Chain transfer kinetics of acid/base switchable n -aryl- n -pyridyl dithiocarbamate RAFT agents in methyl acrylate, n -vinylcarbazole and vinyl acetate polymerization

Article abstract of DOI:10.1021/ma300616g

The structures of the "Z" and "R" substituents of a RAFT agent (Z-C(S)S-R) determine a RAFT agent's ability to control radical polymerization. In this paper we report new acid/base switchable N-aryl-N-pyridyl dithiocarbamates (R = -CH₂CN, Z = -N(Py)(Ar)) which vary in substituent at the 4-position of the aryl ring and the use of these to control molecular weight and dispersity. In their protonated form, the new RAFT agents are more effective in controlling polymerization of the more activated monomer, methyl acrylate (MA), whereas in their neutral form they provide more effective control of the polymerization of less activated monomers, N-vinyl carbazole (NVC) and vinyl acetate (VAc). For each polymerization, the apparent chain transfer coefficient (C_tr^app) shows a good correlation with Hammett parameters. Dithiocarbamates with more electron-withdrawing aryl ring substituents have the higher C_tr ^app. This demonstrates the influence of polar effects on C _tr^app and supports the hypothesis that the activity of these RAFT agents is determined by the availability of the lone pair of the dithiocarbamate nitrogen. Published 2012 by the American Chemical Society.

Full text of DOI:10.1021/ma300616g

Article
pubs.acs.org/Macromolecules
Chain Transfer Kinetics of Acid/Base Switchable N-Aryl-N-Pyridyl
Dithiocarbamate RAFT Agents in Methyl Acrylate, N-Vinylcarbazole
and Vinyl Acetate Polymerization
Daniel J. Keddie,* Carlos Guerrero-Sanchez, Graeme Moad,* Roger J. Mulder, Ezio Rizzardo,
and San H. Thang
CSIRO Materials Science and Engineering, Bag 10, Clayton South VIC 3169, Australia
S
* Supporting Information
ABSTRACT: The structures of the “Z” and “R” substituents of a RAFT
agent (Z−C(S)S−R) determine a RAFT agent’s ability to control radical
polymerization. In this paper we report new acid/base switchable N-aryl-N-
pyridyl dithiocarbamates (R = −CH2CN, Z = −N(Py)(Ar)) which vary in
substituent at the 4-position of the aryl ring and the use of these to control
molecular weight and dispersity. In their protonated form, the new RAFT
agents are more effective in controlling polymerization of the more activated
monomer, methyl acrylate (MA), whereas in their neutral form they provide
more effective control of the polymerization of less activated monomers, N-
vinyl carbazole (NVC) and vinyl acetate (VAc). For each polymerization, the
apparent chain transfer coefficient (Ctrapp) shows a good correlation with
Hammett parameters. Dithiocarbamates with more electron-withdrawing aryl ring substituents have the higher Ctrapp. This
app
demonstrates the influence of polar effects on Ctr and supports the hypothesis that the activity of these RAFT agents is
determined by the availability of the lone pair of the dithiocarbamate nitrogen.
Scheme 1. Canonical Structures of Dithiocarbamates
INTRODUCTION
■
The level of control over polymer molar mass and dispersity
achievable using the reversible addition−fragmentation chain
transfer (RAFT) process is dependent on the intrinsic structure
of the monomer and RAFT agent (1) employed.1−5
1).6−8 For instance, N,N-diaryl dithiocarbamates have been
shown to give some (but less than ideal) control over the
polymerization of both MAMs and LAMs.9,10 Further reports
on use of N,N-diaryl dithiocarbamates for the control of the
polymerization of either MAMs11,12 or LAMs13 demonstrate
the potential utility in these types of RAFT agent.
Recently, we have reported acid/base switchable RAFT
agents (4, R′ = CH3) which possess a pyridyl moiety capable of
moderating the electron density, and hence reactivity, of the
thiocarbonyl group (see Scheme 2).14−16 The pyridinium
species 5 allows for control over the polymerization of MAMs,
while the neutral species 4 controls the polymerization of
LAMs. Factors that affect the acid/base equilibria and hence the
extent of protonation of the RAFT agent in solution have a
marked effect on the dispersity of the polymers obtained during
MAM polymerization.16 Importantly, the switchable RAFT
protocol also allows for the preparation of block copolymers
Monomers can be broadly divided into two classes based on
their reactivity. RAFT polymerization of “more-activated
monomers” (MAMs, e.g. (meth)acrylates, (meth)acrylamides
and styrenics) are best controlled by RAFT agents that possess
high transfer coefficients, such as dithioesters (1, Z = alkyl or
aryl) or trithiocarbonates (Z = SR′).1−4 However, these same
RAFT agents give inhibition or retardation in the polymer-
ization of “less-activated monomers” (LAMs; such as vinyl
esters, vinyl amides). Alternately, RAFT agents where the
thiocarbonyl group is comparatively electron rich, such as
xanthates (1, Z = OR′) and dithiocarbamates (1, Z = NR′2), due
to donation of the heteroatom lone-pair into the thiocarbonyl
(see Scheme 1), have lower transfer coefficients and give good
control over the polymerization of LAMs but poor to no
control over MAMs.1−4 Their ability to control MAM
polymerization can be improved via the incorporation of
substituents such as electron-withdrawing groups strategically
placed on the heteroatom of the ‘Z’-group that reduce the
contribution of the zwitterionic canonical form 3 (see Scheme
Received: March 27, 2012
Revised: April 30, 2012
Published XXXX by the American Chemical
Society
A
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(Mw) molar masses were evaluated using Waters Empower Pro
software. The GPC columns were calibrated with low dispersity
polystyrene standards (Polymer Laboratories) and molecular weights
are reported as polystyrene (PS) equivalents. A third order polynomial
was used to fit the log Mp vs time calibration curve, which was linear
across the molecular weight range 2 × 102 to 2 × 106 g mol−1. GPC
measurements for poly(N-vinylcarbazole) samples was performed on a
Shimadzu system equipped with a CMB-20A controller system, a SIL-
20A HT autosampler, a LC-20AT tandem pump system, a DGU-20A
degasser unit, a CTO-20AC column oven, a RDI-10A refractive index
detector, and 3× Waters Styragel columns (HT2, HT3, HT4, each 300
mm × 7.8 mm providing an effective molecular weight range of 100−
600000). N,N-dimethylformamide (DMF) (containing 0.45% w/v
lithium bromide (LiBr)) at 80 °C (flow rate: 1 mL min−1. Number
(Mn) and weight-average (Mw) molar masses were evaluated using
Shimadzu LC Solution software. The GPC columns were calibrated
with low dispersity PS standards (Polymer Laboratories) ranging from
3100 to 650000 g mol−1 and molar masses are reported as polystyrene
equivalents. A third order polynomial was used to fit the log Mp vs
time calibration curve, which was linear across the molecular weight
ranges.
Scheme 2. Acid/Base Switchable RAFT Agents
incorporating both MAM and LAM monomer units (i.e.,
poly(MAM)-block-poly(LAM))14−16 which are generally un-
obtainable using the conventional RAFT methodology.
Herein, we show that new acid/base “switchable” N-aryl-N-
pyridyl dithiocarbamates of structure 4, where R′ is aryl or
pyridyl, possess enhanced activity in both the acidified and
neutral forms when compared to that of the parent class (R′ =
CH3). These specific RAFT agents (4) were synthesized as the
incorporation of aryl substituents as R′, due to their electron
withdrawing character, should enhance their activity and
improve control over molar mass (Mn) and dispersity (Đ).
We also demonstrate how manipulation of the RAFT agent
structure influences chain transfer kinetics during homopoly-
merization of methyl acrylate (MA), N-vinylcarbazole (NVC)
and vinyl acetate (VAc) and how this in turn affects the
properties of the polymers obtained.
Synthesis of N-Aryl-N-Pyridyl Amines 11−14. The N-aryl-N-
pyridyl amines 11−14 were prepared by Pd-catalyzed amination of 4-
chloropyridine with substituted anilines, employing a method adapted
from that reported by Nolan and co-workers.20 The synthesis of N-(4-
methoxyphenyl)pyridin-4-amine (11) is shown as an example, below.
Modifications in the amine used, yield and characterization data for the
additional examples can be found below.
N-(4-Methoxyphenyl)pyridin-4-amine (11). KOtBu (4.9 g, 43.5
mmol, 1.5 equiv), Pd2(dba)3 (316 mg, 0.58 mmol, 2 mol %), 1,3-
bis(2,6-diisopropylphenyl)imidazolium chloride (232 mg, 0.58 mmol,
2 mol %), 4-chloropyridine (3.3 g, 29 mmol), and p-anisidine (7)
(3.92 g, 32 mmol, 1.1 equiv) were dissolved in 1,4-dioxane (90 mL)
and degassed by sparging nitrogen (30 min), sealed under nitrogen
and heated at 100 °C for 20 h. The reaction was diluted with HCl
(200 mL, 1 M), washed with Et2O (2 × 100 mL) and basified (pH
∼14) with NaOH (6.25 M). The resulting solution was extracted with
CHCl3 (3 × 100 mL), washed with brine, dried (Na2SO4) and solvent
removed under reduced pressure. The crude product was recrystallized
from acetonitrile (MeCN) to give N-(4-methoxyphenyl)pyridin-4-
EXPERIMENTAL SECTION
■
Materials. All solvents were of analytical reagent grade unless
otherwise stated. 4-Aminobenzonitrile, 4-aminopyridine, aniline, p-
anisidine, bromoacetonitrile, carbon disulfide (CS2), 4-chloropyridine
hydrochloride, 1,4-dioxane (anhydrous), dodecylbenzenesulfonic acid
(DDBSA), 4-fluoroaniline, MA, potassium tert-butoxide (KOtBu),
pyridine, VAc, and NVC were purchased from Sigma-Aldrich and used
as received unless otherwise stated. 2,2′-Azobis[2-methyl propionitrile]
(AIBN) was purchased from Acros and purified by recrystallization
twice from methanol prior to use. All deuterated solvents were
obtained from Cambridge Isotope Laboratories. MA and VAc were
filtered through neutral alumina activity I (70−230 mesh) and
fractionally distilled under reduced pressure immediately before use. 4-
Chloropyridine was prepared by neutralization of 4-chloropyridine
hydrochloride with aqueous sodium hydroxide (25% w/v), extracted
with diethyl ether (Et2O) and dried (Na2SO4) and the solvent
removed immediately prior to use. Cyanomethyl methyl(pyridin-4-
yl)carbamodithioate (21),14 4,4′-dipyridylamine (15),17 tris-
(benzylideneacetone) dipalladium(0) (Pd2(dba)3),18 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (IPr·HCl)19 were prepared
by the reported literature procedures. Analytical thin layer
chromatography (TLC) was performed on Merck Aluminum oxide
F254 neutral TLC plates. Preparative column chromatography was
performed using Merck Aluminum oxide 90 neutral (mesh 70−230)
Brockmann activity III (prepared from activity I by addition of 6% w/
w of H2O).
1
amine (11) as yellowish-brown crystals (3.99 g, 20.0 mmol, 69%). H
NMR (400 MHz, CDCl3): δ 3.82 (s, 3H, OCH3), 5.96 (br s, 1H, NH),
6.65 (d, J = 6.0 Hz, 2H, pyridyl ArH), 6.91 (d, J = 8.8 Hz, 2H, ArH),
7.13 (d, J = 8.8 Hz, 2H, ArH), 8.22 (d, J = 6.0 Hz, 2H, pyridyl ArH);
13C NMR (50 MHz, CDCl3): δ 55.5 (OCH3), 108.6 (PyC-3), 114.8
(ArC-3), 125.2 (ArC-2), 132.1 (ArC-1), 150.3 (PyC-2), 151.9 (ArC-4),
157.1 (PyC-4). LR−EI−MS: m/z 200 (M+, 85%), 185 (100). HR−
EI−MS: m/z = found 200.0941 M+ (calcd 200.0944 for C12H12ON2).
N-Phenylpyridin-4-amine (12). Coupling of aniline (8) (2.9 mL,
2.97 g, 32 mmol, 1.1 equiv) with 4-chloropyridine, following the above
procedure, gave N-phenylpyridin-4-amine (12) as colorless off white
1
crystals (3.93 g, 22.9 mmol, 79%). H NMR (400 MHz, CDCl3): δ
6.19 (br s, 1H, NH), 6.81 (d, J = 5.8 Hz, 2H, pyridyl ArH), 7.13 (t, J =
7.7 Hz, 1H, ArH), 7.19 (d, J = 7.7 Hz, 2H, ArH), 7.36 (t, J = 7.7 Hz,
2H, ArH), 8.28 (d, J = 5.8 Hz, 2H, pyridyl ArH). 13C NMR (50 MHz,
CDCl3): δ 109.5 (PyC-3), 121.6 (ArC-2), 124.1 (ArC-4), 129.5 (ArC-
3), 139.6 (ArC-1), 150.4 (PyC-2), 150.5 (PyC-4). LR−EI−MS: m/z
170 (M+, 100%), 169 (50). HR−EI−MS: m/z = found 170.0840 M+
(calcd 170.0838 for C11H10N2).
N-(4-Fluorophenyl)pyridin-4-amine (13). Coupling of 4-fluo-
roaniline (9) (3.56 g, 3.03 mL, 32 mmol, 1.1 equiv) with 4-
chloropyridine, following the above procedure, gave N-(4-
fluorophenyl)pyridin-4-amine (13) as off white crystals (4.35 g, 23.1
mmol, 80%). 1H NMR (400 MHz, DMSO-d6): δ 6.79 (m, 2H, pyridyl
ArH), 7.12−7.22 (m, 4H, ArH), 8.14 (m, 2H, pyridyl ArH), 8.71 (br s,
1H, NH). 13C NMR (50 MHz, DMSO-d6): δ 109.0 (PyC-3), 116.2
(ArC-3, J(C−F) = 22.4 Hz), 122.8 (ArC-2, J(C−F) = 8.1 Hz), 136.9 (ArC-
1, J(C−F) = 2.5 Hz), 150.3 (PyC-2), 150.7 (PyC-4), 158.2 (ArC-4, J(C−F)
= 239.3 Hz). LR−EI−MS: m/z 188 (M+, 100%), 187 (35). HR−EI−
MS: m/z = found 188.0744 M+ (calcd 188.0744 for C11H9N2F).
Characterization. Nuclear magnetic resonance (NMR) spectra for
structural assignments were obtained with a Bruker Avance 200 or 400
MHz spectrometer (1H 200 or 400 MHz, 13C 50 or 100 MHz). Where
required, carbon spectra were assigned by use of HSQC and HMBC
experiments. High resolution electron impact (HR-EI) mass spectra
recorded using a ThermoQuest MAT95XP operating at 70 eV using
perfluorokerosene (PFK) as a reference. Gel permeation chromatog-
raphy (GPC) for poly(methyl acrylate) and poly(vinyl acetate)
samples was performed on Waters Alliance e2695 liquid chromato-
graph equipped with a Waters 2414 differential refractometer and 3×
mixed C and 1 mixed E PLgel columns (each 300 mm × 7.5 mm)
from Polymer Laboratories. The eluent was tetrahydrofuran (THF) at
30 °C (flow rate: 1 mL min−1). Number (Mn) and weight-average
B
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4-(Pyridin-4-ylamino)benzonitrile (14). Coupling of 4-amino-
benzonitrile (10) (3.78 g, 32 mmol, 1.1 equiv) with 4-chloropyridine,
following the above procedure, gave 4-(pyridin-4-ylamino)benzonitrile
yl)carbamodithioate (19) as an orange solid (1.85 g, 6.0 mmol, 45%).
1H NMR (400 MHz, DMSO-d6): δ 4.32 (s, 2H, CH2CN), 7.71 (m,
2H, pyridyl ArH), 8.73 (m, 2H, pyridyl ArH); 13C NMR (50 MHz,
DMSO-d6): δ 23.5 (CH2CN), 117.2 (CH2CN), 123.5 (PyC-3), 150.9
(PyC-4), 152.3 (PyC-2)), 196.9 (CS). MS (HR−EI): 286.0340 m/z
[M]+ (C13H10N4S2 requires 286.0347).
1
(14) as yellowish-brown crystals (3.00 g, 15.4 mmol, 53%). H NMR
(400 MHz, DMSO-d6): δ 7.05 (m, 2H, pyridyl ArH), 7.28 (m, 2H,
ArH), 7.71 (m, 2H, ArH), 8.30 (m, 2H, pyridyl ArH), 9.35 (br s, 1H,
NH). 13C NMR (50 MHz, DMSO-d6): δ 102.8 (ArC-4), 111.7 (PyC-
3), 118.2 (ArC-2), 119.8 (CN), 134.2 (ArC-3), 145.9 (ArC-1), 148.4
(PyC-4), 150.9 (PyC-2). LR−EI−MS: m/z 195 (M+, 100%), 194 (36).
HR−EI−MS: m/z = found 195.0791 M+ (calcd 195.0791 for
C12H9N3).
RAFT Agent Synthesis. The cyanomethyl N-aryl-N-pyridyl
dithiocarbamate RAFT agents 16−20 were prepared by modification
of the previously reported procedure.14,16 The synthesis of
cyanomethyl (4-methoxyphenyl)(pyridin-4-yl)carbamodithioate (16)
provided is typical. The pyridylaniline used, the yield and character-
ization data are listed below for 17−20.
C y a no m e t hy l ( 4 - C y a no p h e n y l ) ( p y r i d i n - 4- y l ) -
carbamodithioate (20). Reaction of N-(4-cyanophenyl)pyridin-4-
amine (14) (2.62 g, 13.4 mmol) with KOtBu, CS2, and
bromoacetonitrile in dry THF gave cyanomethyl 4-(cyanophenyl)-
(pyridin-4-yl)carbamodithioate (20) as an orange solid (1.85 g, 6.0
1
mmol, 45%). H NMR (400 MHz, CDCl3): δ 4.09 (s, 2H, CH2CN),
7.32 (m, 2H, pyridyl ArH), 7.52 (m, 2H, ArH), 7.78 (m, 2H, ArH),
8.74 (m, 2H, pyridyl ArH). 13C NMR (50 MHz, CDCl3): δ 23.4
(CH2CN), 113.4 (ArCN), 115.0 (CH2CN), 117.4 (ArC-4), 122.2
(PyC-3), 129.2 (ArC-2), 133.9 (ArC-3), 146.7 (ArC-1), 150.6 (PyC-4),
151.9 (PyC-2), 195.3 (CS). MS (HR−EI): 310.0338 m/z [M]+
(C15H10N4S2 requires 310.0341).
RAFT Polymerization. Polymerization of Methyl Acrylate (MA).
The procedure for polymerization of MA in the presence of RAFT
agent 16 is shown below, that using RAFT agents 17−21 can be found
in the Supporting Information.
Preparation of Poly(methyl acrylate) Using Cyanomethyl (4-
Methoxyphenyl)(pyridin-4-yl)carbamodithioate (16) at 70 °C: With
and without Acid. Stock solution I of AIBN (17.5 mg) in MeCN was
prepared in a 50 mL volumetric flask. Stock solution II of
dodecylbenzenesulfonic acid (DDBSA) (220.5 mg) in MeCN was
prepared in a 5 mL volumetric flask. Reaction mixture A (with acid)
was prepared by addition of cyanomethyl (4-methoxyphenyl)(pyridin-
4-yl)carbamodithioate (16) (21.3 mg), stock solution I (1 mL), stock
solution II (0.5 mL), MA (1.20 g) and MeCN to a volume of 5 mL
was prepared in a volumetric flask. Reaction mixture B (without acid)
was prepared by addition of cyanomethyl (4-methoxyphenyl)(pyridin-
4-yl)carbamodithioate (16) (21.3 mg), stock solution I (1 mL), MA
(1.20 g) and MeCN to a volume of 5 mL was prepared in a volumetric
flask. Each reaction mixture was divided in two and transferred to
ampules which were degassed by three repeated freeze−evacuate−
thaw cycles and sealed. The ampules were heated at 70 °C for either 2
or 6 h.
Polymerization of N-Vinyl Carbazole (NVC). The polymerization
of NVC was performed by preparing stock solutions of known
concentration which were placed in ampules, degassed via three
freeze−evacuate−thaw cycles, flame-sealed, and heated for the
designated time period. An example, using RAFT agent 16 is shown
below (further examples using RAFT agents 17−21 can be found in
the Supporting Information).
Preparation of Poly(N-vinylcarbazole) Using Cyanomethyl (4-
Methoxyphenyl)(pyridin-4-yl)carbamodithioate (16) at 60 °C. A
stock solution containing cyanomethyl (4-methoxyphenyl)(pyridin-4-
yl)carbamodithioate (16) (24.3 mg), NVC (2.12 g), AIBN (8.3 mg)
and 1,4-dioxane to a volume of 5 mL was prepared in a volumetric
flask. The reaction mixture was divided in three and transferred to
ampules which were degassed by three repeated freeze−evacuate−
thaw cycles and sealed. The ampules were heated at 60 °C for 2, 6, or
20 h (see Supporting Information for details of additional polymer-
izations).
Polymerization of Vinyl Acetate (VAc). The procedure for
polymerization of VAc with dithiocarbamate RAFT agents 16−21
was adapted from a previously reported method.15 Syntheses were
performed using a Chemspeed Swing-SLT automated synthesiz-
er.16,21,22 Stock solutions of VAc, RAFT agents 16−21, and AIBN in
MeCN were prepared (see Supporting Information for more details),
degassed by sparging with nitrogen for 5 min, and placed inside the
automated synthesizer, directly into the reactor vessels. The reaction
solutions were further degassed through three freeze−evacuate−thaw
cycles, the reaction vessels backfilled with nitrogen and heated at 70
°C with vortex stirring (150 rpm); the temperature of the reflux
condensers on top of the reactors was set to 7 °C. Time zero (t0) for
kinetic data was taken to be the moment the reactors reached 70 °C.
At pre-established times, monomer conversions and molecular weights
Cyanomethyl (4-Methoxyphenyl)(pyridin-4-yl)-
carbamodithioate (16). KOtBu (1.54 g, 13.8 mmol, 1.03 equiv)
was added to a solution of N-(4-methoxyphenyl)pyridin-4-amine (11)
(2.68 g, 13.4 mmol) in dry tetrahydrofuran (THF, 90 mL) and the
resultant mixture stirred for 1 h at room temperature (RT).
Subsequently, CS2 (2.4 mL, 3.1 g, 40.1 mmol, 3 equiv) was added
dropwise and the reaction mixture stirred for a further 16 h at RT,
after which bromoacetonitrile (1.1 mL, 1.58 g, 13.4 mmol, 1 equiv)
was added and the mixture stirred for a further 24 h. The mixture was
diluted with H2O and subsequently extracted with CHCl3, the
combined organics washed with H2O, dried (Na2SO4) and the solvent
removed under reduced pressure. The residue was dissolved in a
minimal volume of CHCl3 and purified by column chromatography
(neutral Al2O3 activity III, 50% EtOAc/50% n-hexane) to give
cyanomethyl (4-methoxyphenyl)(pyridin-4-yl)carbamodithioate (16)
as a orange gum that solidified upon standing (3.52 g, 11.2 mmol,
1
83%). H NMR (400 MHz, CDCl3): δ 3.85 (s, 3H, OCH3), 4.06 (s,
2H, CH2CN), 6.98 (m, 2H, ArH), 7.30 (m, 2H, ArH), 7.35 (m, 2H,
pyridyl ArH), 8.65 (m, 2H, pyridyl ArH). 13C NMR (50 MHz,
CDCl3): δ 23.3 (CH2CN), 55.6 (OCH3), 115.3 (ArC-3), 115.5
(CH2CN), 121.6 (PyC-3), 129.8 (ArC-2), 134.5 (ArC-1), 151.1 (PyC-
2), 152.5 (PyC-4), 160.6 (ArC-4), 196.9 (CS). LR−EI−MS: m/z
315 (M+, 24%), 275 (11), 243 (21), 211 (62), 176 (100). HR−EI−
MS: m/z = found 315.0497 M+ (calcd 315.0495 for C15H13N3OS2).
Cyanomethyl Phenyl(pyridin-4-yl)carbamodithioate (17).
Reaction of N-phenylpyridin-4-amine (12) (2.28 g, 13.4 mmol) with
KOtBu, CS2 and bromoacetonitrile in dry THF gave cyanomethyl
phenyl(pyridin-4-yl)carbamodithioate (17) as an orange solid (3.14 g,
11.0 mmol, 82%). 1H NMR (400 MHz, CDCl3): δ 4.07 (s, 2H,
CH2CN), 7.35 (m, 2H, pyridyl ArH), 7.38 (m, 3H, ArH), 7.41 (m, 2H,
ArH), 8.67 (m, 2H, pyridyl ArH). 13C NMR (50 MHz, CDCl3): δ 23.3
(CH2CN), 115.5 (CN), 121.8 (PyC-3), 128.5 (ArC-3), 130.1 (ArC-4),
130.2 (ArC-2), 142.2 (ArC-1), 151.4 (PyC-2), 152.1 (PyC-4), 196.3
(CS). LR−EI−MS: m/z 285 (M+, 42%), 245 (79), 213 (99), 211
(62), 176 (100). HR−EI−MS: m/z = found 285.0385 M+ (calcd
285.0389 for C14H11N3S2).
Cyanomethyl (4-Fluorophenyl ) ( p y r i d i n- 4- y l )-
carbamodithioate (18). Reaction of N-(4-fluorophenyl)pyridin-4-
amine (13) (2.52 g, 13.4 mmol), with KOtBu, CS2 and
bromoacetonitrile in dry THF gave cyanomethyl (4-fluorophenyl)-
(pyridin-4-yl)carbamodithioate (18) as an orange solid (2.48 g, 8.2
1
mmol, 61%). H NMR (400 MHz, CDCl3): δ 4.07 (CH2CN), 7.18
(m, 2H, ArH), 7.34 (m, 2H, pyridyl ArH), 7.39 (m, 2H, ArH), 8.69
(m, 2H, pyridyl ArH). 13C NMR (50 MHz, CDCl3): δ 23.4 (CH2CN),
115.4 (CN), 117.4 (ArC-3, J(C−F) = 23.2 Hz), 121.8 (PyC-3), 130.5
(ArC-2, J(C−F) = 9.2 Hz), 138.2 (ArC-1, J(C−F) = 3.4 Hz), 151.5 (PyC-
2), 151.9 (PyC-4), 162.9 (ArC-4, J(C−F) = 252.3 Hz), 196.5 (CS).
LR−EI−MS: m/z 303 (M+, 24%), 263 (34), 231 (61), 176 (100).
HR−EI−MS: m/z = found 303.0287 M+ (calcd 303.0295 for
C14H10FN3S2).
Cyanomethyl Di(pyridin-4-yl)carbamodithioate (19). Reac-
tion of 4,4′-dipyridylamine (15) (2.29 g, 13.4 mmol) with KOtBu, CS2
and bromoacetonitrile in dry THF gave cyanomethyl di(pyridin-4-
C
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of the formed polymers were followed by sampling 75 μL from the
reaction mixtures with the automated liquid handling system into
NMR tubes and GPC vials. GPC and NMR samples for analysis were
prepared with the automated liquid handling system of the synthesizer
at the end of the experiment by adding the corresponding GPC
(THF) and NMR (deuterated chloroform, CDCl3) solvents.
Scheme 3. Synthesis of N-Aryl-N-pyridyl Amines 11−14 and
the Structure of 15
Calculation of Apparent Transfer Coefficients (Ctrapp) of RAFT
Agents. Transfer coefficients of the RAFT agents 16-21 in
polymerization of MA, NVC and VAc, and 16-H+−21-H+ in the
polymerization of MA were calculated numerically from the slope of a
plot of ln[monomer] versus ln[RAFT] using the method of
Walling.7,23,24 The concentrations of monomer and RAFT agent
were obtained via in situ 1H NMR. Prior to polymerization each
reaction mixture was degassed via three freeze/pump/thaw cycles and
sealed in an NMR tube equipped with a PTFE screw cap. Reaction
conditions and reagent concentrations were the same as those used in
the preparative experiments, but used the deuterated analogue of the
reaction solvent (see Supporting Information for plots).
pyridine in PCl3 at 140 °C according to the literature
procedure.17
Transformation of the N-aryl-N-pyridyl amines to their
corresponding cyanomethyl dithiocarbamates was achieved by
adapting the standard method used for alkylation of
carbodithioate salts.14,34 Best yields were obtained using the
non-nucleophilic base KOtBu to deprotonate the amines, which
were reacted with CS2 to give the carbodithioate salt.
Subsequent alkylation by addition of bromoacetonitrile to the
reaction mixture gave the cyanomethyl RAFT agents 16-20 in
moderate to high yields (see Scheme 4). The lower yields
RESULTS AND DISCUSSION
■
Previous work has shown that N,N-diaryl dithiocarbamates
offer some utility in controlling the polymerization of both
MAMs11,12 and LAMs.9,10,13 For example, diethylmalon-2-yl
N,N-diphenyl dithiocarbamate 6 gave the targeted molar mass
in the polymerization in both ethyl acrylate (EA) and VAc
polymerization but delivered polymers of only slightly lowered
dispersity (Đ = ca. 1.40).9 The result for polymerization of VAc
may be considered typical for the conversion and polymer-
ization conditions and reflects the high reactivity of the
propagating radical (PVAc•). However the control over acrylate
polymerization is poor with respect to that provided by more
suitable RAFT agents, such as dithioesters25,26 and trithiocar-
bonates,27−30 which give much lower dispersity (i.e., Đ = ca.
1.1). In this work, we sought to prepare an acid/base
“switchable” N-aryl-N-pyridyl dithiocarbamates of structure 4
(R′ = aryl or pyridyl) which could give improved control of
acrylates by use of the pyridinium species 5, while inspecting
how these changes in structure affected the ability for the same
RAFT agents, in their neutral form to control the polymer-
ization of LAMs, in particular, NVC and VAc. The cyanomethyl
R-group was selected as it has been shown to be a relatively
good homolytic leaving group that is effective in reinitiating
polymerization of both MAMs and LAMs.14
Scheme 4. Synthesis of RAFT Agents 16−19 and the
Structure of RAFT Agent 21
obtained for the N-(4-cyanophenyl)-N-pyridyl 20 and the N,N-
dipyridyl 19 are attributed to decreased nucleophilicity of the
amide anions of 14 and 15 toward CS2 due to the strongly
electron-withdrawing substituents on the aryl ring. Low
solubility of 14 and 15 in THF also reduces the rate of
reaction and hence the yield of the desired RAFT agents (19
and 20). The use of n-BuLi as the base, as previously employed
in the synthesis of N-methyl-N-pyridyl dithiocarbamates such
as 21,14 gave very poor or no yield (ca. 0−5%) and the crude
reaction products contained significant amounts of dark colored
highly polar components. This was circumvented by using
KOtBu under otherwise identical reaction conditions.
RAFT Agent Synthesis. The RAFT agent precursors, N-
aryl-N-pyridyl amines 11−14, were synthesized by the Pd-
catalyzed Buchwald-Hartwig amination31 of 4-chloropyridine
(see Scheme 3 below).20 The products were obtained in high
purity and in moderate to high yields without need for
chromatographic separation. Higher yields of the desired
products were obtained using the freshly neutralized free base
of 4-chloropyridine than were obtained with the HCl salt as
reported by Nolan and co-workers.20 The reduced yield
observed for the synthesis of the nitrile derivative 14 is
attributed to lower reactivity of the aniline starting material 10
due to it being more electron deficient.32,33 N,N-Dipyridyl-
amine (15) was prepared by reaction of 4-aminopyridine with
Apparent Transfer Coefficient (Ctrapp). Determination of
26
app
the apparent transfer coefficient (Ctr
)
allows for the direct
comparison of RAFT agent activity. This was achieved using
the method of Walling23 by plotting the natural log of
monomer concentration against the natural log of RAFT agent
concentration (ln[M] vs ln[RAFT]) which gives a straight line
26
app
with gradient Ctr
.
The derivation of the Walling method
includes an assumption that chain transfer is irreversible. For
application of this method in a RAFT context, where chain
transfer is reversible, it should be noted that only those
processes that lead to consumption of the initial RAFT agent
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will be observed. While the actual transfer constant may be
higher by several orders of magnitude in the case of more active
RAFT agents, we have demonstrated that the values of Ctr
For protonation of the pyridyl moiety of the RAFT agents
16−21, DDBSA which contains a long C12 aliphatic chain, was
used to improve solubility of the charged species (16-H+−21-
H+). TsOH, used as a “switching” acid in previous work,14−16
did not allow for complete dissolution of some of the more
polar protonated RAFT agents, such as 19-H+. Polymerization
of MA in the presence of the acid “switched” RAFT agents 16-
H+−21-H+ furnished PMA of significantly lower dispersity (ca.
1.1) than did the neutral RAFT agents (see Table 1, entries
1B−6B and Figure 2a), approaching that of PMA produced
using dithioester26 or trithiocarbonate28,36 RAFT agents. The
reactions displayed a linear increase in Mn with conversion
app
obtained provide a good guide to relative reactivities of RAFT
agents.7,26,35 The transfer coefficients are, therefore, called
apparent transfer coefficients (Ctrapp). The polymerizations
were monitored by in situ NMR, with the concentration of
monomer and RAFT agent determined by the disappearance of
the vinylic (MA, 5.7−6.5 ppm in MeCN-d3; NVC, 4.8−5.5
ppm in 1,4-dioxane-d8; VAc, 4.1−5.0 ppm in MeCN-d3) and
cyanomethyl protons (∼4.0−4.1 ppm in MeCN-d3, ∼3.9 ppm
in 1,4-dioxane-d8), respectively (see Supporting Information for
ln[M] vs ln[RAFT] plots, Figures S1−S4).
Polymerization of Methyl Acrylate. In our previous
work, we showed that in the presence of a stoichiometric
amount of strong acid (i.e., p-toluenesulfonic acid)14−16 N-4-
pyridyl-substituted dithiocarbamates (4, R′ = CH3) have
increased activity, due to a decrease in the electron density of
the thiocarbonyl group. This provides improved control during
the polymerization of MAMs.
app
(Figure 2a) and the Ctr were again found to increase for the
RAFT agents possessing stronger electron withdrawing
substituents in the 4-position of the aryl ring. The significant
decrease in dispersity is attributed to a significant increase in
app
Ctr of the protonated RAFT agents with respect to the
neutral analogues, allowing for enhanced equilibration between
the growing chains during polymerization. This can be seen by
comparison of the evolution of molar mass in the presence of
acid (Figure 2b, calculated from in situ NMR)) to that in the
absence of acid (Figure 1b) where the results compare much
more closely to the theoretical (dotted line in both Figure 1b
and Figure 2b). This shows an increase in the rate of the RAFT
pre-equilibrium process in the presence of acid, which
culminates in lower dispersity polymers.
Overall, the polymerization of MA is well controlled by
switchable cyanomethyl N-aryl-N-pyridyl dithiocarbamates 16−
20 in the presence of one equivalent of DDBSA, giving
polymers with lower dispersity than those achieved using the
protonated N-methyl-N-pyridyl dithiocarbamate 21.
To ascertain the effect of this on the polymerization of MA,
controlled with N-aryl-N-pyridyl RAFT agents 16−20, we
performed experiments both with and without acid (see
Scheme 5 and Table 1). Experiments were also performed
Scheme 5. Polymerization of MA with RAFT Agents 16−21,
16-H+−21-H+, and 19-2H+
Polymerization of N-Vinyl Carbazole. Polymerization of
NVC has been previously reported to be most suitably
controlled using xanthates39−41 and has therefore been
classified among the LAM class of monomers.3,14 As such, it
might be expected that some inhibition/retardation would be
observed with increase in Ctrapp, toward that of the more active
RAFT agents (e.g., dithiobenzoates or pyrrolocarbodi-
thioates).39 For polymerization of NVC in the presence of
the dithiocarbamates 16−21 dispersities decrease as RAFT
agent activity increases (see Scheme 6 and Table 2) with no
significant change in the rate of polymerization (i.e., no
inhibition or retardation). As was observed in MA polymer-
ization, the N-aryl-N-pyridyl compounds 16−20 delivered
lower dispersity than the N-methyl-N-pyridyl analogue 21.
The lowest dispersities and the molar mass evolution from
NMR closest to theoretical were obtained from the polymer-
using N-methyl-N-pyridyl analogue14 21 to aid in a direct
comparison with our previous work. In the absence of acid, the
N-aryl-N-pyridyl compounds give reasonable control over MA
polymerization (see entries 2A−6A, Table 1), with a linear
increase in Mn with conversion observed (see Figure 1a) and
dispersities in line with results previously reported for other
N,N-diaryl dithiocarbamates.10−12 In comparison, the polymer-
ization of MA with N-methyl-N-pyridyl RAFT agent 21 gives
polymers of relatively high dispersity and a nonlinear increase
app
izations controlled with the RAFT agents with the highest Ctr
app
(see Figure 3, parts a and b). These results suggest that RAFT
agents with higher activity may provide better control over
NVC polymerization. Unfortunately, NVC is incompatible with
protonated form of switchable RAFT agents,14,16 as acid, in the
presence of trace amounts of water, promotes hydrolysis of the
monomer to carbazole and acetaldehyde.42 For this reason,
polymerization of NVC could only be performed in the
presence of the RAFT agents in their neutral form. Upon
inspection of the evolution of Mn with conversion it can be seen
that at high conversion the dispersity increases (see Figure 3a)
and the Mn deviates significantly from the theoretical, when
initiator derived chains are not taken into account (dashed line,
Figure 3a). Because of the high initiator to RAFT agent ratio
(1:1.5) used to obtain polymer at high conversion, there is a
large proportion of initiator derived chains. When this factor is
of Mn with conversion. In this case the Ctr is less than one,
indicating the addition of more than one monomer unit per
active cycle and relatively poor control over the polymerization
(see entry 1A, Table 1 and Figure 1a). The Ctrapp values for the
RAFT agents were found to increase as the electron density on
the dithiocarbamate nitrogen decreased as a consequence of
stronger electron withdrawing substituents at the 4-position of
the aryl ring. The effect that the Ctrapp has upon the evolution of
the molar mass with monomer conversion, from in situ NMR
(calculated as Mn
[RAFT]t) × MWM), can be appreciated from Figure 1b, with
RAFT agents with highest Ctr more closely resembling the
= ([M]0 − [M]t)/([RAFT]0 −
(NMR)
app
theoretical molecular weight evolution (dashed line, Figure 1b).
As anticipated, the RAFT agents with higher activity provided
polymers of lower dispersity.
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a
Table 1. Details of MA Polymerization Using Standard Experimental Methods
b
c
b
d
app
entry
1A
RAFT agent
R′
time (h)
Mn
Mn(calcd)
Đ
convn (%)
Ctr
21
Me
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
2
6
22 900
22 900
12 600
16 700
11 700
15 000
12 300
16 000
8320
11 200
15 000
11 000
15 100
11 500
14 800
11 700
15 000
7100
1.66
1.69
1.47
1.52
1.42
1.33
1.46
1.35
1.35
1.20
1.33
1.33
1.30
1.19
1.16
1.19
1.16
1.10
1.13
1.10
1.14
1.09
1.32
1.11
1.09
1.10
63
85
62
86
65
84
66
85
40
78
41
62
57
82
58
83
49
77
49
80
30
72
23
60
38
63
0.9
1.9
2A
3A
4A
5A
6A
1B
2B
3B
4B
5B
6B
5C
16
4-MeOPh
Ph
17
2.9
18
4-FPh
3.0
19
4-pyridyl
4-NCPh
Me
5.0
15 000
8740
13 800
7300
20
5.2
10 800
8620
10 900
10 100
14 400
10 300
14 600
8700
21-H+
16-H+
17-H+
18-H+
19-H+
20-H+
19−2H+
6.9
14 400
8990
4-MeOPh
Ph
10.1
12.5
17.0
34.0
26.0
e
14 700
6790
13 400
7560
13 500
8700
4-FPh
14 400
5990
14 100
5300
4-pyridyl
4-NCPh
4-pyridyl
14 700
2340
12 700
4100
10 500
5650
10 600
6730
13 200
11 100
a
b
[MA] = 2.79 M, [RAFT] = 1.35 × 10−2 M, [AIBN] = 4.0 × 10−4 M in MeCN, T = 70 °C, Mn (theory) = 17800 at 100% conv. GPC THF eluent,
c
PS equivalents. Mn(calcd) = ([M]0 − [M]t)/([RAFT]0 + df([I]0(1 − ekdt)) × MWmonomer, (where d is number of chains formed by radical−radical
̂
termination (= 1),37 f is the initiator efficiency (= 0.7)38 and kd = 3.77 × 10−5 s−1, calculated from Arrhenius parameters)37 Calculated from plot of
d
e
app
ln [MA] vs ln [RAFT] from in situ NMR polymerization experiments. Ctr was unable to be measured accurately due to low solubility of RAFT
agent at t0.
Figure 1. Evolution of (a) number-average molar mass and dispersity from GPC with conversion of MA using neutral RAFT agents 16 (blue
diamonds), 17 (dark red triangles), 18 (green triangles), 19 (purple hexagons), 20 (gray crosses), 21 (red circles), theoretical Mn excluding initiator
derived chains (---), theoretical Mn including initiator derived chains (•••) and (b) evolution of molar mass from in situ NMR calculated from RAFT
agent and MA conversion (Mn = {([MA]0 − [MA]t)/([RAFT]0 − [RAFT]t)} × MWMA).
included in calculation of the theoretical Mn value (dotted line,
Figure 3a), the evolution of the experimental Mn for each of the
polymerizations is matched more closely, with the remaining
discrepancy is most likely attributable to the difference between
PNVC and the PS GPC standards. Despite this, NVC
polymerization in the presence of RAFT agents 16-20 are
well controlled at lower conversion (t ≤ 6 h), providing PNVC
of low dispersity (ca. 1.1−1.15). RAFT agent 21 was found to
give comparatively higher dispersity (cf. 1.25) in all cases.
Polymerization of Vinyl Acetate. For polymerization of
VAc in the presence of the RAFT agents 16−21 (Scheme 7)
very good control was observed for lower monomer
conversions as evidenced by low dispersities (see Table 3 and
Figure 4a) and close correlation of the experimental molar
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Figure 2. Evolution of (a) number-average molar mass and dispersity from GPC with conversion of MA using protonated RAFT agents 16-H+ (blue
diamonds), 17-H+ (dark red triangles), 18-H+ (green triangles), 19-H+ (purple hexagons), 20-H+ (gray crosses), 21-H+ (red circles), theoretical Mn
excluding initiator derived chains (---), theoretical Mn including initiator derived chains (•••) and (b) evolution of molar mass from in situ NMR
calculated from RAFT agent and MA conversion (Mn = {([MA]0 − [MA]t)/([RAFT]0 − [RAFT]t)} × MWMA).
and head-to-head propagation.46,47 Slight retardation is
Scheme 6. Polymerization of NVC with RAFT Agents 16−21
observed in rate of polymerization in the presence of N-aryl-
N-pyridyl RAFT agents 16−18 (conversion ∼65% at 14 h)
with respect to that with the N-methyl-N-pyridyl dithiocarba-
mate 21 (conversion 87% at 14 h) (see Figure 5). More
substantial retardation is observed with the more active N-aryl-
N-pyridyl RAFT agents 19 and 20 (conversion ∼20% at 14 h).
The retardation is most likely due to fragmentation of the
RAFT intermediate being slow with respect to propagation of
VAc. Intermediate radical stability is expected to increase with
increasing RAFT agent activity.
masses with the theoretical values calculated from NMR
(Figure 4b). However, for higher conversion dispersities
increase significantly which is attributed to the occurrence of
the usual side reactions associated with radical polymerization
of VAc, such as chain transfer to monomer,43 and polymer44,45
Correlation between Hammett Coefficients and
Apparent Transfer Coefficients (Ctrapp). For the N-aryl-N-
pyridyl RAFT agents 16−20 the value of Ctrapp and the control
over molecular weight increase and the breadth of the
a
Table 2. Details of NVC Polymerization Using Standard Experimental Methods
b
c
b
d
app
entry
1
RAFT agent
R′
time (h)
Mn
Mn(calcd)
Đ
convn (%)
Ctr
21
Me
2
6
3460
12 500
16 400
2310
1330
16 400
21 500
1330
1.27
1.24
1.23
1.18
1.15
1.19
1.16
1.14
1.19
1.14
1.13
1.18
1.10
1.12
1.22
1.11
1.10
1.20
5
65
96
5
23.3
47.6
56.0
68.5
20
2
2
3
4
5
6
16
17
18
19
20
4-MeOPh
Ph
6
10 900
17 200
2390
12 400
21 500
1330
49
96
5
20
2
6
11 000
16 900
2490
12 400
21 200
1330
49
95
5
20
2
4-FPh
6
12 500
17 800
2340
13 900
21 900
1870
55
98
7
20
2
4-pyridyl
4-NCPh
111
6
11 000
17 400
1470
10 900
21 000
530
43
94
2
20
2
117
6
8690
7840
31
20
17 900
20 300
91
a
b
[NVC] = 2.19 M, [RAFT] = 1.54 × 10−2 M, [AIBN] = 1.01 × 10−2 M in 1,4-dioxane, T = 60 °C, Mn(theory) = 27500 at 100% convn. GPC DMF
c
e
̂
eluent. Mn(calcd) = ([M]0 − [M]t)/([RAFT]0 + df([I]0(1 − kdt)) × MWmonomer (where d is number of chains formed by radical−radical
termination (=1),37 f is the initiator efficiency (= 0.7),38 and kd = 9.6 × 106 s−1).37 Calculated from plot of ln [NVC] vs ln [RAFT] from in situ
d
NMR polymerization experiments.
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Figure 3. Evolution of (a) number-average molar mass and dispersity from GPC with conversion of NVC using neutral RAFT agents 16 (blue
diamonds), 17 (dark red triangles), 18 (green triangles), 19 (purple hexagons), 20 (gray crosses), 21 (red circles), Mn (theory) excluding initiator
derived chains (---), Mn (theory) including initiator derived chains (•••) and (b) evolution of molar mass from in situ NMR calculated from RAFT
agent and NVC conversion (Mn = {([NVC]0 − [NVC]t)/([RAFT]0 − [RAFT]t)} × MWNVC).
dithiocarbamate nitrogen. We note that there is also an
excellent correlation of the Hammett coefficient49−51 with the
1H NMR chemical shift of the N−H of the precursor anilines
(see Figure 6).
Scheme 7. Polymerization of VAc with RAFT Agents 16−21
In order to gain further mechanistic understanding, we
examined the relationship between the electronic demand of
the 4-aryl substituent and Ctrapp through construction of a series
of Hammett plots49 (Figure 7) for each set of polymerization
experiments. For the polymerization of the MAM, MA, the
molecular weight distribution decreases as the electron
withdrawing character of the 4-subsituent increases. A trend
for increase in RAFT agent activity with the electron
withdrawing character of Z has also be observed for other
classes of RAFT agent.48 These observations are consistent
with the activity of the N-aryl-N-pyridyl RAFT agents 16−20
being dictated by the availability of the lone pair of the
app
measured Ctr of the N-aryl-N-pyridyl RAFT agents 16−20
correlates well with the Hammett coefficients of the 4-aryl
substituents,49−51 in both neutral (dotted line, Figure 7a) and
protonated (solid line, Figure 7a) forms. We postulate that the
measured Ctr for 19-H+ in MA polymerization is elevated
app
from its true value due to the presence of a small amount of the
a
Table 3. Details of VAc Polymerization Using High-Throughput Experimental Methods
b
c
b
d
app
entry
1
RAFT agent
R′
time (h)
Mn
Mn(calcd)
Đ
convn (%)
Ctr
21
Me
3
10
14
3
7900
11 000
10 800
2240
9880
8860
3850
9850
10 400
3930
8750
9770
710
7420
12 300
12 500
1930
8810
9750
3120
8370
9460
2970
8230
9320
450
1.39
1.60
1.66
1.31
1.37
1.67
1.19
1.36
1.51
1.18
1.44
1.48
1.19
1.19
1.22
1.15
1.19
1.22
50
85
87
13
61
68
21
58
66
20
57
65
3
41.7
92.9
124
153
320
2
3
4
5
6
16
17
18
19
20
4-MeOPh
Ph
10
14
3
10
14
3
4-FPh
10
14
3
4-pyridyl
4-NCPh
10
14
3
3170
4150
630
2600
3440
220
18
24
1.5
13
20
e
>320
10
14
2870
4350
1880
2870
a
b
[VAc] = 6.35 M, [RAFT] = 3.6 × 10−2 M, [AIBN] = 3.6 × 10−3 M in MeCN, T = 70 °C, Mn(theory) = 15200 at 100% convn. GPC THF eluent.
c
e
̂
Mn(calcd) = ([M]0 − [M]t)/([RAFT]0 + df([I]0(1 − kdt)) × MWmonomer, (where d is number of chains formed by radical−radical termination
(=1),37 f is the initiator efficiency (=0.7)38 and kd = 3.77 × 10−5 s−1, calculated from Arrhenius parameters)37 Calculated from an plot of ln [VAc] vs
d
e
ln [RAFT] from in situ NMR polymerization experiments. An exact value of Ctrapp was unable to be calculated as data could not be accurately fitted.
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Figure 4. Evolution of (a) number-average molar mass and dispersity from GPC with conversion of VAc using neutral RAFT agents 16 (blue
diamonds), 17 (dark red triangles), 18 (green triangles), 19 (purple hexagons), 20 (gray crosses), 21 (red circles), theoretical Mn excluding initiator
derived chains (---), theoretical Mn including initiator derived chains (•••) and (b) evolution of effective molar mass from in situ NMR calculated
from RAFT agent and VAc conversion (Mn = {([VAc]0 − [VAc]t)/([RAFT]0 − [RAFT]t)} × MWVAc).
coefficients for the 4-aryl substituent.49−51 Note that the
transfer coefficient of 20 in VAc polymerization (see entry 6,
Table 3) could not be determined accurately as excessive
instrument noise frustrated attempts to fit the experimental
data. However, the monomer conversion and the dispersity of
app
the polymer formed suggest that Ctr for 20 is greater than
app
Ctr for 19, as would be expected based on the Hammett
correlation (entries 5 and 6, Table 3, a graphical comparison of
the NMR integration data can be found in the Supporting
Information, Figure S4).
From these Hammett correlation plots (Figure 7), a
predictable relationship between RAFT structure and activity
can be observed, where the activity of dithiocarbamates in
RAFT polymerization can be correlated to the availability of the
nitrogen lone pair.
CONCLUSION
Figure 5. VAc conversion at 14 h for RAFT agents 16−21, illustrating
■
app
increased retardation with increasing Ctr
.
In this contribution, we have shown how alterations to RAFT
agent structure can lead to appreciable (and, in hindsight,
predictable) changes in reactivity. This was facilitated by use of
N-aryl-N-pyridyl dithiocarbamates that contain strategically
placed substituents with differing electronic demand.
Synthesis of a series of N-aryl-N-pyridyl dithiocarbamates
differing in substituent on the 4-position of the aryl ring was
achieved in two steps. The functional N-aryl-N-pyridyl amines
were prepared by Pd-catalyzed amination of 4-chloropyridine
with the appropriate 4-substituted aniline in moderate to high
yield. These were converted to the dithiocarbamate RAFT
agents 16-21 by alkylation of the carbodithioate salts formed by
reaction of the aniline with CS2 in the presence of KOtBu.
The apparent chain transfer coefficients of dithiocarbamates
16-21 (Ctrapp) in MA, NVC and VAc polymerizations were
found to increase with decreasing thiocarbonyl electron density.
Figure 6. Correlation plot of the Hammett coefficient of the 4-aryl
substituent of the amines 11 (blue diamonds), 12 (dark red triangles),
13 (green triangles), 15 (purple hexagons), 14 (gray crosses), and
their NH chemical shift in DMSO-d6.
app
The magnitude of Ctr of the N-aryl-N-pyridyl RAFT agents
dipyridium species 19-2H+. RAFT agent 19-2H+ would be
16−20 in MA (in the presence or absence of acid), NVC and
VAc polymerization correlated well with Hammett coefficients,
indicating a predictable relationship between RAFT agent
structure and activity. As expected, the protonated RAFT
agents 16-H+−21-H+ offered significantly improved control
with respect to the neutral 16−21, in polymerization of the
MAM MA. Best control was obtained with cyano-functional 20-
app
expected to have a higher Ctrapp. Attempts to measure the Ctr
of 19-2H+ in MA polymerization directly were unsuccessful as
the doubly protonated RAFT agent was not completely soluble
in the reaction mixture. For the polymerization of the LAMs,
NVC (Figure 7b) and VAc (Figure 7c), the Ctrapp values of the
RAFT agents were also found to correlate well with Hammett
I
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Figure 7. Correlation plots between the Hammett coefficient of the 4-aryl substituent of RAFT agents 16 (blue diamonds), 17 (dark red triangles),
18 (green triangles), 19 (purple hexagons), 20 (gray crosses), and their apparent transfer coefficients in (a) MA polymerization with neutral (16−20
(---)) or protonated (16-H+−20-H+ ()) RAFT agents, (b) in NVC polymerization (16−20 ()) and (c) in VAc polymerization (16−19 (),
app
20 omitted as Ctr was unable to be calculated accurately).
H+ or pyridyl-functional 19-H+. For NVC polymerization, the
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ASSOCIATED CONTENT
* Supporting Information
■
(14) Benaglia, M.; Chiefari, J.; Chong, Y. K.; Moad, G.; Rizzardo, E.;
Thang, S. H. J. Am. Chem. Soc. 2009, 131, 6914−6915.
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S
Polymer synthesis details and apparent transfer coefficient
calculation plots. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: daniel.keddie@csiro.au (D.J.K.); graeme.moad@csiro.
au (G.M.).
■
(18) Djakovitch, L.; Wagner, M.; Hartung, C. G.; Beller, M.; Koehler,
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Notes
The authors declare no competing financial interest.
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Mar, M. J.; Hoogenboom, R.; Schubert, U. S. Appl. Surf. Sci. 2006, 252,
2555−2561.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the Capability Develop-
ment Fund of CSIRO Materials Science and Engineering for
financial support. D.J.K. acknowledges the Office of the Chief
Executive of CSIRO for an OCE postdoctoral fellowship. The
authors also acknowledge Jo Cosgriff for aiding in some NMR
experiments.
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