Reactive & Functional Polymers 73 (2013) 360–368

Contents lists available at SciVerse ScienceDirect

Reactive & Functional Polymers

journal homepage: www.elsevier .com/ locate / react

Polymerization behavior of methylol-functional benzoxazine monomer

Mohamed Baqar a, Tarek Agag b,⇑,1, Hatsuo Ishida b, Syed Qutubuddin a,b

a Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USAb Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

a r t i c l e i n f o a b s t r a c t

Article history:Available online 14 May 2012

Keywords:Methylol groupBenzoxazine resinNetwork structureKineticsCatalyst

1381-5148/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.reactfunctpolym.2012.04.0

⇑ Corresponding author.E-mail address: taa16@case.edu (T. Agag).

1 On leave from Tanta University, Egypt.

This study focuses on methylol functional benzoxazines as precursors to build a network structure utiliz-ing both benzoxazine and resole chemistry. The first part is a review of systems that contain methylolgroups which play a role on their crosslinking formation. The polymerization mechanism and propertiesof resoles will be highlighted as the most abundant polymers that are characterized by polymerizationthrough condensation reaction of methylol group. In the second part, the effect of incorporating methylolgroup into benzoxazine monomers is studied. Differential scanning calorimetry (DSC) is used to study theeffect of methylol group on the rate of polymerization. Kissinger and Ozawa methods using non-isother-mal DSC at different heating rates show that methylol monomer exhibits lower average activation energycompared to the un-functionalized monomer. The effect of adding catalysts into the monomers is alsostudied. p-Toluene sulfonic acid (PTSA) is found to be more efficient than 1-methyl-imidazole (IMD)and lithium iodide (LiI) in the case of methylol monomer due to its ability of accelerating both the meth-ylol condensation and ring-opening polymerization. Additionally, thermal behavior of the monomers isstudied using thermogravimetric analysis (TGA).

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The diversity of the applications of crosslinking polymers foradhesives, coatings, and electric and electronic materials originatesfrom their network structures. Generally, the network structure ofthe polymers consists of long macromolecular chains crosslinkedwith each other by chemical or physical bonding. The crosslinkedpolymers are characterized by their three-dimensional networkstructures which are responsible for dimensional stability, chemi-cal resistance, and improved thermal and physical properties. Thepolymerization process can take place by different methods,including heat, chemical reaction, or irradiation [1,2]. Therefore,polymers can be formed from functional precursors by reactionsbetween their functionalities resulting in bond formation. Someof those functionalities include acetylene, propargyl, nitrile, allyl,epoxy, furan, methacryloyl and methylol. In order for the networkstructure to be formed, monomer has to possess at least two ormore polymerizable groups per molecule [3]. For example, a net-work of three-dimensional crosslinked epoxy resin is achieved bypolymerizing the linear epoxy resins in the presence of curingagents such as amines, anhydrides and mercaptans [4].

In this review, some special features of thermosetting polymerswherein methylol functionality plays a role towards network

ll rights reserved.17

formation are summarized. Furthermore, the effect of incorporat-ing methylol group into benzoxazine structure together withadding a catalyst is emphasized. The kinetic study of the polymer-ization of benzoxazine methylol monomer was conducted usingboth Kissinger and Ozawa methods.

2. Traditional phenolic resin (resoles)

Phenol–formaldehyde is considered to be the first man-madepolymer since Baekeland developed a commercial manufacturingprocess in 1907 [5]. Two classes of traditional phenolic resinsknown as resoles and novolacs are synthesized by varying thecatalyst type and the molar ratio of formaldehyde to phenol. Thedifference between these classes is that resoles are heat polymer-izable whereas novolacs require addition of a crosslinking agentfor polymerization. The synthesis reaction of resoles is exothermicand normally run under basic conditions with ratio of formalde-hyde:phenol higher than 1. The initial product is monomethylol-phenols followed by a mixture of dimethylolphenols andtrimethylolphenols. Phenol monomers substituted with methylolgroups can then react with themselves in a basic medium and inthe absence of formaldehyde via condensation to yield higherorder compounds. The condensation reactions produce diph-enylmethane derivative compounds with aromatic rings connectedby methylene or dimethylene ether bridges [6]. Dimethylene etherbridges are found to be negligible in the polymerized resoles [7].Different studies reveal the hypothesis that quinone methide exists

OHCH2OH

OCH2OH

-H2O

-OH

K1

K2

CH2

O OCH2

OHCH2OH

OCH2OH

OCH2OH

CH2HO

OHCH2OH

CH2HOOH

CH2OH

OCH2OH

K3

+H2O

K4

+OH

K5

K6

+ H2O

+ H2O

K1,K2 <<K3 ,K4,K5,K6

Quinone methide

Scheme 1. The mechanism of methylol condensation reaction.

OH

CH2O+

OH

Cat.

OH OH

ΔΔCH2OH

1,2,3

OH

OH

OHOH

OH

Scheme 2. Synthesis of resoles.

M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368 361

in the form of a resonance between quinoid and benzenoid struc-ture [8,9] and the condensation reaction can be illustrated as inScheme 1.

Due to the high reactivity of para-methylol groups in the con-densation reaction, methylene bridges formed are mainly in theform of ortho–para (o,p) and para–para (p,p) bridges with theortho–ortho (o,o) bridges rarely forming [10,11]. The typical syn-thesis and polymerization of resoles are shown in Scheme 2.

The longevity of the dominant resole resins is due to their excel-lent fire resistance and superb cost effectiveness along with goodthermal and mechanical properties. On the other hand, the use ofresoles as the traditional phenolic resins is associated with manydisadvantages including poor shelf life, use of strong acid or base cat-alyst for polymerization, and the reaction by-products producedduring polymerization, such as water and formaldehyde, which leadto void formation and volume shrinkage in the resulting polymer [6].

3. Benzoxazine resins

Benzoxazine resins commonly known as polybenzoxazines area recently developed class of polymers that has a wide range ofmechanical and physical properties which can be tailored to vari-ous applications. The diversity of their applications comes from

the extremely flexible molecular design, near zero shrinkage uponpolymerization and excellent thermal and mechanical properties[12]. On the other hand, there are a few properties where improve-ment is desired. For example, benzoxazine resins derived frommonomeric benzoxazines show brittleness as a common disadvan-tage with other thermosetting resins. Furthermore, the require-ment of somewhat high temperature for polymerization toproceed is an inconvenience for certain applications. To minimizethese problems, various approaches including catalysts [13–15]and benzoxazines with special side groups [16–20] have beendeveloped to promote the polymerization at lower temperatureand increase the crosslinking density. Efficient initiator and cata-lytic systems include acids, bases and transition metals [21–23].These are all well-known initiators and catalysts used for cationicring-opening polymerization. On the other hand, among the func-tional groups that increase the network density of benzoxazineresins are acetylene [24], nitrile [25], propargyl [26], allyl [27], fur-an [28], epoxy [29], maleimides and norbornene [30,31], methacry-loyl [32,33] and methylol [34].

In continuation of our recent study regarding methylol benzox-azine monomers, the crosslinking kinetic of this class of monomersis investigated. The ring-opening polymerization temperature inthe presence of various catalysts is also studied.

362 M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368

3.1. DSC theoretical analysis

The literature indicates that different techniques implementedto study the polymerization kinetics of thermosetting polymers in-clude differential scanning calorimetry (DSC) [35–37], Fouriertransform infrared spectroscopy (FT-IR) [38], and rheokinetic mea-surements [39]. Generally, the non-isothermal or isothermal DSCmethods have been considered as the most utilized techniquesfor evaluation of kinetic parameters. The relatively short periodof time required to obtain the kinetic data is considered as themain advantage of using non-isothermal DSC technique; however,the complexity of the mathematical analysis of the temperatureintegral is inherent to this approach. On the other hand, the useof isothermal method is associated with some disadvantages, suchas the instability of the DSC heat flow at the beginning of the mea-surement which leads to experimental errors. Therefore, in the cur-rent study, non-isothermal DSC method is used to study thecrosslinking reactions of the synthesized monomers and find outthe kinetic parameters for the polymerization reaction.

In this study, the measured heat flow from the DSC experiment(dH/dt) is assumed to be proportional to the reaction rate (da/dt).Moreover, the reaction rate at a given temperature is only a func-tion of the conversion regardless of the exact reaction mechanismand given by the following expression:

dadt¼ b

dadT¼ kðTÞf ðaÞ ð1Þ

where b ¼ dTdt is the constant heating rate, f(a) is a conversion func-

tional depending on the reaction mechanism and k(T) is a reactionrate constant depending on the temperature and calculated fromArrhenius law as expressed in following equation:

kðTÞ ¼ Aexpð� Ea

RTÞ ð2Þ

where A is the pre-exponential factor, Ea is the activation energy, Tis the absolute temperature, and R is the universal gas constant.

3.1.1. The Kissinger methodThis method is based on the fact that the temperature of the

exothermic peak Tp varies with the heating rate. In addition, it as-sumes that the maximum reaction rate occurs at the temperatureof the exothermic peak. Therefore, a linear relationship betweenthe logarithms of b

T2p

against the inverse of the temperature of theexothermic peak is used to obtain the activation energy. The meth-od is based on the following equation:

lnb

T2p

!¼ ln

Q pAREa

� �� Ea

RTpð3Þ

where Tp is the exothermic reaction peak, Ea is the activation energyand

Q p ¼ �df ðaÞ

da

� �a¼ap

3.1.2. The Ozawa methodThis method is based on the assumption that the degree of con-

version at temperature of the exothermic peak for different heatingrates is constant. Therefore, at the same conversion, the slope ofthe linear relationship between the logarithm of the heating rate(ln b) and the inverse of the temperature of the exothermic peak(Tp) is used to obtain Ea.

ln b ¼ lnAEa

R

� �� ln FðaÞ � 5:331� 1:052

Ea

RT

� �ð4Þ

FðaÞ ¼Z a

0

daf ðaÞ ð5Þ

where F(a) is a constant function

3.2. Experimental section

3.2.1. Materials4-Hydroxybenzyl alcohol (pHBA) (98%), phenol (99%), 1-

methyl-imidazole (IMD) (99%), p-toluidine (pt) (98%) and lithiumiodide (LiI) were obtained from Sigma–Aldrich and used as re-ceived. Paraformaldehyde (96%) was obtained from Acros OrganicsUSA. P-Toluenesulfonic acid (PTSA) (99%) was obtained from FlukaChemie Gmbh, Switzerland. Toluene, acetone and chloroform wereobtained from Fisher and used as received.

3.2.2. Preparation of 3-p-tolyl-3,4-dihydro-2H-benzo[e][1,3]oxazine[abbreviated as P-pt]

Into a 50 mL flask, p-toluidine (1.10 g, 10 mmol), phenol (0.95 g,10 mmol) and paraformaldehyde (0.75 g, 20 mmol) with a molarratio of 1:1:2, respectively, were mixed in toluene (14 mL). Themixture was then heated at 100 �C for 10 h. The reaction was al-lowed to cool to room temperature followed by evaporation to re-move the solvent. The product was then re-dissolved in chloroformand base washed three times with 1 N sodium hydroxide solutionfollowed by one wash with water. The solution was then dried oversodium sulfate anhydrous, filtered and evaporated to remove thechloroform. The product was then re-crystallized from toluene toafford a white crystalline product (yield: 1.64 g, 73%).

IR spectra (KBr, cm�1): 1222 (asymmetric stretching of C–O–C),1153 (asymmetric stretching of C–N–C), 941 (out-of-plane vibra-tion, benzene ring to which oxazine is attached).

1H NMR dH (600 MHz, CDCl3, ppm) 2.22 (s, Ar–CH3), 4.53 (s, Ar–CH2–N), 5.27 (s, N–CH2–O), 6.76–7.08 (m, Ar).

3.2.3. Preparation of (3-p-tolyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methanol [abbreviated as pHBA-pt]

Into a 50 mL flask, p-toluidine (1.10 g, 10 mmol), pHBA (1.25 g,10 mmol) and paraformaldehyde (0.76 g, 20 mmol) with a molarratio of 1:1:2, respectively, were mixed in toluene (15 mL). Themixture was then heated at 100 �C for 10 h. The reaction was al-lowed to cool to room temperature followed by evaporation to re-move the solvent. The product was then re-dissolved in chloroformand base washed three times with 0.5 N sodium bicarbonate solu-tion followed by once with water. The solution was then dried oversodium sulfate anhydrous, filtered and evaporated to remove thechloroform. The product was then re-crystallized from toluene toafford a white crystalline product (yield: 1.84 g, 72%).

IR spectra (KBr, cm�1): 1502 (stretching of trisubstituted ben-zene ring), 1475 (bending of CH2 of methylol), 1230 (asymmetricstretching of C–O–C), 1035 (stretching of C–O of methylol), 943(out-of-plane vibration, benzene ring to which oxazine isattached).

1H NMR dH (600 MHz, CDCl3, ppm) 1.86 (s, –OH), 2.24(s, Ar–CH3), 4.52 (s, Ar–CH2–OH), 4.55 (s, Ar–CH2–N), 5.29(s, N–CH2–O), 6.74–7.23 (m, Ar).

3.2.4. Characterization and measurements1H NMR spectra were acquired on a Varian Oxford AS600 at a

proton frequency of 600 MHz. The solvent used for NMR measure-ment was deuterated chloroform with tetramethylsilane as aninternal standard. The number of transients was 64. A relaxationtime of 10 s was used for the integrated intensity determinationof 1H NMR spectra. Fourier transform infrared spectroscopic(FT-IR) spectra were acquired on a Bomem Michelson MB100which was equipped with a deuterated triglycine sulfide (DTGS)

M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368 363

detector at a resolution of 4 cm�1 with 32 coadditions. Kineticanalysis was performed using differential scanning calorimetry(DSC) from TA Instruments, DSC model 2920. The temperaturewas ramped at four different heating rates of 2, 5, 10, and 20 �C/min. The samples were heated from room temperature up to300 �C using ultra pure nitrogen with a flow rate of 60 mL/min.All samples were crimped in hermetic aluminum pans with lids.The thermal stability was studied by thermogravimetric analysis(TGA) using TA Instruments High Resolution 2950 Thermogravi-metric Analyzer. All samples were heated from ambient tempera-ture to 900 �C at a heating rate of 10 �C/min under a purging gas(nitrogen or air) of 90 mL/min for all tests.

Fig. 1. 1H NMR spectra of (top) pHBA-pt and (bottom) P-pt.

5001000150020002500300035004000

Abs

orba

nce

Wavenumber (cm-1)

1501

1475

1230

1035

943

1222 941

1153N

O

NO

HO

3340

Fig. 2. FT-IR spectra of (top) pHBA-pt and (bottom) P-pt.

3.3. Results and discussion

3.3.1. Preparation of benzoxazine monomersMethylol functional benzoxazine monomer has been synthe-

sized through Mannich condensation reaction of methylol-func-tional phenol and aromatic amine, in the presence ofparaformaldehyde as depicted in Scheme 3. In this method, 4-hydroxybenzyl alcohol (pHBA), p-toluidine and paraformaldehydewith a molar ratio of 1:1:2 were used to produce methylol func-tional benzoxazine monomer abbreviated as pHBA-pt. To studythe effect of methylol group into benzoxazine structure, non-func-tionalized benzoxazine monomer, abbreviated as P-pt, was pre-pared using similar route but with phenol instead of pHBA.

The structure of the monomers was confirmed by 1H NMR andFT-IR spectra. The 1H NMR spectra are depicted in Fig. 1. The spec-tra show the typical resonances attributed to benzoxazine struc-ture, (Ar–CH2–N–, –N–CH2–O–), for pHBA-pt and P-pt monomersat 4.53 and 5.27 ppm, and 4.55 and 5.29 ppm, respectively. In addi-tion, the presence of methylol group (–CH2OH) in pHBA-pt is con-firmed through the resonance of –CH2– and –OH of methylol groupat 4.52 and 1.86 ppm, respectively.

In addition to 1H NMR, FT-IR was used to confirm the structureof the monomers as depicted in Fig. 2. For pHBA-pt, observation ofthe band at 1502 cm�1 due to the stretching of trisubstitutedbenzene ring, at 1475 cm�1 due to the bending of CH2 of methylol,at 1230 cm�1 due to the asymmetric stretching of C–O–C, at 1143cm�1 due to the asymmetric stretching of C–N–C, at 1035 cm�1

due to the stretching of C–O of methylol, and at 943 cm�1 due tothe out-of-plane vibration of benzene ring to which oxazine is at-tached confirms the formation of the monomer. The presence ofmethylol group in the synthesized monomers is also confirmedthrough the very broad absorption peaks centered at 3340 cm�1

due to the OH stretching mode. However, the spectrum of thenon-functionalized monomer (P-pt) shows the correspondingbands at 1222 cm�1 due to the asymmetric stretching of C–O–C,at 1153 cm�1 due to the asymmetric stretching of C–N–C, and at941 cm�1 due to the out-of-plane vibration of benzene ring towhich oxazine is attached [40,41].

3.3.2. Polymerization behavior of the monomersDSC was used to study the polymerization behavior of the

monomers. Since the structure of pHBA contains methylol group,the structure of pHBA-pt contains both methylol and benzoxazine

OH

R+ CH2O

NH2

R=H or CH2OH

+2

Scheme 3. Preparation of methylol-fu

structure; however, the reference material, P-pt, contains onlybenzoxazine group. Therefore, the DSC thermograms of pHBA, P-pt and pHBA-pt represent the polymerization behavior throughmethylol, benzoxazine ring-opening, and a combination of bothmethylol and benzoxazine ring-opening, respectively. Fig. 3 showsthe DSC thermograms of pHBA, P-pt and pHBA-pt, where P-ptshows one sharp exothermic peak with onset at 240 �C and maxi-mum centered at 261 �C. In addition, pHBA shows a wide broadexothermic peak with onset at 154 �C and a maximum centeredat 200 �C. However, in the case of pHBA-pt, the onset of exothermicpeak was observed at 209 �C with a maximum exothermic at232 �C. No peak that corresponds to the methylol condensation ob-served at 200 �C for pHBA-pt was observed despite the fact thatpHBA-pt contains methylol group as well. This might be due tothe dilution effect of the methylol groups as well as the benzox-azine groups by increasing the molecular size of pHBA-pt. Theaforementioned DSC results indicate that the condensation

100 oC/10hToluene N

O

R

R=H; P-ptR=CH2OH; p HBA-pt

nctional benzoxazine monomers.

0 100 200 300

Hea

t flo

w

Temperature (°C)

exo

endo

232 oC

261 oC

259 J/g

121 J/g

317 J/g

200 oC

NO

NO

HO

OHHO

Fig. 3. DSC thermograms of pHBA, P-pt and pHBA-pt.

0 50 100 150 200 250 300

Hea

t flo

w

Temperature (°C)

exo

endo

2 °C/min

20 °C/min

10 °C/min

5 °C/min

NO

HO

Fig. 4. DSC thermograms of pHBA-pt at different heating rates.

0 50 100 150 200 250 300

Hea

t flo

w

Temperature (°C)

exo

endo

5 °C/min

10 °C/min

2 °C/min

20 °C/minN

O

Fig. 5. DSC thermograms of P-pt at different heating rates.

0 50 100 150 200 250 300

Hea

t flo

w

Temperature (°C)

exo

endo

2 °C/min

20 °C/min10 °C/min

5 °C/min

OHHO

Fig. 6. DSC thermograms of pHBA at different heating rates.

364 M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368

polymerization through methylol groups as shown in pHBA, has abroad pattern confirming the occurrence of polymerization reac-tion over a wide range of temperatures, whereas P-pt polymerizesin a narrow temperature range. The narrower pattern of exother-mic peak for the polymerization behavior of P-pt is an indicationof the monomer’s higher sensitivity to temperature for acceleratingthe crosslinking reaction than pHBA and pHBA-pt. Ishida and Allen[42] indicate that the broadness of exothermic polymerizationpeak is also an indication of phenolic impurities coming from theoligomers or any other initial phenolic compound during the prep-aration of the monomers. However, the purity of the monomers inthis study is confirmed to be high as judged from the 1H NMR andthe presence of sharp endothermic peak due to melting in the DSCthermogram as shown earlier in Figs. 1 and 3. Therefore the pres-ence of impurities is unlikely cause of this behavior. The heat ofpolymerization of pHBA and pHBA-pt were 121 and 259 J/g,respectively. However, P-pt as a monomer with no methylol group,exhibits a higher heat of polymerization of 317 J/g. This may beattributed to the endothermic nature of methylol condensation[34]. It is interesting to note that the two distinctive polymeriza-tion mechanisms, condensation polymerization of methylol andcationic ring-opening polymerization of oxazine ring, seem to takeplace in a concerted manner. Due to the symmetric line shape ofpHBA-pt exotherm, coincidental yet independent occurrence ofthose two polymerization processes is highly unlikely. Detailedconsideration of the effect of hydrogen bonding on the polymeriza-tion will be reported elsewhere.

Non-isothermal DSC of the monomers was used to evaluateinformation about the polymerization reaction such as the onsetof polymerizing temperature, exothermic peak temperature andthe heat of polymerization. In this study, it is assumed that com-plete conversion is achieved for experiments at heating rates of2, 5, 10 and 20 �C/min. The DSC thermograms of pHBA-pt, P-pt,and pHBA are depicted in Figs. 4–6, respectively. The figures showa shift of the exothermic peak to a higher temperature by increas-ing the heating rate. In addition, the systems show that the heatingrates have slight influence on the amount of exothermic reaction.

Based on the Kissinger and Ozawa methods for non-isothermalanalysis, the activation energy is obtained from the logarithmicplots of heating rate versus the reciprocal of the absolute peaktemperature of the monomers as depicted in Fig. 7. In addition, agood linear relationship between the heating rate and the inverse

of the exothermic peak temperature is observed and the resultsare summarized in Table 1. The average activation energy valuesobtained from the Kissinger and Ozawa methods are not signifi-cantly different. For example, the average activation energies forpolymerization of pHBA-pt, P-pt and pHBA using the Kissingermethod are 83.1, 101.2 and 93.4 kJ/mol, respectively. However,the Ozawa method shows the values of average activation energiesfor polymerization of pHBA-pt, P-pt and pHBA as 96.0, 115.6 and

-3

-2

-1

0

1

2

3

4

-12

-11

-10

-9

-8

-7

-6

-5

0.0018 0.0019 0.002 0.0021 0.0022 0.0023

ln (

β)

ln (

β/T

2 )

1/T (K-1)

Fig. 7. Plots for determination of averaged activation energy of (D) P-pt, (h) pHBA-pt and (O) pHBA using (————) Kissinger method and (33) Ozawa method.

Table 1Average activation energy (Ea) of the monomers obtained by the Kissinger and Ozawamethods.

Ea (kJ/mol) pHBA-pt P-pt pHBA

Kissinger method 83.1 101.2 93.4Ozawa method 96.0 115.6 106.4

125 175 225 275

Hea

t flo

w

Temperature (°C)

exo

endo

IMDLiIPTSANo Cat.

No Cat.

IMD

LiI PTSA

NO

NO

HO

Fig. 8. DSC thermograms of (top) pHBA-pt and (bottom) P-pt mixed with (3 mol%)of p-toluenesulfonic acid (PTSA), 1-methyl-imidazole (IMD) and lithium iodide (LiI).

Table 2Effects of catalysts on polymerization of P-pt and pHBA-pt.

Sample code P-pt pHBA-pt

Catalyst No. cat. IMD PTSA LiI No. cat. IMD PTSA LiI

Tp (�C) 261 256 211 189 232 231 188 201DH (J/g) 317 281 163 369 259 256 157 283Td5% (�C) 128 139 121 146 179 173 131 165Td10% (�C) 148 157 144 179 194 191 162 182Y300 �C (%) 11 25 58 78 83 71 81 82

Tp: exothermic peak temperature.DH: the amount of exothermic peak.Td5%: 5% weight loss temperatures.Td10%: 10% weight loss temperatures.Y300 �C: Residual weight loss at 300 �C.

M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368 365

106.4 kJ/mol, respectively. Non-functionalized benzoxazine mono-mer (P-pt) exhibits the highest activation energy whereas methylolbenzoxazine monomer (pHBA-pt) shows the lowest. This variationin the average activation energies originates from the difference inthe polymerization mechanisms of the monomers. For example, inthe case of non-functionalized monomers, the polymerization pro-ceeds through opening of heterocyclic ring which is the only reac-tive site for polymerizing. However, methylol monomers undergopolymerization through both heterocyclic ring-opening and meth-ylol condensation which occur simultaneously. Therefore, benzox-azine monomers polymerize through only ring-opening are moresensitive to temperature than monomers polymerizing via bothmethylol and ring-opening mechanisms.

3.3.3. The role of catalyst on polymerization of P-pt and pHBA-ptThe effect of adding catalyst on the polymerization and thermal

stability behavior of P-pt and pHBA-pt was studied using DSC andTGA. In this study, the polymerization behavior after adding p-tol-uenesulfonic acid (PTSA), 1-methyl-imidazole (IMD) and lithiumiodide (LiI) were tested. To these monomers, 3.0 mol% of desiredcatalyst was added and dissolved in acetone using sonication untilclear solution was obtained. The mixture was then dried at roomtemperature in a vacuum oven to remove the solvent. It shouldbe mentioned that under this condition no polymerization tookplace. The resulting homogeneous mixtures were placed in a DSCpan and heated under nitrogen flow using heating rate of 10 �C/min to study the polymerization behavior as depicted in Fig. 8.The summary of the results is shown in Table 2. Both monomersshow the highest exothermic peak temperature when no catalystwas added. For example, the exothermic peaks of P-pt and pHBA-pt were 261 and 232 �C, respectively. However, adding IMD as abase catalyst shifts the exothermic peak slightly to a lower temper-ature of 256 and 231 �C for P-pt and pHBA-pt, respectively. Theheat of polymerization also shows a slight decrease compared tothe control in both P-pt and pHBA-pt. On the other hand, addingLiI shifts the exothermic peak significantly to lower values of 189and 201 �C for P-pt and pHBA-pt, respectively. However, the heat

of polymerization was increased. By adding PTSA as an acid cata-lyst, the exothermic peak shifted to as low as 211 and 188 �C forP-pt and pHBA-pt, respectively. The heat of polymerization showsalso a significant decrease compared to the control in both P-pt andpHBA-pt. Furthermore, pHBA-pt shows a very broad exothermicpeak compared to the control indicating that the polymerizationoccurs in an early stage at lower temperatures. Although LiI showsa significant decrease of the exothermic peak in the case of P-pt,PTSA was more efficient for polymerizing the methylol monomer.This is attributed to the catalytic effect of PTSA for both methylolcondensation and ring-opening polymerization. Therefore, a signif-icant shift to low temperatures in the case of PTSA stimulates itsuse as an efficient catalyst for these monomers.

In summary, the effects of catalysts on the polymerization ofthese two monomers appear to be complex. The efficient catalystssuch as PTSA and LiI, might affect benzoxazine polymerization andmethylol condensation for pHBA-pt. These catalysts might com-pete with the hydrogen bonding effect of methylol on the oxazinering-opening. Thus, despite lower polymerization temperature ofpHBA-pt without added catalyst, the exotherm peak temperaturewith the effective catalysts did not show substantial differencefrom the P-pt case. On the other hand, these two catalysts showdramatic exotherm peak reduction for P-pt. They might also stim-ulate the reactivity of the bond insertion onto the phenolic ben-zene ring, in particular at the para-position with respect to the

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

20

40

60

80

100

120

0 300 600 900

Der

ivat

ive

wei

ght l

oss

(%/°

C)

Res

idua

l wei

ght (

%)

Temperature (°C)

Poly (P-pt)Poly (pHBA-pt)Poly (pHBA)

Fig. 10. TGA thermograms of poly(P-pt) and poly(pHBA-pt) under nitrogen.

366 M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368

oxygen atom, in addition to the usual acceleration of the oxazinering-opening reactions.

The thermal behavior of unpolymerized monomers in the pres-ence of various catalysts was also studied using TGA as depicted inFig. 9. The 5% and 10% weight loss temperatures (Td5 and Td10) andweight loss at 300 �C were determined from the graph. For theunpolymerized control monomers, where no catalyst was added,one-step weight loss was observed for P-pt monomer below400 �C. However, two-steps weight loss was observed in the caseof unpolymerized pHBA-pt. Since the methylol functionality isthe only difference between pHBA-pt and P-pt, methylol condensa-tion is the possible reaction that takes place in one of the steps.Moreover, it is well known that water is a by-product during themethylol condensation reaction in the same way as in traditionalphenolic resin (resole) [43,44]. Therefore, methylol condensationand benzoxazine ring-opening polymerization are features ofmethylol functional benzoxazine monomer. Adding catalyst accel-erates the polymerization reactions as shown earlier from the DSCthermogram (Fig. 8).

In general, the TGA thermograms of unpolymerized P-pt andpHBA-pt show improvement in the weight loss beyond 200 �C byadding catalyst which is attributed to the formation of crosslinkingnetwork structure. For example, by adding LiI to P-pt, the DSC ther-mogram (Fig. 8) shows that this system exhibits the lowest exo-thermic peak compared to other systems. This result is consistentwith the TGA thermogram which reveals the lowest weight lossamong other systems due to the early formation of crosslinkingnetwork structure. However in the case of pHBA-pt, adding LiI stillshows significant improvement in the exothermic peak but almostidentical TGA behavior to the system without catalyst was ob-served. This implies that the addition of LiI accelerates the ring-opening reaction of benzoxazine which has no attributed weightloss or any kind of volatile compounds. On the other hand, theDSC thermograms show that adding IMD to either P-pt or pHBA-pt exhibits a slight shift in the exothermic peak temperature. Forexample, although the DSC shows a slight shift in the exothermicpeak, the thermal behavior of P-pt monomer in the presence or ab-sence of IMD was almost identical. Since there is no volatile orbyproduct compound associated with the ring-opening polymeri-zation of oxazine ring, this implies that the addition of IMD has

0

20

40

60

80

100

0 300 600 900

Res

idua

l wei

ght (

%)

Temperature (°C)

No Catalyst

pHBA-pt (3% LiI)

pHBA-pt (3%IMD)

pHBA-pt (3% PTSA)

NO

HO

Fig. 9. TGA thermogram of unpolymerized benzoxazine monomers in the absence of cataand in presence of 3 mol% PTSA (– –).

no significant improvement in the thermal behavior and mightaccelerate some kind of degradation reactions. Similarly, for thepHBA-pt, adding IMD slightly shifts the exotherm peak and conse-quently slight drop in the weight loss which might be attributed tothe methylol condensation and some degradation reactions.

By adding PTSA as an acid catalyst to P-pt, there is a significantshift in the exothermic peak as seen earlier from the DSC but theTGA indicates an early improvement in the char yield. The in-creased char yield arises from the early crosslinking structureformed due to the ring-opening polymerization. On the other hand,using PTSA as a catalyst for pHBA-pt, the DSC shows significantshift of exothermic peak towards the lowest temperature com-pared to other systems. However, the TGA of unpolymerizedpHBA-pt reveals the early weight loss which is attributed to theearly methylol condensation reaction and formation of water as a

0

20

40

60

80

100

0 300 600 900

Res

idua

l wei

ght (

%)

Temperature (°C)

No Catalyst

P-pt (3% LiI)

P-pt (3% IMD)

P-pt (3% PTSA)

NO

lyst (33), in the presence of 3 mol% LiI (. . .. . .), in the presence of 3 mol% IMD (- - -)

Table 3TGA results for the polymers.

Sample code Td5% (nitrogen) (�C) Td10% (nitrogen) (�C) Td5% (air) (�C) Td10% (air) (�C) Char yield Y800 �C (nitrogen) (%)

Poly(pHBA) 373 401 390 411 58Poly(P-pt) 299 349 325 368 41Poly(pHBA-pt) 347 376 370 404 57

Y800 �C: weight loss at 800 �C.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

20

40

60

80

100

120

0 300 600 900

Der

ivat

ive

wei

ght l

oss

(%/°

C)

Res

idua

l wei

ght (

%)

Temperature (°C)

Poly (P-pt)Poly (pHBA-pt)Poly (pHBA)

Fig. 11. TGA thermograms of poly(P-pt) and poly(pHBA-pt) under air.

M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368 367

byproduct. Although there is an early weight loss using PTSA, thiscatalyst still show high efficiency among others for pHBA-pt due tothe significant shift of the exothermic peak and early methylol con-densation reaction together with ring-opening polymerization ofoxazine ring.

3.3.4. Thermal properties of the crosslinked polymersThe thermal degradation of poly(P-pt), poly(pHBA-pt) and

poly(pHBA) was further studied by TGA after their polymerizationusing temperature profile as follows: 100 �C/2 h, 125 �C/2 h,150 �C/2 h, 175 �C/2 h, 200 �C/1 h, 225 �C/1 h and 250 �C/1 h.Fig. 10 illustrates the TGA thermograms of the polymers undernitrogen and a summary of the results is shown in Table 3. The5% and 10% weight loss temperatures, Td5 and Td10, were deter-mined from the graph. The TGA analysis shows that the introduc-tion of methylol group as additional crosslinking site intobenzoxazine structure leads to a significant enhancement in thethermal properties of polybenzoxazine including the char yield,Td5% and Td10%. For example, in the case of poly(P-pt), the Td5 andTd10 are 299 and 349 �C, respectively, whereas poly(pHBA-pt)shows higher values of 347 and 376 �C, respectively. However inthe case of poly(pHBA), the Td5 and Td10 are 373 and 401 �C, respec-tively. Furthermore, the polymer prepared from methylol mono-mer shows significant improvement in the residual weight at800 �C under nitrogen, which is defined here as the char yield,compared to the control. The char yield of poly(P-pt) was 41% incomparison to 57% for poly(pHBA-pt) and 58% for poly(pHBA).The difference in the decomposition behavior of the polymers isattributed to the nature of their structures. For example, traditionalpolybenzoxazines prepared from non-functionalized monomer,

such as poly(P-pt), are characterized by the presence of Mannichbase bridges as repeating units which is known to degrade firstwith the loss of amine-related compounds [45]. However, in thecase of polybenzoxazine derived from methylol benzoxazinemonomers, such as poly(pHBA-pt), a combination of Mannich base,methylene and dimethylene ether are present as repeating units inthe structure. Therefore, the decomposition behavior of poly-benzoxazine prepared from methylol monomer is taking placethrough multi degradation steps in comparison to the traditionalpolybenzoxazines. The aforementioned result confirms the highercrosslinking density in the polymers prepared from methylol ben-zoxazine precursor in comparison to polymers synthesized fromnon-functionalized benzoxazine precursor.

The thermoxidative stability of the polymers derived from bothmethylol and non-functionalized benzoxazine monomer, as evalu-ated by TGA under air atmosphere, is shown in Fig. 11. The TGA re-sults, as shown in Table 3, indicate that poly(P-pt) shows thelowest Td5% and Td10% compared to poly(pHBA-pt) and poly(pHBA).For example, in the case of poly(P-pt), the Td5 and Td10 are 325 and368 �C, respectively, whereas poly(pHBA-pt) as a methylol mono-mer shows higher values of 370 and 404 �C, respectively. In addi-tion, poly(pHBA) exhibits the highest Td5 and Td10 with values of390 and 411 �C. Therefore, this enhancement in the thermoxidativestability of poly(pHBA-pt) compared to poly(P-pt) comes from thehigher crosslinking network formed due to the polymerization bybenzoxazine ring-opening and methylol condensation.

4. Conclusions

The incorporation of methylol group into benzoxazine structurewas successfully achieved and found to lower the polymerizationtemperature compared to the non-functionalized monomer. Themonomer is polymerized through both ring-opening polymeriza-tion of oxazine ring and condensation of methylol groups. Themethylol monomer shows lower activation energy compared tothe non-functionalized monomer as calculated using Kissingerand Ozawa methods. The variation in the average activation ener-gies comes from the difference in the polymerization mechanismof the monomers. Therefore, monomers polymerized via ring-opening of benzoxazine precursors are more sensitive to tempera-ture than monomers polymerized through both ring-opening ofoxazine ring and methylol condensation. In addition, catalyzedbenzoxazine study shows that systems using LiI and PTSA are effi-cient in reducing the polymerization temperatures. However, PTSAis more efficient in the case of methylol monomers due to its abil-ity to accelerate both methylol condensation and ring-openingpolymerization. Furthermore, the TGA reveals higher char yieldand Td5, Td10 over the typical non-functionalized polybenzoxazines,suggesting the higher thermal stability of the synthesized polymer.

Acknowledgments

M. Baqar acknowledges the Ministry of Higher Education andScientific Research of Libya and Azzaytuna university-Libya for ascholarship.

368 M. Baqar et al. / Reactive & Functional Polymers 73 (2013) 360–368

References

[1] G. Tillet, B. Boutevin, B. Ameduri, Prog. Polym. Sci. 36 (2011) 191–217.[2] P. Kubisa, S. Penczek, Prog. Polym. Sci. 24 (1999) 1409–1437.[3] K. Dusek, M. Duskova-Smrckova, Prog. Polym. Sci. 25 (2000) 1215–1260.[4] Z. Ma, J. Gao, J. Phys. Chem. B 110 (2006) 12380–12383.[5] L.H. Baekeland, 1907. U.S. Patent 942,809.[6] L. Gardziella, L.A. Pilato, A. Knop, Phenolic Resins: Chemistry, Applications,

Standardization, Safety and Ecology, Springer, Berlin, 2000.[7] M.G. Kim, Y. Wu, L.W. Amos, J. Polym. Sci. Part A: Polym. Chem. 35 (1997)

3275–3285.[8] A. Knop, L.A. Pilato, Phenolic Resins, Springer, Berlin, 1985. pp. 40–46.[9] M. Higuchi, T. Urakawa, M. Morita, Polymer 42 (2001) 4563–4567.

[10] D.D. Werstler, Polymer 27 (1986) 750–756.[11] J.H. Freeman, C.W. Lewis, J. Am. Chem. Soc. 76 (1954) 2080–2087.[12] H. Ishida, T. Agag (Eds.), Handbook of Benzoxazine Resins, Elsevier,

Amsterdam, 2011.[13] F. Kasapolglu, I. Cianga, Y. Yagci, T. Takeichi, J. Polym. Sci. Part A: Polym. Chem.

41 (2003) 3320–3328.[14] A. Sudo, T. Endo, J. Polym. Sci. Part A: Polym. Chem. 47 (2009) 4847–4858.[15] J. Dunkers, H. Ishida, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 1913–1921.[16] R. Andreu, J.A. Reina, J.C. Ronda, J. Polym. Sci. Part A: Polym. Chem. 46 (2008)

3353–3366.[17] B. Kiskan, L. Demirel, O. Kamer, Y. Yagci, J. Polym. Sci. Part A: Polym. Chem. 46

(2008) 6780–6788.[18] R. Kudoh, A. Sudo, T. Endo, Macromolecules 42 (2009) 2327–2329.[19] A. Sudo, L.C. Du, S. Hirayama, T. Endo, J. Polym. Sci. Part A: Polym. Chem. 48

(2010) 2777–2782.[20] H. Oie, A. Sudo, T. Endo, J. Polym. Sci. Part A: Polym. Chem. 48 (2010) 5357–

5363.[21] H. Ishida, Y. Rodriguez, J. Appl. Polym. Sci. 58 (1995) 1751–1760.

[22] A. Sudo, R. Kudoh, H. Nakayama, K. Arima, T. Endo, Macromolecules 41 (2008)9030–9034.

[23] A. Sudo, S. Hirayama, T. Endo, J. Polym. Sci. Part A: Polym. Chem. 48 (2010)479–484.

[24] H.J. Kim, Z. Brunovska, H. Ishida, Polymer 40 (1999) 1815–1822.[25] Z. Brunovska, H. Ishida, J. Appl. Polym. Sci. 73 (1999) 2937–2949.[26] T. Agag, T. Takeichi, Macromolecules 34 (2001) 7257–7263.[27] T. Agag, T. Takeichi, Macromolecules 36 (2003) 6010–6017.[28] Y.L. Liu, C.I. Chou, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 5267–5282.[29] S. Rimdusit, P. Jongvisuttisun, C. Jubsilp, W. Tanthapanihakoon, J. Appl. Polym.

Sci. 111 (2009) 1225–1234.[30] H. Ishida, S. Ohba, Polymer 46 (2005) 5588–5595.[31] L. Jin, T. Agag, H. Ishida, Eur. Polym. J. 46 (2010) 354–363.[32] L. Jin, T. Agag, Y. Yagci, H. Ishida, Macromolecules 44 (2011) 767–772.[33] B. Koz, B. Kiskan, Y. Yagci, Polym. Bull. 66 (2011) 165–174.[34] M. Baqar, T. Agag, H. Ishida, S. Qutubuddin, J. Polym. Sci. Part A: Polym. Chem.

50 (2012) 2275–2285.[35] H. Ishida, Y. Rodriquez, Polymer 36 (1995) 3151–3158.[36] D. Rosu, C.N. Cascaval, F. Mustata, Thermochim. Acta 383 (2002) 119–127.[37] S. Vyazovkin, N. Sbirrazzuol, Macromolecules 29 (1996) 1867–1873.[38] P. Bartolomeo, J.F. Chailan, J.L. Vernet, Eur. Polym. J. 37 (2001) 659–670.[39] K.S.S. Kumar, C.P.R. Nair, K.N. Ninan, Thermochim. Acta 441 (2006) 150–155.[40] J. Dunkers, H. Ishida, Spectrochim. Acta 51A (1995) 1061–1074.[41] T. Holopainen, L. Alvila, J. Rainio, T.T. Pakkanen, J. Appl. Polym. Sci. 69 (1998)

2175–2185.[42] H. Ishida, D.J. Allen, J. Appl. Polym. Sci. 79 (2001) 406–417.[43] M.F. Grenier-Loustalot, S. Larroque, P. Grenier, D. Bedel, Po1ymer 31 (1996)

955–964.[44] J. Lubczak, React. Funct. Polym. 38 (1998) 51–59.[45] G. Reiss, J.M. Schwob, G. Guth, M. Roche, B. Lande, in: B.M. Culbertson, J.E.

McGrath (Eds.), Advances in Polymer Synthesis, Plenum Press, New York,1985, pp. 27–49.