Synthesis of polybenzoxazine precursors using thiols: Simultaneous thiol-ene and ring-opening reactions

Article abstract of DOI:10.1002/pola.26202

A new class of polybenzoxazine precursors of combined molecular structure of benzoxazine in the open and ring form has been developed. Thermally curable benzoxazine networks were obtained by simultaneous photoinduced thiol-ene and Catalytic Opening of the Lateral Benzoxazine Rings by Thiols (COLBERT). The thiol-ene reactions were initiated by photolysis of a free radical photoinitiator, 2, 2-dimethoxy-2-phenyl acetophenone (DMPA), and the competing COLBERT reaction was triggered by thiol compound, 1,2-ethanedithiol, present in the reaction mixture. The extent of reactions was evaluated by conducting experiments both under UV irradiation and in dark using model benzoxazines. The precursor soft films (pre-P(B-ala-DTE)) were prepared by irradiating solutions of diallyl functional benzoxazine (B-ala), 1,2-ethanedithiol and DMPA. The obtained pre-P(B-ala-DTE) films were then cured through thermally activated ring opening reaction of remaining benzoxazine groups yielding a more rigid and tough film. Thermal and mechanical properties of the films were investigated by DSC, DMA and TGA and compared with a typical polybenzoxazine, P(B-ala).

Full text of DOI:10.1002/pola.26202

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Synthesis of Polybenzoxazine Precursors Using Thiols: Simultaneous
Thiol–Ene and Ring-Opening Reactions
Zeynep Beyazkilic, Muhammet U. Kahveci, Binnur Aydogan, Baris Kiskan, Yusuf Yagci
Faculty of Science and Letters, Department of Chemistry, Istanbul Technical University, Maslak 34469, Istanbul, Turkey
Correspondence to: Y. Yagci (E-mail: yusuf@itu.edu.tr)
Received 10 April 2012; accepted 29 May 2012; published online
DOI: 10.1002/pola.26202
ABSTRACT: A new class of polybenzoxazine precursors of com-
bined molecular structure of benzoxazine in the open and ring
form has been developed. Thermally curable benzoxazine net-
works were obtained by simultaneous photoinduced thiol-ene
and Catalytic Opening of the Lateral Benzoxazine Rings by Thi-
ols (COLBERT). The thiol-ene reactions were initiated by pho-
tolysis of a free radical photoinitiator, 2, 2-dimethoxy-2-phenyl
acetophenone (DMPA), and the competing COLBERT reaction
was triggered by thiol compound, 1,2-ethanedithiol, present in
the reaction mixture. The extent of reactions was evaluated by
conducting experiments both under UV irradiation and in dark
using model benzoxazines. The precursor soft films (pre-P(B-
ala-DTE)) were prepared by irradiating solutions of diallyl func-
tional benzoxazine (B-ala), 1,2-ethanedithiol and DMPA. The
obtained pre-P(B-ala-DTE) films were then cured through ther-
mally activated ring opening reaction of remaining benzoxa-
zine groups yielding a more rigid and tough film. Thermal and
mechanical properties of the films were investigated by DSC,
DMA and TGA and compared with a typical polybenzoxazine,
C
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P(B-ala).
2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: benzoxazine; polybenzoxazine; crosslinking; ring-
opening polymerization; thiol-ene; thermosets; phenolic resin
INTRODUCTION Benzoxazine-based phenolic thermosets have
gained an increasing interest in the field of polymer research
owing to the superior properties that overcome several short-
comings of conventional novolac and resole-type phenolic res-
ins. These materials offer low water absorption, high char yield,
resistance against flame, high modulus, high strength, high
glass transition temperatures, chemical resistance, long shelf
life, they have very limited volumetric change upon curing and
strong acid catalysts are not required for curing.1 Moreover, the
molecular design flexibility of benzoxazines is enormous. Basi-
cally, the synthesis of a monomer is carried out by the reaction
of any suitable phenols, formaldehyde, and primary amines (ali-
phatic or aromatic).2–15 Synthesis of polybenzoxazine network
is a thermally induced ring-opening polymerization of precur-
sor 1,3-benzoxazines which can be accomplished without any
initiator or curing agent (Scheme 1).16–19
zoxazines, additional sites such as allyl-,7,22 acetylenyl-,23–26
nitrile-,27 and propargyl25,28 groups have been introduced into
benzoxazine monomers. These groups take part in further
crosslinking reactions, yielding three-dimensional networks that
exhibit high thermal and mechanical stability, and high solvent
and moisture resistance. Similarly, incorporating some functional
groups such as carboxylic and phenolic groups allows lowering
curing temperature. The side-, main-, and end-chain polymeric
benzoxazine precursors or blending benzoxazines with a high-
performance polymer, filler, and fiber have been successfully
used to reduce brittleness and improve thoughness.29–45
Thiol–ene reactions, originally developed for UV-curing appli-
cation, are versatile techniques for the synthesis of numer-
ous polymeric structures and considered to represent some
characteristic related to the click reactions.46 Relative to the
other click reactions, thiol–ene chemistry provides improved
tolerance to oxygen and water, rapid reaction, and significant
versatility in ene functional groups.47,48 Moreover, these
reactions can be conducted thermally and photochemically at
room temperature without any catalyst. Typical applications
of thiol–ene reactions include modification of polymers and
surfaces, preparation of bioconjugated polymers, synthesis of
star polymers, dendrimers, dental materials, etc.49–56
Despite their various attractive properties, polybenzoxazines
have drawbacks and limitations related to their processabil-
ity, brittleness, high curing temperatures (ꢀ200ꢁC or more),
and low degree of polymerization for mono-functional
monomers.20,21
To overcome these problems, various modification strategies
have been proposed. To improve the performance of polyben-
C
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SCHEME 1 Thermally activated ring-opening polymerization of
SCHEME 3 Proposed mechanism for the ring opening of ben-
bisbenzoxazines.
zoxazine by thiols.
Thiol–ene reactions are fundamentally based on free radical
reactions, but proceed through a step-growth process. As
presented on the example of polymer functionalization in
Scheme 2, the thiol–ene reaction is established by the radical
formation, hydrogen abstraction, and thiyl radical addition to
the olefinic bond. The primary radical addition and coupling
reactions reduces the efficiency of the overall process.
study, a bisbenzoxazine-based ene-monomer, namely diallyl
benzoxazine (B-ala), and a difunctional thiol compound, 1,2-
ethanedithiol, were utilized in photoinduced thiol–ene and
COLBERT reactions. As will be discussed below, these com-
peting reactions essentially yield thermally curable benzoxa-
zine networks as precursor films. Subsequently, the precur-
sor films were subjected to thermal treatment for the
conventional ring-opening reaction of benzoxazine units. The
films were thermally and mechanically characterized before
and after thermal process.
In a recent publication, benzoxazines were reported to
undergo catalytic ring-opening reaction at ambient condi-
tions by thiols. The process is known as ‘‘Catalytic Opening
of the Lateral Ben-zoxazine Rings by Thiols’’ (COLBERT)
which proceeds via a two-step acid catalyzed nucleophilic
addition.57 In the first step, the benzoxazine amine is proto-
nated by thiol to form an ionic intermediate. Then, immedi-
ate attack by sulfide on the methylene carbon results in the
ring opening of the benzoxazine (Scheme 3).
EXPERIMENTAL
Materials
The reagents and solvents including 4,40-isopropylidenedi-
phenol (Aldrich, 97%), paraformaldehyde (Acros, 96%), allyl
amine (Aldrich, 98%), sodium hydroxide (Acros, >97%), so-
dium sulfate (Acros, 99%), 1,4-dioxane (Aldrich, 99%), tolu-
ene (Aldrich, 99%), methanol (MeOH, Aldrich, 99%), diethy-
lether (Carlo Erba, 99.8%), N, N-dimethyl formamide (DMF,
Merck, 99.8%), and chloroform (Acros, >99%) were used as
received for the synthesis of the benzoxazine derivative (B-
ala). Difunctional thiol compound, 1,2-ethanedithiol (Merck,
99%), and free radical photoinitiator, 2,2-dimethoxy-2-phe-
nylacetophenone (DMPA, Ciba), were also used as received.
However, when the benzoxazine moiety is equipped with the
allyl functionalities, these investigations do not answer the
question whether the added thiol groups react directly with
benzoxazine according to COLBERT reaction and whether
thiol–ene reactions are dominant process upon photolysis in
the presence of a photoinitiator. Also, both and other reac-
tions may prevail. Therefore, detailed polymerization and
model studies by using appropriately functionalized mole-
cules were performed to provide information that would be
useful in confirming the validity of the both mechanisms. In
this connection, it is worth to mention the work of Agag
et al. who demonstrated that two independent reactions of
benzoxazines may proceed simultaneously when benzoxa-
zines are equipped with suitable functionalities.58 In this
Characterization
1H NMR spectrum of B-ala was recorded in CDCl3 with
Si(CH3)4 as an internal standard, using a Bruker AC250
(250.133 MHz) instrument. Fourier transform infrared
(FTIR) spectra of the precursor and cured films were
obtained with Perkin-Elmer FTIR Spectrum One spectrome-
ter. Curing of the films was monitored on a Perkin Elmer Di-
amond differential scanning calorimetry (DSC) with a heating
rate of 10ꢁCꢂminꢃ1 under nitrogen flow. Thermal gravimetric
analysis (TGA) was performed on Perkin-Elmer Diamond
TA/TGA with a heating rate of 10ꢁCꢂminꢃ1 under nitrogen
flow. Dynamic viscoelastic measurements were conducted on
Perkin Elmer dynamic mechanical analyzer model DMS6100
at 1 Hz at a heating rate of 5ꢁCꢂminꢃ1. Molecular weight of
oligomeric linear polymer was determined by gel permeation
chromatography (GPC) instrument, Viscotek GPCmax Auto-
sampler system, consisting of a pump, three ViscoGEL GPC
columns (G2000HHR, G3000HHR, and G4000HHR), a Viscotek
UV detector, and a Viscotek differential refractive index de-
tector with a tetrahydrofuron at a flow rate of 1.0 mL minꢃ1
at 30ꢁC. Both detectors were calibrated with PS standards,
SCHEME 2 Polymer functionalization by thiol–ene reactions.
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SCHEME 4 Synthesis of B-ala monomer.
having narrow molecular weight distribution. Data were ana-
lyzed using Viscotek OmniSEC Omni-01 software.
stepwise at 80ꢁC for 5 h and 110ꢁC for 3 h, and then at 160,
180, and 200ꢁC for 2 h each in an air oven. The films then
were postcured at 240ꢁC for 1 h resulted in transparent
brown films.
Synthesis of 6,60-(Propane-2,2-diyl)bis(3-allyl-3,4-
dihydro-2H-benzo[e][1,3] oxazine) (B-ala)
In a 250-mL flask, allyamine (30.8 g, 0.54 mol) was dis-
solved in 200 mL of 1,4-dioxane at room temperature. The
solution was cooled in an ice bath, followed by portion-wise
addition of paraformaldehyde (32.5 g, 1.08 mol) during 10
min of stirring in an ice bath. Then, bisphenol-A (61.6 g,
0.27 mol) was added to the cooled solution. The temperature
was raised and refluxed for 24 h. After the removal of 1,4-
dioxane under vacuum, the resulted crude product was puri-
fied by dissolving in 200 mL of diethyl ether and washing
various times with 0.1 N sodium hydroxide, and finally two
times with distilled water. Pale yellowish oily product was
obtained after drying with NaSO4, filtering and evaporating
diethyl ether. The oily viscous product was dissolved in 20
mL of MeOH; water was added to this solution until it
became turbid, and then the mixture was refrigerated. The
precipitated sticky mass was obtained after decantation and
subsequently washing with water. The mass was dried under
RESULTS AND DISCUSSION
To use in the proposed reactions, first, allyl functional ben-
zoxazine (B-ala) was synthesized through the Mannich and
respective ring closure reactions between 4,40-isopropylide-
nediphenol (bisphenol-A) and allylamine in the presence of
formaldehyde in one pot (Scheme 4).
We started our investigation by studying the photolysis of B-
ala (ene-monomer) and 1,2-ethanedithiol in the presence of
a typical free radical photoinitiator DMPA. It should be
pointed out that photochemically, benzoxazines can be poly-
merized only by a cationic route. As stated in the INTRO-
DUCTION section, it is expected that thiols could participate
in thiol–ene reactions with the allylic groups present in the
structure. DMPA generates alkoxybenzyl and benzoyl radicals
by a-cleavage under UV irradiation. Both radicals can
abstract labile hydrogens of the thiol compound, leading to
the formation of thiyl radicals (Scheme 5). Subsequently, the
thiyl radicals readily react and couple with allylic group. As
both components are difunctional, such step-growth reac-
tions are expected to form linear polymeric structures.
1
vacuum at 60ꢁC for 24 h to afford solid (Yield, 62%). The H
NMR spectrum of B-ala is as follows: d (CDCl3) 6.94 (dd, J ¼
8.3 and 2.1 Hz, 2H, ArH); 6.79 (dd, J ¼ 2.0 Hz, 2H, ArH);
6.67 (dd, J ¼ 8.3 Hz, 2H, ArH); 5.89 (multiplet, 2H, CH2
¼
CHA); 5.23 (dd, J ¼ 12.2 and 1.5 Hz, 2H, CH¼¼CH2); 5.17
(dd, J ¼ 5.3 and 1.3 Hz, 2H, CH¼¼CH2); 4.82 (s, 4H,
NACH2AO); 3.93 (s, 4H, ArACH2ꢃ); 3.37 (d, 4H,
CH2¼¼CHACH2ꢃ); 1.58 (s, 6H, ACH3).
Hydrogen abstraction reactions of either triplet excited state
or photochemically generated radicals from benzoxazine
methylene bridge should also be considered.59 Both path-
ways would give rise to additional reactions with thiol and
oxazine groups (Scheme 6).
Preparation of Precursor Films [pre-P(B-ala-DTE)]
by Photolysis
In contrast to the expected linear polymer formation, upon
photolysis soft but mostly crosslinked polymeric materials
were obtained, indicating, in addition to thiol–ene reactions,
participation of simultaneous ring-opening reactions. Some
of the benzoxazine groups may interact from the ring side
Typical procedure for the preparation of the precursor films
is as follows: B-ala (1.0 g, 2.6 ꢄ 10ꢃ3 mol), 1,2-ethanedithiol
(214 mL, 2.6 ꢄ 10ꢃ3 mol), and photoinitiator DMPA (6.7 mg,
2.6 ꢄ 10ꢃ5 mol, 1 % equiv of thiol reagent) were dissolved
in a minimal amount of the solvent, chloroform, required to
dissolve the photoinitiator. For the preparation of flexible
R
films, the mixture was placed equally into TeflonV plates and
exposed to light continuously at room temperature for 6 h in
a Rayonet merry-go-round photo reactor in which the sam-
ple was surrounded by a circle of 16 lamps emitting light
nominally at 350 nm. The light intensity at the location of
the sample was measured by a Delta Ohm model HD-9021
radiometer (I ¼ 3.0 mWꢂcmꢃ2).
Thermal Curing of B-ala and pre-P(B-ala-DTE) Films
Films of bisallyl benzoxazine were prepared by solvent cast-
R
ing of B-ala (50% w/v in DMF) over TeflonV plates. Curing
SCHEME 5 Photoinduced thiol–ene reaction of B-ala.
of both B-ala and pre-P(B-ala-DTE) films was performed
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SCHEME 6 Possible photoinduced side reactions of benzoxazines in the presence of DMPA.
with thiols and undergo ring-opening reaction as shown in
Scheme 3. Either or both reactions would essentially yield
branched and networked polymers, possessing both ring and
open benzoxazine structures (Scheme 7).
stretching of allyl group at 3084 cmꢃ1 slightly decreases after
the reaction of B-ala and thiol compound. The presence of the
bands at 1495, 925, and 1225 cmꢃ1 clearly confirms that
there is remaining unopened oxazine rings. Notably, after the
thermal treatment of [pre-P(B-ala-DTE)], while the absorption
band at 1495 cmꢃ1 shifts to 1474 cmꢃ1, the bands at 1225
and 925 cmꢃ1 disappear completely (Fig. 1).
In this process, besides insoluble crosslinked structures,
hardly soluble oligomeric products were also formed. The
molecular weights of these oligomers were found to be Mn
¼
1000–2000. The relatively low molecular weights
To get more insight to the polymerization mechanism and
evaluate the structure of the final network, we conducted
some model experiments using benzoxazine compounds with
and without allyl groups, P-ala and P-a, respectively. These
compounds were separately irradiated 350 nm for 24 h in
the presence of monofunctional thiol compound, propane-1-
thiol and photoinitiator, DMPA under the same experimental
conditions to that applied for obtaining [pre-P(B-ala-DTE)].
Depending on the presence of ally functionality, possible
reactions involving both COLBERT and/or thiol–ene reactions
are shown in Scheme 8.
observed with the linear polymers are probably owing to the
chain transfer reactions favored by oxygen. Indeed, the
experiments conducted under oxygen-free conditions yielded
insoluble polymers.
The occurrence of simultaneous thiol–ene and COLBERT ring-
opening reactions was confirmed by FTIR spectral analysis.
Figure 1 shows FTIR spectra of the starting compound B-ala,
the film [pre-P(B-ala-DTE)] (Precursor Poly(B-ala-Dithiol-
ethane) obtained by the photolysis in the presence of 1,2-
ethanedithiol and photoinitiator DMPA and thermally post-
cured film [P(B-ala-DTE)]. The pure monomer B-ala exhibits
characteristic absorption bands of the allyl groups emerging
at 3084 cmꢃ1 (stretching of ¼¼CAH) and 1644 cmꢃ1 (stretch-
ing of C¼¼C) (Fig. 1). Moreover, spectrum also reveals asym-
metric stretching of CAOAC at 1225cmꢃ1 and tri-substituted
benzene ring modes at 925 and 1495 cmꢃ1 belonging to the
absorptions of benzoxazines. The absorption of ¼¼CAH
Experimentally, samples were taken from reaction mixtures
at different intervals and analyzed by NMR spectroscopy. Fig-
1
ures 2 and 3 show H NMR spectra of solutions prior to irra-
diation and after 24 h of irradiation. In both experiments, ox-
azine ring opening was observed which was particularly
intense for P-ala monomer (Fig. 3). However, the thiol–ene
reaction also took place although most of the thiols reacted
SCHEME 7 Photoinduced curing of B-ala in the presence of DMPA and 1,2-ethanedihiol forming pre-P(B-ala-DTE).
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FIGURE 2 1H NMR spectrum of model reaction for P-a.
became more rigid and tough owing to the ring-opening po-
lymerization of the remaining benzoxazines (Scheme 9).
FIGURE 1 FTIR spectra of (a) B-ala, (b) pre-P(B-ala-DTE), and
Thermal postcurings of both the pure B-ala film and the film
obtained by the thiol–ene reaction were monitored by DSC.
A typical unsymmetrical broad exotherm with a shoulder
was observed during the curing of pure B-ala film. The ring-
opening exotherm of the oxazines was observed around
230ꢁC as a shoulder, whereas the exotherm centered around
278ꢁC was attributed to thermal cure of the allyl groups
[Fig. 4(a)]. On the other hand, two exotherms were observed
on thermogram of pre-P(B-ala-DTE) around 175 and 260ꢁC
[Fig. 4(b)]. The first exotherm with onset temperature at
140ꢁC and exotherm maximum at 175ꢁC was attributed to
exothermic thiol–ene reaction between unreacted allyl func-
tionality of benzoxazines and unreacted thiol groups,
whereas the second exotherm with onset at 225ꢁC and peak
at 260ꢁC was resulted from thermal curing of the benzoxa-
zine and residual allylic double bonds.
(c) [P(B-ala-DTE)].
with oxazine rings that is proved by the shift of the peak
belonging NACH2ꢃC¼¼CH at 3.37–2.70 ppm. Using the inte-
grated area ratios of the peaks at 2.70 and 3.37 ppm, it is
calculated that 14% of the allyl groups reacted with thiol
compound after 24 h. Based on the results of the model
experiments, it is evident that COLBERT reaction is predomi-
nant for P-ala monomer. In contrast, the corresponding reac-
tion with P-a monomer occurred in a much less extend. The
same model reactions were also performed in dark. NMR
analysis of the reaction products gave strong evidence for
the occurrence of the ring-opening reaction between thiol
compound and oxazine ring. These observations are in agree-
ment with the previous report which confirms in dark, indi-
cating the independency of COLBERT reaction from light.
This result is in agreement with the previous reports about
COLBERT reaction.57
Dynamic mechanical analysis (DMA) of pre-P(B-ala-DTE) and
P(B-ala-DTE) was examined, and compared with polybenzox-
azine P(B-ala). Figures 5–7 show the temperature depend-
ence of the storage modulus (E0), loss modulus (E00) and tan
d for pre-P(B-ala-DTE), P(B-ala-DTE), and P(B-ala), respec-
tively. The Tg values as defined by the E00 and tan d peaks
are summarized in Table 1. For the precursor polymer, pre-
P(B-ala-DTE), two Tgs were observed at 12 and 123ꢁC from
the maximum of loss modulus and at 30 and 151ꢁC from the
maximum of tan d. The storage modulus decreased sharply
It should be noted that after thiol treatment some of the
benzoxazine rings preserved and unreacted thiol functional-
ities were also determined. Upon heating, the soft film
SCHEME 8 Photoinduced reactions of benzoxazine monomers
with propan-1-thiol in the presence of DMPA.
FIGURE 3 1H NMR spectrum of model reaction for P-ala.
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SCHEME 9 Thermally activated post curing of pre-P(B-ala-DTE).
FIGURE 6 Dynamic mechanical analysis of P(B-ala-DTE) poly-
merized at 240ꢁC for 1 h in air: (h) E0, (*) E00, and (~) tan d.
FIGURE 4 DSC profiles of (b) pure B-ala film and (a) pre-P(B-ala-
DTE) during their postcuring.
and augmented rigidity of the polymer because of increased
crosslinking density in certain domains. It is quite likely that
the network contains two different macromolecular chains:
polymers rich in thiol and polybenzoxazine. This statement
was evidenced by the observation of microphase separation
in DMA investigations of the cured films since two Tgs were
detected for P(B-ala-DTE) at 150 and 255ꢁC from the maxi-
mum of loss modulus and at 170 and 245ꢁC from the maxi-
mum of tan d. In contrast, the typical polybenzoxazine,
P(B-ala), exhibited only one Tg at relatively higher temperature,
at ca. 2 and 115ꢁC. According to these results, it could be
assumed that the polymer obtained by thiol reactions, pre-
P(B-ala-DTE), was not completely crosslinked and still
contained some oligomeric parts. The presence of these
oligomeric structures was proved by GPC analysis of the
soluble part. Thermal treatment of pre-P(B-ala-DTE) resulted
in ring-opening polymerization of the benzoxazine moieties
FIGURE 5 Dynamic mechanical analysis of pre-P(B-ala-DTE)
polymerized under k > 350 nm light for 6 h in air: (h) E0, (*)
E00, and (~) tan d.
FIGURE 7 Dynamic mechanical analysis of P(B-ala) polymerized
at 240ꢁC for 1 h in air: (h) E0, (*) E00, and (~) tan d.
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TABLE 1 Thermal Properties of Benzoxazine Polymers
Weight-Loss
Temperature
(ꢁC)
Tg (ꢁC)
Polymer
E00
Tan d
T5%
T10%
Char Yield (%)
Pre-P(B-ala-DTE)
P(B-ala-DTE)
P(B-ala)
12 and 123
150 and 255
276
30 and 151
170 and 245
298
–
–
–
255
334
300
363
17
25
276ꢁC from the maximum of loss modulus and 298ꢁC from
the maximum of tan d. The storage modulus started to
decrease at ca. 280ꢁC. The lower Tg for P(B-ala-DTE) com-
pared to P(B-ala) may be attributed to the presence of cross-
linked parts of pre-P(B-ala-DTE), leading to a lower crosslink-
ing density.
mers by successive photochemical and thermal means exhib-
ited comparable thermal stability to classical polybenzoxa-
zines owing to the higher crosslinked density.
ACKNOWLEDGMENTS
The authors thank Istanbul Technical University for financial
support. One of the authors (M. U. Kahveci) thanks TUBITAK
(Turkish Scientific and Technological Council) for the financial
support by means of a graduate program.
Thermal stabilities of the films were investigated by TGA under
nitrogen atmosphere. The comparative TGA thermograms of
P(B-ala-DTE) and P(B-ala) are shown in Figure 8. The results
are also summarized in Table 1. The 5 and 10% weight loss
temperatures (T5% and T10%) for P(B-ala) are at 334 and
363ꢁC, respectively. Incorporation of linear alkyl groups into the
polybenzoxazines decreases aromatic content of the materials
and consequently their thermal stabilities.30 Therefore, the T5%
and T10% for P(B-ala-DTE) were around 255 and 300ꢁC, respec-
tively. Moreover, the similar fashion was observed for char
yields of P(B-ala) (25%) and P(B-ala-DTE) (17%) at 800ꢁC.
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