Synthesis, characterization, and computational study of potential itaconimide-based initiators for atom transfer radical polymerization

Article abstract of DOI:10.1039/c4ra08981b

Atom transfer radical polymerization (ATRP) has been a promising technique to provide polymers with well-defined composition, architecture, and functionality. In most of the ATRP processes, alkyl halides are used as an initiator. We report the synthesis o

Full text of DOI:10.1039/c4ra08981b

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PAPER
Synthesis, characterization, and computational
study of potential itaconimide-based initiators for
atom transfer radical polymerization†
Cite this: RSC Adv., 2014, 4, 48163
a
b
c
a
*
Chetana Deoghare, C. Baby, Vishnu S. Nadkarni, Raghu Nath Behera
a
*
and Rashmi Chauhan
Atom transfer radical polymerization (ATRP) has been a promising technique to provide polymers with well-
defined composition, architecture, and functionality. In most of the ATRP processes, alkyl halides are used
as an initiator. We report the synthesis of three possible potential initiators, N-phenyl(3-bromo-3-methyl)
succinimide, N-phenyl(3-bromo-4-methyl)succinimide, and N-phenyl(3-bromomethyl)succinimide for
ATRP of N-phenylitaconimide (PI) and methyl methacrylate (MMA). These functionalized alkyl halides,
having structural similarity with PI, were characterized by FTIR, HRMS, 1H, 13C NMR spectroscopy, and
elemental analysis. The equilibrium constants for the ATRP activation/deactivation process (KATRP) of
these alkyl halides as well as a commercial ATRP initiator (ethyl-a-bromoisobutyrate) were determined
using UV-Vis-NIR and DOSY NMR spectroscopy. Alternatively, these compounds and similar alkyl halides
(R-X) were investigated using density functional theory for their possible chain initiation activity for the
ATRP process. The B3LYP functional and 6-31+G(d)/LanL2DZ basis set was used for the prediction of
geometries and energetics associated with the homolytic R–X bond dissociation. The relative value of
KATRP and its variation with system parameters (such as substituent, temperature, and solvent) was
investigated. We found a good agreement between the experimentally determined and theoretically
calculated KATRP values. Our experiments show that the newly synthesized initiator N-phenyl(3-bromo-
3-methyl)succinimide performs better than the commercially available initiator ethyl-a-bromoisobutyrate
for the atom transfer radical copolymerization of PI and MMA.
Received 20th August 2014
Accepted 2nd September 2014
DOI: 10.1039/c4ra08981b
www.rsc.org/advances
halides, a-haloesters, a-haloketones, a-halonitriles, sulfonyl
halides, and iniferters. The basic mechanism of the ATRP
Introduction
Atom transfer radical polymerization (ATRP)1 is a popular and
robust controlled radical polymerization (CRP) technique for
the preparation of polymers with controlled architecture and
site-specic functionality. Like other CRP methods,2–5 ATRP is
controlled by the equilibrium between propagating radicals and
dormant species (mostly in the form of initiating alkyl halides
or macromolecular species).6 Various types of initiators have
been used in ATRP,2,7,8 e.g., halogenated alkanes, benzylic
process (Scheme 1) involves homolytic cleavage of the alkyl
halide (R–X) bond (activation step) by a transition metal
complex in its lower oxidation state (the activator, MtnY/L),
reversibly generating (with rate constant of activation, kact) the
propagating radical (Rc) and the transition metal halide complex
in its higher oxidation state (the deactivator, X–Mtn+1Y/L). In the
deactivation step (with rate constant of deactivation, kdeact), the
halide atom (X) is transferred back from the activator to the
propagating radical, also through homolytic bond dissociation.9
A successful ATRP process requires uniform growth of all the
chains and small contribution of the terminated chain. This can
be achieved through fast initiation and rapid reversible deac-
tivation and also by preserving the chain end functionality.10 For
aDepartment of Chemistry, Birla Institute of Technology and Science, Pilani – K. K.
Birla Goa Campus, Zuarinagar
– 403726, Goa, India. E-mail: rbehera@goa.
bits-pilani.ac.in; rchauhan@goa.bits-pilani.ac.in; Fax: +91 832 2557033; Tel: +91
832 2580331/153
bSophisticated Analytical Instrument Facility, Indian Institute of Technology Madras,
Chennai, Tamilnadu – 600036, India
cDepartment of Chemistry, Goa University, Taleigao Plateau, Goa – 403206, India
† Electronic supplementary information (ESI) available: Supplementary data
associated with this manuscript: FTIR, 1H, 13C NMR, and HRMS spectra of
synthesized alkyl bromides; graphs related to the experimental determination
of the equilibrium constant of ATRP; and 1H NMR spectra of reaction mixture
of copolymers of PI and MMA. B3LYP/6-31+G(d)/LanL2DZ optimized the gas
phase geometries of all the studied compounds. See DOI: 10.1039/c4ra08981b
Scheme 1 Mechanism of transition metal-catalyzed ATRP.
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F(Y) ¼ 2kt(KATRP)2t + c0
(4)
an ATRP initiator to be efficient, the rate of initiation should be
faster than the rate of propagation, and there should be
minimum side reactions. However, a very reactive initiator may
provide too many radicals that will terminate in the early stage
and slow down the overall process. Thus, the initiator reactivity
should be comparable to the monomer reactivity. One way of
achieving this is to employ an initiator that structurally mimics
the dormant species.2,6,11 To obtain a well-dened polymer with
narrow molecular distribution, the halide group must rapidly
and selectively migrate between the growing chain and the
transition metal complex (the catalyst). Thus, the initiator
should be carefully selected in accordance with the structure
where,
ꢀ
ꢁ
2
I0C0
1
FðYÞ ¼
2
C0 ꢁ I0
C0 ðC0 ꢁ YÞ
ꢀ
ꢁ
2
I0 ꢁ Y
þ
þ
ln
I0C0ðC0 ꢁ I0Þ
C0 ꢁ Y
!
1
for C0sI0
(5)
2
I0 ðC0 ꢁ YÞ
and,
FðYÞ ¼
and reactivity of the monomers and metal complexes.12
A
2
C0
C0
1
normal ATRP process has one notable limitation in that the
catalysts used are sensitive to air and other oxidants.13 In order
to overcome this drawback, Matyjaszewski's group14 has devel-
oped an improved ATRP technique, namely, an activator
generated by electron transfer atom transfer radical polymeri-
zation (AGET-ATRP). In a typical AGET-ATRP system, a transi-
tion metal complex in its higher oxidation state such as CuII
complex is used as a catalyst instead of CuI complex for a
normal ATRP system. The CuI complex is generated in situ from
ꢁ
þ
for C0 ¼ I0 (6)
3
2
C0 ꢁ Y
3ðC0 ꢁ YÞ
ðC0 ꢁ YÞ
In the above eqn (2)–(6), Y ¼ concentration of deactivating
species (X-CuIIL), I0 ¼ initial concentration of initiator (R–X), C0
¼ initial concentration of catalyst (CuIY/L), R is the concentra-
tion of radical (Rc), kt is the termination rate constant, and t is
time. The KATRP can be calculated (eqn (4)) from the slope of the
rffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
slope
2kt
CuII complex using
ethylhexanoate.14a
a
reducing agent such as tin(II)-
plot F(Y) vs. t KATRP
¼
, provided that kt is known.
Theoretically, the relative values of KATRP can be estimated
from the homolytic bond dissociation energy (BDE) of the
initiating R–X under certain conditions.20 The atom transfer
equilibrium of the ATRP process.
Several experimental15 as well as theoretical15b,16 studies have
reported the kinetic and thermodynamic parameters of ATRP
for various systems. The rate of an ATRP process depends on the
concentration of monomer [M] and propagating radical [Pc].
The radical concentration depends on the position of the
(7)
equilibrium and equilibrium constant of ATRP (KATRP ¼ kact
/
can be viewed as the sum of the following two equilibrium
processes,7,9 viz. (i) the homolytic bond dissociation of alkyl
halide (eqn (8)) and (ii) X–Mtn+1Y/L bond formation (hal-
idophilicity, eqn (9)), so that KATRP ¼ KRX ꢀ KX.
kdeact). Experimentally, KATRP can be determined from the
polymerization kinetics. For example, in a CuIY/L (Y ¼ Cl/Br)-
catalyzed polymerization reaction when excess deactivating
species (X-CuIIL) is used and the concentration of other species
such as activator, initiator, and monomer do not change
signicantly, the rate of propagation (Rp) is given as17
KRX
c
c
*
ðiÞ R ꢁ X )
R þ X
(8)
KX
Rp ¼ kp[M][Pc] ¼ kpKATRP[M][I]0 ꢀ [CuIY/L]/[X-CuIIL] (1)
ðiiÞ MtnY=L þ Xc )
X ꢁ Mtnþ1Y=L
(9)
*
where kp ¼ rate constant of propagation, [I]0 ¼ initial concen-
tration of initiator R–X, and [CuI] ¼ concentration of activator.
Using eqn (1), KATRP can be determined provided that kp is
known.
Alternatively, KATRP can also be determined from the rate of
formation of a dormant species/persistent radical using the
Fischer–Fukuda equation for the persistent radical effect (eqn
(2) and (3)):15b,18
The value of the equilibrium constant KX depends on the
type of catalyst/ligand (MtnY/L) and halogen, X. For similar
conditions and using the same catalytic system (similar KX), the
overall equilibrium constant KATRP will depend on the ener-
getics of alkyl halide R–X, and a knowledge of the equilibrium
constant KRX alone will enable prediction of the relative value of
9,20a,20b
KATRP
.
Several studies have been performed on ATRP of methyl
methacrylate (MMA) using ethyl-a-bromoisobutyrate (4-Br) as
the initiator.21 We are interested in the copolymerization of N-
phenylitaconimide (PI) and MMA. Recently, we used the ATRP
method to copolymerize PI with MMA (in anisole at 80 ꢂC using
CuBr/bipyridine catalyst) using the commercially available
Y ¼ (6kt(KATRP)2I0 C02)1/3t1/3
.
(2)
(3)
2
ꢀ
ꢁ
1=3
KATRPI0C0
R ¼
tꢁ1=3
6kt
The Fischer–Fukuda equation can be modied15b,19 to take initiator 4-Br, which resembles the monomer MMA.22 The
into account the changes in catalyst and initiator concentration molecular weight of the obtained copolymer was found to be
(eqn (4)–(6)):
less (3412 g molꢁ1) than that obtained by a conventional poly-
merization method (23 300 g molꢁ1),23 which may have
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occurred due to the chosen initiator. Previous studies23,24 on water, followed by vacuum distillation over sodium
such copolymer systems have shown higher reactivity ratios for benzophenone.
itaconimides monomers as compared to MMA. This necessi-
The FTIR spectra were recorded on a Shimadzu DR-8031
tates the need to explore de novo initiators for ATRP of PI and FTIR spectrophotometer in the region from 4000 to 400 cmꢁ1
MMA. It will be of interest to copolymerize the same system with using a KBr pellet. The 1H NMR and 13C NMR spectra were
different initiators obtained from renewable resources that have obtained by dissolving the samples in deuterated chloroform
structural similarities with the other monomer (i.e., the (CDCl3) using a Brucker AV III 500 MHz FT-NMR and a Brucker
itaconimide).
DRX500 spectrometer, respectively. Chemical shis (d) are given
In this study, we report the synthesis of three alkyl halides relative to tetramethylsilane (TMS). Diffusion ordered spec-
[N-phenyl(3-bromo-3-methyl)succinimide, N-phenyl(3-bromo-4- troscopy (DOSY) NMR measurements were performed on a
methyl)succinimide, and N-phenyl(3-bromomethyl)succinimide] Bruker Avance (AV III) 500 MHz NMR spectrometer, equipped
that are structurally similar to PI. These alkyl halides were with a 5 mm broadband observe (BBO) z-axis gradient probe
prepared from itaconic acid.25 The KATRP values of the synthe- which delivers a maximum gradient strength of 50 G cmꢁ1
.
sized alkyl halides and 4-Br were determined using UV-Vis-NIR Experiments were carried out with active temperature regula-
spectroscopy. These alkyl halides, as well as some similar tion at 25 ꢂC. Self-diffusion coefficients were measured using
halides and some well-known ATRP initiators, were also inves- the stimulated echo, bipolar gradient (stebpgp1s) pulse
tigated using density functional theory (DFT) before attempting sequence. Generally, the diffusion coefficient (D) is experi-
to use them as initiators for the ATRP copolymerization. The mentally determined by monitoring the signal intensity decay
predicted initiator, N-phenyl(3-bromo-3-methyl)succinimide, in a 1D pulsed-eld gradient spin-echo experiment (PFGSE)
was successfully tested for the copolymerization of PI and spectrum as a function of the applied gradient strength.28 In the
MMA via the AGET-ATRP process.
two-dimensional DOSY experiment,29,30 the decay of magneti-
zation as a function of increasing gradient intensity (i.e., the
gradient ramp) was observed. The gradient ramp in our exper-
iment was adjusted between 2 and 95% strength of the gradient
amplier using 16 equidistant steps. Each experiment was
acquired with a spectral width of 4500 Hz and 16k complex
Experimental section
Chemicals and materials
Itaconic acid (99.0%) and phosphorus pentoxide (95.0%) were points. A diffusion time (D) of 100 ms was used, and the dura-
used as supplied; chloroform (99.5%), acetic anhydride (98%), tion of the gradient pulse (d) was 1 ms. Aer Fourier transform
aniline (99.5%), and acetone (99.0%) were puried by distilla- and baseline correction, the spectra were processed with DOSY
tion. Anhydrous sodium acetate (99.5%) was dried over ame. processing tools from the Bruker Topspin 2.1 soware package.
Silica gel for column chromatography was used as supplied. The data were analyzed using variable gradient tting routines,
Dichloromethane (DCM, 99.0%) and toluene (99.0%) were dried and in all cases, the proton resonances were t with a single
using fused calcium chloride and puried by distillation. 2,20- exponential decay function using peak intensities. HRMS was
Bipyridine (Bpy) (99.5%) and acetonitrile dry (99.5%) were used recorded using the 1290 Innity UHPLC System, 1260 Innity
as supplied. All of the above chemicals were obtained from S. D. Nano HPLC with Chipcube, and 6550 iFunnel Q-TOF. Elemental
Fine Chem Limited, Mumbai, India. Itaconic anhydride (I) was analysis of the compounds was done using a Vario Micro Cube
synthesized from itaconic acid using the known methods elemental analyzer. The spectroscopic measurements were
previously reported.26 HBr gas was prepared by the bromination obtained with
a Jasco V-570 UV-Vis-NIR spectrometer.
of 1,2,3,4-tetrahydro-naphthalene (99.0%, Sigma-Aldrich, Ban- Molecular-mass characteristics of the copolymers were deter-
galore, India) with bromine (95.0%, S. D. Fine Chem Limited, mined by gel permeation chromatography (GPC) in THF as an
Mumbai, India) using a previously published procedure.27 eluent at a ow rate of 0.75 mL minꢁ1 and column temperature
Ethyl-a-bromoisobutyrate (98.0%), CuBr2 (99.9%), and tin(II) of 25 ꢂC with an Agilent 1260 HPLC-GPC system equipped with
ethylhexanoate [Sn(EH)2] (95.0%) were obtained from Sigma- column: PL gel 5 micron Mixed D: 300 mm ꢀ 7.5 mm with a
Aldrich Chemicals Private Limited, Bangalore, India and used differential refractometer. Polystyrene standards with a molec-
as supplied. CuBr (99.9%, Sigma-Aldrich Chemicals Private ular weight of 103 to 105 g molꢁ1 were used for calibration.
Limited, Bangalore, India) was puried by stirring overnight in
glacial acetic acid and washing with absolute ethanol and
diethyl ether, followed by drying under vacuum. Methyl meth-
Computational details
acrylate (99.0%, S. D. Fine Chem Limited, Mumbai, India) was Gaussian09 31 was used as the source program for all theoretical
puried by washing with 5% NaOH solution to remove the calculations. All the geometries were fully optimized using the
inhibitor, followed by repeated washing with distilled water hybrid B3LYP exchange correlation functional32 with the 6-
until the pH was neutral. It was then kept on anhydrous 31+G(d) basis set, except for iodine, where we used the LanL2DZ
magnesium sulfate to remove any traces of water followed by basis set. Frequency calculations were performed for all the
vacuum distillation with calcium hydride. Anisole (99.0%, SRL compounds to conrm (no imaginary frequencies) the
Private limited, Mumbai, India) was puried by washing with stationary points as minima on the potential energy surface.
5% NaOH solution and then with distilled water. It was then The scaling factor used for the frequency calculation was
kept on anhydrous potassium carbonate to remove any traces of 0.9613.33 We have used the Gauge-Independent Atomic Orbital
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(GIAO) method inbuilt in the Gaussian09 soware for
1H NMR (CDCl3) d: 7.4 (m, 2H, C6H5), 7.3 (m, 2H, C6H5), 7.2
computing NMR properties. All of the 1H NMR calculations (m, 1H, C6H5), 6.4 (m, 1H, H1 of vinylic CH2), 5.6 (m, 1H, H2 of
were carried out using the HF/6-311+G(2d,p) method from vinylic CH2), 3.4 (m, 2H, CH2 of ring).
ꢂ
B3LYP/6-31+G(d) optimized geometry. All the systems contain-
N-Phenylcitraconimide (IV), (6.2 g, yield 25%, mp 100 C). IR
ing unpaired electrons were optimized with spin unrestricted (KBr, cmꢁ1): 3082 (aromatic C–H stretch), 3069 (alkenyl C–H
formalism. The spin contamination was found to be negligible stretch), 2923 (C–H stretch), 1701 and 1717 (>C]O of imide),
(the mean value of the S2 operator was close to the theoretical 1641 (>C]C< stretch of double bond in ring), 1593, 1505, 1409
value of 0.75 for all radicals). A spin–orbit correction term was (aromatic >C]C< stretch), 876 (oop C–H bending).
applied for X ¼ Cl, Br, and I due to their atomic nature.34 The
1H NMR (CDCl3) d: 7.4 (m, 2H, C6H5), 7.3 (m, 2H, C6H5), 7.2
3
DFT calculation includes only the average energy of the ground (m, 1H, C6H5), 6.4 (q, 1H, 3J ¼ 1.6 Hz, CH), 2.1 (d, 3H, J ¼ 1.6
2
2
state P term; the extra stability of the real ground-state P3/2 Hz, CH3).
term is taken from the literature values (0.8, 3.5, and 7.3 kcal
The FTIR and 1H NMR spectra of compounds III and IV are
molꢁ1 for Cl, Br, and I respectively).35 Solvent effects were given in ESI Fig. S1 to S4.† The detailed characterization of
studied using Tomasi's polarizable continuum model (PCM).35 compounds III and IV has previously been reported.23,36
A. Synthesis of compounds II–IV (Scheme 2)
B. Synthesis of bromo substituted succinimides (Scheme 3)
1. Synthesis of N-phenylitaconamic acid (II). In a one-litre
3. Preparation of N-phenyl(3-bromomethyl)succinimide (1a-
kettle equipped with a mechanical stirrer, I (30 g, 0.27 mol) Br). To the solution of III (2 g, 0.01 mol) in 50 mL of toluene,
was dissolved in 100 mL of acetone. To this solution, aniline (25 HBr gas was purged for 24 h at 80 ꢂC in the presence of
mL, 0.27 mol), dissolved in 100 mL of acetone, was slowly added benzoyl peroxide, with continuous stirring that was extended
with vigorous stirring. Aer the N-phenylitaconamic acid star- for an additional 12 h. The crude product obtained by
ted precipitating out, the stirring was continued for another 12 concentrating the reaction mixture with a rotary evaporator
h. The obtained precipitate was ltered, dissolved in a saturated was puried by column chromatography using petroleum
solution of sodium bicarbonate, and reprecipitated with 5 M ether and diethyl ether as the solvent system, followed by
HCl. The precipitate was ltered, washed with distilled water, crystallization with DCM and hexane. The compound was
ꢂ
and dried in an oven at 80 C, giving II (19.0 g, yield 63%, mp dissolved in a minimal amount of DCM, and hexane was
145–150 ꢂC).
added dropwise until the solution was turbid. The mixture
2. Synthesis of N-phenylitaconimide (III) and N-phenyl- was then kept on ice to obtain crystals of 1a-Br (1.0 g, yield
ꢂ
citraconimide (IV). To the solution of II (25 g, 0.14 mol) in 100 50%, mp 177 C).
mL of acetone, acetic anhydride (25 mL, 0.25 mol) and
IR (KBr, cmꢁ1): 3069 (aromatic C–H stretch), 1784, and 1703
anhydrous sodium acetate (10 g, 0.12 mol) were added. The (>C]O of imide), 1590, 1491, and 1447 (aromatic >C]C<
reaction mixture was reuxed until a clear solution was stretch), 758 (oop C–H bending), 595 (C–Br stretch).
obtained, and reuxing with stirring was continued for
1H NMR (CDCl3) d: 7.3 (m, 1H, C6H5), 7.4 (m, 2H, C6H5), 7.5
another 5 h. The reaction mixture was then cooled to room (m, 2H, C6H5), 4.0 (dd, 1H, 1J ¼ 8.4 Hz, 2J ¼ 4.5 Hz, CH2), 3.7 (dd,
temperature and poured in an excess of ice cold water. The 1H, 1J ¼ 8.4 Hz, 2J ¼ 3.5 Hz, CH2), 3.5 (m, 1H, CH), 2.9 (dd, 1H, 1J
precipitate obtained was ltered and washed with a saturated ¼ 14.8 Hz, 2J ¼ 5.0 Hz, CH2), 3.1 (dd, 1H, 1J ¼ 14.8 Hz, 2J ¼ 9.5
solution of sodium bicarbonate followed by distilled water. Hz, CH2).
The crude product was puried by column chromatography
using 10% ethyl acetate in hexane as the eluent, giving III and 129.33, 128.97, 128.52, and 131.74 (C6H5), 41.20 (CH2Br), 33.38
13C NMR (125 MHz, CDCl3) d: 175.81 and 174.52 (>C]O),
IV Scheme 2.
(CH), 32.32 (CH2).
N-Phenylitaconimide (III), (12.5 g, yield 50%, mp 115 ꢂC). IR
Elemental analysis (CHNOBr): found – %C, 49.02; %H, 3.71;
(KBr, cmꢁ1): 3098 (aromatic C–H stretch), 3050 (alkenyl C–H %N, 5.13; %O, 11.88; %Br, 30.26; calculated for C11H10NO2Br:
stretch), 2994, 2955 (C–H stretch), 1789 and 1714 (>C]O of %C, 49.28; %H, 3.76; %N, 5.22; %O, 11.94; %Br, 29.80.
imide), 1663 (>C]C< stretch of double bond in ring), 1593,
HRMS (EI): (m/z) 267.7751 ([M]+; 95%), 269.4687 ([M + 2]+;
1501, 1453 (aromatic >C]C< stretch), 761 (oop C–H bending). 90%); calculated mass for C11H10NO2Br; 268.1066, observed
mass for C11H10NO2Br; 268.6219.
Scheme 2 Synthesis of N-phenylitaconimide (III) and N-phenylcitraconimide (IV).
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amount of DCM, and hexane was added dropwise until the
solution was turbid. The mixture was then kept on ice to obtain
ꢂ
crystals of 3a-Br (1.3 g, yield 65%, mp 109 C).
IR (KBr, cmꢁ1): 3048 (aromatic C–H stretch), 1751 and 1714
(>C]O of imide), 1591, 1501 and 1452 (aromatic >C]C<
stretch), 760 (oop C–H bending), 512 (C–Br stretch).
1H NMR (CDCl3) d: 7.3 (m, 1H, C6H5), 7.4 (m, 2H, C6H5), 7.5
(m, 2H, C6H5), 3.5 (d, 1H, 1J ¼ 18.8 Hz, CH2), 3.2 (d, 1H, 1J ¼ 18.8
Hz, CH2), 2.1 (s, 3H, CH3).
13C NMR (125 MHz, CDCl3) d: 174.74 and 171.65, 131.47,
129.34, 129.04, and 126.26 (C6H5), 51.29 (C–Br), 47.68 (CH2),
27.47 (CH3).
Elemental analysis (CHNOBr): found – %C, 50; %H, 3.82; %
N, 5.36; %O, 12.05; %Br, 28.77; calculated for C11H10NO2Br: %
C, 49.28; %H, 3.76; %N, 5.22; %O, 11.94; %Br, 29.80.
HRMS (EI): (m/z) 267.8351 ([M]+; 25%), 269.4401 ([M + 2]+;
25%); calculated mass for C11H10NO2Br; 268.1066, observed
mass for C11H10NO2Br; 268.5376.
Scheme 3 Synthesis of bromo-substituted succinimides.
The FTIR, 1H, 13C NMR, and HRMS data of synthesized
compounds 1a-Br, 2a-Br, and 3a-Br are given in ESI Fig. S5 to
S16.†
4. Preparation of N-phenyl(3-bromo-4-methyl)succinimide (2a-
Br). To the solution of IV (2 g, 0.01 mol) in 50 mL of toluene, HBr
gas was purged for 24 h at 80 ꢂC in the presence of benzoyl
peroxide, with continuous stirring that was extended for an
additional 12 h. The crude product obtained by concentrating
the reaction mixture with a rotary evaporator was puried by
column chromatography using petroleum ether and diethyl
ether as the solvent system, followed by crystallization with
DCM and hexane. The compound was dissolved in a minimal
amount of DCM, and hexane was added dropwise until the
solution was turbid. The mixture was then kept on ice to obtain
C. Synthesis of copolymers of PI and MMA using N-
phenyl(3-bromo-3-methyl)succinimide as the initiator. The
reaction for the synthesis of copolymers of PI and MMA using
AGET-ATRP is shown in Scheme 4. A solution of PI (1.87 g, 0.01
mol) and MMA (4.0 mL, 0.04 mol) in 10 mL of dry anisole was
poured into a three-neck 100 mL round-bottom ask equipped
with a reux condenser and a magnetic bead. To this reaction
mixture, CuBr2 (56 mg, 0.25 mmol), Sn(EH)2 (41 mL, 0.125
mmol), and 2,20-bipyridine (117.14 mg, 0.75 mmol) were added.
The reaction mixture was frozen under nitrogen, thawed, and
degassed under vacuum; this cycle was repeated twice. 3a-Br (67
mg, 0.25 mmol) was dissolved in 4 mL of dry anisole in a
separate two-neck 100 mL round-bottom ask and was
degassed with a freeze–pump–thaw cycle twice, and was then
charged to the three-neck reaction ask through a nitrogen-
purged syringe. The mixture was again subjected to a freeze–
thaw–vacuum cycle three times. The reaction ask was now
ꢂ
crystals of 2a-Br (1.3 g, yield 65%, mp 116 C).
IR (KBr, cmꢁ1): 3077 (aromatic C–H stretch), 1791 and 1716
(>C]O of imide), 1591, 1501 and 1444 (aromatic >C]C<
stretch), 763 (oop C–H bending), 520 (C–Br stretch).
1H NMR (CDCl3) d: 7.3 (m, 1H, C6H5), 7.4 (m, 2H, C6H5), 7.5
(m, 2H, C6H5), 4.9 (d, 1H, 2J ¼ 7.6 Hz, CH), 3.3 (m, 1H, CH), 1.5
2
(d, 3H, J ¼ 7.2 Hz, CH3).
13C NMR (125 MHz, CDCl3) d: 174.74 and 171.65, 131.45,
129.37, 129.07, and 126.28 (C6H5), 48.29 (C–Br), 40.68 (CH),
14.47 (CH3).
ꢂ
immersed in a hot oil bath maintained at 80 C. The polymer-
ization was terminated by adding the reaction mixture in excess
of methanol. The copolymer that precipitated out was ltered
and washed with hot methanol to remove any unreacted
monomers. The obtained copolymer was dissolved in 100 mL
acetone and passed through an alumina bed to remove the
catalyst. It was concentrated using a rotary evaporator and
reprecipitated with methanol. The obtained copolymer was
Elemental analysis (CHNOBr): found – %C, 49.27; %H, 3.76;
%N, 5.04; %O, 11.92; %Br, 30.01; calculated for C11H10NO2Br:
%C, 49.28; %H, 3.76; %N, 5.22; %O, 11.94; %Br, 29.80.
HRMS (EI): (m/z) 267.9651 ([M]+; 60%), 269.4657 ([M + 2]+;
60%); calculated mass for C11H10NO2Br; 268.1066, observed
mass for C11H10NO2Br; 268.7154.
5. Preparation of N-phenyl(3-bromo-3-methyl)succinimide (3a-
Br). To the solution of III or IV (2 g, 0.01 mol) in 50 mL of DCM,
HBr gas was purged for 24 h at room temperature in the absence
of benzoyl peroxide, with continuous stirring that was extended
for 12 h. The crude product obtained by concentrating the
reaction mixture with a rotary evaporator was puried by
column chromatography using petroleum ether and diethyl
ether as the solvent system, followed by crystallization with
DCM and hexane. The compound was dissolved in a minimum
Scheme 4 Reaction scheme for the synthesis of copolymer of PI and
MMA via AGET-ATRP using 3a-Br as the initiator.
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then dried under vacuum. The nal yield of the copolymer was molecular weight (M ¼ 175.18 g molꢁ1) and density (r ¼ 1.28 g
obtained using a gravimetric method. The reactions under cmꢁ3), the molar volume can be calculated as Vm ¼ M/d ¼
similar conditions were carried out in ten different reaction 136.97 cm3 molꢁ1, and using eqn (11), a value of d ¼ 6.11 ꢀ 10ꢁ8
asks and quenched at different time intervals, i.e., 5 h, 10 h, 15 cm is obtained. With this estimation of the reaction distance
h, 20 h, 25 h, 30 h, 35 h, 45 h, and 50 h.
(d), eqn (10) can be rewritten (for the phenylsuccinimide
D. Determination of the ATRP equilibrium constant radical) as:
(KATRP). The KATRP of the studied alkyl bromides were deter-
mined using eqn (4) and (5). In these equations, the parameters
I0 and C0 are known, and Y ¼ [BrCuIIBpy] and kt are determined
2ktD ¼ 2.31 ꢀ 1014 ꢀ D.
(12)
The diffusion coefficient D can be obtained from the classic
Einstein–Stokes equation or some of its variations40 or it can be
directly calculated using pulsed eld gradient NMR spectro-
scopic techniques.41 The latter and more accurate approach was
employed in the present study. DOSY allows the measurement
of translational diffusion of molecules in solution. The
measurement of diffusion is carried out by observing the
attenuation of the NMR signals during a pulsed eld gradient
experiment. The degree of attenuation is a function of the
magnetic gradient pulse amplitude and occurs at a rate
proportional to the diffusion coefficient of the molecule. In
practice, a series of NMR diffusion spectra are acquired as a
function of the gradient strength. It can be observed that the
intensities of the resonances follow an exponential decay. The
slope of this decay is proportional to the diffusion coefficient
according to the equation.
as follows.
6. Determination of [BrCuIIBpy]. To a dry, three-neck round-
bottom ask, dry acetonitrile (30 mL) was added, and
nitrogen was bubbled for ve min. CuIBr (15.06 mg, 3.7 mM)
and the ligand Bpy (32.80 mg, 7.0 mM) were then added to the
ask, and the contents were stirred for 30 minutes to ensure
complete dissolution of CuIBr and formation of complex. The
absorbance of the obtained solution (at l ¼ 745 nm for
BrCuIIBpy complex) was set to zero. To this solution, 4-Br (70
mM, 308 mL) was added, and the resulting mixture was stirred at
room temperature (25 ꢂC). The absorbance of this solution was
measured aer every 15 min. A linear increase in the absor-
bance was observed, which indicated an increase in the
concentration of the BrCuIIBpy complex. The same experiment
was repeated using 50 mM of 4-Br.
Similarly, the same experiments were performed using all
other synthesized alkyl bromides, viz. 1a-Br, 2a-Br, and 3a-Br, at
two different concentrations (i.e., 70 mM and 50 mM). The plots
of absorbance vs. time at 745 nm are given in ESI Fig. S17 to
S20.† The concentration of the BrCuIIBpy complex was deter-
mined using values of the extinction coefficient for the
BrCuIIBpy complex, which were determined separately.
To determine the extinction coefficient, different solutions
of BrCuIIBpy complex in acetonitrile were prepared, with known
concentrations ranging from 0.2 mM to 4.6 mM. The absor-
bance for each solution was measured using a quartz cuvette at
lmax ¼ 745 nm. The extinction coefficient calculated from the
slope of the curve in the plot of concentration vs. absorbance
I ¼ I0 exp(ꢁDg2g2d2D ꢁ d/3 ꢁ p/2
(13)
where I and I0 are the signal intensities in the presence and
absence of gradient, respectively, g is the gyromagnetic ratio, g
is the strength of the diffusion gradients, D is the diffusion
coefficient of the observed spins, d is the duration of the
diffusion gradient, and D is the diffusion time. All signals cor-
responding to the same molecular species will decay at the same
rate. The processing soware evaluates this decay behavior and
extracts the diffusion coefficient out of the signal decay curve. In
two-dimensional DOSY NMR, spectra are presented with the
chemical shi along the x-axis and the diffusion constant along
the y-axis. The DOSY NMR spectrum of SI and its diffusion
variable gradient are given in ESI Fig. S22 to S24.† From these
data, the diffusion coefficient of SI in deuterated acetonitrile
was found to be 1.65 ꢀ 10ꢁ5 cm2 sꢁ1. Correspondingly, the
value of the termination rate constant 2kt for SI in deuterated
acetonitrile at 25 ꢂC was obtained as 3.81 ꢀ 109 Mꢁ1 sꢁ1, which
is similar to the value used in the literature. This value of 2kt was
used for the determination of the KATRP values using acetonitrile
as the solvent for the above synthesized alkyl bromides.
8. Determination of KATRP using UV-Vis-NIR spectroscopy. The
KATRP values of the above synthesized alkyl bromides, i.e., 1a-Br,
2a-Br, 3a-Br and commercially available 4-Br, were determined
using the modied Fischer–Fukuda equation for the persistent
radical effect (eqn (4)). The values of F(BrCuIIBpy) were obtained
by substituting the values for the concentration of Y, C0 and I0
into eqn (4). The plots of F(BrCuIIBpy) vs. time (t) are linear in
nature for all the above mentioned alkyl bromides, and these
are given in Fig. 1 for the 70 mM concentration. Similar plots for
the 50 mM concentration are given in ESI Fig. S25.† The KATRP
(ESI Fig. S21†) was found to be 327 Mꢁ1 cmꢁ1
.
7. Determination of termination rate constant (kt). In the
literature,37 a constant value (2.5 ꢀ 109 Mꢁ1 sꢁ1) of kt has been
assumed for small radicals in the calculation of KATRP. However,
a more accurate value can be theoretically calculated using the
following formula:38
2ktD ¼ 3.78 ꢀ 1021 ꢀ D ꢀ d
(10)
where D is the diffusion coefficient of the radical in a given
medium and d is the reaction distance. The latter parameter can
be approximated using the approach of Gorrell and Dubois,19,39
according to the equation
ꢀ
ꢁ
1=3
Vm
NA
d ¼
(11)
where Vm is the molar formula of a stable model compound
structurally resembling the radical, and NA is Avogadro's
number. We used N-phenylsuccinimide (SI) as our model
compound for the above synthesized alkyl halides. Taking its
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Fig. 1 Plots of F(Y) ¼ F[BrCuIIBpy] vs. time (s) for the alkyl bromides at 70 mM, (a) 4-Br, (b) 1a-Br, (c) 2a-Br, and (d) 3a-Br.
Table 1 The KATRP values for R–X homolytic bond cleavage of the studied alkyl bromides in acetonitrile at 25 ꢂC determined using UV-Vis-NIR
spectroscopy. Experimental conditions: [R–X] : [CuBr] : [Bpy] ¼ 70 and 50 mM : 3.5 mM : 7.0 mM. The KATRP values, calculated using DFT
method, are given in the last column
R–X
KATRP at 70 mM
KATRP at 50 mM
Average KATRP
KATRP calculated
4-Br
1.40 ꢀ 10ꢁ09
0.89 ꢀ 10ꢁ18
1.07 ꢀ 10ꢁ11
1.39 ꢀ 10ꢁ09
1.18 ꢀ 10ꢁ09
1.39 ꢀ 10ꢁ18
1.29 ꢀ 10ꢁ11
1.40 ꢀ 10ꢁ09
1.29 ꢀ 10ꢁ09
1.14 ꢀ 10ꢁ18
1.18 ꢀ 10ꢁ11
1.40 ꢀ 10ꢁ09
3.93 ꢀ 10ꢁ09
2.77 ꢀ 10ꢁ18
1.37 ꢀ 10ꢁ11
1.42 ꢀ 10ꢁ09
1a-Br
2a-Br
3a-Br
values of synthesized alkyl bromides, i.e., 1a-Br, 2a-Br, 3a-Br, by the hydrobromination of III or IV with HBr gas in the absence
and commercially available 4-Br, were determined from the of benzoyl peroxide at room temperature using DCM as the
slope (plot of F(BrCuIIBpy) vs. t) and the above calculated value solvent. The hydrobromination reaction follows Markovnikov's
of 2kt. The results are summarized in Table 1.
addition (electrophilic addition reaction) of HBr across the
double bond in the absence of peroxide and anti-Markovnikov's
addition (free radical addition reaction) of HBr in the presence
of peroxide.42
Results and discussion
We also studied the bond dissociation enthalpies (BDEs) and
other energetics for the four series of alkyl halides 1, 2, 3, and 4
(Fig. 2). The chosen test set of alkyl halides includes (a) species
that mimic the dormant chain ends in the polymerization of PI
In this work, we report the synthesis of compounds 1a-Br and
2a-Br by the hydrobromination of III and IV, respectively, with
HBr gas in the presence of benzoyl peroxide at 80 ꢂC using
toluene as the solvent, while compound 3a-Br was synthesized
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Fig. 2 The alkyl halides, 1 (1a-X to 1d-X), 2 (2a-X to 2d-X), 3 (3a-X to
3d-X), and 4-X (X ¼ Cl, Br, I) investigated in this study.
and MMA (including the synthesized alkyl bromides 1a-Br, 2a-
Br, and 3a-Br), and (b) some common ATRP initiators (series 4).
The effect of the structure (e.g., primary, secondary, and tertiary
alkyl halides), substituent, and medium on the R–X bond
dissociation of these alkyl halides was studied, and a compar-
ison of their performance with known potential initiators was
evaluated. All the gas phase-optimized geometries (Cartesian
coordinates) of the studied compounds and radicals are given
in ESI Tables S1 and S2,† respectively.
Fig. 3 Correlation plot of theoretical and experimental values of
selected IR frequencies (in cmꢁ1) of the studied alkyl bromides.
Frequencies corresponding to C–C (both aliphatic and aromatic),
C]C (aromatic), C–H (both aliphatic and aromatic), C]O, C–N, and
C–Br are considered.
Homolytic bond dissociation enthalpies and free energies
The bond dissociation enthalpy (BDE) data for the studied
compounds is displayed in Table 3. They range from 304.25 kJ
molꢁ1 to 170.64 kJ molꢁ1 for series 1, 263.26 kJ molꢁ1 to 127.72
kJ molꢁ1 for series 2, 250.70 kJ molꢁ1 to 108.96 kJ molꢁ1 for
series 3, and 257.07 kJ molꢁ1 to 115.16 kJ molꢁ1 for series 4 alkyl
halides in gas phase. The R–X homolytic BDEs of alkyl chlorides
were found to be higher than the corresponding alkyl bromides,
which are in turn higher than their iodide counterparts. This
variation is parallel to the decreasing ionic character of halides,
Cl > Br > I. The decrease in BDEs was approximately 3–22 kJ
molꢁ1 from Cl to Br, but approximately 110–140 kJ molꢁ1 from
Structural features of alkyl halides and alkyl radicals
We used the B3LYP/6-31+G(d)/LanL2DZ method in our study.
This level of theory has been used in the literature for studying
similar processes.20a–c To check the accuracy of our computa-
tional results, we compared the selected IR frequencies and
calculated 1H NMR values with those obtained from experi-
ments with the alkyl bromides 1a-Br, 2a-Br, and 3a-Br. Corre-
lation plots between experimentally determined and
theoretically calculated values for selected IR frequencies and
1H NMR of these alkyl bromides are displayed in Fig. 3 and 4,
respectively. We found a good correlation between the experi-
mentally determined and theoretically calculated IR frequency
values (Fig. 3, R2 ¼ 0.99) and 1H NMR d values (Fig. 4, R2 ¼ 0.99).
This indicates that the chosen method of calculation is quite
appropriate for the studied compounds.
The R–X bond distances (rR–X) for the studied compounds in
gas phase as well as in two polar solvents are given in Table 2.
ꢀ
ꢀ
They are between the ranges 1.809 A to 2.202 A for series 1, 1.821
ꢀ
ꢀ
ꢀ
ꢀ
A to 2.224 A for series 2, 1.844 A to 2.261 A for series 3, and 1.855
ꢀ
ꢀ
A to 2.278 A for series 4 alkyl halides. The R–X bond distances
follow the trends 1 < 2 < 3 < 4 for a given X and Cl < Br < I for a
given series. The R–X bond lengths systematically increase for
all the studied alkyl halides with increasing polarity of the
medium and follow the trend gas phase < anisole < acetonitrile
for a given R–X. The dihedral angles for the R–X bond with
respect to the ring (or with respect to C(O)OC2H5 for 4) are
between 137ꢂ–177ꢂ for series 1 and 4, and between 171ꢂ–172ꢂ
for series 2 and 3. The carbon atom bearing the unpaired
electron for all the radicals was found to be planar in the sense
that the sum of the three bond angles at this carbon was found
to be 360ꢂ in all cases. The spin densities at this carbon varied
from 0.87 to 1.17 in the order 1 > 2 > 3 z 4.
Fig. 4 Correlation plot of theoretical and experimental d (in ppm)
values of 1H NMR of studied alkyl bromides.
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48 kJ molꢁ1 for series 4). This indicates that the entropy factor
contributing to the series 1, 2, and 3 alkyl halides (and similarly
for the series 4) is nearly the same. This is understandable from
the structural similarities among alkyl halides of series 1, 2, and
3 (and similarly, series 4). The free energies vary according to 4
z 3 < 2 < 1 for a given X and follows the trends Cl > Br > I for a
given series. The free energies, like BDEs, increase with
decreasing R–X bond lengths. This is obvious from the corre-
lation plot (Fig. 6) between the free energies and enthalpies. In
the presence of polar solvents, the variation of free energies
follows the trends gas phase > anisole > acetonitrile for the 2, 3,
and 4 series alkyl halides, while it is gas phase < anisole <
acetonitrile, for series 1 alkyl halides. This trend is similar to the
trends found in BDEs.
Table 2 The R–X bond lengths of studied alkyl halides in gas phase
and solution phase at 25 ꢂC
ꢀ
rR–X (A)
R–X
Gas
Anisole
Acetonitrile
1a-Cl
1a-Br
1a-I
1b-Cl
1b-Br
1c-Cl
1c-Br
1c-I
1d-Cl
1d-Br
1d-I
2a-Cl
2a-Br
2a-I
2b-Cl
2b-Br
2c-Cl
2c-Br
2c-I
2d-Cl
2d-Br
2d-I
3a-Cl
3a-Br
3a-I
3b-Cl
3b-Br
3c-Cl
3c-Br
3c-I
1.809
1.966
2.203
1.810
1.966
1.810
1.967
2.203
1.809
1.965
2.202
1.821
1.979
2.225
1.821
1.979
1.822
1.979
2.225
1.821
1.978
2.224
1.844
2.008
2.262
1.844
2.009
1.845
2.009
2.263
1.844
2.008
2.261
1.855
2.017
2.278
1.817
1.973
2.210
1.818
1.973
1.818
1.974
2.267
1.817
1.972
2.202
1.824
1.981
2.227
1.824
2.014
1.825
1.982
2.227
1.824
1.980
2.226
1.848
2.014
2.266
1.848
2.014
1.849
2.013
2.267
1.847
2.013
2.266
1.865
2.025
2.287
1.822
1.977
2.213
1.822
1.977
1.822
1.977
2.270
1.821
1.977
2.213
1.825
1.982
2.228
1.825
2.016
1.826
1.982
2.229
1.825
1.981
2.228
1.850
2.016
2.267
1.849
2.016
1.851
2.016
2.270
1.849
2.015
2.269
1.869
2.029
2.292
Relative equilibrium constants (KATRP
)
As discussed in the introduction, the overall equilibrium
constant (KATRP) can be given as the product of two equilibrium
Table 3 Enthalpy and free energy for R–X homolytic cleavage of
studied alkyl halides in gas phase and solution phase at 25 ꢂC
DH (kJ molꢁ1
)
DG (kJ molꢁ1
)
R–X
Gas
Anisole Acetonitrile Gas
Anisole Acetonitrile
1a-Cl 304.25 309.03 312.42
1a-Br 282.14 285.92 289.09
262.30 267.55 270.73
240.78 243.14 247.92
130.99 132.72 135.14
262.92 273.03 271.26
1a-I
171.1
173.88 176.37
1b-Cl 304.29 306.67 312.67
1b-Br 282.54 284.00 289.42
1c-Cl 304.51 309.27 312.75
1c-Br 282.65 286.18 289.48
244.74 254.1
248.66
3d-Cl
3d-Br
3d-I
4-Cl
4-Br
263.29 266.56 266.54
241.11 242.47 243.93
131.45 133.45 133.62
262.23 278.44 272.18
240.34 243.22 246.32
130.64 134.26 135.03
223.23 221.16 220.06
212.67 210.36 209.71
1c-I
171.31 174.16 176.77
1d-Cl 303.79 323.71 312.62
1d-Br 281.88 285.78 289.21
4-I
1d-I
170.64 173.72 176.46
2a-Cl 263.26 261.18 260.68
2a-Br 252.48 249.94 249.11
2a-I
2b-Cl 263.55
2b-Br 253.05
2c-Cl 263.56 261.67 261.14
2c-Br 252.86 250.51 249.56
2c-I
2d-Cl 262.22 260.06 259.76
2d-Br 251.6 248.79 248.34
2d-I 127.72 125.24 124.59
3a-Cl 250.7 247.39 245.92
3a-Br 247.88 243.71 242.06
3a-I 110.61 106.07 104.31
3b-Cl 251.07 247.62 246.46
3b-Br 248.42 244.18 242.59
3c-Cl 251.48 248.16 246.66
128.76 126.04 125.23
89.49
223.58
211.54
223.80 222.49 219.97
213.15 212.15 209.36
87.24
—
—
86.62
216.27
200.68
Br to I, for a given series. For a given halide, the trend in BDEs
was 3 z 4 < 2 < 1, which is nearly according to the stability of the
corresponding alkyl free radical generated. The BDE increases
as we go from series 2 to series 1 as compared to going from
series 3 to 2. For a given series of alkyl halides, the BDEs
decrease while the corresponding R–X bond distance increases
(Table 2) as we go from X ¼ Cl to X ¼ I. The BDEs correlate well
with the R–X bond lengths; as the R–X bond length increases,
the BDEs decrease (Fig. 5). Solvent polarity has a different effect
for 1ꢂ (series 1) alkyl halides than 2ꢂ and 3ꢂ alkyl halides. The
trends are gas phase > anisole > acetonitrile for 2, 3 and 4 series
alkyl halides, while the trend is gas phase < anisole < acetoni-
trile for series 1 alkyl halides. Thus, with increasing polarity of
the medium, the relatively stable radicals (the 2, 3 and 4 series)
become more unstable, while the less stable radical 1 series
becomes more stable.
—
—
260.93
251.99
129.05 126.47 125.8
89.70
87.43
87.31
222.29 220.69 223.86
211.83 208.9
88.14 86.13
208.66 204.27 202.69
205.12 199.94 198.21
68.86
208.01 204.38 203.1
206.26 199.3 197.32
209.61 207.29 203.48
205.76 201.86 196.88
69.54
207.07 202.72 203.66
204.06 198.95 197.71
67.58
209.9
208.93
85.24
64.63
64.28
3c-Br 248.7
3c-I 110.83 106.47 104.8
245.02 243.04
64.95
63.82
3d-Cl 249.42 245.81 244.9
3d-Br 246.95 242.58 241.26
3d-I
4-Cl
4-Br
4-I
108.96 104.45 102.84
257.07 255.98 255.53
247.87 246.44 245.9
115.16 112.71 111.66
62.81
208.81 205.31
62.26
The free energies for the studied alkyl halides (Table 3)
exhibit nearly a constant difference with the enthalpy data
(approximately 42 kJ molꢁ1 for series 1, 2, 3 and approximately
200.87 199.29 195.68
69.3 66.64 62.98
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in orders of magnitude of 10ꢁ4 to 10ꢁ16 for the gas phase at
25 ꢂC. All of the alkyl halides of series 3 and many from series 2
(2a-Cl, 2b-Cl, 2c-Cl, 2d-Cl, 2a-I, 2b-I, 2c-I, and 2d-I) have
comparable KATRP values with those of corresponding
commercial initiators (series 4). However, the alkyl halides of
series 1 have KATRP values much smaller than those of series 4
alkyl halides. The KATRP data follows trends 4 z 3 > 2 >> 1 for a
given X, and Cl < Br < I for a given series. With increasing
temperature from 25 ꢂC to 80 ꢂC, the values of KATRP increase by
an order of magnitude (103 to 109). A different substituent in the
phenyl ring at the para position has no signicant effect on
bond lengths, bond dissociation enthalpy (BDEs), free energy,
or the KATRP of the 1, 2, and 3 series of alkyl halides. If the
medium of the system is changed, the trends of KATRP shi from
gas phase > anisole > acetonitrile for series 1 alkyl halides, and
gas phase < anisole < acetonitrile for the rest of the alkyl halides
(series 2, 3, and 4). We obtained good correlation in the varia-
tion of KATRP and R–X bond lengths as well as variation of KATRP
with BDEs. These correlation plots are displayed in Fig. 7 and 8,
respectively. Fig. 7 implies that as the R–X bond length
increases, ꢁlog KATRP decreases, i.e., log KATRP increases. This
occurs because as the R–X bond length increases (bond
weakens), homolysis is easier, and therefore, KATRP increases.
Thus, iodides are better initiators than bromides, which in turn,
are better initiators than chlorides. However, for a given X, there
is little variation in the R–X bond length, yet there is an order of
magnitude difference in KATRP. This is mainly due to the
structural difference (primary, secondary, and tertiary) of alkyl
halides. A similar explanation holds for the correlations
between BDEs and KATRP as shown in Fig. 8.
Fig. 5 Correlation plot of bond dissociation enthalpies with R–X bond
distances of the studied alkyl halides.
constants, KRX and KX; the latter depends on a catalytic system
and halogen radical. Thus, for a given catalytic system and a
given X, KX will be a constant, and the relative value of KATRP can
be obtained from KRX. We have calculated the KRX of the alkyl
halides from their free energy using a standard textbook
formula.43 We estimated the KX ¼ (KATRP/KRX) values for the
series 4 alkyl halides from their literature KATRP values (1.50 ꢀ
10ꢁ6 for 4-Cl,7 3.93 ꢀ 10ꢁ9 for 4-Br,15b and 2.2 ꢀ 10ꢁ8 for methyl-
2-iodopropanoate7) and then used this KX value for the rest of
the compounds of a given X. We could have used the KATRP value
of 4-Br that was determined experimentally in this work for this
purpose. However, for chlorides and iodides, only the literature
values were used. Thus, we decided to use the literature values
for all the studied alkyl halides. The values of KATRP, so
obtained, are displayed in Table 4. A quick glance at Table 4
shows that the values of KATRP for the studied alkyl halides differ
The experimentally determined and theoretically calculated
values of KATRP for the studied alkyl bromides were found to be
comparable. The reported experimentally calculated KATRP value
for 4-Br is 3.93 ꢀ 10ꢁ09, which is quite comparable with our
experimentally calculated value of 1.29 ꢀ 10ꢁ09. Similarly, the
theoretically calculated KATRP values in acetonitrile at 25 ꢂC for
1a-Br, 2a-Br, and 3a-Br are 2.77 ꢀ 10ꢁ18, 1.37 ꢀ 10ꢁ11, and
1.42 ꢀ 10ꢁ09, respectively, which are in good agreement with
our experimentally determined KATRP values (1.14 ꢀ 10ꢁ18
,
1.18 ꢀ 10ꢁ11, and 1.40 ꢀ 10ꢁ09, respectively). The results are
summarized in Table 1. All the observations of experimentally
determined values of KATRP for the above mentioned alkyl
bromides (using the Fischer–Fukuda equation for the persistent
radical effect) are given in ESI Fig. S17 to S25.†
The KATRP values for 3a-Br were found to be comparable with
the commercially available initiator 4-Br. Hence, ATRP was
carried out using this initiator with the modied procedure
(AGET-ATRP) to overcome the sensitivity of the catalyst (CuIBr)
towards air and other oxidants.
AGET-ATRP of PI and MMA using 3a-Br as an initiator
The copolymerization of the monomers PI and MMA was
carried out using the mole fraction ratio of 2 : 8 in the presence
of CuBr2/Bpy/Sn(EH)2 in anisole at 80 ꢂC using 3a-Br as the
initiator. The percent conversion of monomer was calculated
gravimetrically by pouring the reaction mixture aer 5 h, 10 h,
Fig. 6 Variation of enthalpies with the free energies for the R–X bond
dissociation process for the studied alkyl halides.
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Table 4 Relative values of KATRP for R–X homolytic bond cleavage of
studied alkyl halides in gas phase and in solution phase
KATRP
R–X
Gas, 25 ꢂ
C
Gas, 80 ꢂ
C
Anisole, 25 ꢂ Acetonitrile, 25 ꢂ
C C
1a-Cl 9.92 ꢀ 10ꢁ16 2.03 ꢀ 10ꢁ07 7.68 ꢀ 10ꢁ17 5.19 ꢀ 10ꢁ18
1a-Br 4.00 ꢀ 10ꢁ16 1.86 ꢀ 10ꢁ08 8.18 ꢀ 10ꢁ17 2.77 ꢀ 10ꢁ18
1a-I
1.18 ꢀ 10ꢁ15 5.59 ꢀ 10ꢁ11 5.30 ꢀ 10ꢁ11 6.96 ꢀ 10ꢁ17
1b-Cl 7.72 ꢀ 10ꢁ16 1.58 ꢀ 10ꢁ07 8.40 ꢀ 10ꢁ18 4.18 ꢀ 10ꢁ18
1b-Br 8.11 ꢀ 10ꢁ17 4.32 ꢀ 10ꢁ09 9.82 ꢀ 10ꢁ19 2.06 ꢀ 10ꢁ18
1c-Cl 6.65 ꢀ 10ꢁ16 1.38 ꢀ 10ꢁ07 1.14 ꢀ 10ꢁ16 2.81 ꢀ 10ꢁ17
1c-Br 3.50 ꢀ 10ꢁ16 1.86 ꢀ 10ꢁ08 1.07 ꢀ 10ꢁ16 1.39 ꢀ 10ꢁ17
1c-I
9.82 ꢀ 10ꢁ16 4.71 ꢀ 10ꢁ11 1.76 ꢀ 10ꢁ16 1.28 ꢀ 10ꢁ16
1d-Cl 1.02 ꢀ 10ꢁ15 2.03 ꢀ 10ꢁ07 9.50 ꢀ 10ꢁ19 2.89 ꢀ 10ꢁ18
1d-Br 4.78 ꢀ 10ꢁ16 2.42 ꢀ 10ꢁ08 7.92 ꢀ 10ꢁ17 5.30 ꢀ 10ꢁ18
1d-I
1.36 ꢀ 10ꢁ15 6.25 ꢀ 10ꢁ11 1.27 ꢀ 10ꢁ16 7.26 ꢀ 10ꢁ17
2a-Cl 6.94 ꢀ 10ꢁ09 1.07 ꢀ 10ꢁ01 1.03 ꢀ 10ꢁ08 3.90 ꢀ 10ꢁ09
2a-Br 3.36 ꢀ 10ꢁ11 2.62 ꢀ 10ꢁ04 4.53 ꢀ 10ꢁ11 1.37 ꢀ 10ꢁ11
Fig.
8
Correlation plot of relative KATRP values with R–X bond
2a-I
2.20 ꢀ 10ꢁ08 7.21 ꢀ 10ꢁ05 2.20 ꢀ 10ꢁ08 2.20 ꢀ 10ꢁ08
enthalpies of studied alkyl halides.
2b-Cl 6.02 ꢀ 10ꢁ09 9.45 ꢀ 10ꢁ02
2b-Br 5.31 ꢀ 10ꢁ11 2.01 ꢀ 10ꢁ04
—
—
1.80 ꢀ 10ꢁ08
5.25 ꢀ 10ꢁ10
2c-Cl 5.50 ꢀ 10ꢁ09 8.65 ꢀ 10ꢁ02 6.01 ꢀ 10ꢁ09 4.04 ꢀ 10ꢁ09
2c-Br 2.77 ꢀ 10ꢁ11 2.22 ꢀ 10ꢁ04 2.20 ꢀ 10ꢁ11 1.58 ꢀ 10ꢁ11
15 h, 20 h, 25 h, 30 h, 35 h, 45 h, and 50 h in methanol. The
copolymer that precipitated out was washed with hot methanol,
dried, and weighed. The FTIR spectrum of the PI-MMA copol-
ymer (ESI Fig. S26†) shows the >C]C< stretching of the phenyl
ring at 1598, 1501, and 1456 cmꢁ1, pinpointing the incorpora-
tion of the phenylitaconimide in the copolymer. The absence
of characteristic absorption bands at 1663–1665 cmꢁ1 due to
>C]C< bond stretching indicates the absence of monomers.
The characteristic absorption bands at 1789 and 1714 cmꢁ1 are
due to carbonyl groups of imide (>C]O of imide), the absorp-
tion peak at 1229 cmꢁ1 is due to C–O stretching of MMA, and
the peak at 2850 cmꢁ1 is due to the C–H stretch of –CH3. In the
1H NMR spectrum of the PI-MMA copolymer in CDCl3 (ESI
Fig. S27†), the peaks in the region d ¼ 7.6–7.2 ppm are due to
the phenyl ring of PI. The –OCH3 of the side chain is observed at
d ¼ 3.8 ppm, whereas the signal at d ¼ 3.6 ppm may be attrib-
uted to the methylene group adjacent to the carbonyl group of
the side chain. Similarly, the –CH3 of the side chain is observed
at d ¼ 0.8 ppm. The two methylene groups of the polymer
2c-I
2.02 ꢀ 10ꢁ08 6.73 ꢀ 10ꢁ05 2.04 ꢀ 10ꢁ08 1.67 ꢀ 10ꢁ08
2d-Cl 1.01 ꢀ 10ꢁ08 1.46 ꢀ 10ꢁ01 1.24 ꢀ 10ꢁ08 8.42 ꢀ 10ꢁ10
2d-Br 4.72 ꢀ 10ꢁ11 3.49 ꢀ 10ꢁ04 8.16 ꢀ 10ꢁ11 1.88 ꢀ 10ꢁ11
2d-I
3.79 ꢀ 10ꢁ08 1.16 ꢀ 10ꢁ04 3.44 ꢀ 10ꢁ08 3.84 ꢀ 10ꢁ08
3a-Cl 2.47 ꢀ 10ꢁ06 1.72 ꢀ 10+01 9.36 ꢀ 10ꢁ06 4.30 ꢀ 10ꢁ06
3a-Br 7.06 ꢀ 10ꢁ10 4.11 ꢀ 10ꢁ03 3.03 ꢀ 10ꢁ09 1.42 ꢀ 10ꢁ09
3a-I
9.05 ꢀ 10ꢁ05 9.46 ꢀ 10ꢁ02 2.01 ꢀ 10ꢁ04 2.00 ꢀ 10ꢁ04
3b-Cl 3.22 ꢀ 10ꢁ06 2.30 ꢀ 10+01 8.96 ꢀ 10ꢁ06 3.65 ꢀ 10ꢁ06
3b-Br 4.46 ꢀ 10ꢁ10 2.64 ꢀ 10ꢁ02 3.93 ꢀ 10ꢁ09 2.03 ꢀ 10ꢁ09
3c-Cl 1.68 ꢀ 10ꢁ06 1.23 ꢀ 10+01 2.77 ꢀ 10ꢁ06 3.13 ꢀ 10ꢁ06
3c-Br 5.46 ꢀ 10ꢁ10 3.35 ꢀ 10ꢁ03 1.40 ꢀ 10ꢁ09 2.42 ꢀ 10ꢁ09
3c-I
6.89 ꢀ 10ꢁ05 7.30 ꢀ 10ꢁ02 1.76 ꢀ 10ꢁ04 2.17 ꢀ 10ꢁ04
3d-Cl 4.70 ꢀ 10ꢁ06 3.01 ꢀ 10+01 1.75 ꢀ 10ꢁ05 2.91 ꢀ 10ꢁ06
3d-Br 1.08 ꢀ 10ꢁ09 6.17 ꢀ 10ꢁ03 4.52 ꢀ 10ꢁ09 1.73 ꢀ 10ꢁ09
3d-I
1.52 ꢀ 10ꢁ04 1.43 ꢀ 10ꢁ01 4.19 ꢀ 10ꢁ04 4.07 ꢀ 10ꢁ04
4-Cl 1.50 ꢀ 10ꢁ06 1.56 ꢀ 10+01 1.50 ꢀ 10ꢁ06 1.50 ꢀ 10ꢁ06
4-Br 3.93 ꢀ 10ꢁ09 2.29 ꢀ 10ꢁ02 3.93 ꢀ 10ꢁ09 3.93 ꢀ 10ꢁ09
4-I
7.59 ꢀ 10ꢁ05 1.05 ꢀ 10ꢁ01 8.93 ꢀ 10ꢁ05 3.05 ꢀ 10ꢁ04
Table 5 The variation in molecular weight, PDI of copolymer, and %
conversion of monomer with time for copolymerization of PI and MMA
via AGET-ATRP. Experimental conditions: [PI]/[MMA]/[3a-Br]/[CuBr2]/
[Bpy]/[Sn(EH)2] ¼ 20/80/1/1/3/0.5, in anisole at 80 ꢂ
C
Molecular weight
S. no. Time (h) Yield (%) % Conversion (Mn) g molꢁ1
PDI
1
2
3
4
5
6
7
8
9
10
5
10
15
20
25
30
35
40
45
50
17.89
25.89
34.75
43.61
55.03
64.91
73.25
80.40
90.97
98.81
17
25
35
43
54
65
73
80
90
98
3292
4316
4858
5086
6772
7167
8174
10 518
12 347
13 224
1.32
1.56
1.50
1.37
1.32
1.34
1.41
1.53
1.47
1.30
Fig. 7 Correlation plot of relative KATRP values with R–X bond lengths
for the studied alkyl halides.
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Fig. 9 Plot of % conversion of monomer with time for AGET-ATRP of
Fig. 11 Plot of ln{[M]0/[M]t} with time for AGET-ATRP of PI and MMA.
PI and MMA. Experimental conditions: [PI]/[MMA]/[3a-Br]/[CuBr2]/ Experimental conditions: [PI]/[MMA]/[3a-Br]/[CuBr2]/[Bpy]/[Sn(EH)2] ¼
[Bpy]/[Sn(EH)2] ¼ 20/80/1/1/3/0.5, in anisole at 80 ꢂC.
20/80/1/1/3/0.5, in anisole at 80 ꢂC.
backbone are observed in the region d ¼ 2.8–1.7 ppm and d ¼ 1,1,2,2-tetrachloroethane. For the concentration determination
1.4–0.9 ppm, respectively.
of PI, the intensity of the peaks at d ¼ 6.5 was compared with the
The molecular weights of the copolymers were determined peaks for the standard at d ¼ 5.9 ppm. A linear plot (Fig. 11) is
using GPC. The details of the percent conversion of monomers, observed for ln{[M]0/[M]t} vs. time, further conrming that the
polydispersity index (PDI), and molecular weights are given in polymerization is occurring under controlled radical polymeri-
Table 5. A typical linear variation in the plots of % conversion of zation conditions. The 1H NMR spectra of the reaction mixture
monomers with time (Fig. 9) is observed, which is characteristic for the copolymerization of PI and MMA in CDCl3 at various
of controlled radical polymerization. The molecular weight of time intervals are given in ESI Fig. S28.† The PDI of the copol-
the copolymer also increases linearly with % conversion of ymer of PI and MMA was found to be 1.30 with 98% monomer
monomers (Fig. 10). The concentration of the unreacted
monomer (PI) in the reaction mixture was determined using 1H
conversion.
Recently, we reported the ATRP of PI and MMA using 4-Br as
NMR spectra of the reaction mixture recorded at various time the initiator, where we achieve only 50% conversion of mono-
intervals in the presence of a known amount of the standard, mer with 1.40 PDI for copolymers.22b Compared to this, the
current study demonstrates that the molecular weight of
copolymer, PDI ratio, and % conversion of monomer have been
improved using 3a-Br as the initiator. In addition, 3a-Br has
better control over the polymerization of PI and MMA as
compared to 4-Br.
Conclusions
We have reported the synthesis of three alkyl bromides
(N-phenyl(3-bromo-3-methyl)succinimide, N-phenyl(3-bromo-4-
methyl)succinimide, and N-phenyl(3-bromomethyl)succinimide)
that are structurally similar to PI for possible chain initiation
activity in the ATRP of PI and MMA. The KATRP of these alkyl
bromides as well as a commercially available initiator (ethyl-2-
bromoisobutyrate) was determined using UV-Vis-NIR spectros-
copy, and kt for radicals was determined using DOSY NMR
spectroscopy.
We have also studied the structural and thermodynamic
properties (bond dissociation enthalpies and free energies) of
these alkyl bromides (along with some similar alkyl halides and
some common ATRP initiators) using density functional theory.
Variation of the substituent in the phenyl ring at the para
Fig. 10 Plot of molecular weight of copolymer with % conversion of
monomer for AGET-ATRP of PI and MMA. Experimental conditions:
[PI]/[MMA]/[3a-Br]/[CuBr2]/[Bpy]/[Sn(EH)2] ¼ 20/80/1/1/3/0.5, in ani-
sole at 80 ꢂC.
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position of the studied alkyl halides has very little effect on
bond length, bond dissociation enthalpy (BDEs), free energy, or
KATRP. As expected, the R–X bond lengths increase in the order
Cl < Br < I for a given R and 4 z 3 < 2 < 1 for a given X, while the
BDEs and free energies follow the opposite trends. With an
increase in the polarity of the medium, the R–X bond distances
increase (BDEs and free energies decrease) for series 2, 3, and 4,
while opposite trends are observed for series 1 (primary) alkyl
halides. Relative values of KATRP were extracted from free energy
values with respect to common ATRP initiators. It was found
that values of KATRP slightly decrease with increasing solvent
polarity and increase signicantly with increasing temperature.
The value of KATRP for series 3 and many of the alkyl halides of
series 2 (2a-Cl, 2b-Cl, 2c-Cl, 2d-Cl, 2a-I, 2b-I, 2c-I, 2d-I and two of
2001, 101, 3661; (c) C. Barner-Kowollik, T. P. Davis,
J. P. Heuts, M. H. Stenzel, P. Vana and M. Whittaker, J.
Polym. Sci., Part A: Polym. Chem., 2003, 41, 365.
6 N. V. Tsarevsky and K. Matyjaszewski, Chem. Rev., 2007, 107,
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7 A. Mullar and K. Matyjaszewski, Radical Polymerization,
Controlled and Living Polymerization, WILEY-VCH Verlag
GmbH and Co. KGaA, Weinheim, 2009, pp. 103–166.
8 (a) T. Otsu, J. Polym. Sci., Part A: Polym. Chem., 2000, 38, 2121;
(b) W. Zhang, C. Wang, D. Li, Q. Song, Z. Cheng and X. Zhu,
Macromol. Symp., 2008, 261, 23; (c) R. Nicolay, Y. Kwak and
K. Matyjaszewski, Macromolecules, 2008, 41, 4585.
9 W. A. Braunecker and K. Matyjaszewski, Prog. Polym. Sci.,
2007, 32, 93.
the synthesized bromides 2a-Br and 3a-Br) are comparable to 10 (a) M. E. Levere, N. H. Nguyen and V. Percec, Macromolecules,
the commercially available initiator 4-Br for the ATRP process.
We found good agreement between our experimentally
determined and theoretically calculated KATRP values for 4-Br,
2012, 45, 8267; (b) N. H. Nguyen, M. E. Levere, J. Kulis,
M. J. Monteiro and V. Percec, Macromolecules, 2012, 45,
4606; (c) M. Zhong and K. Matyjaszewski, Macromolecules,
2011, 44, 2668; (d) Y. Wang, M. Zhong, Y. Zhang and
A. J. D. Magenau, Macromolecules, 2012, 45, 8929; (e)
Y. Wang, N. Soerensen, M. Zhong, H. Schroeder,
M. Buback and K. Matyjaszewski, Macromolecules, 2013,
46, 683.
ꢂ
1a-Br, 2a-Br, and 3a-Br in acetonitrile at 25 C. The copolymer-
ization of PI and MMA was successfully carried out using one of
our synthesized alkyl bromides, 3a-Br, as the initiator in anisole
at 80 ꢂC via AGET-ATRP. Comparisons with our recent work on
copolymerization of the same systems with the commercially
available initiator 4-Br demonstrates that our newly synthesized 11 (a) W. Tang and K. Matyjaszewski, Macromolecules, 2007, 40,
3a-Br performs more advantageously than 4-Br in terms of
controlling the rate of polymerization, % conversion of mono-
mer, and PDI of obtained copolymers.
1858; (b) B. M. Rosen and V. Percec, J. Polym. Sci., Part A:
Polym. Chem., 2008, 46, 5663.
12 G. Odian, Principles of Polymerization, Wiley Interscience,
Staten Island, 4th edn, 2004, pp.198–349.
13 G. Moineau, P. Dubois, R. Jerome, T. Senninger and
P. Teyssie, Macromolecules, 1998, 31, 545.
Acknowledgements
CD and RC are thankful to DST, New Delhi (Grant no. SR/FT/CS- 14 (a) W. Jakubowski and K. Matyjaszewski, Macromolecules,
053/2009) for funding. The support from BITS, Pilani – K. K.
Birla Goa Campus is gratefully acknowledged. VSN wishes to
thank the UGC, New Delhi for their support under XII Plan
2005, 38, 4139; (b) K. Min, H. F. Gao and K. Matyjaszewski,
J. Am. Chem. Soc., 2005, 127, 3825; (c) K. Min, H. F. Gao
and K. Matyjaszewski, J. Am. Chem. Soc., 2006, 128, 10521.
grants and Prof. S. G. Tilve, Department of Chemistry, Goa 15 (a) T. Pintauer, P. Zhou and K. Matyjaszewski, J. Am. Chem.
University for useful discussions.
Soc., 2002, 124, 8196; (b) W. Tang, N. V. Tsarevsky and
K. Matyjaszewski, J. Am. Chem. Soc., 2006, 128, 1598; (c)
W. Tang, Y. Kwak, W. Braunecker, N. V. Tsarevsky,
M. L. Coote and K. Matyjaszewski, J. Am. Chem. Soc., 2008,
130, 10702; (d) D. A. Singleton, D. T. Nowlan, N. Jahed and
K. Matyjaszewski, Macromolecules, 2003, 36, 8609; (e)
I. Degirmenci, S. Eren and V. Aviyente, Macromolecules,
2010, 43, 5602; (f) A. P. Haehnel, M. Schneider-Baumann,
K. U. Hiltebrandt, A. M. Misske and C. Barner-Kowollik,
Macromolecules, 2013, 46, 15.
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