Control of methyl methacrylate radical polymerization via Enhanced Spin Capturing Polymerization (ESCP)

Article abstract of DOI:10.1016/j.polymer.2010.06.040

The nitrone mediated polymerization of methyl methacrylate (MMA) via the enhanced (termination) spin capturing polymerization (ESCP) process is made possible via the addition of small amounts of styrene (between 5 and 10 vol.%) to the reaction mixture. Efficient control over the molecular weight between 7000 and 57,000 g mol^-1 (at 60 °C) yields macromolecules that feature a mid-chain alkoxyamine functionality and are rich in methyl methacrylate. The collated kinetic and molecular weight data allow for a deduction of the spin capturing constant, C_SC, in the range between 0.15 and 0.30. During the ESCP process, the number average molecular weight, M_n, of the formed mid-chain functional polymer is constant up to high monomer to polymer conversions (i.e. 80%). The high degree of alkoxyamine mid-chain functionality present in the generated polymeric material is evidenced via a subsequent nitroxide-mediated polymerization process employing the formed ESCP polymer, indicating a chain extension from 37,700 to 118,000 g mol^-1 with a concomitant reduction in polydispersity (from 2.3 to 1.5).

Full text of DOI:10.1016/j.polymer.2010.06.040

Polymer 51 (2010) 3821e3825
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Polymer Communication
Control of methyl methacrylate radical polymerization via Enhanced Spin
Capturing Polymerization (ESCP)
Lin Zang , Edgar H.H. Wong c, Christopher Barner-Kowollik a,*, Thomas Junkers a
a
a
,
,b,
**
a
Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18,
6128 Karlsruhe, Germany
Institute for Materials Research (IMO), Universiteit Hasselt, Agoralaan, Gebouw D, 3590-Diepenbeek, Belgium
7
b
c
Centre for Advanced Macromolecular Design, The University of New South Wales, Sydney 2052, Sydney, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2010
Received in revised form
The nitrone mediated polymerization of methyl methacrylate (MMA) via the enhanced (termination)
spin capturing polymerization (ESCP) process is made possible via the addition of small amounts of
styrene (between 5 and 10 vol.%) to the reaction mixture. Efficient control over the molecular weight
between 7000 and 57,000 g mol (at 60 C) yields macromolecules that feature a mid-chain alkoxy-
amine functionality and are rich in methyl methacrylate. The collated kinetic and molecular weight data
allow for a deduction of the spin capturing constant, CSC, in the range between 0.15 and 0.30. During the
16 June 2010
ꢀ1
ꢁ
Accepted 22 June 2010
Available online 30 June 2010
n
ESCP process, the number average molecular weight, M , of the formed mid-chain functional polymer is
Keywords:
Methyl methacrylate (MMA)
Nitrone/nitroxide
Enhanced (termination) spin capturing
polymerization (ESCP)
constant up to high monomer to polymer conversions (i.e. 80%). The high degree of alkoxyamine mid-
chain functionality present in the generated polymeric material is evidenced via a subsequent nitroxide-
mediated polymerization process employing the formed ESCP polymer, indicating a chain extension from
ꢀ
1
37,700 to 118,000 g mol with a concomitant reduction in polydispersity (from 2.3 to 1.5).
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
and a macronitroxide is formed. These macronitroxides subse-
quently undergo a fast termination reaction in a second reaction
step to form the final polymer product, representing macromole-
cules with a mid-chain alkoxyamine function. The general process
is depicted in Scheme 1. The ESCP reaction is thus by itself
a controlled, yet not a polymerization with living characteristics
[4,6]. The product, however, is a polymer bearing an alkoxyamine in
the middle of the chain and thus of a dormant nature. Therefore,
release of macroradicals upon heating to higher temperatures
Even with the established methods for controlled/living poly-
merization such as the reversible addition fragmentation transfer
(
RAFT) process [1], atom transfer radical polymerization (ATRP) [2]
and nitroxide-mediated polymerization (NMP) [3], there is never-
theless a constant strive for the establishment of novel methods
that efficiently control radical polymerizations. A recent enlarge-
ment of the polymer chemist’s toolbox is the enhanced (termina-
tion) spin capturing polymerization (ESCP) method [4e6], which
was developed on the basis of thioketone-mediated polymerization
ꢁ
(exceeding 100 C) and subsequent chain extension in a conven-
tional nitroxide-mediated polymerization is feasible.
[
7]. ESCP makes use of nitrones to control the functionality and the
We have recently demonstrated that ESCP functions efficiently
and yields polymers with well-predictable molecular weight and
high end group fidelity using various phenyl nitrones [5,8]. In
addition, coupling of polymer chains that have been generated via
ATRP was successful where the principle of ESCP is applied in the
absence of monomer and thus growing chains [9]. So far, most
examples of ESCP focused on polymers based on acrylates, styrene
and N-isopropyl acrylamide as monomers. For these systems, good
control was instantly achieved. As will be discussed below in detail,
no successfully controlled polymerization of methacrylates have
been reported to date. Polymerizations did occur in presence of
nitrones, however, no effective control over molecular weight was
observed. As methacrylates are however an important class of
chain length of polymers obtained in-situ in a radical polymeriza-
tion. Thereby, the addition of growing macroradicals to a nitrone is
in competition with the addition of monomer units. When a prop-
agating radical adds to a nitrone, chain growth of the macromole-
cule is stopped (hence the control over molecular weight of the
obtained polymer depends on the concentration of the mediator)
*
Corresponding author. Tel.: þ49 721 608 5641; fax: þ49 721 608 5740.
*
* Corresponding author. Tel.: þ32 11 26 8318; fax: þ32 11 26 8399.
E-mail addresses: christopher.barner-kowollik@kit.edu (C. Barner-Kowollik),
thomas.junkers@uhasselt.be (T. Junkers).
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.06.040

3822
L. Zang et al. / Polymer 51 (2010) 3821e3825
N+
R
N
R
N
R
R
O-
O
R
O
Scheme 1. Underpinning key reaction sequence in the Enhanced Spin Capturing polymerization (ESCP) process.
monomers, it is a matter of priority to explore avenues that over-
come the existing limitations. In here, we report on the successful
control of methyl methacrylate polymerization via usage of a-tert-
avoid background polymerization, a solution of THF/hydroquinone
is added to the pans after weighing. The samples are subjected to
characterization without further treatment.
butyl phenylnitrone (PBN, see Scheme 1), a nitrone that was
previously employed in our studies to control other classes of
monomers. The otherwise uncontrolled polymerization is con-
verted into a controlled polymerization by adding small amounts of
styrene to the polymerization mixture e a procedure that has also
been used in the framework of conventional NMP of MMA [10e13].
2.4. Block copolymer formation via NMP
The ESCP polymer (37,700 g molꢀ1) obtained from a similar
procedure as described above is precipitated in cold methanol to
remove excess AIBN and PBN from the polymer. The obtained solid
is then transferred into a Schlenck tube and dissolved in fresh
styrene in a molar ratio of 1:3000 (100 mg copolymer in 1 mL fresh
styrene). The tube is subsequently sealed and oxygen is removed
via three freeze-pump-thaw cycles. For the NMP reaction, the tube
2
. Experimental section
2.1. Materials
ꢁ
is then brought into a pre-heated oil bath at 110 C. After 100 min,
Methyl methacrylate (MMA, Acros 99%) and styrene (STY, Acros
the tube is removed from the oil bath and cooled down in an ice/
water bath. The mixture is subsequently treated as the ESCP
samples to isolate the polymer and to determine the polymer to
monomer conversion.
9
9%) were freed from inhibitor by percolating the monomer of
0
a column filled with basic alumina. The initiator 2,2 -azobsisobu-
tyronitrile (AIBN, Acros, 98%) was recrystallized twice from meth-
anol. All chemicals for the synthesis of the nitrone were obtained
from SigmaeAldrich and used as received.
2.5. Size exclusion chromatography
2.2. Preparation of
a-phenyl-N-tert-butylnitrone (PBN)
Gel Permeation Chromatography (GPC) measurements were
performed on a Polymer Laboratories PL-GPC 50 Plus Integrated
1
8.94 g hydrogen peroxide urea adduct (UHP, 0.2 mol) dissolved
in 100 mL methanol are added dropwise to a cooled solution of
6.33 g N-tert-butylbenzylamin (0.1 mol) and 13.20 g sodium
System, comprising an autosampler, a PLgel 5
m
m
m bead-size guard
m Mixed E column
column (50 ꢂ 7.5 mm) followed by one PLgel 5
1
(300 ꢂ 7.5 mm), three PLgel 5 m Mixed C columns (300 ꢂ 7.5 mm)
m
tungstate) 0.04 mol in 150 mL methanol. The temperature of the
reaction mixture is monitored and kept below 0 C throughout the
and a differential refractive index detector using THF as the eluent
ꢁ
ꢁ
ꢀ1
at 35 C with a flow rate of 1 mL min . The apparatus was directly
¼ 700 e 2$106 g molꢀ
1
,
addition of the peroxide. Subsequently, the slightly yellow
suspension is brought to ambient temperature and subsequently
stirred overnight. The solvent is then removed in vacuo and dis-
solved in 200 mL dichloromethane. The residual solid is dissolved
in 200 mL of water. The water phase is extracted with dichloro-
methane (3 ꢂ 150 mL) and the combined organic layers are then
dried with sodium sulfate and the solvent is removed in vacuo. The
calibrated using polyMMA standards (M
PPS Mainz).
p
3. Results and discussion
MMA is not easily controlled via the NMP process. If carried out
under standard reaction conditions, no control over molecular
weight and functionality is observed over the course of the poly-
merization [15]. Two problems have been resolved over the years
that prevent an effective living polymerization of methacrylates via
nitroxide control. Firstly, the alkoxyamine bond that is formed
upon the reaction between an active propagating radical and the
nitroxide is not sufficiently stable to establish an effective equilib-
rium between active and dormant chains [15]. Thus, the reaction
proceeds largely uncontrolled. Secondly, termination between
growing chains and the controlling nitroxide may also occur via
a disproportionation reaction, which is favored in methacrylate
polymerizations in general [16]. Thereby, nitroxide is eliminated
from the system e shifting the equilibrium towards the active
radical and thus uncontrolled side e and irreversible terminated
polymer chains are created. Two approaches have been used in the
past to overcome this problem. Guillaneuf et al. [15] identified
a nitroxide (i.e. 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-
yloxyl nitroxide, DPAIO) that forms sufficiently stable alkoxyamines
with growing methacrylate chains and is at the same time suffi-
ciently unreactive as to not undergo significant disproportionation
reactions. In the second approach, Charleux and coworkers ach-
ieved control over methyl methacrylate polymerization using the
pure nitrone
a-phenyl-N-tert-butylnitrone (PBN) is directly
1
obtained and no further purification is required. H-NMR
250 MHz):
m, 3H, 3H
0.5 (N-tert-butyl),131.0 (CH]N),130.0,129.7,128.7,128.3 (Ph). The
procedure was adapted from ref [14].
(
(
7
d
¼ 8.22e8.18 (m, 2H, 2H
a
), 7.44 (s, 1H, H
). C-NMR (75 MHz):
c
), 7.34e7.27
1
3
b
), 1.49 (s, 9H, -C(CH
3
)
3
d
¼ 28.3,
2
.3. Synthesis of ESCP polymers
The initiator AIBN (0.067 g, 0.04 mol Lꢀ1) is dissolved in 9.5 mL
MMA and 0.5 mL styrene as well as the required amount of PBN are
added to the mixture. The solution is then divided into 6 portions
and filled into glass vials which are subsequently sealed by a rubber
septum. Via purging the samples with nitrogen for approximately
1
0 min in an ice bath, oxygen is removed from the samples. The
vials are subsequently placed into a pre-heated thermomixer at
ꢁ
6
0 C. After predetermined reaction times the samples are removed
from the heat and cooled down in an ice bath. The contents of the
vials are transferred into aluminum pans and the conversion is
determined gravimetrically by weighting the samples before and
after evaporation of the residual monomer in a vacuum oven. To

L. Zang et al. / Polymer 51 (2010) 3821e3825
3823
commercial SG1-nitroxide via addition of styrene as a comonomer
in amounts between 4 and 8% [10]. Styrene as the comonomer has
the function to allow for stable alkoxyamine bond formations upon
reaction between a styrene-terminated growing chain and the
control agent. As was demonstrated, only relatively small amounts
of styrene are required to achieve conditions under which control
over the polymerization is achieved. Thus, even though the
obtained polymer contained a certain percentage of styrene, it is
relatively close to pure polyMMA.
Inspired by the elegant copolymerization approach introduced
by Charleux and colleagues [10], we in here evaluate the same
approach for the control of MMA polymerizations via ESCP. The
situation is here however somewhat different. No equilibrium
between active and dormant species is established in ESCP and
even if it is, the control over the polymerization is not compro-
mised. In addition, disproportionation reactions only play a minor
role. The reaction step that controls the molecular weight of the
residual polymer is the addition of growing chains to the nitrone.
The subsequent termination reaction has only a lesser impact on
the size of the polymer. Disproportionation reactions affect the
uniformity of functional groups, in contrast the molecular weight of
combination and disproportionation products only differ by a factor
of two. Nevertheless, addition of small amounts of styrene also has
a beneficiary influence on the ESCP of MMA.
higher molecular weights are observed than for comparable
monomers where average molecular weights between 2000 and
ꢀ
1
30,000 g mol are commonly obtained [4,5]. Further, it can be
observed that the molecular weight distributions are slightly
bimodal. Closer inspection of the distributions (by differentiating
the distributions and comparison of the thereby identified points
of inflection) reveals that all distributions (also the one obtained in
absence of PBN) are mixtures of two distributions differing in
a factor of 2 in size. Thus, it may be concluded that the observed
bimodality is a result from disproportionation reactions occurring
during the polymerizations; clearly, the styrene content is not high
enough to allow for almost exclusive termination by combination
(of macronitroxides and growing macroradical chains). Regardless,
the molecular weight control remains largely unaffected as dis-
cussed above.
From the obtained molecular weights, the rate of addition of the
growing chains to the nitrone, kad,macro, and the so-called spin
capturing constant CSC (which is defined as the ratio of kad,macro
over the propagation rate k ) can be derived in the following
p
fashion
l
$kad;macro : cPBN
ꢀ1
ꢀ1
N
DP
¼
DP
þ
(1)
the
n
k
p
: cM
ꢀ1
is the inverse of the degree of polymerization, DP
N
ꢀ
1
where DP
n
Fig. 1 depicts molecular weight distributions of poly(MMA-co-
STY) polymers obtained from ESCP at low monomer conversions
with various concentrations of PBN as control agent. For the sake of
simplicity, mixtures of MMA and STY were made on a volume-
basis. For the molecular weight distributions depicted in Fig. 1, the
polymerizations were conducted with a content of 10 vol.% styrene
inverse of the degree of polymerization that is obtained in an
uncontrolled reaction (zero nitrone concentration) and c is the
monomer concentration. The factor was derived in our previous
M
l
studies as 0.5 [4], which represents exclusive termination via
combination (as is usually the case in ESCP). For termination
exclusively via disproportionation, l would reach unity. Thus, the
‑
1
in the reaction mixture, initiated by 0.04 mol L AIBN. The nitrone
ꢀ1
true value for the polymerizations at hand is between 0.5 and 1
with the exact value being dependent on the styrene content as this
will influence the number of possible disproportionation events.
Fig. 2 shows the data from Fig. 1 e transformed into average
degrees of polymerization plotted against the nitrone concentra-
tion e in addition to a further data-set obtained with 5 vol.%
styrene in the reaction mixture according to Equation (1). Both data
sets display the same trend and may be regarded to be identical
within the scatter of the data. From a joint fit the combined value of
concentration was varied between zero and 0.4 mol L as indi-
cated within the figure. As noted e for practical reasons e the
experiments in the current work were carried out based on
volume percentages. As the density and molar mass of both
monomers are very similar, volume and mole percentages are very
similar, too. Overall, a good control over molecular weight is ach-
ieved. The average molecular weight of the samples varies
ꢀ1
between 57,000 g mol for the uncontrolled reaction (featuring
ꢀ
1
a nitrone concentration of zero) and close to 7000 g mol for the
ꢀ1
l
$CSC ¼ 0.15 is obtained under the assumption of a monomer
highest PBN concentration of 0.4 mol L . The polydispersities
range between 1.5 for the highest PBN content to about 1.8 at the
lowest concentration, as is typical for ESCP polymers [5]. Thus,
ꢀ
1
concentration of 9.0 mol L
(which is derived from the bulk
ꢀ
1
ꢁ
density (0.90 g mL at T ¼ 60 C) [17] of MMA; as molecular weight
and density of styrene and MMA are very similar (4% difference),
0
.010
0
0
0
0
0
.4 mol·L–1
increasing cPBN
.3 mol·L–1
0.009
.16 mol·L–1
.08 mol·L–1
.04 mol·L–1
0
0
.008
.007
no nitrone
0.006
0
0
0
0
0
0
.005
.004
.003
.002
.001
.000
5
1
% Sty
0% Sty
joint fit
0
.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35 0.40
2
.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
c
PBN
log(M / g·mol–1)
Fig. 2. Inverse of the degree of polymerization of low-conversion polymer samples
taken from PBN-mediated polymerizations of MMA at 60 C with 5 and 10 vol.%
styrene content, respectively, as a function of PBN concentration. The straight line gives
ꢁ
Fig. 1. Molecular weight distributions of polymer made via ESCP from MMA-STY
copolymerizations at 60 C with 10 vol.% styrene content and PBN as the mediating
ꢁ
agent. All samples were taken at low (<5%) monomer to polymer conversions.
the joint fit to both data sets.

3824
L. Zang et al. / Polymer 51 (2010) 3821e3825
ꢁ
ꢀ1
ꢀ1
the error introduced via such an assumption is negligible). Thus,
the -decoupled value of CSC lies between 0.15 and 0.30. From this
at 60 C controlled by 0.08 mol L PBN and 0.16 mol L , respec-
tively. The samples shown in the figure were taken at different
reaction times, where the longest reaction time was about 3.5 h in
case of the sample at 80% monomer conversion for the higher PBN
concentration. From a low CSC, one would usually expect decreasing
molecular weight with increasing nitrone concentration. The CSC
value determined above is however only an average value and the
true value (if only chains are counted that have a terminal styrene
group) is in fact closer to unity or slightly above. Thus, it comes as
no surprise that throughout the polymerization an almost constant
molecular weight is found. For the polymerization with a nitrone
l
value, absolute addition rates of the propagating macroradicals to
the nitrone can be estimated. The average propagation rate of
MMA-STY copolymerizations at the given monomer composition at
ꢁ
ꢀ1 ꢀ1
5
7
7
C is close 500 L mol
s [18] from which an addition rate
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
5 L mol
s
< kad,macro < 150 L mol
s
is derived.
Generally, the spin capturing constant, CSC, is of higher practical
value than the addition rate coefficient, kad,macro, because it directly
allows for the prediction of the average molecular weight for any
given nitrone concentration (so does
l$CSC in case the ratio of
ꢀ
1
combination to disproportionation does not change). Thus, an exact
knowledge of kad,macro is not strictly required and CSC is sufficient.
concentration of 0.08 mol L , an average molecular weight of
39,000 g mol is found in the conversion regime of up to 80%
monomer consumption. For the doubled concentration of PBN,
ꢀ
1
Even under the assumption of
compared to values obtained for the same parameter in the homo-
l
¼ 0.5, CSC is 0.30 and hence very low
a decreased molecular weight is identified as expected at
ꢀ
1
polymerization of styrene [5], where a value of 1.7 (or a kad,macro of
29,900 g mol . The reactivity ratios in the MMA-STY system are
relatively close to an ideal copolymerization. Thus, no large
composition drift occurs during the polymerization and the control
over the reaction works as efficiently as at low conversions.
It should also be noted that the alkoxyamines formed during the
ESCP with PBN may feature a comparatively high decomposition
rate, thus leading to an NMP-like equilibrium at relatively low
temperatures. However, the almost constant molecular weight
with increasing monomer conversion demonstrates that this is e at
ꢀ1 ꢀ1
5
80 L mol
s , respectively) was deduced at the same temperature.
The propensity of a growing chain to add to PBN in the MMA/STY
mixtures is thus about 5e10 times lower compared to styrene
homopolymerization. Such a decrease can be explained via the
assumption that growing chains with a MMA terminus do not effi-
ciently add to the nitrone to form a nitroxide. Based on literature
reactivity ratios of the styrene/MMA system (r
¼ 0.489) [19], one can derive (via the LewiseMayo equation) that
approximately 15% of all chains have a terminal styrene group for the
0% styrene mixture and about 10% of chains for the 5% styrene
1
¼ 0.493 and
r
2
ꢁ
least at 60 C e not (yet) the case. A hybrid ESCP/NMP polymeri-
1
zation may however occur when the temperature is increased.
The above observations all indicate that the control over
molecular weight is reached in the copolymer system with
qualitatively the same features as in conventional ESCP reactions
with other monomers accompanied by the alteration of an
overall reduction in the observable CSC. While the above is
a generally satisfying result, the question remains how well the
functionality of the chains is controlled. As discussed above, one
can expect that from disproportionation reactions unfunctional-
ized chains are created that cannot be reactivated via the NMP
mechanism.
A reliable way to test for the degree of functionality is to perform
a chain extension via the NMP mechanism. After freeing the ESCP
polymer from any excess nitrone and thermal initiator via precip-
itation, fresh monomer is added to the isolated polymer and a NMP
reaction is started via heating the sample to 110 C. Under such
conditions, only chains that carry an intact alkoxyamine function
(hence chains that were formed upon combination of
mixture. Thus, every chain must on average undergo 6 to 7 propa-
gation steps before an addition to the nitrone can take place. If one
additionally assumes that no penultimate unit effect occurs, the
average addition rate in the MMA-STY copolymerization can be
calculated based on the homopolymerization kad,macro. Following
ꢀ
1 ꢀ1
this assumption, an addition rate of approximately 90 L mol
s
is
expected for the polymerization containing 10% styrene and about
ꢀ
1 ꢀ1
6
0 L mol
s
for the 5% mixture. Both values are relatively close to
the observed value provided above. Even though the pleasing
agreement between the values might to some degree be coincidence,
it may nevertheless be concluded that the assumption of an almost
non-adding MMA chain terminus to the nitrone is largely valid. Such
a conclusion is also in-line with the observation of completely
uncontrolled polymerizations of MMA in the presence of PBN when
no comonomer is added, which is to be expected when the nitrone
does not take part in the reaction.
ꢁ
Fig. 3 depicts the evolution of the molecular weight with
increasing conversion for polymerizations with 5% styrene content
ESCP polymer
NMP block copolymer
6
5
4
3
2
1
0 000
0 000
0 000
0 000
0 000
0 000
0
0
.08 mol·L–1 PBN
.16 mol·L–1 PBN
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
0
20
40
60
80
100
log (M / g·mol–1
)
monomer conversion, X
Fig. 3. Monomer to polymer conversion dependence of the number average molecular
Fig. 4. Molecular weight distribution of polymer made via ESCP of MMA with 5 vol.%
ꢁ
weight, M
n
, in the bulk ESCP of MMA with 5 vol.% styrene content at 60 C, mediated
styrene as comonomer and the distribution of the polymer after reactivation of the
ꢀ
1
ꢁ
with PBN and initiated by 0.04 mol L AIBN.
alkoxyamine bonds at 110 C and chain extension with pure styrene via NMP.

L. Zang et al. / Polymer 51 (2010) 3821e3825
3825
macronitroxides and growing chains) can be reactivated and chain
extended. All other material is dead and will not react further. The
outcome of such an assessment is depicted in Fig. 4. The full line
provides the molecular weight distribution of polymer made under
the same conditions as employed in the study of the conversion
polymerization of MMA via the ESCP process, the molecular weight
is constant as a function of the monomer to polymer conversion.
These results indicate that the addition of small fractions of styrene
to methyl methacrylate allows e similar to the previously
demonstrated control of methyl methacrylate via NMP e for
a functioning ESCP process. The kinetic and molecular weight data
obtained in the present study allowed for the estimation of the spin
ꢀ1
dependence with 0.08 mol L PBN. The dotted line represents the
ꢁ
distribution of polymer after NMP at 110 C employing pure styrene
ꢁ
as the monomer for chain extension. The fresh monomer was
added in a ratio of 1:3000, based on the assumption of quantita-
tively mid-chain alkoxyamine functional ESCP polymer, i.e.
featuring no functionality defects from disproportionation. The
figure shows that an efficient chain extension has taken place and
that the majority of polymers thus carried the required alkoxy-
amine functionality in the middle of their chains. The resulting
polymer is thus of a triblock copolymer structure with the sequence
p(MMA-co-STY)epSTYe p(MMA-co-STY). The average molecular
capturing constant, CSC, in between 0.15 and 0.30 at 60 C. Impor-
tantly, the number average molecular weight as a function of
monomer to polymer conversion remains constant up to high
conversions (>80%), demonstrating that the process is capable of
providing uniform polymeric material over a wide conversion
range. The dormant nature of the ESCP generated poly(methyl
acrylate)-co-polystyrene species was demonstrated via a subse-
quent chain extension by an NMP process at elevated temperatures,
which afforded a triblock copolymer. Thus, the present contribution
provides a convenient avenue for expanding the ESCP process to
the important class of methacrylate-type monomer systems and
allows for the macromolecular design of mid-chain functional
alkoxyamine methyl methacrylate rich polymers.
ꢀ1
weight increased in the given example from 37,700 g mol to
ꢀ
1
1
18,000 g mol , which fits reasonably well with the theoretically
ꢀ1
expected molecular weight of 100,500 g mol
(based on the
styrene monomer conversion of 19%). The polydispersity of the
NMP-chain extended polymer reads 1.5, well below the value of 2.3
of the starting ESCP polymer, further attesting the living nature of
the chain extension.
Acknowledgements
From the above experiment, one can conclude that a reason-
ably well functionalized polymer is obtained from ESCP despite
the disturbing influences of disproportionation. The NMP-chain
extended distribution shows however a (slight) tailing on the low-
molecular weight side of the distribution, which indicates that
a proportion of chains did not react in the NMP process. SEC only
provides a mass-weighted distribution, thus a quantification of
the percentage of chains that did not react is fraught with
complications. Overall, it should be noted that more data is
required to assess the efficiency of block copolymer formation and
the current example is only meant to demonstrate that the ESCP
polymer is of overall satisfying functional fidelity and that chain
extensions can in principle be performed. Additionally, the chain
extension allows for insights into the microstructure of the chains
that have terminated the macronitroxide to form the alkoxy-
C.B.-K. acknowledges funding from the Karlsruhe Institute of
Technology (KIT) in the context of the Excellence Initiative for
leading German universities, the German Research Council (DFG),
and the Ministry of Science and Arts of the state of Baden-Würt-
temberg. E.H.H.W. and C.B.-K. are thankful for a postgraduate
scholarship from UNSW central funding.
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molecular weights ranging from over 50,000 to 7000 g mol
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[
Although the molecular weight distributions display a bimodality
caused by the disproportionation events prevalent during the
8