Spin capturing with "Clickable" nitrones: Generation of miktoarmed star polymers

Article abstract of DOI:10.1021/ma100263k

A novel nitrone (α-4-(3-(trimetliylsilyl)prop-2-ynyloxy)-N-tert-butyl nitrone) with an alkyne "click" function is synthesized and employed in enhanced spin capturing polymerization (ESCP) as well as in radical coupling reactions between polymers preformed by atom transfer radical polymerization (ATRP) to generate midchain functionalized polymers. Both techniques allow for the facile introduction of chemical functionalities into a polymer midchain position and hence provide an attractive synthetic avenue for the construction of complex macromolecular architectures. Such a strategy is evidenced by the efficient use of polystyrene and poly(isobornyl acrylate) featuring an alkoxyamine midchain function in polymer-polymer conjugation reactions with azide-terminated polymers, yielding 3-arm star (co)polymers via the Cu-catalyzed alkyne/azide cycloaddition reactions. The successful formation of star-block co- and homopolymers is confirmed by conventional size exclusion chromatography (SEC) as well as via hyphenated liquid absorption chromatography under critical conditions (LACCC)-SEC techniques. The synthetic approach reported herein demonstrates that click functional nitrones can efficiently be employed as macromolecular construction agents in modular reactions.

Full text of DOI:10.1021/ma100263k

Macromolecules 2010, 43, 3785–3793 3785
DOI: 10.1021/ma100263k
Spin Capturing with “Clickable” Nitrones: Generation of Miktoarmed
Star Polymers
Edgar H. H. Wong,†,‡ Martina H. Stenzel,‡ Thomas Junkers,*,†,§ and
Christopher Barner-Kowollik*,†
†Preparative Macromolecular Chemistry, Institut fu€r Technische Chemie und Polymerchemie, Karlsruhe
Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany, and ‡Centre for Advanced
Macromolecular Design (CAMD), School of Chemical Sciences and Engineering, The University of New South
Wales, Sydney, NSW 2052, Australia. §Current address: Institute for Materials Research (IMO), Universiteit
Hasselt, Agoralaan, Gebouw D, 3590 Diepenbeek, Belgium.
Received February 3, 2010; Revised Manuscript Received March 4, 2010
ABSTRACT: A novel nitrone (R-4-(3-(trimethylsilyl)prop-2-ynyloxy)-N-tert-butyl nitrone) with an alkyne
“click” function is synthesized and employed in enhanced spin capturing polymerization (ESCP) as well as in
radical coupling reactions between polymers preformed by atom transfer radical polymerization (ATRP) to
generate midchain functionalized polymers. Both techniques allow for the facile introduction of chemical
functionalities into a polymer midchain position and hence provide an attractive synthetic avenue for the
construction of complex macromolecular architectures. Such a strategy is evidenced by the efficient use of
polystyrene and poly(isobornyl acrylate) featuring an alkoxyamine midchain function in polymer-polymer
conjugation reactions with azide-terminated polymers, yielding 3-arm star (co)polymers via the Cu-catalyzed
alkyne/azide cycloaddition reactions. The successful formation of star-block co- and homopolymers is
confirmed by conventional size exclusion chromatography (SEC) as well as via hyphenated liquid absorption
chromatography under critical conditions (LACCC)-SEC techniques. The synthetic approach reported
herein demonstrates that click functional nitrones can efficiently be employed as macromolecular construc-
tion agents in modular reactions.
Introduction
transfer radical polymerization (ATRP).21 Both ESCP and the
radical coupling reaction, although slightly differing from
The advent of controlled radical polymerization, CRP,1-4 has
allowed for the preparation of complex polymeric materials of
variable composition (e.g., homopolymers, block copolymers)
and topologies (e.g., linear, star, brush, comb) with controllable
architectures and wide ranging properties under milder condi-
tions than living ionic polymerization.5,6 In tandem with the
application of modular synthetic approaches (click chemistry),7,8
which can be highly efficient for polymer-polymer conjugation
reactions,9-12 there exists a great degree of flexibility in the
generation of complex macromolecules. However, despite the
availability of these chemistries, the synthesis of such materials
often requires multiple strategically designed synthetic pathways
and/or chemicals, which may thus not always be easily achiev-
able. Thus, novel procedures and methodologies are conti-
nuously developed with the aim of providing less demanding
synthetic alternatives toward designing well-defined complex
macromolecular architectures.
Recently, we have reported the development of a nonliving
CRP technique termed enhanced spin capturing polymerization
(ESCP)13-15 using nitrones as molecular weight control agents
based on previous studies concerning thioketone-mediated poly-
merization16-19 as well as laser-induced marking of polymer
chains in pulsed laser polymerization experiments.20 In addition,
we have also introduced a nitrone-mediated radical coupling
reaction for ligating two polymer chains prepared via atom
another;the former employs nitrones directly in a polymeriza-
tion medium while the latter uses nitrones to enforce coupling of
two preformed polymeric chains;feature one general common-
ality: The generation of macromolecules with so-called midchain
functionalities. Generation of polymers having a functional
group in the middle of the polymer chain is rare despite a
few recent examples including the cobalt-mediated radical cou-
pling22,23 method, where dienes act as linkages between two
polymeric building blocks preformed by cobalt-mediated radical
polymerization. In the case of ESCP the generated alkoxyamine
is centered in a midchain position, and the polymer product can
be employed in further polymerization reactions via chain exten-
sion, for e.g. by nitroxide-mediated polymerization, giving access
to triblock copolymer structures.13,15 In addition, as both the
nitroxide formation and radical-based reactions are tolerant
toward a large number of functional groups, secondary function-
alities such as alkynes can be introduced concurrently into the
middle of a polymer chain via the alkoxyamine (derived from the
nitrone molecule), thus allowing for post polymer-polymer
conjugation reactions. It is this feature that will be exploited
and described in detail in the current contribution.
In the present article, we report the synthesis of an N-tert-
butyl-R-phenyl PBN-type nitrone 1 featuring a functional alkyne
group that can undergo cycloaddition reactions with an azide-
terminated species via the Cu-catalyzed alkyne/azide cycloaddi-
tion, CuAAC, reaction.9,10 Variable derivatives of nitrones
have been previously synthesized and applied in polymerization
reactions, most notably in the work by Detrembleur and
Jerome24 as well as Grishin and co-workers25 in their in situ
*Corresponding authors: e-mail christopher.barner-kowollik@
kit.edu, Tel þ49 721 608 5641, Fax þ49 721 608 5740 (C.B.K.); e-mail
thomas.junkers@uhasselt.be, Tel þ32 11 26 8318, Fax þ32 11 26 8399
(T.J.).
r 2010 American Chemical Society
Published on Web 03/16/2010
pubs.acs.org/Macromolecules

3786 Macromolecules, Vol. 43, No. 8, 2010
Wong et al.
Scheme 1. Spin Capturing of Macroradicals with Alkyne-Bearing Nitrone 1 and the Subsequent Cu-Catalyzed Click Reaction Forming 3-Arm Star
(Co)polymersa
a I denotes the polymer end group resulting from the employed initiator, and R represents the monomer side group (COOR for iBoA or phenyl for
styrene).
nitroxide-mediated polymerization reactions. However, to the
best of our knowledge, the alkyne-bearing nitrone 1 has not
previously been prepared and employed in modular polymer
synthesis. To demonstrate the utility of the nitrone 1, polystyrene
made from ESCP employing 1 and ATRP-made poly(isobornyl
acrylate) that was subjected to the nitrone-mediated radical
coupling reaction have been prepared. Both midchain functio-
nalized polymer types were then subsequently coupled to azide-
terminated polymers in CuAAC reactions to generate 3-arm star
(co)polymers (see Scheme 1).
hydroxide pellets (KOH, Sigma-Aldrich, 85%), dichloro-
methane (Sigma-Aldrich, 99.9%), magnesium sulfate (MgSO4,
Sigma-Aldrich, 97%), silver chloride (Sigma-Aldrich, 99.5%),
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, Aldrich, 98%),
chlorotrimethylsilane (Aldrich, 97%), sodium hydrogen carbo-
nate (NaHCO3, Sigma-Aldrich, 99.7%), N-(tert-butyl)hydroxyl-
amine acetate (Aldrich, 97%), triethylamine (Sigma-Aldrich,
99%), 2,2-azobis(isobutyronitrile) (AIBN, DuPont), methyl
2-bromopropionate (MBrP, Aldrich, 98%), N,N,N0,N00,N00-
pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%),
butyl acetate (Sigma-Aldrich, 99%), copper(II) bromide
(Sigma-Aldrich, 99%), copper powder <75 μm (Cu(0), Aldrich,
99%), toluene (Aldrich, 99.9%), tributyltin hydride (TBTH,
Aldrich, 97%), tetrahydrofuran (THF, Sigma, 99%), dimethyl-
formamide (DMF, Aldrich, 99.8%), tetrabutylammonium
It is important to note that both the ESCP and radical coupling
reactions essentially follow the same reaction mechanism depic-
ted in Scheme 1, where the macroradicals are captured by the
nitrone, forming a stable radical spin trap adduct of the macro-
nitroxide species 3 that undergoes an almost immediate second
capturing step with another macroradical. However, the key
difference between the two methods of preparing midchain
functional alkoxyamines lies in the source of the macroradicals:
In ESCP, macroradicals are generated during the course of a
conventional free radical polymerization process, whereas in
the radical coupling reaction the macroradicals are “stored” in
the form of the halogenated polymers premade via ATRP and are
only released in the presence of a copper/catalyst system. The
products from both processes are virtually identical with the
ATRP route, however, allowing for polymers with narrow
molecular weight distributions. The successful formation of
polymers with a distinct midchain positioned alkoxyamine in
either technique relies strongly on the fast addition rate of the
macroradicals to the nitrone, kad,macro, and the high coupling rate
coefficients between macronitroxides and macroradical species.
In the following, the synthesis of midchain functionalized
polymers will be presented in detail. The characterization of the
formed star (co)polymers will include conventional size exclusion
chromatography techniques as well as two-dimensional LACCC-
SEC chromatography analysis.
fluoride trihydrate (TBAF 3H2O, Aldrich, 97%), and sodium
3
azide (Sigma-Aldrich, 99%) were used as received. Styrene (Sty,
Aldrich, 99%) and isobornyl acrylate (iBoA, Aldrich) mono-
mers were deinhibited by percolating over a column of basic
alumina. Copper(I) bromide (CuBr, Sigma-Aldrich, 98%) was
washed with glacial acetic acid at 80 ꢀC overnight to remove
any soluble oxidized species before being filtered, rinsed with
ethanol, and dried.
Synthesis of 4-(Prop-2-ynyloxy)benzaldehyde (5). 4-Hydroxy-
benzaldehyde (15.14 g, 0.124 mol), KOH (12.3 g, 0.186 mol),
and propargyl bromide (27.6 g, 0.186 mol) were weighed into a
100 mL round-bottom flask, and the mixture was dissolved in
60 mL of ethanol and mixed thoroughly. The solution was
heated under reflux at 80 ꢀC for 20 h before being allowed to
cool down to ambient temperature. Solvent and unreacted
propargyl bromide were removed under high vacuum, yielding
a dark brown solid. The solid was mixed with water (50 mL) and
extracted with diethyl ether (3 ꢀ 50 mL). The organic fraction
was dried with MgSO4 and filtered, and the solvent was later
removed in vacuo. The recovered solids were crystallized in
ethanol, yielding brown crystals (yield: 68%, 10.3 g). 1H NMR
(300 MHz, CDCl3, 25 ꢀC, δ, ppm): 9.90 (CHdO, s, 1H),
7.84-7.87 (Ph, d, 2H), 6.94-6.97 (Ph, d, 2H), 4.78 (O-CH2-,
s, 1H), 2.55-2.57 (-CtCH, t, 1H). 13C NMR (75 MHz, CDCl3,
25 ꢀC, δ, ppm): 191.2 (CHdO), 162.8, 132.3, 131.0, 115.6 (Ph),
56.3 (O-CH2-), 78.4 (-Cꢁ), 76.7 (ꢁCH).
Experimental Section
Materials. 4-Hydroxybenzaldehyde (Aldrich, 98%), pro-
pargyl bromide solution (Fluka, 80% in toluene), potassium

Article
Synthesis of 4-(3-(Trimethylsilyl)prop-2-ynyloxy)benzalde-
Macromolecules, Vol. 43, No. 8, 2010 3787
Synthesis of PS-N3. To a 100 mL round-bottom flask that
contains CuBr (143 mg), Sty (50 mL) and MBrP (112 μL) were
added, and the solution was degassed by purging with nitrogen
for 30 min in an ice bath. Degassed PMDETA (418 μL) was
transferred into the flask via a degassed syringe. The reaction
mixture was heated at 100 ꢀC in a thermostated oil bath for
40 min, and the polymerization was stopped by immersing
the sealed flask in an ice bath before opening to air. A sample
was withdrawn from the solution and analyzed by NMR to
determine the conversion (∼10% monomer conversion was
obtained). The polymer was purified similarly to PiBoA I,
yielding 3.5 g of polymer with Mn=2700 g mol-1 and PDI=
1.14 (by THF-SEC analysis). The bromine-terminated PS-Br
was converted to PS-N3 in identical fashion to PiBoA-N3.
Radical Coupling Reaction. PiBoA I (860 mg, 200 μmol),
nitrone 1 (122 mg, 400 μmol), and Cu(0) (12.8 mg, 200 μmol)
were dissolved in 1.6 mL of toluene in a glass vial (to make up a
0.1 M solution of the ATRP polymer) and purged with nitrogen
for 10 min in an ice bath. A degassed solution of 400 μL of
toluene with PMDETA (43 μL, 200 μmol) was transferred into
the glass vial via a degassed syringe. The reaction mixture was
heated to 60 ꢀC in an electrical thermomixer for 3 h. The
resulting PiBoA II was purified by passing over a column of
silica gel and reprecipitated twice in cold methanol (Mn=7700 g
mol-1 and PDI=1.16 (by THF-SEC analysis)).
Polymer Quenching. PiBoA II (77 mg, 10 μmol) was dissolved
in 538 μL of DMF before adding 2 mmol of TBTH (538 μL) such
that the ratio of polymer to quencher is 1:200. The solution was
heated to 125 ꢀC for 5 h. Under these conditions, the alkoxy-
amine bond is cleaved and TBTH transfers its proton to the
radicals, preventing recombination. As the hydride is a strong
transfer agent, quantitative quenching can be assumed consid-
ering the high concentration of the quencher and the relatively
high temperature. The solvent was removed by evaporation in a
fume cupboard before the quenched polymer was analyzed via
THF-SEC (Mn=5200 g mol-1 and PDI=1.19).
hyde (6). The trimethylsilyl protection step was adapted from
the literature procedure.26 To a two-neck 100 mL round-bottom
flask that contained the aldehyde 5 (7 g, 37.8 mmol) and silver
chloride (1.09 g, 7.56 mmol) was first added 60 mL of dry
dichloromethane followed by 1,8-diazabicyclo[5.4.0]undec-7-
ene (11.5 g, 75.6 mmol). The reaction mixture was then heated
under reflux at 40 ꢀC and chlorotrimethylsilane (12.3 g, 113
mmol) added dropwise, and the contents were stirred for 2 days.
The mixture was allowed to cool to ambient temperature and
diluted with 150 mL of n-hexane. The organic phase was
subsequently washed successively with aqueous NaHCO3, 2 M
HCl, and water before being dried over anhydrous MgSO4,
filtered, and concentrated under high vacuum. Beige solids were
recovered, and the product was used in the next step without
1
further purification (yield: 89%, 6.2 g). H NMR (300 MHz,
CDCl3, 25 ꢀC, δ, ppm): 9.88 (CHdO, s, 1H), 7.82-7.87 (Ph, d,
2H), 7.05-7.08 (Ph, d, 2H), 4.76 (O-CH2-, s, 1H), 0.15 (TMS,
s, 9H). 13C NMR (75 MHz, CDCl3, 25 ꢀC, δ, ppm): 191.1
(CHdO), 162.9, 132.1, 130.8, 115.5 (Ph), 57.1 (O-CH2-), 94.1
(-Cꢁ), 99.2 (ꢁCH), -0.08 (TMS).
Synthesis of r-4-(3-(Trimethylsilyl)prop-2-ynyloxy)-N-tert-
butyl Nitrone (1). The aldehyde 6 (6 g, 25.8 mmol), N-(tert-
butyl)hydroxylamine acetate (5 g, 33.4 mmol), and excess
MgSO4 (as drying agent to absorb the water byproduct) were
weighed into a 250 mL round-bottom flask before dissolving
with 100 mL of chloroform. Triethylamine (3.8 g, 33.4 mmol)
was added dropwise into the solution. Stirring of the solution at
room temperature was stopped after 3 days. Chloroform solvent
was removed under high vacuum. Approximately 80 mL of
water was added to the flask, and the suspension was extracted
with ethyl acetate (3 ꢀ 60 mL). The organic fraction was dried
with MgSO4 and filtered. Ethyl acetate was removed under high
vacuum to yield light yellow solids which were later crystallized
in concentrated ethanol, yielding light yellow crystals (yield:
64%, 3.84 g). 1H NMR (300 MHz, CDCl3, 25 ꢀC, δ, ppm): 1.60,
(N-tert butyl, s, 9H), 7.47 (CHdN, s, 1H), 8.27-8.30 (Ph, d,
2H), 6.98-7.01 (Ph, d, 2H), 4.72 (O-CH2-, s, 1H), 0.15 (TMS,
s, 9H). 13C NMR (75 MHz, CDCl3, 25 ꢀC, δ, ppm): 28.7, 70.5
(N-tert butyl), 129.7 (CHdN), 159.2, 130.9, 125.0, 115.0 (Ph),
57.0 (O-CH2-), 93.5 (-Cꢁ), 99.8 (ꢁCH), 0.03 (TMS).
ESCP Reactions. All ESCP reactions are conducted in a
similar manner as previously reported in literature.13-15
ATRP Reaction. To a 50 mL round-bottom flask containing
CuBr (50 mg), iBoA (7.5 mL) and MBrP (26 μL) were added,
and the solution was degassed by purging with nitrogen for
10 min in an ice bath. A degassed solution of 2.5 mL of butyl
acetate with PMDETA (147 μL) was transferred into the flask
via a cannula. The reaction mixture was heated at 75 ꢀC in a
thermostated oil bath for 30 min, and the polymerization was
stopped by immersing the sealed flask in an ice bath before
opening to air. A sample was withdrawn from the solution and
analyzed by NMR to determine the conversion (∼35% mono-
mer conversion was obtained). The contents in the flask were
poured into a beaker containing CuBr2 (150 mg) in THF and
was left overnight in high vacuum to remove any solvent and
unreacted monomer. Fresh THF was then added to the beaker,
and the polymer PiBoA I was purified by passing the polymer
solution over a column of silica gel in order to remove all copper
complexes. The copper-free polymer solution was concentrated
and subsequently reprecipitated twice in cold methanol, yielding
2.1 g of polymer with Mn=4300 g mol-1 and PDI=1.21 (by
THF-SEC analysis).
Alkyne Deprotection. The typical procedure for the deprotec-
tion of the alkyne is as follows: PiBoA II (530 mg, 66 μmol) was
dissolved in 1 mL of THF. In another glass vial, TBAF 3H2O
3
(209 mg, 663 μmol) was dissolved in 2.3 mL of THF. Both
solution mixtures are purged with nitrogen for 20 min in an ice
bath. The solution in the glass vial containing TBAF was
transferred to the glass vial containing the polymer solution
via a cannula, and the combined mixture was stirred for 24 h at
ambient temperature. The resulting PiBoA II (after TMS re-
moval) was reprecipitated twice in cold methanol. The molecu-
lar weight distributions (MWDs) of the polymers before and
after TMS removal are almost identical. TBAF, which was used
in 10 times excess, allowed for quantitative removal of the TMS
group as confirmed by 1H NMR analysis.
Click Reactions. In a general Cu-catalyzed alkyne/azide
cycloaddition (CuAAC) reaction, polymers bearing midchain
alkyne functionalities (5 μmol) and PS-N3 (or PiBoA-N3) in a
1:1 mole ratio were dissolved in 1.5 mL of a THF:DMF mixture
(2:1 volume ratio) containing CuBr (50 μmol) and were purged
with nitrogen for 20 min in an ice bath. PMDETA (50 μmol) was
injected into the sealed solution with a degassed syringe, and the
reaction mixture was stirred for 48 h at room temperature. The
clicked product was purified by passing over a column of silica
gel in order to remove all copper complexes. The copper-free
polymer solution was concentrated and subsequently reprecipi-
tated twice in cold methanol before any characterization was
carried out.
Synthesis of PiBoA-N3. PiBoA I (430 mg, 100 μmol) was
dissolved in a 2 mL mixture of THF:DMF (1:1 volume ratio)
before adding an excess of sodium azide (65 mg, 1 mmol). The
mixture was stirred for 2 days, and PiBoA-N3 was reprecipitated
twice in cold methanol and washed exhaustively with water. 1H
NMR and ESI-MS analysis confirmed the formation of the
polymer product.
Characterization by THF Size Exclusion Chromatography
(THF-SEC). Analysis of the MWDs of the polymer samples
were performed on a Polymer Laboratories PL-GPC 50 Plus
Integrated System, comprising an autosampler, a PLgel 5.0 μm
bead-size guard column (50 ꢀ 7.5 mm), followed by three PLgel
5 μm Mixed-C columns (300 ꢀ 7.5 mm) and a differential
refractive index detector using tetrahydrofuran (THF) as the

3788 Macromolecules, Vol. 43, No. 8, 2010
Wong et al.
Scheme 2. Synthesis of the Nitrone 1a
a Reactants and conditions: (a) propargyl bromide, KOH, ethanol,
reflux at 75 ꢀC, 20 h; (b) chlorotrimethylsilane, AgCl, DBU, dichloro-
methane, reflux at 40 ꢀC, 2 days; (c) N-(tert-butyl)hydroxylamine
acetate, triethylamine, chloroform, rt, 3 days.
eluent at 35 ꢀC with a flow rate of 1 mL min-1. The SEC system
was calibrated using linear narrow polystyrene standards ran-
ging from 160 to 6 ꢀ 106 g mol-1 (polystyrene (K=14.1 ꢀ 10-5
dL g-1 and R=0.70),27 poly(isobornyl acrylate) (K=5.0 ꢀ 10-5
dL g-1 and R=0.745)28). Polymer concentrations were in the
Figure 1. Molecular weight distribution (MWD) of polystyrene sam-
ples prepared via ESCP at 60 ꢀC at variable concentrations of nitrone 1
(upper part). The inverse of DPn is plotted as a function of nitrone
concentration for the determination of CSC and kad,macro in the ESCP
of styrene with nitrone 1 (lower part). Note that polymer samples
prepared at low monomer conversions (<5%) are used to deter-
mine DPn.
range of 3-5 mg mL-1
.
Characterization by Nuclear Magnetic Resonance (NMR)
Spectroscopy. All NMR spectra were recorded on a Bruker
DPX spectrometer (300 MHz).
lization procedures were sufficient to ensure high product purity
as confirmed by 1H NMR and 13C NMR analysis.
Characterization by 2D LACCC-SEC. Two-dimensional
chromatography was carried out on a commercial system from
PSS (Mainz). In the first dimension (LACCC), the sample is
injected on a C18 LC Column (Agilent Technologies ZORBAX
Eclipse XDB-C18) with THF:MeOH=68:32 as the eluent29 at a
flow rate of 0.1 mL min-1. After passing UV detection
(SECcurity GPC1200) samples are transferred via a switch valve
onto the second dimension (SEC), which is run on pure THF at a
flow rate of 3 mL min-1. SEC separation is achieved by using a
high-speed column (SDV, linear M, 20 ꢀ 50 mm) provided by
PSS. Polymer concentrations are detected via anevaporative light
scattering detector (SECcurity ELS4000), and the resulting data
are processed using the 2D-module of the WinGPC software.
ESCP and 3-Arm Star Formation. Nitrone 1 is sub-
sequently employed in the ESCP of styrene with 4 ꢀ 10-2
mol L-1 of the initiator AIBN at 60 ꢀC. The upper part of
Figure 1 depicts the molecular weight distribution (MWD)
of the polystyrene samples made via ESCP at variable
nitrone concentrations, cnitrone, while the lower part of the
figure shows the plot of the inverse of degree of polymeriza-
tion, DPn, of the polystyrene samples as a function of cnitrone
.
Both the upper and lower part of Figure 1 clearly show the
polystyrene samples having decreasing molecular weights
and DPn when larger amounts of nitrones are used. Bearing
in mind that the ESCP process involves competitive reac-
tions between the macroradicals adding to the nitrone and
chain propagation events, at higher nitrone concentrations
the propagating chains have a higher tendency to react with
the nitrone, thus resulting in shorter chains being captured
by the nitrone and the subsequently formed macronitroxide.
A satisfactory linear fit is obtained from the plot in the lower
part of Figure 1, which is in good agreement with eq 1, where
DP¥ is the DPn for a conventional radical polymerization (in
the absence of nitrones), kp is the propagation rate coeffi-
cient, and cm is the monomer concentration.
Results and Discussion
One of the most common procedures to synthesize nitrones is
via the condensation of aldehydes or ketones with N-monosub-
stituted hydroxylamines.30 The commercially available benzalde-
hyde derivatives with various additional functional groups such
as hydroxyl and halides attached to the phenyl ring allow for
facile postchemical modifications, making this the method of
choice for the synthesis of nitrone 1. As shown in Scheme 2, the
conversion of 4-hydroxybenzaldehyde into nitrone 1 is a straight-
forward process, where the hydroxyl group was first reacted with
propargyl bromide via nucleophilic substitution to introduce the
alkyne functionality, followed by the protection of the alkyne
with a trimethylsilyl group before finally converting the aldehyde
function into a nitrone by reacting with N-(tert-butyl)hydroxyl-
amine acetate. Over the course of the synthesis, no tedious
chromatographic product separation was required, and crystal-
0:5kad,macrocnitrone
kpcm
-1
-1
DPn
¼ DP¥
þ
ð1Þ
The results depicted in Figure 1 indeed support the notion
that the ESCP mechanism is in place with the new nitrone 1.

Article
Macromolecules, Vol. 43, No. 8, 2010 3789
Table 1. Peak and Average Molecular Weight Data of the Experi-
mental MWDs Shown in Figure 2
polymer
Mp/g mol-1
Mn/g mol-1
Mw/Mn
PS-N3
PSESCP
3000
7900
9400
2700
5300
7400
1.14
1.59
1.40
PClick
I
Figure 2. Molecular weight distribution (MWD) of the original poly-
styrene model compounds PSESCP and PS-N3 and the corresponding
clicked 3-arm starpolystyreneproduct, PClick I. TheMWD oftheTPClick
I is a result of the theoretical convolution of the MWD of the starting
materials computed via the PREDICI software as described pre-
viously.34
We have demonstrated earlier that the molecular weights of
ESCP-derived polymers can be accurately described by
eq 1.13,14 Since eq 1 bears some resemblance to the
Mayo equation for conventional chain transfer polymeriza-
Figure 3. Molecular weight distributions (MWD) of poly(isobornyl
acrylate) before nitrone coupling (PiBoA I), after coupling (PiBoA II),
and after quenching of the alkoxyamine bond (PiBoA III).
tion,31,32 the definition of the spin capturing constant, CSC
,
was derived for ESCP where CSC=kad,macro/kp. The CSC of
nitrone 1 in the ESCP of styrene at 60 ꢀC is determined to be
0.80 from the slope of the linear fit of the plot in Figure 1
(where cm of styrene33 is ∼8.3 mol L-1). In order to achieve a
constant average molecular weight, Mn, up to high conver-
sions, a CSC value of close to unity is required such that
nitrone and monomer are consumed equally over the course
of the polymerization. Although not shown explicitly in the
present work, the Mn of the polystyrene samples stayed
approximately constant up to high monomer to polymer
conversions (maximum conversion tested ∼50%) since
CSC = 0.80 is close to unity. From CSC, kad,macro can be
deduced from the linear fit provided kp is known. Hence,
material will still be inevitable because the PS-N3 is con-
taminated with dead polymeric material from the ATRP
process. Also depicted in Figure 2 is the simulated MWD of
TPClick I as a result of the convolution of the MWD between
PSESCP and PS-N3 generated via the PREDICI simulation
software. Apart from the slight tailing discussed above,
almost identical distributions for the theoretical convolution
and the experimental SEC trace are found, which is further
indication that the formation of the 3-arm star was success-
ful. The simulated MWD in this case (as well as for further
examples in the current) gives an estimation on how clicked
distributions should look like under perfect conditions and
therefore serves as an excellent guideline in assessing the
efficacy of polymer-polymer conjugation reactions.34 It
should, however, be noted that the simulations were carried
out without assuming column band broadening or hydro-
dynamic volume effects (caused by the star structure of the
polymers under investigation).
In Table 1, the Mn of PClick I approximates the sum of the
individual Mn values of 2700 and 5300 g mol-1 of PSESCP and
PS-N3, respectively, which is within the experimental accu-
racy of the SEC, taking into account the tailing in the MWD
of PClick I. The shift in MWD together with the systematic
increase in the Mn of PClick I clearly indicates a successful
click reaction and thus the formation of 3-armed star poly-
styrene.
Radical Coupling Reactions and 3-Arm Star Formations.
Another way of generating midchain functionalized poly-
mers in a facile manner (besides ESCP) is by activating
preformed ATRP polymers in the presence of a nitrone
and a suitable copper/ligand catalyst system. Poly(isobornyl
acrylate), PiBoA I (prepared by ATRP), is reacted with a
2-fold excess of nitrone 1 with Cu(0)/PMDETA at 60 ꢀC for
3 h. During the course of the reaction, macroradicals are
freed and reacted with nitrone 1 before coupling with
another macroradical, giving access to PiBoA II bearing an
alkyne functionality in its midchain position as described in
Scheme 1. The MWDs of the ATRP-made PiBoA I and the
nitrone-coupled PiBoA II are illustrated in Figure 3. As
expected, the molecular weight of the polymer increases
due to the coupling reaction.
k
ad,macro for the addition of styrenic macroradicals to nitrone
1 is determined to be 270 L mol-1 s-1 with kp of styrene33 at
60 ꢀC being 340 L mol-1 s-1. Knowledge of CSC and kad,macro
is important as these numbers allow for a good estimation of
target molecular weights in the later synthesis of polymers on
a larger scale.
To investigate the formation of 3-arm star polymer, a low
molecular weight polystyrene model compound, PSESCP
,
which is prepared via ESCP with 4 ꢀ 10-1 mol L-1 of nitrone
1 (and after deprotection of the alkyne function), is ligated
with an azide-terminated polystyrene, PS-N3, via a CuAAC
reaction. Figure 2 shows the MWDs of the polystyrene
samples before and after the click reaction. Qualitative
analysis of Figure 2 reveals a distinct shift in the molecular
weight of the conjugated product, PClick I, toward higher
molecular weights. Only a small amount of tailing toward the
lower molecular weight side is observed which is most likely
due to leftover unfunctionalized dead chains from the origi-
nal bromine-terminated polystyrene made via ATRP that is
employed as the precursor in synthesizing PS-N3. This
1
notion is confirmed by H NMR analysis, as the original
polystyrene made via ATRP (where the PS-N3 is derived
from) only features 90% bromine functionality. As a result,
the following postmodification step in converting from the
bromine-terminated polystyrene to the PS-N3 leads to the
loss of at least 10% in azide end-group functionality. Thus,
under the assumption that the conjugation reaction does
proceed to full conversion, around 10% of “unreacted”

3790 Macromolecules, Vol. 43, No. 8, 2010
Wong et al.
Collated in Table 2 are the Mn of the SEC traces of the
poly(isobornyl acrylate) samples in Figure 3. The doubling
of the molecular weight of the ATRP polymer from Mn of
4300 to 7700 g mol-1 provides an indication of a successful
coupling reaction between the two identical polymer chains.
To further prove that a coupling reaction occurred, Figure 4
depicts the full NMR spectra of PiBoA I and PiBoA II; the
insets show the end- and midgroup regions of these samples.
The peak corresponding to the proton in an R-position to the
bromine atom in PiBoA I (b0, 4.1-4.3 ppm) quantitatively
disappears, and only the end group originating from the
ATRP initiator employed in the preparation of the starting
polymer (a0, 3.5-3.7 ppm) remains in PiBoA II. Both the
SEC and NMR analysis thus confirm that the Br-terminated
polymers are reduced and are effectively coupled.
However, the successful coupling reaction by itself does
not necessarily indicate that the macroradicals undergo the
coupling reaction as described in Scheme 1, as conventional
termination by combination of the macroradicals would also
lead to similar MWDs as PiBoA II. Therefore, further
analysis of Figure 4 reveals the occurrence of the peak
corresponding to the trimethylsilyl protons of the formed
alkoxyamine (c0, 0.1-0.3 ppm) in PiBoA II that is not
initially present in PiBoA I, supporting the notion that the
coupling reaction did in fact proceed via the enhanced spin
capturing mechanism. In addition, the ratio of integration of
the peaks c0 to a0 yields a value of 1.2, which corresponds to
∼90% of midchain functionalization with respect to the
nitrone coupling efficiency. To further prove this hypothesis,
PiBoA II is quenched by heating in the presence of a radical
scavenger such as tributyltin hydride15,21 to determine if the
nitrone is truly incorporated into the polymer chain: The
alkoxyamine centered in the middle of the polymer chain
undergoes cleavage at elevated temperatures (>100 ꢀC). Via
the action of the radical scavenger present in the quenching
mixture, polymer chains of approximately half of that of the
coupled product PiBoA II should be obtained, i.e., reverting
the full distribution back into its original shape (PiBoA I).
The MWD of the quenched product PiBoA III in Figure 3
shows exactly such behavior as almost the starting distribu-
tion of the ATRP polymer PiBoA I is again obtained after
quenching. As can be seen in Table 2, the Mn of PiBoA III is
also similar to that of PiBoA I within experimental error,
although one may assert that the former is marginally higher
than the latter. A reasonable explanation for this slight
mismatch is that the polymer chains of the quenched sample
have an added nitrone 1 unit (with a molecular weight of
303.4 g mol-1) during the coupling reaction, which thus leads
to the PiBoA III having slightly higher molecular weights as
compared to PiBoA I. Despite the slight discrepancy in the
Mn between the quenched sample and the original ATRP
polymer, it is safe to state that the nitrone-mediated coupling
reaction must proceed to a considerable degree, based on the
collective results. The generated midchain functionalized
Table 2. Average and Peak Molecular Weight Data of Poly(isobornyl
acrylate) Samples (PiBoA I-III) Shown in Figure 2
polymer
Mp/g mol-1
Mn/g mol-1
Mw/Mn
PiBoA I
PiBoA II
PiBoA III
4200
8700
4700
4300
7700
5200
1.21
1.16
1.19
Figure 4. 1H NMR spectra of PiBoA I and PiBoA II samples given in Figure 3.

Article
Macromolecules, Vol. 43, No. 8, 2010 3791
Figure 6. Molecular weight distributions (MWDs) of the poly(isobor-
nyl acrylate) samples, PiBoA-N3, PiBoA II, and PClick III, as well as the
simulated distribution of the clicked product, TPClick III.
Figure 5. SEC traces of the alkyne bearing polymer, PiBoA II, azide-
terminated polymer, PS-N3, and the conjugation product, PClick II. The
theoretical MWD of the conjugate distribution, PClick II, is also
T
included.
Mn value of 10 900 g mol-1 of the star homopolymer is also
equivalent to the sum of the Mn of the starting polymers
PiBoA II (Mn=7700 g mol-1) and PiBoA-N3 (Mn=4300 g
mol-1). In addition, the SEC trace of PClick III matches
perfectly with the theoretical distribution, although a small
tailing is observed in this case, too. The tailing can most
probably be attributed to the inherent dead polymers present
in the PiBoA-N3 for exactly the same reason as that to the
Table 3. Peak and Average Molecular Weight Data of the Polymer
Samples Shown in Figures 5 and 6
polymer
Mp/g mol-1
Mn/g mol-1
Mw/Mn
PS-N3
3000
4200
8700
13200
12700
2700
4300
7700
11900
10900
1.14
1.21
1.16
1.19
1.14
PiBoA-N3
PiBoA II
PClick II
PClick III
1
PS-N3. H NMR analysis confirms that only 89% of the
polymer chains in PiBoA-N3 are functionalized with the
azide end group. Moreover, the same polymer precursor
which is used in synthesizing PiBoA-N3, i.e., the ATRP
polymer PiBoA I which has only 89% bromine end groups,
is subjected to the nitrone-mediated radical coupling reac-
tion to generate PiBoA II. Again, the dead chains that are
actually present in both PiBoA II and PiBoA-N3 formed
most likely during the ATRP reaction itself (and parti-
ally from the radical coupling reaction in the case of
PiBoA II), contributing to the tailing in the distribution of
PClick III.
2D LACCC-SEC Chromatographic Analysis. Despite the
adequacy of conventional SEC techniques in indicating the
successful formation of star copolymer via click reactions, it
is nonetheless difficult to assess the purity of the copolymer
by judging solely from its molar masses since the copolymer
distribution also varies in terms of its chemical composition
and/or functionality. Hence, chromatographic characteriza-
tion of copolymers in more than one dimension is required.
Two-dimensional LACCC-SEC chromatography is an ap-
propriate tool for such purposes, as it allows for simulta-
neous yet independent measurement of a polymer’s chemical
composition and constituting molar mass. The strength of
LACCC-SEC has been demonstrated as an analytical pro-
cedure allowing for an efficient analysis of block copoly-
mers.35,36 In LACCC-SEC, polymer mixtures are first solely
separated according to their chemical composition in the
HPLC dimension, and the respective eluents are then sub-
jected to further separation according to their hydrodynamic
volumes by normal SEC.
PiBoA II can therefore be subsequently employed in click
reactions to form miktostar structures.
Likewise to PSESCP, the alkyne function of the PiBoA II
model compound is deprotected prior to the conjugation
reaction. Figure 5 depicts the individual SEC traces of the
starting polymers, PS-N3 and PiBoA II, as well as the experi-
mental and simulated clicked star MWDs, PClick II and
TPClick II. Inspection of Figure 5 shows a monomodal
distribution of PClick II at higher molecular weights without
any pronounced shoulders and only minor tailings toward
the lower molecular weight side, which is a good initial
indication of a successful click reaction when conjugating
two polymer samples of relatively narrow polydispersities
(∼PDI < 1.2). When comparing the experimental and
theoretical MWDs of the conjugate product, both distribu-
tions match well;further proof of a well-working click
reaction. The molecular weights of the polymer samples in
Figure 5 are listed in Table 3. The Mn of the miktoarm
star polymer is in good agreement with the sum of the
measured Mn values of the individual starting polymers, all
in all strongly pointing toward the successful click reaction
and thus the formation of star polymers. When the experi-
mental and theoretical MWDs of the click product are
examined, a slightly broader distribution is seen in the
experiment compared to the prediction. This tailing to both
sides of the distribution may easily be explained by the
imperfect separation of block copolymers on a conventional
SEC system, which could not be taken into account in the
simulation.
In an attempt to provide more evidence on the purported
synthesis strategy in the generation 3-arm star (co)polymers,
a third reaction involving the ligation between PiBoA II and
another azide-terminated poly(isobornyl acrylate), PiBoA-
N3, is performed. Figure 6 depicts the MWDs of the polymer
samples involved in this click reaction as well as the theore-
tical convolution of the starting materials, TPClick III. Similar
to what is observed in Figures 2 and 5, a monomodal
distribution of the clicked product, PClickIII, occurs at higher
molecular weights compared to the starting polymers. The
The critical conditions for poly(isobornyl acrylate) are
utilized to perform the analysis in the first dimension under
critical conditions. Under such critical conditions, the en-
tropic and enthalpic effects between the poly(isobornyl
acrylate) samples and the packing column cancel each other
out, and therefore polymers with different molar masses
elute at the same retention volume. In other words, the
polymers are now no longer separated by size but according
to their chemical heterogeneity in the first (LACCC) dimen-
sion. Figure 7 shows the 2D chromatograms of the PiBoA II,

3792 Macromolecules, Vol. 43, No. 8, 2010
Wong et al.
Figure 7. LACCC-SEC chromatograms of PiBoA II and the subsequent clicked star products. The y-axis is the LACCC elution volume (not scaled),
and the x-axis corresponds to the elution volume in the SEC dimension.
Figure 8. LACCC-SEC chromatograms in 3D view of the overlaps between PiBoA II with PClick II (a) and with PClick III (b). The x, y, and z axis
correspond to SEC, LACCC and concentration dimensions, respectively.
PClick II, and PClick III samples. Although the LACCC scales
(y-axis) of the three chromatograms are not scaled abso-
lutely, a comparison indicates that the peaks of PiBoA II and
PClick III are identical, i.e., approximately having the same
elution volume. This is in line with the fact that both these
samples are composed of the same isobornyl acrylate mono-
mer units and the only difference being in polymer topology
and molar masses. Thus, because of the same chemical
composition, there is no apparent chemical separation in
the LACCC dimension. However, upon comparing PiBoA II
and PClick II, a distinctive change in elution volume on the
LACCC dimension is observed in which the star copolymer
PClick II elutes faster compared to the linear homopolymer of
PiBoA II. The additional attachment of the polystyrene
block alters the chemical structure of the clicked star, thus
allowing for an efficient separation by LACCC analysis.
Also observable in Figure 7c is a shoulder peak correspond-
ing to unfunctionalized polystyrene. Integration of the peaks
provided quantitative data on the composition of the final
clicked product PClick II. Unfunctionalized PS that occurs in
the chromatogram is close to 10% of the final product, which
is in agreement with the aforementioned NMR results that
indicated about 10% of dead polymers present in the azide-
terminated polystyrene.
For a clearer and more illustrative comparison between
the starting polymer PiBoA II and the formed conjugated
products in Figure 7, overlays of their chromatograms are
plotted in a 3D view as shown in Figure 8. An additional
concentration axis is included into the 2D chromatograms,
making the 3D view possible. The results in Figure 8
strengthen the conclusions drawn from Figure 7 as distin-
guishable distributions between PiBoA II and PClick II on the
LACCC scale are obtained, whereas such clear differences
are not visible between PiBoA II and PClick III.
Conclusions
The novel compound R-4-(3-(trimethylsilyl)prop-2-ynyloxy)-
N-tert-butyl nitrone 1 was prepared and successfully utilized in
ESCP and nitrone-mediated radical coupling reactions to pro-
duce polystyrene and poly(isobornyl acrylate), respectively, with
alkyne functions in the middle of the polymer chains. The
subsequent CuAAC reactions with azide-terminated polymers
to form 3-arm star (co)polymers were also successful and their
formation pathways were confirmed by conventional SEC tech-
niques as well as 2D LACCC-SEC analysis. The technology
developed in the present study, which includes the combination of
the relatively facile generation of midchain functionalized poly-
mers via ESCP or radical coupling reactions, together with the
efficient orthogonal conjugation reactions lay the foundation for
the development of new promising synthetic strategies in the
generation of complex macromolecular architectures via nitrone-
mediated polymerization processes.
Acknowledgment. C.B.-K. acknowledges funding from the
Karlsruhe Institute of Technology (KIT) in the context of the
Excellence Initiative for leadingGerman universities, the German
ResearchCouncil(DFG), andthe MinistryofScience andArtsof
€
the state of Baden-Wurttemberg. T.J. is grateful for support from
the Fonds der Chemischen Industrie. E.H.H.W. and C.B.-K. are
thankful for a postgraduate scholarship from UNSW central
funding. M.H.S. acknowledges a future fellowship from the
ARC.
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