Macromolecules, Vol. 36, No. 20, 2003
Controlled Chain Branching 7447
Sch em e 1. Mech a n ism of RAF T P r ocess
AM GEL columns (5 µm, linear, molecular range 5000-106
g/mol), a Waters 410 RI detector, and a Viscotek model T60
detector that gives both viscosity data and light scattering (LS)
results, using THF as the eluent at a flow rate 1 mL/min at
30 °C (LS angle, 90°; laser wavelength, 670 nm).
1H NMR measurements were carried out on a Bruker (500
MHz) NMR instrument using CDCl3 as the solvent and
tetramethylsilane (TMS) as the reference. UV-vis spectra
were recorded on an Agilent-8453 instrument using CCl4 as
solvent. GC-MS was measured on a Finnigan Voyager instru-
ment. Element analysis was carried on a Heraeus 1106
instrument.
Monomer conversions were determined by thermogravimet-
ric analysis (TGA) of samples taken from the reaction mixture
on a NETZSCH TG-209 instrument under a nitrogen atmo-
sphere. The temperature was elevated from 25 to 510 °C at a
rate of 20 K min-1. The weight loss above 275 °C gave the
polymer content.
In copolymerization study, monomer conversions of styrene
and 6 were measured at different times by a Magna-550 FT-
IR instrument using a KBr pellet of fixed thickness.
Syn th eses of Dith ioester s. Benzyl 4-Vinyldithiobenzoate,
6. An ice-bathed Grignard solution prepared from 4-bromo-
styrene (9.15 g, 50 mmol) and magnesium ribbon (1.32 g, 55
mmol) in 35 mL of THF was vigorously stirred while a solution
of carbon disulfide (3.80 g, 50 mmol) in 20 mL of dry THF was
dropped in. The resulting dark red mixture was warmed at
50 °C for 1 h before cooled to room temperature. A solution of
benzyl bromide (9.41 g, 50 mmol) in 20 mL of THF was then
dropped in. The temperature was maintained at 50 °C for 4
h. The whole process was conducted under a nitrogen atmo-
sphere.
In this paper, we synthesized a RAFT monomer, 6,
which consists of a dithioester moiety and a double bond,
and copolymerized it with styrene to prepare highly
branched polystyrene. The branched structure was
controlled in terms of molecular weight and polydisper-
sity of primary chains as well as the degree of branching
(DB) and the distribution of branch points along the
primary chain. The structure 6 not only is different from
1 and 2 in that it can mediate controlled/“living” radical
polymerization due to the dithioester moiety but also
is different from SCVP inimers since it cannot initiate
polymerization. Therefore, initiation by a radical initia-
tor or by thermal initiation of styrene is necessary.
Furthermore, the location of dithioester moiety at
branch point facilitates the analysis of the resulting
polymers by aminolysis. Our previous work demon-
strated that polymerizable nitroxyl stable radicals, such
as 715 and 8,16 act as branching agents in radical
polymerization as well. However, the dithioester pen-
dent group in the present work has wider monomer
tolerance, and the synthetic principle can be extended
to the synthesis of polyacrylates, etc.
After distillation of the solvent, the resulting thick red oil
(about 10 mL) was mixed with 50 mL of 10% aqueous solution
of NH4Cl and extracted with ethyl ether (3 × 30 mL). The
organic layer was dried on MgSO4 overnight and condensed.
The resulting red oil was purified by chromatography through
an Al2O3 (neutral, 60 mesh) column (eluent: cyclohexane) and
recrystallization from petroleum ether. Red crystals: yield 4.7
g (35%, based on 4-bromostyrene); mp 49.3 °C. UV-vis max
(CCl4): 332 and 507 nm. 1H NMR: δ (ppm) 4.59 (s, 2H, CH2,),
5.38 (d, 1H, CH2dCH, J ) 10.9 Hz), 5.86 (d, 1H, CH2dCH, J
) 17.5 Hz), 6.73 (q, 1H, CH2dCH), 7.24-7.40 (m, 7H, ArH),
8.00 (d, 2H, ArH, J ) 6.7 Hz). FT-IR: v (cm-1) ) 1230 and
1053 (CdS), 3088, 916, 990 (dC-H), 1660 (CdC vinyl). GC-
MS (EI): m/e ) 77, 91, 147, 237, 270. Calcd for C16H14S2: C,
71.07; H, 5.22; S, 23.71. Found: C, 71.91; H, 5.31; S, 23.95.
Benzyl Dithiobenzoate, 9. The procedure is similar to that
in the literature.17-19 The product is red oil. Yield: 60% (based
1
on bromobenzene). UV-vis max (CCl4): 302 and 504 nm. H
NMR: δ (ppm) ) 4.59 (s, 2H, CH2), 7.31-7.39 (m, 7H, ArH),
7.51 (t, 1H, ArH(para)), 7.98-8.00 (d, 2H, ArH(ortho), J ) 8.0
Hz). FT-IR: v (cm-1) ) 1226 and 1044 (CdS). GC-MS (EI):
m/e ) 77, 91, 121, 211, 244. Calcd for C14H12S2: C, 68.81; H,
4.95; S, 26.24. Found: C, 68.59; H, 5.14; S, 24.99.
Exp er im en ta l Section
Ma ter ia ls. Styrene (Shanghai Yonghua Special Chemicals,
99%) was distilled under reduced pressure. Tetrahydrofuran
(Shanghai Feida, 99.5%) was dried over CaH2 and distilled
from Na/benzophenone. 2,2′-Azobis(isobutyronitrile) (AIBN)
(Shanghai 4th Factory of Chemicals, 99%) was recrystallized
from methanol. 4-Bromostyrene (ACROS, 96%) was dried over
CaH2. Carbon disulfide (Shanghai 4th Factory of Chemicals,
99%) was purified by vigorously shaking with KMnO4 (0.4 wt
% based on CS2), followed by filtration and distillation to collect
a colorless fraction. Benzyl bromide (Shanghai Wulian Chemi-
cals, 98%) was used as received. Bromobenzene (Shanghai
Chemical Reagent Co. Ltd., 99.5%) was dried over CaCl2 and
distilled before use.
Mea su r em en ts. The number-average molecular weight,
Mn, and polydispersity index, PDI, of all samples were
measured by gel permeation chromatography (GPC) through
three Waters Styragel columns (pore size: 102, 103, and 104
Å) in series calibrated by narrow polystyrene standard. THF
was used as the eluent at a flow rate of 1 mL/min at 40 °C.
The elution of the sample was detected by a Waters 410 RI
detector. Additional measurements of Mn and PDI were
performed on a Waters 590 GPC system equipped with two
P olym er iza tion . The polymerizations were carried out in
bulk at various molar ratio of dithioester to monomer, 1/10,
1/50, 1/100, 1/200, and 1/300. Taking the 1/300 system as an
example, a solution of 6 (0.27 g, 1.0 mmol) in styrene (31.20
g, 0.30 mol) was degassed by three freeze-pump-thaw cycles.
Then the mixture was thermostated at 120 °C under a nitrogen
atmosphere. The samples taken from the system at reaction
times of 0.5, 1, 2, 3, 5, 7, 9, 11, 16, and 24 h were quenched
into liquid nitrogen to stop the polymerization. Monomer
conversion (at 24 h): 83% (by TGA). The final reaction mixture
was precipitated into methanol, and a pink solid was obtained.
Mn ) 21.7 × 104 (by LS) and 6.45 × 104 g/mol (by GPC); PDI
) 4.6 (by LS) and 9.3 (by GPC).
The copolymerization study was carried out under the same
reaction conditions. The reaction mixture at different times
was injected into a KBr cell of fixed thickness for the measure-
ment of FT-IR.
Clea va ge of th e P r od u ct. A solution of resulting branched
polymer (1 g, Mn,GPC ) 84 900, PDI ) 1.99) and 1 mL of
ethylamine in THF (3 mL) was stirred at ambient temperature