Polymer crystal engineering for control of stereochemical structure of polymers: Stereospecific monomer synthesis and stereospecific solid-state polymerization

Article abstract of DOI:10.1080/15421400590958539

We have successfully synthesized diisotactic and disyndiotactic polymers of (Z,Z)- and (E,E)-muconates with various benzyl ester groups. Muconic acid was conveniently converted into its corresponding ester derivatives without EZ isomerization when reacted with benzyl bromides in the presence of potassium carbonate in hexamethylphosphoramide (HMPA) at room temperature. Several monomers undergo photopolymerization in the crystalline state to give stereoregular polymers according to the monomer configuration and the ester substituents. X-ray single crystal structure analysis of the monomer and polymer crystals has revealed the process of the topochemical polymerization. In the monomer crystals with 4-alkoxybenzyl groups as the ester substituents, a columnar structure is formed by the alternate stacking of monomer molecules with the aid of weak hydrogen bonds such as CH/π and CH/O intermodular interactions. The alternate stacking is appropriate for syndiotactic polymerization, being different from the crystal structures of many other ester monomers which provide a diisotactic polymer due to the translational monomer stacking in a column, as is seen in the crystals of the 4-chloro-, 4-bromo-, and 4-nitro-substituted benzyl esters. Weak and flexible intermolecular interactions provide o variety of crystal structures and different type of molecular stacking leading to the different tacticity of the polymers.

Full text of DOI:10.1080/15421400590958539

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Polymer Crystal Engineering
for Control of Stereochemical
Structure of Polymers:
Stereospecific Monomer
Synthesis and Stereospecific
Solid-State Polymerization
Akikazu Matsumoto a & Toshihiro Tanaka a
a Department of Applied Chemistry, Graduate
School of Engineering , Osaka City University, and
PRESTO, Japan Science and Technology Agency
(JST), Sugimoto , Sumiyoshi-ku, Osaka, Japan
Published online: 31 Aug 2006.
To cite this article: Akikazu Matsumoto & Toshihiro Tanaka (2005) Polymer Crystal
Engineering for Control of Stereochemical Structure of Polymers: Stereospecific
Monomer Synthesis and Stereospecific Solid-State Polymerization, Molecular Crystals
and Liquid Crystals, 440:1, 215-222, DOI: 10.1080/15421400590958539
To link to this article: http://dx.doi.org/10.1080/15421400590958539
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Mol. Cryst. Liq. Cryst., Vol. 440, pp. 215–222, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590958539
Polymer Crystal Engineering for Control of
Stereochemical Structure of Polymers: Stereospecific
Monomer Synthesis and Stereospecific Solid-State
Polymerization
Akikazu Matsumoto
Toshihiro Tanaka
Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, and PRESTO, Japan Science and Technology
Agency (JST), Sugimoto, Sumiyoshi-ku, Osaka, Japan
We have successfully synthesized diisotactic and disyndiotactic polymers of
(Z,Z)- and (E,E)-muconates with various benzyl ester groups. Muconic acid was
conveniently converted into its corresponding ester derivatives without EZ isomer-
ization when reacted with benzyl bromides in the presence of potassium carbonate
in hexamethylphosphoramide (HMPA) at room temperature. Several monomers
undergo photopolymerization in the crystalline state to give stereoregular polymers
according to the monomer configuration and the ester substituents. X-ray single
crystal structure analysis of the monomer and polymer crystals has revealed
the process of the topochemical polymerization. In the monomer crystals with
4-alkoxybenzyl groups as the ester substituents, a columnar structure is formed
by the alternate stacking of monomer molecules with the aid of weak hydrogen
bonds such as CH=p and CH=O intermolecular interactions. The alternate stack-
ing is appropriate for syndiotactic polymerization, being different from the crystal
structures of many other ester monomers which provide a diisotactic polymer due
to the translational monomer stacking in a column, as is seen in the crystals of the
4-chloro-, 4-bromo-, and 4-nitro-substituted benzyl esters. Weak and flexible inter-
molecular interactions provide a variety of crystal structures and different type of
molecular stacking leading to the different tacticity of the polymers.
Keywords: crystal engineering; CH=p interaction; controlled radical polymerization;
isotactic polymer; solid-state photoreaction; syndiotactic polymer; tacticity; topochemical
polymerization; weak hydrogen bond; X-ray structure
Address correspondence to Akikazu Matsumoto, Department of Applied Chemistry,
Graduate School of Engineering, Osaka City University, and PRESTO, Japan Science
and Technology Agency (JST), Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan.
E-mail: matsumoto@a-chem.eng.osaka-cu.ac.jp
215

216
A. Matsumoto and T. Tanaka
INTRODUCTION
We have recently investigated polymer structure control and organic
material design based on polymer crystal engineering [1,2]. The struc-
tures and properties of crystalline materials are designed using
pre-organized molecules through various intermolecular interactions
such as hydrogen bonds, p=p, CH=p, CH=O, and halogen interac-
tions [3–5]. In the previous papers [6–9], we have reported the features
and mechanisms of the topochemical polymerization of 1,3-diene
monomers including some ester, ammonium, and amide derivatives
of muconic and sorbic acids. We have already proposed the topo-
chemical polymerization principles for diene monomers on the basis
of the crystallographic data accumulated for various kinds of diene
monomers. The combination of several intermolecular interactions is
˚
useful for the construction of molecular packing appropriate for 5 A
stacking in order to facilitate the topochemical polymerization in the
crystalline state [6]. Furthermore, we have successfully synthesized
a novel disyndiotactic polymer of di(4-alkoxybenzyl) (Z,Z)- and (E,E)-
muconates [10]. To understand the relationship between the crystal
structure and the solid-state reactivity is most important for further
development and a more sophisticated design of new crystalline mate-
rials by polymer crystal engineering. Actually, however, the EZ isom-
erism of muconic derivatives importantly influences the photoreaction
behavior in the crystalline state. Consequently, the stereoselective
synthesis of muconic esters as monomers is necessary for the design
and control of polymer structures. The ZZ isomers of muconic
derivatives readily isomerize to the corresponding EZ and EE isomers,
but the reverse hardly proceeds. A few synthetic routes have been
reported for ZZ muconic derivatives without EZ isomerization [11,12].
In this paper, we report the stereospecific monomer synthesis and
solid-state polymerization of various benzyl muconate derivatives to
reveal the molecular stacking in the crystals as well as the tacticity
of the resulting polymers during topochemical polymerization.
EXPERIMENTAL
Monomer Synthesis
The general procedure for the esterifications using a phase transfer
catalyst is as follows. (Z,Z)-Muconic acid (711 mg, 5.0 mmol), tetra-
butylammonium hydrosulfate (3.73 g, 11 mmol), and potassium hydro-
xide (1.23 g, 22 mmol) were dissolved in 20 mL of 1,2-dichloroethane
and 20 mL of water in a 100-mL flask. 4-Bromobenzyl bromide (2.75 g,

Stereospecific Solid-State Polymerization
217
11 mmol) was added and the mixture stirred at room temperature for
3 days. The reaction mixture was poured into 100 mL of water, and the
crude product was extracted with two portions of 100 mL of chloro-
form. After the chloroform solution was washed with water and then
dried over sodium sulfate, the chloroform was evaporated under
reduced pressure, providing a crude white solid. The solid was sub-
jected to short-path silica gel column chromatography using chloro-
form, providing di(4-bromobenzyl) muconate. Yield 20.1 g (84%),
1
isomer ratio of ZZ:EZ ¼ 82:18 (determined by H NMR spectroscopy).
The general procedure for the esterification with HMPA is as fol-
lows. To (Z,Z)-muconic acid (1.01 g, 7.11 mmol) in 10 mL of HMPA in
a 150-mL flask equipped with a calcium chloride tube, were added
4-bromobenzyl bromide (3.91 g, 15.6 mmol) and potassium carbonate
(1.47 g, 10.6 mmol) and the mixture stirred at room temperature for
1 day. The reaction mixture was poured into 100 mL of water, and the
crude product was washed with methanol to afford di(4-bromobenzyl)
muconate, yield 2.70 g (79%), with an isomer ratio of ZZ:EZ ¼ ꢀ100:0.
The product obtained was further purified by recrystallization. The
EZ-muconates can be prepared from (E,Z)-muconic acid, similarly.
For the synthesis of EE-monomers, the crude products (as a mixture
of isomers in chloroform) were photoirradiated with a high-pressure
mercury lamp at room temperature in the presence of a small amount
of iodide. The ester derivatives other than 4-bromobenzyl esters were
prepared similarly.
Polymerization
The monomer crystals were photoirradiated with a high-pressure mer-
cury lamp (Toshiba SHL-100-2, 100 W, Pyrex filter) at a distance of
10 cm under atmospheric conditions at room temperature. After
irradiation, the polymers were isolated by removal of the unreacted
monomer with chloroform.
X-ray Crystallography
Single crystal X-ray data were collected on a Rigaku R-AXIS RAPID
Imaging Plate diffractometer using Mo-Ka radiation monochromated
by graphite. The structures were solved by a direct method with
the program SIR92 and refined using full-matrix least-squares
procedures. All calculations were performed using the Crystal Struc-
ture crystallographic software package from the Molecular Structure
Corporation.

218
A. Matsumoto and T. Tanaka
RESULTS AND DISCUSSION
Stereospecific Monomer Synthesis
Table 1 summarizes the results for the synthesis of di(4-bromobenzyl)
muconate using (Z,Z)-muconic acid as the starting material. When
muconic esters were prepared by classic methods, it was difficult to
obtain ZZ isomers efficiently without EZ isomerization. For example,
the dehydration of muconic acid and 4-bromobenzyl alcohol in the pres-
ence of a catalytic amount of sulfuric acid under reflux in benzene for 3 h
resulted in ester formation in a low yield (8%) with a molar ratio of
ZZ:EZ ¼ 4:96 (run 6). When the muconic acid chloride was prepared
from (Z,Z)-muconic acid and thionyl chloride by reflux in dichloro-
methane for 1 h and was then allowed to react with 4-bromobenzyl
alcohol in the presence of triethylamine, a mixture of the esters was
isolated in 28% yield (ZZ:EZ:EE ¼ 82:14:4) (run 5). The ZZ isomer
was isolated by silica gel chromatography, but the EZ and EE isomers
can hardly be separated from each other.
The undesirable results of esterification have prompted us to inves-
tigate other methods for suppressing the isomerization. To develop
reactions under milder conditions, we first adopted a quaternary
ammonium carboxylate salt as the phase transfer catalyst (runs
1–3). Another method is the reaction of the carboxylic acid with benzyl
bromides in the presence of bases in a highly polar aprotic solvent such
as hexamethylphosphoramide (HMPA) (run 4).
Muconic acid was dissolved with KOH (4.4 equiv.) and tetrabutyl-
ammonium hydrosulfate (2.2 equiv.) in 1,2-dichloroethane and water,
followed by addition of 2.2 equiv. of 4-bromobenzyl bromide and
TABLE 1 Stereospecific Synthesis of Di(4-bromobenzyl) Muconate Monomer
Run
no.
Yield
(%)
Isomer ratio
(ZZ:EZ:EE)
Reaction conditionsa
1
2
3
4
5
6
84
58
56
79
28
8
30:70:0
30:70:0
82:18:0
ꢀ100:0:0
82:14:4
4:96:0
KOH(4.4 equiv.), PTC, r.t., 3 days
KOH(4.4 equiv.), PTC, reflux, 1 h
K2CO3(2.2 equiv.), PTC, r.t., 3 days
K2CO3(1.5 equiv.), HMPA, r.t., 1 day
SOCl2, CH2Cl2, reflux, 1 h
H2SO4(cat), benzene, reflux, 3 h
aPhase transfer catalyst (PTC) method: (Z,Z)-muconic acid 1 g, 4-bromobenzyl bromide
(2.2 equiv.), tetrabutylammonium hydrosulfate (2.2 equiv.), 1,2-dichloroethane 20 mL,
water 20 mL. Hexamethyl-phosphoramide (HMPA) method: (Z,Z)-muconic acid 1 g,
4-bromobenzyl bromide (2.2 equiv.), potassium carbonate (1.5 equiv.), HMPA 10 mL.

Stereospecific Solid-State Polymerization
219
TABLE 2 Stereospecific Polymerization of Substituted Benzyl Muconates in
the Solid State
Run no.
Configuration
Ester groups (R)
Stacking structure
Photo-product
1
2
3
4
5
ZZ-
ZZ-
ZZ-
ZZ-
2-Methoxybenzyl
3-Methoxybenzyl
4-Methoxybenzyl
4-Ethoxybenzyl
ZZ-
–
–
EE-Isomer
EE-Isomer
s-Polymer
s-Polymer
Translational
Alternate
Alternate
4-Butoxybenzyl
i-Polymer
i-Polymer
i-Polymer
i-Polymer
6
7
8
9
ZZ-
ZZ-
ZZ-
ZZ-
4-Chlorobenzyl
4-Bromobenzyl
4-Nitrobenzyl
4-Methylbenzyl
Translational
Translational
Translational
–
No Reaction
EE-
EE-
EE-
EE-
EE-
EE-
EE-
EE-
10
11
12
13
14
15
16
17
18
2-Methoxybenzyl
3-Methoxybenzyl
4-Methoxybenzyl
4-Ethoxybenzyl
4-Butoxybenzyl
4-Chlorobenzyl
4-Bromobenzyl
4-Nitrobenzyl
–
No Reaction
i-Polymer
s-Polymer
s-Polymer
s-Polymer
No Reaction
No Reaction
No Reaction
No Reaction
Translational
Alternate
Alternate
–
–
–
–
–
EE-
4-Methylbenzyl
SCHEME 1

220
A. Matsumoto and T. Tanaka
FIGURE 1 Intermolecular interaction in the monomer crystals. (a) Alternate
stacking of di(4-methoxybenzyl) (Z,Z)-muconate. (b) Translational stacking of
di(4-chlorobenzyl) (Z,Z)-muconate. (c) Translational stacking of di (4-nitrobenzyl)
(Z,Z)-muconate.
stirring for 3 days at room temperature to afford the ester in an
isolated yield of 84% with an isomer ratio of ZZ:EZ ¼ 30:70. When
K2CO3 was used as the weak base, the isomerization was suppressed;
ZZ:EZ ¼ 82:18. Furthermore, we have found that (Z,Z)-muconic acid
was readily converted into the corresponding ester without EZ
isomerization by the reaction of the potassium muconate with benzyl
halides in HMPA [13,14]. This procedure for the esterification and
the subsequent isolation was very simple and convenient, as is shown
in the experimental section.
Stereospecific Polymerization in the Solid State
The photopolymerization was carried out under UV irradiation with a
high-pressure mercury lamp for 8 h at room temperature. The results

Stereospecific Solid-State Polymerization
221
FIGURE 2 (a) Crystal structure of poly(4-methoxybenzyl muconate). (b)
ORTEP drawing. The benzyl moieties are omitted for clarity. (c) Stereochemical
relationship for racemo-disynidiotactic polymer.
of the reactions are shown in Table 2. Some ZZ monomers provided the
corresponding EE isomers (runs 1, 2). Several other monomers under-
went photopolymerization to give polymers with highly controlled
stereochemical structures, i.e., meso-diisotactic-2,5-trans and
racemo-disyndiotactic-2,5-trans polymers, as shown in Scheme 1.
The X-ray single crystal structure analysis has successfully
revealed the stacking structure of the monomer molecules in the crys-
tals and also the stereochemistry of the polymer chains in the polymer
crystals [7,10]. Figure 1 shows intermolecular interactions for the
alternate and translational molecular stacking of benzyl muconate
monomers bearing the methoxy, chloro, and nitro substituents at
the para position of their benzyl group. For the alternate stacking,
CH=p interaction is observed between the benzene ring and the
methoxy hydrogens along the specific axis, which is the same direction
as that for the formation of polymer chains. The alternate packing of
the monomer molecules results in syndiotactic polymerization upon
photoirradiation. The 4-methoxy and ethoxy substitutions induce
the alternate stacking, irrespective of the monomer configuration
(runs 3, 4, 12, and 13), whereas the 4-butoxybenzyl derivatives of ZZ
muconate resulted in the translational packing leading to the
diisotactic polymerization (run 5), similarly to the 4-chloro, 4-bromo,
and 4-nitrobenzyl (Z,Z)-muconates (runs 6–8). The corresponding EE

222
A. Matsumoto and T. Tanaka
isomers showed no reactions under similar photoirradiation conditions
(runs 14–17). The 2-methoxy derivatives (runs 1, 10) and the ZZ iso-
mer of the 3-methoxy derivative (run 2) as well as the 4-methylbenzyl
derivatives (runs 9, 18) gave no polymers, but the 3-methoxy (E,E)-
muconate polymerized to give a diisotactic polymer under c-radiation
by the appropriate translational monomer stacking in the crystals
(run 11). We have directly confirmed the topochemical polymerization
process by the single crystal structure analysis of both the monomer
and polymer crystals. Figure 2 shows the crystal structure of poly-
(4-methoxybenzyl muconate), from which we can determine the
stereochemical structure of the polymer chain; i.e., racemo-disyndio-
tactic-2,5-trans structure as well as a polymer chain assembly in the
crystals.
Thus, we have demonstrated that weak intermolecular inter-
actions such as CH=p and halogen–halogen interactions are important
in constructing of the stacking structures for polymerization and that
the stacking structure depends on the position and structure of the
substituents in the benzyl groups. Further detailed investigation of
crystal structure analysis, molecular packing, and polymer properties
will be reported elsewhere in the future.
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