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exhibited by the end groups on linear pNIPAAm,24 it is
expected that this form of Tcp control is even more effective
for highly branched pNIPAAm because there are more end
groups available for highly branched polymers. While copoly-
merization of pNIPAAm with various comonomers in highly
branched systems has been briefly explored,12,17 compari-
sons to linear models have not been done. By exploring
these three models of LCST control on highly branched pNI-
PAAm, we demonstrate that these methods not only have dif-
ferent efficiency in controlling the LCST, but can also have
unexpected effects on the polymer product.
ery hour for the first 4 h and then allowed to proceed over-
night. The samples were then frozen and lyophilized.
End-Group Modification
pNIPAAm was synthesized similarly to the methods
described above. For instance, a 10.3 g mixture of 100:1:0.5
ratio of NIPAAm:1:AIBN was placed in a sealed 25-mL
round-bottom flask equipped with a magnetic stir bar. The
mixture was purged with nitrogen for 15 min and 10 mL of
nitrogen-purged 1,4 dioxane was added. The solution was
ꢁ
reacted at 65 C for 48 h and was quenched by exposure to
EXPERIMENTAL
air. The pNIPAAm was precipitated into diethyl ether and
collected via filtration. The pNIPAAm was then dissolved in
nanopure water, and dialyzed as previously described with a
2000 MWCO membrane dialysis cassette. The sample was
then frozen and lyophilized.
Materials
N-Isopropylacrylamide was purchased from TCI America and
recrystallized in 9:1 hexanes:benzene. 4(5)-Imidazole dithio-
carboxylic acid, cesium carbonate, dimethyl acrylamide
(DMA), acrylamide (AAm), acrylic acid (AAc), 1-hexylamine,
5,50-dithiobis(2-nitrobenzoic acid), tributylphosphine, 1, 4
dioxane, 3-methyl-3-pentanol (3Me3PenOH) were purchased
from Sigma Aldrich and used without further purification.
The freeze-dried pNIPAAm was then was then subjected to
aminolysis using hexylamine. Thiol functionality was main-
tained using tributylphosphine. Briefly, 1 g of pNIPAAm was
reacted with 660 lL of 1-hexylamine and 247 lL of tributyl-
phosphine in 25 mL of 1, 4 dioxane under nitrogen for 2 h.
The product was precipitated in cold ether, filtered, and
dried in vacuo. An Ellman’s assay was conducted to confirm
the presence of thiols.25 In brief, 100 lL of 100 lM solution
of lysed pNIPAAm in 0.1 M Tris buffer, pH 8 was reacted
with 100 lL of 4 mg/mL of 5,50-dithiobis(2-nitrobenzoic
acid) in Tris buffer. Absorbance was measured at 410 nm on
a Beckman DTX 880 Multimode Plate Reader and was com-
pared to standards made with known concentrations of L-
cysteine.
4-Vinylbenzylimidazoledithioate (1) Synthesis
Synthesis of 1 was modified from the procedure set forth by
Carter et al.12 Briefly, 2.2 g of 4(5)-imidazole dithiocarboxylic
acid and 15.4 g of cesium carbonate were dissolved in 45
mL of dimethylformamide. The solution was purged with
nitrogen and stirred for 30 min. 4-Vinylbenzyl chloride (1.69
mL) was added to the reaction vessel and was reacted for
70 h. The raw product was then filtered to remove excess
cesium carbonate. The filtrate was diluted with 500 mL of
nanopure water and extracted with 200 mL of dichlorome-
thane twice. The DCM mixture was subsequently concen-
trated using a rotary evaporator to reduce the volume to
ꢀ50 mL. The mixture was then passed through a silica col-
umn with 2.5% methanol in DCM and then again through an
alumina column with 2% methanol in DCM. The appropriate
fraction was collected and the resulting product was dried,
yielding bright orange crystalline product. The product 1
was confirmed (see Supporting Information) using 1H NMR
(400 MHz, CDCl3, d): 7.8 (d, 2H); 7.3 (d, 2H); 6.6 (q, 1H); 5.6
(d, 1H); 5.15 (d, 1H); 4.5 (s, 1H).
End groups were introduced using thiol-Michael addition. A
1:1.2 ratio of thiols to -enes were conjugated using 1-hexyl-
amine as the base. In a typical reaction, 300 mg of cleaved
pNIPAAm was dissolved in 5 mL of Tetrahydrofuran (THF)
and 60 lL of DMA or 40 lL of AAc was added along with 50
lL of 1-hexylamine. The solutions were reacted at 40 ꢁC
overnight and dried in a vacuum oven. They were then redis-
solved in nanopure water, dialyzed as previously described
using 2000 MWCO dialysis cassettes, and lyophilized. Conju-
gation was confirmed using gel permeation chromatography
1
(GPC) and H NMR (see Supporting Information).
Tacticity Control
Polymerization of NIPAAm was carried out with 1 in the
presence and absence of 3Me3PenOH to control tacticity.
Two ratios of 3Me3PenOH were tested: 4:1 and 10:1 of
3Me3PenOH:NIPAAm. For example, under the 4:1 condition,
a 1.03 g mixture of 100:1:0.5 ratio of NIPAAm:1:AIBN and 4
mL of 3Me3PenOH was placed in a sealed 25-mL round-bot-
tom flask equipped with a magnetic stir bar. The mixture
was purged with nitrogen for 15 min and 10 mL of nitro-
gen-purged 1, 4 dioxane was added. The solution was
Random Copolymer Synthesis
Copolymerization of NIPAAm was carried out with 1. Copoly-
mers of pNIPAAm with DMA, acrylamide (AAm), and AAc
were synthesized with varying amounts of comonomer. For
example, a 1.03 g mixture of 90:10:1:0.5 ratio of NIPAA-
m:AAc:1:AIBN was placed in a sealed 25-mL round-bottom
flask equipped with a magnetic stir bar. The mixture was
purged with nitrogen for 15 min and 10 mL of nitrogen-
purged 1, 4 dioxane was added. The solution was reacted at
65 ꢁC for 48 h and was quenched by exposure to air. The
copolymers were precipitated in diethyl ether and collected
via filtration. Successful copolymerization was confirmed
using proton NMR.
ꢁ
reacted at 65 C for 48 h and was quenched by exposure to
air. The pNIPAAm was precipitated into diethyl ether and
collected via filtration. The pNIPAAm was then dissolved in
nanopure water, and dialyzed with a 2000 MWCO membrane
dialysis cassette. During dialysis, the water was changed ev-
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JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 2068–2078
Chang, Kai
