Nitroxide-mediated polymerization of N-isopropylacrylamide: Electrospray ionization mass spectrometry, matrix-assisted laser desorption ionization mass spectrometry, and multiple-angle laser light scattering studies on nitroxide-terminated poly-N-isopropylacrylaniides

Article abstract of DOI:10.1021/ma050343n

Nitroxide-mediated controlled living free radical polymerization of N-isopropylacrylamide using highly sterically hindered 2,2,6,6-tetraethylpiperidin-4-on-N-oxyl 1 is described. In addition, an improved synthesis for nitroxide 1 is presented. Poly-N-isopropylacrylamides (PNIPAMs) prepared are analyzed by multiple-angle laser light scattering. Moreover, the nitroxide-terminated PNIPAMs are characterized using electrospray ionization mass spectrometry, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and Fourier transform ion cyclotron MALDI-MS. Careful MS analysis reveals that chain-end degradation of nitroxide-terminated PNIPAMs occurs during MALDI analysis. A mechanism for chain end degradation is presented.

Full text of DOI:10.1021/ma050343n

Macromolecules 2005, 38, 6833-6840
6833
Nitroxide-Mediated Polymerization of N-Isopropylacrylamide:
Electrospray Ionization Mass Spectrometry, Matrix-Assisted Laser
Desorption Ionization Mass Spectrometry, and Multiple-Angle Laser
Light Scattering Studies on Nitroxide-Terminated
Poly-N-isopropylacrylamides
Tobias Schulte,† Kai Oliver Siegenthaler,† Heinrich Luftmann,†
Matthias Letzel,‡ and Armido Studer*,†
Organisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Corrensstrasse 40,
D-48149 Mu¨nster, Germany, andUniversita¨t Bielefeld, Organische Chemie I, Postfach 100131,
D-33501 Bielefeld, Germany
Received February 17, 2005; Revised Manuscript Received June 2, 2005
ABSTRACT: Nitroxide-mediated controlled living free radical polymerization of N-isopropylacrylamide
using highly sterically hindered 2,2,6,6-tetraethylpiperidin-4-on-N-oxyl 1 is described. In addition, an
improved synthesis for nitroxide 1 is presented. Poly-N-isopropylacrylamides (PNIPAMs) prepared are
analyzed by multiple-angle laser light scattering. Moreover, the nitroxide-terminated PNIPAMs are
characterized using electrospray ionization mass spectrometry, matrix-assisted laser desorption ionization
time-of-flight mass spectrometry (MALDI-TOF-MS), and Fourier transform ion cyclotron MALDI-MS.
Careful MS analysis reveals that chain-end degradation of nitroxide-terminated PNIPAMs occurs during
MALDI analysis. A mechanism for chain end degradation is presented.
Introduction
on NMP of NIPAM are unknown. Herein we present
results on successful controlled radical polymerization
of NIPAM using our recently introduced alkoxyamine
2, which is readily prepared from the highly sterically
hindered nitroxide 1 (Figure 1).12 Moreover, we present
an improved synthesis for nitroxide 1. Careful electro-
spray ionization mass spectrometry (ESI-MS), matrix
assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF-MS), and Fourier transform
ion cyclotron MALDI-MS (FTICR-MALDI-MS) analyses
have been used for the characterization of nitroxide-
terminated PNIPAMs.13 In addition, we will show that
multiple-angle laser light scattering (MALLS) is supe-
rior to classical gel permeation chromatography (GPC)
analysis to determine the molecular weight of PNIPAM.
Poly-N-isopropylacrylamide (PNIPAM) is a highly
interesting polymer that in aqueous solution undergoes
phase separation upon temperature increase.1,2 The
transition from a hydrophilic hydrated to a hydrophobic
phase-separated state occurs at the lower critical solu-
tion temperature (LCST). PNIPAM is currently one of
the most popular polymers investigated. Numerous
publications on PNIPAM appeared during the last 5
years.3 Many applications using PNIPAM for the con-
struction of thermoresponsive smart materials have
been published.4 PNIPAM is generally prepared via
free radical polymerization of N-isopropylacrylamide
(NIPAM). However, classical radical polymerization
techniques do not allow the control of the molecular
weight of PNIPAM. Moreover, broad polydispersities are
obtained. For sophisticated PNIPAM-containing materi-
als, however, defined molecular weight and narrow
polydispersities of PNIPAM are highly desirable.
Controlled free radical polymerization techniques
have been intensively investigated during the past few
years. Nitroxide-mediated polymerizations (NMP),5 atom
transfer radical polymerizations (ATRP),6 and reversible
addition fragmentation chain transfer (RAFT) polymer-
izations7 among others are radical polymerization tech-
niques that allow the preparation of polymers with
defined molecular weight and narrow polydispersities.
Whereas for ATRP8 and RAFT9,10 several papers on the
controlled radical polymerization of PNIPAM were
published, only a single example on a successful nitrox-
ide-mediated radical polymerization of NIPAM has
appeared in the literature to date.11 Systematic studies
Experimental Section
Materials. 2-Ethylbutene (Aldrich, 95%), chlorosulfonyl
isocyanate (Aldrich, 98%), N,N-(dimethylamino)pyridine (Flu-
ka, g99%), di-tert-butyl dicarbonate (Acros, 99%), methylmag-
nesium bromide (Aldrich, 3 M in Et2O), trifluoroacetic acid
(Merck, g 99%), and diethyl ketone (Aldrich, 99+%) were used
as purchased. N-Isopropyl acrylamide (Aldrich, 99+%) was
recrystallized twice from n-hexane to remove the stabilizer.
Et2O and THF were distilled over K/Na, benzene was distilled
over sodium, and CH2Cl2 was distilled over CaH2 before use.
All other chemicals were used as received.
General. 1H NMR (500 MHz, 300 MHz, 200 MHz) and 13
C
NMR (125 MHz, 75 MHz, 50 MHz) spectra were recorded on
an AMX 500 (Bruker), ARX 300 (Bruker), or ARX 200
(Bruker). Chemical shifts δ in ppm are relative to SiMe4 as
an internal standard. Thin-layer chromatography: silica gel
60 F254 plates (Merck); detection with UV or dipping into a
solution of KMnO4 (1.5 g in 333 mL of 1 M NaOH) or a solution
of Ce(SO4)2‚H2O (10 g), phosphormolybdic acid hydrate (25 g),
concentrated H2SO4 (60 mL), and H2O (940 mL), followed by
heating. Flash chromatography (FC): silica gel 60 (40-63 µm,
Merck or Fluka), at ca. 0.4 bar. Melting points: 510 apparatus
(Bu¨chi); uncorrected. IR spectra were recorded on an IR 750
* To whom correspondence should be addressed. E-mail:
studer@uni-muenster.de.
† Westfa¨lische Wilhelms-Universita¨t Mu¨nster.
‡ Universita¨t Bielefeld.
10.1021/ma050343n CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/15/2005

6834 Schulte et al.
Macromolecules, Vol. 38, No. 16, 2005
h at RT. Solid NaHCO3 was added to adjust the pH ap-
proximately to a value of 7. The heterogenic mixture was
stirred for another 30 min. The phases were separated, and
the aqueous phase was extracted with CH2Cl2 (2 times). The
combined organic phases were dried over MgSO4, and the
solvent was removed under reduced pressure. Drying in vacuo
afforded 3 (7.80 g, 75%) as a pale-yellow viscous oil. IR (film):
3240w (NH), 2967s, 2939m, 2880m (CH), 1751w (CdO),
1460m, 1415m, 1374m, 1116m, 948m, 665m cm-1 1H NMR
.
Figure 1. Nitroxide 1 and alkoxyamine 2 used in the present
(200 MHz, CDCl3): δ ) 6.75 (bs, 1 H, NH), 2.63 (s, 2 H,
CH2CO), 1.70 (q, J ) 7.5 Hz, 4 H, CH2CH3), 0.93 (t, J ) 7.5
Hz, 6 H, CH2CH3). 13C NMR (75 MHz, CDCl3): δ ) 165.3 (CO),
148.3 (C), 43.7 (CH2CO), 29.2 (CH2CH3), 28.2 (CH2CH3), 8.4
(CH2CH3). MS (EI): 128 (2, [M + H]+), 98 (20), 84 (67), 69
(45), 56 (100), 55 (23), 43 (16), 42 (13), 41 (23), 28 (22). HRMS
(EI) calcd for C7H13NO (M+): 127.0997. Found: 127.1002.
N-(tert-Butoxycarbonyl)-4,4-diethylazetidin-2-one (4).
The â-lactam 3 (7.66 g, 60.2 mmol) was dissolved in CH2Cl2
(55 mL), and NEt3 (10.2 mL, 72.3 mmol) and DMAP (736 mg,
6.02 mmol) were added. A solution of di-tert-butyl dicarbonate
(15.8 g, 72.3 mmol) in CH2Cl2 (20 mL) was added, and the
reaction mixture was stirred at RT overnight. The mixture was
washed with saturated NH4Cl, H2O, and brine. The organic
phase was dried over MgSO4, and the solvent was removed
under reduced pressure. Purification by flash chromatography
(FC) (pentane/methyl-tert-butyl ether (MTBE), 8:2) afforded
4 (13.42 g, 98%) as a pale-yellow viscous oil. IR (film): 2971s,
2939m (CH), 1807s (CdO, BOC), 1718s (CdO, lactam), 1332w,
study.
(Nicolet Magna) or a IFS-200 (Bruker). Size-exclusion chro-
matography (SEC) was carried out with tetrahydrofuran (THF)
or N,N-dimethylformamide (DMF) as eluents at a flow rate of
1.0 mL/minute at room temperature on a system consisting
of a L-6200A Intelligent Pump (Merck Hitachi), a set of two
PLgel 5-µm MIXED-C columns (300 × 7.5 mm, Polymer
Laboratories, linear range of molecular weight: 200-2 × 106
g/mol), and a RI-101 detector (Shodex, halogen lamp) or a
Knauer differential refractometer (λ ) 950 ( 30 nm) detector.
Data were acquired through a PL Datastream unit (Polymer
Laboratories) and analyzed with Cirrus GPC software (Poly-
mer Laboratories) or with PSS WinGPC compact V 7.20
software based upon calibration curves built upon polystyrene
standards (Polymer Laboratories Polystyrene Medium MW
Calibration Kit S-M-10) with peak molecular weights ranging
from 500 to 3 × 106 g/mol.
For MALDI-MS, 2,5-dihydroxybenzoic acid (DHB) (Aldrich)
was used as the matrix and NaBF4 (Aldrich) was added to
improve ionization. The samples were prepared by mixing THF
solutions of the polymer and matrix (50 mg/mL) with saturated
methanolic NaBF4 in a typical ratio of 1:50:2 (w/w/w, polymer/
matrix/salt).
Mass spectra were measured with: Quattro LCZ (Waters-
Micromass) electrospray mass spectrometer with nanospray
inlet; Reflex IV (Bruker) MALDI-TOF mass spectrometer;
APEX III (Bruker) FT-ICR mass spectrometer used for exact
mass determination with MALDI. Scans (250) were added,
each scan contained the ions of 3 laser shots which were
collected within the hexapole and transferred to the ICR cell
after cooling with Ar atoms. The data aquisition used 256k
datapoints/scan.
For ESI measurements, the samples were diluted in metha-
nol (approximately 0.05 mg/mL) and sprayed with a nanospray
needle with internal contact. The capillary and cone voltage
was adjusted for maximum signal intensity (typical values 1.3
kV, respectively, 35 V). MALDI samples where prepared by
mixing equal volumes of solutions of 1 mg/mL polymer in THF
and 10 mg/mL of DHB containing 10 µL of a saturated
methanolic solution of NaBF4. The mixture (1 µL) was applied
to the target and gently evaporated. Spectra (100-200) were
taken on different locations of the sample spot and added.
Laser intensity was adjusted as low as possible except for the
decomposition experiment.
MALLS was performed with a PSS SLD 7000 (laser: P )
30 mW, λ ) 660 nm, angles ) 35, 50, 75, 105, 130, 145°) at a
cell temperature of about 23 °C. The light-scattering detector
was used in combination with the GPC apparatus described
above, and the concentration detector was thus a Knauer
differential refractometer (λ ) 950 ( 30 nm). The PNIPAM
samples were dissolved in THF at a concentration of 0.8-1.7
mmol/L (referring to Mn,theor.). The refractive index increment
used (dn/dc ) 0.0942 ( 0.00041 mL/g) was determined by
Polymer Standards Service (PSS) in Mainz (Germany) using
a PSS dn/dc 2010 Ablenkrefraktometer. The samples were
dissolved in THF (1-8 g/L), and the measurements were
performed at a temperature of 30 °C using a laser with a
wavelength of 620 nm.
1254m, 1162s, 1094m, 1054m (C-O), 870m, 776m cm-1 1H
.
NMR (200 MHz, CDCl3): δ ) 2.72 (s, 2 H, CH2CO), 1.89 (q, J
) 7.5 Hz, 4 H, CH2CH3), 1.51 (s, 9 H, C(CH3)3), 0.95 (t, J )
7.5 Hz, 6 H, CH2CH3). 13C NMR (75 MHz, CDCl3): δ ) 168.6
(CO), 155.2 (CO), 85.9 (C), 58.3 (C), 46.5 (CH2CO), 29.9
(C(CH3)3), 29.2 (CH2CH3), 9.0 (CH2CH3). MS (ESI): 250 (82,
[M + Na]+), 226 (20), 194 (100), 182 (28), 150 (52). HRMS (ESI)
calcd for C12H21NO3 (M+): 227.1521. Found: 227.1523.
1,1-Dimethylethyl ester-(1,1-diethyl-3-oxobutyl)-car-
bamic acid (5). â-Lactam 4 (16.35 g, 71.93 mmol) was
dissolved under argon in THF (250 mL), and the solution was
cooled to -40 °C. A solution of methylmagnesium bromide (3
M in Et2O, 31.20 mL, 93.51 mmol) was slowly added, and the
reaction mixture was stirred for 2 h at -40 °C. Saturated
NH4Cl was added, and after warming to RT, the mixture was
extracted with CH2Cl2 (2 times). The combined organic phases
were dried over MgSO4 and the solvent was removed under
reduced pressure. Purification by FC (pentane/MTBE 19:1)
afforded 5 (14.89 g, 85%) as a colorless oil. IR (film): 3365w
(NH), 2973s, 2939m, 2882m (CH), 1716s (CdO), 1506s, 1457m,
1366m, 1250m, 1170s, 1088m (CsO), 784m cm-1 1H NMR
.
(200 MHz, CDCl3): δ ) 4.66 (bs, 1 H, NH), 2.86 (s, 2 H,
CH2CO), 2.15 (s, 3 H, C(O)CH3), 1.91-1.52 (m, 4 H, 2 ×
CH2CH3), 1.42 (s, 9H, C(CH3)3), 0.81 (t, J ) 7.5 Hz, 6 H, 2 ×
CH2CH3). 13C NMR (50 MHz, CDCl3): δ ) 210.2 (C(O)CH3),
156.4 (C(O)C(CH3)3), 80.6 (C), 59.0 (C), 48.8 (CH2CO), 33.8
(C(O)CH3), 30.2 (C(CH3)3), 29.6 (CH2CH3), 9.4 (CH2CH3). MS
(ESI): 266 (60, [M + Na]+), 250 (22), 210 (100), 194 (20), 166
(93), 140 (30), 108 (48). HRMS (ESI) calcd for C13H25NO3
(M+): 243.1834. Found: 243.1832.
2,2,6,6-Tetraethyl-1,2,5,6-tetrahydro-4-methylpyrimi-
dine (6). The BOC-protected â-aminoketone 5 (5.61 g, 23.1
mmol) was dissolved in CH2Cl2 (11.4 mL). Trifluoroacetic acid
(11.4 mL, 148 mmol) was added, and the reaction mixture was
stirred for 2 h at RT. The liquids were removed in vacuo at 60
°C to afford the deprotected â-aminoketone as the trifluoro-
acetate salt as a viscous oil. The salt was dissolved in MeOH
(11.0 mL), and diethyl ketone (12.2 mL, 115 mmol) was added.
The solution was cooled to -78 °C, liquid ammonia (∼30 mL)
was added, and the reaction mixture was stirred at RT
overnight in a sealed glass tube. The sealed glass tube was
opened at -78 °C, and the mixture was allowed to warm to
RT with stirring in order to remove ammonia. The residue was
dissolved in CH2Cl2, and the solution was washed with
saturated NaHCO3 (2 times). The organic phase was dried over
MgSO4, and the solvent was removed under reduced pressure.
Purification by FC (CH2Cl2/acetone, 1:1) afforded 6 (3.86 g,
4,4-Diethylazetidin-2-one (3). 2-Ethylbutene (10 mL, 82
mmol) was dropwise added at room temperature (RT) (syringe
pump) over a period of 1 h to a solution of chlorosulfonyl
isocyanate (7.1 mL, 82 mmol) in ether (40 mL). After it was
stirred for an additional hour at RT, a saturated solution of
Na2SO3 (80 mL) was added and the mixture was stirred for 2

Macromolecules, Vol. 38, No. 16, 2005
Polymerization of N-Isopropylacrylamide 6835
80%) as a pale-yellow oil. IR (film): 2965s, 2937m, 2877m
(CH), 1672s (CdN), 1460m, 1370m, 1332m, 1167m, 1057m,
933m, 526m cm-1. 1H NMR (300 MHz, CDCl3): δ ) 1.99 (s, 3
H, CH3), 1.83 (s, 2 H, CH2), 1.66-1.49 (m, 4 H, 2 × CH2CH3),
1.47-1.33 (m, 2 H, CH2CH3), 1.30-1.18 (m, 2H, CH2CH3),
0.88-0.79 (m, 12 H, 4 × CH2CH3). 13C NMR (75 MHz,
CDCl3): δ ) 164.0 (CdN), 74.1 (NCN), 51.9 (C), 39.2
(CH2CdN), 33.5 (CH2CH3), 31.4 (CH2CH3), 28.4 (CH3CdN),
8.3 (CH2CH3), 7.5 (CH2CH3). MS (ESI): 210 (84, [M]+),
195 (36), 192 (18), 154 (100), 140 (30), 126 (19), 86 (38).
HRMS (ESI) calcd for C13H26N2 (M+): 210.2096. Found:
210.2094.
Scheme 1. Synthesis of Alkoxyamine 2a
2,2,6,6-Tetraethyl-4-piperidinone (7). Tetrahydropyri-
midine 6 (2.59 g, 12.3 mmol), NH4Br (48 mg, 0.49 mmol), and
diethyl ketone (6.50 mL, 61.6 mmol) were dissolved in
MeOH (25 mL). Concentrated HCl (1.0 mL) was added
dropwise at 0 °C, and the solution was stirred overnight at
RT. Aqueous HCl (18%, 3.0 mL) was added, and the mix-
ture was stirred for 3 days at 60 °C. After addition of aqueous
K2CO3 (40%, until solution turned basic), MeOH was evapo-
rated under reduced pressure. The remaining aqueous solu-
tion was extracted with CH2Cl2 (3 times), and the combined
organic phases were dried over K2CO3. The solvent was
removed under reduced pressure and the crude product was
purified by FC (pentane/diethyl ether, 5:1) to afford 7 (1.41 g,
54%) as a yellow oil. The analytical data are in agreement
with those reported in the literature.12 The experimental
procedure for synthesis of alkoxyamine 2 from 7 via 1 is
described in ref 12.
Typical Procedure for the Polymerization of NIPAM.
In a Schlenk tube, the alkoxyamine initiator 2 (6 mg, 0.02
mmol) and NIPAM (201 mg, 1.78 mmol) were dissolved in
benzene (1.0 mL). The solution was deoxygenated by argon
bubbling for several minutes. The polymerization was carried
out under argon at 125 °C for 16 h. The resulting mixture was
cooled to RT, dissolved in a small amount of acetone, and
precipitated from Et2O. The polymer was filtered and dried
on air. Conversion was evaluated by 1H NMR spectroscopy
comparing the vinylic monomer resonance signal at δ ) 5.31
(dd, 1 H, 2J ) 10.1 Hz, 3Jcis ) 2.2 Hz, CHdCHcisHtrans) ppm as
well as the iso-propylic monomer signal at δ ) 4.26-4.09 (m,
1 H, CH(CH3)2) with the iso-propylic resonance of the polymer
at δ ) 4.32-4.01 (brm, 1 H, CH(CH3)2) ppm, additionally the
resonance of the methyl groups of the monomer (δ ) 1.01 (d,
6 H, 3J ) 6.6 Hz, CH(CH3)2) ppm) and the polymer (δ ) 1.45-
1.02 (brm, 6 H, CH(CH3)2) ppm) were set into relation;
molecular weight was determined by MALDI-MS and MALLS.
Conversion ) 77%. MALDI: Mn ) 8100 g/mol, MALLS: Mn )
9700 g/mol calculated from Mw ) 11 300 g/mol with polydis-
persity index (PDI) ) 1.17.
a Boc2O ) di-tert-butyl dicarbonate, DMAP ) N,N-(dimeth-
ylamino)pyridine, THF ) tetrahydrofuran, TFA ) trifluoro-
acetic acid, Cu(OTf)2 ) copper(II)triflate, 4,4′-di(t-Bu)bipy )
4,4′-di-tert-butyl bipyridine.
Polymerization of NIPAM using 2 and Charac-
terization of Nitroxide-Terminated PNIPAMs. Po-
lymerizations of NIPAM were conducted in perdeuter-
ated benzene (1.78 M) using 1, 0.6, and 0.4 mol % of
alkoxyamine 2 as initiator/regulator at 125 °C (oil bath
temperature) in sealed tubes. Reaction time was varied
from 4 to 36 h. After the mixture reached the desired
reaction time, the mixture was allowed to cool to room
temperature and conversion was determined by 1H
NMR spectroscopy. The reaction mixture was redis-
solved in acetone and precipitation of PNIPAM was
achieved upon addition to Et2O. The polymer was then
dried on air and was analyzed using GPC using THF
or DMF as the mobile phase. Polymerizations in t-BuOH
were not successful. It is known that PNIPAM forms
insoluble aggregates in this and other solvents.19 Re-
sults are presented in Table 1.
As expected for a living polymerization, conversion
is increasing upon increasing the reaction time from 4
to 20 h (runs 1-5). The theoretical molecular weight
was calculated after determining the conversion gravi-
metrically from the known amount of added alkoxyamine
regulator. Narrow PDIs were obtained for PNIPAM;
however, the experimentally determined molecular
weight (GPC) did not follow the rules of a living process.
In fact, molecular weight leveled at about 2000 g/mol if
THF was used as a mobile phase. For higher molec-
ular weight PNIPAM, DMF was also tested as sol-
vent for GPC analysis. However, determined molecular
weight was far higher than expected (runs 5-7). We
assumed that aggregation of PNIPAM during chroma-
tography may be the reason for these results. In-
deed, it has been reported in the literature that analy-
sis of PNIPAM using GPC is difficult.9 We tested sev-
eral literature protocols for sample preparation such as
a Li salt addition; however, GPC results were not
reproducible.
Results and Discussion
Improved Synthesis of Nitroxide 1. â-Lactam 3
was readily prepared from commercially available 2-eth-
ylbutene and chlorosulfonyl isocyanate in 75% yield
according to literature procedures (Scheme 1).14 Boc
protection with Boc-anhydride in CH2Cl2 using N,N-
(dimethylamino)pyridine (DMAP) as a catalyst afforded
4 in excellent yield. Ring opening with MeMgBr at -40
°C provided â-aminoketone 5. N-Deprotection was readily
accomplished using trifluoroacetic acid (TFA) to give the
corresponding TFA salt in quantitative yield. Treatment
of this salt with diethyl ketone and NH3 in MeOH
afforded tetrahydropyrimidine 6 in 80% yield.15 Rear-
rangement/hydrolysis of 6 was performed in MeOH(aq)
HCl in the presence of diethyl ketone and a catalytic
amount of NH4Br to give 7.16 Oxidation17 and alkoxy-
amine synthesis18 according to known procedures even-
tually afforded 2. In contrast to our previous procedure
where only small amounts of alkoxyamine have been
obtained in each run,12 gram quantities of 2 can readily
be prepared using the novel synthetic route.
We therefore decided to analyze PNIPAM by MALDI-
MS. Results obtained by MALDI-MS measurements are
included in Table 1. In addition, the MALDI-MS spectra
of the samples obtained in the polymerizations stopped
after 4, 8, 12, and 16 h are depicted in Figure 2. All of
the polymers exhibited differences between the molec-
ular weight peaks in MALDI spectra that were equal
to the monomer weight of the NIPAM monomer unit.

6836 Schulte et al.
Macromolecules, Vol. 38, No. 16, 2005
Table 1. Polymerization of NIPAM Using Alkoxyamine 2 (in C6D6 Using 1 Mol % 2 at 125 °C)
time
(h)
conversion
(%)
Mn,th
(g/mol)
Mn,exp(GPC)
(g/mol)
PDI
(GPC)
Mn,exp (MALDI)
PDI
(MALDI)
Mw (MALLS)
entry
(g/mol)
(g/mol)
1
2
3
4
5
4
8
12
16
20
40
54
73
77
87
4 500
6 100
8 300
8 700
9 700
2 300a
2 100a
2 000a
1 900a
1 900a
22 400b
28 800b
39 200b
1.16a
1.16a
1.16a
1.17a
1.17a
1.17b
1.18b
1.26b
4 700
6 100
6 800
8 100
7 700
1.21
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
11 300
12 100
6c
36
36
73
78
13 800
22 100
16 200
24 500
7d
a GPC in THF and polystyrene as standard. b GPC in DMF and polystyrene as standard. c Initiator 2 (0.6 mol %) was used. d Initiator
2 (0.4 mol %) was used. n.d. ) not determined.
Figure 2. MALDI-MS of PNIMPAM obtained for polymerizations stopped after 4, 8, 12, and 16 h, respectively (from the top).
MALDI conditions: the samples were prepared by dissolving the polymer, DHB, and NaBF4 in THF. About 1 µL of the solution
was applied to the MALDI target and evaporated. To gain representative information, the spot was probed at several locations
and 100-200 spectra were accumulated.
In contrast to the data obtained by GPC analysis, MS-
determined molecular weight is increasing from 4700
to 8100 g/mol upon increasing the reaction time from 4
to 16 h. Polymerization for 20 h delivered PNIPAM with
a slightly lower molecular weight (run 5). Probably, the
high laser energy used to desorb high molecular weight
PNIPAM leads to chain degradation. Indeed, also the
higher molecular weight PNIPAM showed a mass of
about 8000 g/mol by MS analysis. Therefore, analysis
of PNIPAM under our MS conditions is only possible to
a molecular weight of up to 8000 g/mol. PDI was
determined by careful reflectron analysis on a low

Macromolecules, Vol. 38, No. 16, 2005
Polymerization of N-Isopropylacrylamide 6837
Figure 3. ESI-MS analysis of the early stage of a 2-mediated NIPAM polymerization.
molecular weight sample (run 1, Mn,exp ) 4700 g/mol).
As with GPC, a narrow PDI (1.21) was extracted from
this experiment.
doubly, and triply charged PNIPAMs. We were very
pleased to observe that all the peaks identified in the
spectrum belong to styryl radical initiated nitroxide-
terminated PNIPAMs. No other peaks were identified.
This demonstrates the absence of any detectable amount
of termination or other side reactions and is therefore
an experimental proof that at least in the early stage
of the polymerization nitroxide 1 is able to perfectly
mediate the polymerization of NIPAM.
To determine the molecular weight of PNIPAM with
a mass of >8000 g/mol, we decided to use MALLS
analysis. Studies were performed in THF as solvent.20
The MALDI-determined molecular weight could be well
reproduced by MALLS analysis (entry 4: MALDI, Mn
) 8100 g/mol; MALLS, Mn ) 9700 g/mol calculated from
Mw ) 11 300 g/mol with PDI ) 1.17). The higher
molecular weight PNIPAM-delivered values for Mw
which reasonably agree with a controlled living NIPAM
polymerization (runs 5-7). Hence we can state that
analysis of PNIPAM with Mn < 8000 g/mol can be
perfectly performed using MALDI-MS techniques,
whereas for PNIPAM with Mn > 8000 g/mol MALLS is
the method of choice.
To better understand the 1-mediated NIPAM polym-
erization (livingness, end-group determination), we
analyzed the early stage of the polymerization process
using the ESI-MS technique. The linear mode of the
TOF analyzer is superior for the investigation of higher
MWs, but in this mode and by the relatively high laser
intensity the resolution is not sufficient at >3000 Da
for an unambiguous end-group analysis.21 For the ESI-
MS studies, NIPAM polymerization was repeated under
the above-described conditions (1% alkoxyamine, ben-
zene (1.78 M) at 125 °C) and samples were taken every
10 min during the first 60 min of the reaction. These
samples were directly subjected to ESI-MS analysis. The
spectra are presented in Figure 3. One can readily see
the progress of the polymerization in the first 60 min.
The mean molecular weight is steadily increasing. As
in the MALDI spectra, the mayor peaks are separated
by 113 mass units (NIPAM monomer).
To get a better resolution of higher molecular weight
PNIPAM (up to 3000 g/mol), the FTICR-MALDI-MS
technique was also included in the present study. The
sample preparation was identical to the Reflex IV
measurements described above. In Figure 5, part of a
spectrum of a PNIPAM sample is depicted. As with the
MALDI-TOF experiments described above, the 113
series were clearly identified. However, determination
of the exact mass of the individual peaks revealed that
the peaks do not correspond to styryl radical initiated
nitroxide-terminated PNIPAM species (simulated exact
masses are also included in Figure 5). Presumably
chain-end degradation occurred during mass analysis.
Indeed, it has previously been reported that MALDI-
MS analysis on polystyrene and poly-n-butyl acrylate
prepared by NMP leads to chain end degradation.13,22,23
Surprisingly, the peaks do not correspond to the ex-
pected chain-end degradation products 8 derived from
alkoxyamine C-O-bond homolysis with subsequent
H-transfer to nitroxide (Scheme 2). The peaks clearly
correspond to protonated methylene-terminated
PNIPAMs 11.22 Simulated and experimental deter-
mined data fit perfectly (see Figure 5). We believe that
N-O-bond homolysis to generate the polymeric alkoxyl
radical 9 occurs under laser irradiation. â-Fragmenta-
tion will then afford radical 10, which upon H-loss leads
to the observed polymers 11. To the best of our knowl-
edge, such a degradation mechanism has not been de-
scribed before. We believe that this degradation mech-
anism may be general for nitroxide-terminated poly-
As example, part of the mass spectrum obtained from
the sample taken after 50 min is discussed in more
detail in Figure 4. Importantly, the peaks were unam-
biguously assigned. We found 113 series for singly,

6838 Schulte et al.
Macromolecules, Vol. 38, No. 16, 2005
Figure 4. Part of the ESI mass spectrum of the sample taken after 50 min.
Scheme 2. Suggested Mechanism for the Chain-End
Degradation during MS Analysis
Figure 5. Characteristic region of a high resolution FT-ICR
and not during the polymerization process, we ran
MALDI experiments on low molecular weight polymers.
With these low MW samples, the onset of ion formation
is at low laser intensity (∼0.5 MW/cm2). A characteristic
region of the spectra is presented in Figure 6. The first
experiment was conducted using ∼0.5 MW/cm2 laser
intensity. No chain-end degradation was obtained under
these conditions. The major peak at 784.6 (m/z) corre-
mass spectrum and the corresponding simulated spectrum.
mers. In fact, degradation products identified during
MALDI-TOF-MS analysis on TEMPO-terminated poly-
n-butyl acrylates can also be explained by this mecha-
nism.22
To prove whether the fragmentation suggested in
Scheme 2 is induced by the laser desorption/ionization

Macromolecules, Vol. 38, No. 16, 2005
Polymerization of N-Isopropylacrylamide 6839
Figure 6. MALDI measurements on a low molecular weight PNIPAM sample using different laser intensities (above, ∼0.5 MW/
cm2; below, ∼4.0 MW/cm2).
sponds to protonated oligomer 12 containing alkoxyamine
2 and 4 NIPAM units. Upon increasing the laser
intensity to ∼4.0 MW/cm2, degradation products were
clearly identified in the mass spectrum. We were
pleased to observe that methylene-terminated PNIPAMs
of type 11 appeared as major decomposition products
among others. The peak at 797.7 corresponds to the
protonated chain end degraded 13, and the 819.7 peak
belongs to the corresponding Na+-complexed oligomer
13. These results unambiguously show that chain-end
degradation occurs during MALDI mass analysis if high
laser intensity is used.
nated polymers during MALDI-analysis. Future experi-
ments will show the generality. Chain-end degradation
can be suppressed during MALDI-analysis using low
laser intensities.
Acknowledgment. We thank the Deutsche Fors-
chungsgemeinschaft (STU 280/1-3) for funding. The
editor, Prof. Coleen Pugh, is acknowledged for helpful
comments.
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We showed that alkoxyamine 2 can be used as an
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MA050343N