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X. Liu et al. / Reactive & Functional Polymers 70 (2010) 972–979
for at least 3 days before use. Ethyl 2-bromoisobutyrate (98+%) was
purchased from Alfa Aesar. Diethyl ether, n-hexane, iodine plate
(99.8+%) and magnesium turnings (British Drug Houses Laboratory
Chemicals Group, England) were used without further purification.
The scCO2 apparatus was designed and fabricated by our group to
withstand a high pressure and temperature.
then carried out at 60 °C for 3 h. All of the above operations were
carried out under an argon atmosphere. Ethyl 2-bromoisobutyrate
(7.00 g, 0.04 mol) was added to the resultant dark-brown mixture
over 15 min, after which the reaction mixture was maintained at
80 °C for 60 h. Ice-cold water (50 mL) was then added and the or-
ganic products were repeatedly extracted with diethyl ether
(100 mL total). The combined ether extracts were washed with
water and brine and dried over anhydrous magnesium sulfate for
24 h. After removal of the solvent using a rotary evaporator and
purification by column chromatography on silica with n-hexane/
diethyl ether (9:1) as the eluent, ECPDB was obtained as a red oil
(34.2% yield). 1H NMR (CDCl3) results are listed for d (ppm): 1.21
(t, 3H, CH2CH3), 1.41 (a) (s, 3H, CHa CCHb), 1.66 (b) (s, 3H,
2.2. Pre-treatment of ramie fiber cellulose
Ramie fiber was cut into 3.2 g segments and dried at 60 °C over-
night (the chemical formula of the cellulose fiber is (C6H10O5)n.
There are three hydroxyl groups in each unit of cellulose fiber.
Therefore, for 3.2 g of cellulose fiber, the number of active hydroxyl
groups was ((3.2 g/162 g/mol) ꢀ 3 = 0.06 mol). These were mixed
in a two-neck round bottom flask with sufficient NMP to cover
the ramie fiber. After ultrasonic treatment for 30 min at 25 °C,
the ramie fiber was swollen for another 24 h and then used for
the next step without any further pre-treatment.
3
3
CHa CCHb), 4.03 (q, 2H, OCH2CH3), 7.28 (dd, 2H, m-ArH), 7.40 (dd,
3
3
1H, p-ArH), 7.86 (d, 2H, o-ArH). 13C NMR (CDCl3), d (ppm): 13.57
(CH3CH2), 60.82 (CH3CH2), 172.41 (C@O), 54.69 (CH3CCH3), 24.82
(CH3CCH3), 221.21 (C@S), 136.2 4 (ArAC1), 128.10 (o-ArC), 126.13
(m-ArC), 131.87 (p-ArC).
2.3. Synthesis of cellulose ester (cellulose-OBIB) using
2-bromoisobutyryl bromide (BIBB)
2.6. Synthesis of cellulose-g-PTFEMA via RAFT technique mediated by
free CTA in scCO2
A solution containing an excess of BIBB (22.25 mL, 0.18 mol)
and 35 mL of NMP (0.36 mol) was used as the proton scavenger
and was added dropwise to the swollen ramie fiber under an argon
atmosphere at 0 °C (ice-water bath). The molar ratios of cellulose
hydroxyl group, BIBB and NMP were 1:3:9. The mixture was
heated with slow stirring at 60 °C in an oil bath for 24 h, with
the flask sealed against the ingress of air. The nut-brown product
was rinsed thoroughly several times with anhydrous ethanol to
completely remove any unreacted BIBB and salt. The yellow final
product (termed cellulose-OBIB) was vacuum dried at 60 °C over-
night. The analytical data for this product were as follows: EA:
Br, 20.78 wt%. Average degree of substitution (DS) at the esterifica-
tion step = 1.00 (28.47% substitution).
Cellulose-CTA (0.18 g, 0.30 mmol), free CTA (0.0804 g, 0.30
mmol), and the initiator (AIBN) (0.005 g, 0.03 mmol) were added
to a 30 mL reactor. The reactor was purged with low pressure CO2
and alternately degassed several times with a vacuum pump to re-
move the air in the system. After sealing the outlet of the reaction
system, TFEMA (12.6 g, 75 mmol) was added to the reactor at a con-
stant CO2 flow rate (approximately 1 mL minꢁ1). Pressurized CO2
was then added to fill approximately half of the reactor, and heating
of the reactor was initiated. The molar ratios of TFEMA:CTA:initia-
tor were kept constant in all reactions at 250:1:0.1. Finally, the
reaction system was pressurized to 25 MPa by feeding additional
CO2 and heated to 70 °C with stirring for the scheduled reaction
time. After completion of the reaction, the reactor was cooled with
ice/water and the CO2 was released when the temperature of the
reactor had decreased to room temperature. The crude product
was collected and rinsed with THF to remove the unreacted mono-
mer PTFEMA not attached to the surface of ramie fiber and residual
initiator. Monomer conversion in the solution was determined by
1H NMR spectroscopy. The number-average molecular weight
(Mn) and polydispersity index (Mw/Mn) of homo-PTFEMA were
measured with SEC after precipitation in n-hexane. The crude solid
product was Soxhlet extracted with THF for 72 h to remove the sol-
uble materials noted above. Finally, the dark-yellowish product
known as cellulose-g-PTFEMA was vacuum dried at 60 °C over-
night. The monomer conversion was 90% from 1H NMR. The grafting
ratio was determined using Eq. (1) as Gcellulose-g-PTFEMA = 27%.
2.4. Synthesis of cellulose-based RAFT chain transfer agent
(cellulose-CTA)
Phenylmagnesium bromide was prepared from magnesium
turnings (1.50 g, 0.062 mol) and bromobenzene (6.7 mL, 0.064 mol).
The solution was warmed to 40 °C in a two-neck flask and an excess
of a mixture of carbon disulfide (18.7 mL, 0.31 mol) and anhydrous
THF (10 mL) was added dropwise over 20 min. The dark-brown
mixture was allowed to react at 40 °C for 3 h. All of the above
operations were carried out under an argon atmosphere. Then
2.40 g of cellulose-OBIB was added to the flask together with
30 mL of anhydrous THF. The reaction mixture was heated at
80 °C for 24 h. The solid product was washed thoroughly with dilute
hydrochloric acid and THF and subjected to a Soxhlet extraction to
completely remove the unreacted reagents. The resulting dark-
yellow sample (termed cellulose-CTA) was vacuum dried at 60 °C
overnight. The analytical data for this product were as follows: EA:
Br, 6.12 wt%; S, 10.86 wt%. The average degree of conversion of the
OH group to cellulose-CTA in this step was 0.27 (8.97% substitution).
2.7. Grafting ratio
The grafting ratio (wt.%) of cellulose-g-PTFEMA was calculated
using the formula
À
Á
ꢀ
Gðwt%Þ ¼ Wcellulose-g-PTFEMA ꢁ Wcellulose-CTA ꢀ 100 Wcellulose-CTA
ð1Þ
2.5. Synthesis of free CTA 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate
(ECPDB)
where Wcellulose-g-PTFEMA is the dry weight of the graft copolymer
and Wcellulose-CTA is the dry weight of the macromolecular RAFT
chain transfer agent (cellulose-CTA).
The synthesis of the free chain transfer agent ECPDB employed
in the RAFT process was modified according to the literature [33]. A
solution of phenylmagnesium bromide made from bromobenzene
(6.28 g, 0.040 mol) and magnesium turnings (1.00 g, 0.041 mol) in
dry THF(60 mL) was warmed to 40 °C, and carbon disulfide (3.05 g,
0.041 mol) was added dropwise over 20 min at a rate such that the
reaction temperature was maintained at 40 °C; the reaction was
2.8. Characterization
2.8.1. Elemental analyses (EA)
The surfaces of the samples were analyzed for Br, S and F con-
tent by energy dispersive spectrometry (EDS) (EDAX Co. Ltd., USA).