Stereogradient polymers formed by controlled/living radical polymerization of bulky methacrylate monomers

Article abstract of DOI:10.1002/anie.200805168

(Chemical Equation Presented) Life RAFT: A bulky methacrylate monomer, triphenylmethyl methacrylate (TrMA), was polymerized with reversible addition-fragmentation chain transfer (RAFT) agents. Stereogradient polymers in which the isospecificity increased

Full text of DOI:10.1002/anie.200805168

Angewandte
Chemie
DOI: 10.1002/anie.200805168
Polymerization
Stereogradient Polymers Formed by Controlled/Living Radical
Polymerization of Bulky Methacrylate Monomers**
Kenji Ishitake, Kotaro Satoh, Masami Kamigaito,* and Yoshio Okamoto
The stereochemistry of monomer additions can be recorded
on a living polymer chain because the chain continues its
growth throughout the polymerization process. If the stereo-
chemistry of the monomer addition can be changed during the
living polymerization, it will be reflected accordingly in the
changes in the tacticity along the chain. However, unless the
chain is living, the products will only be mixtures of the
polymer chains with different tacticities and various chain
lengths.
merization ranges from 64% to 99% and is dependent on the
monomer concentrations, temperatures, and solvents, which is
most probably due to changes in the kinetic and thermody-
namic control.[9] In particular, the isotacticity increases with
the decreasing monomer concentration, which suggests the
possibility of preparing stereogradient polymers by giving the
chains a living nature (Scheme 1).
A stereogradient polymer is defined as a polymer in which
the tacticity gradually varies along the chain, and is of interest
because of the continuous changes in its physical and thermal
properties. This type of stereocontrolled polymer is rather
new, with only a few examples reported to date;[1–3] this is
partly because the synthetic methodology requires both living
and stereospecific polymerization and also requires a steady
change in the stereospecificity.
A methacrylate monomer with extremely bulky pendant
groups, such as triphenylmethyl (TrMA) and its analogues,
undergoes anionic polymerization to give highly isotactic
polymers.[4,5] Furthermore, asymmetric anionic polymeri-
zations with chiral initiators afford optically active polymers
with a stable one-handed helical conformation,[6,7] which are
commercialized as a chiral support for HPLC.[8] Even radical
polymerizations generate isotactic-rich polymers because of
steric repulsion between the bulky ester groups of an
incoming monomer and of the growing chain end, which
results in a rigid helical conformation of the polymer chain
maintained by the bulky substituents, although the isotacticity
is slightly lower than for anionic polymerizations.[4] The
isotacticity (mm) of poly(TrMA) obtained by radical poly-
Scheme 1. Stereogradient polymer by RAFT polymerization of triphenyl-
methyl methacrylate.
[*] K. Ishitake, Dr. K. Satoh, Prof. Dr. M. Kamigaito
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)
Fax: (+81)52-789-5112
We thus investigated the controlled/living radical poly-
merization[10] of TrMA by using reversible addition–fragmen-
tation chain transfer (RAFT) agents with varying monomer
concentrations, temperatures, and solvents for the sponta-
neous formation of the stereogradient polymers. TrMA was
polymerized by using 2,2’-azobisisobutyronitrile (AIBN) in
the presence of cumyl dithiobenzoate (CDB)[11] at 608C. The
polymerization occurred smoothly and proceeded at almost
the same rate for methyl methacrylate (MMA) under the
same conditions (see Figure S1 in the Supporting Informa-
tion). The size-exclusion chromatograms (SEC) of the poly-
(TrMA) showed broad and bimodal molecular weight dis-
tributions (MWDs; dotted SEC curves in Figure 1) because of
the aggregation of the rigid and less soluble polymers,[12]
which were then converted into poly(MMA) by acid hydrol-
E-mail: kamigait@apchem.nagoya-u.ac.jp
Prof. Dr. Y. Okamoto
EcoTopia Science Institute, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)
[**] This work was supported in part by a Grant-in-Aid for Young
Scientists (S) (no. 19675003) from the Japan Society for the
Promotion of Science, a Grant-in-Aid for Scientific Research on
Priority Areas “Advanced Molecular Transformation of Carbon
Resources” (no. 17065008) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan), and the Global COE
Program “Elucidation and Design of Materials and Molecular
Functions.”
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805168.
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depended on the monomer conversion, but
were almost independent of the structure of
the RAFT agents. The isotactic contents
(mminst) for a certain part in the chains were
then calculated from the differential increase
in the mmcum value versus the chain length.
The instantaneous values of the isotacticity
(mminst) gradually increased with the normal-
ized chain lengths from 55% to finally over
95% (Figure 2), which indicates the forma-
tion of isotactic-rich stereogradient polymers.
This means that a decrease in the monomer
concentration changes the propagation into a
more isotactic enchainment.
Figure 1. Mn, Mw/Mn, and SEC curves of poly(MMA) obtained from the RAFT polymeri-
~
~
*
,
*
zation of TrMA ( , ) and MMA (
) with CDB/AIBN in toluene at 608C: [M]0 =1.0m;
The effects of the monomer concentration
on the polymerization were then investigated
with CDB in toluene at 608C. Upon decreas-
ing the initial monomer concentrations, poly-
merizations became slower and the final
[M]0/[CDB]0/[AIBN]0 =100:1:0.5. The diagonal line indicates the calculated Mn , assuming
the formation of one polymer chain per CDB molecule. The dotted SEC curves are elution
diagrams of poly(TrMA).
ysis of the trityl group, followed by methylation with
trimethylsilyl diazomethane. The obtained poly(MMA)
showed unimodal SEC curves, and the number-average
molecular weights (Mn) increased with the conversion and
were very close to the calculated values. Slightly broader
MWDs suggest a slower addition-fragmentation process for
this bulky monomer. Trithiocarbonate-type RAFTagents also
enabled a similar molecular weight control (Figure 2). Thus,
the controlled/living radical polymerization of TrMA was
attained by using these RAFT agents.
conversions became lower (Figure 3a). However, the final
monomer concentrations reached almost the same value (60–
The tacticity was measured from the 1H or 13C NMR
spectra of the poly(MMA) obtained from poly(TrMA)
(Figure S2 in the Supporting Information). All the polymers
showed a predominant isotacticity, which increased with the
consumption of TrMA. The entire or cumulative triad
isotacticity (mmcum) obtained for each conversion increased
from approximately 55% to 65%. Figure 2 shows a plot of the
mmcum value against the normalized chain length, that is, the
chain length of the polymers obtained for each conversion
relative to those at the final conversions. The mmcum values
Figure 3. Effects of initial monomer concentrations on the accessible
conversion (a) and instantaneous mm triad contents (b) in the RAFT
~
,
*
polymerizations of TrMA in toluene at 608C: [TrMA]0 =1.0 ( ,
~
~
*
*
c), 0.50 ( , , c), and 0.25 ( , , c) M;
[TrMA]0/[CDB]0/[AIBN]0 =100:1:0.5.
70 mm) independent of the initial concentrations,
which indicated the presence of the equilibrium
monomer concentration for such a bulky mono-
mer even at 608C.[13] In all cases, the molecular
weights increased with the conversion, which
shows that the molecular weight control is
achievable irrespective of the monomer concen-
trations (see Figure S3a in the Supporting Infor-
mation).
The instantaneous isotacticity value (mminst
)
also increased as the monomer conversion
increased and was higher for a lower initial
monomer concentration at the same monomer
conversion (see Figure S3b in the Supporting
Information). Plots of mminst versus the remain-
ing monomer concentration for different values
of initial monomer concentration practically
follow the same curve, which gradually increased
Figure 2. Dependences of cumulative and instantaneous mm triad contents on the
normalized chain length (a) and SEC curves of poly(MMA) (b) from the RAFT
polymerizations of TrMA in toluene at 608C; [TrMA]0 =1.0m. [TrMA]0/[RAFT agent]
0/[AIBN]0 =100:1:0.5; RAFT agent: 2-cyanoprop-2-yl ethyl trithiocarbonate (CPETC;
,
*
), 2-cyano-4-methoxy-4-methylpentan-2-yl ethyl carbonotrithioate (CMMETC; ),
*
and CDB ( ).
1992
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Chemie
as the instantaneous monomer concentration decreased
(Figure 3b). These results indicate that the tacticity is
governed by the remaining monomer concentration.
conformation of poly(TrMA).[15] The linear plot also gave the
ceiling temperature value of TrMA as 1048C at [M]0 = 1.0m.
This is also reasonable because almost no polymers were
obtained at 1008C for [M]0 = 1.0m.
We further utilized the unique isotactic-rich stereogra-
dient RAFT polymerization for a block copolymerization to
synthesize another novel stereocontrolled polymer. Thus we
first polymerized MMA with CDB to prepare the syndiotac-
tic-rich living poly(MMA) with RAFT moieties at the chain
ends and then employed it as a macroinitiator for the RAFT
polymerization of TrMA (Figure 5). After the copolymeriza-
The effects of the temperature were also examined on the
polymerizations. Upon raising the temperature, the initial
polymerization rate increased while the final monomer
conversions became lower. The mminst value increased with
temperature (see Figure S4 in the Supporting Information).
These results suggest that lowering the monomer concen-
tration or increasing the polymerization temperature makes
the formation of the more thermodynamically stable isotactic
propagating chain ends more favorable by the reversible
polymerization–depolymerization equilibrium.
The polymerization–depolymerization equilibrium was
further examined by increasing the temperature during the
polymerization. We first conducted the RAFT polymerization
at 608C and increased the temperature to 808C when the
conversion reached 70%. Upon increasing the temperature,
the monomer conversion decreased and remained constant at
around 60% (see Figure S5 in the Supporting Information).
The molecular weights also decreased and became constant
while the molecular weight distributions became broader.
This result also supports the presence of the polymerization–
depolymerization equilibrium by the reversible formation of
the growing radical species.
We then measured the final accessible monomer con-
versions by varying the temperatures (40–1008C) and the
initial monomer concentrations (0.25–1.0m) to obtain the
equilibrium monomer concentration ([M]eq) at each temper-
ature (see Table S1 in the Supporting Information). We
plotted the logarithmic monomer concentrations versus the
reciprocal of the temperatures (Figure 4). The plot showed a
good linear relationship, which indicated the propagation–
depropagation equilibrium for this a-substituted bulky mo-
nomer. The enthalpy (DHp) and entropy (DSp) values for the
polymerizations of TrMAwere calculated from this linear plot
(ln[M]eq = DHp/RTÀDSp/R), which yielded values of DHp =
À60.4 kJmolÀ1 and DSp = À160.5 JmolÀ1 KÀ1.[14] The enthalpy
value is almost the same as those of other methacrylate
monomers (DHp(MMA) = À56 kJmolÀ1, DHp(ethyl metha-
crylate) = À60 kJmolÀ1) while the entropy value is higher
(DSp(MMA) = À118 JmolÀ1 KÀ1, DSp(ethyl methacrylate) =
À124 JmolÀ1 KÀ1), probably because of the rigid main-chain
Figure 5. Stereoblock poly(MMA) obtained from the RAFT block
copolymerization of MMA and TrMA in toluene at 608C. For poly-
(MMA) macroinitiator: [MMA]0 =7.0m, [AIBN]0 =5.0 mm,
[CDB]0 =35 mm; for poly(MMA-b-TrMA): [TrMA]0 =1.0m,
[AIBN]0 =6.0 mm, [PMMA-CDB]0 =12 mm.
tion, we converted the trityl substituent into a methyl group
and obtained the homopoly(MMA) with narrow MWDs. The
cumulative and instantaneous mm contents versus the
normalized chain length are also shown in Figure 5. The
isotacticity was abruptly changed at the blocking point, after
which it gradually increased along the chain. The differential
spectrum of these polymers also indicates the formation of the
isotactic-rich segments in the second polymerization (cumu-
lative mm/mr/rr= 61:26:13; see Figure S6 in the Supporting
Information). These show the formation of the stereoblock
PMMA[16] with syndiotactic and isotactic-stereogradient seg-
ments.
In conclusion, we have demonstrated the formation of
stereogradient polymers by RAFT polymerization of a bulky
methacrylate monomer, in which the isospecificity gradually
increased with a decrease in the monomer concentrations.
This is caused by the propagation–depropagation equilibrium,
which can convert a less stable growing polymer terminal with
the r conformation into the more stable m form especially at a
lower monomer concentration. Further studies are now
directed to the controlled/living radical copolymerization of
bulky methacrylates with other monomers for the synthesis of
novel stereocontrolled polymers.[17]
Received: October 22, 2008
Revised: January 10, 2009
Figure 4. Dependence of equilibrium monomer concentration ([M]eq)
on temperature in the RAFT polymerizations of TrMA in toluene.
Published online: February 4, 2009
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Keywords: block copolymers · polymerization · RAFT ·
stereogradient polymers · tacticity control
.
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