Influence of end groups on the stimulus-responsive behavior of poly[oligo(ethylene glycol) methacrylate] in water

Article abstract of DOI:10.1021/ma1005759

The influence of the chemical structure of both end groups onto the lower critical solution temperature (LCST) of poly[oligo(ethylene glycol) monomethyl ether methacrylate] (POEGMA) in water was systematically investigated. POEGMA of M_n = 3550 g/mol and M_w/M_n = 1.14 prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization was equipped with two different functional end groups in a one-step postpolymerization reaction combining activated esters, functional amines, and functional methane thiosulfonates. As end groups, n-propyl, n-hexadecyl, di(n-octadecyl), poly(ethylene glycol)-550 (PEG), 1H,1H-perfluorononyl, azobenzene, and trimethylethylammonium groups were systematically combined with methyl, n-hexadecyl, and 1H,1H,2H,2H-perfluorooctyl groups. Polymers were characterized by gel permeation chromatography, dynamic light scattering, and turbidimetry. Hydrophobic end groups at either end of the polymer chain decreased the LCST. For hydrophobic groups at both ends of the chain their influence was additive. Two large hydrophobic end groups allowed micelle formation below the LCST and an LCST higher than to be expected from nonaggregated polymers. The strongest hydrophobic effect was found for rigid aromatic end groups, which was attributed to their incompatibility with the flexible polymer chain. Charged end groups increased the LCST and could compensate for the effect of hydrophobic end groups at the opposite end group. PEG end groups could mask a hydrophobic influence of the opposite end group and stabilized the LCST.

Full text of DOI:10.1021/ma1005759

4
638 Macromolecules 2010, 43, 4638–4645
DOI: 10.1021/ma1005759
Influence of End Groups on the Stimulus-Responsive
Behavior of Poly[oligo(ethylene glycol) methacrylate] in Water
†
,§
†,§
Peter J. Roth, Florian D. Jochum, F. Romina Forst, Rudolf Zentel, and
†
†
,†,‡
Patrick Theato*
†
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany, and
‡
School of Chemical and Biological Engineering, World Class University (WCU) program of Chemical
Convergence for Energy and Environment (C2E2), Seoul National University, 151-744 Seoul, South Korea.
§
Equal contribution of both authors
Received March 15, 2010; Revised Manuscript Received April 17, 2010
ABSTRACT: The influence of the chemical structure of both end groups onto the lower critical solution
temperature (LCST) of poly[oligo(ethylene glycol) monomethyl ether methacrylate] (POEGMA) in water
was systematically investigated. POEGMA of M = 3550 g/mol and M /M = 1.14 prepared by reversible
n
w
n
addition-fragmentation chain transfer (RAFT) polymerization was equipped with two different functional
end groups in a one-step postpolymerization reaction combining activated esters, functional amines, and
functional methane thiosulfonates. As end groups, n-propyl, n-hexadecyl, di(n-octadecyl), poly(ethylene
glycol)-550 (PEG), 1H,1H-perfluorononyl, azobenzene, and trimethylethylammonium groups were system-
atically combined with methyl, n-hexadecyl, and 1H,1H,2H,2H-perfluorooctyl groups. Polymers were
characterized by gel permeation chromatography, dynamic light scattering, and turbidimetry. Hydrophobic
end groups at either end of the polymer chain decreased the LCST. For hydrophobic groups at both ends
of the chain their influence was additive. Two large hydrophobic end groups allowed micelle formation
below the LCST and an LCST higher than to be expected from nonaggregated polymers. The strongest
hydrophobic effect was found for rigid aromatic end groups, which was attributed to their incompatibility
with the flexible polymer chain. Charged end groups increased the LCST and could compensate for the effect
of hydrophobic end groups at the opposite end group. PEG end groups could mask a hydrophobic influence
of the opposite end group and stabilized the LCST.
Introduction
is intended to change a distance between both of its end groups
through its chain collapse, it is important to know the influence of
Stimulus responsive polymers that have a lower critical solu-
tion temperature (LCST) in water are gaining an increasing
interest in recent research. Below the LCST, the polymers are
well soluble in water and adopt an extended chain conformation.
At the LSCT, the polymers become hydrophobic and undergo a
chain collapse. Above the LCST, the polymers are insoluble in
water. If the concentration of polymer is sufficiently high, a
clouding and eventual precipitation of the material can be
observed upon heating a clear solution above the LCST. This
enables precise measurements of LCSTs through turbidimetry.
Usually, the transition occurs quite sharp within a very small
temperature range and is fully reversible. Because of this abrupt
and well inducible change from a swollen coil to a hydrophobic
collapsed globule, polymers with an LCST are often termed
both (modified) end groups onto the LCST. The end groups may
also be used to adjust the LCST, for instance for micellar drug
2
4
carrier applications. Furyk et al. attributed the molecular
weight influence onto the LCST solely onto the influence of the
2
5
end groups, which is stronger the lower the molecular weight is.
2
6
Several studies, including theoretical ones, have thus elucidated
1
1,13,27-29
the influence of end groups on the LCST.
The most
investigated stimulus responsive polymer in water is poly[N-
2
4,25,30-32
isopropylacrylamide] (PNIPAM),
from which also
thermoresponsive bioconjugates have been prepared through
4
,33,34
end-group modification.
Nonlinear PEG analogues, such as poly[oligoethylene glycol
monomethyl ether methacrylate] (POEGMA), have recently been
receiving a lot of interest as stimulus responsive polymers relevant
“
smart” materials and are employed and probed in a variety of
18,35
1-4
for biomedical applications.
Precision in the polymerization
applications such as thermosensitive bioconjugates, molecular
actuators, drug delivery, tunable optical devices, or chroma-
tographic separation.
5
6,7
8
of OEGMA has been achieved for example by anionic poly-
36,37
merization
38-42
9,10
and atom transfer radical polymerization.
As
Although LCST values found in the
a consequence, POEGMA polymers and especially copolymers
proved to feature an adjustable thermoresponsive behavior that is
literature for a given polymer are usually in unison, measured
LCST values of that polymer can vary depending on concentra-
39,40
1
1-13
14,15
16,17
comparable to that of PNIPAM, 42which can also be utilized in
tion, molecular weight,
or incorporation of comonomers.
In many applications, stimulus responsive polymers are con-
salt concentration,
tacticity,
41
1
4,18
hydrogels and molecular brushes.
However, a systematic survey of the influence of the end groups
on its thermoresponsive behavior has not been conducted yet.
The synthesis of polymers with two different functional end
groups is challenging, and as a consequence, some studies have
investigated the influence of only one functional end group
1
9,20
21,22
23
jugated to proteins,
fluorescent dyes,
or surfaces with
their end groups. In these setups, and especially when the polymer
1
3,27,28
onto the LCST.
Stimulus responsive polymers with two
*
Corresponding author: e-mail theato@uni-mainz.de; Ph þ49-6131-
3
926256; Fax þ49-6131-3924778.
different functional end groups have been prepared by living
pubs.acs.org/Macromolecules
Published on Web 04/27/2010
r 2010 American Chemical Society

Article
cationic ring-opening polymerization, atom transfer radical
Macromolecules, Vol. 43, No. 10, 2010 4639
29
N-(2-Aminoethyl)-4-(2-phenyldiazenyl)benzamide was synthe-
sized as described in the literature.
13,28
24,25,30
55
polymerization (ATRP),
living cationic polymerization, or reversible addition-fragmen-
tation chain transfer (RAFT) polymerization in combination
free radical polymerization,
43
Sodium methanethiosulfonate was synthesized according to a
literature procedure from sodium methylsulfinate and sulfur.
44
56
13,28,29,43
29,43
S-1H,1H,2H,2H-Perfluorooctyl methanethiosulfonate. 3.15 g
23.5 mmol) of sodium methanethiosulfonate was dissolved in
mL of dry DMF. 5 g (11.7 mmol) of 1H,1H,2H,2H-perfluoro-
octyl bromide was separately dissolved in 2 mL of dry DMF.
Both solutions were combined and stirred at 40 °C. Upon
mixing, some precipitation occurred. After 3 h, two liquid
phases had formed. Stirring at 40 °C was however continued
overnight. After that time, there was one liquid phase. The
solvent was removed at reduced pressure, and the light-yellow
residue was extracted with diethyl ether. By removing the diethyl
with functionalized initiators,
terminating agents,
or
24,44
(
5
chain transfer agents.
These approaches, however, require a
separate polymerization procedure, with distinct initiation and
termination reactions, for each end group combination of interest.
It is therefore not possible to completely eliminate influences of the
molecular weight or the molecular weight distribution onto the
LCST. Many reports on end group influences deal with the effect
of hydrophobic end groups, which generally reduce the LCST,
24,30,45,46
similar to the incorporation of hydrophobic comonomers.
Preparing various heterotelechelic polymers with identical
degrees of polymerization and polydispersity indices may be
achieved by postpolymerization end-group modifications of a
polymer with two separately addressable reactive end groups.
Examples following the concept of click chemistry include thiol-
ether, 4.51 g (84.2%) of product was obtained which could be
1
used without further purification. H NMR, CDCl
3
, 300 MHz,
-), 2.64 (m, 2 H,
CF CH -). C NMR, CDCl , 75 MHz, δ = 50.47 (CH -),
δ /ppm = 3.35 (m, 5 H, -CH
3
and -SCH
2
13
-
3
-
-
2
2
3
3
2.32 (t, J = 16.5 Hz, -CF
2
CH
, 376 MHz, δ = -81.3 (3 F),
114.7, -122.3, -123.2, -123.7, -126.5 (2 F each). MS (FD)
2
-), 27.03 (t, J = 3.5 Hz,
3
1
47
ene/thiol-yne reactions, activated esters, azide with acetylene
19
SCH
2
-). F NMR, CDCl
3
27,48,49
cycloaddition,
thiocarbonyl with dienophile cycloaddi-
50
48
51
tion, pyridyl disulfides, methane thiosulfonates, or Michael
m/z (%): 457.59 (100.00), 458.60 (12.59), 459.60 (9.43).
S-Hexadecyl methanethiosulfonate was prepared in analogy
to the above procedure from 2.66 g (8.71 mmol) of hexadecyl
49
addition.
In this study, we investigate the influence of both end groups
of heterotelechelic POEGMA (M = 3550 g/mol) synthesized via
n
bromide and 2.39 g (17.8 mmol) of sodium methanethiosulfo-
1
nate. 2.26 g (77.2%) of product was obtained. H NMR, CDCl
RAFT polymerization. The use of a pentafluorophenyl (PFP)
ester modified chain transfer agent (CTA) afforded a polymer
with a PFP ester and a dithioester end group. Postpolymerization
functionalization by combining functional amines and functional
methane thiosulfonates (MTS) enabled to selectively modify both
end groups in a single step with very high conversions. This way, a
library of polymers differing only in their end groups but with the
same degree of polymerization and same polydispersity indices
could be obtained. The influence of the end groups including
different hydrophobic, charged, fluorophilic, and uninfluential
moieties and combinations thereof onto the LCST was system-
atically investigated.
3
,
300 MHz, δ = 3.28 (s, 3 H, -SO CH ), 3.13 (t, J = 7 Hz, 2 H,
2
3
-CH S-), 1.76 (m, 2 H, -CH -CH S-), 1.21 (m, 24 H
2
2
2
-CH
2
-), 0.84 (t, J = 7 Hz, 3 H, CH
3
-CH -).
2
General Procedure for r,ω-Functionalization of Polymers. Ina
typical run, 76 mg (20 μmol) of PFP-POEGMA-DTE (P1) was
dissolved in 1 mL of chloroform. 20 equiv (400 μmol) of the
respective MTS reagent (methyl, hexadecyl or 1H,1H,2H,2H-
perfluorooctylmethanethiosulfonate) dissolvedin1mLofchloro-
form was added, and the mixture was stirred for 1 min. Then,
5
0 equiv (1 mmol) of the respective amine (n-propyl, n-hexadecyl,
dioctadecyl, poly(ethylene glycol), or 1H,1H-perfluorononyl) was
injected. The mixture was stirred overnight at room tempera-
ture. For (2-aminoethyl)trimethylammonium chloride and N-(2-
aminoethyl)-4-(2-phenyldiazenyl)benzamide as amines, the reac-
tions were performed in DMF instead of chloroform. For (2-
aminoethyl)trimethylammonium chloride, triethylamine (0.5 mL)
was additionally added as auxiliary base and cosolvent.
Experimental Section
Methods. Gel permeation chromatography (GPC) was per-
formed on 2 mg/mL THF solutions on MZ-Gel SDplus columns
to determine the polystyrene equivalent molecular weight and
1
13
19
the polydispersity index (PDI) M
w
/M
n
. H, C, and F NMR
For work-up, one of two different procedures was applied.
For reactions with amines and MTS reagents soluble in diethyl
ether (all except poly(ethylene glycol) amine, N-(2-aminoethyl)-
4-(2-phenyldiazenyl)benzamide, and (2-aminoethyl)trimethy-
lammonium chloride), the completed reactions were dried in
vacuum, and water was added to the residue. After several
extractions with diethyl ether to remove any side products,
the aqueous phase was extracted with chloroform to transfer
the polymeric product into the organic phase. Upon removal of
the chloroform, pure end-group-functionalized polymers were
obtained in 50-80% yields. An alternative work-up procedure
applicable to all reactions was dialysis against methanol through
3500 g/mol molecular weight cutoff membranes for 3 days with
solvent changes twice a day. This afforded pure polymers in
40-80% yields.
spectra were measured on a 400 MHz instrument by Bruker on
CDCl3 solutions at room temperature. Cloud points were
determined using 10 mg/mL aqueous solutions (Millipore
grade) and were observed by optical transmittance at a wave-
length of λ = 632 nm through a 1 cm quartz cell using a Jasco
V-630 photospectrometer with a Jasco ETC-717 Peltier element
with a heating rate of 1 °C/min. The transmitted light was
recorded versus the temperature, and the LCST given is the
temperature at 50% transmittance. Dialysis membranes were
purchased from Roth (Germany) and had a molecular weight
cutoff of 3500 g/mol. Dynamic light scattering was measured on
a Malvern Zetasizer Nano in a low volume glass cuvette (45 μL)
at 20 °C on 1 g/L solutions in Millipore water at an angle of 90°.
R-Pentafluorophenyl, ω-dithioester poly[oligo(ethylene glycol)
monomethyl ether methacrylate], PFP-POEGMA-DTE (P1),
was prepared by RAFT polymerization of commercial oli-
goethylene glycol monomethyl ether methacrylate with a mo-
lecular weight of about 300 g/mol utilizing pentafluorophenyl-
GPC data (polystyrene equivalent molecular weight and
polydispersity index M /M ) are summarized in Table 1 to-
w
n
gether with the LCST value of each polymer. NMR, CDCl
3
, 400
MHz, δ/ppm: poly[oligoethylene glycol methacrylate]: H, 4.07
(-COOCH -), 3.64, 3.60, 3.52 (-OCH CH O-), 3.36
1
(
chain transfer agent, as described elsewhere. M (GPC, poly-
4-phenylthiocarbonylthio-4-cyanovalerate) (PFP-CTA) as a
2
2
2
5
2
13
(-OCH ), 1.96-1.65 (-CH -), 1.00, 0.81 (-CH ). C, 177.2
n
3
2
3
styrene equivalent) = 3550 g/mol, PDI = 1.14, M
800 g/mol.
Dioctadecylamine was prepared according to the literature.
Amino-terminated poly(ethylene glycol) (PEG-NH ) was pre-
n
(NMR) =
(-COO-), 71.8, 70.4, 68.3 (-OCH
2
CH
2
O-), 63.8 (-COO-
3
CH -), 59.0 (-OCH ), 54.5 (-CH
2
3
2 3
-), 44.6 (-(CH )C-
5
3
(COO-)-), 18.5, 16.2 (-CH ). R-(Trimethylammonium)ethyl
3
1
þ
amide: H, 3.79 (-CH NHCO-), 3.73 ((CH ) N CH -),
2
2
3 3
2
þ
13
pared according to a literature procedure from PEG mono-
methyl ether with a molecular weight of 550 g/mol.
3.30 ((CH
3
)
3
N -), 2.41 (-NHCOCH
2
-). C, 65.5 ((CH
3 3
) -
5
4
þ
þ
N CH
2
-), 54.5 ((CH
3
)
3
N -), 34.4 (-CH
2
NHCO-), 30.9

4
640 Macromolecules, Vol. 43, No. 10, 2010
Roth et al.
Table 1. End Groups, Measured Polystyrene Equivalent and Theoretical Molecular Weights, Polydispersity Indices, LCSTs, and Hydrodynamic
Radii of Polymers Synthesized
1
2
a
-1
b
-1
a
c
R end group (R )
ω end group (R )
M
n,GPC /g mol
M
n,theor /g mol
PDI
LCST/°C
H
R /nm
d
P1
P2
P3
P4
P5
P6
P7
P8
C
6
F
5
O-
-SCSPh
3550
3270
3270
3100
3230
3370
3440
3590
3900
3820
3480
3620
3620
3590
3660
4020
4330
4800
3400
3550
3840
4050
4180
3350
3560
3680
3530
3740
3870
3390
3600
3720
3740
3950
4070
3810
4150
3650
1.14
1.11
1.11
1.13
1.14
1.12
1.10
1.13
1.11
1.13
1.16
1.12
1.17
1.14
1.13
1.14
1.15
1.14
1.16
42.8
62.7
62.4
62.1
58.3
50.1
49.7
53.6
45.8
44.7
66.3
62.5
62.1
50.9
51.7
50.3
48.9
48.9
49.1
3.28
3.57
CH
CH
CH
C
C
C
C
C
C
(CH
(CH
(CH
C
C
C
3
3
3
(OCH
(OCH
(OCH
2
2
2
CH
CH
CH
2
2
2
)
)
)
11NH-
11NH-
11NH-
-CH
3
-C16H
33
-CH
-CH
2
CH
2
2
2
2
C
C
C
C
6
6
6
6
F
F
F
F
13
13
13
13
3
3
3
H
H
H
7
NH-
3
3.31
3.77
7
NH-
NH-
-C16
H
33
7
-CH
-CH
2
CH
16
16
16
H
H
H
33NH-
33NH-
33NH-
3
P9
-C16
H
33
P10
P11
P12
P13
P14
P15
P16
P17
P18
-CH
-CH
2
CH
4.28
3.28
þ
þ
þ
3
3
3
)
)
)
3
3
3
N CH
N CH
N CH
2
2
2
CH
CH
CH
2
NH-
2
NH-
2
NH-
3
-C16
H
33
-CH
-CH
2
CH
8
8
8
F17CH
F17CH
F17CH
2
2
2
NH-
NH-
NH-
N-
N-
3
-C16
H
33
8.51
8.62
-CH
-CH
-CH
-CH
2
3
2
3
CH
2
C
C
6
6
F
13
13
(C18
(C18
H
H
37
)
)
2
37
2
CH
2
F
7.83
P19
a
6 5 6 4 2 2
C H NdNC H CONH(CH ) NH-
b
Polystyrene equivalent molecular weight and polydispersity index determined by GPC. Theoretical molecular weight determined from the
c
M (GPC) of P1 and the molecular weight of the end groups assuming 100% conversion. Hydrodynamic radius in water at 20 °C measured by dynamic
n
d 2
light scattering. For P1, no disulfide bridge between polymer chain and R end group; structure given in Scheme 1.
1
Scheme 1. Synthesis of r,ω-Heterotelechelic Polymers Based on Pen-
tafluorophenyl Esters and Functional Methane Thiosulfonates Together
with Structures and Abbreviations of End Groups
R-Dioctadecylamide: H, 3.24 ((-CH
)
2 2
NCO-), 2.45 ((-CH ) -
2
2
1
3
NCOCH
7.1 ((-CH
CH CH -),14.1(CH -).R-2-[4-(Phenyldiazenyl)benzoylamino]-
2
-), 1.28 (CH
3
CH
2
-), 1.25 (-CH
2
-), 0.89 (CH
3
-). C;
4
(
2
)
2
NCO-), 31.7 (CH
3
CH CH -),29.6 (-CH
2
2
2
-), 22.5
3
2
3
1
1
ethylamide: H, 7.96, 7.91, 7.50. ω-methyl disulfide: H, 2.36
13
1
(
-SCH
3
3
). C, 24.2 (-SCH ). ω-Hexadecyl disulfide: H, 2.61
(
-SCH -), 1.55 (-SCH CH -), 1.25 (-CH -), 0.86 (CH -).
2
2
2
2
3
1
3
C, 39.5 (-SCH
-SCH CH -), 22.5 (CH
H,2H-Perfluorooctyl disulfide: H, 2.78 (-SCH -), 2.40 (-SC-
19
2
-), 32.1 (CH
3
CH
2
CH
2
2
-), 29.7 (-CH -), 28.8
(
2
2
2
3
CH
2
-), 14.1 (CH -). ω-1H,1H,
3
1
2
1
3
H CH -) C, 31.1 (-SCH CH -), 28.5 (-SCH -). F, -81.1
2
2
2
2
2
(
3 F), -114.1, -122.2, -123.2, -123.6, -126.5 (2 F each).
End-group conversions as calculated from NMR integrations
were between 90% and 97% for R end-group conversions and
between 89% and 98% for ω end-group conversions, based on
the presence of terminal phenyl dithioester in the starting
polymer PFP-POEGMA-DTE.
Results and Discussion
Synthesis. Starting from POEGMA with R pentafluoro-
47
phenyl (PFP) ester and ω dithioester end groups, the
synthesis of polymers with two different functional end
groups proceeded via a one-pot, one-step reaction employing
an excess of a functional primary or secondary amine and a
5
1
functional methane thiosulfonate (MTS) (see Scheme 1). In
1
the following, the amide residues (R ) are termed R end
group, whereas the disulfide residues (R ) will be referred to
2
as ω end groups. Commercially available methyl-MTS was
used. MTS reagents with hexadecyl and 1H,1H,2H,2H-
perfluorooctyl residues were synthesized in two steps from
sulfur, sodium methylsulfinate, and the respective alkyl
57
bromides, adopted from the literature (see Scheme 2).
Because of a simple purification through either extraction
or dialysis, a library of heterotelechelic polymers could easily
be obtained. In order to systematically assess the influence of
the end groups on the LCST of POEGMA, such of different
sizes and of different polarities, including fluorophilic resi-
dues were probed. Perfluorinated alkyl chains are very
1
(
0
2
0
3
2
-NHCOCH
.87 (CH
2
-). R-n-Propylamide: H, 3.15 (-CH
2
2
NHCO-),
NHCO-),
1
3
-). R-Hexadecylamide: H, 3.15 (-CH
.27 (-NHCOCH -), 1.45 (-CH CH NHCO-), 1.24 (-CH -),
2
2
2
2
5
8
59,60
13
hydrophobic, but fluorocarbon blocks
or chains with
F atoms have been shown to phase separate
.85 (CH
1.4 (-NHCOCH
6.7 (-CH CH CH NHCO-), 22.5 (CH CH -), 14.0 (CH -).
3
-). C, 39.5 (-CH
2
NHCO-), 31.7, (CH
3
CH
2 2
CH -),
6
15 or 17
1
62-64
2
-), 29.6 (-CH
2
-), 29.5 (-CH CH
2
2
NHCO-),
from regular alkyl chains. A 1H,1H-perfluorononylamide
(with 17 F atoms, C9F17) and a 1H,1H,2H,2H-perfluorooc-
tyl disulfide (with 13 F atoms, C8F13) were thus included
2
2
2
3
2
3
19
R-1H,1H-Perfluorononylamide: F, -80.6 (3 F), -118.1 (2 F),
121.8 (6 F), -122.6 (2 F), -123.3 (2 F), -126.0 (2 F).
-

Article
Macromolecules, Vol. 43, No. 10, 2010 4641
1
13
Figure 1. Exemplary H/ C HSQC NMR spectra of the starting polymer P1 (left) and polymer P10 with R C16, ω C8F13 end groups (right) showing
the complete end-group conversions.
Scheme 2. Synthesis of S-Hexadecyl and S-1H,1H,2H,2H-Perfluoro-
octylmethanethiosulfonate
into the functional end groups of interest. Because of the
postpolymerization strategy, all polymers had exactly the
same average degree of polymerization and the same molec-
ular weight distribution.
Figure 2. Transmission versus temperature plots recorded while heat-
ing 10 mg/mL aqueous polymer solutions at 1 °C/min.
GPC traces of all product polymers P2-P19 were mono-
modal, had a narrow molecular weight distribution, and did
not differ in form or width from the trace of the starting
polymer P1. Polystyrene equivalent molecular weights and
PDIs are given in Table 1. Generally, the experimental
1
13
generally observed. As example, the H/ C HSQC spectrum
of polymer P10 is presented in Figure 1, showing the
quantitative (within NMR accuracy) conversions of the R
PFP ester into a hexadecyl amide and the ω dithioester into a
1H,1H,2H,2H-perfluorooctyl disulfide.
molecular weights M were in good agreement (less than
n
8
% deviation) with the theoretical M expected for quanti-
n
All ω end groups surveyed in this study were connected to
the polymer chain via disulfide links, which were stable under
all synthetic and analytic conditions. The disulfide bridge
did not have any significant influence onto the solubility
behavior of the polymers as could be seen from a comparison
of P19 (Rdiazo, ω methyl disulfide POEGMA) with R diazo, ω
isobutyronitrile POEGMA with the same degree of polymer-
tative end-group conversions, except for PEG R end groups,
where the experimental M was significantly lower than
n
expected, and for dioctadecyl R end groups, where the
molecular weights found were higher than expected. Never-
theless, these shifted curves were monomodal and narrow.
The divergence may be explained by the blocklike structure
of the PEG-terminated polymers and the 3-arm-star-like
structure of the dioctadecyl-terminated polymers.
UV-vis measurements (not shown) indicated a complete
removal of the terminal dithioesters from each polymer
P2-P19 due to the absence of its strong absorbance band
centered at 302 nm.
52
ization, as reported previously by us. P19 had an LCST of
49.1 °C, very close to 49.9 °C found for the isobutyronitrile
analogue which had originated from the common method to
65
remove the reactive dithioester end groups with AIBN.
LCST Values. Representative cloud point heating plots of
2
polymers P2, P5, P8, P11, P14, and P17 (all with R =
NMR spectroscopy could be used to quantify the conver-
sions at each end group. Conversions were generally above
methyl) are shown in Figure 2. The curves showed sharp
transitions from clear solutions (100% transmittance) to
turbid mixtures (0% transmittance) within a narrow range
of about 3 °C upon heating and only very small hystereses
upon cooling (not shown). These observed phase separations
were fully reversible for all polymers. The LCST values (50%
transmittance) of all polymers are given in Table 1 and are
plotted in Figure 3A,B. Both plots contain the same data;
9
fraction of heterotelechelic chains. Generally, each end
0% at each end group, yielding polymers with a high
1 13
group produced characteristic signals in H, C, or
19
F
NMR. Additionally, a quantitative shift of main-chain
located protons directly influenced by the end groups, such
as the methylene group adjacent to the R amide, was

4
642 Macromolecules, Vol. 43, No. 10, 2010
Roth et al.
(LCST 58.3 °C) and P6 (LCST 50.1 °C) (blue curve in
Figure 3a) and a 7.8 °C decrease for the same ω end group
change for the R hexadecyl polymers P8 (LCST 53.6 °C)
and P9 (LCST 45.8 °C) (green curve in Figure 3a). These
differences are in good agreement with the LCST difference
observed when going from an R PEG (P2) to an R C16 (P8)
end group of 9.1 (4.4 þ 4.7 °C). The perfluorooctyl (C8F13)
ω end group had a very similar effect as the hexadecyl end
group, with LCST for the perfluorinated group being in
average 0.72 ( 0.50 °C lower for all five pairs compared (see
red and green curves in Figure 3b).
Combination of Hydrophobic R and ω End Groups. The
blue and green curves in Figure 3a (representing the LCST of
R C3 polymers P5-P7 and the R C16 polymers P6-P10,
respectively) run parallel, with the differences between R C3
and R C16 end groups being 4.7, 4.3, and 5.0 °C for C1, C16,
and C8F13 ω end groups, respectively. These similar differ-
ences show that the influence of two end groups appears to be
additive; the introduction of an R C16 group for an R C3 end
group caused an LCST decrease of 4.67 ( 0.35 °C in addition
to a decrease caused by any of the three ω end groups. This
observation suggests that LCST values may easily be tuned
through a combination of two hydrophobic end groups. A
goal would be to develop a quantitative model and an
increment system based on this, from which LCST values
could be predicted.
Large Hydrophobic ω End Groups. Combination of two
hydrophobic end groups in order to reduce the LCST,
however, was found to have certain limitations: When look-
ing at the LCST data in Figure 3a,b, three temperatures
stand out because they seem too high. The orange curve in
Figure 3a, representing polymers P14-P16 with R 1H,1H-
perfluorononyl amide (C9F17) end groups, starting with
polymer P14 at an LCST of 50.9 °C, surprisingly rises when
going from ω C1 to ω C16 to 51.7 °C for P15. The LCST of
polymer P16 with ω C8F13 end groups with 50.3 °C is lower
than that of P15, but unexpectedly 5.6 °C higher than P10
(with R C16 and ω C8F13 end groups) (see course of green
curve in Figure 3b). The third temperature appearing at first
glance too high is the LCST of P18 with R dioctadecyl
(DiC18) and ω C8F13 end groups of 48.9 °C, which is the
same as for P17 with R dioctadecyl (DiC18) and ω C1 end
groups (brown triangles in Figure 3a and right side of
Figure 3b). A reasonable explanation for this behavior is a
microphase separation causing the large hydrophobic resi-
dues to form aggregates such as micelles. In that case, a single
polymer chain does not have to provide solubility for its
hydrophobic end group, but several polymer chains solubi-
lize a collapsed sphere. The LCST of such systems has been
Figure 3. LCST values connected with lines according to (A) same R
end groups with the x-axis giving the respective ω end groups and (B)
same ω end groups with the x-axis giving the respective R end groups.
however, for clarity, in Figure 3A, the values are grouped
1
according to same R (R ) end groups, with the x-axis giving
2
the respective ω (R ) end groups, while in Figure 3B, each set
of data points connected with lines represent polymers
sharing the same ω end group. In the following the influence
of the different end groups will be discussed.
Influence of PEG and Hydrophobic R End Groups. We first
consider the case where only one end group was changed
while the other remained the same. The black line in
Figure 3b (and the transmittance plots in Figure 2) shows
the change of LCST in a series with ω methyl end groups
and varying R end groups. The LCST of polymer P2 with an
R PEG end group was 62.7 °C, very close to the literature
1
8
value of about 64 °C, presumably, because PEG is chemi-
cally very similar to the side chains of the oligo(ethylene
glycol) methacrylate polymer and thus does not influence
its solubility behavior. This was contrary to the end-group
functionalization of poly[2-isopropyl-2-oxazoline] with
oligo(ethylene glycol), which was reported to decrease the
24,29
known to be higher than of singly dissolved chains.
We
thus employed dynamic light scattering (DLS) to measure
the hydrodynamic radii of selected heterotelechelic polymers
in water at 20 °C, at which all polymers gave clear solutions.
The values are also given in Table 1. The sizes of polymers
P1, P2, P5, P8, P10, and P11 all showed hydrodynamic radii
in the order of 3.3-4.3 nm, as to be expected from isolated
polymers of around 3500 g/mol. Polymers P15, P16, and
P18, which all had shown unexpectedly high LCST, all
produced monodisperse size distributions around 7.8-8.6
nm, significantly larger than all other measured polymers.
The aggregation behavior of triphilic (hydrophilic, lipophi-
lic, and fluorophilic components) polymers into super-
2
9
LCST. As to be expected, the more hydrophobic the R end
group became, the lower was the LCST. With an R propyl
(
C3) end group (P5), the LCST was 58.3 °C, 4.4 °C lower
than with PEG. With an R hexadecyl (C16) end group (P8),
the LCST was 53.6 °C, 4.7 °C lower than with C3, and with
an R dioctadecyl (DiC18) end group (P17), the LCST was
1
1
4
8.9 °C, again 4.7 °C lower than C16. The H, H-perfluoro-
nonyl (C9F17) R end group (P14), although shorter, had a
stronger influence than the hexadecyl chain and caused an
LCST of 50.9 °C, between those of hexadecyl and dioctade-
cyl residues, showing the higher impact of the perfluorinated
end group.
Influence of Hydrophobic ω End Groups. The same effect
was found for hydrophobic ω end groups. There was an
LCST decrease of 8.2 °C when going from an ω methyl (C1)
to an ω hexadecyl (C16) end group for the R C3 polymers P5
6
6
structures has been the focus of various research projects.
Kubowicz et al. reported the formation of cylindrical
micelles of poly(N-acylethylene imines) with both C8F17
6
4
67
and C16 end groups, while Kyeremateng et al. recen-
tly described that the formation of multicompartment

Article
Macromolecules, Vol. 43, No. 10, 2010 4643
5
9,60,62,68,69
micelles
of triphilic polymers depends on the
its removal via the AIBN method could increase the LCST
for 3.3 °C in this case.
Influence of Charged End Groups. An effect described in the
length of the hydrophilic block and its ability to form loops.
The DLS results found here will require a further profound
characterization of the exact composition of aggregates,
which is beyond the scope of this paper. In short, the
DLS measurements confirmed that the polymers that
showed unexpectedly high LCST were not dissolved on a
molecular level but formed aggregates already at room
temperature. The influence of such phase-separated R
C9F17 or R DiC18 end groups was in the same order of
magnitude as the influence of solubilized R C3 end groups
2
4,27,30
literature is an LCST increase of PNIPAM or poly[2-
isopropyl-2-oxazoline] through the introduction of hydro-
or
2
9
2
philic end groups, such as azides, hydroxyl groups,
7
24,30
2
4
amines. Except for PEG, which produced nearly the lit-
erature LCST of POEGMA, the end groups discussed so far
had all caused an LCST decrease. We therefore next inves-
tigated, whether a charged end group, R-(trimethylam-
monium)ethylamide (abbreviated “(þ)” or “ammonium”),
would also result in a higher LCST of POEGA. Polymer P11,
with an R ammonium and an ω methyl end group, had
an LCST of 66.3 °C. This showed that the charged end
group with a higher hydrophilicity than the polymer chain
could indeed provide water solubility until 2.3 °C above the
literature LCST of 64 and 3.6 °C above the LCST of R PEG,
ω methyl-terminated polymer P2.
(
P6 and P7). Interesting is the direct comparison of R C16, ω
C8F13 polymer P10 (radius 4.28 nm; LCST 44.7 °C) with R
C9F17, ω C16 polymer P15 (radius 8.51 nm; LCST 51.7 °C),
the main difference of which is that a methylene end group
unit of P10 is substituted with a -CF CF - segment in P15.
2
2
Because of this, the critical length of the perfluorinated block
is exceeded, causing P15 to form aggregates and have a
higher LCST.
Combination of Charged and Hydrophobic End Groups. Of
special interest was then the combination of two end groups
with opposite effects;one raising and one lowering the
LCST. The LCST of the R ammonium polymers P12 and
P13, with ω C16 and ω C8F13 end groups, had LCST of 62.5
and 62.1 °C, respectively (black curve in Figure 3a and first
column in Figure 3b). These temperatures were lower than
for ω methyl end groups (P11), showing an influence of both
end groups of these heterotelechelic polymers. Coincidently,
the LCST of R ammonium polymers P12 and P13 were very
close to those found for the R PEG polymers P3 and P4.
However, the difference between ω methyl and ω C16 of
3.8 °C was significantly lower than 8.0 ( 0.2 °C observed for
the uncharged C3 and C16 R end groups. Apparently, for end
groups of conflictive influences, a simple addition of incre-
ments for each end group in order to determine the LCST of
a system composed of a chain and two end groups is not
possible. This shows that a complex quantitative model
would be required to predict the effect of a chemical structure
on the LCST. However, such a model is not available at
present but would clearly be desirable. Still, this series
showed that two end groups with opposite effects can
compensate each other. In this case, the influence of the
charged R ammonium group dominated the influences of
hydrophobic ω C16 or ω C8F13 end groups, with the latter
two having a smaller impact than they do in combination
with hydrophobic R end groups.
Rigid (Aromatic) End Groups. Within the series of poly-
mers P2-P19, the lowest LCST of 44.7 °C was found for P10
with two hydrophobic end groups, not large enough though
to cause a phase separation. Surprisingly, the LCST of
starting polymer P1 with pentafluorophenyl ester and
dithioester end groups was yet lower at 42.7 °C. We attribu-
ted this low value to the difficulty of the polymer to solu-
bilize its end groups. In contrast to (perfluorinated) alkyl
chain end groups, the aromatic groups are rigid and reduce
the end group entropy because of limited possibilities for the
rigid systems to respond to conformational polymer chain
movements. In addition, aromatic rings generally prefer
π-π stacking interactions both over contact with water
and over interactions with the nonaromatic polymer chain.
This further decreases the stability of the modified coil and
favors a precipitation below the LCST of chains with non-
aromatic end groups. In the series of polymers with ω C1 end
groups, the R diazo end group of P19 caused an LCST of
4
9.1 °C, only 0.2 °C above P17 with an R DiC18 end group,
23
showing the impact of the two aromatic rings. Inoue et al.
investigated the molecular weight dependent thermal re-
sponse of poly[ethoxyethyl glycidyl ether] chains grafted
onto gold surfaces. For characterization in aqueous solution,
they reported an LCST decrease of 6.3-14.6 °C (depending
on molecular weight) for (rigid) phenothiazine end groups
as compared to butoxy end group. In a different study,
where functional acetylenes were clicked to azide modified
PNIPAM, the lowest LCST was found for 4-phenoxyphenyl
end groups, when compared to phenyl, octyl, and butyl
Combination of PEG and Hydrophobic End Groups. Final-
ly, we consider the combination of PEG with hydrophobic
end groups. Surprisingly, the LCST drop of 8.0 ( 0.2 °C for
ω C16 as compared to ω C1 end groups, as discussed above,
was not found, when the polymer contained R PEG groups.
Instead, only a decrease from 62.7 °C (P2) to 62.4 °C (P3) of
0.3 °C was measured. In addition, the difference between ω
C8F13 and ω C16 end groups was lowest for R PEG end
groups, with another decrease of 0.3 to 62.1 °C of polymer P4
(see red curve in Figure 3a). In sum, the LCST values of all R
PEG polymers were nearly the same, independent of the ω
end group (red curve in Figure 3A), indicating an ability of
PEG to mask the influence of a hydrophobic group at the
opposite end of the polymer chain onto the LCST. This
suggests that PEGylation, which is already employed for
2
7
groups. These results are in agreement with the findings
of this study.
As dithioesters are very typical RAFT end groups, it is of
interest to consider the influence of PFP esters and dithio-
esters separately. In a previous paper, we reported the LCST
of POEGMA of the same molecular weight with R PFP ester
5
2
and ω isobutyronitrile end groups to be 46.1 °C. This
temperature was 3.8 °C lower than the LCST of R diazo,
ω isobutyronitrile POEGMA, showing that the PFP ester
has a stronger hydrophobic effect than the diazo end group.
This is further supported by the even lower LCST of 39.5 °C
5
2
of R PFP, ω PFP POEGMA. This observation was in
contrast to contact angle measurements of glass surfaces
exhibiting PFP esters, which were less hydrophobic than
dioctadecylamide, octadecylamide, or even octylamide sur-
71
enhancing the therapeutic potential of peptides, proteins,
and drugs, or improving the contents release behavior
7
2
7
3
of copolymer modified liposomes, may also be applied
for LCST stabilization of poly[oligo(ethylene glycol)
methacrylate], thereby broadening the potential of this bio-
compatible polymer through highly functional materials
with well-predictable thermal responses.
7
0
face groups of the same concentration. R PFP, ω isobutyro-
nitrile POEGMA having an LCST of 46.1 °C and R PFP, ω
dithioester POEGMA (P1) having an LCST of 42.8 °C
showed the hydrophobic contribution of the dithioester;

4
644 Macromolecules, Vol. 43, No. 10, 2010
Roth et al.
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Acknowledgment. The Institute of Biophysics of the Univer-
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