Controlled free-radical polymerization of n-butyl acrylate by reversible addition-fragmentation chain transfer in the presence of tert-Butyl dithiobenzoate. A kinetic study

Article abstract of DOI:10.1021/ma049757r

The RAFT polymerization of n-butyl acrylate was studied in the presence of various concentrations of tert-butyl dithiobenzoate (t-BDB) as a chain transfer agent, and various concentrations of AIBN as an initiator at 60 and 90°C. The polymerizations exhibited the expected features of controlled systems. In particular, the linear increase in number-average molar mass (M_n) with monomer conversion, from very low conversion values, indicated a large apparent chain transfer constant to t-BDB. The kinetic study showed two typical characteristics already observed in RAFT polymerizations of various acrylates: the existence of an induction period and a strong rate retardation, both enhanced by the increase in t-BDB initial concentration. ESR spectroscopy was applied at 90°C and revealed the existence of an unexpectedly large concentration of intermediate radicals. The system was examined and discussed considering either the possibility of cross-termination between a propagating macroradical and an intermediate one, with fast fragmentation of the intermediate radicals or the absence of cross-termination but slow fragmentation.

Full text of DOI:10.1021/ma049757r

Macromolecules 2004, 37, 6329-6339
6329
Controlled Free-Radical Polymerization of n-Butyl Acrylate by
Reversible Addition-Fragmentation Chain Transfer in the Presence of
tert-Butyl Dithiobenzoate. A Kinetic Study
Elen a Ch er n ik ova ,†,‡ An d r ey Mor ozov,† Ek a ter in a Leon ova ,† Eliza veta Ga r in a ,†
Vla d im ir Golu bev,† Ch u on g Bu i,‡ a n d Ber n a d ette Ch a r leu x*,‡
Polymer Department, Faculty of Chemistry, Lomonosov Moscow State University,
Leninskie Gori, 117899, Moscow, Russia, and Laboratoire de Chimie des Polyme`res, Unite´ de recherche
associe´e au CNRS (UMR7610), Universite´ Pierre et Marie Curie, Tour 44, 1er e´tage,
4, Place J ussieu, 75252 Paris Cedex 05, France
Received February 4, 2004; Revised Manuscript Received J une 22, 2004
ABSTRACT: The RAFT polymerization of n-butyl acrylate was studied in the presence of various
concentrations of tert-butyl dithiobenzoate (t-BDB) as a chain transfer agent, and various concentrations
of AIBN as an initiator at 60 and 90 °C. The polymerizations exhibited the expected features of controlled
systems. In particular, the linear increase in number-average molar mass (Mn) with monomer conversion,
from very low conversion values, indicated a large apparent chain transfer constant to t-BDB. The kinetic
study showed two typical characteristics already observed in RAFT polymerizations of various acrylates:
the existence of an induction period and a strong rate retardation, both enhanced by the increase in
t-BDB initial concentration. ESR spectroscopy was applied at 90 °C and revealed the existence of an
unexpectedly large concentration of intermediate radicals. The system was examined and discussed
considering either the possibility of cross-termination between a propagating macroradical and an
intermediate one, with fast fragmentation of the intermediate radicals or the absence of cross-termination
but slow fragmentation.
In tr od u ction
alkyl or aryl), trithiocarbonates (Z ) R′S-), and dithio-
carbamates (Z ) R′R′′N-), whereas MADIX requires the
use of xanthates (i.e., dithiocarbonates, Z ) R′O-). The
great diversity of the RAFT agents comes from the
possible combination of various Z and R groups. The
activity of such molecules in controlling the free-radical
polymerization process strongly depends on the nature
of R, of Z, and of the selected monomer.10,11
A wide variety of well-defined homopolymers and
copolymers with complex architectures have become
easily accessible since the advent of controlled free-
radical polymerization (CRP).1-3 Several methods are
available such as nitroxide-mediated polymerization
(NMP),4 atom transfer radical polymerization (ATRP),5,6
and reversible transfer techniques (reversible addition-
fragmentation chain transfer, RAFT,7-11 and macro-
molecular design via the interchange of xanthates,
MADIX12). The reversible transfer methods based on
addition-fragmentation reaction are particularly ver-
satile as they allow to control the homopolymerization
of a great variety of monomers7-15 such as styrenics,
acrylates, methacrylates, vinyl esters, and polar or
water-soluble monomers such as acrylamides16-18 and
acrylic acid.7,8,19,20 Control operates via the introduction
of a specific chain transfer agent (CTA) into the polym-
erization medium. In the RAFT and MADIX methods
these CTA have in common to contain a thiocarbonylthio
function, and their structure can be schemed as Z-
C(dS)-S-R with Z the activating group and R the
leaving/initiating group. The admitted transfer mech-
anism follows two steps. The first one is the addition of
a propagating radical P• to the CdS double bond of the
CTA, which leads to an adduct radical P-S-C•(Z)-S-
R. The second step corresponds to fragmentation of this
intermediate radical into R• and the Z-C(dS)-S-P
dormant species. The latter also plays the role of a CTA,
undergoing the same addition-fragmentation reactions
as the Z-C(dS)-S-R initial compound via a P-S-
C•(Z)-S-P intermediate radical (Scheme 1). For the so-
called RAFT method the CTA can be dithioesters (Z )
Kinetics of the polymerizations based on the RAFT
process have been extensively studied, in particular for
styrene, and an important aspect, commonly revealed
by literature data, is the significant rate retarda-
tion.10,11,13,14 The reason for this retardation is not
completely clear so far. On one hand, slow fragmenta-
tion of the initial intermediate radical (see Scheme 1)
and/or the influence of the nature of the leaving group
(such as slow reinitiation) were suggested to be respon-
sible for this phenomenon.21-24 Actually, the ongoing
debate mainly focuses on the magnitude of the frag-
mentation rate constant.25,26 On the other hand, the
possibility of irreversible or reversible termination (by
combination27 or possible disproportionation28) of the
intermediate radical with another (macro)radical in the
polymerization medium was put forward.27-33 To this
point, a definitive conclusion is still expected since
experimental indications supporting either the exis-
tence27,31-33 or the absence28 of cross-termination prod-
ucts were reported. A study devoted to the early stage
of the polymerization of styrene in the presence of
cyanoisopropyl dithiobenzoate showed that the first
addition step to the RAFT agent was extremelly selec-
tive (called initialization period).34 During that step the
RAFT agent was almost completely converted into its
single monomer adduct before the formation of larger
molar mass living chains. For this reason, in a system
where the rate constant of addition of the initiating
† Lomonosov Moscow State University.
‡ Universite´ Pierre et Marie Curie.
10.1021/ma049757r CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/30/2004

6330 Chernikova et al.
Macromolecules, Vol. 37, No. 17, 2004
Sch em e 1. Mech a n ism of P olym er iza tion Ba sed u p on Rever sible Ad d ition -F r a gm en ta tion Ch a in Tr a n sfer
radical (leaving radical from the RAFT agent) to the
monomer would be lower than the rate constant of
propagation, an induction period might then be ob-
served.
of the SEC peak).14 These high molar mass homopoly-
mers were still living as they retained the dithioben-
zoate group, which precludes their simple formation by
irreversible coupling between propagating macroradi-
cals.14 Polymerizations were found to be very slow with
respect to reactions carried out in the absence of chain
transfer agent. Indeed, more than 10 h was typically
required to reach high conversions.14,36,38,39 This was
particularly true when dithiobenzoate RAFT agents
were used. Indeed, changing to a transfer agent with
dithioacetate or phenyldithioacetate functionality en-
abled the polymerization to be faster and even controlled
at ambient temperature.37 Polymerization rate signifi-
cantly decreased when the amount of RAFT agent was
increased. In addition, a long induction period was
observed,38 the duration of which increased when the
concentration of RAFT agent was increased. Induction
and retardation were correlated to the structure of the
RAFT agent, in particular that of the R leaving group
in dithiobenzoate transfer agents, as exemplified by the
work of Perrier et al.38 Indeed, the longer induction
periods were observed in this order: R ) cumyl (com-
plete inhibition at 60 °C) > phenylethyl > cyanoisopro-
pyl, although the reverse would be expected on the basis
of the stability of the leaving radical itself. In contrast,
In comparison to styrene, much less work has been
devoted to the polymerization kinetics of other mono-
mers. For instance, the features of the polymerization
of acrylates, which represent an important class of
monomers, have already been reported, but generally
as part of a more general study.7-15,35 Only a very small
number of articles exclusively focused on acrylate RAFT
polymerization, in particular methyl acrylate28,36-38 and
n-butyl acrylate.39 All of these articles reported very
similar characteristics as far as polymerization kinetics
were concerned. In general, with most RAFT agents
used, polymerizations led to well-controlled homopoly-
mers with predictable molar mass and narrow molar
mass distribution due to very large chain transfer
constants. In some cases, a bimodal SEC (size exclusion
chromatography) distribution has been observed at high
conversion,10,13,14 depending on the nature of the RAFT
agent (i.e., with chain transfer agents exhibiting very
large chain transfer constants, the final polymer had
sufficiently narrow molar mass distribution to reveal
the existence of a shoulder on the high molar mass side

Macromolecules, Vol. 37, No. 17, 2004
Polymerization of n-Butyl Acrylate 6331
Ta ble 1. Exp er im en ta l Con d ition s for th e Va r iou s
P olym er iza tion s of n -Bu tyl Acr yla te in Bu lk , In itia ted by
AIBN in th e P r esen ce of ter t-Bu tyl Dith ioben zoa te
no induction period was observed with a poly(methyl
acrylate) macro-RAFT agent. All of these results allowed
the authors to conclude that slow fragmentation of the
first intermediate radical was responsible for this induc-
tion period.
expt
temp (°C)
[AIBN]0 (mol L-1
)
[t-BDB]0 (mol L-1
)
1a
1b
1c
1d
1e
1f
1g
2a
2b
2c
2d
2e
3a
3b
3c
3d
3e
3f
3g
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
6
60
60
60
60
60
60
60
60
60
60
60
60
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
3 × 10-2
1 × 10-2
3 × 10-3
1 × 10-3
3 × 10-4
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
1 × 10-3
3 × 10-2
1 × 10-2
3 × 10-3
1 × 10-3
3 × 10-4
1 × 10-4
1 × 10-1
3 × 10-2
1 × 10-2
3 × 10-3
1 × 10-2
1 × 10-2
1 × 10-1
3 × 10-2
1 × 10-2
3 × 10-3
1 × 10-3
3 × 10-4
0
This work is devoted to the kinetic study of the RAFT
homopolymerization of n-butyl acrylate in the presence
of tert-butyl dithiobenzoate as a reversible chain trans-
fer agent. Such transfer agent has not been used very
often.40-43 In particular, it has never been used for the
homopolymerization of alkyl acrylates, only in the
random copolymerization of methyl acrylate with vinyl-
idene chloride,42 in which it induced a severe rate
retardation when compared with other dithiobenzoate
RAFT agents such as benzyl dithiobenzoate and 1-
(ethoxycarbonyl)eth-1-yl dithiobenzoate. The purpose of
this work is to examine the kinetic features of such
polymerization and to understand the reasons for the
induction and rate retardation phenomena.
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-1
3 × 10-2
1 × 10-2
3 × 10-3
1 × 10-3
3 × 10-4
0
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-2
1 × 10-2
3 × 10-1
3 × 10-1
3 × 10-1
3 × 10-1
1 × 10-1
3 × 10-2
Exp er im en ta l P a r t
Ma t er ia ls. n-Butyl acrylate (n-BuA, Acros, 99%) was
distilled under vacuum before use. Azobis(isobutyronitrile)
(AIBN, Aldrich, 99%) was recrystallized from ethanol and dried
under vacuum. tert-Butylmercaptan (Aldrich, 99%) and S-
(thiobenzoyl)thioglycolic acid (Aldrich, 99% purity) were used
as received. Benzene (SDS, 99.5%) and tetrahydrofuran (THF,
Prolabo, 99%) were distilled on CaH2 before use.
Syn th esis of ter t-Bu tyl Dith ioben zoa te (t-BDB). Syn-
thesis of the tert-butyl dithiobenzoate RAFT agent was per-
formed according to a procedure previously described in the
literature.19 S-(Thiobenzoyl)thioglycolic acid (10.6 g, 0.05 mol)
was dissolved in dilute alkaline solution containing 2 equiv of
NaOH (4.0 g, 0.1 mol in 400 mL of H2O). Then, tert-butyl-
mercaptan (4.95 g, 0.055 mol) was added under stirring at
room temperature. The mixture was stirred for a period of 15
h. Afterward, tert-butyl dithiobenzoate, which separated out
as a heavy, dark, pink oil, was extracted with ether (1 × 600
mL, 1 × 300 mL). The organic extracts were washed with 0.1
N aqueous NaOH (3 × 300 mL) and water (3 × 300 mL), then
dried over MgSO4, filtered, and evaporated. Then the product
was left overnight under vacuum (0.01 mmHg, 40 °C) to
eliminate the residual tert-butylmercaptan (bp ) 63 °C). The
yield of tert-butyl dithiobenzoate (t-BDB), which remained as
a dark pink oil after purification, was 90%. 1H NMR (d6-
acetone; 200 MHz) δ (ppm): 1.69, 7.34, 7.47, 7.87. 13C NMR
(d6-acetone; 50 MHz) δ (ppm): 28.24, 52.63, 127.19, 128.91,
132.64, 147.71, 231.10. 1H and 13C NMR did not reveal the
presence of extra peaks, which indicates the absence of
impurities that could be accurately quantified.
7
The experimental conditions are summarized in Table 1.
P olym er iza tion Kin etics. Polymerization kinetics was
investigated at the Polymer Department of Moscow State
University using a Calvet type differential automatic calorim-
eter DAK-1-1a. An ampule containing 0.2 mL of the poly-
merization medium was put into the working cell. Another
ampule containing the same volume of a previously polymer-
ized reaction medium was put in the reference cell. The
amount of released heat was measured in isothermal condi-
tions at 60 or 90 °C. The conversions were calculated by
integration of the calorimetric curves, using the molar en-
thalpy of n-butyl acrylate polymerization:44 ∆H ) -76 kJ
mol-1. For all experiments, gravimetry was used to measure
conversion of the samples selected for molar mass analysis.
An a lytica l Tech n iqu es. Average molar mass and molar
mass distribution of the polymers were measured by size
exclusion chromatography (SEC). The analyses were carried
out in THF solution. For experiments 1a, 1c, 3a, and 3c they
were carried out at 40 °C, with a flow rate of 1 mL min-1
.
P olym er iza tion Con d ition s. For investigation of the
influence of t-BDB on the polymerization kinetics, the solution
containing AIBN (10-3 mol L-1) in freshly distilled n-BuA was
prepared first, and then this solution was added to a selected
amount of t-BDB. Changing t-BDB concentration was achieved
by diluting the prepared reaction mixture (AIBN, t-BDB,
n-BuA) with the initial solution of AIBN in monomer. For
investigation of the influence of AIBN on kinetics, the reaction
mixtures were prepared according to the same procedure, but
keeping t-BDB concentration the same (10-2 mol L-1) and
varying AIBN concentration. Afterward, a known amount of
each solution was poured into an ampule (approximately 0.2
mL) and degassed down to 5 × 10-3 mmHg residual pressure
using four freeze-vacuum-thaw cycles. The ampules were
immersed, for the required period of time, into an oil bath
thermostated at 60 or 90 °C. After polymerization, the reaction
mixtures were cooled in liquid nitrogen and diluted with
benzene. The polymers were extracted by lyophilization from
benzene.
Separation was performed using a GPC Vorsaule PSS SDV
8 × 300 mm, 5 µm, linear column. The apparatus is equipped
with a refractive index detector (LDC Analytical refractoMoni-
tor IV), a UV detector operating at 254 nm (Waters 484), a
Viscotek VE 7510 GPC degasser, a Waters 515 pump, and a
Viscotek VE 5200 autosampler. Chromatograms were recorded
using the TriSec Data Acquisition System software from
Viscotek. Molar masses were derived from a calibration curve
based on polystyrene standards (300-1.09 × 106 g mol-1),45
and calculation was performed using the TriSec 3.0 GPC
Software from Viscotek. For experiments 1b, 1c, 3b, and 3c
some analyses (at medium and high conversions) were carried
out at 35 °C with a flow rate of 0.9 mL min-1, using a Waters
liquid chromatograph equipped with a RI410 refractive index
detector and three columns packed with Ultrastyragel with
pore dimensions 104 and 105 Å and a Waters Styragel HR 5E
linear column (2000-4 × 106 g mol-1). Chromatograms were
processed with a Data Module-730 integrator using calibration
curve based on polystyrene standards.
Samples for ESR experiments were prepared similarly; the
Polymers obtained in experiment 3a were analyzed by
amount of reaction mixture in the ampule was 70 µL.
MALDI-TOF mass spectrometry performed using
a

6332 Chernikova et al.
Macromolecules, Vol. 37, No. 17, 2004
PerSeptive Biosystems Voyager Elite (Framingham, MA) time-
of-flight mass spectrometer. This instrument is equipped with
a nitrogen laser (337 nm, 3 ns pulse), a delayed extraction,
and a reflector. It was operated at an accelerating potential
of 25 kV in reflector mode. The MALDI-TOF mass spectra
represent averages over 2048 consecutive laser shots (3 Hz
repetition rate). The polymer solutions (2-5 g L-1) were
prepared in THF. The matrix, 1,8-dihydroxy-9[10H]-anthra-
cenone (dithranol), was also dissolved in THF (25 g L-1). A 10
µL portion of the polymer solution was mixed with 20 µL of
the matrix solution. A sodium iodide solution (10 µL of a
solution at 20 g L-1 in THF) was finally added to favor
ionization by cation attachment. A 1 µL portion of the final
solution was deposited onto the sample target and allowed to
dry in air at room temperature. Polystyrene standards of
known structure (purchased from Polymer Standards Service)
were used to calibrate the mass scale using the two-point
calibration software 3.07.1 from PerSeptive Biosystems.
Detection of the intermediate radicals was carried out at
the Polymer Department of Moscow State University by ESR
spectroscopy using a RE-1307 instrument, equipped with a
universal X-band (9.4 GHz) cavity using 100 kHz field modu-
lation, 30 mW microwave power, and amplitude in field
modulation 0.03 G. The reaction mixture prepared according
to the procedure described above was poured into a molybdenic
glass ampule with an inner diameter equal to 1.8 mm. The
experiment was carried out at 90 °C directly in the resonator
of the spectrometer. Data processing and simulations were
carried out using respectively the EPR (v. 2.3) and Bruker
EPRSRC software packages. The amount of paramagnetic
centers was determined by comparison of the integral of the
ESR signal of the sample with that of a carbon black standard
of known spin number.
F igu r e 1. Molar mass (a) and polydispersity index (b) vs
conversion for n-BuA polymerizations in bulk with AIBN as
an initiator and t-BDB as a reversible chain transfer agent.
See Table 1 for experimental conditions. Straight line: Mn
MWt-BDB + (conv[M]0/[t-BDB]0)MWmonomer
)
Resu lts
.
1. Mola r Ma ss Ch a r a cter istics a n d Str u ctu r e of
th e P olym er s. a . Aver a ge Mola r Ma ss a n d Mola r
Ma ss Dist r ib u t ion . Whereas classical radical poly-
merization of n-BuA usually leads to the formation of
cross-linked insoluble polymer at high conversions (the
same was observed for experiments 1g and 3g), in the
presence of a sufficient concentration of t-BDB as a
RAFT agent, polymerizations proceeded without the
formation of gel; i.e., the recovered polymers were
always fully soluble (it is for instance the case in
experiments 1a-1d and 3a-3c after 60-70% monomer
conversion).
F igu r e 2. Size exclusion chromatograms of the poly(n-BuA)
polymers prepared at 60 °C with AIBN as an initiator and
t-BDB as a transfer agent (experiment 1c). [AIBN]0 ) 10-3
Molar mass characteristics were determined for the
systems containing 10-3 mol L-1 of AIBN and different
concentrations of t-BDB at 60 °C (experiments 1a-1c)
and 90 °C (experiments 3a-3c). Figure 1a shows that
for all of the investigated systems, independently of the
temperature, Mn increased linearly with monomer
conversion, in good agreement with the theory of living/
controlled polymerization (theoretical Mn was calculated
from the ratio of monomer initial concentration over
RAFT agent initial concentration). At the same time the
polymers were characterized by low values of the
polydispersity indexes (PDI), which remained signifi-
cantly below 1.5 throughout the polymerization (see
Figure 1b). These results fully confirmed that the
polymerizations proceeded via a controlled mechanism,
which is also illustrated in Figure 2 (experiment 1c) by
the shift of the SEC chromatograms toward larger molar
masses, with the progress of monomer conversion.
However, the quality of control slightly deteriorated at
large monomer conversions as shown by the PDI, which
extended from 1.07 to 1.20 over a broad conversion
range, but increased up to 1.33 at high conversions
(Figure 1b). In addition, it can be seen for experiment
mol L-1; [t-BDB]0 ) 10-2 mol L-1
.
1c after 60% conversion (Mn > 70 000 g mol-1) that Mn
simultaneously increased above the theoretical line
(Figure 1a) and that a shoulder appeared on the high
molar mass side of the chromatograms (Figure 2). The
appearance of this shoulder was also observed in experi-
ments 1b, 1e, and 1f, above ∼50-60% conversion. This
feature has already been observed in most RAFT
polymerizations of alkyl acrylates (see the Introduction)
and might be assigned either to branched structures as
a result from intermolecular chain transfer to polymer
or possibly to cross-termination products.
In RAFT polymerization, the important kinetic pa-
rameter that governs the linear increase of molar mass
with monomer conversion is Ctr, the apparent chain
transfer constant to the RAFT agent, namely t-BDB in
this work. As shown in Figure 1a, there is a good match
between the experimental molar masses and the theo-
retical ones, even at very low monomer conversions. It
is for instance the case for experiments 1a-1c and

Macromolecules, Vol. 37, No. 17, 2004
Polymerization of n-Butyl Acrylate 6333
F igu r e 3. MALDI-TOF-MS spectrum of poly(n-BuA) obtained
in experiment 3a at 12% conversion (Mn ) 1300 g mol-1; PDI
) 1.23).
Sch em e 2. Str u ctu r e of th e P oly(n -bu tyl a cr yla te)
P olym er Ch a in s Obta in ed by Con tr olled F r ee-Ra d ica l
P olym er iza tion in th e P r esen ce of t-BDB a s a
Rever sible Ch a in Tr a n sfer Agen t
Ta ble 2. Th eor etica l a n d Exp er im en ta l Mola r Ma ss (fr om
MALDI-TOF -MS; Mon oisotop ic P ea k ) of Na + Ion ized
P oly(n -bu tyl a cr yla te) fr om Exp er im en t 3a (See Sch em e
2 for P olym er Str u ctu r e)
sample characteristics
(from SEC)
n
theor m/z
exptl m/z
conv ) 12%
9
10
11
1385.80
1513.88
1641.96
1385.60
1513.59
1641.70
Mn ) 1300 g mol-1
PDI ) 1.23
conv ) 19%
12
13
14
1770.05
1898.13
2026.22
1769.49
1897.61
2025.66
Mn ) 2200 g mol-1
PDI ) 1.20
F igu r e 4. Kinetics of n-BuA polymerization in bulk at 60 °C
(a, b) and 90 °C (c) in the presence of AIBN as an initiator
and t-BDB as a transfer agent. See Table 1 for experimental
conditions.
conv ) 26%
15
16
17
2154.30
2282.38
2410.47
2153.57
2281.70
2409.73
Mn ) 3300 g mol-1
PDI ) 1.17
both ends (see Table 2). In the examined low-conversion
samples, no other peak series with cyanoisopropyl end
group or with structure corresponding to classical ter-
mination product or to 3-arm or 4-arm stars (that would
form by termination involving the intermediate radicals)
could be detected in the corresponding spectra. This does
not mean they did not exist, but simply the experimen-
tal conditions were not designed in order to target their
observation.
2. P olym er iza tion Kin etics. Figure 4a-c displays
the typical kinetic curves of the polymerization of n-BuA
in the presence of various concentrations of t-BDB at
60 °C (experiments 1a-1g) and 90 °C (experiments 3a-
3g). From Figure 4b at 60 °C it can be seen that all
conversion vs time plots exhibited a pronounced S-shape
in the presence of CTA, with an induction period that
increased with the increase in t-BDB initial concentra-
tion (for instance, see experiment 1b; at [t-BDB] ) 0.03
mol L-1 and 60 °C, polymerization started only after 2
h of heating). Moreover, increasing t-BDB concentration
resulted in the significant decrease of the polymerization
rate. The same shape was also observed in the poly-
merizations at 90 °C, but in this case, the induction
period was strongly reduced, being usually less than 10
min (Figure 4c, experiments 3a-3f).
conv ) 31%
18
19
20
2538.55
2666.63
2794.72
2537.82
2665.97
2794.10
Mn ) 4500 g mol-1
PDI ) 1.11
3a-3c above approximately 3% monomer conversion.
This result indicates that the RAFT agent is completely
consumed in the early stages of the polymerization and
hence that Ctr is large. The estimation of Ctr using the
method suggested by Chong et al.10 gave us a value of
about 100. More accurate determination of Ctr by direct
measurement of the conversion of both the RAFT agent
and the monomer by NMR and gas-phase chromatog-
raphy is planned.
b. Ch a in -En d Str u ctu r e An a lyzed by MALDI-
TOF -MS. Polymers obtained in experiment 3a were
examined by MALDI-TOF-MS. A typical spectrum is
presented in Figure 3 for a polymer sample exhibiting
12% conversion. A single series could be observed with
molar mass between the peaks corresponding to the
molar mass of one monomer unit (128.08 u). Comparison
of the experimental m/z with the calculated molar
masses corresponding to the structure in Scheme 2 led
to the conclusion that a large majority of the poly(n-
butyl acrylate) macromolecular chains exhibited the
expected structure with the RAFT agent fragments at

6334 Chernikova et al.
Macromolecules, Vol. 37, No. 17, 2004
From the kinetic analysis, we determined the appar-
ent external orders of the reaction with respect to AIBN
and t-BDB concentrations, using for each experiment
the maximum value of the polymerization rate, after the
induction period. For the experiments conducted at 60
°C and characterized by a long induction period (experi-
ments 1 and 2), the polymerization rate was calculated
for the same monomer conversion (the difference in
initiator consumption could be neglected due to low
value of AIBN decomposition rate constant, i.e., kd
)
8.5 × 10-6 s-1).44 For experiments 3 and 4 carried out
at 90 °C, where the induction period was significantly
shorter, calculations have been done for the same
polymerization time equal to 10 min. From the depen-
dence of the maximum initial polymerization rate vs the
initial concentrations of AIBN and t-BDB (Figure 5,
experiments 1-4) the apparent orders with respect to
AIBN and t-BDB were determined; they were close to
0.5 and -0.5, respectively, irrespective of the temper-
ature (for t-BDB, they were -0.50 ( 0.10 at 60 °C and
-0.59 ( 0.07 at 90 °C, whereas for AIBN, they were
0.52 ( 0.02 at 60 °C and 0.59 ( 0.03 at 90 °C).
3. ESR An a lysis of th e Rea ction Med iu m a t 90
°C. ESR qualitative and quantitative analysis of the
intermediate radicals in n-BuA polymerization in the
presence of cumyl dithiobenzoate RAFT agent was
previously reported by Hawthorne et al.35 In the present
work, the polymerization medium was also analyzed by
ESR. Heating the reaction mixture of n-BuA with AIBN
and t-BDB at 90 °C resulted, within 15-20 s of heating,
in the appearance of a signal, the intensity of which
increased with time and was strongly affected by AIBN
and t-BDB initial concentrations. The early typical
spectrum (Figure 6a) consisted of a 1:3:3:1 quadruplet
with complex hyperfine structure. Upon cooling the
sample to 60 °C, the signal disappeared completely but
reappeared with similar intensity after heating it again
up to 90 °C. The signal was the same when n-BuA was
replaced by another monomer such as styrene, N-
vinylpyrrolidone, or vinyl acetate or even when it was
replaced by the corresponding deuterated monomers
(Rd-styrene, d8-styrene, or Rd-vinyl acetate). These
results indicate, as we shall see below, that the R-proton
of the polymeric radical added to t-BDB did not signifi-
cantly influence the hyperfine structure of the resulting
adduct radical spectrum. In the absence of added
monomer (ESR spectra recorded in benzene or ethyl-
benzene solutions), the signal intensity was lower by
approximately 2 orders of magnitude.
F igu r e 5. Bilogarithmic plot of polymerization rate (Rp) vs
concentration of AIBN or t-BDB at 60 °C (open symbols,
experiments 1 and 2) and 90 °C (black symbols, experiments
3 and 4). Effect of [t-BDB]0: slope ) -0.50 ( 0.10 at 60 °C
and -0.59 ( 0.07 at 90 °C. Effect of [AIBN]0: slope ) 0.52 (
0.02 at 60 °C and 0.59 ( 0.03 at 90 °C. See Table 1 for
experimental conditions.
affect the ESR spectrum, then it was necessary to
assume hyperfine coupling with the δ-H of the leaving
tert-butyl group. To check this assumption, the spectrum
•
of I2 was simulated using the following parameters:
g-factor ) 2.0038 G, 9 tert-butyl δ-H with aδ-H ) 0.425
G, 1 γ-H with aγ-H ) 0.1 G, 2 benzenic ortho-H with
ao-H ) 3.65 G, 2 benzenic meta-H with am-H ) 1.34 G,
1 benzenic para-H with ap-H ) 3.99 G (Figure 6b). A
reasonably good fit with the experimental spectrum
shown in Figure 6a was observed. The larger hyperfine
coupling constant to the δ-H of the tert-butyl group with
respect to the γ-H of the added oligoradical was rather
unexpected and would require further investigation. The
absence of strong effect of the γ-H has already been
reported by Alberti et al.46 for the polymerization of
styrene in the presence of benzyl (diethoxyphosphoryl)-
dithioformate as a RAFT agent.
At longer times, the ESR spectrum gradually de-
formed (Figure 6c, polymerization time: 20 min; and
finally Figure 6d, polymerization time: 100 min) mainly
•
•
due to the change in radical structure (from I2 to I3 )
and possibly to the increase in chain length and medium
viscosity. In particular, the spectrum shown in Figure
•
6d was assigned to radical I3 (Scheme 1C). Hyperfine
structure was essentially influenced by the interaction
between the unpaired electron and the hydrogens of the
benzene ring. As mentioned above, negligible splitting
resulting from coupling to the two γ-H from the attached
polymer chains was detected. This was corroborated by
simulation (Figure 6e, g-factor ) 2.0043 G, 2 γ-H with
aγ-H ) 0.1 G, 2 benzenic ortho-H with ao-H ) 3.66 G, 2
benzenic meta-H with am-H ) 1.34 G, 1 benzenic para-H
with ap-H ) 3.96 G). Assignment of the spectrum shown
in Figure 6d was further confirmed by the results
obtained with poly(n-BuA) taken as a RAFT agent.
When poly(n-BuA) was isolated after polymerization,
purified, and then used as a RAFT agent in the
polymerization of a new portion of freshly distilled
monomer, the ESR spectrum observed at 90 °C and
At the early stage of the reaction, it is very likely that
the observed spectrum was directly related to the
species formed by the addition of an initiating radical
or a propagating oligoradical to t-BDB (Scheme 1). In
particular, in the presence of monomer, it is reasonable
to suppose that the ESR signal mainly corresponded to
the addition of an oligomeric radical to t-BDB (Scheme
•
1B, intermediate radical I2 ). The larger concentration
of this radical with respect to I1• formed in the absence
of monomer might be justified by either a better
chemical stability or a faster rate of formation. The ESR
spectrum displayed in Figure 6a after 3 min reaction
•
supposed to correspond to radical I3 was exactly the
same as that displayed in Figure 6d. The same spectrum
was also previously observed in the polymerization of
n-BuA in the presence of cumyl dithiobenzoate as a
RAFT agent.35 Since this RAFT agent differs from
t-BDB by only the nature of the leaving group, struc-
tures of the intermediate radicals I3• should be identical
in both cases, which explains the similarity in the ESR
spectra.
•
was thus assigned to the intermediate radical I2 . The
general envelope is characteristics of an interaction of
the unpaired electron with the hydrogen atoms of the
benzene ring. However, this is not sufficient to explain
the observed complex hyperfine structure. Since, as it
was shown above, the γ-H of the added radical did not

Macromolecules, Vol. 37, No. 17, 2004
Polymerization of n-Butyl Acrylate 6335
F igu r e 6. ESR spectra of the intermediate radicals formed in experiment 5a at 90 °C. (a) Experimental; time ) 3 min. (b)
Simulation (g ) 2.0038 G, 9 tert-butyl δ-H with aδ-H ) 0.425 G, 1 γ-H with aγ-H ) 0.1 G, 2 benzenic ortho-H with ao-H ) 3.65 G,
2 benzenic meta-H with am-H ) 1.34 G, 1 benzenic para-H with apH ) 3.99 G). The line width of the spectrum was set to 0.18 G,
and the line shape was Lorentzian/Gaussian ) 0.5. (c) Experimental; time ) 20 min. (d) Experimental; time ) 100 min. (e)
Simulation (g ) 2.0043 G, 2 γ-H with aγ-H ) 0.1 G, 2 benzenic ortho-H with ao-H ) 3.66 G, 2 benzenic meta-H with am-H ) 1.34
G, 1 benzenic para-H with ap-H ) 3.96 G). The line width of the spectrum was set to 1.08 G, and the line shape was Lorentzian/
Gaussian ) 0.8.
The influence of the concentrations of AIBN and
t-BDB on the kinetics of formation of the intermediate
by 1-2 orders of magnitude than the concentration
reported for n-BuA polymerization in the presence of
cumyl dithiobenzoate.35
•
•
radicals I2 and I3 at 90 °C is displayed in Figure 7a,b
(the overall concentration was given owing to superim-
posed ESR signals). All of the kinetic curves can be
divided into two parts, as already observed previously
for styrene.47,48 At the beginning the overall concentra-
tion of intermediate radicals increased with time,
reached a maximum concentration, and then decreased.
The time to reach the maximum increased when
[t-BDB]0 was increased or when [AIBN]0 was decreased.
The maximum concentration of intermediate radicals
increased with the increase in both AIBN and t-BDB
initial concentrations; it was in the 0.2 × 10-5-1.5 ×
10-5 mol L-1 range for the studied experiments (experi-
ments 5a-5d, 6, and 7). Such a concentration is
unexpectedly large for a free radical species; it is larger
Discu ssion
The presented experimental results concerning the
polymerization kinetics might give useful information
on the mechanism of the n-BuA/t-BDB polymerization
system proceeding via RAFT. They are discussed in this
part, in particular, (i) the existence of an induction
period, (ii) the large concentration of intermediate
radicals in the early stage of the reaction, and (iii) the
rate retardation accompanied by a -0.5th order of the
polymerization rate with respect to the RAFT agent. On
the basis of the previous literature, all those results
have to be examined considering either the possibility
of cross-termination with fast fragmentation or the

6336 Chernikova et al.
Macromolecules, Vol. 37, No. 17, 2004
absence of cross-termination, but slow fragmentation.
All kinetic equations with definition of the terms used
are reported in the Appendix at the end of this article.
In d u ction P er iod . The ESR experiments showed
that in the earliest part of the polymerization the
intermediate radicals, I•, accumulated in the reaction
medium, i.e., d[I•]/dt > 0. On the basis of the kinetic
analysis (see Appendix), the evolution with time of the
concentration of the propagating radicals P• reads as
shown in eq 1 (from eq A11 in the Appendix or Eq A22
when kt′ ) 0), where Ri is the initiation rate, kt the rate
constant of termination between propagating macro-
radicals, and k′t the rate constant of cross-termination
between a propagating macroradical and a I• adduct
radical.
d[P•]
dt
d[I•]
dt
) Ri - kt[P•]2 - 2k′[P•][I•] -
(1)
t
Consequently, during the I• accumulation period, the
concentration of the propagating radical P• would
increase quite slowly and remain lower than in a similar
reaction carried out in the absence of RAFT agent. Such
situation should persist as long as d[I•]/dt > 0. There-
fore, the initial formation of the intermediate radicals
might be the cause of the existence of an induction
period and of the observed S-shaped conversion vs time
plots, without the need to invoke cross-termination to
explain it. Such behavior is compatible with slow
fragmentation, but the experimental results do not
allow, at that stage, to give an estimation of the extent
of the fragmentation rate constant. The rate of addition
of the propagating radicals to the thiocarbonyl groups
of the RAFT agents, i.e., kadd[P•][RX] and ka′ dd[P•][PX],
also played an important contribution in the kinetics
of that period (see Scheme 1; RX ) t-BDB, PX ) polymer
chain terminated by the dithiobenzoate end group).
In contrast, the selectivity proposed by McLeary et
al.,34 which would initially form a larger proportion of
the initiating tert-butyl radical with respect to P•, might
not lead in our case to an initialization period with
reduced propagation rate. Indeed, the rate constant of
addition of the tert-butyl radical to an acrylate monomer
(kp1) is larger than the rate constant of propagation of
n-butyl acrylate, kp (kp1 ) 1.7 × 106 L mol-1 s-1 for the
addition of the tert-butyl radical to methyl acrylate at
60 °C,49 whereas kp ) 3.4 × 104 L mol-1 s-1 for the
propagation of n-butyl acrylate at 60 °C50). Conse-
quently, the expelled tert-butyl radical might add at
least one monomer unit, and the formed oligoradical
would then react quite rapidly with the t-BDB chain
F igu r e 7. Overall concentration of the intermediate radicals
vs time in n-BuA polymerization with AIBN as an initiator
and t-BDB as a transfer agent at 90 °C: (a) [AIBN]0 ) 10-2
mol L-1, effect of [t-BDB]0. (b) [t-BDB]0 ) 3 × 10-1 mol L-1
,
effect of [AIBN]0. See Table 1 for experimental conditions.
Simultaneously, the (k-add + kfr) value might be smaller
than for the polymerization of styrene, indicating a
slower fragmentation. It is however not possible from
our experimental data to conclude on the absolute range
of the fragmentation rate constants.
P olym er iza tion Ra te. It was shown by Kwak et
al.27,32 that cross-termination can explain the rate
retardation observed in the polymerizations based on
RAFT, in agreement with the following equation de-
scribing the stationary concentration of propagating
radicals as a function of initiator and RAFT agent initial
concentrations (see Appendix for the complete kinetic
analysis; eqs A1-A16).
1/2
Ri/kt
[P•] )
(2)
k′
t
1 + 2 Kapp[RX]
(
)
0
kt
•
transfer agent to produce the intermediate radical I2 .
The kinetic data determined after the induction period
(i.e., 0.5th and -0.5th order with respect to AIBN and
t-BDB, respectively) are in good agreement with such
calculated stationary concentration of propagating radi-
cals.
Ma xim u m Con cen t r a t ion of t h e In t er m ed ia t e
Ra d ica ls. The large concentration of I• measured at the
maximum increased with the increase in both AIBN and
t-BDB initial concentrations. This is in good qualitative
agreement with the theoretical predictions (see Ap-
pendix). This large concentration of intermediate radi-
cals might initially correspond to a large concentration
In the absence of cross-termination, the concentration
of propagating radicals would be lower than in a
classical system as long as the intermediate radicals
accumulate (i.e., d[I•]/dt > 0). However, after the maxi-
mum concentration in I•, no rate retardation is expected.
Indeed, on the basis of eq 1 and of the equations given
in the Appendix, when d[I•]/dt < 0, [P•] should not be
reduced with respect to a similar polymerization in the
absence of RAFT agent, considered at the same poly-
merization time (same Ri).
•
of I2 , caused by large AIBN and t-BDB initial concen-
trations (see Figure 7 and Table 1) together with a large
preequilibrium constant kadd/(k-add + kfr). In the studied
n-BuA/t-BDB system, kadd, the addition rate constant
of an acrylate terminated oligomer to t-BDB, might be
quite large (possibly larger than kp by 1 or 2 orders of
magnitude, if one considers the fast consumption of
t-BDB discussed at the beginning of this article).

Macromolecules, Vol. 37, No. 17, 2004
Polymerization of n-Butyl Acrylate 6337
•
Con clu sion s
d[I2 ]
) kadd[P•][RX] - (k-add + kfr)[I2 ] - k′[P•][I2 ]
•
•
t
The RAFT polymerization of n-butyl acrylate was
studied in the presence of various concentrations of
tert-butyl dithiobenzoate (t-BDB) as a chain transfer
agent and various concentrations of AIBN initiator at
60 and 90 °C. The polymerizations exhibited the
expected features of a controlled system. In particular,
the linear increase in number-average molar mass (Mn)
with monomer conversion, from very low conversion
values, indicated a large chain transfer constant to
t-BDB. The kinetic study showed two typical charac-
teristics already observed in RAFT polymerizations of
acrylates: the existence of an induction period and a
strong rate retardation, both enhanced by the increase
in t-BDB initial concentration. ESR spectroscopy was
applied at 90 °C and revealed the existence of an
unexpectedly large concentration of intermediate radi-
cals. In the early stage of the polymerization, such a
radical was attributed to that formed in the preequi-
librium by the direct addition of poly(n-butyl acrylate)
dt
(A1)
•
d[I3 ]
) ka′ dd[P•][PX] - 2k-′ add[I3 ] - k′[P•][I3 ] (A2)
•
•
t
dt
•
•
d[I•]
d[I2 ] d[I3 ]
)
+
) kadd[P•][RX] -
dt
dt
dt
(k-add + kfr)[I2 ] - k′[P•][I2 ] + ka′ dd[P•][PX] -
•
•
t
•
•
2k-′ add[I3 ] - k′[P•][I3 ] (A3)
t
d[I•]
dt
) (kadd[RX] + ka′ dd[PX])[P•] - (k-add + kfr)[I2 ] -
•
2k-′ add[I3 ] - k′[P•]([I2 ] + [I3 ]) (A4)
•
•
•
t
d[I•]
dt
[RX]
[PX]
) kadd
+ k′
[RX]0[P•] -
add
(
)
•
[RX]0
[RX]0
oligoradical to t-BDB (I2 ). With the progress of monomer
•
•
•
conversion and t-BDD consumption, it changed into I3 ,
[I2 ]
[I•]
[I3 ]
containing two polymeric fragments attached to the
sulfur atoms.
(k-add + kfr)
+ 2k′
[I•] - k′[P•][I•] (A5)
-add
t
•
(
)
[I ]
All the kinetics features of the n-BuA/t-BDB system
were examined considering two potential interpreta-
tions: either the possibility of cross-termination with
fast fragmentation or the absence of cross-termination,
but slow fragmentation. It appeared that the induction
period was not caused by a selectivity in radical addi-
tion, but by an initially slow increase in the concentra-
tion of the propagating radicals, due to the concomitant
formation of the intermediate radicals. Such behavior
was rather compatible with a slow fragmentation, at
d[I•]
) kadd,app[P•][RX]0 - kfr,app[I•] - k′[P•][I•] (A6)
t
dt
Ri is the initiation rate, kt the rate constant of termina-
tion between propagating macroradicals, and k′ the
t
rate constant of cross-termination between a propagat-
ing macroradical and a I• adduct radical. [RX]0 is the
initial concentration of the RAFT agent (i.e., t-BDB in
this work; R ) tert-butyl and X ) dithiobenzoate). The
apparent rate constants of addition and fragmentation
are respectively
•
least of the intermediate radical I2 . In the same way,
the high concentration of intermediate radicals was
explained by a high apparent equilibrium constant,
which is also compatible with slow fragmentation,
together with fast addition of the propagating radicals
to the t-BDB RAFT agent. The further rate retardation
was however better explained by the existence of cross-
termination.
[RX]
[PX]
kadd,app ) kadd
+ k′
(A7)
(A8)
add
[RX]0
[RX]0
•
•
[I2 ]
[I•]
[I3 ]
[I•]
kfr,app ) (k-add + kfr)
+ 2k′
-add
Ack n ow led gm en t . The authors are indebted to
the French Ministery of Research and Technology for
the financial support of E. Chernikova. This research
was also supported by the Russian Foundation for
Basic Research (Project 02-03-32183). They thank J .
Belleney from UPMC for the MALDI-TOF mass spec-
trometry analyses. B.C. is personally grateful to Dr. L.
Marx and Prof. A. Rassat for interesting and fruitful
discussions.
The evolution rate of the propagating macroradical
concentration with time can then be expressed as
follows:
d[P•]
) Ri - kt[P•]2 - k′[P•][I•] - kadd[P•][RX] +
t
dt
(k-add + kfr)[I2 ] - k′add[P•][PX] + 2k-′ add[I3 ] (A9)
•
•
•
•
d[P•]
dt
d[I2 ] d[I3 ]
) Ri - kt[P•]2 - 2k′[P•][I•] -
-
Ap p en d ix
t
dt
dt
(A10)
1. F ir st Ca se: Existen ce of Cr oss-Ter m in a tion
a lon g w ith F a st F r a gm en ta tion of th e In ter m ed i-
a te Ra d ica ls. Th e Stea d y-Sta te Con d ition s Ap p ly
for Bot h t h e P r op a ga t in g a n d t h e In t er m ed ia t e
Ra d ica ls. Consider that the addition-fragmentation
equilibrium starts from the second step (Scheme 1B),
with RX the RAFT agent, PX the polymer chains
d[P•]
dt
d[I•]
dt
) Ri - kt[P•]2 - 2k′[P•][I•] -
(A11)
t
In all those equations the homotermination between two
intermediate radicals was not considered.32 In addition,
fast reinitiation by the expelled tert-butyl radical was
supposed, owing to a very large rate constant of addition
to the monomer49 and to a large initial concentration of
the latter.
terminated by the dithiobenzoate end group, P•
)
t-Bu-(n-BuA)i•, or P• ) NC-C(CH3)2-(n-BuA)i• (with i )
•
1, 2, 3, ... small value) the propagating radical, and I2
and I3 the intermediate radicals ([I•] ) [I2 ] + [I3 ]).
•
•
•

6338 Chernikova et al.
Macromolecules, Vol. 37, No. 17, 2004
) Ri - kt[P•]2 - kadd[P•][RX] +
d[P•]
dt
When the overall concentration of intermediate radi-
cals reaches the steady state (at and after the maximum
concentration), the following equations can be written:
(k-add + kfr)[I2 ] - k′add[P•][PX] + 2k-′ add[I3 ] (A21)
•
•
d[I•]
dt
) kadd,app[P•][RX]0 - kfr,app[I•] - k′[P•][I•] ) 0
d[P•]
dt
d[I•]
dt
t
) Ri - kt[P•]2 -
(A22)
(A12)
At the maximum concentration of [I•]
kadd,app
[I•] )
[P•][RX]0
(A13)
kfr,app + k′[P•]
d[I•]
) 0
dt
t
(A23)
(A24)
Only if fragmentation is fast enough with respect to
cross-termination can the following simplified equation
be written, with Kapp the apparent equilibrium constant.
k
add,app[P•][RX]0 - kfr,app[I•] ) 0
k
add,app[P•][RX]0
k
add,app[P•][RX]0
[I•]max
)
) Kapp[P•][RX]0 (A25)
[I•] )
) Kapp[P•][RX]0 (A14)
kfr,app
kfr,app
At this stage, if one considers steady-state conditions
for the propagating radicals
Then, according to the equations below, the concen-
tration of the propagating radicals should undergo
reduction with respect to a classical polymerization
conducted in the absence of RX, only if cross-termination
occurs.27,32
d[P•]
d[I•]
) Ri - kt[P•]2 -
) Ri - kt[P•]2 ) 0 (A26)
dt
dt
and then
d[P•]
dt
) Ri - kt[P•]2 - 2k′[P•][I•] ) 0
(A15)
t
[P•] ) (Ri/kt)1/2
(A27)
1/2
Ri/kt
[P•] )
) r(Ri/kt)1/2 (A16)
This leads to
k′
t
1 + 2 Kapp[RX]
(
)
1/2
0
Ri
kt
[I•]max ) Kapp
[RX]0
(A28)
( )
kt
The term r corresponds to a retardation factor, which
can be defined as r ) (1 + 2(k′/kt)Kapp[RX]0)-1/2, where
Before the maximum in intermediate radicals concen-
tration, d[I•]/dt > 0, whereas after the maximum, d[I•]/
dt < 0. In this latter stage, the production of propagating
radical would be faster at a given polymerization time
than in the same polymerization conducted without any
RAFT agent. A larger than expected concentration
might then be observed ([P•] > (Ri/kt)1/2).
t
Ri is the initiation rate proportional to [AIBN]0.
Similarly, the concentration of intermediate radicals
can be written in the following way.
1/2
Ri/kt
[I•] ) Kapp[P•][RX]0 ) Kapp[RX]0
k′
t
1 + 2 Kapp[RX]
(
)
0
kt
Refer en ces a n d Notes
(A17)
(1) Controlled Radical Polymerization; Matyjaszewski, K., Ed.;
ACS Symp. Ser. 1998, 685.
(2) Controlled/ Living Radical Polymerization: Progress in ATRP,
NMP, and RAFT; Matyjaszewski, K., Ed.; ACS Symp. Ser.
2000, 768.
(3) Advances in Controlled/ Living Radical Polymerization; Maty-
jaszewski, K., Ed.; ACS Symp. Ser. 2003, 854.
(4) Hawker, C. J .; Bosman, A. W.; Harth, E. Chem. Rev. 2001,
101, 3661.
(5) Matyjaszewski, K.; Xia, J . Chem. Rev. 2001, 101, 2921.
(6) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101,
3689.
(7) Chiefari, J .; Chong, Y. K. B.; Ercole, F.; Krstina, J .; J effery,
J .; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C.
L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998,
31, 5559.
(8) Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J .; Chong, Y.
K.; Moad, G.; Thang, S. H. Macromolecules 1999, 32, 6977.
(9) Moad, G.; Chiefari, J .; Chong, Y. K.; Krstina, J .; Mayadunne,
R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int.
2000, 49, 993.
(10) Chong, Y. K.; Krstina, J .; Le, T. P. T.; Moad, G.; Postma, A.;
Rizzardo, E.; Thang, S. H. Macromolecules 2003, 36, 2256.
(11) Chiefari, J .; Mayadunne, R. T. A.; Moad, C. L.; Moad, G.;
Rizzardo, E.; Postma, A.; Skidmore, M. A.; Thang, S. H.
Macromolecules 2003, 36, 2273.
(12) Charmot, D.; Corpart, P.; Adam, H.; Zard, S. Z.; Biadatti, T.;
Bouhadir, G. Macromol. Symp. 2000, 150, 23.
The decrease in [I•] with time, after the maximum,
might then be assigned to a decrease in Ri, owing to
the fast consumption of AIBN at 90 °C.
2. Secon d Ca se: Cr oss-Ter m in a tion Does Not
Exist; Stea d y-Sta te Con d ition s Do Not Ap p ly for
th e In ter m ed ia te Ra d ica ls. If cross-termination does
not exist, the only way to explain rate retardation is to
consider a slow fragmentation process; in that case,
steady-state concentrations of the intermediate radicals
cannot be considered anymore.
•
d[I2 ]
) kadd[P•][RX] - (k-add + kfr)[I2 ] (A18)
•
dt
•
d[I3 ]
•
) ka′ dd[P•][PX] - 2k′-add[I3 ]
(A19)
dt
•
•
d[I•]
dt
d[I2 ] d[I3 ]
)
+
) kadd,app[P•][RX]0 - kfr,app[I•]
(A20)
dt
dt

Macromolecules, Vol. 37, No. 17, 2004
Polymerization of n-Butyl Acrylate 6339
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