Macromolecules 2010, 43, 1185–1187 1185
DOI: 10.1021/ma902416h
1
isolated as diethyl ether-insoluble parts in 58% yield. H
A Highly Reactive Benzoxazine Monomer,
1-(2-Hydroxyethyl)-1,3-Benzoxazine: Activation of
Benzoxazine by Neighboring Group Participation of
Hydroxyl Group
and 13C NMR spectra of 2a revealed that it was a phenolic
polymer having Mannich linkage (-aromatic-CH2-NR-
CH2-aromatic-) (Figure 1).
The obtained polymer 2a was treated with tert-butyl
isocyanate to transform the hydroxyl groups into the corre-
sponding urethanes for the purpose of estimation of the
number- and weight-average molecular weights (Mn and
Mw) of the resulting polymer 3a by size exclusion chromato-
graphy (SEC). Previously, we reported that Mn and Mw of
poly(N-phenyl-1,3-benzoxazine) were significantly under-
estimated by SEC due to the polarity of phenolic OH group,
and its transformation into less polar tert-butylurethane
permitted more reasonable estimation of Mn and Mw.20
Similarly to these previous observations, SEC-estimated
Mn and Mw of 2a having free OH groups were estimated to
be 130 and 210, respectively, while those of 3a having tert-
butylurethane moieties were 2300 and 3600.
Results. Effect of Hydroxyl Group on the Polymerization
Behavior of 1a. As shown in Figure 2a, the polymerization of
1a was much faster than that of N-n-propylbenzoxazine (1c).
Since the bulkiness of the n-propyl group (-CH2CH2CH3) is
comparable to that of 2-hydroxylethyl group (-CH2CH2-
OH), the remarkably high reactivity of 1a would be caused
by participation of polar and potentially nucleophilic hydro-
xyl group in the polymerization mechanism (vide infra).
N-Methylbenzoxazine (1b), which has a sterically less hin-
dered methyl group, was more reactive than 1c; however, 1b
was less reactive than 1a to imply that a certain role of the
hydroxyl group dominated the polymerization over the steric
factor. In comparison with the hydroxyl group in 1a, the
2-methoxyethyl group in benzoxazine 1d had no effect to
enhance reactivity. By masking OH as methyl ether, its
polarity and nucleophilicity were lost, and at the same
time, bulkiness was increased to result in the slow polymeri-
zation of 1d.
Ryoichi Kudoh, Atsushi Sudo, and Takeshi Endo*
Molecular Engineering Institute, Kinki University, Kayanomori,
Iizuka, Fukuoka 820-8555, Japan
Received October 31, 2009
Revised Manuscript Received January 7, 2010
Introduction. Benzoxazines have received considerable
attention because their ring-opening polymerizations afford
the corresponding polymers having excellent properties such
as high mechanical strength,1 thermal stability,2 and dur-
ability under a humid environment.3 Benzoxazine mono-
mers can be easily synthesized from various phenols, amines,
and formaldehyde.4 This versatile synthetic method has
allowed development of various benzoxazines having func-
tional groups5 such as allyl,6 propargyl,7,8 diacetylene,9
nitrile,10,11 furyl,12 epoxide,13 amino,14 and siloxy,15 which
were effectively used for various chemical modifications
of the benzoxazine monomers and the corresponding poly-
mers including cross-linking reactions. In addition, several
benzoxazines16,17 and its naphthalene-containing analogue
(naphthoxazine)18 having hydroxyl group have been deve-
loped. These literatures described convenient use of hydroxyl
group for polycondensation and initiation of ring-opening
polymerization of lactone. Recently, thermally induced
polymerizations of benzoxazines having a 2-(2-hydro-
xyethoxy)ethyl group (=CH2CH2OCH2CH2OH) have been
reported:19 Because of the presence of OH group, the poly-
merizations of these monomers proceeded smoothly at rela-
tively low temperature. Hydrogen bonding offered by OH
may be a reason for it.
The difference in reactivity between 1a and N-methyl-
benzoxazine (1b) was more clearly observed when their
polymerizations were performed at lower temperature,
120 °C. As shown in Figure 2b, 1a was smoothly consumed
and its conversion reached 90% at 6 h, while 1b was
consumed only in 20% in 6 h.
Next, polymerization of 1b was performed with adding an
equimolar amount of 2-(N,N-dimethylamino)ethanol (DMAE)
to clarify whether such an amino alcohol can accelerate polym-
erization of benzoxazine in an intermolecular manner or not.
As shown in Figure 2b, by adding DMAE, the polymerization
of 1b was accelerated effectively; however, it was still slower than
the polymerization of 1a, to suggest that hydroxyl group in 1a
would activate the benzoxazine moiety in an intramolecular
manner.
Mechanism. The remarkable difference in reactivity bet-
ween 1a and 1b let us postulate a mechanism where the hydroxyl
group has an important role to promote the ring-opening
reaction of 1a (Scheme 2): First, 1a would come to equili-
brium with the corresponding zwitter ionic intermediate (ZI-1)
having an iminium moiety and a phenoxide via rever-
sible heterolytic bond scission of the cyclic N,O-acetal
moiety.21-23 Another type of zwitterionic intermediate is a
carbocationic one (ZI-2), which is a tautomer of ZI-1. At this
stage, the hydroxyl group would react with the imminium
Herein, we report polymerization behavior of 3-(2-hydro-
xyethyl)-1,3-benzoxazine, a benzoxazine having a hydroxyl
group. Our discovery of its high reactivity and the role of
hydroxyl group in activating benzoxazine are described.
Experimental Section. Synthesis of 1,3-Benzoxazine Having
Hydroxyl Group 1a. 1a was synthesized from p-cresol,
2-aminoethanol, and formaldehyde (Scheme S-1 in the
Supporting Information). Alcohols were suitable solvents
for the synthesis, and particularly, 2-methoxyethanol was
the most suitable one because of its high boiling point
that allowed the operation of the reaction at higher tem-
perature (Table S-1 and Figure S-1 in the Supporting
Information). The 1H and 13C NMR spectra of 1a are
shown in Figure 1.
Thermally Induced Polymerization of 1a. Thermally in-
duced polymerization of benzoxazine having hydroxyl group
1a was carried out at 150 °C (Scheme 1). 1H NMR analysis of
the resulting mixture revealed that more than 90% of 1a was
consumed within 1 h, indicating that the reactivity of 1a
was remarkably higher than other N-alkylbenzoxazines
1b (N-methyl-1,3-benzoxazine) and 1c (N-n-propyl-1,3-
benzoxazine) (Figure 2a). The formed polymer 2a was
*To whom correspondence should be addressed: Fax þ81-948-22-
7210, e-mail tendo@mol-eng.fuk.kindai.ac.jp.
r 2010 American Chemical Society
Published on Web 01/13/2010
pubs.acs.org/Macromolecules
Kudoh, Ryoichi
