Stereospecific radical polymerization of N-triphenylmethylmethacrylamides leading to highly isotactic helical polymers

Article abstract of DOI:10.1021/ja035871y

N-Triphenylmethylmethacrylamide (TrMAM) and N-[di(4-butylphenyl)phenylmethyl]methacrylamide (DBuTrMAM) were polymerized using radical initiators. The polymer PTrMAM obtained from TrMAM was insoluble in the common solvents, but was dissolved in sulfuric acid to quantitatively give polymethacrylamide. The ¹H NMR spectrum of PTrMAM in sulfuric acid-d₂ indicated that the derived polymethacrylamide was nearly 100% isotactic. On the other hand, DBuTrMAM yielded the polymer PDBuTrMAM, which was soluble in THF and chloroform and whose NMR pattern in sulfuric acid-d₂ was similar to that of PTrMAM. The polymerization of TrMAM and DBuTrMAM in an optically active solvent, (-)- or (+)-menthol, afforded the optically active polymers having a prevailing one-handed helical structure. The monomers were not solvolized in a methanol-chloroform mixture. TrMAM was not polymerized by anionic initiators. Copyright

Full text of DOI:10.1021/ja035871y

Published on Web 09/17/2003
Stereospecific Radical Polymerization of N-Triphenylmethylmethacrylamides
Leading to Highly Isotactic Helical Polymers
Naohiro Hoshikawa, Yuji Hotta, and Yoshio Okamoto*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity,
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received April 30, 2003; E-mail: okamoto@apchem.nagoya-u.ac.jp
Many stereoregular polymers are known to have a helical
conformation in the solid state, which may be maintained even in
solution if a side chain is very bulky.1 Triarylmethyl methacrylates
including triphenylmethyl methacrylate and 1-phenyldibenzosubryl
methacrylate are known to produce stable one-handed helical
polymers through not only an anionic process2 but also a radical
process under chiral conditions.3 The one-handed helical polymers
exhibit high chiral recognition, which enables the resolution of many
enantiomers when used as chiral stationary phases for HPLC.4
However, the ester bonds of the polymethacrylates are readily
cleaved in methanol, particularly under acidic condition, which
makes it difficult to study and stably use the polymers for a long
time. On the other hand, amide bonds are usually stronger than the
corresponding ester bonds against hydrolysis. N-Triphenylmethyl-
Figure 1. 400 MHz 1H NMR spectra of PMAMs (A, obtained from radical
polymerization of MAM at 60 °C in methanol; B, derived from PTrMAM
(run 2 in Table 1); C, derived from PDBuTrMAM (run 8 in Table 1)) (at
60 °C in D2SO4). The peaks (0.85, 1.2, 2.35 ppm in spectrum C) are due
to the butyl protons on the triphenylmethyl residue of the byproduct during
sulfuric acid-induced decomposition.
methacrylamide (TrMAM) was synthesized and polymerized more
than 20 years ago.5 However, its sterical structure including
stereoregularity has not yet been determined.
assignment for the radically obtained polymethacrylamide has
already been reported,9 and a syndiotactic (rr)-rich polymer is
formed as shown in Figure 1. On the other hand, the polymethacryl-
amide obtained from PTrMAM (run no. 2 in Table 1) shows a
completely different NMR pattern, and only a methyl peak due to
the isotactic (mm) sequence at 1.05 ppm and two sets of doublets
due to the methylene group at 1.60 and 1.95 ppm were clearly
observed, indicating that the original PTrMAM is highly isotactic.
When PDBuTrMAM was dissolved in D2SO4, an analogous
spectrum pattern was observed as shown in Figure 1C. These results
indicate that the radically obtained poly(N-triphenylmethylmeth-
acrylamide)s are nearly 100% isotactic.
The PDBuTrMAMs prepared in (+)- and (-)-menthol were
optically active, and their optical rotation was opposite to that of
menthol. This suggests that the optical rotation is not due to the
incorporation of menthol residue in the polymer. The polymer
obtained at 0 °C showed the highest optical activity. The melting
point of (-)-menthol is 43 °C, but all of the polymerization solution
was homogeneous before polymerization was initiated. As the
polymerization proceed, the solution became heterogeneous except
for run 8 in Table 1. The circular dichroism (CD) spectra of the
optically active PDBuTrMAMs are shown in Figure 2. Split-type
CD peaks were observed at 230 and 245 nm, and their pattern is
different from that for the highly one-handed helical poly-
(triphenylmethyl methacrylate) that has peaks with the same CD
sign around this area.10 The CD intensity of PDBuTrMAM seems
to be lower than that of the one-handed helical poly(triphenylmethyl
methacrylate) with a specific rotation of [R]D +300°. This indicates
that the one-handedness of the PDBuTrMAMs may not be high.
Recently, we found that Lewis acids, such as the rare earth
triflates like Y(OTf)3 and Yb(OTf)3, significantly increase the
isotactic specificity during the radical polymerization of the
acrylamide and methacrylamide derivatives.6-8 This prompted us
to study the polymerization of bulky methacrylamides that may
produce highly isotactic helical polymers. In the present study, we
prepared two N-triphenylmethylmethacrylamides, TrMAM and
N-[di(4-butylphenyl)phenylmethyl]methacrylamide (DBuTrMAM),
and polymerized them using radical initiators under various
conditions, particularly chiral conditions.
The results of the radical polymerization of TrMAM and
DBuTrMAM are summarized in Table 1. In most cases, the
polymers were obtained in good yields. PTrMAM was totally
insoluble in common organic solvents, but PDBuTrMAM was
soluble in chloroform and tetrahydrofuran (THF). Both polymers
were dissolved in concentrated sulfuric acid to quantitatively
produce polymethacrylamide in a short time at room temperature,
which was recovered by precipitation in cold methanol. No polymer
was yielded in the polymerization of TrMAM using n-BuLi in
toluene at -78 °C.
The tacticity of the polymers was able to be estimated from the
1H NMR spectrum of the derived polymethacrylamide in D2SO4
(Figure 1). The NMR spectrum of the polymethacrylamide prepared
by the conventional radical polymerization of methacrylamide in
1
methanol at 60 °C is also shown for comparison. The H NMR
9
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J. AM. CHEM. SOC. 2003, 125, 12380-12381
10.1021/ja035871y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Radical Polymerization of TrMAM and DBuTrMAMa
temp
(°C)
yieldb
(%)
M c
[R]D
[R]365
d
d
n
c
run
monomer
TrMAM
TrMAM
TrMAM
solvent
(×104)
M /M
w n
(deg)
(deg)
1
2
3
THF
THF
THF
-40
0
60
0
81
79
85
61
67
85
89
64
91
90
86
90
73
75
69
73
4
5
6
7
8
9
10
11
12
13
14
15
16
TrMAM
TrMAM
TrMAM
TrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
DBuTrMAM
toluene/(+)-menthole
toluene/(-)-menthole
toluene/(+)-menthole
toluene/(-)-menthole
THF
0
60
60
0
20
20
0
7.4
20.1
13.0
44.0
56.0
13.5
14.7
19.3
16.5
3.7
hexane/THF/(+)-mentholf
hexane/THF/(-)-mentholf
toluene/(+)-mentholg
toluene/(-)-mentholg
toluene/(+)-mentholg
toluene/(-)-mentholg
toluene/(+)-mentholg
toluene/(-)-mentholg
12
14
22
17
-3.5
+3.2
-22
+17
-14
+11
-11
+12
-13
+11
-60
+59
-37
+33
-30
+32
0
20
20
40
40
6.5
8.5
4.4
2.9
a [Monomer]0 ) 0.5 mol/L; [initiator]0 ) 0.02 mol/L; initiator, n-Bu3B/air (run 1), AIBN (runs 2-16, UV irradiation at 0 and 20 °C); time, 24 h.
b MeOH-insoluble part (runs 1-7); hexane-insoluble part (runs 8-16). c Determined by SEC in THF at 40 °C (polystyrene standard). d In THF at room
temperature. e Toluene/menthol ) 2/5 (v/v). f Hexane/THF/menthol ) 1/2/12 (v/v/v). g Toluene/menthol ) 1/100 (v/v).
This indicates that TrMAM is much stronger toward hydrolysis
and the obtained polymer must be more stable when used as a chiral
stationary phase for HPLC with polar alcoholic solvents. This
stability must be very valuable for the introduction of various
functional groups on the phenyl groups. The soluble polymers seem
to be interesting polymer samples that can be used to study the
solution properties of the polymers.
Acknowledgment. This study was supported in part by the 21st
Century COE Program ”Nature-Guided Materials Processing” and
a Grant-in-Aid for Scientific Research (B) No. 14350485 by the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
Figure 2. CD spectra of PDBuTrMAMs (runs 11-16 in Table 1) in THF
at room temperature.
Supporting Information Available: Experimental details for the
monomer synthesis, polymerization, and characterization (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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Figure 3. CD spectra of PTrMAMs (runs 4-7 in Table 1) in Nujol at
room temperature (a, run 5; b, run 7; c, run 6; d, run 4).
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Because PTrMAMs are insoluble in solvents, the CD spectra in
solution cannot be measured, but the spectra were obtained for the
polymer randomly dispersed in Nujol (Figure 3).11 The spectral
pattern is similar to that of the PDBuTrMAM obtained in the same
menthol, and the CD intensity was also high for the polymer
prepared at 0 °C. These results support the formation of the
prevailing one-handed helical PTrMAM in menthol.12
TrMAM was not solvolyzed at all in methanol-chloroform (1:
1) at 35 °C even after 1 week, whereas the half-life of triphenyl-
methyl methacrylate under the same conditions is only 15 min.13
(7) Habaue, S.; Isobe, Y.; Okamoto, Y. Tetrahedron 2002, 58, 8205.
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(12) A possible helical structure is shown in the TOC graphic. The polymer
has a 7/2 helical conformation.
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