A synthetic approach to a novel class of fluorine-bearing reversible addition-fragmentation chain transfer (RAFT) agents: F-RAFT

Article abstract of DOI:10.1071/CH05069

A synthetic route is described to a novel class of reversible addition-fragmentation chain transfer (RAFT) agents bearing a fluorine Z-group. Such F-RAFT agents are theoretically predicted to allow living free radical polymerization of various monomers wi

Full text of DOI:10.1071/CH05069

Rapid Communication
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 437–441
A Synthetic Approach to a Novel Class of Fluorine-Bearing
Reversible Addition–Fragmentation Chain Transfer (RAFT)
Agents: F-RAFT
Alexander Theis,A Martina H. Stenzel,A Thomas P. Davis,A Michelle L. Coote,B,C
and Christopher Barner-KowollikA,C
A Centre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial
Chemistry, University of New South Wales, Sydney NSW 2052, Australia.
B Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia.
C Corresponding authors. Email: camd@unsw.edu.au; mcoote@rsc.anu.edu.au
A synthetic route is described to a novel class of reversible addition–fragmentation chain transfer (RAFT) agents
bearing a fluorine Z-group. Such F-RAFT agents are theoretically predicted to allow living free radical polymeriza-
tion of various monomers without affecting the rate of polymerization, and should also facilitate the construction
of block copolymers from monomers with disparate reactivity. The class of F-RAFT agents is exemplified by the
example of benzyl fluoro dithioformate (BFDF) in styrene free-radical polymerizations and the process is shown
to induce living polymerization.
Manuscript received: 7 April 2005.
Final version: 3 May 2005.
S
S
S
S
S
S
Pn
R
Introduction
ϩ
ϩ
Pn
R
Pn
R
C
Z
C
Z
C
Z
M
M
Reversible addition–fragmentation chain transfer (RAFT)
polymerization[1–3] has—along with other equally important
living free radical techniques[4,5]—revolutionized free-
radical polymerization as it allows for the generation of com-
plex macromolecular architectures such as comb, star, and
block copolymers with narrow polydispersities. RAFT poly-
merization is increasingly finding applications for generating
novel structures and materials in bioengineering and nano-
technology applications. Lowe et al. used copolymers made
by RAFT to stabilize transition metal nanoparticles[6] and
materials based on nano- and micro-porous polymers have
also been reported.[7,8] Other applications include the man-
ufacture of biocompatible nanocontainers for drug delivery
applications.[9]
The RAFT approach was developed by the CSIRO
group,[1] by combining their earlier work on addition–
fragmentation reactions of macromonomers[10] with the
radical chemistry of small organic molecules of Zard and
coworkers.[11] In a typical RAFT process, thiocarbonylthio
compounds (known as RAFT agents, see Scheme 1)
reversibly react with the growing polymeric radical via the
chain transfer reaction depicted in Scheme 1. This reversible
addition–fragmentation equilibrium is superimposed on a
conventional free-radical polymerization process. Ideally, the
chain-transfer process should be fast and the intermediate
RAFT–adduct radical should be short lived. Because of the
Propagating RAFT agent
radical
RAFT–adduct
radical
polyRAFT
agent
Leaving
group
Scheme 1. Degenerative chain transfer—basis of the RAFT
processes.
rapid transfer of the growing polymeric radicals between their
free and dormant forms, living characteristics (i.e., a lin-
ear increase of molecular weight with monomer-to-polymer
conversion) are imparted on the polymerizing system.
One of the outstanding challenges is the design of RAFT
agents capable of controlling monomers with disparate reac-
tivities. Currently, the RAFT agents used for controlling
monomers with unstable propagating radicals (such as vinyl
acetate) are not suitable for controlling monomers with sta-
ble propagating radicals (such as styrene), and vice versa.
The problem arises because unstable propagating radicals
(as in vinyl acetate polymerization) are poor radical leav-
ing groups from the RAFT–adduct radical. As a result, the
RAFT agents used to control the more stable systems (such
as styrene) retard or inhibit its polymerization. Current RAFT
agents for vinyl acetate address this problem through the use
of lone-pair donor alkoxy or amino Z-groups. These promote
fragmentation by stabilizing the other fragmentation product,
the RAFT agent itself, by resonance (Scheme 2).
© CSIRO 2005
10.1071/CH05069
0004-9425/05/060437

438
A. Theis et al.
S
C
ϪS
C
Experimental
Benzyl Chloro Dithioformate (BCDF)
O
N
S
S
O
S
S
ϩ
Thiophosgene (15.00 g, 130 mmol, Aldrich, 97%) and benzyl mercap-
tan (16.20 g, 130 mmol, Lancaster, 99%) were dissolved in 30 mL of
carbon disulfide (Ajax Unilab). The solution was stirred in a flask with
a gas outlet for 48 h at room temperature. The HCl gas produced was
passed through a bubble counter and subsequently neutralized in alka-
line potassium permanganate solution. After the reaction had finished,
the solvent was evaporated and the product was distilled under vac-
uum to yield 13.31 g (50%) of benzyl chloro dithioformate (BCDF;
yellow liquid), bp 104◦C/0.4 mbar. δH (300 MHz, CDCl3) 4.44 (2H,
s, CH2), 7.25–7.40 (5H, m, C6H5). δC (75 MHz, CDCl3) 45.45 (1C,
CH2), 128.15 (1C, para C6H5), 128.82/129.20 (4C, ortho/meta C6H5),
133.08 (1C, –C6H5), 196.35 (1C, C=S). νmax (ATR)/cm−1 3085vw
(CH valence), 3061w (CH valence), 3028m (CH valence), 2914vw
(CH2 valence), 1600vw (ar CH valence), 1494m (CH rocking), 1452m
(CH rocking), 1399vw(br) (CH2 bending), 1236w(br) (CH2 wagging),
1195w (CH bending), 1095vs (C=S valence), 1070m (ar, def), 1028w
(ar, def), 915vw (CH twist), 853vw (CH2 rocking), 824w (ar, def), 770vs
(as, Cl–C–S valence), 693s (CH wagging). λmax/nm (CH2Cl2) 307
(log ε 4.13).
S
C
ϪS
C
N
ϩ
Scheme 2. Resonance stabilization in xanthates and dithiocarbamates.
S
S
F
S
S
R
F
General structure
BFDF
Scheme 3. Novel class of F-RAFT agents: General structure and
structural image of BFDF.
Unfortunately, by stabilizing the S=C bond, its reactivity
towards radical addition is reduced, and the agents are not
sufficiently reactive for controlling monomers with relatively
stable propagating radicals.As a result, existing RAFT agents
are not suitable for the production of well-defined styrene–
vinyl acetate block copolymers.*
Benzyl Fluoro Dithioformate (BFDF)
A mixture of 5.81 g (100 mmol) of potassium fluoride (Ajax Unilab,
>97%, dried under vacuum before use), 0.26 g (10 mmol) of 18-crown-
6 (Aldrich, 99%), and 30 mL of anhydrous acetonitrile (Ajax Univar,
>99.5%) was stirred under nitrogen with the exclusion of light for 1 h.
Subsequently, 9.31 g (46 mmol) of BCDF was added within 10 min.
After the solution was stirred at 40◦C for 6 h, the solid content was
filtered off, and the solvent was evaporated under vacuum. The crude
product was distilled at 0.4 mbar and purified by chromatography over
silica gel with toluene to yield 4.3 g (50%) of the pure BFDF (pale
In order to be capable of controlling monomers with
disparate reactivities, it is necessary to find RAFT agent
substituentsthatpromotefragmentationofunstablepropagat-
ing radicals by destabilizing the RAFT–adduct radical rather
than stabilizing the RAFT agent. Recently, high-level ab
initio molecular orbital calculations were used to address this
problem.[13] It was demonstrated that a fluorine Z-substituent
significantlydestabilizestheRAFT–adductradical, relativeto
those derived from all known RAFT agents.[14] It was further
shown that this destabilization should be sufficient to pro-
mote non-retarded fragmentation of the RAFT–adduct radical
without stabilizing the RAFT agent. It was proposed that, pro-
vided appropriate R-groups were chosen, RAFT agents with
a fluorine Z-group (denoted as ‘F-RAFT’ agents) should be
capable of controlling monomers with disparate reactivities.
Although F-RAFT agents appear promising, they have
been entirely designed by computational studies and it is
of course necessary to demonstrate that: (a) they can be
synthesized; and (b) they can control polymerization. A
feasible approach to the synthesis of fluoro dithioformates
starting from the corresponding chloro dithioformates has
been previously reported by Sturm and Gattow.[15] However,
based on the calculations for S=C(F)SCH3,[13] the reported
S=C(F)SC2H5 and S=C(F)S(n-C3H7) agents are unlikely to
be successful in a polymerization context. It is thus necessary
to prepare novel F-RAFT agents (Scheme 3) S=C(F)SR with
appropriateleavinggroups, andexaminetheirpolymerization
behaviour. The aim of the present work is thus to demonstrate
a viable synthetic route to F-RAFT agents, and to examine
their ability to control styrene polymerization.
4
yellow liquid). δH (300 MHz, CDCl3) 4.41 (2H, d, JHF 1.1, CH2),
7.30–7.45 (5H, m, C6H5). δC (75 MHz, CDCl3) 43.14 (1C, d, JCF
5.1, CH2), 128.26 (1C, para C6H5), 128.85/129.12 (4C, ortho/meta
3
1
C6H5), 133.32 (1C, – C6H5), 205.75 (1C, d, JCF 361.9, C=S). δF
(282 MHz, CDCl3) 71.56 (s). νmax (ATR)/cm−1 3087vw (CH valence),
3063w (CH valence), 3030m (CH valence), 2921vw (CH2 valence),
1602vw (ar, C–C valence), 1495m (CH rocking), 1453m (CH rock-
ing), 1419vw(br) (CH2 bending), 1249w (br) (CH2 wagging), 1202s
(CH bending), 1160vs (C=S valence), 1070m (ar, def), 1013m (ar, def),
994vs (as, F–C–S valence), 916vw (CH twist), 857vw (CH2 rocking),
814vw (ar, def.), 764m (CH wagging), 693s (CH wagging). λmax/nm
(CH2Cl2) 288 (log ε 4.12).
Polymerizations
Styrene (Aldrich, 99%) was purified by drying for one day over molecu-
lar sieves (4 Å) and subsequent distillation under vacuum.The thermally
decaying initiator 2,2ꢀ-azoisobutyronitrile (AIBN, Aldrich, 99%) was
purified twice by crystallization from ethanol. A solution of 10.00 g of
styrene, 57.5 mgofBFDF, and12.4 mgofAIBNwaspreparedandmixed
thoroughly. The solution was subsequently subjected to four freeze–
pump–thaw cycles to remove any residual oxygen. A small amount of
solution was then transferred under nitrogen atmosphere into a 2-mm
optical path length Infrasil cell (Starna Optical) which was subsequently
sealed with a rubber septum. Monomer conversions were determined
using on-line Fourier-transform near infrared (FT-NIR) spectroscopy
by following the decrease of the intensity of the first vinylic stretching
overtone of the monomer [νmax (styrene)/cm−1 6134].The FT-NIR mea-
surements were performed using a Bruker IFS66\S Fourier-transform
spectrometer equipped with a tungsten halogen lamp, a CaF2 beam
splitter, and a liquid nitrogen-cooled InSb detector. Each spectrum in
*It should be noted that xanthates and dithiocarbamates can be used to control monomers such as styrene, provided the alkoxy or amino Z-group is designed so
as to minimize the resonance stabilization of the C=S bond. For example, xanthates in which the alkoxy group is substituted with strong electron withdrawing
groups that reduce the lone pair donating ability of the oxygen (for example, see ref. [12a]) or dithiocarbamates in which the lone pair of the nitrogen in the
dithiocarbamates is included as part of an aromatic system (for example, see ref. [12b]), can control styrene. However, when modified in this way, such agents
do appear to be suitable for controlling vinyl acetate.

Novel Class of Fluorine-Bearing RAFT Agent
439
the spectroscopic region of 8000–4000 cm−1 was calculated from the
co-added interferograms of 50 scans with a resolution of 2 cm−1. For
conversion determination, a linear baseline was selected between 6200
and 6100 cm−1. The integrated absorbance between these two points
was subsequently used to calculate the monomer-to-polymer conver-
sion by Beer–Lambert’s law. In regular intervals, a small sample was
withdrawn from the reaction mixture with an airtight syringe that had
been flushed three times with nitrogen gas. The sample was transferred
into a flask containing tetrahydrofuran (THF) with hydroquinone as
inhibitor and immediately subjected to size-exclusion chromatography
(SEC) analysis.
an initial increase to high molecular weight material occurs.
Such behaviour has been observed also with other RAFT
agents and has been termed hybrid behaviour.[16] Hybrid
behaviour is frequently observed when the addition rate
S
S
HS
S
CS2, ϪHCl
Cl
ϩ
48 h, room temp.
Cl
Cl
Molecular Weight Analysis
S
S
S
S
Molecular weight distributions were measured by size-exclusion chro-
matography (SEC) on a Shimadzu modular system comprised of an auto
injector, a Polymer Laboratories (PL) 5.0 µm bead-size guard column
(50 × 7.5 mm2), followed by three PL columns (105, 104, and 103 Å)
and a differential refractive index detector. The eluent was THF at 40◦C
with a flow rate of 1 mL min−1.The system was initially calibrated using
acetonitrile, 18-crown-6
ϩ
ϩ
KCl
KF
Cl
F
6 h, 40ЊC
Scheme 4. Synthetic route to BFDF.
narrow polystyrene standards ranging from 540 to 2 × 106 g mol−1
.
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
Results and Discussion
5%
Conversion
10%
20%
30%
40%
In contrast to the known dithioester or xanthate RAFT
reagents, which are usually prepared by a nucleophilic
attack of carbon disulfide by organometallic compounds
or alcoholates, the synthetic approach to halogen bearing
RAFT reagents requires a completely different strategy. Since
recently published results suggest that (because of a homo-
anomeric effect) a benzyl R-group would be a good leaving
group for styrene polymerization,[14] we prepared BFDF in
an analogous reaction to that described elsewhere,[15] using
the commercially available benzyl mercaptan. In a first step,
benzyl chloro dithioformate was prepared by treating thio-
phosgene with equivalent amounts of benzyl mercaptan.
Subsequently, the remaining chlorine atom was replaced by
fluorine in a Finkelstein analogous reaction. The reaction
sequence is depicted in Scheme 4.
The overall yield of both synthetic steps is 25%.According
to the 1H NMR data, the BFDF is free of any residual BCDF
and the purity is higher than 95%. The assignment of the IR
absorption bands was done by means of density functional
theory calculated vibrations at the B3-LYP 6-31G(d) level of
theory.
To test the ability of the novel RAFT agent to induce
living free radical polymerization, we polymerized styrene
using 2.81 × 10−2 mol L−1 BFDF with AIBN ([AIBN]0
6.86 × 10−3 mol L−1) as initiator. At 5, 10, 20, 30, and 40%
conversion, samples were taken and the molecular weight
distribution was determined.
Fig. 1 depicts the evolution of the full molecular weight
distributions with monomer-to-polymer conversion in a
BFDF-mediated RAFT polymerization of styrene at 80◦C.
The distributions are mono-modal and shift to higher molec-
ular weights with increasing conversion. For a more detailed
analysis, a molecular weight versus conversion plot is given
in Fig. 2, with the corresponding Mn values being collated in
Table 1 alongside the associated polydispersities (PDIs).
Inspection of Figs 1 and 2 clearly demonstrates that
the novel class of RAFT agent induces living/controlled
behaviour. However, inspection of Fig. 2 also shows that
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2
log(M)/g molϪ1
Fig. 1. Evolution of the full molecular weight distributions in
BFDF-mediated styrene free-radical polymerizations at 80◦C. The
initial BFDF concentration was 2.81 × 10−2 mol L−1, with an AIBN
concentration of 6.86 × 10−3 mol L−1
.
1.40
1.35
1.30
1.25
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
0.0
0.1
0.2
0.3
Conversion
0.4
0.5
Fig. 2. Number-average molecular weight, Mn, versus monomer-
to-polymer conversion in BFDF-mediated styrene free radical poly-
merizations at 80◦C. The initial BFDF concentration was 2.81 ×
10−2 mol L−1, with an AIBN concentration of 6.86 × 10−3 mol L−1
.
The full line represents the best fit for the molecular weight versus
time evolution, the dotted line depicts the theoretically expected val-
ues. The upper part of the figure gives the corresponding polydispersity
indices, PDIs.

440
A. Theis et al.
coefficient to the initial RAFT agent is too low in comparison
with the propagation rate coefficient or the propagating radi-
cal, polystyrene, leaves the adduct radical in preference to the
R-group, i.e., Bz. Assuming that the rate of the reverse reac-
tion between the leaving group R• and the polymeric RAFT
agent is negligible, the chain transfer constant, Ctr = ktr/kp,
can be estimated by the degree of polymerization of the
polymer formed instantaneously, DPinnst, using Eqn 1.
addition–fragmentation reaction (2), for R = 1-PhEt and
R = Bz.†
•R + S=C(F)SCH3 → R SC•(F)S CH3
(2)
These two systems were selected to model the two steps,
(a) and (b), of the chain-transfer reaction that occurs dur-
ing the initial equilibration period in the RAFT process (see
Scheme 5). Although only the thermodynamics have been
considered in the present case, it has been shown elsewhere
that these radical additions to C=S bonds are virtually bar-
rierless, and the trends in the kinetics follow those in the
thermodynamics.[17] The calculated equilibrium constants
(L mol−1) for each of the reactions are displayed in Scheme 5,
from which it is seen that, on a thermodynamic basis, the
1-PhEt group has a sevenfold preference for fragmenta-
tion over the Bz group, consistent with the observed hybrid
behaviour. This trend differs from that reported recently[14]
for the R = Bz and 1-PhEt substituents in CH3SC•(CH3)SR
and CH3SC•(Ph)SR RAFT–adduct radicals. In this previous
work it was shown that the Bz R-group is a better leav-
ing group than 1-PhEt, despite the latter radical being both
more bulky and more stable. This counter-intuitive trend in
the CH3SC•(CH3)SR and CH3SC•(Ph)SR radicals was pre-
viously attributed to an interesting homo-anomeric effect;
the current results suggest that the fluorine Z-group is suffi-
cient to diminish the effect and thereby reverse the order of
leaving group ability. This ability to enhance or diminish the
homo-anomeric effect, and hence manipulate the leaving
group ability without changing the R-group itself, may prove
useful in other RAFT-agent design problems, such as revers-
ing the block order in block copolymers. Of course, whether
this will work in practice will depend on whether the original
[M0]
Ctr =
(1)
(DPinnst − 1) · [RAFT0]
Extrapolation of the determined molecular weights to zero
conversion yields a DPninst value of 92. Hence, via Eqn 1, a
value of 3.4 for Ctr is returned, indicating a relatively slow
transfer.
To determine whether the slow transfer is caused by
a slow monomer addition or a poor leaving group, fur-
ther molecular orbital studies have been undertaken. The
equilibrium constant at 80◦C was calculated for the model
Table 1. Monomer-to-polymer conversions, number-average
molecular weight, Mn, and polydispersity, PDI, in BFDF-mediated
styrene free-radical polymerization at various polymerization
times
The initial BFDF concentration was 2.81 × 10−2 mol L−1, with the
AIBN concentration being 6.86 × 10−3 mol L−1
Time [min]
Conversion [%]
Mn
[g mol−1
Mtnheo
[g mol−1
]
PDI
]
24
50
107
195
362
5.17
10.75
20.21
30.22
40.51
10 000
10 400
12 000
14 000
15 600
1673
3478
6539
9778
13 108
1.36
1.36
1.34
1.29
1.27
CH2
S
S
S
S
S
S
(a)
(b)
ϩ
ϩ
C
F
C
H2
C
F
C
H2
C
F
H3C
H3C
S
S
C
F
CH3
model of (a)
S
S
CH3
C
F
ϩ
K
353ϭ 1.2 ϫ 10Ϫ5
S
S
CH2
H3C
C
F
CH2
S
S
model of (b)
H3C
C
F
ϩ
K
353ϭ 8.3 ϫ 10Ϫ5
Scheme 5. Model addition–fragmentation reactions (for details see text).
†Equilibrium constants (353 K, L mol−1) were calculated at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory using the standard textbook formulae,
based on the statistical thermodynamics of an ideal gas under the rigid-rotor/harmonic oscillator approximation. Full details of the calculations, together with
complete geometries in the form of GAUSSIAN archive entries, are provided in the Accessory Material.

Novel Class of Fluorine-Bearing RAFT Agent
441
block order was strongly influenced by the homo-anomeric
effect (as in the ordering of 2-cyanopropyl compared with
cumyl). This would not be expected to be the case when
the radical stabilities are very dissimilar (as in styrene–
vinyl acetate block copolymers). However, there may be
some applications where this manipulation of the homo-
anomeric effect may be exploited; further investigations of
this possibility are now underway.
Recently, we have shown how hybrid behaviour can indeed
be used (in cases where the initial RAFT agent R-group is
identical in electronic structure to the propagating radical)
to estimate the magnitude of the addition rate coefficient.[18]
It would be thus instructive to synthesize an F-RAFT agent
with a 1-PhEt R-group, which is analogous to a propagating
polymer chain with chain length one, and exploit the pres-
ence and, if applicable, the magnitude of the hybrid behaviour
under this condition.
We further compared the leaving group ability of several
R-groups with that of the 1-PhEt group. Based on these calcu-
lations we found that a better R-group for this system would
be 2-cyanopropyl, since the equilibrium constant of reac-
tion (1) at 80◦C (8.4 × 10−6 L mol−1) is lower than those
for Bz and even 1-PhEt. This group is also known to be
a good re-initiator in styrene polymerization, and it could
be predicted that the F-RAFT agent S=C(F)SC(CH3)2CN
would induce less or even no hybrid behaviour in styrene
polymerizations. Synthetic procedures towards a range of
R-groups are currently being developed in our laboratories.
of the Australian Partnership for Advanced Computing and
the Australian National University Supercomputer Facility.
We thank Dr Leonie Barner and Mr Istvan Jacenyik for their
excellent management of CAMD.
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Conclusions
In the present work we present a viable synthetic pathway
to a new class of RAFT agent, F-RAFT agents, and exem-
plify it in the case of benzyl fluoro dithioformate (BFDF).
F-RAFT agents had previously been theoretically predicted
to be suitable for the control of monomers with disparate
reactivities; which could in turn provide a means of prepar-
ing block copolymers from such monomers. In the present
work, we demonstrate that such agents are capable of con-
trolling stable monomers, such as styrene. Experiments to
test its behaviour in the polymerization of vinyl acetate and
further optimize the R-group are currently underway in our
laboratories.
Accessory Materials
Full details of the calculations, together with complete
geometries in the form of GAUSSIAN archive entries, are
available from the author or, until June 2010, the Australian
Journal of Chemistry.
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Acknowledgments
We gratefully acknowledge financial support from the Aus-
tralian Research Council in the form of two Discovery Grants
to C.B.-K. and M.H.S., as well as to M.L.C. T.P.D. acknowl-
edges receipt of an Australian Professorial Fellowship.
M.L.C. also acknowledges generous allocations of comput-
ing time on the Compaq Alpha server and the Linux Cluster