Macromolecules, Vol. 38, No. 21, 2005
Dendrimer-like Star-Branched PMMA 8703
mL) solution containing polymers at room temperature, fol-
lowed by cooling to 0 °C to precipitate the coupled polymer
(1.27 g, 85%). The starting PMMA used in excess in the
reaction was readily recovered from the supernatant solution.
The resulting polymer was purified by reprecipitation from
THF to hexane and freeze-drying from its absolute benzene
solution to afford G-1 (1.13 g, 76%).
g, 0.461 mmol for SMP group) was reacted with a mixture of
LiBr (2.06 g, 23.7 mmol) and (CH3)3SiCl (3.50 mL, 27.5 mmol)
in acetonitrile/chloroform (15 mL/60 mL) at 40 °C for 24 h.
The polymer was precipitated in methanol, reprecipitated
twice from THF to methanol, and freeze-dried from its absolute
benzene solution for 24 h. After the workup similar to that
mentioned above, the resulting polymer was then treated with
LiBr (3.96 g, 45.6 mmol) in acetone (50 mL) at reflux temper-
ature for 1 h. The objective G-2-Br was obtained by precipita-
tion in methanol and purified by reprecipitation twice, followed
by freeze-drying (1.00 g, 81%).
1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 16H, -CH2-O-
), 3.7-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.5 (m, -CH2-C(CH3)-), 0.90 (s, 72H, -C(CH3)3), 0.04 (s, 48H,
Si(CH3)2).
Synthesis of First-Generation Dendrimer-like Star-
Branched PMMA with Eight Benzyl Bromide (BnBr)
Moieties, G-1-Br. The title polymer, G-1-Br, was synthesized
by treatment of G-1 with (CH3)3SiCl-LiBr to transform the
terminal tert-butyldimethylsilyloxymethylphenyl (SMP) groups
into benzyl bromide (BnBr) functionalities. The procedure is
as follows: Under nitrogen, G-1 (0.790 g, 0.457 mmol for SMP
group) was dissolved in a mixed solvent of acetonitrile (15 mL)
and chloroform (60 mL), followed by addition of LiBr (2.01 g,
23.1 mmol) and (CH3)3SiCl (3.65 mL, 28.7 mmol) to the
solution. The reaction mixture was allowed to stir at 40 °C for
24 h under nitrogen and then quenched with water (5 mL).
After removal of the solvents, the resulting mixture was
extracted with chloroform, and the organic layer was washed
with water and concentrated. The resulting polymer was
purified by reprecipitation twice from THF to methanol and
freeze-drying from its absolute benzene solution. It was
1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 32H, -CH2-Br), 3.7-
3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-0.6 (m,
-CH2-C(CH3)-).
Synthesis of G-3. The title G-3 was synthesized by the
coupling reaction of G-2-Br with the R-functionalized living
anionic PMMA under the similar conditions to those employed
in the synthesis of G-2.
1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 64H, -CH2-O-
), 3.7-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.5 (m, -CH2-C(CH3)-), 0.90 (s, 288H, -C(CH3)3), 0.04 (s,
192H, Si(CH3)2).
Synthesis of G-3-Br. The title G-3-Br was synthesized by
the transformation reaction with (CH3)3SiCl-LiBr, followed
by the retransformation with LiBr under the identical condi-
tions with those employed in the synthesis of G-2-Br.
1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 64H, -CH2-Br), 3.7-
3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-0.5 (m,
-CH2-C(CH3)-).
1
observed by H NMR analysis that a small resonance at 4.53
ppm assignable to the methylene protons of benzyl chloride
was present. Therefore, the resulting polymer (0.74 g) and LiBr
(3.54 g, 40.7 mmol) were dissolved in acetone (50 mL) and
refluxed for an additional 1 h to retransform the chloride into
bromide. After cooling to room temperature, the solvent was
removed under reduced pressure. The resulting polymer was
purified by the reprecipitation from THF to methanol and
freeze-drying from its absolute benzene solution to afford G-1-
Br (0.63 g, 80%).
Furthermore, G-4, G-4-Br, G-5, G-5-Br, G-6, G-6-Br, and
G-7 were synthesized according to the coupling and transfor-
mation reactions similar to those employed as mentioned
above. A 3.0-fold excess of the R-functionalized living PMMA
was used in the coupling reaction for the synthesis of G-4 and
G-5, respectively. A 3.5-fold excess and a 5.0-fold excess of the
R-functionalized living PMMA were used for the synthesis of
G-6 and G-7. A 250-fold excess of (CH3)3SiCl-LiBr was
employed for the synthesis of G-6-Br. In the retransformation
reaction of G-4-Br, G-5-Br, or G-6-Br with a 50-fold excess
of LiBr in acetone at refluxing for 1 h, undesirable dimeric
products were formed in 5-10% yields. Therefore, they were
removed by fractionation with SEC. The characterization
results of the resulting dendrimer-like star-branched PMMAs
are as follows:
1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 16H, -CH2-Br), 3.7-
3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-0.5 (m,
-CH2-C(CH3)-).
After this treatment, the resonance at 4.53 ppm correspond-
ing to the benzyl chloride methylene protons completely
disappeared.
Synthesis of Second-Generation Dendrimer-like Star-
Branched PMMA, G-2. The title polymer, G-2, was synthe-
sized by the coupling reaction of G-1-Br with R-functionalized
living anionic PMMA initiated with the functionalized anion
prepared from sec-BuLi and 1. The procedure is as follows:
Under high-vacuum conditions, MMA (2.14 g, 21.4 mmol) was
polymerized with the functionalized anionic initiator prepared
from sec-BuLi (0.662 mmol) and 1 (0.951 mmol) in the presence
of LiCl (2.10 mmol) in THF (20.5 mL) at -78 °C for 0.5 h. A
THF solution (12.7 mL) of G-1-Br (0.630 g, 0.347 mmol for
BnBr moiety) was added to the resulting living PMMA solution
at -78 °C, and the mixture was allowed to react at -40 °C for
24 h. After quenching with degassed methanol, the mixture
was poured into hexane to precipitate the polymers. The
resulting polymers were characterized by SEC to directly
compare their peak areas. They were redissolved in THF, and
the resulting THF solution was poured into methanol. Fortu-
nately, the objective coupled polymer was precipitated selec-
tively, while the deactivated PMMA used in excess was soluble
in methanol due to the low molecular weight (Mn ) 3.77 kg/
mol). The coupled polymer was further purified by reprecipi-
tation twice from THF to methanol and freeze-drying from its
absolute benzene solution to afford G-2 (1.48 g, 77%).
1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 32H, -CH2-O-
), 3.7-3.5 (m, -O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.6 (m, -CH2-C(CH3)-), 0.90 (s, 144H, -C(CH3)3), 0.04 (s,
96H, Si(CH3)2).
G-4: 1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 128H,
-CH2-O-), 3.8-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-
C(CH3)-), 1.1-0.5 (m, -CH2-C(CH3)-), 0.90 (s, 576H,
-C(CH3)3), 0.04 (s, 384H, Si(CH3)2).
G-4-Br: 1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 128H, -CH2-
Br), 3.7-3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.5 (m, -CH2-C(CH3)-).
G-5: 1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 256H,
-CH2-O-), 3.8-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-
C(CH3)-), 1.1-0.5 (m, -CH2-C(CH3)-), 0.90 (s, 1152H,
-C(CH3)3), 0.04 (s, 768H, Si(CH3)2).
G-5-Br: 1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 256H, -CH2-
Br), 3.8-3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.5 (m, -CH2-C(CH3)-).
G-6: 1H NMR: δ 7.2-6.6 (m, aromatic), 4.65 (s, 512H,
-CH2-O-), 3.7-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-
C(CH3)-), 1.1-0.5 (m, -CH2-C(CH3)-), 0.90 (s, 2304H,
-C(CH3)3), 0.04 (s, 1536H, Si(CH3)2).
G-6-Br: 1H NMR: δ 7.2-6.6 (m, Ar), 4.45 (s, 512H, -CH2-
Br), 3.7-3.4 (m, O-CH3), 2.1-1.6 (m, -CH2-C(CH3)-), 1.1-
0.5 (m, -CH2-C(CH3)-).
G-7: 1H NMR: δ 7.2-6.6 (m, aromatic), 4.66 (s, 1024H,
-CH2-O-), 3.7-3.4 (m, -O-CH3), 2.1-1.6 (m, -CH2-
C(CH3)-), 1.1-0.5 (m, -CH2-C(CH3)-), 0.90 (s, 4608H,
-C(CH3)3), 0.04 (s, 3072H, Si(CH3)2).
After the coupling reaction, the polymers were first precipi-
tated in hexane for comparing their SEC peak areas. Then,
the coupled products were readily isolated in near quantitative
yields by the selective precipitation from THF to methanol,
since the deactivated PMMAs used in excess were soluble in
methanol due to their low molecular weights (Mn ) 3.5-4.1
Synthesis of Second-Generation Dendrimer-like Star-
Branched PMMA with 16 BnBr Moieties, G-2-Br. The title
polymer, G-2-Br, was synthesized by the same procedures as
those employed in the synthesis of G-1-Br. At first, G-2 (1.25