Synthesis of polymers bearing 1,3-benzoxazine moiety in the side chains from poly(allylamine) and their crosslinking behaviors

Article abstract of DOI:10.1002/pola.24754

A polymer bearing 1,3-benzoxazine moiety in the side chain was synthesized successfully from poly(allylamine) based on a stepwise strategy consisted of three steps: (1) treatment of poly(allylamine) with salicylaldehyde to convert the amino group in the s

Full text of DOI:10.1002/pola.24754

Synthesis of Polymers Bearing 1,3-Benzoxazine Moiety in the Side Chains
from Poly(allylamine) and Their Crosslinking Behaviors
HIROAKI OIE, ATSUSHI SUDO, TAKESHI ENDO
Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka 820-8555, Japan
Received 23 March 2011; accepted 27 April 2011
DOI: 10.1002/pola.24754
Published online 16 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A polymer bearing 1,3-benzoxazine moiety in the
side chain was synthesized successfully from poly(allylamine)
based on a stepwise strategy consisted of three steps: (1) treat-
ment of poly(allylamine) with salicylaldehyde to convert the
amino group in the side chain into the corresponding o-(imino-
methyl)phenol moiety, (2) reduction of the o-(iminomethyl)phe-
nol to obtain the corresponding o-(aminomethyl)phenol
moiety, and (3) formation of 1,3-benzoxazine moiety by the
reaction of the o-(aminomethyl)phenol with formaldehyde. The
content ratio of benzoxazine moieties and o-(aminomethyl)phe-
nol moieties in the polymer were tunable by varying amount
of formaldehyde. The presence of o-(aminomethyl)phenol
moieties exhibited a significant promoting effect on the cross-
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linking reaction. 2011 Wiley Periodicals, Inc. J Polym Sci Part
A: Polym Chem 49: 3174–3183, 2011
KEYWORDS: 1,3-benzoxazine; crosslinking; functionalization of
polymer; networks; ring-opening polymerization
INTRODUCTION Benzoxazines are a class of heterocyclic
compounds capable of undergoing ring-opening polymeriza-
tion to give the corresponding polymers, which exhibit excel-
lent mechanical strength,1 thermal stability,2 and durability
under humid environment.3 Benzoxazines can be easily syn-
thesized by heating a solution of the corresponding phenolic
compounds, amines, and formaldehyde.4 This simple and
versatile synthetic method has supported development of a
wide range of benzoxazine-type monomers5–9 involving
curable multifunctional ones.10,11
side chain modification of polymer precursors bearing reac-
tive side chains. For example, poly(4-vinylphenol) was used as
a polymeric precursor bearing plural phenol moieties, which
were condensed with aniline and formaldehyde to give the
corresponding benzoxazine-functionalized polystyrene.26,27 In
another example, the copper-catalyzed azide-alkyne coupling
reaction was efficiently used for the attachment of benzoxa-
zine moieties to a polystyrene.28
Polymers bearing amino groups in the side chains are attrac-
tive precursors for synthesizing polymers bearing benzoxa-
zines in the side chains. Poly(allylamine) is one of such poly-
meric precursors that can be easily obtained by radical
polymerization of allylamine hydrochloric acid.29 However,
there has not been reported the use of poly(allylamine) for
benzoxazine synthesis. The current contribution describes
our successful use of poly(allylamine) as a precursor for the
efficient approach to polymers bearing benzoxazine moieties
in the side chains. The present synthetic route that con-
verted the primary amino group into the corresponding ben-
zoxazine moiety consisted of three steps, i.e., 1) condensation
of the amino group with salicylaldehyde into the correspond-
ing o-(iminomethyl)phenol, 2) its reduction into the corre-
sponding o-(aminomethyl)phenol, and 3) its cyclocondensa-
tion with formaldehyde to give 1,3-benzoxazine moiety. All
the steps proceeded without side reactions to give new lin-
ear polymers bearing benzoxazine pendants, and the final
step with varying amount of formaldehyde allowed efficient
control of content of benzoxazine moieties. Details of the
Recently, polymers bearing 1,3-benzoxazine moieties have
been attracted much attention because of their improved
processability and their potential use as prepolymers that
undergo crosslinking reactions to afford the corresponding
networked polymers with high crosslinking density.12,13 Sev-
eral polymers bearing benzoxazine moieties in the main
chains have been synthesized by various strategies including
polycondensation of bisphenols, diamines and formalde-
hyde,14–16 polycondensation of a diol bearing benzoxazine
moieties with diacid chlorides,17,18 and polyaddition of a bisal-
kyne bearing benzoxazine moieties with bisazide based on the
copper (I)-catalyzed alkyne-azide coupling reaction.19,20
Besides, polymers bearing benzoxazine moieties in the side
chains have been developed: For example, benzoxazines bear-
ing polymerizable moieties such as epoxide,21 maleimide,22
propargyl,23 and methacryl groups24,25 have been synthesized
and selectively polymerized into the corresponding benzoxa-
zine-functionalized polymers. Another reliable approach is
Additional Supporting Information may be found in the online version of this article. Correspondence to: T. Endo (E-mail: tendo@mol-eng.fuk.
kindai.ac.jp)
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ARTICLE
synthesis and the crosslinking reactions of the obtained poly-
mers are described herein.
Reaction of Poly(allylamine) with Di-tert-butyl
dicarbonate30,31
To the 17.5 wt % aqueous solution of PAA (5.71 g, number
of amino groups 17.5 mmol), a solution of di-tert-butyl dicar-
bonate (4.58 g, 21.0 mmol) in dioxane (10 mL) was added
and the mixture was stirred at room temperature. After 8 h,
1M aqueous solution of sodium hydroxide (50 mL) was
added and the mixture was stirred at room temperature for
12 h. The resulting precipitate was collected by filtration
with suction, washed with water, dissolved in THF (15 mL)
and poured into hexane (400 mL).
EXPERIMENTAL
Materials
All reagents and solvents were used as received. Allylamine,
n-propylamine, paraformaldehyde, formaldehyde (37% in
water), 2,20-azobis(2-methylpropionamidine) dihydrochloride
(V-50), salicylaldehyde, sodium borohydride, p-cresol, hydro-
chloric acid (35% in water) and sodium hydroxide were
purchased from WAKO Pure Chemical Industries.
The resulting precipitate was collected by filtration with suc-
tion, washed with hexane and dried at room temperature
under vacuum to yield 2 (2.21 g, 86%) as a white solid: 1H
NMR (in CDCl3) d 5.60 (br, 1H, ANHA), 3.07 (br, 2H,
ANACH2AC<), 1.43 (br, 12H, ACH2ACH<, ACH2ACH<,
AC(CH3)3); 13C NMR (in CDCl3) d 157.01 (C¼¼O), 78.98
(AC(CH3)3), 44.53 (ANACH2AC<), 36.09 (ACH2ACH<),
33.48 (ACH2ACH<), 28.79 (AC(CH3)3); IR (KBr) 3365,
2979, 2932, 1699, 1521, 1456, 1392, 1366, 1275, 1251,
1173 cm-1.
Measurements
1H NMR spectra were recorded in chloroform-d (CDCl3) on a
JEOL EX-300 (1H, 300 MHz) spectrometer, with tetramethyl-
silane (TMS) as an internal standard, acquisition period ¼
5.45 sec; pulse delay ¼ 1.55 sec; accumulation times ¼ 32.
Chemical shifts d and coupling constants J are given in ppm
and Hz, respectively. Thermo-gravimetric analysis (TG) and
differential scanning calorimetric analysis (DSC) were per-
formed with a SEIKO EXSTAR6000 (Seiko Instruments). IR
spectra were obtained on a JASCO FT/IR-460 plus. Number-
and weight- average molecular weight (Mn and Mw) were
estimated from size exclusion chromatography (SEC), per-
formed on a Tosoh chromatograph model HLC-8220GPC
equipped with consecutive Tosoh TSKgel columns
(SuperAW4000, SuperAW3000 and SuperAW2500) using
DMF as an eluent at the flow rate of 0.5 mL/min after cali-
bration with polystyrene standards.
Reaction of Poly(allylamine) with p-Cresol and
Formaldehyde
The 17.5 wt % aqueous solution of PAA (0.65 g, number of
amino groups 2.00 mmol) was dried at room temperature
under vacuum for 12 h. The residual solid was dispersed in
toluene (20 mL), and then p-cresol (0.24 mg, 2.2 mmol) and
paraformaldehyde (0.34 g, 5.6 mmol) were added. The mix-
ture was refluxed with azeotropic removal of water with using
a Dean-Stark apparatus. The resulting precipitate was col-
lected by filtration with suction, washed with toluene and
dried to yield 3 (0.30 g) as a white solid: IR (KBr) 2916, 2851,
Synthesis of Poly(allylamine) (PAA)
PAA was synthesized according to the article.29 To allylamine
(22.8 g, 0.40 mol), prechilled hydrochloric acid (43.8 g,
0.42 mol) was slowly added at 0 ꢀC and the mixture was
stirred at 0 ꢀC for 1 h. The resulting solution was concen-
trated under reduced pressure at room temperature to
adjust 70 wt % aqueous solution of allylamine hydrochlor-
ide. V-50 (651 mg, 2.40 mmol) was added to the solution,
and the mixture was degassed by three freeze-pump-thaw
cycles and heated at 50 ꢀC for 50 h under N2. After cooling
to room temperature, the reaction mixture was poured into
methanol (800 mL).
1670, 1617, 1501, 1457, 1252, 1227, 1118, 936, 814 cmꢁ1
.
Synthesis of 2-[(n-Propylimino)methyl]phenol (4)
To a solution of n-propylamine (3.25 g, 55.0 mmol) in metha-
nol (500 mL), salicylaldehyde (6.10 g, 50.0 mmol) was added
and the mixture was stirred at room temperature for 1 h.
The resulting solution was concentrated under reduced pres-
sure and dried at room temperature under vacuum to yield
4 (8.19 g, 99%) as a yellow oil: 1H NMR (in CDCl3) d 13.69
(s, 1H, AOH), 8.34 (s, 1H, ACH¼¼NA), 7.32–7.23 (m, 2H,
ArAH), 6.95 (d, 1H, J ¼ 8.2 Hz, ArAH), 6.87 (t, 2H, J ¼ 7.4
Hz, ArAH), 3.56 (t, 2H, J ¼ 6.4 Hz, ACH2ACH2ACH3), 1.73
The resulting precipitate was collected by filtration with suc-
tion, washed with methanol and dried at room temperature
under vacuum to yield PAA-HCl 1 (27.5 g, 29.4 mmol, 73%)
as a white solid: 1H NMR (in D2O)
d 3.11 (br, 2H,
(m, 2H, ACH2ACH2ACH3), 0.98 (t, 3H,
J
¼
7.4 Hz,
164.59
ACH2ANH3þ), 2.09 (br, 1H, ACH2ACH<), 1.57 (br, 2H,
ACH2ACH<).
ACH2ACH2ACH3); 13C NMR (in CDCl3)
d
(ACH¼¼NA), 161.34 (Ar), 131.82 (Ar), 131.12 (Ar), 118.77
(Ar), 118.21 (Ar), 116.78 (Ar), 60.99 (ACH2ACH2ACH3),
23.95 (ACH2ACH2ACH3), 11.52 (ACH2ACH2ACH3); IR
(neat) 2963, 2931, 1633, 1583, 1497, 1462, 1417, 1338,
1280, 1210, 1151, 1054, 1032, 1013, 977, 851, 755 cm-1.
Prechilled 50 wt % aqueous solution of sodium hydroxide
(12.3 g, 30.8 mmol) was slowly added to 1 and the mixture
was stirred at 0 C.
ꢀ
After 1 h, the reaction mixture was diluted by adding water
to adjust concentration of PAA to 17.5 wt %: 1H NMR (in
D2O) d 2.64 (br, 2H, ACH2ANH2), 1.56 (br, 1H, ACH2ACH<),
1.27 (br, 2H, ACH2ACH<). This aqueous solution of PAA
coꢀntaining about 30 mmol NaCl was stored in refrigerator at
0 C until use.
Synthesis of N-(n-Propyl)-1,3-benzoxazine (6)
To a solution of 4 (2.45 g, 15.0 mmol) in THF (150 mL), a
suspension of sodium borohydride (1.13 g, 30.0 mmol) in
methanol (4.5 mL) was added and the mixture was stirred
at room temperature for 0.5 h. Water (15 mL) was added
SYNTHESIS OF POLYMERS BEARING 1,3-BENZOXAZINE MOIETY, OIE, SUDO, AND ENDO
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
and the resulting mixture was stirred at room temperature
for 12 h. After THF and methanol were removed under
reduced pressure, the obtained residue were dissolved in
diethylether (20 mL) and washed with water (60 mL) three
times.
ACH2ACH<); 13C NMR (in CDCl3) d 166.03 (ACH¼¼NA),
161.29 (Ar), 132.24 (Ar), 119.42 (Ar), 118.62 (Ar), 117.92
(Ar), 63.06 (ANACH2AC<), 36.99 (ACH2CH<), 33.88
(ACH2CH<); IR (KBr): 2915, 1630, 1580, 1497, 1459, 1277,
1210, 1150, 1041, 849, 754 cm-1.
Diethylether was removed under reduced pressure to yield
crude 5 (2.57 g): 1H NMR (in CDCl3) d 7.16 (t, 1H, J ¼ 5.9
Hz, ArAH), 6.99 (d, 1H, J ¼ 5.5 Hz, ArAH), 6.83 (d, 1H, J ¼
6.0 Hz, ArAH), 6.77 (t, 1H, J ¼ 5.6 Hz, ArAH), 3.99 (s, 2H,
ANACH2AAr), 2.65 (t, 2H, J ¼ 7.1 Hz, ANACH2ACA), 2.17
(s, 1H, NAH), 1.56 (m, 2H, ACH2ACH2ACH3), 0.94 (t, 3H,
J ¼ 7.4 Hz, ACH2ACH2ACH3); 13C NMR (in CDCl3) d 158.32
(Ar), 128.23 (Ar), 128.09 (Ar), 122.59 (Ar), 118.57 (Ar),
115.95 (Ar), 52.36 (ANACH2AAr), 50.23 (ACH2ACH2ACH3),
22.44 (ACH2ACH2ACH3), 11.41 (ACH2ACH2ACH3); IR
(neat) 3320, 3299, 2960, 2933, 1612, 1590, 1492, 1415,
1259, 1186, 1150, 1104, 1034, 754 cm-1.
Synthesis of Polymer Bearing o-(Aminomethyl)phenol
Moieties (8)
To a solution of 7 (3.87 g, number of o-(iminomethyl)phenol
groups ¼ 24.0 mmol) in THF (240 mL), a suspension of
sodium borohydride (1.82 g, 48.0 mmol) in methanol (7 mL)
was added and the mixture was stirred at room temperature.
After 0.5 h, water (50 mL) was added. The resulting mixture
was stirred at room temperature for 12 h. THF and methanol
were removed under reduced pressure to obtain a sticky
precipitate on the wall of the apparatus. The supernatant
aqueous layer was removed by decantation, and the precipi-
tate was dissolved in THF (30 mL) and poured into water
(800 mL).
The crude 5 was dissolved in THF (150 mL), and to this so-
lution, formalin (1.34 g, 16.5 mmol) was added. The mixture
was stirred at room temperature for 1 h, and then concen-
trated under reduced pressure.
The resulting precipitate was collected by filtration with suc-
ꢀ
tion, washed with water and dried at 50 C under vacuum to
1
yield 8 (3.71 g, 95%) as a white solid: H NMR (in CDCl3) d
7.04–6.68 (br, 4H, ArAH), 5.52 (br, 1H, AOH), 3.79 (br, 2H,
ANACH2AAr), 2.44 (br, 2H, ANACH2AC<), 1.52–1.12 (br,
3H, ACH2ACH<, ACH2ACH<); 13C NMR (in CDCl3) d 158.27
(Ar), 128.86 (Ar), 122.84 (Ar), 119.32 (Ar), 116.23 (Ar),
53.24 (ANACH2AAr, ANACH2AC<), 36.53 (ACH2ACH<),
32.88 (ACH2ACH<); IR (KBr) 3303, 2846, 2916, 1590,
1489, 1474, 1457, 1411, 1256, 1182, 1102, 1036, 750 cm-1.
The obtained residue was fractionated by column chroma-
tography (hexane/ethyl acetate ¼ 4/1) to yield 6 (1.74 g,
1
66%) as a colorless oil: H NMR (in CDCl3) d 7.12 (t, 1H, J ¼
7.4 Hz, ArAH), 6.95 (d, 1H, J ¼ 7.1 Hz, ArAH), 6.86 (t, 1H,
J ¼ 7.2 Hz, ArAH), 6.77 (d, 1H, J ¼ 8.2 Hz, ArAH), 4.87 (s,
2H, ANACH2AOA), 3.99 (s, 2H, ANACH2AAr), 2.71 (t, 2H,
J ¼ 7.5 Hz, ACH2ACH2ACH3), 1.59 (m, 2H, ACH2ACH2ACH3),
0.93 (t, 3H, J ¼ 7.4 Hz, ACH2ACH2ACH3); 13C NMR (in CDCl3)
d 154.30 (Ar), 127.23 (Ar), 127.12 (Ar), 120.09 (Ar),
119.95 (Ar), 116.13 (Ar), 82.08 (ANACH2AOA), 52.93
Syntheses of Polymers Bearing 1,3-Benzoxazine
Moieties (9)
To a solution of 8 (1.47 g, amount of o-(aminomethyl)phenol
group ¼ 9.0 mmol) in THF (90 mL), formalin (1.46 g,
18.0 mmol) was added and the mixture was stirred at room
(ANACH2AAr),
49.87
(ACH2ACH2ACH3),
21.04
(ACH2ACH2ACH3), 11.34 (ACH2ACH2ACH3); IR (neat) 2959,
2934, 2873, 1618, 1588, 1502, 1378, 1344, 1321, 1223, 1143,
1120, 939, 909, 814, 745 cm-1.
temperature. After
2 h, volatiles were removed under
reduced pressure. The resulting residue was dissolved in
THF (10 mL) and poured into methanol (200 mL).
Synthesis of Polymer Bearing o-(Iminomethyl)phenol
Moieties (7)
The resulting precipitate was collected by filtration with suc-
tion, washed with methanol and dried at room temperature
under vacuum to yield 9a (1.40 g, 89%) as a white solid: H
NMR (in CDCl3) d 7.01–6.71 (br, 4H, ArAH), 4.63 (br, 2H,
ANACH2AOA), 3.73 (br, 2H, ANACH2AAr), 2.45 (br, 2H,
ANACH2AC<), 1.61–0.97 (br, 3H, ACH2ACH<, ACH2ACH<);
13C NMR (in CDCl3) d 154.52 (Ar), 127.89, (Ar) 120.53 (Ar),
116.58 (Ar), 83.13 (ANACH2AOA), 57.13 (ANACH2AAr),
The 17.5 wt % aqueous solution of PAA (9.79 g, number of
amino groups 30.0 mmol) containing sodium chloride was
dried under vacuum for 12 h. The obtained residue was dis-
solved in methanol (300 mL). To the solution, salicylalde-
hyde (4.03 g, 33.0 mmol) was added and the mixture was
stirred at room temperature for 3 h. After methanol was
removed under reduced pressure, THF (50 mL) was added,
and precipitated sodium chloride was removed by filtration
with suction. The resulting THF solution of PAA was concen-
trated under reduced pressure, and the obtained residue
was redissolved in THF (30 mL) and poured into methanol
(800 mL).
1
50.71
(ANACH2AC<),
39.73
(ACH2ACH<),
31.49
(ACH2ACH<); IR (KBr) 2905, 2851, 1607, 1583, 1488, 1456,
1333, 1221, 1137, 1106, 1034, 1024, 924, 749 cm-1.
The polymers 9b and 9c were prepared similarly from 8
(0.49 g, amount of o-(aminomethyl)phenol group
¼
3.0 mmol). 0.18 g of formalin (2.25 mmol) and 0.12 g of for-
malin (1.50 mmol) were used for the synthesis of 9b and 9c,
respectively. 9b (0.49 g, 95% yield) and 9c (0.48 g, 94%)
were isolated as hexane-insoluble parts.
The resulting precipitate was collected by filtration with suc-
tion, washed with methanol and dried at room temperature
under vacuum to yield 7 (4.14 g, 85%) as a yellow solid: H
1
NMR (in CDCl3) d 13.40 (br, 1H, AOH), 8.08 (br, 1H,
ACH¼¼NA), 7.16–6.70 (br, 4H, ArAH), 3.38 (br, 2H,
1
9b: H NMR (in CDCl3) d 7.00–6.77 (br, 4H, ArAH), 4.65 (br,
ANACH2AC<),
1.86–1.30
(br,
3H,
ACH2ACH<,
1.5H, ANACH2AOA), 3.74 (br, 2H, ANACH2AAr), 2.42 (br,
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2)/(formula weight of the repeating unit of 2)ꢂ(formula
weight of the repeating unit of PAA). The 1H- and 13C NMR
spectra of the polymers 1, PAA and 2, which supported their
chemical structures, are shown in Figure S1 (in Supporting
Information).
Synthesis of a Polymer Bearing Benzoxazine Moieties 3
by a Conventional Method
Conventionally, various benzoxazines have been synthesized
by heating a mixture of the corresponding amines, phenols,
and formaldehyde. With expecting that this conventional
method would allow us to transform PAA directly into a
polymer bearing benzoxazine moieties, a mixture of PAA,
p-cresol and paraformaldehyde in toluene was heated with
azeotropic removal of formed water (Scheme 2). As a result,
a solid insoluble in organic solvents such as chloroform, tet-
rahydrofuran (THF), dimethylsulfoxide, N,N-dimethylforma-
mide, and methanol was obtained. IR analysis of the solid
revealed that it was a networked polymer 3 bearing benzox-
azine32,33 and triazine34 moieties (Fig. 1), to imply that the
amino groups of PAA and formaldehyde underwent conden-
sation, and the cyclotrimerization of the resulting imines
afforded the triazine rings as crosslinking points. Generally,
triazines undergo thermal dissociation into the correspond-
ing imines, which react with phenols to give the correspond-
ing Mannich-type adducts as precursors of benzoxazines.35
However, heating the networked polymer 3 in a toluene
solution of p-cresol for additional 9 h did not induce any
consumption of the triazine moieties, suggesting that the
insoluble nature of 3 suppressed motion of its triazine
moieties to avoid their efficient thermal dissociation. From
these results, we concluded that the conventional method for
SCHEME 1 Synthesis of poly(allylamine).
2H, ANACH2AC<), 1.53–1.00 (br, 3H, ACH2ACH<,
ACH2ACH<).
1
9c: H NMR (in CDCl3) d 7.06–6.71 (br, 4H, ArAH), 4.64 (br,
1H, ANACH2AOA), 3.76 (br, 2H, ANACH2AAr), 2.42 (br,
2H, ANACH2AC<), 1.52–1.10 (br, 3H, ACH2ACH<,
ACH2ACH<).
RESULTS AND DISCUSSION
Synthesis of Poly(allylamine) (PAA)
PAA was synthesized by (1) the radical polymerization of
allylamine hydrochloride and (2) treatment of the resulting
PAA-HCl salt 1 with an aqueous solution of sodium hydrox-
ide (Scheme 1).29 To estimate of the molecular weight of
PAA by size exclusion chromatography (SEC), PAA was trans-
formed into a less polar derivative 2 by the reaction of the
amino side chain with di-tert-butyl dicarbonate.30,31 The esti-
mated number average molecular weight (Mn) of 2 was
33,400, from which Mn of PAA was calculated to be 12,000
based on the equation, (Mn of PAA) ¼ (SEC-estimated Mn of
SCHEME 2 1-Step syntheses of polymers bearing benzoxazine
moieties from poly(allylamine).
FIGURE 1 IR spectra of the insoluble polymers 3.
SYNTHESIS OF POLYMERS BEARING 1,3-BENZOXAZINE MOIETY, OIE, SUDO, AND ENDO
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
benzoxazine synthesis was quite difficult to be applied to
selective synthesis of PAA-derived polymers bearing benzox-
azine moieties.
Stepwise Approach to Polymers Bearing Benzoxazine
Moieties in the Side Chains
The difficulty in synthesizing our target polymer bearing
benzoxazine pendants by the conventional method prompted
us to investigate a more reliable synthetic approach based
on cascading selective chemical transformations starting
from PAA: Ronda and coworkers36,37 have reported a step-
wise approach that enabled efficient synthesis of several
benzoxazines. The starting materials used therein are amines
and o-formyl phenols, which underwent condensation to
afford the corresponding o-(iminomethyl)phenols. The
obtained o-(iminomethyl)phenols can be readily reduced by
sodium borohydride into o-(aminomethyl)phenols, and their
SCHEME 3 A stepwise synthesis of benzoxazine.
FIGURE 2 1H and 13C NMR spectra of (a) N-(n-Pr) benzoxazine 6 and (b) polymer 9a.
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TABLE 1 Syntheses of Polymers 9 Bearing Benzoxazine
Moieties in the Side Chain
b
Amount of Composition
Entry Polymer CH2O (eq) x: ya
Mn
Yield (%) (Mw/Mn)b
1
2
3
9a
9b
9c
2.0
92:8
89c
95d
94d
27,100 (2.3)
30,000 (2.3)
32,500 (2.3)
0.75
0.50
72:28
50:50
a
b
c
Determined by 1H NMR.
Estimated by SEC (eluent ¼ DMF, polystyrene standards).
Methanol-insoluble parts.
Hexane-insoluble parts.
d
1
predicted positions. The H and 13C NMR spectra of 4 and 5
are shown in Figures S2 and S3 (in Supporting Information).
The successful model reactions prompted us to apply the
same stepwise approach to the synthesis of our target poly-
mers bearing benzoxazine moieties in the side chains. The
stepwise chemical transformation of the amino group in the
side chain of PAA into o-(aminomethyl)phenol moiety is
shown in Scheme 4. The first step was the condensation of
the amino groups in PAA and salicylaldehyde. The quantita-
tive conversion of PAA into the corresponding polymer 7
bearing o-(iminomethyl)phenol moieties was confirmed by
1H NMR. Treatment of 7 with sodium borohydride resulted
in the successful reduction of the o-(iminomethyl)phenol
moieties to afford the corresponding polymer 8 bearing
o-(aminomethyl)phenol moieties. The polymer 7 and the cor-
responding model compound 4 shared common spectro-
scopic features, that is, the 1H NMR signals for the imine
proton and the phenol proton were observed at around
8 and 14 ppm, respectively, and the 13C NMR signal for the
C¼¼N group was observed at around 166 ppm (Fig. S2 in
Supporting Information). Similarly, the comparison of the
spectra of the polymer 8 with those of the model compound
5 confirmed its chemical structure (Fig. S3 in Supporting
Information). Mn and Mw/Mn of the polymers shown in
Scheme 4 were estimated by SEC.
SCHEME 4 Synthesis of polymer 8 bearing o-(aminomethyl)-
phenol moieties as a precursor for synthesizing polymers bear-
ing benzoxazine moieties.
treatment with formaldehyde gave the corresponding
benzoxazines.
Before applying this stepwise approach to the present
synthesis of PAA-derived polymers bearing benzoxazine
moieties, a series of the relevant reactions starting from
n-propylamine as a model of PAA were performed to investi-
gate the efficiency in the reaction steps and to clarify the
spectroscopic features of the resulting compounds (Scheme 3):
In the first step, n-propylamine and salicylaldehyde were
condensed into o-(iminomethyl)phenol 4. In the second step,
the imino group of 4 was successfully reduced by sodium
borohydride at room temperature. The reaction mixture was
analyzed by 1H NMR to confirm the quantitative conversion
of 4 into o-(aminomethyl)phenol 5, which was then used in
the final cyclocondensation step without purification. The
cyclocondensation of 5 with formaldehyde smoothly pro-
ceeded at room temperature to give N-propyl benzoxazine 6.
After confirming the quantitative conversion of 5 into 6 by
1H NMR, 6 was isolated by column chromatography. Figure
2(a) shows the 1H NMR spectrum of 6, which indicated
characteristic two singlet signals at 4.87 and 3.99 ppm at-
tributable to the methylene protons of the N,O-acetal and the
benzyl protons, respectively. The 13C NMR also supported
the chemical structure of 6 by showing three signals for the
propyl group and two signals for the benzoxazine ring at the
With using the polymer 8 as a precursor, the target polymers
9 bearing benzoxazine moieties were synthesized based on
the cyclocondensation reaction of o-(aminomethyl)phenol
moieties with formaldehyde (Scheme 5). In this reaction
step, content of the benzoxazine moieties in the formed poly-
mer 9 was successfully controlled by varying amount of
formaldehyde used for the reaction (Table 1). In entry 1, an
TABLE 2 DSC Profiles of the Crosslinking Reactions of
Polymers 9
Polymer
Tonset Tpeak top Heat Evolution
Entry (Composition x: y)a (ꢀC)b
(ꢀC)b
(J/g)b
1
2
3
9a (92:8)
9b (72:28)
9c (50:50)
181
162
150
251
242
227
480
310
270
a
Determined by 1H NMR.
Obtained by DSC in a dynamic mode (heating rate ¼ 10 ꢀC/min).
SCHEME 5 Syntheses of polymers
9 bearing benzoxazine
b
moieties.
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FIGURE 3 (a) DSC and (b) TG thermograms of the polymer 9a.
excess amount of formaldehyde (2.0 equivalent to the
amount of o-(aminomethyl)phenol moieties) was used. The
cyclocondensation reaction proceeded smoothly at room
temperature to afford the polymer 9a bearing benzoxazine
moieties. The 1H NMR of 9a indicated a broad signal at
around 4.6 ppm attributable to the methylene protons of the
N,O-acetal moiety [Fig. 2(b)]. By comparison of the intensity
of this signal with the integrated signal intensity for the aro-
matic protons, the composition ratio x:y of 9a was calculated
to be 92:8. The presence of the methylene group between
the nitrogen and oxygen atoms was also confirmed by the
13C NMR spectrum [Fig. 2(b)]. In entries 2 and 3, reduced
amounts of formaldehyde (0.75 and 0.5 equivalents to the
number of the o-(aminomethyl)phenol moieties, respectively)
were used. As a result, the corresponding polymers 9b and
9c, of which content ratios x:y were 72:28 and 50:50,
respectively, were obtained. These content ratios were deter-
mined by 1H NMR similarly to the case of 9a. The corre-
sponding spectra are shown in Figure S4 (in Supporting
Information).
DSC measurement [Table 2 and Fig. 3(a)]. In parallel, weight
loss behavior was also investigated by TG measurement [Fig.
3(b)]. In the DSC thermogram, two exothermic peaks were
ꢀ
observed, and their peak top temperatures were 251 C and
301 ꢀC, respectively. The former one was attributable to
ring-opening reaction of benzoxazine moieties to crosslink
the polymer 9a into the corresponding networked polymer,
while the latter one was attributable to degradation of the
unconsumed benzoxazine moieties (Scheme S1 in Supporting
information).38 In fact, the TG thermogram clarified that the
weight loss gradually occurred from 244 ꢀC until 342 ꢀC,
and this temperature range was in good accordance with the
temperature range of the second exothermic peak observed
in the DSC chart.
Next, rate of the consumption of the benzoxazine moieties
was evaluated. Polymer 9a was heated at 180 ꢀC or 200 ꢀC
for t (hours), and the resulting sample was analyzed by DSC
to detect heat evolution DH(t) due to reactions of the resid-
ual benzoxazine moieties. By varying t, a series of the corre-
sponding DSC profiles were collected, from which DH(t) val-
ues were obtained (Fig. 4). As increasing the heating time,
the exothermic peak in a range from 210 to 270 ꢀC dimin-
ished to imply the consumption of the benzoxazine moieties.
Conversions were calculated according to the following
Thermally Induced Crosslinking Reaction of Polymers 9
Bearing Benzoxazine Moieties
With using polymer 9a, thermally induced ring-opening reac-
tion behavior of benzoxazine moieties was investigated by
FIGURE 4 Heat evolution profiles of the polymer 9a after heat treatment (a) at 180 ꢀC and (b) at 200 ꢀC.
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ARTICLE
TABLE 3 Thermal Properties of the Crosslinked Polymers
Obtained by Heating Polymer 9a (x: y 5 92:8) at 180 or 200 8C
Residual
Temp.
(ꢀC)
Time
(h)
Td5
(ꢀC)a
Td10
(ꢀC)a
Weight Ratio
at 450 ꢀC (%)a,b
Entry
1c
2
3
a
–
–
275
290
295
309
335
341
45
41
43
180
200
12
4
Measured by TG/DTA.
b
c
[Weight after heating up to 450 ꢀC]/[initial weight].
Data for the pristine 9a.
equation, conversion of benzoxazine moieties ¼ 100(1 ꢁ
DH(t)/ DH(0)), and were plotted against t (Fig. 5). Upon heat-
ing 9a at 180 ꢀC, the benzoxazine moieties were gradually
consumed and the conversion reached 80% at 12 h. Much
faster consumption of the ꢀ benzoxazine moieties was
achieved by heating 9a at 200 C.
FIGURE 5 Time-conversion relationships for ring-opening reac-
tions of the polymer 9a. The conversions of benzoxazine moi-
eties were calculated according to the following equation,
conversion ¼ 100(1 ꢁ DH(t)/DH(0)).
SCHEME 6 Plausible reaction
pathways involved in the cross-
linking reaction of polymer 9.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The thermal stabilities of the samples obtained by heating
9a were investigated (Table 3). In the TG/DTA analysis of
9a, temperature for 5% weight loss (Td5) and that for 10%
weight loss (Td10) were observed at 275 ꢀC and 309 ꢀC,
source would be the reason for the promoted crosslinking
reactions of 9b and 9c.
SUMMARY
ꢀ
respectively (entry 1). By heating 9a atꢀ 180 C for 12 h, Td5
ꢀ
From poly(allylamine) as a starting material, a sequence of
modifications of the amino pendants allowed development of
a series of new crosslinkable polymers bearing benzoxazine
moieties in the side chains. In the final step of the synthetic
route, the benzoxazine moieties were prepared from the
corresponding o-(aminomethyl)phenol moieties by their
reactions with formaldehyde, and thus the content ratio
between benzoxazine and o-(aminomethyl)phenol moieties
were controllable by varying amount of formaldehyde. The
polymer with 8% o-(aminomethyl)phenol content readily
underwent the crosslinking reaction by heating it at 200 ꢀC.
On the other hand, the polymers with higher o-(aminome-
thyl)phenol contents were crosslinkable at lower tempera-
tures due to the dual role of o-(aminomethyl)phenol as a
reactant and a proton source that promoted the ring-opening
reaction of the benzoxazine moieties.
and Td10 increased to 290 C and 335 C, respectively (entry
2). Furthermore, higher Td5 and Td10 values were achieved
by heating 9a at 200 ꢀC for 4 h (entry 3). These results
clarified that the crosslinking reaction of 9a based on the
ring-opening reactions of the benzoxazine moieties contrib-
uted to the improvement of the thermal stability.
Based on these results, a cast film of polymer 9a was pre-
pared from its THF solution on a glass plate and heated at
ꢀ
200 C for 4 h under air. The resulting polymer film became
insoluble in organic solvents such as DMF, THF, and chloro-
form, to imply 9a was crosslinked successfully.
Next, DSC analyses of the polymers 9b and 9c in a dynamic
scan mode (heating rate ¼ 10 ꢀC /min) were performed.
These polymers had higher contents of o-(aminomethyl)phe-
nol moiety than 9a, and its effects on the crosslinking behav-
iors were of our interest. Supporting Information Figure S5
shows the resulting DSC profiles, and some essential data
extracted from them are listed in Table 2. As increasing the
content of o-(aminomethyl)phenol moiety in the order of 9a
(8%) < 9b (28%) < 9c (50%), the peak top temperatures
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