Universal polymer analysis by 1H NMR using complementary trimethylsilyl end groups

Article abstract of DOI:10.1021/ja102096u

New degenerative chain transfer agents, namely 4-(trimethylsilyl)benzyl 4′-(trimethylsilyl)butane-dithioate, 4-(trimethylsilyl)benzyl 3′-(trimethylsilyl)propyl trithiocarbonate and their 3-(trimethylsilyl) benzyl isomers, that are two-fold labeled with complementary trimethylsilyl (TMS) markers, were designed and shown to be powerful tools for universal polymer analysis by conventional ¹H NMR spectroscopy. Their use in controlled free radical polymerization, here the reversible addition- fragmentation chain transfer (RAFT) method, resulted in polymers with low polydispersities up to high molar masses, as well as with defined complementary TMS end groups. Thus, routine ¹H NMR spectra allowed facile determination of the molar masses of polymers of various chemical structures up to at least 10⁵ g/mol, and simultaneously provided crucial information about the content of end groups that is typically >95% when polymerizations are correctly performed. Polymerizations were carried out in various solvents for two standard monomers, namely n-butyl acrylate and styrene, as well as for two specialty monomers, so-called inimers, namely 2-(2-chloropropionyloxy)ethyl acrylate and 2-(2-chloropropionyloxy)ethyl acrylamide. The complementary end group markers revealed marked differences in the suitability of commonly used solvents for RAFT polymerization. The results demonstrate-beyond good polymerization control-that the new RAFT agents are universal, powerful tools for facile polymer analysis by routine ¹H NMR spectroscopy, of their absolute molar masses as well as of the content of end groups. This is crucial information, e.g., for the synthesis of high-quality telechelics and, in particular, of block copolymers, which is difficult to obtain by other methods. Preliminary screening experiments indicate that similar uses can be envisaged for analogous ATRP systems.

Full text of DOI:10.1021/ja102096u

Published on Web 06/08/2010
1
Universal Polymer Analysis by H NMR Using Complementary
Trimethylsilyl End Groups
Michael Pa¨ch,† Daniel Zehm,‡ Maik Lange,‡ Ina Dambowsky,‡ Jan Weiss,‡ and
Andre´ Laschewsky*,†,‡
Fraunhofer Institute of Applied Polymer Research, Geiselbergstrasse 69,
D-14476 Potsdam-Golm, Germany, and UniVersity of Potsdam, Institute of Chemistry,
Karl-Liebknecht-Strasse 25, D-14476 Potsdam-Golm, Germany
Received March 11, 2010; E-mail: andre.laschewsky@iap.fhg.de
Abstract: New degenerative chain transfer agents, namely 4-(trimethylsilyl)benzyl 4′-(trimethylsilyl)butane-
dithioate, 4-(trimethylsilyl)benzyl 3′-(trimethylsilyl)propyl trithiocarbonate and their 3-(trimethylsilyl)benzyl
isomers, that are two-fold labeled with complementary trimethylsilyl (TMS) markers, were designed and
1
shown to be powerful tools for universal polymer analysis by conventional H NMR spectroscopy. Their
use in controlled free radical polymerization, here the reversible addition-fragmentation chain transfer
(RAFT) method, resulted in polymers with low polydispersities up to high molar masses, as well as with
1
defined complementary TMS end groups. Thus, routine H NMR spectra allowed facile determination of
the molar masses of polymers of various chemical structures up to at least 105 g/mol, and simultaneously
provided crucial information about the content of end groups that is typically >95% when polymerizations
are correctly performed. Polymerizations were carried out in various solvents for two standard monomers,
namely n-butyl acrylate and styrene, as well as for two specialty monomers, so-called inimers, namely
2-(2-chloropropionyloxy)ethyl acrylate and 2-(2-chloropropionyloxy)ethyl acrylamide. The complementary
end group markers revealed marked differences in the suitability of commonly used solvents for RAFT
polymerization. The results demonstratesbeyond good polymerization controlsthat the new RAFT agents
are universal, powerful tools for facile polymer analysis by routine 1H NMR spectroscopy, of their absolute
molar masses as well as of the content of end groups. This is crucial information, e.g., for the synthesis of
high-quality telechelics and, in particular, of block copolymers, which is difficult to obtain by other methods.
Preliminary screening experiments indicate that similar uses can be envisaged for analogous ATRP systems.
the synthesis of telechelics9-15 and, in particular, of block
copolymers3,16 from a wide range of (functional) monomers
Introduction
Controlled free radical polymerization (CRP),1-3 in particular
the methods of atom transfer radical polymerization (ATRP),4,5
nitroxyl-mediated polymerization (NMP),6 and reversible
addition-fragmentation chain transfer polymerization (RAFT),7,8
have emerged during the past decade as potent methods to tailor
polymers. Key to each of these processes are specially designed
mediators which (i) reversibly terminate the active end of the
growing chain and (ii) may confer specific functionalities to
the polymer chain ends. Major applications of CRP are therefore
without the need of protecting groups. At the heart of all CRP
methods for block polymer syntheses are polymeric intermedi-
ates, which carry the active moiety of the CRP agent and thus
allow the addition of the subsequent block. Accordingly, the
determination not only of the molar mass but also of the content
of end groups is important for the successful synthesis of high-
quality block copolymers by sequential CRP techniques.
However, this information requires mostly the use of heavy and
expensive specialized equipment as well as of cumbersome and
time-consuming analytical procedures. This is particularly true
† Fraunhofer Institute of Applied Polymer Research.
‡ University of Potsdam, Institute of Chemistry.
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A R T I C L E S
Pa¨ch et al.
when monomers bearing functional groups are used and other
than linear homopolymers are to be analyzed. Mostly, therefore,
only apparent molar masses and polydispersities are determined
by size exclusion chromatography using standard polymersssuch
as polystyrene or poly(methylmethacrylate)sfor calibration. In
contrast, the true molar masses and, especially, the degrees of
end group functionality are only assumed to be appropriate on
the basis of indirect observations, due to the lack of convenient
analytical tools.
A priori, end group analysis is a universal method to
determine molar masses of homo- as well as copolymers.
Importantly, end group analysis is also valid in the case of
associating polymers, for which most other methods fail. The
use of this method requires only that the average number of
end groups per macromolecule is known and that the end groups
can be reliably identified and quantified. In practice however,
both conditions are normally difficult to fulfill. The situation is
different for polymers made by living polymerizations or CRP,
as all macromolecules should in very good approximation bear
either exactly one or exactly two defined end groups, when using
a monofunctional initiator. In the case of the thiocarbonyl-based
RAFT process, the situation is particularly advantageous. The
moderating RAFT agent that can be conveniently presented as
R-S-C(dS)-Z, will confer one defined initiating end group
R and one defined terminating end group Z to each polymer if
the polymerization is conducted correctly.17 Experimentally, the
number-average molar mass Mn can then be calculated via end
group analysis as
should preferentially make use of the R rather than of the Z-end
group. Importantly, if both end groups R and Z can be quantified
independently, the degree of the end group functionality can
be deduced from the ratio [Zinc]/[Rinc].24,25
Several methods for end group analysis and molar mass
determination of RAFT-made polymers have been applied, such
as UV absorption,20,21,26-32 SEC with UV detection,19,33 elemental
15,24,25,30,34-45
analysis,24,30 as well as H NMR spectroscopy.
1
1
Among these, UV absorption and H NMR spectroscopy are
preferable, as they ask only for standard equipment, and their
signals provide easily quantitative information. Especially NMR
analysis is attractive due to its easy accessibility and the
relatively short measurement times. Still, sufficiently intense
end group signals that are not obscured by signals from any
other groups are required. Hence, compounds which are effective
CRP agents and, in addition, exhibit intense NMR signals in a
spectral region free of interferences by common solvents and
polymers are desirable. For the sake of best signal-to-noise
ratios, singlet signals are preferred. So far, several examples
1
for end group analysis Via H NMR spectroscopy have been
reported. However, the RAFT agents used typically had only
NMR marker groups, which showed up in the range of 0.8-8
ppm, i.e. in the range where many polymers show signals of
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Menxp ) Mmonomer × [CRU]/[Rinc] + MCTA
(1a)
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with Mmonomer and MCTA being the molar masses of the monomer
and the RAFT chain transfer agent, respectively, and [CRU],
[Rinc], and [Zinc] being the concentrations of the constitutional
repeat units (CRUs), the R-groups, and the Z-groups incorpo-
rated in the polymers, respectively. Equation 1a is based on
the conditions (1) that the amount of initiator-derived polymer
chains is much smaller than the ones initiated by R residues
and (2) that uncontrolled chain transfer to the solvent, the
monomer, and impurities is negligible. Both conditions can be
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initiator vs RAFT agent,8,17 and of the solvent. In eq 1b, one
must further assume that the extent of all reactions inducing a
loss of thiocarbonyl moieties is negligible, too. This assumption
is less reliable, as several side reactions of thiocarbonates and
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1
New RAFT Agents for Polymer Analysis by H NMR
A R T I C L E S
their CRUs and which give broad and low intensity signals.
Hence, the NMR markers introduced by the RAFT agents have
been typically useful for particular cases only, often allowing
only the quantification of either the Z36,38-40,46 orseven more
seldom sof the R group,45,47,48 and were relatively imprecise
for Mn values above 10,000.
reactions, which otherwise would have been hard to detect, let
alone to analyze and to avoid by adapting the polymerization
procedures.
Experimental Section
The methods and chemicals used are specified in the Supporting
Information. 4-(Trimethylsilyl)benzyl bromide (4-TBzB) and 3-(tri-
methylsilyl)benzyl bromide (3-TBzB),59,60 3-trimethylsilyl-1-pro-
panethiol,61 as well as 2-(2-chloropropionyloxy)ethyl acrylate
(ClPEA)57 were synthesized according to published procedures.
2-(2-Chloropropionyloxy)ethyl acrylamide (ClPEAm) was prepared
in two steps from acryloylchloride, ethanol amine, and 2-chloro-
propionic acid chloride. RAFT agents 4-(trimethylsilyl)benzyl 4′-
(trimethylsilyl)butane-dithioate (1) and 3-(trimethylsilyl)benzyl 4′-
(trimethylsilyl)butane-dithioate (2) were synthesized by addition
of CS2 to the Grignard derivative of 3-chloropropyltrimethylsilane
and subsequent S-alkylation by the corresponding trimethylsilyl-
benzylbromides. The analogous synthesis of 4-(trimethylsilyl)benzyl
3′-(trimethylsilyl)propyl trithiocarbonate (3) and 3-(trimethylsilyl)-
benzyl 3′-(trimethylsilyl)propyl trithiocarbonate (4) started from
3-(trimethylsilyl)-1-propanthiol. The synthetic procedures for the
new compounds and their analytical data are specified in
the Supporting Information, as are the detailed conditions for the
polymerization reactions.
An outstanding example for a NMR label is the trimethylsilyl
(TMS) group, as it (i) can be conveniently introduced, (ii) is
sufficiently stable when attached to carbon, and (iii) has nine
equivalent protons giving rise to an intense singlet signal in a
spectral region that is usually NMR silent (0-0.5 ppm).
Moreover, the proton signals of TMS groups attached to alkyl
residues appear at 0.00-0.05 ppm, whereas signals of aryl-
bound TMS groups are typically found between 0.2 and 0.4
ppm. The observed deshielding of protons in aryl-bound TMS
groups can be accounted for by ring current effects.49,50
Therefore, TMS groups on both R- and Z-group may display
two separated singlets and, thus, provide information about
molar mass and end group functionality simultaneously. This
seems most useful inasmuch as losses of Z-groups during
synthesis and workup can occur and may be quantified
conveniently by comparing the signals of the R- and Z-groups
(Vide infra).
Results and Discussion
Herein, we report on the synthesis of four novel, doubly TMS-
labeled RAFT agents as well as their use in polymerizations of
two standard monomers, namely n-butyl acrylate and styrene,
and of two specialty monomers, so-called inimers (initiator-
monomer) as used e.g. for self-condensing vinyl polymeriza-
tion,51-54 or molecular brushes,55-58 namely the inimers 2-(2-
chloropropionyloxy)ethyl acrylate and 2-(2-chloropropiony-
loxy)ethyl acrylamide, in various solvents. Dithioesters as well
as trithiocarbonates with an aromatic R-group, substituted with
a TMS-label in the 3- and 4-position, and an aliphatic Z-group,
containing a terminal TMS-label, were prepared, which proved
to be suitable for the controlled polymerization of polymers with
a molar mass of up to 105 g/mol. Moreover, the TMSR- and
TMSZ-resonances provided information about the molar mass
Design of the RAFT Agents. Numerous types of RAFT agents
have been prepared and tested in polymerization reactions.8,17,62
Common choices for the R- and the Z-group are benzyl and
alkyl, aryl or S-alkyl groups, respectively. RAFT agents with a
benzyl or aryl moiety, for instance, may be used to evaluate
the molar mass for poly(alkyl(meth)acrylate)s,30,34-36,63 poly(alkyl-
(meth)acrylamide)s25,26,39,44 or poly(acrylonitrile)38 by 1H NMR
since there is no spectral overlap of the end group and the
polymer signals. Still, for high molar mass polymers one
aromatic end group is generally not suitable for end group
analysis due to the complex signal pattern and to the low
intensity of the useful signals of the aromatic protons. Also, in
most reports on end group analysis Via 1H NMR, only one end
group, mostly the less reliable Z-group, has been used. In any
case, when using polymers with aromatic moieties in the CRUs,
such as styrene and its derivatives, the overlap of the end group
and polymer signals prevents generally molar mass determina-
tion by NMR spectroscopy. Obviously, these limitations become
even more severe in the case of copolymers, due to the higher
diversity of NMR signals that may superpose a possible end
group signal.
1
as well as the content of end groups from routine H NMR
spectra. This allowed the identification of unexpected side
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1
applicable RAFT agents for facile end group analysis Via H
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(namely, 0-0.5 ppm). The chemical structures of the new RAFT
agents are shown in Figure 1. The reinitiating benzylic R-groups,
being TMS-labeled on the aryl residue in the 3- or 4-positions,
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A R T I C L E S
Pa¨ch et al.
Figure 1. (a) General structure of RAFT agents used; (b) structure of the two-fold TMS-labeled RAFT agents synthesized.
Figure 2. Chemical structure of the monomers investigated.
spacer between the aliphatic TMS residue and the thiocarbonyl
moieties was chosen because exploring studies with a shorter
spacer group to produce benzyl 2-(trimethylsilyl)ethanedithioate
indicated important side reactions involving the TMS group (cf.
Supporting Information). The RAFT agents were thus synthe-
sized starting from 3-chloropropyltrimethylsilane for 1 and 2,
and from trimethylallylsilane for 3 and 4. The TMS-free
analogous dithioesters64-66 and trithiocarbonates58,67,68 have
already been used successfully in RAFT polymerizations of
styrenic and acrylic monomers, suggesting the effectiveness of
1-4 in RAFT polymerizations.
To demonstrate the usefulness of the end group labeling with
the new RAFT agents, a series of styrenic, acrylate, and
acrylamide monomers were studied (Figure 2). First, polymer-
izations were performed on styrene (St) and n-butyl acrylate
(BuA), as these standard polymers can be easily analyzed by
SEC calibrated by appropriate standards to give correct absolute
molar mass values. Indeed, the Mark-Houwink-Sakurada
parameters of poly(St) and poly(BuA) are virtually identical
in THF, so that calibration by poly(St) standards provides also
true molar masses for poly(BuA).69,70 Subsequently, two
specialty monomers (“inimers”), namely the acrylate ClPEA
and the acrylamide ClPEAm were polymerized and analyzed.
While the synthesis of ClPEA was described before,57,58
ClPEAm was newly synthesized.
Polymerization of n-Butyl Acrylate (BuA). First, CTA1 was
used for RAFT polymerization of BuA in THF, benzene, and
ethyl acetate. The results are given in Table 1 (entries 1-10).
For all the RAFT agents 1-4, polymerization of BuA in benzene
or ethyl acetate provided poly(BuA) with monomodal molar
mass distributions and relatively low polydispersities of about
1.1-1.2 (Table 1, entries 3-10), strongly suggesting good
control over the polymerization process. The number average
molar mass values Mn, which were calculated theoretically and
which were measured by SEC, agreed well, as did the Mn values
1
determined by H NMR end group analysis using either the
TMSR or the TMSZ label (see Figure 3). From their relative
intensity ratios, it is seen that Z/R is close to unity (Table 1).
Thus, the RAFT end groups are very well preserved. Therefore,
the polymers made have not only a very high end group
functionality, if aimed at uses as telechelics, but may serve also
as high quality macro RAFT agents for preparing block
copolymers (Vide infra).
In order to learn about the limits of molar mass determination
by 1H NMR, poly(BuA)580 was synthesized (Table 1, entry 7).
Even in this case, the TMS signals were sufficiently intense to
allow the convenient determination of Mn (Figure 3b) The values
determined Via the TMSR as well as Via the TMSZ signals agreed
very well with both the theoretically expected one and the SEC
result, demonstrating that the TMS-labeled RAFT agents are
indeed powerful tools to determine molar masses of at least up
to 105 g/mol with good precision. Moreover, the comparison
of the results obtained from both labels allows a fast and efficient
check for preservation of the RAFT end groups. This is a most
precious, but nontrivial, information that is otherwise extremely
difficult to obtain.
In THF, polymerization provided poly(BuA)41 and poly-
(BuA)191 with monomodal molar mass distributions, too, but
with somewhat larger polydispersities of 1.6 and 1.3, respec-
tively (Table 1, entries 1 and 2) than in benzene or ethylacetate.
The simultaneous labeling of the R- and Z-end groups by the
complementary TMS groups enabled us to determine the Z/R
ratio of poly(BuA)191 to be only 0.80, thus indicating a ∼20%
loss of CTA functionality during the polymerization. This partial
loss of the dithioester groups implies that a subsequent block
(64) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le,
T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.;
Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559–5562.
(65) Chiefari, J.; Mayadunne, R. T. A.; Moad, C. L.; Moad, G.; Rizzardo,
E.; Postma, A.; Skidmore, M. A.; Thang, S. H. Macromolecules 2003,
36, 2273–2283.
(66) Nozari, S.; Tauer, K.; Ali, A. M. I. Macromolecules 2005, 38, 10449–
10454.
(67) Bowes, A.; McLeary, J. B.; Sanderson, R. D. J. Polym. Sci., Part A:
Polym. Chem. 2007, 45, 588–604.
(68) Rotzoll, R.; Nguyen, D. H.; Vana, P. Macromol. Symp. 2009, 275/
276, 1–12.
(69) Beuermann, S.; D. A. Paquet, J.; McMinn, J. H.; Hutchinson, R. A.
Macromolecules 1996, 29, 4206–4215.
(70) Schmitt, B. Investigations on the mechanism of the anionic polym-
erization of acrylates in the presence of aluminum alkyl compounds
in toluene. Ph.D. Thesis; Johannes-Gutenberg-Universita¨t Mainz:
Mainz, Germany, 1999.
9
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1
New RAFT Agents for Polymer Analysis by H NMR
A R T I C L E S
Table 1. RAFT Polymerization of Styrene (St) and n-Butyl Acrylate (BuA) at 65 °C, Initiated by AIBN, and Characterization of the Polymers
poly(St) and poly(BuA)a
SECc
end group analysis
DPnd
b
entry
monomer
CTA
solvent
t [h]
%
Mtnheor
Mn
PDI
Mnd
Z/R
1
2
3
4
5
6
7
8
BuA
BuA
BuA
BuA
BuA
BuA
BuA
BuA
BuA
BuA
St
St
St
St
St
St
St
St
St
1
1
1
1
1
1
1b
2
3
4
1
2
3
4
1
1e
2
3
4
THF
0.5
2
2
4
2
4
9
2
2
18
78
73
89
68
93
56
53
66
51
30
30
32
32
60
79
68
70
75
92
5000
20300
18700
23100
17400
24200
72300
14000
17300
13400
6100
6100
7100
7100
11500
74100
13400
15100
16200
39300
4300
17500
23000
28500
22800
26200
66500
16900
20400
16000
6600
6700
6400
7300
11800
61200
14200
14200
15600
36000
1.62
1.30
1.13
1.16
1.12
1.15
1.17
1.13
1.13
1.14
1.41
1.46
1.45
1.47
1.23
1.22
1.22
1.21
1.20
1.21
5700
25000
21800
26500
20000
25400
74600
15400
19500
15000
7600
7800
8300
8500
11900
77600
14800
15700
16800
41000
41
192
167
206
153
195
580
117
150
114
69
72
76
78
110
743
139
147
157
192
0.98
0.80
0.99
0.93
0.99
0.97
0.98
0.99
0.98
0.99
0.99
0.99
0.99
0.98
0.97
0.94
0.98
0.99
0.98
0.94
THF
benzene
benzene
EtAc
EtAc
EtAc
EtAc
EtAc
EtAc
bulk
bulk
bulk
bulk
bulk
bulk
bulk
bulk
bulk
9
10
11
12
13
14
15
16
17
18
19
20
2
12
12
12
12
22.5
69
22.5
22.5
22.5
16
BuA
poly(St)147
EtAc
a Polymerization conditions: [monomer]/[CTA]/[AIBN] ) 200:1:0.1. b Conversion determined by 1H NMR analysis of the crude product. c RI
detection, calibrated using polystyrene standards. d By 1H NMR, using the intensity of the TMSR signal. e Modified polymerization conditions:
[monomer]/[CTA]/ [AIBN] ) 1000:1:0.1.
Figure 3. 1H NMR spectra of poly(BuA)153 (a) and poly(BuA)580 (b) in CDCl3.
copolymerization would provide a mixture of diblock copolymer
and dead homopolymer, which are generally difficult to separate
from each other. Therefore, it seems advisible to avoid THF as
solvent in the RAFT process, if very high-quality polymers are
required. These results agree with the findings of a recent
degradation study of RAFT end groups in THF.23 In the latter
study, a radical degradation mechanism due to hydroperoxides
was proposed. Keeping in mind that we distilled the THF freshly
from NaK/benzophenone prior to polymerization and worked
under inert gas all the time, one may think about alternative
explanations for the observed loss of end groups (which,
however, is beyond the scope of this work). In any case, it seems
that THF, although appearing convenient by virtue of its good
dissolving power for many polymers, is not the solvent of first
choice to prepare high-quality block copolymers by the RAFT
process.
initiated polymerization of styrene and subsequent analysis of
the polymers by H NMR spectroscopy (Figure 4). All RAFT
1
polymerizations provided poly(St) with monomodal mass
distributions and relatively low polydispersities of 1.2-1.5. The
1
molar masses determined by integrating the H NMR signals
of both TMS labels of the R- and the Z-groups were in good
agreement with both the theoretically expected Mn values and
the molar masses measured by SEC analysis calibrated with
polystyrene standards (Table 1, entries 11-19). The comparison
of the intensities of the TMSR and TMSZ groups indicated again
ratios of Z/R close to unity, i.e. excellent preservation of the
RAFT end groups. Even the molar mass of poly(St)743 of about
70,000 could still be determined conveniently. Hence, as for
poly(BuA), TMS-labeled RAFT agents are powerful tools for
a convenient molar mass determination of polystyrenes, too.
1
However, in contrast to poly(BuA), the H NMR signals of
Polymerization of Styrene. Whereas benzylic R-groups are
rarely suitable for the end group analysis of polystyrene and its
derivatives due to the spectral overlap of end group and polymer
signals, RAFT agents 1-4 worked perfectly for the thermally
both TMS labels in the poly(St) samples did not show defined
singlets but weresto our surprisessplit into complex signal
groups each (Figure 5a-d). Similar findings were reported for
9
J. AM. CHEM. SOC. VOL. 132, NO. 25, 2010 8761

A R T I C L E S
Pa¨ch et al.
Figure 4. 1H NMR spectra of poly(PS)110 (a) and poly(PS)743 (b) in CDCl3.
1
Figure 5. Comparison of the H NMR signals of the TMS end group labels of poly(St) samples made with RAFT agents (a) 1, (b) 2, (c) 3, and (d) 4, or
by ATRP using for initiation (e) 4-(trimethylsilyl)benzyl bromide and (f) 3-(trimethylsilyl)benzyl bromide, respectively.
TMS-terminated oligomeric styrene chains before71 a detailed
NMR study revealing that the complex signal groups arose from
stereo isomerism, i.e. from CRU sequences with different
tacticities at the chain ends. Noteworthy, while the TMS end
groups in the latter report were directly placed at the polymer
chain end, in our systems the TMS groups are separated by
nine bonds from the first stereo center in the polymer backbone.
This indicates that the peak splitting is a combined effect of
stereoisomerism with the anisotropy effects of the adjacent
aromatic rings.49,50
(71) Bheda, M. C.; Gibson, H. W. Macromolecules 1991, 24, 2703–2708.
9
8762 J. AM. CHEM. SOC. VOL. 132, NO. 25, 2010

1
New RAFT Agents for Polymer Analysis by H NMR
A R T I C L E S
Table 2. Bulk Polymerization of Styrene (St) by ATRP at 110 °C, and Analytical Data of the poly(St) obtaineda
SECc
end group analysis
DPd
b
entry
initiator
t [h]
%
Mtnheor
Mn
PDI
Mnd
Z/Re
1
2
3
4
5
6
4-TBzB
4-TBzB
4-TBzB
3-TBzB
3-TBzB
3-TBzB
1
2
3
1
2
3
38
55
64
36
54
65
4200
6000
6900
4000
5900
7000
3600
5400
6300
3300
5600
7000
1.09
1.11
1.11
1.12
1.14
1.12
3600
5200
6300
3300
5400
6700
35
50
60
32
52
64
1.0
0.8
0.9
0.9
0.9
0.8
a Polymerization conditions: [monomer]/[initiator]/[CuBr]/[dNbpy] ) 100:1:1:2. b Conversion determined by 1H NMR analysis of the crude product.
d
e
c Calibrated using polystyrene standards. By H NMR, using the intensity of the TMSR signal. by H NMR, based on the relative the intensities of the
1
1
TMSR signal and of the >CHBr end group signal at about 4.6 ppm.
Indeed, a closer comparison of the spectra of the poly(St)
samples made with the four RAFT agents 1-4 revealed that
the R- and the Z-group split systematically (Figure 5). While
the TMSZ signals were virtually identical for all polymers
bearing a given Z group (Figure 5a-b and 5c-d), the splitting
of TMSR signals depended on the substitution pattern of the
trimethylsilylbenzyl group, i.e., the TMS-label in 3- and
4-position led to different peak patterns (cf. Figure 5a and c
with Figure 5b and d). Although sensitivity and precision of
the TMS group integrals inevitably suffer somewhat for higher
molar masses due to the signal splitting (in comparison to the
analysis of poly(BuA)), the TMS signals were still useful to
determine the molar mass of the polymers as shown by
comparison with SEC results (Table 1). Also, the analysis of
the R/Z ratio was still possible.
and 4-(trimethylsilyl)benzylbromide (4-TBzB) were employed
to initiate the ATRP polymerization of styrene (Table 2). Again,
the TMS signals of the initiator derived end groups in the
poly(St) samples thus prepared exhibited the same peak splitting
(Figure 5e,f) as observed in the RAFT polymerization when
using agents 1-4.
Also, the Mn values derived from NMR end group analysis
were in good agreement with the SEC results, and polydisper-
sities were relatively low (1.1-1.2). Importantly, preliminary
ATRP polymerization experiments of acrylate using initiator
4-TBzB resulted in polymers exhibiting a well-defined singlet
signal for the TMS label, as observed when polymerizing BuA
in the presence of RAFT agents 1-4. All these findings
corroborate the explanation of the observed splitting of the end
group signals in the case of poly(St). Noteworthy, the poly(St)
samples made Via ATRP contain inherently a -CH(Aryl)-Br
group at the active end of the polymer chain. This group gives
rise to a distinct 1H NMR signal at about 4.6 ppm and therefore
can also provide information about the relative amounts of
initiating and terminating end groups in the polymers (Table
2). However, this signal is in a spectral region that is more prone
to be superposed by signals of the polymer chain and is
inevitably broad and weak. Therefore, it is difficult to quantify
reliably, in particular for molar masses above 10,000. Accord-
ingly, the use of a TMS end group marker is also helpful for
molar mass analysis when applying ATRP, but is more limited
in the informations provided, in comparison to the case for
RAFT. In the case of third major CRP method, namely of NMP
polymerization, however, one may envisage the use of analo-
gously substituted alkoxamine initiators, with complementary
TMS markers located on the N- and O-substituents, respectively,
for molar mass analysis as exemplified for RAFT polymerization
in this study.
Polymerization of ClPEA and ClPEAm. In order to demon-
strate the general utility of the new TMS-labeled RAFT agents
for determining molar masses conveniently Via 1H NMR
analysis, we chose 1 in a second phase to investigate the RAFT
polymerization of two unconventional monomers, so-called
inimers, namely of acrylate ClPEA and acrylamide ClPEAm,
for which appropriate polymer standards for SEC calibration
do not exist. The polymerization of such monomers gives rise
to densely functionalized reactive polymer backbones, which
afford the preparation of molecular brushes Via “grafting from”
processes.72,73
The explanation for the splitting of the TMS signals was
corroborated by a chain extension experiment. The preparation
of poly(St)-b-poly(BuA) using poly(St)147 (Table 1, entry 18)
as macro-RAFT agent (Table 1, entry 20) gave a well-defined
singlet for the TMSZ signal at about 0 ppm, while the complex
pattern of the aryl bound TMSR signal at about 0.25 ppm was
retained. Due to the separation of the Z-end group from the
poly(St) block by the added poly(BuA) block, the TMSZ group
is no more influenced by adjacent phenyl rings, whereas
inherently, the situation has not changed for the TMSR group.
Importantly, the monomodal mass distribution as well as the
relatively low polydispersity of the block copolymer indicated
good polymerization control and efficiency of the macro-RAFT
agent for chain extension. Moreover, even in the block
copolymer the resolved NMR-signals enabled us to verify the
Mn values directly by end group analysis, and to establish the
ratio R/Z being well above 90% yet (Table 1, entry 20).
Inevitably, the R/Z ratio of 0.94 is somewhat lower for the chain
extended polymer than for the macro RAFT agent employed
(Z/R ) 0.99) due to a certain number of chain termination
reactions occurring. Still, these analytical data reveal that the
diblock copolymer made would be perfectly suited as a high
quality macro RAFT agent for a subsequent additional chain
extension reaction of a third block. Commonly in the past, molar
masses of such block copolymers made by CRP methods were
mostly determined only indirectly. The average comonomer
1
composition of the block copolymer is analyzed Via H NMR
spectra, and the molar mass is calculated with the assumption
that the Mn value of the first block has been fully preserved.
Also, the extent of active end groups for the chain extension
has been mostly not known. Obviously, the situation is much
more advantageous when using RAFT agents 1-4 due to the
two-fold TMS labels.
The results of the polymerization of both ClPEA and
ClPEAm are listed in Table 3. Initially, the inimer ClPEA was
(72) Zhang, M.; Mu¨ller, A. H. E. J. Polym. Sci Part A: Polym. Chem. 2005,
43, 3461–3481.
To rule out any peculiar effects of the RAFT process on the
end group signals, 3-(trimethylsilyl)benzylbromide (3-TBzB)
(73) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci.
2008, 33, 759–785.
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A R T I C L E S
Pa¨ch et al.
Table 3. Synthesis and Characterization of poly(ClPEA) and poly(ClPEAm) by RAFT Polymerization at 65 °C, Initiated by AIBNa
SEC
end group analysis
DPc
b
%
entry
monomer
CTA
solvent
t [h]
Mtnheor
Mnapp
PDI
Mnc
Z/R
1
2
3
4
5
6
ClPEA
ClPEA
ClPEA
ClPEA
ClPEAm
ClPEAm
1
1
1
1
1
1
THF
THF
C6H6
EtAc
THF
EtAc
2
15
12
8
2
13
56
96
69
25
77
86
23000
39000
29000
10000
31000
36000
4900d
3400d
1.67
1.79
1.22
1.38
1.17
1.12
30000
47000
31000
10000
37000
33000
112
192
151
47
154
159
0.48
0
0.90
0.93
0.40
0.93
19000d
7000d
17000e
25000e
a Polymerization conditions: [monomer]/[CTA]/[AIBN] ) 200:1:0.1. b Conversion determined by 1H NMR analysis of the crude product. c By 1H
NMR, using the intensity of the TMSR signal. d SEC in THF (RI detection), based on calibration with polystyrene standards. e SEC in NMP (RI
detection), based on calibration with polystyrene standards.
polymerized in THF, providing poly(ClPEA)112 and poly-
(ClPEA)192 (Table 3, entries 1-2). Comparison of the theoreti-
cally calculated molar masses Mtnheor with the apparent molar
mass values derived from SEC analysis shows strong differ-
ences, which is a priori not surprising in the light of the
calibration by polystyrene standards. The comparison of Mntheor
with the experimental Mn values derived from end group analysis
using the TMSR marker match much better, still, there is a
20-30% difference. In combination with the observed poly-
dispersities of 1.6-1.8, this points to moderate control only over
the polymerization process. This finding is also reflected in the
low Z/R ratios. This was in the case of poly(ClPEA)112 about
0.5 only, indicating a massive 50% loss of chain transfer
functionality. For poly(ClPEA)192 (Table 3, entry 2) the loss
of end group functionality is even virtually complete, as the
Z-group-bound TMS moiety could no more be detected in the
NMR spectra. Note that, in the latter case, polymerization of
ClPEA was conducted nearly to completion.
Similarly for the polymerization of ClPEAm in THF (Table
3, entry 5), the apparent molar mass values derived from SEC
analysis deviate substantially from the theoretically calculated
molar masses Mtnheor, while the latter values and experimental
Mn (Via the TMSR marker) are relatively close. Moreover, the
ratio Z/R of poly(ClPEAm) obtained from end group analysis
indicated about 60% loss of the RAFT active groups in the
polymer formed. These results clearly demonstrate the particular
merit of the TMS labels for polymer analysis especially in the
case of more complex monomers, as these data cannot be
deduced from most other molar mass analysis methods. In
particular, SEC analysis is not only blind toward such complica-
tions due to loss of active end groups but even may (wrongly)
make believe that high-quality macro RAFT agents were
prepared due to the low measured polydispersity of 1.2.
membrane osmometry in toluene. Osmometry provided a value
of 35,500 for Mn, that compares very well with the value of
31,000 derived from end group analysis. These findings
demonstrate that the TMS-labeled RAFT agents are well suited
to control the polymerization even of complex monomers, such
as inimers ClPEA and ClPEAm, and simultaneously allow
characterizing them easily and efficiently.
Conclusions
Four novel two-fold labeled RAFT agents with complemen-
tary trimethylsilyl (TMS) markers on the benzylic R- and on
the alkyl Z-groups were designed. They proved to be suitable
chain transfer agents for the controlled radical polymerization
of various standard monomers such as styrene and n-butyl
acrylate as well as of complex monomers such as the inimer
acrylate ClPEA and acrylamide ClPEAm. Moreover, the
complementary TMS marker groups allowed a facile analysis
of the absolute molar masses of the polymers and of their end
1
group functionality, and this with routine H NMR apparatus,
without the need for expensive specialized equipment or for
complicated and time-consuming methods. The viability of
molar mass determination by routine 1H NMR spectroscopy by
virtue of the end group markers was validated by a comparison
of these analytical data with the results SEC for standard
polymers polystyrene and poly(n-butyl acrylate), for which
appropriate calibration standards exist. Preliminary studies
demonstrated that the same concept can be extended to other
CRP methods, such as ATRP polymerizations, too. In addition,
the relative intensities of the complementary TMS marker groups
at the R- and ω-positions of the polymer chain proved a
powerful tool to determine the remaining end group functional-
ity, information that is very difficult to obtain otherwise. This
enabled us to judge the suitability of various solvents for
controlled RAFT polymerizations (revealing that the widely used
solvent THF is not ideal) and provided a direct measure of the
active polymer chain ends, which are needed, e.g., for the
synthesis of block copolymers.
Different from polymerization in THF, the RAFT polymer-
izations in benzene or ethyl acetate provided poly(ClPEA) and
poly(ClPEAm) not only with monomodal mass distribution and
relatively low polydispersities of 1.1-1.2 but also with the
desired high content of end groups (Table 3, entries 3, 4, and
6; see also Figures S3 and S4 [Supporting Information]). This
Finally, we note that the aryl-bound TMS groups may allow
additional functionalization Via postpolymerization modification
of the end group-labeled polymers by, for instance, ipso-borode-
silylation74,75 or ipso-bromodesilylation.76
1
is indicated by the Z/R ratios >0.9. Furthermore, H NMR
spectroscopy enabled us to determine easily the absolute values
of Mn, whereas the molar masses measured by SEC analysis
are only apparent because of the calibration by PS standards
and are therefore, at best, approximate only. The experimentally
determined values by end group analysis agree very well with
the theoretically calculated ones, while the apparent molar
masses derived from SEC analysis inevitably deviate from the
true values. The good agreement of the theoretically calculated
and end group analyzed experimentally determined molar
masses was corroborated for the poly(ClPEA)151 sample by
Acknowledgment. We thank Stefano Bruzzano (Fraunhofer
IAP, Potsdam-Golm) and Christoph Wieland (Universita¨t Potsdam)
(74) Qin, Y.; Cheng, G.; Sundararaman, A.; Ja¨kle, F. J. Am. Chem. Soc.
2002, 124, 12672–12673.
(75) Parab, K.; Venkatasubbaiah, K.; Ja¨kle, F. J. Am. Chem. Soc. 2006,
128, 12879–12885.
(76) Han, Y.; Walker, S. D.; Young, R. N. Tetrahedron Lett. 1996, 37,
2703–2706.
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8764 J. AM. CHEM. SOC. VOL. 132, NO. 25, 2010

1
New RAFT Agents for Polymer Analysis by H NMR
A R T I C L E S
for help with SEC analysis in THF, Marlies Gra¨wert and Helmut
Schlaad (Max-Planck-Institut for Colloids and Interfaces, Potsdam-
Golm) for help with SEC measurements in NMP, and Jens
Beckmann (Freie Universita¨t Berlin) for fruitful discussions.
Financial support was provided from Deutsche Forschungsgemein-
schaft DFG, Grants LA611/4 and LA611/8.
analytical data of all new compounds and their intermediates,
as well as on model compound benzyl 2-(trimethylsilyl)-
ethanedithioate, including a putative mechanism for the observed
side reaction. Conditions and protocols for the polymerizations,
and selected SEC elugrams of the polymers prepared. This
information is available free of charge via the Internet at http://
pubs.acs.org.
Supporting Information Available: Scheme of the synthetic
pathways to compounds 1-4. Details on the synthesis and
JA102096U
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J. AM. CHEM. SOC. VOL. 132, NO. 25, 2010 8765