conventional 
 phenolics are cross - linked products of low molecular weight precursors , 
 typically formed through a condensation reaction .
the versatility 
 of their structures and the fact that they display desirable properties 
 such as good heat resistance , flame - retardant properties , low dielectric 
 constants , and the production of these materials is relatively inexpensive .
however , there are some shortcomings associated 
 with these materials in that they have poor toughness properties , 
 they have a poor shelf life , and the process of polymerization of 
 these materials produces byproducts and requires , in many cases , the 
 use of a strong acid or base catalyst to effect cure . in the search for higher performance replacements for phenolic 
 resins
many materials have received attention , and published reports 
 of the preparation of aromatic oxazines , or benzoxazines , date back 
 some 60 years ; although commercial exploitation 
 of the corresponding polymers has only come about relatively recently , 
 they are now receiving a great deal of academic and industrial interest .
poly(bis - benzoxazine)s ( sometimes simply 
 referred to as polybenzoxazines ) are a family of thermosetting polymers 
 that are made up through step growth ring - opening polyaddition from 
 bis - benzoxazine monomers ( figure 1 ) , which 
 are in turn the products of the mannich reaction between a bis - phenol , 
 formaldehyde , and a primary amine .
the 
 monomer  oligomer ratio in the yield can also be influenced 
 by using an excess of formaldehyde and amine during the synthesis , 
 causing the products to form via a different mechanism and resulting 
 in a greater proportion of monomer in the product .
this , in turn , affects the properties of the resin before , 
 during , and after cure ( the presence of oligomers bearing hydroxyl 
 groups in the chain is known to enhance the reactivity of the benzoxazine ) .
polymerization 
 of bisbenzoxazines through ring - opening and cross - linking and representative 
 network .
commercial benzoxazines are currently 
 being evaluated for use in the aerospace industry ( in which they would 
 replace or augment phenolic polymers in secondary applications such 
 as interior paneling ) .
polybenzoxazines appear to incorporate the 
 best properties from conventional phenolics and may find application 
 in a number of their traditional niches , while improving on shelf 
 life and offering the potential for greater toughness properties through 
 their greater molecular flexibility ; the relative cheapness of the 
 monomer is also an important factor influencing their adoption . however , 
 while polybenzoxazines display many benefits over conventional phenolics , 
 the relatively low fracture toughness that is achieved by cured polymers 
 ( a kic value of ca .
0.510.54 mpam is typical , although later 
 linear polymers achieve higher kic values ) 
 and the comparatively poor reactivity 
araldite mt35600 commences 
 its reaction at 180200 c ( dsc , 10 k / min ) in the absence 
 of additive ( and 160 c with the addition of 5 wt % 3,3-thiodiphenol)are 
 still problems in advanced aerospace applications when compared with 
 competitor resins . unlike many other commercial thermosetting 
 resins , which evolve condensation products such as water or ammonia , 

benzoxazine monomers react relatively cleanly to form a polymer with 
 few reaction byproducts , and although 
 a number of studies have been published , the exact manner of the polymerization 
 reaction to form a network has not been fully elucidated . in our ongoing 
 research programmes ,
the ultimate aim is to examine and increase our 
 understanding of the influence of the benzoxazine monomer structure 
 on reactivity , fracture toughness , and thermal stability . in this 
 paper
we present the results of a study into the influence of the 
 nature of the additive on the reaction mechanism .
significant differences 
 in the mechanism of the network growth are observed via raman spectroscopy 
 when different initiators are employed , leading to significant differences 
 in the thermal and mechanical properties that are observed .
the monomers were characterized fully using both spectroscopic and 
 chromatographic techniques , as it is well - known that monomer purity 
 ( in particular , the oligomer content ) 
 can have a significant effect on both cure mechanism and kinetics . 

analysis using high performance liquid chromatography ( hplc ) suggested 
 that ba - a was markedly more pure ( ca .
95% ) than bf - a ( ca . 80% and 
 a mixture of isomers ) , but this was misleading .
subsequent discussion 
 with the materials supplier confirmed that bis - benzoxazine monomers 
 are prone to undergo degradation via ring - opening on the surface of 
 the column during reverse - phase analysis , and so these data can not 
 be used to determine absolute purity .
h and c nmr spectroscopic analyses were performed in parallel on the monomers 
 ( e.g. , using dept-135 , hsqc , and hmbc pulse sequences ) , and these 
 supported the observation that ba - a was the more pure , containing 
 a lower concentration of unreacted starting materials , but not disproportionately 
 so .
the dsc data for the  uninitiated  
 polymerizations are displayed in figure 2 and 
 given in table 1 , from which it can be seen 
 that ba - a offers a potentially relatively wide processing window ( i.e. , 
 the gap between tm = 43 c and the 
 onset of polymerization ca .
scanning dsc data for 
 the thermally initiated polymerization of the benzoxazine monomers 
 ( 10 k / min , nitrogen ) : ( a ) ba - a recrystallized , ( b ) ba - a as received , 
 ( c ) bf - a as received , and ( d ) bt - a as received .
tg1 = possible glass 
 transition or melting transition of monomer determined using a heating 
 rate of 10 k min ; top = observed onset of polymerization ; tmax = temperature of exothermic peak maximum ; tep = observed end of polymerization ; hp = enthalpy of exothermic peak ( both as j / g of monomer 
 and kj / mol of benzoxazine ring ) ; tg2 = glass transition temperature of polymer determined 
 using a heating rate of 10 k min ; tdp = 3,3-thiodiphenol ; 
 tda = 3,3-thiodipropionic acid . the thermal data obtained in a previous paper showed that the onset of the thermal polymerization 
 was lowest for the monomers formed from aniline and incorporating 
 tdp and 4,4-biphenol ( with relatively broad exotherms spanning 160270 
 c , tmax 180 and 200 c , respectively ) , 
 with more flexible bridging groups or conjugated ring moieties . in 
 the current work , the monomer based on 3,3-thiodiphenol ( bt - a ) 
 is also the most reactive of those studied .
narrower , more symmetrical 
 exotherms were previously recorded at higher temperatures ( tmax = 215 and 225 c ) for the monomers 
 based on dihydroxyphenyl ether and bisphenol a , respectively , with 
 more flexible bridging groups and/or electron donating moieties .
conversely , 
 the highest onset temperature ( 180 c ) was recorded for the monomer 
 based on bisphenol af6 , with a highly electronegative bridge 
 ( i.e. , a hexafluoroisopropyl moiety ) .
it was notable that the enthalpies 
 recorded for the polymerizations of the majority of the monomers were 
 practically the same ( 147.9  7.4 or 74 kj / mol bz ring ) and compared 
 favorably with literature values ( of 144.7 or 72.4 kj / mol bz ring ) .
the generation of kinetic information 
 from thermal data in thermosetting polymers was reviewed extensively 
 in the context of epoxy resins by barton . of these
, the kissinger method relates 
 the peak maximum temperature , tmax , obtained 
 using dynamic dsc and the activation energy ( ea ) for data obtained at different heating rates ( figure 3).1where rmax is the rate of reaction at 
 the peak maximum derived from dsc and max is the 
 conversion at the peak maximum .
scanning dsc data under nitrogen for ba - a 
 at a variety of heating rates . using this method ,
the kinetics of the polymerizations were 
 modeled using dynamic dsc at a variety of heating rates , from which 
 an activation energy of 81 kj mol was calculated 
 for the monomer ba - a ( as received ) , which correlated well with literature 
 values ( table 2 ) .
the data are presented for 
 the  as - received  ba - a sample ( figure 4a ) and following recrystallization ( figure 4b ) .
kissinger (  ) and ozawa (  ) plots for the polymerization 
 of ( a ) ba - a and ( b ) recrystallized ba - a .
however , the dsc data show the reaction mechanism is more 
 complex with the reaction evolving from being chemically controlled 
 to physically controlled as the viscosity increases throughout the 
 reaction .
consequently , the kinetics were then determined using the 
 following equation , which is better suited to an autocatalytic system 
 than an nth - order model:2where  is the degree 
 of conversion ,  the heating rate , a the pre - exponential 
 factor , r the gas constant , 8.314 j/(mol k ) , n and m the orders of reaction in the initial 
 and later stages of reaction , respectively .
this equation was 
 solved by multiple linear regression to yield values of a , m , and n using the activation 
 energy previously determined ; these values were found to differ significantly 
 from those found in the literature .
for example , jubsilp et al . achieved average values of 1.7 for n and 0.8 for m for a ba - a benzoxazine and ishida 
 and rodriguez achieved average values 
 of 1.4 for n and 1.0 for m. following 
 recrystallization from ethanol , ba - a was reanalyzed using dsc , from 
 which it was observed that the melting temperature , polymerization 
 onset temperature , and peak maximum were all raised by some 15 k , 
 suggesting the removal of one or more impurities ( figure 2a , b ) .
when the same kinetic analysis was repeated 
 on the recrystallized monomer ba - a , average values of 1.89 and 0.89 
 for n and m , respectively , were 
 obtained which were more in line with the literature values for a 
 10 k / min heating rate .
therefore , commercial ba - a contains components 
 that are affecting the cure kinetics of the benzoxazine , in the absence 
 of the additive .
this is supported by the value of ea for recrystallized ba - a ( 94 kj mol ) , which compares favorably with reported values ( 102116 
 kj mol ) for an analogous material prepared in 
 the laboratory .
it is known that commercial 
 benzoxazine monomers may be held at elevated temperature to yield 
 soluble , low molecular weight oligomers , in which the presence of hydroxyl groups increase the reactivity . 

an autocatalytic model was used to calculate values for n and m. although bis - benzoxazines 
 will polymerize through opening of the heterocyclic ring when subjected 
 to heat , the use of initiators or catalysts is preferred since highly 
 pure monomers are comparatively unreactive ( onset temperature of 225 
 c , at 10 k / min dsc scan , in the absence of deliberate catalysis / initiation ) . 

samples were also analyzed using dsc in order to establish the relative 
 effects of the additives , and the thermal data for the  uninitiated  

polymerization of ba - a are displayed in figure 5 along with the corresponding mixtures containing each of the additives 
 ( 10 mol % ) .
the dsc thermograms for the acid - initiated reactions 
 show several almost sequential thermal events , unlike the uninitiated 
 system ( which yields a near gaussian exotherm with a high temperature 
 tail covering several almost simultaneous events ) ; this becomes more 
 apparent when the heating rate is further reduced to 5 k / min ( figure 3 ) .
dsc data ( 10 k / min , under nitrogen ) for ba - a ( a ) in the 
 absence of catalyst , with 10 mol % tdp and with 10 mol % tda .
3545 c along with a more diffuse endotherm 
 associated with the fusion of the additive in the mixture containing 
 tdp .
the monomer is most sensitive to the addition of the diacid ( tda ) , 
 but although the onset of the polymerization and the peak maximum 
 are moved significantly to a lower temperature , the end point of the 
 exotherm does not move along with this , leading to a broad exotherm . 

the additive has the greatest impact on the initial stages of reaction 
 ( ring opening ) , but the later processes associated with cross - linking 
 are not affected to the same degree .
speed of cure is a concern with 
 polybenzoxazines , which are sluggish to react when highly pure , and 
 this may hinder the more widespread use of these materials despite 
 the interest in their thermomechanical and physical properties ( especially 
 fst ) . using modelmaker ( version 3.0.4 , cherwell scientific ltd . ) , 
 peaks were fitted to the plots and activation energy determined to 
 compare to those obtained using traditional methods .
the curves chosen 
 for peak fitting were 15 mol % tda and 12 mol % tdp , as they gave 
 the best improvement in temperature of onset , while still being able 
 to discern the different reaction peaks .
the best fit was obtained 
 for the initiated systems when three peaks were used , especially in 
 the tdp samples where the reactions are much more drawn out , but a 
 fourth peak needed to be included in the first peak to prevent the 
 fit distorting in the tdp plots , suggesting there may be more reactions 
 occurring than are being observed .
the 5 k / min experiments show that 
 the initiated systems give very different models . in the uninitiated 
 system
the best peak fit was found using two peaks , which is consistent 
 with two processes : ring - opening and bridge forming reactions . in 
 the tdp plot ( figure 6 ) the initial stages 
 of reaction are being drawn out , so that several consecutive reactions 
 occur : i.e. , the first ( and smaller second ) peak is associated with 
 ring - opening ; the third peak , which accounts for the bulk of the reaction , 
 is associated with bridge forming , and the last peak is structural 
 rearrangement or possible beginnings of degradation .
conversely , the 
 tda plot shows a more simultaneous reaction occurring where several 
 reactions are occurring at once .
the results of peak deconvoluting 
 are debatable , but the models presented here represent the best fitting 
 models ( using regression coefficient ) with the smallest number of 
 peaks .
peak fitting for dsc data ( 5 k / min , under nitrogen ) for ba - a ( a ) 
 in the absence of catalyst and ( b ) with 12 mol % tdp and ( c ) with 
 10 mol % tda . using the kissinger method 
 ( 1 ) ,
the activation energy of for each peak 
 was calculated ( table 3 ) , and the activation 
 energies for the second and third peaks in tdp are similar to those 
 calculated for uninitiated ba - a .
the activation energy for the second 
 peak of tda is slightly lower but of a similar magnitude , suggesting 
 a similar reaction .
the activation energy of 
 the first peak is some 20 kj mol lower than the 
 subsequent peak in both instances ; this is due to the initiator instigating 
 the ring - opening reaction .
it is apparent that the degree of catalysis 
 and the nature of the catalyst have an influence on the progress of 
 the reaction , although this is more marked in the case of the diacid 
 ( e.g. , the reduction is visible in both the onset temperature and 
 the peak maximum of the polymerization exotherm when compared with 
 the uncatalyzed monomer and the formulations containing the diphenol ) 
 ( figure 7 ) .
furthermore , the glass transition 
 temperatures of the tda - cured polybenzoxazines ( tg2 , recorded during the dsc rescan experiment ) are significantly 
 higher ( and proportionate to additive concentration ) , where the values 
 are unchanged for the tdp - cured analogues .
this supports the suggestion 
 that the additives are giving rise to different polymerization mechanisms , 
 since the cross - link density does not alter significantly for the 
 tdp - initiated blends .
plots of glass transition temperature , tmax , and h ( j / g ) for ba - a initiated 
 with ( a ) tda and ( b ) tdp as a function of additive concentration .
data from dynamic mechanical thermal analysis were acquired to 
 explore the cross - link densities generated in the different network 
 structures .
representative storage modulus curves for pba - a when initiated 
 with tdp and tda are overlaid for comparison in figure 8 ; similar trends in the data were observed for the other polybenzoxazines . 

the glass transition ( -transition ) is clearly visible in the 
 loss modulus data at ca .
125225 c ( tmax = 175 c at 2 k / min ) with a drop in the storage 
 modulus of around 3500 mpa ; a -transition is visible between 
 50 and 100 c .
the polybenzoxazine that had been cured using 
 a 5 wt % addition of tdp differs in several important respects : ( i ) 
 the tg occurs at slightly lower temperatures 
 as the tdp content increases , ( ii ) the magnitude of the tan  
 peak increases relative to the loss modulus peak as tdp ( 5 wt % ) is 
 added , but decreases slightly at 10 wt % ; ( iii ) the magnitude of the 
 -transition increases as with tdp concentration .
these observations 
 ( which are similar for the other polybenzoxazines studied ) indicate 
 the emergence of a different cured network with the use of the additive ; 
 the effect on modulus and damping behavior ( figure 9 ) is clearly demonstrated .
dynamic viscoelasticity data for thermally 
 cured polybenzoxazines : ( a ) pba - a , ( b ) pba - a with 5 wt % tdp , ( c ) 
 pba - a with 10 wt % tdp , and ( d ) pba - a with 10 wt % ( tda ) . n.b . :
scale 
 relates to plot ( a ) other data offset progressively downward by 1000 
 mpa for clarity .
dynamic viscoelasticity 
 data ( tan  ) for thermally cured polybenzoxazines : ( a ) pba - a ) , 
 ( b ) pba - a with 5 wt % tdp , ( c ) pba - a with 10 wt % tdp , and ( d ) pba - a 
 with 10 wt % ( tda ) . n.b .
: scale relates to plot ( a ) other data offset 
 progressively downward by 1.0 for clarity .
the breadth of the tan  peaks show differences in 
 the damping behavior of the polybenzoxazines , and the peak widths 
 represent the temperature ranges over which the glass transition temperatures 
 occur .
thus , the broadest tan  peak ( ba - a ) can be attributed 
 to more heterogeneous networks containing both highly and less densely 
 cross - linked regions . this , in turn , 
 results in a broad distribution of molecular mobilities or relaxation 
 times ( this is seen to be broadest for the  uninitiated  

pba - a ) . the cross - link density (  ) for each polybenzoxazine 

was calculated from the dmta data using eq 3:3where  is taken as unity and ge is the storage modulus strictly from a sample 
 at equilibrium , but is taken at te , where te = tg + 50 k. this equation is technically most appropriate for lightly cross - linked 
 materials so it should only be used as a comparison between similar 
 materials .
the cross - link density values are shown for the benzoxazines 
 following polymerization in both the presence and absence of the additive 
 ( table 4 ) .
examination of the data for the 
 neat polybenzoxazines indicates that the three commercial monomers 
 develop very similar cross - link densities upon thermal cure , which 
 is consistent with the structures differing only by the nature of 
 the central bridge : isopropyl and methylene ; the sulfur - bridged monomer 
 bt - a develops a higher cross - link density . for comparison , values 
 of cross - link density of ( 1.110.5 )
 10 mol cm have been reported depending on the type 
 of benzoxazine and the functionality .
typically , ba - a is quoted as 
 having a cross - link density of 1.1  10 mol 
 dm , and the higher value found for the ba - a may 
 be due to the higher oligomers or impurities as evidenced by the discrepancies 
 in the reaction kinetics . however , it was not possible to obtain enough 
 material to carry out dmta on the polymer arising from the recrystallized 
 monomer .
allen and ishida obtained a 
 cross - link density of 47 mol m for polybenzoxazines 
 based on long chain aliphatic diamines .
of the two additives employed 
 in this work , tda has the greatest impact on the cross - link density 
 according to the dmta data ; e.g. , it leads to an increase in the tg of some 20 k ( pba - a ) , 30 k ( pbf - a ) , and 20 
 k ( pbt - a ) .
polybenzoxazine degradation 
 can be broken down into three events , although the processes are not entirely discrete as evidenced by 
 the breadth of the peaks .
there is a lower temperature event ( 200220 
 c ) for the initiated pba - a systems that is much higher for the 
 pba - a alone ( around 290 c ) .
this first degradation stage is 
 associated with the breakdown of the mannich bridge and loss of the 
 pendant amino groups and is most prominent for the formulations containing 
 the lowest concentrations of initiators , suggesting there are more 
 pendant moieties in these systems .
this might be due to the initiator 
 preventing reaction at the arylamine ring or affecting the chain growth , 
 although this growth in weight loss may also be due to loss of initiator 
 from the polymer matrix .
the second degradation peak is associated 
 with the breakdown of the polymer chain , leading to release of substituted 
 phenols , and is the largest contributor to the thermal degradation 
 process . the observed rise in the cross - link density and tg is also reflected in the measurement of initial thermal 
 stability ( table 5 ) of the cured ( but uninitiated ) 
 polybenzoxazines ;
e.g. , the t5% rises 
 from 258 c (  uninitiated  pba - a ) to 294 c 
 ( pba - a + 5% tda ) and from 263 c (  uninitiated  
 pbf - a ) to 290 c ( pbf - a + 5% tda ) .
although pbt - a is initially 
 the more stable polymer , the mass loss tends to reverse beyond ca . 
 30% , suggesting a different degradation mechanism .
work is currently 
 underway within our group to examine this aspect and will be reported 
 in a future paper .
the presence of 10 wt % tda reduces thermal stability 
 and leads to a more rapid loss in mass but provides an increase in tg .
the char yields are ordered in the following 
 way : pbt - a > pbf - a  pba - a with the polymers containing 
 the highest alkyl component providing the lowest char .
this order 
 of stability was confirmed by visual observations made on thermally 
 cured and deliberately charred samples .
all samples analyzed under nitrogen 
 at a heating rate of 10 k / min .
the introduction 
 of tdp is beneficial at the 5 wt % level of incorporation : the initial 
 stability ( t5% ) is raised for both pba - a 
 and pbf - a but has no apparent effect on pbt - a . at higher mass losses 
 ( e.g. , 30% ) ,
the stability of both pbf - a and pbt - a is elevated , but 
 the effect on pba - a is to reduce stability .
tdp is less influential 
 at 10 wt % ( where it leads to a reduction in both cross - link density 
 and thermal stability for all the polymers , accompanied by the release 
 of volatiles during cure ) .
tda can be incorporated at 5 and 10 wt 
 % and leads to enhanced thermal stability in pba - a , but a reduction 
 in the char yield of pbf - a and also pbt - a .
the differences in cross - link 
 density observed , coupled with the thermal behavior , may reflect a 
 different initiation mechanism as the pka = 6.6 for tdp is greater than for tda 
 ( pka = 4.11 ) .
wang and ishida also saw improved char 
 yield and tg when lewis acids were used 
 to initiate the ring - opening reaction of ba - a and , when considering 
 the first - derivative plot of the weight change from tga data , suggested 
 that a different degradation mechanism occurred in the polybenzoxazines 
 that arose from initiated and uninitiated mechanisms .
this was attributed 
 by the authors to the formation of a different polymer architecture 
 caused by the presence of the initiator .
dunkers and ishida discuss the use of both strong and weak protonic 
 catalysts and found that a slightly different polymerization mechanism 
 occurs depending on the pka of the acid . 
 for acids with pka in the range
0.704.43 , 
 this was explained by the formation of a stable iminium ion at early 
 conversions , but generally the stronger the acid , the more rapidly 
 the monomer was consumed and more rapidly converted to the tetra - substituted 
 ring of the product , which is consistent with our findings .
thermal 
 analysis provides information about physical changes brought about 
 during the thermal treatment of the monomer , but ( without hyphenation ) 
 no chemical information .
consequently , the cure of monomer ba - a was 
 monitored initially in situ using raman spectroscopy 
 using a heated cell ( ramped rapidly to 180 c and held isothermally 
 for 120 min ; spectra being taken at intervals of 7.5 min ) , but while 
 a stacked spectral plot could be produced as a function of cure time 
 ( figure 10 ) the fluorescence background produced 
 by the thermal degradation at an impurity level caused the signal 
 to be increasingly swamped between 1800 and 3400 cm so that any data in this region could not be used quantitatively . 

it is apparent that the vibrational spectra of the monomers and propagating 
 species are quite complex and assignments in the literature are occasionally 
 contradictory , but arguably , the most diagnostically useful bands 
 in the spectra are those associated with the constitution of the benzoxazine 
 ring , given that the ring opens to form an oligomer during polymerization . 
 the band corresponding to the presence of the c 
c 
 asymmetric stretch will be present only in the benzoxazine and can 
 be found at 10701200 cm and the symmetric 
 stretch at 800860 cm .
similarly , the bands 
 for c  o  c stretching can be found at ca .
stacked raman spectra for the polymerization 
 of ba - a acquired in situ ( comprising 16 spectra , 
 sampling rate 1 every 7.5 min ) .
the peaks can not be assigned unequivocally to pure monomer 
 or dimer / oligomer as the same functional groups appear in both species 
 ( and some studies have reached tentative assignments ) , but while the basic bands can be assigned with confidence the data 
 from several published studies ( table 6 ) are 
 sometimes contradictory on key points .
consequently , it was decided 
 to employ chemometrics techniques ( particularly principal components 
 analysis , pca ) as our group has already demonstrated the benefits 
 of this approach in unrelated areas of polymer chemistry .
pca is a 
 multivariate analysis technique that 
 utilizes all the data and uses a holistic approach to determine similar 
 and dissimilar spectra .
the extraction of the principal components 
 ( pcs ) , which account for the variance in data , enables the particular 
 analysis to be simplified but makes no suppositions about the chemical 
 meaning of the data .
this technique has been shown to yield useful information in the correlation of infrared 
 spectra of cyanate ester / bismaleimide blends with thermomechanical 
 properties and has also been employed to probe the mechanism by which our previous complexes have undergone 
 dissociation .
figure 11 shows 
 the pca data arising from the raw raman spectra for the time course 
 of uncatalyzed ba - a in a series of in situ measurements . 

the data show that two principal components ( pcs ) account for the 
 variation in the data . a plot of pc1 ( accounting for 97% ) vs pc2 ( accounting 
 for 3% ) shows regular incremental changes in the data until spectrum 
 7 ( i.e. , after 52 min at 180 c ) , at which point there is a large 
 change in the plot , indicating that a significant change in the curing 
 polymer is occurring at this point .
most of the spectral changes observed 
 during the experiment occur in regions associated with the aromatic 
 rings and the phenolic moiety .
the main band changes are reductions 
 at 700800 cm ( c  h out - of - plane 
 bending vibrations ) , 9901050 cm ( c  h 
 in - plane vibrations ) , and 11701250 cm ( c  n  c 
 stretch ) and increases at 12601300 cm ( c  o 
 stretch ) , 14001500 cm ( c  h deformations ) , 
 and 15301630 cm ( skeletal vibrations such 
 as quadrant stretching bands ) .
these changes in the bands associated 
 with aryl moieties upon heating is consistent with the monomer undergoing 
 ring - opening leading to changes in the rotational freedom of the phenyl 
 ring .
the scores plot of pc-2 versus pc-1 indicates that the 
 spectra are very similar and show very little difference during the 
 first five acquisitions .
spectrum 6 begins to differ , and then there 
 is a progressive change in the spectral characteristics , with pc-1 
 becoming increasingly influential , up to spectrum 9 . from spectrum 
 10 onward until the end of the experiment
there is a change in the 
 trend , perhaps related to the change from chemical to physical control 
 ( and from chain extension to cross - linking ) .
similar plots were obtained 
 for the range of bis - benzoxazine monomers and support the observation 
 that similar changes are observed in key peaks in the spectra during 
 cure ( this , in turn , suggests that the  uninitiated  

thermal cure mechanism is largely independent of the nature of the 
 bis - phenol of the benzoxazines studied here ) .
unfortunately , 
 the samples could only be cured up to 180 c for 2 h in the in situ raman experiment , due to instrumental limitations 
 making it impossible to monitor reactions cured / postcured at 200 c . 
 consequently , in order to validate the two experimental methods , it 
 was decided to perform an ex situ analysis ( i.e. , 
 curing individual samples in a fan - assisted oven to differing degrees 
 of cure following the cure schedule2 h at 180 c  and 
 analyzing these samples individually with respect to cure time ) before 
 comparing the results of the two methods .
the data may be compared 
 directly since the sampling intervals are the same and it is clear 
 that there is no significant effect on the pathway of the reaction 
 by changing the sampling method .
there is inevitably a slight delay / variation 
 in this method associated with removal of the samples from the oven , 
 quenching them in liquid nitrogen before introducing them into the 
 raman spectrometer . for comparison ,
the data for the uninitiated ba - a 
 ( as received ) and following recrystallization ( figure 11 , bottom ) are presented , showing graphically the differences 
 in the scores plot ( marked by a reduction in the significance of pc-2 
 is evident and a more symmetrical presentation of the data for the 
 latter ) .
pca data from cure of ba - a as received ( top ) and recrystallized 
 ( below ) .
raman data from in situ experiment showing 
 ( left ) scores for pc2 versus pc1 and ( right ) loadings for pc1 . n.b .
: 
 the first data point ( monomer ) has been omitted from this treatment .
the bis - benzoxazine monomers were each formulated with tdp or tda 
 at 10 mol % addition , and ex situ analysis was performed 
 using the same conditions ( heated at 2 k / min ) . using the same data 

treatment , the pca results arising from these spectra are presented 
 for the polymerization reaction initiated with tdp ( figure 12 ) and tda ( figure 13 ) .
these 
 treatments indicate that the initiated systems obey very different 
 kinetics during the early stages of the reaction ; e.g. , the changes 
 in the scores plots occur over a markedly shorter time scale during 
 both the early and later stages of reaction when compared with the 
 uninitiated blend ( confirming that the effect of the additives is 
 to induce a faster rate of cure ) .
furthermore , in the case of tdp 
 only one pc is required to describe the data ( whereas in the case 
 of tda two pcs are needed : pc-1 covers > 98% and pc-2 >
however , 
 it is possible to state that the final networks yielded different 
 spectra in the presence of the additives , possibly due to a different 
 termination mechanism , increased cross - link density , and consequent 
 differences in thermo - oxidative stability ( discussed below in the 
 context of the tga data ) .
pca data from in situ cure 
 of ba - a ( as received ) with 10 mol % tdp .
raman data showing ( top left ) 
 scores for pc2 versus pc1 and ( top right ) loadings for pc1 . n.b . : 

pca data from in situ cure of ba - a ( as received ) with 10 mol % tda .
raman data showing 
 ( top left ) scores for pc2 versus pc1 and ( top right ) loadings for 
 pc1 . n.b . :
the first , second , and third data points were omitted from 
 this treatment .
the difference in the 
 scores plots suggests differences in the mechanisms . from an overlay 
 plot of pc1 for the different samples ( figure 14 ) ,
there are spectral regions ( 15501700 cm ) that overlay well in terms of intensity , suggesting that the observed 
 differences in intensity are real .
this would indicate that during 
 the initial stages of the polymerization reaction the greatest reductions 
 are in the bands due to aromatic substitution .
for instance , reductions 
 are observed in the c  h out - of - plane deformation at 750 and 
 793 cm attributed by dunkers and ishida to 1,2,4-trisubstituted rings and 999 cm ( mono- , 1,3- , or 1,3,5-trisubstitution perhaps in 
 the arylamine ) .
overlay of plots of pc1 for ba - a , ba - a in the presence 
 of tda ( 10 mol % ) , and ba - a in the presence of tdp ( 10 mol % ) .
this is accompanied by reductions 
 in the bands at 1031 cm ( from c  c ring 
 stretch or c  o  c stretch ) and 1102 ( possibly c  n  c stretch ) and 1208 cm ( c  n stretch ) , 13001310 cm ( c  o stretch ) , and 14501500 cm ( ch2 scissoring ) . over the same period , increases in 
 band intensity are observed for 1050 ( c  o stretch ) and 1114 
 cm ( c  n stretch ) and 1518 and 1619 cm ( aromatic ring breathing ) , while bands appear to 
 shift between 1150 and 1250 cm . to explore the 
 mechanisms further , selected individual spectral bands within pc1 
 for the thermal polymerization of ba - a in the absence of additive 
 and with both tdp and tda were monitored as a function of reaction 
 time ( figure 15 ) over a total period of 185 
 min .
variation in intensity of selected spectral bands ( cm ) as a function of reaction time for ba - a ( a ) in the absence of additive , 
 ( b ) in the presence of tdp ( 5 mol % ) , and ( c ) in the presence of tda 
 ( 5 mol % ) . the profiles for ba - a 
 when thermally initiated or in the presence of tdp are quite similar , 
 albeit with the initiated polymerization occurring earlier .
at the 
 point at which crossover occurs the sample appears to be undergoing 
 some molecular change , e.g. , gelation causing a change in the kinetics , 
 as this is the same point at which the parabola changes gradient in 
 the scores plot ( 6080 min for the tdp - initiated reaction ; 
 100120 min for the thermally initiated reaction ) .
this demonstrates 
 the sluggish nature of the reaction in the absence of initiators or 
 catalysts ; the observations would have been retarded still further 
 on the purified ba - a monomer .
there are reductions in the bands at 
 793 , 999 , 1032 , and 1104 cm ( associated with the 
 ring - closed benzoxazine structure ) from about 85 min in the tdp - initiated 
 ba - a and from 35 to 115 min in the ba - a alone .
all the aforementioned 
 bands appear to drop at a similar rate , while the intensity of the 
 band at 751 cm decreases more rapidly and earlier 
 in the polymerization reaction .
conversely , the bands at 1052 , 1312 , 
 and 1487 cm , which are attributed to aromatic 
 ring functions , increase at approximately the same rate , and bands 
 at 1619 and 1581 cm initially increase at the 
 same rate , before 1581 cm increases its growth . 

these observations imply that the bands associated with 1581 and 751 
 cm are related , as are those at 999 and 1619 cm .
when compared with ba - a alone the differences in 
 the growth of the band at 1581 and 1619 cm are 
 much more marked for the tdp initiated system , with 1581 cm increasing more rapidly and to a greater intensity , whereas in ba - a 
 alone the increase in 1619 cm is greater than 
 tdp , relative to many of the other bands .
a similar analysis 
 of the spectral data relating to the ba - a initiated with tda yields 
 quite different results .
for instance , the bands at 999 and 1619 cm are negatively correlated , as are 1581 and 751 cm .
this observation is confirmed by the pc1 loadings 
 plots as these are the most intense peaks ( either positive or negative ) 
 and have similar loadings although 1581 cm is 
 slightly greater .
the increase in the degree of substitution of the 
 newly formed phenolic ring can be monitored by observing the reduction 
 in the intensities of the characteristic phenolic bands at 999 and 
 1032 cm ; the latter is assigned to the c  o  c 
 stretching mode of the benzoxazine ring .
thus , these may relate to the changing nature of 
 the benzene ring substitution through benzoxazine ring - opening and 
 bridge - forming .
dunkers and ishida also observe 
 a band at 1581 cm in a monofunctional benzoxazine 
 with alkylamine groups , but this apparently shifts to 1600 cm in the corresponding dimer , suggesting that this 
 band is related to the closed ring structure alone ; the band at 1620 
 cm is reported in both instances .
the same authors 
 note that the oxazine ring typically gives rise to a band around 960920 
 cm due to an out - of - plane b2 mode with 
 medium to strong intensity in the ir spectrum .
the changes in 
 peak intensities show a different pattern for the tda initiated monomer 
 ( figure 15c ) : the bands at 751 , 793 , 1032 , 
 and 1104 cm increase at a similar rate , but 999 
 cm appears to increase more rapidly and over a 
 longer period of time .
the bands at 1312 , 1487 , and 1052 cm increase at the same rate , but 1052 cm reaches 
 a much lower ultimate intensity . in fact , in ba - a , tdp , and tda , the 
 difference in intensity between 1312 and 1487 cm and 1052 cm increase from nearly 0 to 1 arbitrary 
 unit .
the bands at 1312 and 1487 cm are most likely 
 to reflect aromatic functions or possibly the methylene groups in 
 the bridges , but 1052 cm is probably a c  o  c 
 stretching mode . examining figure 15c ,
the relationship between 1581 and 1619 cm differs from 751 and 999 cm ; 
 1581 and 751 cm appear to be associated , as are 
 1619 and 999 cm ( unlike both ba - a and tdp ) . the thermal initiation and the tdp initiation of ba - a conform to 
 a similar polymerization mechanism , but initiation shifts the reaction 
 to a lower temperature regime and the resulting polymer networks display 
 similar cross - link density and glass transition temperatures ( spanning 
 a wider range  implying a more heterogeneous network ) . the initiation 
 of ba - a with tda progresses via a different mechanism involving the 
 paired bands at 999 and 1581 cm and 751 and 1619 
 cm , which are all aromatic vibration modes .
the 
 tda works more efficiently , leading to the opening of more benzoxazine 
 rings , and propagation is less linear ( more cluster - like ) prior to 
 cross - linking .
the result is that there are more points of reaction 
 leading to a higher cross - link density and consequent higher tg .
the effect of additive concentration [ tda ] 
 is initially really marked but reaches an apparent plateau at 1015 
 mol % .
a combination of thermal 
 and spectroscopic analysis has proven successful in offering support 
 to the proposed ring - opening mechanisms associated with benzoxazine 
 monomers .
the presence of the additive improves the temperature of 
 polymerization , by encouraging the ring - opening reaction at a lower 
 temperature , and its presence also appears to improve the thermal 
 stability of the final polymer .
spectral and thermal data suggest 
 both initiated and uninitiated blends undergo the same mechanism : 
 suggesting a step growth mechanism whereby the rings open and an active 
 center reacts so there may be several centers active at one time . 

the selection of an initiator based on its pka appears to be able to influence both the nature of the network 
 architecture and potentially the final properties of the polybenzoxazine 
 as the tg ( determined from dmta ) is elevated 
 when 3,3-thiodipropionic acid ( tda ) is used in preference 
 to 3,3-thiodiphenol .
tda is found to be the superior initiator , 
 offering a more rapid polymerization mechanism and a higher glass 
 transition temperature in the cured polybenzoxazines .
the monomers based on 
 bisphenol a ( ba - a ) , a mixture of 4,4- , 2,4- , and 2,2-bisphenol 
 f ( bf - a ) , 3,3-thiodiphenol ( bt - a ) , and additives 3,3-thiodiphenol 
 ( tdp ) and 3,3-thiodipropionic acid ( tda ) were all supplied 
 by huntsman advanced materials ( basel , switzerland ) and having been 
 characterized using h nmr , raman spectroscopy , and elemental 
 analysis were generally used as received without further purification . 

monomer ( ba - a ) was recrystallized from ethanol ( 96% ) prior to dsc 
 analysis when examining the kinetics of reaction .
the structures and 
 designations for the monomers studied are given in table 7 ; the corresponding polybenzoxazine of ba - a is designated 
 pba - a . where 
 additives were used , these were first ground ( together with the monomer ) 
 and then heated to 90100 c ( to melt the additive ) and 
 degassed for 23 h under vacuum depending on the level of foaming . 

where additives were not used , monomers were first degassed at 100120 
 c ( 23 h depending on the level of foaming ) and cured 
 in aluminum dishes ( 55 mm diameter , depth 10 mm ) in a fan - assisted 
 oven : heating at 2 k / min to 180 c ( 2 h isothermal ) + heating 
 at 2 k / min to 200 c ( 2 h isothermal ) followed by a gradual cool 
 ( 3 k / min ) to room temperature .
cured samples were cut to the correct 
 token size for analysis using a diamond saw . for the dynamic cure 
 study
100 c ( 1 h ) 
 and cured in aluminum dishes ( 55 mm diameter , depth 10 mm ) and transferred 
 to a fan - assisted oven set at 90 c .
the samples were allowed 
 to equilibrate before being heated at 2 , 8 , or 15 k / min to 180 c 
 ( 2 h isothermal ) + heating at 2 k / min to 200 c ( 2 h isothermal ) 
 followed by a gradual cool ( 3 k / min ) to room temperature . cured samples 
 were cut to the correct token size for analysis using a diamond saw . 

producing flat , void - free plaques for dmta provided a big challenge 
 as the benzoxazines were difficult to degas , and the addition of the 
 initiators compounded the problem as they seemed to undergo degradation 
 during cure to produce gas bubbles ; this was particularly problematic 
 at higher concentrations of tda .
vibrational spectra were obtained using a perkinelmer system 2000 
 ft - nir - raman spectrometer operating at 250500 mw ( nd : yag laser ) 
 and a perkinelmer ftir system 2000 spectrometer .
samples that were 
 analyzed in situ during the cure process using raman 
 spectroscopy employed a heated cell that was ramped rapidly from 50 
 to 180 c and held isothermally for 120 min ; spectra being taken 
 at intervals of 7.5 min .
in contrast , samples analyzed ex 
 situ were heated in a fan - assisted oven that was ramped rapidly 
 to 180 c and held isothermally for 120 min ; samples being taken 
 at intervals of 7.5 min and quenched in liquid nitrogen .
samples for 
 ir analysis were cast from chloroform ( gpr ) on to kbr disks and the 
 solvent removed in a fan oven ( 50 c , 30 min ) . for each measurement , 
 10 spectra were obtained at a resolution of 4 cm and coadded to produce the final spectrum .
chemometrics analysis 
 ( pca ) was carried out on the spectral data using the unscrambler x , 
 v10.1 software ( camo , oslo ) .
nuclear magnetic resonance ( nmr ) spectra 
 ( including dept-135 , hsqc and hmbc pulse sequences to assign individual 
 environments ) were obtained for h ( 500 mhz ) and c ( 75 mhz ) using a bruker drx500 ft - nmr at 298 k in cdcl3 with tms .
high performance liquid chromatography ( hplc ) was undertaken 
 using a varian 920-lc using c18 column and uv detection 
 (  = 254 nm ) .
samples were dissolved in hplc grade chcl3 , and the mobile phase ( flow rate 0.7 ml / min , 40 c ) 
 was 100% chcl3/100% methanol gradient ( both hplc grade ) . 

differential scanning calorimetry ( dsc ) was undertaken using a ta 
 instruments q1000 running ta q series advantage software on samples 
 ( 4.0  0.5 mg ) in hermetically sealed aluminum pans .
experiments 
 were conducted at a heating rate of 5 , 8 , 10 , 12 , and 15 k / min from 
 room temperature to 300 c ( heat / cool / heat ) under flowing nitrogen 
 ( 50 cm / min . ) . in order to gauge the reactivity of the 
 monomer in the bulk
thermogravimetric analysis ( tga ) was performed on a ta 
 q500 on milled , cured resin samples ( 5.5  0.5 mg ) in a platinum 
 crucible from 20 to 1000 c at 10 k / min in air and nitrogen ( 40 
 cm / min ) .
dynamic mechanical thermal analysis ( dmta ) ( in 
 single cantilever mode at a frequency of 1 hz ) was carried out on 
 cured neat resin samples ( 2 mm  10 mm  17 mm ) using a 
 ta q800 in static air .
 three bis - benzoxazine monomers based 
 on the aniline derivatives of bisphenol a ( ba - a ) , bisphenol f ( bf - a ) , 
 and 3,3-thiodiphenol ( bt - a ) are examined using a variety of 
 spectroscopic , chromatographic , and thermomechanical techniques . 
 the 
 effect on the polymerization of the monomers is compared using two 
 common compounds , 3,3-thiodiphenol ( tdp ) and 3,3-thiodipropionic 
 acid ( tda ) , at a variety of loadings . 
 it is found that the diacid 
 has a greater effect on reducing the onset of polymerization and increasing 
 cross - link density and tg for a given 
 benzoxazine . 
 however , the addition of > 5 wt % of the diacid had 
 a detrimental effect on the cross - link density , tg , and thermal stability of the polymer . the kinetics 
 of the polymerization of ba - a were found to be well described using 
 an autocatalytic model for which values of n = 1.64 
 and m = 2.31 were obtained for the early and later 
 stages of reaction ( activation energy = 81 kj / mol ) . 
 following recrystallization 
 the same monomer yielded values n = 1.89 , m = 0.89 , and ea = 94 kj / mol 
 ( confirming the influence of higher oligomers on reactivity ) . 
 the 
 choice of additive ( in particular the magnitude of its pka ) appears to influence the nature of the network formation 
 from a linear toward a more clusterlike growth mechanism . 
Introduction
Results
and Discussion
Conclusions
Experimental Section