% DMAEMA and 20% DMAEMA, as shown below. The former
was found to have a final monomer conversion of 85.52 % and the latter 94.7 %, corresponding
to DP’s of 259 and 281. The final NMR spectrum of each polymerization is shown below (NMR
1-3).
The polymer was dissolved in methylene chloride, then precipitated in a 90% hexane-10%
diethyl ether solution a total of three times. The remaining hexane/diethyl ether solution was
removed via rotary evaporation, and the polymer was placed under a high vacuum overnight. 1H-
NMR spectroscopy was conducted to ensure the desired polymer had been made. The following
tables show the three random statistical copolymers that were synthesized using a similar