Liquid crystalline properties of polyguanidines

Article abstract of DOI:10.1021/ma0493527

Modified polyguanidines were prepared, and their liquid crystalline properties were studied using optical polarizing microscopy and X-ray diffraction. Parameters examined include chirality, the uniformity of the lengths of side chains, and attached side-chain mesogens. If the side chains on the repeat units are identical, then polymer has a more ordered structure in both solution and the solid state. Uniform lengths of the side chains are also important. Poly(N,N′-di-n-hexylguanidine), poly-I, exhibited a lyotropic smectic texture in contrast to a nematic texture of poly(N-(rac)-2-phenylethyl)- N′-methylguanidine) (poly-(rac-II)). Optically pure poly(N-((R)-2- phenylethyl)-2-methylguanidine) (poly-(R-II)) formed a cholesteric texture, whereas the corresponding racemic polyguanidine, poly-(rac-II), formed a nematic texture. Additionally, poly-(R-II) displayed a mesophase at a lower critical concentration than either poly-I or poly-(rac-II), implying that poly-(R-II) is suffer than these two other derivatives. Polyu(N-6-((4′-methoxyphenylazo) phenyl-4-oxy)hexyl-N′-n-hexylguanidine) (poly-IV), one of combined liquid crystalline structures (liquid crystalline backbone plus liquid crystalline side chains), displayed a lyotropic nematic texture presumably due to the strong dipolar-dipolar interaction between the main chain and side chains that folds the appendages parallel to the molecular axis. Poly(N-12-((4′- methoxybiphenyl-4-oxy)dodecyl-N′-n-dodecylguanidine) (poly-V) and poly(N,N′-di-n-dodecylguanidine) (poly-VI) exhibited thermotropic liquid crystalline behavior.

Full text of DOI:10.1021/ma0493527

8286
Macromolecules 2004, 37, 8286-8292
Liquid Crystalline Properties of Polyguanidines
J eon gh a n Kim a n d Br u ce M. Nova k *
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695
Ala n J oh n Wa d d on
Department of Polymer Science & Engineering, University of Massachusetts,
Amherst, Massachusetts 01003
Received April 1, 2004; Revised Manuscript Received August 13, 2004
ABSTRACT: Modified polyguanidines were prepared, and their liquid crystalline properties were studied
using optical polarizing microscopy and X-ray diffraction. Parameters examined include chirality, the
uniformity of the lengths of side chains, and attached side-chain mesogens. If the side chains on the
repeat units are identical, then polymer has a more ordered structure in both solution and the solid
state. Uniform lengths of the side chains are also important. Poly(N,N′-di-n-hexylguanidine), poly-I,
exhibited a lyotropic smectic texture in contrast to a nematic texture of poly(N-(rac)-2-phenylethyl)-N′-
methylguanidine) (poly-(r a c-II)). Optically pure poly(N-((R)-2-phenylethyl)-N′-methylguanidine) (poly-
(R-II)) formed a cholesteric texture, whereas the corresponding racemic polyguanidine, poly-(r a c-II),
formed a nematic texture. Additionally, poly-(R-II) displayed a mesophase at a lower critical concentration
than either poly-I or poly-(r a c-II), implying that poly-(R-II) is stiffer than these two other derivatives.
Poly(N-6-((4′-methoxyphenylazo)phenyl-4-oxy)hexyl-N′-n-hexylguanidine) (poly-IV), one of combined liquid
crystalline structures (liquid crystalline backbone plus liquid crystalline side chains), displayed a lyotropic
nematic texture presumably due to the strong dipolar-dipolar interaction between the main chain and
side chains that folds the appendages parallel to the molecular axis. Poly(N-12-((4′-methoxybiphenyl-4-
oxy)dodecyl-N′-n-dodecylguanidine) (poly-V) and poly(N,N′-di-n-dodecylguanidine) (poly-VI) exhibited
thermotropic liquid crystalline behavior.
chromatography. Polymerizations were conducted in an inert
atmosphere drybox or on vacuum lines.
In tr od u ction
Because of their stiffness, many helical polymers show
lyotropic main-chain liquid crystalline properties since
the helix behaves as a mesogen. Lyotropic mesophases
of polypeptides,1-10 polyisocyanides,11-14 and polyiso-
cyanates15-22 have been identified and reported. During
our efforts to develop new helical materials, we found
that carbodiimides can be polymerized in a living
fashion using a variety of transition metal com-
plexes.23,24 The polycarbodiimides (or polyguanidines)
are rigid chain helical polymers with high inversion
barriers and long persistence lengths. In keeping with
these intramolecular characteristics, we found that
these polymers display a range of liquid crystalline
phases. We recently reported on the lyotropic smectic
liquid crystalline properties of poly(N,N′-di-n-hexyl-
guanidine), poly-I.25 Poly-I displays a lyotropic layered
structure a over broad concentration range (g16% (w/
w)) in toluene at ∼45 °C.26 The smectic layering arises
from the dense, uniform corona formed by the hexyl side
chains, which allows the chains to align in a uniform
fashion.25 In this study, polyguanidines with various
stereochemical and molecular architectural modifica-
tions were prepared, and their lyotropic liquid crystal-
line properties were examined. Most polyguanidines are
limited to lyotropic studies because these polymers
selectively decompose back to their monomers at tem-
peratures below any melt transitions. Herein, we report
two thermotropic exceptions to this generalization.
Syn th esis of Mon om er s. N-Meth yl-N′-(R)-(+)-(r)-m eth -
ylben zylth iou r ea . The same procedure for the preparation
of N,N′-di-n-hexylurea was employed as reported previously.27
N-Meth yl-N′-((R)-(+)-(r)-m eth ylben zyl)ca r bod iim id e,
R-II. A dry 1 L round-bottom flask was charged with 500 mL
of acetone, 10.0 g of N-methyl-N′-((R)-(+)-(R)-methylbenzyl)-
thiourea (51.5 mmol), and a magnetic stir bar. The resulting
solution was heated to reflux. 44.6 g of mercury oxide (0.206
mol, 4 equiv) was added in five equivalent portions to the hot
N-methyl-N′-(R)-(+)-(R)-methylbenzylthiourea solution over 1
h, and the resulting solution was heated at refluxed for an
additional hour. The solution turned to dark green, and a
solution was filtered through a glass filter. The acetone was
distilled off to give light yellow oil. The oil was washed with
500 mL of n-hexane, and the residue was removed under
vacuum. The resulting oil was distilled under reduced pressure
to give N-methyl-N′-((R)-(+)-(R)-methylbenzyl)carbodiimide as
a clear, colorless liquid. Yield: 4.90 g (51%); bp ) 55 °C (0.1
1
Torr). H NMR (300 MHz, CDCl3): δ (ppm) 1.48 (d, 3H, CH3),
2.89 (s, 3H, CH3), 4.62 (q, 1H, CH), 7.3 (m, 5H, Ar). IR (neat):
3055 (m), 2908 (s), 2935 (s), 2124 (vs), 1492 (s), 1452 (s), 1421
(s), 1372 cm-1 (s). 13C NMR (CDCl3): 173.72, 152.37, 132.11,
131.89, 131.47, 130.90, 130.27, 129.98, 37.34, 28.73. Anal.
Calcd for C10H12N2: C, 74.95; H, 7.55; N, 17.50. Found: C, 75.21;
H, 7.43; N, 17.36.
N,N′-Di-n -d od ecylu r ea . A 250 mL round-bottom flask was
charged with 8.40 g of n-dodecylamine (0.110 mol), a magnetic
stir bar, and 150 mL of reagent grade dry toluene. The
resulting solution was cooled to 0 °C by the use of an ice bath.
A dry pressure equalizing additional funnel was charged with
50 mL of toluene and 8.40 g of 1,1′-carbonyldiimidazole (51.8
mmol) and was then placed on the 250 mL round flask. The
whole apparatus was placed under a nitrogen atmosphere. The
1,1′-carbonyldiimidazole solution was added dropwise over the
course of 30 min, and the resulting solution was refluxed for
3 h. The solution was cooled to room temperature. The white
solid N,N′-di-n-dodecylurea was obtained by removing the
Exp er im en t
Gen er a l. All glassware was either flame-dried or oven-dried
overnight. Unless otherwise noted, all solvents were reagent
grade and used without further purification except for drying
either over 3 Å molecular sieves or by inert atmosphere
10.1021/ma0493527 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/25/2004

Macromolecules, Vol. 37, No. 22, 2004
Polyguanidines 8287
toluene using rotary evaporator and recrystallized in ethanol.
Yield: 20.3 g (95%). This product was carried forward without
further purification.
chloroform was used to ensure complete transfer of n-hexyl-
amine. The resultant solution was stirred for 1 h. The white
solid, N-6-bromohexyl-N′-n-hexylurea, was obtained by remov-
ing the chloroform using rotary evaporator and then recrystal-
lized in ethanol. Yield: 12.3 g (97%). 1H NMR (300 MHz,
CDCl3): (ppm) 0.89 (t, 3H, CH3), 1.2-1.6 (m, 14H, CH2), 1.85
(t, 2H, CH2), 3.15 (m, 4H, CH2), 3.40 (t, 2H, CH2), 4.22 (s, br,
2H, NH). IR (KBr pellet): 3420 (s, br), 2955 (w), 2930 (m),
2869 (m), 1660 (s), 1630 cm-1 (s).
N -6-(4′-Me t h o x y b ip h e n y l-4-o x y )h e x y l-N ′-n -h e x y l-
u r ea . To a 1 L round-bottom flask was added 2.50 g of N-6-
bromohexyl-N′-n-hexylurea (8.14 mmol, 1 equiv) and 100 mL
of ethanol as a solvent. The whole apparatus was placed under
a nitrogen atmosphere. To a 250 mL round-bottom flask was
added 1.63 g of 4′-methoxybiphenyl-4-ol (8.14 mmol, 1 equiv),29
0.460 g of potassium hydroxide (8.20 mmol, 0.99 equiv), 40
mL of ethanol, and 10 mL of water. The 4′-methoxybiphenyl-
4-ol solution was added to the N-6-bromohexyl-N′-n-hexylurea
solution. The resulting solution was heated to reflux overnight.
The solution was filtered to give white powder. The white
powder was washed with fresh hot ethanol. Yield: 1.63 g
(50%). 1H NMR (300 MHz, CDCl3): δ (ppm) 0.89 (t, 3H, CH3),
1.2-1.6 (m, 14H, CH2), 1.85 (t, 2H, CH2), 3.20 (m, 4H, CH2),
3.85 (s, 3H, CH3), 4.01 (t, 2H, CH2), 4.18 (d, br, 2H, NH), 6.95
(m, 4H, Ar), 7.50 m, 4H, Ar). IR (NaCl pellet): 3320 (s, br),
3300 (s, br), 3050 (w), 2955 (w), 2930 (m), 2869 (m), 1660 (s),
1630 cm-1 (s).
N-6-(4′-Meth oxybip h en yl-4-oxy)h exyl-N′-n -h exylca r bo-
d iim id e, III. The same procedure for the preparation of N,N′-
di-n-hexylcarbodiimide was employed.27 The quantities of
reagents used were 1.92 g of N-6-(4′-methoxybiphenyl-4-oxy)-
hexyl-N′-n-hexylurea (4.50 mmol, 1 equiv), 1.48 g of triphenyl-
phosphine (5.64 mmol, 1.25 equiv), 0.290 mL of bromine (0.900
g, 5.64 mmol, 1.25 equiv), 1.97 mL of triethylamine (1.43 g,
11.3 mmol, 2.5 equiv), and 55 mL of dichloromethane. The
crude product from evaporation of n-pentane is recrystallized
in warm acetonitrile to give white powder. Yield: 1.1 g (60%).
1H NMR (300 MHz, CDCl3): δ (ppm) 0.89 (t, 3H, CH3), 1.2-
1.6 (m, 14H, CH2), 1.85 (t, 2H, CH2), 3.25 (m, 4H, CH2), 3.85
(s, 3H, CH3), 4.01 (t, 2H, CH2), 6.95 (m, 4H, Ar), 7.40 (m, 4H,
Ar). 13C NMR (CDCl3): δ (ppm) 173.99, 168.24, 163.35, 133.43,
133.26, 133.03, 132.94, 132.56, 131.23, 121.68, 121.23, 121.02,
120.85, 87.69, 60.70, 57.64, 57.55, 37.84, 37.24, 37.18, 37.01,
31.80, 31.52, 30.90, 30.24, 18.32. IR (neat): 3067 (w), 3037 (w),
2932 (vs), 2865 (vs), 2151 (vs), 2127 (vs), 1608 (w), 1503 (s),
1467 (m), 1338 (m), 1276 (s), 1252 cm-1 (s). Anal. Calcd for
N,N′-Di-n -dodecylcar bodiim ide, VI. A dry 500 mL round-
bottom flask was charged with 300 mL of dichloromethane,
15.7 g of triphenylphosphine (59.9 mmol, 25% excess), and a
magnetic stir bar. A dry pressure equalizing addition funnel
was charged with 30 mL of dichloromethane and 3.09 mL of
bromine (59.9 mmol, 25% excess) and was then placed on the
500 mL round-bottom flask. The whole apparatus was placed
under a nitrogen atmosphere, and the triphenylphosphine
solution was cooled to 0 °C. The bromine solution was added
dropwise over the course of 30 min, and the resulting solution
was allowed to stir for an additional 10 min. To the resulting
suspension of dibromotriphenylphosphorane, 16.7 mL of tri-
ethylamine (0.120 mol, 26% excess) was added. In a similar
fashion, 18.5 g of N,N′-di-n-dodecylurea (47.8 mmol) was added
in five equivalent portions to the 0 °C suspension over the next
hour. One hour after the last addition of the urea, 100 mL of
water was poured to the round-bottom flask in order to extract
the triethylammonium hydrobromide, and organic and aque-
ous phases were separated using a separatory funnel. The
dichloromethane was distilled off using a rotary evaporator.
Addition of 300 mL of n-hexane to the viscous, dark brown oil
served to remove the triphenylphosphine oxide. The n-hexane
was distilled off to give light yellow oil. The remaining
triphenylphosphine oxide was removed in acetonitrile. N,N′-
Di-n-dodecylcarbodiimide was obtained through vacuum distil-
1
lation. Yield: 13.8 g (76%); bp ) 122 °C (0.1 Torr). H NMR
(300 MHz, CDCl3): δ (ppm) 0.90 (t, 6H, CH3), 1.35 (m, 36H,
CH2), 1.55 (m, 4H, CH2), 3.11 (t, 4H, CH2). 13C NMR (CDCl3):
171.89, 53.26, 36.88, 36.73, 36.05, 35.96, 35.78, 35. 67, 34.99,
34.04, 32. 89, 32.54, 31.81. IR (neat): 2923 (vs), 2852 (vs),
2129.6 (vs), 1466.7 (m), 1440.3 cm-1 (m). Anal. Calcd for
C25H50N2: C, 79.29; H, 13.31; N, 7.40. Found: C, 79.02; H, 13.44;
N, 7.54.
N-Meth yl-N′-((()-(r)-m eth ylben zyl)th iou r ea . The same
procedure for the preparation of N-methyl-N′-((R)-(+)-(R)-
methylbenzyl)thiourea was employed.
N-Meth yl-N′-((()-(r)-m eth ylben zyl)ca r bod iim id e, r a c-
II. The same procedure for the preparation of N-methyl-N′-
((R)-(+)-(R)-methylbenzyl)carbodiimide was employed.
7-Br om oh ep ta n oyl Ch lor id e. To a 500 mL Schlenk flask
was added 125 g of 7-bromoheptanoic acid (0.600 mol, 1
equiv)28 and 142 g of thionyl chloride (0.231 L, 1.19 mol, 2
equiv). The mixture was heated to 50 °C and then stirred for
4 h. The excess thionyl chloride was removed using a rotary
evaporator to give yellow liquid. 7-bromoheptanoyl chloride
was obtained under reduced pressure. Yield: 121 g (89%); bp
C
26H36N2O2: C, 76.43; H, 8.88; N, 6.86. Found: C, 76.22; H,
8.87; N, 6.67.
4-(4-Meth oxyp h en yla zo)p h en ol. To a 100 mL round-
bottom flask was added 10.0 g of p-anisidine (81.2 mmol, 1
equiv) and 20.0 mL of water. 6.70 mL of 37% hydrochloric acid
(81.2 mmol, 1 equiv) was added to the p-anisidine solution.
The solution was cooled below 10 °C. To a 100 mL beaker was
added 5.60 g of sodium nitrite (81.2 mmol, 1 equiv) and 50.0
mL of water. The sodium nitrite solution was cooled below 10
°C. The sodium nitrite solution was added dropwise to the
cooled p-anisidine solution. After allowing the solution to stir
for 2 h at 10 °C, the solution was added dropwise to the
solution of 7.64 g of phenol, 3.25 g of sodium hydroxide, 9.00
g of sodium carbonate, and 100 mL of water at 10 °C. 1 h after
the last addition of diazonium salt solution, the solution was
neutralized using 37% of hydrochloric acid. The suspended
solid was filtered to give dark brown solid. The crude product
was recrystallized in mixed solvent of methanol and water
(50%) to give the brown solid. Yield: 14.76 g (80%); mp ) 149.8
°C. 1H NMR (300 MHz, CDCl3): δ (ppm) 3.95 (s, 3H, CH3),
6.84 (d, 2H, Ar), 6.92 (d, 2H, Ar), 7.75-7.79 (m, 4H, Ar). 13C
NMR (CDCl3): δ (ppm) 170.24, 167.31, 152.43, 151.78, 133.98,
132.74, 130.29, 128.33, 120.66, 120.01, 119.19, 118.72, 60.80.
N-6-(4-(4-Met h oxyp h en yla zo)p h en yloxy)h exyl-N′-n -
h exylu r ea . The same procedure for the preparation of N-6-
(4′-methoxybiphenyl-4-oxy)-hexyl-N′-n-hexylurea was em-
ployed. The quantities of reagents used were 10.0 g of 4-(4-
methoxyphenylazo)phenol (44.0 mmol, 1 equiv), 13.5 g of N-6-
bromohexyl-N′-n-hexylurea (44.0 mmol, 1 equiv), 2.47 g of
1
) 60 °C (0.1 Torr). H NMR (300 MHz, CDCl3): δ (ppm) 1.3-
1.5 (m, 4H, CH2), 1.70-1.80 (m, 2H, CH2), 1.80-1.90 (m, 2H,
CH2), 2.9 (t, 2H, CH2), 3.40 (t, 2H, CH2). IR (neat): 2938 (s),
2861 (s), 1799 (vs), 1459 (w), 1406 (w), 953 (w) cm-1
.
6-Br om oh exyl Isocya n a te. To a 500 mL Schlenk flask
equipped with a condenser and nitrogen inlet was added 121
g of 7-bromoheptanoyl chloride (0.530 mol, 1 equiv) followed
by 100 mL of toluene. 69.5 g of azidotrimethylsilane (42.6 mL,
0.580 mol, 1.1 equiv) was added to the mixture and stirred at
room temperature for 30 min. The temperature was increased
to 50 °C for 30 min and then refluxed for 20 h. The yellow
liquid was distilled under reduced pressure to give colorless
1
liquid. Yield: 60.1 g (55%); bp ) 60 °C (0.15 Torr). H NMR
(300 MHz, CDCl3): (ppm) 1.4-1.19 (m, 8H, CH2), 3.3 (t, 2H,
CH2), 3.45 (t, 2H, CH2). 13C NMR (CDCl3): 133.35, 51.13, 45.36,
37.78, 33.38, 32.23, 31.99. IR (neat): 2928 (s), 2869 (m), 2257
cm-1 (vs). Anal. Calcd for C26H36N2O2: C, 40.80; H, 5.87; Br,
38.77; N, 6.80; O, 7.76. Found: C, 41.19; H, 5.85; Br, 38.42; N,
6.55; O, 7.99.
N-6-Br om oh exyl-N′-n -h exylu r ea .
A 250 mL round-
bottom flask was charged with 8.60 g of 6-bromohexyl iso-
cyanate (41.7 mmol), a magnetic stir bar, and 50 mL of reagent
grade dry chloroform. The resulting solution was cooled to 0
°C by the use of an ice bath. Over the next 30 min, 5.50 mL of
n-hexylamine (4.20 g, 41.3 mmol, 0.99 equiv) was added
dropwise to the cooled solution. An additional 20 mL of

8288 Kim et al.
Macromolecules, Vol. 37, No. 22, 2004
potassium hydroxide (44.0 mmol, 1 equiv), 500 mL of ethanol,
and 10.0 mL of water. Yellow solid. Yield: 11.20 g (57%). This
product was used for the next reaction without further
purification.
N-6-(4-(4-Meth oxyph en ylazo)ph en yloxy)h exyl-N′-n -h ex-
ylca r bod iim id e, IV. The same procedure for the preparation
of N,N′-dibenzylurea was employed. The quantities of reagents
used were 5.00 g of N-6-(4-(4-methoxyphenylazo)phenyloxy)-
hexyl-N′-n-hexylurea (11.2 mol, 1 equiv), 3.67 g of trihpenyl-
phosphine (14.0 mmol, 1.25 equiv), 0.720 mL of bromine (2.24
g, 5.64 mmol, 1.25 equiv), 3.90 mL of triethylamine (2.83 g,
28.0 mmol, 2.5 equiv), and 110 mL of dichloromethane. The
crude product from evaporation of n-pentane was recrystallized
in diethyl ether to give yellow powder. Yield: 3.5 g (70%); mp
1
) 63.1 °C. H NMR (400 MHz, CDCl3): δ (ppm) 0.89 (t, 3H,
CH3), 1.20-1.65 (m, 14H, CH2), 1.85 (m, 2H, CH2), 3.25 (m,
4H, CH2), 3.85 (s, 3H, CH3), 4.05 (t, 2H, CH2), 6.95 (m, 4H,
Ar), 7.40 (m, 4H, Ar). IR (neat): 3099 (w), 3057 (w), 2938 (s),
2861 (s), 2151 (vs), 2133 (vs), 1603 (m), 1579 (m), 15-1 (m),
1465 (m), 1251 (s), 1149 (s), 1030 (m), 845 cm-1 (s). Anal. Calcd
for C26H36N4O2: C, 71.53; H, 8.31; N, 12.86. Found: C, 71.71;
H, 8.46; N, 13.00.
F igu r e 1. Structures of the liquid crystalline polyguanidines
used in this study.
13-Br om otr id eca n oic Acid .30 To a 1 L round-bottom flask
was added 378 mL of ethanol. 17.5 g (0.760 mol) of chopped
sodium metal was slowly added to the flask (sodium metal was
washed with n-hexane before use). 243 g of diethylmalonate
(1.52 mol, 2 equiv) was added to the sodium ethoxide solution.
The resulting solution was added dropwise to the solution of
96.0 g of 11-bromo-1-undecanol (0.380 mol, 0.5 equiv) in 100
mL of ethanol at 25 °C. The resulting solution was heated to
reflux for 5 h. The resultant solution was poured into 1 L of
ice water and extracted with diethyl ether. The extract was
dried over sodium sulfate, and the diethyl ether was distilled
off using a rotary evaporator. The solution of 102 g of
potassium hydroxide in 300 mL of water was added to the
residue and then heated reflux for 4 h. The resultant solution
was poured into 200 mL of ice water, acidified with concen-
trated hydrochloric acid, and then extracted with diethyl ether.
The diethyl ether was distilled off using a rotary evaporator.
800 mL of hydrobromic acid in glacial acetic acid was added
to the residue. The resulting solution was allowed to stir at
room temperature for 15 h and then heated to 100 °C for 8 h.
The solvent was distilled off, and then the residue was heated
to 160 °C until CO2 was completely escaped. A dry 1 L round-
bottom flask was charged with the liquid, 500 mL of glacial
acetic acid, 100 mL of sulfuric acid, and a magnetic stir bar.
The resulting solution was refluxed for 1 day, with a warm
condenser allowing ethyl acetate to escape. The residue was
cooled to room temperature to give the crude solid product. It
was recrystallized in petroleum ether to give white solid. Yield:
90.7 g (81%); mp ) 53 °C. 1H NMR (400 MHz, CDCl3): δ (ppm)
1.3-1.9 (m, 20H, CH2), 2.30 (t, 2H, CH2), 3.33 (t, 2H, CH2).
13C NMR (400 MHz, CDCl3): δ (ppm) 167.22, 36.24, 33.67,
32.13, 30.15, 30.12, 30.10, 30.08, 30.01, 29.80, 29.26, 28.67,
27.33.
N-12-(4′-Meth oxybip h en yl-4-oxy)d od ecyl-N′-n -d od ecyl-
ca r bod iim id e, V. The same procedure for the preparation of
N-6-(4′-methoxybiphenyl-4-oxy)-hexyl-N′-n-hexylcarbodi-
imide was employed. The quantities of reagents used were
3.00 g of N-12-(4′-methoxybiphenyl-4-oxy)dodecyl-N′-n-do-
decylurea (5.78 mmol, 1 equiv), 3.79 g of triphenylphosphine
(14.4 mmol, 2.5 equiv), 1.46 g of triethylamine (14.4 mmol,
2.5 equiv), and 2.30 g of Br2. Yield: 1.42 g (49%). 1H NMR (300
MHz, CDCl3): δ (ppm) 0.89 (t, 3H, CH3), 1.2-1.6 (m, 40H,
CH2), 1.85 (t, 2H, CH2), 3.25 (m, 4H, CH2), 3.85 (s, 3H, CH3),
4.01 (t, 2H, CH2), 6.95 (m, 4H, Ar), 7.40 (m, 4H, Ar). 13C NMR
(CDCl3): δ (ppm) 173.56, 167.35, 164.21, 132.78, 132.33,
132.21, 132.02, 131.88, 131.57, 122.79, 122.12, 121.90, 120.79,
90.24, 61.34, 58.13, 57.83, 37.84, 37.24, 37.18, 37.01, 32.43,
32. 21, 32.04, 31.80, 31.52, 31.22, 31.02, 30.90, 30.78, 30.53,
30.39, 30.24, 30.11, 29.85, 29.58, 29.27, 18.32. IR (neat): 3067
(w), 3037 (w), 2932 (vs), 2865 (vs), 2151 (vs), 2127 (vs), 1608
(w), 1503 (s), 1467 (m), 1338 (m), 1276 (s), 1252 cm-1 (s). Anal.
Calcd for C38H60N2O2: C, 79.17; H, 10.42; N, 4.17. Found: C,
78.73; H, 10.21; N, 4.10.
Gen er a l Meth od for th e P olym er iza tion of Ca r bod i-
im id es. The following description serves as a general method
for the polymerization of carbodiimides having mesogenic side
chains. Prior to sealing ampules, all manipulations were
performed in a glovebox. The highest concentrated solution of
N-6-(4′-m et h oxybiph en yl-4-oxy)h exyl-N′-n -h exylca r bodi-
imide was prepared in chloroform because the yield and the
conversion depend on the concentration of monomer solution
(1.46 g of the monomer dissolved in 8.74 g of chloroform (molar
concentration is 0.609 mol/L)). The prepared solution was
added via syringe to an ampule (10 mL) containing a stirbar.
(Note: these ampules were made from Pasteur pipets (146
mm). The wide end of the pipet was cut just below the
indentation, and a stirbar was inserted into the pipet. The wide
end was then sealed with a glass blower’s torch.) 0.176 g of
the titanium catalyst (0.776 mmol) was added to a volumetric
flask (5 mL) and dissolved in 1 mL of chloroform. 0.100 mL of
the catalyst solution was added to the ampule via syringe and
quickly stirred to ensure proper mixing. The ampule was then
plugged with grease, removed from the glovebox, and cooled
in liquid nitrogen. After the contents froze, the ampule was
sealed with a glass blower’s torch and allowed to warm to room
temperature. The samples were then stirred until gelation.
After the samples have gelled, methanol was added to the
ampules. The color of the solution disappeared, and then white
powder was precipitated. The precipitated polymer was filtered
and then dissolved in chloroform. The polymer was precipi-
tated into stirring methanol. After the precipitation is com-
plete, the polymer is filtered through a Bu¨chner funnel and
dried under reduced pressure. In IR, the characteristic peak
of carbodiimide monomer (stretching of -NdCdN-) was
observed. The unreacted monomer was completely extracted
out in n-hexane, using Soxhlet extraction.
Resu lts a n d Discu ssion
All of the polyguanidines used in this study were
prepared by polymerizing the corresponding carbodi-
imide monomers using dichloro(η5-cyclopentadienyl)-
(dimethylamido)titanium as a catalyst (eq 1).23
The chemical structures of the polyguanidine prepared
are shown in Figure 1.
Most of the carbodiimides were prepared following
literature methods. Monomers III, IV, and V and their

Macromolecules, Vol. 37, No. 22, 2004
Polyguanidines 8289
Sch em e 1a
a
(a) SOCl2, toluene; (b) azidotrimethylsilane, toluene; (c) n-hexyl (III, IV) or n-dodecylamine (V), CHCl3; (d) corresponding
phenol, KOH/ethanol; (e) PPh3, triethylamine, Br2, CH2Cl2.
polymers were synthesized following the multistep route
shown in Scheme 1. The polymerization of carbodiimides
by the titanium half-metallocenes are living, reversible
polymerizations with large, negative ∆Spolym values (e.g.,
-33 to -38 cal mol-1 K-1) and comparatively small
∆Hpolym (-13.5 to -14.1 kcal mol-1). Hence, they have
relatively low ceiling temperatures, Tc (e.g, I, 80 °C; r a c-
II, 153 °C; and R-II, 157 °C).31 Therefore, carbodiimide
polymerizations should be carried out at room temper-
ature and at the highest possible monomer concentra-
tions. All polymerizations proceeded smoothly, and high
yields were obtained in all cases. Liquid monomers I,
r a c-II, R-II, and V were polymerized in bulk, whereas
F igu r e 2. Optical polarizing microscopic image of a 35% (w/
w) solution of poly-(r a c-II) in toluene at room temperature
(500×).
the solid monomers with mesogenic side chains III, IV,
and V were polymerized in concentrated chloroform
solutions. The chemical structures of all the monomers
1
and polymers were confirmed by H NMR, 13C NMR,
IR spectroscopy, and elemental analysis. All of the
resulting polymers except for poly-III showed high
solubilities in common organic solvents are were there-
fore deemed suitable for this study.
Lyotropic main-chain liquid crystallinity of poly-(r a c-
II) and poly-(R-II). From viscosity measurements it was
determined that poly-(r a c-II) displays lyotropic liquid
crystalline phases above a critical concentration of about
20%. The optical polarizing microscopic image of poly-
(r a c-II) obtained from a 35% (w/w) solution in toluene
is shown in Figure 2.
Poly-(r a c-II) exhibited a threaded schlieren texture,
indicating a nematic mesophase. The formation of the
nematic mesophase was confirmed by X-ray diffraction
studies (Figure 3).
In the X-ray diffractogram, a broad peak over a wide
range of angles was observed, which is characteristic
of a nematic mesophase. In contrast, another achiral
polymer, poly-I, showed a lyotropic smectic mesophase
under similar conditions.25 The decrease in order be-
tween poly-I and poly-(r a c-II) is attributed to the
uniformity of the side chains (di-n-hexyl) in the former
and the lack thereof in the latter (methyl and 2-phen-
F igu r e 3. X-ray diffractogram of a 30% (w/w) solution of poly-
(r a c-II) in toluene at room temperature.
ylethyl). The uniform side chains of poly-I form a
relatively impenetrable corona that holds adjacent
polymer chains at a uniform distance apart. The dis-
similar side chains of poly-(r a c-II) are unable to provide
the same effective barrier between backbones, and
lateral disorder arises.
Fixing the chirality of the side chains has a profound
effect on the properties of the polyguanidines, and this
includes their liquid crystalline properties. Homochiral

8290 Kim et al.
Macromolecules, Vol. 37, No. 22, 2004
F igu r e 4. Optical polarizing microscopic image of the 12%
solution (w/w) of poly-(R-II) in chloroform at room temperature
(500×).
F igu r e 5. X-ray diffractograms of poly-III and poly-IV in solid
state at room temperature.
side chains impart a diastereometic relationship be-
tween the right- and left-handed helical conformations,
and if the helix inversion barrier is low enough, then
single-handed helicies will be formed.32,33 The optical
polarizing microscopic image of poly-(R-II) shown in
Figure 4 was obtained from the 12.5% (w/w) solution in
chloroform.
This chiral polymer showed a prototypical cholesteric
mesophase, which results when the helical backbones
adopt a single-handed screw sense.34 It is well-known
that liquid crystalline mesophase formation is depend-
ent on the stiffness of the backbone. Compared to poly-
(r a c-II), poly-(R-II) forms mesophases at much lower
concentrations at identical temperature. This implies
that optically pure polymer is stiffer than the racemic
polymer, and this is consistent with our light scattering
and thermal analysis experiments.26
Lyotr op ic Liqu id Cr ysta llin ity of th e Com bin ed -
Typ e P olygu a n id in es, P oly-III a n d P oly-IV. One
might think of the chiral helical backbone as a scaffold
to build polymers with useful properties for use in fields
such as nonlinear optics. This particular application
would require appending polarizable mesogens as side
chains. As these side chains could impart their own
influence on the intermolecular alignment of these
helices, we decided to explore the properties of polyguani-
dines with mesogenic side chains. The ease of prepara-
tion of carbodiimides with various side chains has
allowed us to examine systems in which two distinct
mesogens, showing different meosophase propensities,
are combined within the same polymer. In this case, the
stiff polyguanidine backbone with dissimilar size side
chains and its preference for nematic phases was
combined with small mesogens that favor smectic phase
formations.
F igu r e 6. Optical polarizing microscopic image from a 21.4%
(w/w) solution of poly-IV in 1,1,2,2-tetrachloroethane at 65 °C
(200×).
poly-IV, indicating poly-III has more ordered structure
of side chains than poly-IV in solid state. It should be
noted that both of these polymers show smaller inter-
chain spacing than that observed for poly-I with its two
n-hexyl side chains. Even though half the side chains
are longer in poly-III and poly-IV, the side-chain
mesogens apparently accommodate a larger degree of
interpenetration than the pure alkyl chains. Unfortu-
nately, the lyotropic liquid crystalline properties of poly-
III could not be investigated because concentrations
exceeding the critical concentration of the polymer in
solution could not be reached due to its poor solubility.
The azo-mesogen appendage proved to be a better
candidate, and the optical polarizing microscopic image
of poly-IV is shown in Figure 6. From small-angle
diffraction studies, it was determined that poly-IV forms
a nematic mesophase. This, combined with the shorter
interchain spacing suggests that the mesogenic side
chains may pack parallel to the main-chain axis.
Three carbodiimide monomers having a mesogenic
side chain were prepared following the synthetic route
summarized in Scheme 1. Monomers III, IV, and V all
have dissimilar side chains and would be expected to
form polymers that adopt nematic phases. However,
the biphenyl and diphenylazo mesogens of the sides
should promote smectic ordering. These monomers were
polymerized to form poly-III and poly-IV, respectively.
The first isolated poly-III and poly-IV were found to be
semicrystalline as shown in their X-ray diffractogram
(Figure 5).
While poly-III shows one strong peak at 2θ ) 7.6°,
which corresponds to a long-range spacing of 11.6 Å.
Poly-IV shows a relatively small and broader peak at
low angle (2θ ) 7.7°), which corresponds to a long-range
spacing of 11.5 Å. Additionally, poly-III shows stronger
and sharper peaks at higher angle region, compared to
Poly-V with its longer C12 space and side chain
proved to be sufficiently soluble to allow us to study the
solution behavior of this derivative possessing the
biphenyl mesogens. Like poly-IV, this derivative also
displayed a lyotropic nematic mesophase. Its more
interesting thermotropic behavior is discussed below.
We conclude from this study, in which we compete
mesogens with opposing propensities for phase behavior,
the backbone appears to dominate (i.e., in this case at
least, the tail does not wag the dog).
Th er m otr op ic Liqu id Cr ysta llin e P r op er ties of
P olygu a n id in es, P oly-V a n d P oly-VI. Polyguanidines
have the unusual property of quantitatively depolymer-
izing back to monomer at relatively low temperatures
(ca. 150-200 °C). Since nearly all of polyguanidines
depolymerize before any melt transitions, thermotropic

Macromolecules, Vol. 37, No. 22, 2004
Polyguanidines 8291
F igu r e 9. Optical polarizing microscopic image of poly-V at
130 °C (200×).
F igu r e 7. DSC thermogram of poly-VI.
F igu r e 10. X-ray diffraction pattern of poly-V at 130 °C.
F igu r e 8. Optical polarizing microscopic image of poly-VI at
118 °C (200×).
The X-ray diffraction patterns were measured at room
temperature and 130 °C. At 130 °C, the X-ray pattern
of poly-V appears the same as that at room temperature,
with both showing low-angle peaks at approximately 2θ
) 5°. This provides evidence for a smectic layered
structure with spacing 18-20 Å that is maintained in
the high-temperature mesophase (Figure 10). At high
angle, the peaks are sharper than seen in poly-VI, which
implies an ordered side chain structure between the
main chains at both room temperature and 130 °C.
Unlike in solution, where nematic phases preferred by
the stiff backbone are favored, the thermotropic meso-
phases appear to be influenced by the side-chain me-
sogens, and smectic layered structures result. In this
case, the tail does appear to wag the dog.
liquid crystalline properties of these materials have
never been observed. When we were preparing these
polyguanidine derivatives with long side chains to
increase the interchain spacing, we found that these
polymers would display melt transitions. Interestingly,
poly-VI shows two phase transitions in the DSC ther-
mogram before its decomposition (Figure 7). The first
transition at 33 °C is assigned to the melting of the C12
alkyl side chains. A second endothermic transition is
observed at 102 °C and corresponds to a thermotropic
mesophase formationsas evidenced by polarized optical
microscopy (Figure 8)sthat persists until the decom-
position temperature of ∼175 °C. X-ray diffraction
studies were performed in the solid state at room
temperature and at 118 °C. At room temperature, a
strong sharp peak at low angle (2θ ) 5.4°) corresponds
to the long-range layer spacing, 16.4 Å, which is
expectedly larger than the 13 Å spacing measured for
the di-n-hexyl case of poly-I. Above the second endo-
therm at 118 °C, no large differences were observed
except for a slight shift of the long-range spacing peak
to lower angle, which is consistent with the extension
of the alkyl side chains at such a high temperature. The
mesophase behavior of this polymer can be described
as helical rods ordered and aligned in molten paraffin.
Unlike poly-VI which possesses equivalent alkyl
chains, poly-V, with its mixed C12 and C12 mesogenic
side chains, shows no side-chain melting in the DSC
trace. It thus appears that the terminal biphenyl
mesogen prevents alkyl side chain crystallization in this
derivative, but it does induce an order of its own (vide
infra). Poly-V does show a reversible phase transition
at 119 °C, and the cross-polarized micrograph provides
evidence of mesophase formation above this tempera-
ture (Figure 9).
Con clu sion
With the goal of establishing some of the rules
governing the liquid crystallinity of the helical polyguani-
dines, we prepared several derivatives and investigated
their lyotropicsand in some cases thermotropics
properties using optical polarizing microscopy and X-ray
diffraction. Each repeat unit of these polymers possesses
two side chains, and if these are identical, the polymer
can adopt a more ordered, layered structure in both
solution and the solid state. Examples include poly-I and
poly-VI, which possess di-n-hexyl and di-n-dodecyl side
chains, respectively. This higher level of ordering is
believed to arise from the formation of a dense corona
barrier that effectively acts to uniformly separate the
helical backbones. When the side chains differ in length,
e.g., poly-(r a c-II) and poly-IV, the uniform layering
breaks down and a simpler nematic structure arises.
Chirality also plays a very important role. Fixed,
homochiral side chains, e.g., poly-(R-II), force all of the
chains to adopt a preferred helical sense, and these
screws now pack in a cholesteric mesophase. Further-

8292 Kim et al.
Macromolecules, Vol. 37, No. 22, 2004
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