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B. Dedeo gꢀ lu et al. / Polymer 54 (2013) 5122e5132
to this level of theory (to within kcal accuracy) can be obtained via
an ONIOM-based procedure in which the inner core is studied at
W1, the core at G3(MP2)-RAD and the full system at ROMP2/6-
For the synthesis of bis(dithiobenzoyl) disulfide, dithiobenzoic
acid (0.136 mol) and a few crystals of iodine were dissolved in 20 ml
of ethyl acetate. DMSO (0.068 mol) was added to this solution
slowly and stirred in the dark overnight. Removal of the solvent
under reduced pressure provided the bis(dithiobenzoyl) disulfide
in 90% yield.
3
11 þ G(3df,2p) levels [39]. The same group has calculated the kadd
, and K values for a variety of R- and Z-groups of small compounds
using high-level ab initio methods [33,41e44]. They have shown
,
k
b
that k
b
and K depend strongly on the nature of R and Z, and can
In the last step, bis(dithiobenzoyl) disulfide (0.059 mol) and
freshly crystallized 2,2 -azobisisobutyronitrile (13.5 g, 0.083 mol)
ꢀ
0
range over 10 orders of magnitude at 60 C [31,33,45]. In general, it
has been shown that each group (R and Z) can have separate con-
tributions in controlling the molecular weight of the polymer
during the process therefore one cannot generalize the substituent
effect on RAFT polymerization [33].
Here, we demonstrate the homopolymerization of a cd-
monomer, 2-acetamidoacrylic acid (AAA), with the RAFT method
using CPDB as a CTA and the first synthesis of its block copolymers
with MMA and methacrylic acid (MAA). Although polymerizations
were successful, there are limitations due to stability of the cd-
radical. Calculations based on density functional theory (DFT) [46]
provided an insight to the origin of this difficulty and opportunity
to compare the kinetic parameters and equilibrium constant of the
AAA polymerization with the polymerization of MAA and N-(prop-
were dissolved in dry ethyl acetate and refluxed under ultrapure
ꢀ
(99.999%) N
2
for 30 min. After an overnight reaction at 65 C, sol-
vent was evaporated and the crude product was isolated as a red oil.
Pure product was obtained in 59% yield after flash chromatography
using pentane:heptane:diethyl ether 9:9:2 as an eluent. CPDB is a
ꢀ
red oil at ambient temperature but crystallizes at ꢁ20 C.
2.4. Synthesis of polymers
In a typical polymerization reaction, monomer, RAFT agent
(CTA) and AIBN were dissolved in 1 ml of dimethyl formamide in a
5 ml round bottomed flask. Flask was sealed with a rubber septum
and purged with pure nitrogen for about 45 min. Then, the flask
ꢀ
1
2
2
-en-2-yl)acetamide using CPDB.
was placed in an oil bath at 70 C. After desired amount of time,
polymers were isolated by precipitation into diethyl ether. Specifics
of the polymerizations are provided in Table 2. All polymers possess
a pink color indicative of the attachment of CPBD to the polymer.
For the block copolymer synthesis, macromolecular CTA agents
. Materials and methods
.1. Materials
n
were used. Amount of CTA agent was calculated based on the M of
the macro-CTA assuming all chain ends are active. Details for the
copolymerizations are given in Table 3.
All reagents were purchased from Aldrich at the highest purity
available and used without further purification unless otherwise
0
stated. 2,2 -Azobisbutyronitrile (AIBN) was recrystallized from
2
methanol. Dimethyl formamide (DMF) was distilled over CaH .
2.5. Characterization
Methacrylic acid and methylmethacrylate were passed through
alumina column to remove the inhibitor. pH 7 buffer was purchased
from Baker. For GPC analysis 0.5% sodium azide (Aldrich) was added
to pH 7 buffer.
Polymer molecular weights were determined by Size Exclusion
Chromatography (SEC). SEC analysis was done on an Agilent 1100
ꢀ
instrument with a refractive index detector at 30 C. PLGel-OH mixed
8
(
m
m column was used with pH 7 buffer. Polyacrylic acid sodium salt
Polymer Labs., Inc.) standards were used for the calibration. PMMA
samples were analyzed with THF eluent and polystyrene standards
2
.2. Synthesis of 2-acetamidoacrylic acid
2
-Acetamidoacrylic acid was prepared according to a literature
report with some modifications [10]. Pyruvic acid (10 ml,
.144 mol), acetamide (3.7 g, 0.07 mol) and 300 ml of toluene were
(Polymer Labs., Inc.) using PLGel-Mixed C column.
Monomer conversions were calculated gravimetrically.
0
added into a three necked round bottomed flask fitted with a
DeaneStark trap and a reflux condenser. The solution was heated in
an oil bath to distill about 4.5 ml water. Then, 130 ml of toluene was
distilled off to concentrate the solution. The crude product was
precipitated upon cooling to room temperature. The orange pre-
cipitate was collected by filtration and dried in a vacuum oven. The
crude yield was 16.88 g. Recrystallization from methanol gave
2
.6. Computational methodology
Geometry optimizations were performed using the density
functional theory (DFT) [48] at the B3LYP/6-31 þ G(d) level of
theory (For a detailed account on these types of basis sets, see, e.g.:
Hehre, W. J.; Radom, L.; von Raque Schleyer, P.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986). All stationary
points have been characterized by a frequency analysis from which
thermal corrections have also been attained. Local minima and first
order saddle points were identified by zero and one imaginary
vibrational frequencies, respectively. All calculations have been
carried out using the Gaussian 03 program package [46]. The acti-
vated complex theory is used to calculate the rate constants. The
activated complex theory pictures a reaction between A and B,
proceeding through the formation of an activated complex, C, in a
rapid pre-equilibrium:
9
.63 g of white crystals in 57% yield.
2.3. Synthesis of chain transfer agent
2
-(2-Cyanopropanyl dithiobenzoate) (CPDB) was prepared ac-
cording to the procedure detailed in the literature [47]. Briefly, first
phenylmagnesium bromide was prepared from bromobenzene
ꢀ
(
0.1 mol) in dry THF (80 ml) using Mg (0.083 mol) at 40 C. Then,
equivalent amount of CS
reagent. After 4 h of reaction, 50 ml of H
2
was added dropwise to the Grignard
O was added gently and
2
the mixture was cooled to room temperature. THF was removed
under pressure and the resulting solution was diluted with water.
After removal of insoluble magnesium salts, aqueous solution was
treated with conc. HCl until brown color disappeared and dithio-
benzoic acid separated as a purple layer. Product was extracted into
dichloromethane. Removal of the solvent under reduced pressure
provided dithiobenzoic acid.
z
A þ B#C /P
The activated complex falls apart by a unimolecular decay into
product, P. The rate constant becomes:
kT RT
h pq
ꢁDGz=RT
k2
¼
k
e