8320
Macromolecules 2003, 36, 8320-8329
Model Compounds Study on the Network Structure of Polybenzoxazines
Ho-Don g Kim a n d Ha tsu o Ish id a *
Department of Macromolecular Science and Engineering, Case Western Reserve University,
Cleveland, Ohio 44106
Received February 13, 2003
1
ABSTRACT: The FT-IR and H NMR spectra of model dimers, having different molecular sizes and pKa,
are investigated in order to understand the differences in the hydrogen-bonded network structures of
polybenzoxazines. The correlation between the -OH‚‚‚N intramolecular hydrogen-bonding interaction
1
and benzoxazine functional groups in the asymmetric dimers is investigated by H NMR spectra. While
the FT-IR spectra of the model dimers indicate that the nature of hydrogen bonding is closely related to
the basicity of the amine constituent, the spectra of the corresponding polymers suggest the existence of
different hydrogen-bonding interactions. The existence of phenolic linkage formation and the stability of
the Mannich base structure during polymerization are investigated by a dimerization reaction. It is
demonstrated that benzoxazines based upon extremely bulky amines do not develop desirable properties
due to the extensive degradation process.
In tr od u ction
intramolecular hydrogen-bonding interactions, was stud-
ied by Ishida et al.6 using molecular modeling of
benzoxazine dimers based on different amine functional
groups. They reported that the compactness of a net-
work structure is related to both the basicity and
bulkiness of the functional amines.
However, the previous studies were based on the
assumption that the bulk of monomer is converted to
polymer upon curing, resulting in a pure polybenzox-
azine structure which solely consisted of Mannich
bridges between phenolic groups. Moreover, despite the
important effect of the amine substituents on hydrogen
bonding, research has been conducted only on a few
simple dimer molecules.6,12 A more detailed study of the
network structures in the polymers using model dimers
will allow for a better explanation of the behavior of the
polymers. Therefore, this paper will investigate the
effect of amine functional groups on the hydrogen
bonding and network structure using a series of model
dimers and polymers.
It is well-known that hydrogen bonds play an impor-
tant role in the wide range of material properties, and
therefore, many studies have been actively done in this
area.1-3 Recently, a novel class of phenolic resins,
polybenzoxazines, which can overcome many shortcom-
ings associated with traditional phenolic resins, has
been synthesized and characterized by Ishida et al.4-7
Since the wide variety of benzoxazine monomers can be
easily obtained by changing the primary amine compo-
nent in Mannich condensation, they are of great interest
for commercial applications. Furthermore, these resins
have many excellent properties, which easily surpass
the phenolic resins, such as excellent mechanical prop-
erties,4,5 high char yield,5 near zero volumetric shrink-
age/expansion upon polymerization,6 low water absorp-
tion4 (despite the large amount of hydroxyl groups in
the backbone structure), excellent resistance to chemi-
cals8 and UV light,9 and amazingly high Tg,10 even with
rather low cross-link density. A possible explanation for
these unusual properties is the formation of extensive
hydrogen-bonding networks.11
Exp er im en ta l Section
All chemicals were obtained from Aldrich with high purity
(above 98%), except for formaldehyde (Fisher, 37% in H2O),
and used as received.
Consequently, the effort to understand the funda-
mental nature of the hydrogen-bonding structure of
polybenzoxazines has been a recent focus of this
laboratory.6,8,12-14 To overcome the instrumental limita-
tion in studying thermoset polymer structures and to
better investigate the complex relation between several
hydrogen-bonding species, a series of model compounds
were synthesized.12,13 Investigating the FT-IR spectra
and X-ray crystallographic structure for N,N-bis(3,5-
dimethyl-2-hydroxybenzyl)methylamine, Dunkers et al.12
proposed a hydrogen-bonding scheme involving both
OH‚‚‚O intermolecular and intramolecular hydrogen
bonding as well as OH‚‚‚N intramolecular hydrogen
bonding. The more precise FT-IR band assignments for
each hydrogen species in benzoxazine model compounds
were reported by Kim et al.13 by comparing the spectra
for simplified asymmetric dimers and an aromatic
amine-based model dimer. Also, they proposed a differ-
ence in network structure between aliphatic amine-
based polybenzoxazines and aromatic amine-based
polybenzoxazines. One possibility for this difference, the
Syn th esis of P olyben zoxa zin e Mon om er s a n d P oly-
m er s. Benzoxazine monomers, based on bisphenol A, were
synthesized and purified according to the procedure of Ning
and Ishida7 or Ishida.15 These benzoxazine monomers were
polymerized without added initiator or catalyst according to
the method reported in a previous paper.8 The monomers used
are summarized in Table 1, and the generalized polymer
structure is shown in Scheme 1.
Syn th esis of Sym m etr ic Dim er s. The symmetric model
dimers for polybenzoxazines were synthesized according to a
previous study using 2,4-dimethylphenol, formaldehyde, and
primary amines.16 All products except for the aniline dimer
were recrystallized three times in hexane. The aniline dimer
was recrystallized from hexane and subsequently purified by
column chromatography with silica gel using hexane/methyl-
ene chloride (20:1) as the eluent. The resulting dimers are
summarized in Table 2, and the dimer formation reaction is
shown in Scheme 2.
Syn th esis of Asym m etr ic Dim er s. The asymmetric model
dimers based on different primary amines (Table 3), which
have only one hydroxyl group in the structure, were synthe-
10.1021/ma030108+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/30/2003