Ind. Eng. Chem. Res. 1994,33, 2265-2271
2265
Thermal Stability of High Performance Poly(ary1 ether sulfones): Structure/Reactivity Relationships in the Pyrolysis of Oligomeric Model Compounds Linda J. Broadbelt and Michael T. Klein' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Barry D. Dean and Stephen M. Andrews Amoco Performance Products, Inc., Alpharetta, Georgia 30202
The model oligomers 2,2-bis[ [4-(phenylsulfonyl)phenoxylphenyllpropane (ISO) and 4,4'-bis [4(phenylsulfonyl)phenoxyl-1,l'-biphenyl(BP) were reacted neat in argon at 425 "C t o compare the thermal stability of poly(ary1 ether sulfones). Sulfur dioxide was the major gas product, B P generating a slightly higher evolution rate. The weight-average molecular weight increased with reaction time for both oligomers, but after 60 min, the rate of increase was higher for BP. A quantitative molecular level explanation of these observations focuses on the reactive isopropylidene link in ISO. Fission of the weak Ph-SO2-Ph bond produces phenyl radicals that abstract hydrogen and add to the oligomer backbone. The latter reaction results in increases in M, and a net weakening of the aryl-SOz bond, which leads to increased rates of SO2 evolution. T h e value of the relative rate, Rrel, of radical addition to H-abstraction was higher for B P than for IS0 due to easily abstractable isopropylidene hydrogens.
Introduction Poly(ary1 ether sulfones) form a class of polymers that have excellent mechanical properties, maintain their desirable properties for extended periods of time, and have high glass transition temperatures. These poly(ary1ether sulfones) (PAES) include aromatic rings linked together by ether and sulfone groups, and their thermal and mechanical properties can be tailored by the addition of different tertiary linkages. Two that are commonly used are the isopropylidene link and the biphenyl link. The resulting high temperature softening point makes their end-use properties attractive but presents special challenges during thermal processing. The glass transition of a PAES is often as high as 270 "C and temperature, Tg,. therefore necessitates high melt processing temperatures to achieve practical melt viscosities. Processing temperatures are often elevated as high as 400 "C. Rose (1974) reported that the biphenyl-containing PAES has a Tgvalue equal to 250 "C. The isopropylidene-containingPAES has a lower Tgof 190 "C, but still must be processed at elevated temperatures. At these high processing temperatures, thermal degradation reactions occur, which can have a deleterious impact on both manufacturing and enduse properties. This has motivated considerable interest in the elucidation of the reaction pathways, kinetics, and mechanisms underlying thermal stability. The literature provides both individual and comparative studies of the thermal stability of PAES polymers with various tertiary linkages. Hale et al. (1967) studied the isopropylidene-containing PAES to assess its thermal stability and end-use properties in comparison to a polymer composed of solely ether linkages or solely sulfone linkages. The experimentally observed formation of volatiles was explained by scission reactions of both the weak isopropylidene C-CH3 and C-SO2 bonds. Levantovskaya et al. (1971) also examined the thermal stability of polysulfone. They reported both SO2 evolution and an increase in viscosity with heating. Scission reactions, followed by hydrogen abstraction to cap phenyl radicals, were proposed to account for the formation of gaseous products. Path-
ways leading to an increase in M, or viscosity were not advanced. The first suggestion of phenyl radical addition to other polymer chains as a cross-linking mechanism was put forth by Davis (1969) to explain the formation of an insoluble gel fraction. The isopropylidene link was considered as only a weak link that was subject to scission reactions. A comparative experimental study of PAES polymers with different tertiary linkages was carried out by Danilina et al. (1974). They found greater yields of evolved SO2 from the biphenyl-containing polymer than for the PAES with the isopropylidene link. Cyclohexadienyl radicals were detected, and their presence supported phenyl radical addition as a reaction leading to cross-linking and gel formation. The delayed gel formation of PAES with the isopropylidene linkage was attributed to the dissociation of H' from the isopropylidene link itself. Crossland et al. (1986) also reported a greater quantity of SO2 for the biphenyl polymer but, curiously, found that PAES with the isopropylidene moiety was, overall, less thermally stable. The first suggestion of competing rates of Habstraction and phenyl radical addition was made by Kuroda et al. (19891, but the isopropylidene link was considered only in terms of scission, not as a source of easily abstractable hydrogen. The present work uses oligomeric mimics of these polymers to examine in detail the specific differences between the stability of PAES polymers containing the biphenyl link and those containing the isopropylidene link. The isopropylidene link itself was viewed as the key structural moiety controlling the rates of SO2 evolution and the growth in M , that correlates with gel formation. Two aryl oligomerswere synthesized that contained sulfone and ether linkages and differed primarily in the identity of their central linkage. One possessed a biphenyl linkage (BP), the other an isopropylidene linkage (ISO). The oligomers IS0 and BP are viewed as representations of two important PAES whose repeat units are shown in Figure l a and Figure lb, respectively. The oligomeric reactants IS0 and BP were chosen because their inter-
o s s s - ~ s s ~ i 9 4 ~ 2 s 3 3 - ~ 2 ~ 5 $ 0 4 .@ ~ 01994 1 0 American Chemical Society
2266 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
v)
a
Figure 1. Repeat units of the poly(ary1 ether sulfones) mimicked by the oligomeric materials (a) IS0 and (b) BP.
BP IS0
. 0
0.4
0
0
0
0.0
0 2,2'-bis[4-(sulfonylphenyl)-4, I-phenylene oxy-4,l-phenylene]-propane(ISO)
0
0
Y
0
0
0.6
100
200
300
TIME (MIN)
Figure 3. Kinetics of IS0 and BP pyrolysis at 425 O C . u
u
u
u
4,4'-bis[4-(sulfonylphenyl)phenoxy]-1 .l'-biphenyl(BP)
Figure 2. Structure and nomenclature of the oligomeric reactants 2,2-bis[[4-(phenylsulfonyl)phenyloxylphenyllpropane and 4,4'-bis[4-(phenylsulfonyl)phenoxyl1,l'-biphenyl.
mediate sizes allowed facile experimental analysis while introducing the physical effects on reactivity evidenced in polymer systems. Quantitative kinetic analysis of the reactions of these two oligomers sought to explain the differences in the thermal stability of poly(ary1 ether sulfones) with different tertiary linkages.
Experimental Section The reactants 2,2-bis[ 14-(phenylsulfonyl)phenyloxylphenyllpropane (designated as IS0 because of the isopropylidene linkage) and 4,4'-bis[4-(phenylsulfonyl)phenoxy]-1,l'-biphenyl (designated as BP because of the biphenyl linkage) are shown in Figure 2. IS0 and BP were synthesized by the reaction of the sodium salt of 4,4'-isopropylidenediphenol and 4,4'-dihydroxybiphenyl, respectively,with 4-chlorophenylphenyl sulfone (Aldrich). Neat pyrolysis reactions were carried out at 425 "C in an inert argon atmosphere to mimic elevated melt processing temperatures. Batch reaction times typically ranged from 5 to 240 min. The reactors were 2 mL glass ampules (Wheaton). Approximately 40 mg of oligomer was placed in the glass ampule, which was purged and flame sealed. This reactor was then placed in an isothermal fluidized sand bath for the predetermined reaction time. The time required to achieve the reaction temperature of 425 OC was determined to be 2 min. After the duration of the reaction time, the reactor was removed and cooled at room temperature. The reactor was reweighed to check for leakage. The reaction products were grouped into three different product fractions: gas fraction, gel fraction, and sol fraction. The defining analytical procedures were as follows. For gas analysis, the vial was opened in a sampling vessel of predetermined volume wherein the gases were allowed to equilibrate for 8 h. Gas aliquots of 1mL were injected into a Perkin-Elmer Sigma 3b chromatograph equipped with a flame photometric detector, allowing quantification of sulfur-containing compounds. The gel fraction was defined by a combined solubility/ filtration protocol. After the gas analysis was completed, the oligomer was extracted from the reaction vial with repeated washings of dichloromethane. This liquid sample
was then passed through 1.0 and 0.45 pm polytetrafluoroethylene filters in series, and the soluble fraction (sol fraction) was collected. The preweighed filter was then dried in a vacuum oven at 80 "C for 24 h. Subsequent weighing provided the weight of the solid fraction. The gel fraction was calculated by dividing the weight of the solid fraction by the initial weight of the oligomer. The molecular weight distribution of the sol fraction was analyzed on a Hewlett Packard 1090M liquid chromatograph using gel permeation chromatography. A Hewlett Packard PL gel linear column was used in series with a Hewlett Packard 50 A column. A refractive index detector was employed. The solvent was dichloromethane at a flow rate of 0.8 mL/min. Calibration was performed using narrow molecular weight distribution polystyrene standards from Scientific Polymer Products. The possibility that low molecular weight liquid products formed was checked by gas chromatography (GC)using an HP5890 gas chromatograph equipped with a flame ionization detector and an Ultra 2 5% cross-linked phenyl methyl silicone capillary column. Dibenzyl ether was added as an external standard.
Experimental Results The rates of disappearance of I S 0 and BP are summarized in Figure 3. The disappearance rates were comparable, as both reactants achieved a conversion of approximately 0.3 after 120 rnin of reaction time. BP appeared to react more slowly thereafter. For example, I S 0 was over 60% converted after 240 min of reaction time, whereas BP was only 50% converted after 225 min. Sulfur dioxide (SOZ)was the major gas product from reaction of both oligomers. The time dependence of its quantitative yield, calculated as the mass of evolved SO2 divided by the mass of SO2 units in the oligomer, is presented in Figure 4. SO2 evolution from BP showed apparent autocatalytic behavior. A t low times the SO2 evolution rate increased as reaction time increased. This lasted until about 150 min, after which came a period of slight decrease. The rate of SO2 evolution from IS0 showed a similar, but less marked, initial increase as reaction time increased. At times greater than 100 min, the rate of SO2 evolution was comparable to that from BP pyrolysis but showed no analogous decrease at long reaction times. Overall, the yields were similar except for the BP autocatalytic period of 80-170 min. The quantity of other sulfur-containing gases was negligible for both oligomers. Moreover,quantitative GC analysis revealed that no liquid phase lower molecular weight products indicative of bond scission formed.
Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 2267 0 0
0 biphenyl
0 isopropylidene
time
(0)
Figure 4. Evolution of SO2 as the major gaseous product from pyrolyses of IS0 and BP. 1400
1200
t EP 0 IS0
*""
0
100
200
TIME (MIN)
Figure 5. Increase in weight-average molecular weight for both BP and IS0 with reaction time.
The time dependence of the weight-average molecular weight (M,) is shown in Figure 5. Clearly, M , increased as reaction time increased. There was little difference in M , between the two oligomers intially. However, after about 60 min, M, for BP increased more rapidly than that for ISO. A value M , = 1400 amu was achieved after a reaction time of 150 min for BP. This is considerably larger than the maximum value of 950 amu obtained by IS0 at the same reaction time. Although the increase in M , shown in Figure 5 for both oligomers is indicative of the formation of a gel fraction precursor (Kuroda et al., 1989; Libanati et al., 1993), no gel formed in the present experiments. Thus the entire distribution of reaction products was composed of molecules small enough to be solubilized by the extraction solvent and pass through the filter pores. This was true even at the highest average molecular weights achieved.
Discussion The experimental differences observed here are consistent with difficulties encountered in melt processing of the analogous polymers. Early-time data extracted during processing reveal no significant differences among candidate polymers, but at some critical processing time, the differences become apparent. The formation of high molecular weight material and a significant gel fraction prevents further melt processing. The underlying mechanism that contributes to polymer thermal stability as defined by molecular weight growth and gel formation manifests itself at longer reaction times as a critical phenomenon. The experimental results shown here are global in nature and therefore present only indirect evidence of the underlying reaction fundamentals. That is, the measurables of gas fraction composition and yield, reactant disappearance rates, and sol fraction molecular weight
distribution are direct consequences, but indirect measures, of the elementary bond-breaking and bond-forming reactions that form cross-links or produce gases. Nevertheless, it is these elementary steps that hold the key to not only improved understanding but also strategies for improving the overall globally observed behavior. The logical starting point for analysis at this level is to focus on the effect of the major structural differences of the two oligomers, namely the aliphatic isopropylidene moiety and the biphenyl moiety that serve as the central link in the IS0 and BP oligomers, respectively. Controlling Elementary Steps. The observed product distribution guides this mechanistic analysis. The evolution of sulfur dioxide in high yield, the simultaneous increase in M,, and the lack of any quantifiable, small products are most telling. This information suggests that two major competing reactions were operative for both oligomers: bond scission that led to the evolution of SO2 and bond formation or addition reactions that led to crosslinks and the increases in molecular weight. The IS0 oligomer had unique reaction paths, available due to the presence of the aliphatic carbon, to be developed below. Common to both oligomers, the high yields of evolved SO2 suggest that the aryl-SO2 bonds in both BP and IS0 are thermally labile at 425 "C. This fission has been proposed to occur through a two-step process (Crossland et al., 1986; Danilina et al., 1974; Davis, 1969; Hale et al., 1967). The cleavage of the first aryl-SO2 bond affords an aryl radical and an aryl-SO20 radical. The rapid release of a molecule of sulfur dioxide yields another aryl radical. The net reaction is represented as eq 1. The identity of oligomer
-
SO,
+ Ph' + aryl'
(1)
the "first-broken" aryl-SO2 bond will be dictated by the relative rates of fission, which are in turn dictated by the differences in bond strength, which are, in general, conversion dependent in the reaction-altered oligomer mass. This elementary step of eq 1would clearly predict firstorder kinetics for SO2formation. However,the net release of SO2 from the oligomers could deviate from simple firstorder behavior because of both physical and chemical effects. "Cage" effects (Koenig and Fischer, 1973; Moore and Pearson, 1981) serve to decrease the rate of SO2 evolution. These could become increasingly important, for example, with the increase in the viscosity of the reaction medium that would occur because of the increase in molecular weight and cross-link density (Kuroda et al., 1989; Moore and Pearson, 1981; Rohr and Klein, 1990). Chemical, or electronic, effects can either increase or decrease the rate of SO2 formation, depending on the impact of the reaction-altered electronic environment of the SO2 bond, which, in turn, would affect the bond strength. The SO2 evolution kinetics observed for both IS0 and BP suggests that both effects were operative. For both oligomers, the rate of SO2evolution increased with reaction time for t < 170min. This suggeststhat, during this period, any physical effects were dominated by chemical factors whose net effect was to decrease the bond strength of the aryl-SO2 bonds. The BP oligomer exhibited a subsequent decrease in the rate of SO2 evolution after 150 min. This, combined with the increasing molecular weight, suggests the intrusion of physical, or cage, effects. The low conversion chemical effects are clearly a result of the reactions of the radicals created by eq 1. The aryl radicals that are formed as a result of this bond fission can pursue several reaction pathways. First, the close prox-
2268 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
imity of the aryl radicals in a melt reaction medium supports the likelihood of radical recombination to form a biphenyl linkage. This reaction would result in no measurable net change in molecular weight. Second, a t low conversion prior to any significant viscosity increase, the radicals would have sufficient mobility to disencounter one another and react with other species. These reactions include aryl radical hydrogen abstraction or radical addition. Hydrogen abstraction would result in no significant change in molecular weight. This leaves radical addition as the key elementary step in the mechanistic scenario underlying the observed release of SO2 and increase in M,. The addition of the aryl radical to an aromatic ring of an oligomeric backbone results in an intermediate cyclohexadienyl radical. The cyclohexadienyl radical can (1) eliminate Ha,to regain aromaticity; (2) recombine with another radical; and (3) abstract hydrogen to form a cyclohexadiene. Addition of an unsubstituted phenyl radical would lead to modest change in M,. However, addition of an aryl radical with the oligomeric substituent would lead to a significant increase in M,. This increase in M , was observed experimentally for both oligomers. This suggests that aryl radical addition is a dominant reaction pathway in aryl ether sulfone systems. Its effect, considered more quantitatively below, was evidently to weaken the aryl-SO2 linkages. The finer-scale differences between the IS0 and BP oligomers could be attributed to the influence of the isopropylidene link on the selectivity to and consequences of phenyl radical addition, as follows. The simple presence of the isopropylidene link can change the selectivity to aryl radical addition by providing other alternative reaction pathways. This would in turn retard the rate of M , increase. Specifically, the methyl hydrogens of the isopropylidene link are a source of easily abstractable hydrogen. They serve as competition for consumption of the aryl radicals. If a phenyl radical or an aryl radical with an oligomeric tail abstracts hydrogen from the isopropylidene link, this caps the radical and leads to no substantial increase in molecular weight. Aryl radical addition, and thus molecular weight growth, are diminished through the appearance of this competitive pathway. This scenario explains the experimentally observed kinetics of M , increase. Figure 5 shows that changes in M , were not as marked for the I S 0 oligomer as for the BP oligomer. This had an indirect, second-order effect on the kinetics of SO2evolution. If aryl radical addition leads to a weakening of the aryl-S02 bond, the rate of evolution of SO2 would be higher for BP than ISO. Again, this was observed experimentally in the region t < 170 min. The foregoing qualitative scenario rests upon two theses: (1)that aryl radical addition weakens the affected aryl-SO2 linkages and (2) that the presence of the isopropylidene linkage lowers the selectivity to aryl radical addition by supplying competitive reaction paths. Clearly, more quantitative analysis aimed at testing these theses would be of use. Kinetics of the Elementary Steps: The Local Environment,Model CompoundApproach. The effect of aryl radical addition on aryl-SO2 bond cleavage can be estimated through the calculation of the changes in the activation energy of bond fission with the addition of a ring substituent. For bond fission, the activation energy is equal to the heat of reaction. Thus the effects of the ring substituents can be made by simply calculating the heat of reaction for the different reactions. The challenge in this approach is that the heats of
Figure 6. Bond fission example of the minimal local environment model compound (LEMC) approach.
formation for oligomericspecies and the radicals that they form upon bond fission are, for the most part, unavailable in the literature. This suggests the value of applied computational quantum chemistry in the estimation of the full range of heat of formation values sought. The MNDO/PM3 parameter set (Stewart, 1990a) was used in this work since it was parametrized for heats of formation of a large database of molecular species. The calculation of heats of formation of radicals requires the use of a linear correction that was developed by comparing experimental values to calculated values. The average signed error of MNDO/PM3 for all organic compounds containing C, H, N, and 0 is +0.21 kcal/mol (Stewart, 1990b) and for all compounds containing sulfur is -1.30 kcal/mol. The BP and IS0 oligomers and their potential adducts presented an additional challenge. CPU resource limitations restrict the total number of atoms that can be handled by the MNDO/PM3 approach. Geometry optimization of these complex molecules would also be tedious. These issues motivated the use of a local environment, model compound approximation, which provides the effect of a substituent whose influence becomes negligible as the distance from the reactive center increases. In its current application, reaction-induced changes in reactivity of the oligomeric reactive moieties were modeled in terms of the effect of substituents in the meta or ortho positions of the immediately adjacent benzene rings. Para substituent effects were only considered for the terminal rings, since more generally, the para position would be occupied by the extended oligomeric chain. Ipso substitution is not considered because the results of Tiecco (1980) revealed the absence of phenylradical addition at the ipso positions for substituted benzenes. A bond fission example of this approach is illustrated in Figure 6. Bond fission of an o-phenyl-substituted arylSO2 bond is modeled as bond fission of the aryl-SO2 bond of o-phenyl diphenyl sulfone. The model compound o-phenyl diphenyl sulfone contains the same meta and ortho environment around the SO2 linkage as in the oligomer. Numerically, the enthalpy of bond fission of the oligomer was obtained as the quantitative value for bond fission of the model compound, the calculations for which being computationally tractable. Table 1 summarizes the substituted aryl-SO2 bond fissions considered and the associated reaction enthalpies. Note that the possibility that phenyl addition led to a cyclohexadiene intermediate is included in the reactions of Table 1. Collectively, the reactions of Table 1provide several trends about the dependence of the magnitude of the substituent effect on the bond and substituent positions. First, the effect of cyclohexadiene was to lower the bond strength by 8 kcal/mol as compared to the aromatic counterpart. The *nearby” aryl402 bond strength is
Ind. Eng. Chem. Res., Vol. 33,No. 10,1994 2269 Table 1. Substituted-Aryl-SO2 Bond Fissions and Associated Reaction Enthalpies Reaction
AHR = E*/(kcal mol-1)
Sites of Unique Reactivity Toward Radical Addition and Hydrogen Abstraction
Number
66.6
1
58.2
2
51.3
3
b
d
c
Model Compounds Used to Represent Unique Reactive Sites Reactive Site 52.2
Model Compound
4
a, b. c 60.7
5
d
60.1
6
66.9
7
e, f
g 65.1
8
68.5
9
71.5
10
lowered by the presence of both ortho and meta phenyl substituents. Reaction 3 shows that the aryl-SO2 bond is weakened by slightly greater than 9 kcal/mol solely by the addition of the ortho phenyl substituent whereas reaction 5 shows that a phenyl substituent in the meta position lowers the bond dissociation energy for the aromatic molecule by 6 kcal/mol. These decreases are due in part to the increased stability of the bicyclicradical as compared to the very reactive phenyl radical. When the interior ring is a cyclohexadiene, there is a 6 kcal/mol lowering of the bond dissociation energy as compared to the fission of the C-SOz bond of the unsubstituted phenylsulfonylcyclohexadiene as shown in reaction 4. This 14 kcal/mol total decrease as compared to the base case of reaction 1 translates into a log&/kl) = 4.4 increase in the rate at 425 "C. It is interesting that the "removed" (terminal) phenylSO2 bond strength is essentially unchanged with the addition of an ortho phenyl Substituent. Moreover, a phenyl substituent in the meta position actually increases the bond dissociation energy. This difference is attributable to the electron withdrawing or electron donating character of the substituents. For example, the ortho substituent has a Hammett parameter of -0.01, which indicates that it is electron donating. The meta phenyl substituent has a Hammett parameter of 0.06, indicating that it is electron withdrawing (Hansch et al., 1973). The electron-deficient sulfur atom, with the highly electronegative oxygens attached, is destabilized by the electronwithdrawing substituent.
Figure 7. Minimal local environment model compound selection for the unique reactive sites of the oligomers.
Overall, the calculations noted above suggest a clear net decrease in the strengthsof the "nearbyn substituted arylSO2 bonds. This explains the increase in the rate of SO2 evolution as the reaction proceeds observed at low conversion. The slight decrease in the rate of SO2 yield in the BP oligomer at long reaction times is attributable to the increasing importance of cage effects. At the longer reaction times, the BP oligomer showed an increase in M, growth, leading to decreased mobility of the fission-derived radicals. This is because the BP oligomer did not have access to the reaction paths made available to IS0 because of the crucial isopropylidene link. Competition between Phenyl Radical Addition and H-Abstraction. The differences in the rates of SO2 evolution and growth in M, observed between BP and I S 0 can now be traced to the combined action of aryl radical addition, which leads to enhanced rates of SO2 evolution, and the ability of the isopropylidene linkage to inhibit this addition by acting as an aryl radical scavenger. The LEMC approximation was thus employed to obtain quantitative estimates of the relative rates of aryl radical addition and H-abstraction. The LEMC approach for aryl radical addition is analogous to that for bond fission. However, the multiplicity of addition and H-abstraction sites increased the possible combinations. Thus, each of the oligomers was examined to determine the sites of unique reactivitytoward radical addition and H-abstraction. Corresponding model compounds were then chosen to represent these unique reactive sites. The reaction sites and their representative model compounds are illustrated in Figure 7. Both hydrogen abstraction and radical addition reactions can occur at all of the aromatic sites. The three distinct reactive sites, a, b, and c, on the terminal benzene ring were modeled by phenyl sulfone. Site d was influenced by the proximity of the oxygen and its possible electronic effects. Thus,
2270 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 Table 2. Possible Addition and H-AbstractionReactions of the ISO-and BP-Derived Phenyl Radicals Reaction RPD Number 2 4
8 8 4
6 4
2
4
8
8
4
Table 3. Calculated Heats of Reaction and Associated Rate Constants for H-Abstractionand Addition Reactions reaction no. MR/(kcal mol-') E*l(kcal mol-') kl(108 L mol-' 8-1) -7.30 1 9.68 0.591 9.83 2 -6.70 1.06 -3.28 10.68 1.14 3 8.85 -10.59 4.28 4 10.57 -3.73 0.621 5 7.24 -17.05 6 10.3 -3.02 0.546 10.75 7 -27.10 8.52 3.42 8 3.74 -26.50 13.5 9 7.32 5.55 -23.08 10 1.68 -30.39 11 119 41.1 2.20 -29.41 12 -34.48 201 13 0.00 In to Table 4. Selectivity of Aryl Radical AI H-Abstractionby Aryl Radical rate additionhate oligomer H-abstraction BP 45.8 IS0 10.6
the oligomers and the use of kinetic parameters on a per atom basis. Table 3 summarizes calculated heat of reaction and associated rate constant with reaction path degeneracy incorporated for each of the reactions of Table 2. Note that the rate constant for H-abstraction by phenyl radical from the aliphatic hydrogens in the isopropylidene link is an order of magnitude greater than that for the other H-abstraction reactions. This is the major contribution to a reduction in the calculated ratio for the IS0 oligomer. The relative rates Rrel of phenyl radical addition to H-abstraction by phenyl radical were estimated by summing the rate constants for each site. That is,
A
diphenyl ether was used as the relevant model compound that distinguished site b from site d. The assignment of the other sites and their corresponding model compounds followsthe same logic. Note that a, b, c, and d are common to the two oligomers. The crucial differences are the presence of sites e and f, in the IS0 oligomer, and a unique site, g, in the BP oligomer. Quantitative estimates of the relative rates of aryl addition and H-abstraction used Polanyi structurereactivity correlations, one for each of the elementary step reaction families. For H-abstraction, a single loglo A value of 8.5 was used. The classic Polanyi (Evans and Polanyi, 1938) parameters of a! = 0.25 and EO= 11.5 kcal/mol due to Semenov (1958) were employed. The radical addition reaction family parameter of loglo A = 7.7 (James and Suart, 1968; Janzen and Anderson, 1975; Libanati, 1992) was used, whereas the structure-reactivity parameters of a! = 0.53 and Eo = 17.75 kcal/mol were chosen from a linear correlation deduced by LaMarca (1992). Thus eqs 2 and 3 correlated the kinetics of H-abstraction and radical addition, respectively. log,,(k/(L mol-' s-)) = 8.5 - (11.5 + O.25AHoR)/8 (2) log,,(k/(L mol-' s-I)) = 7.7 - (17.8 + O.53AHoR)/8 (3) Table 2 delineates the possible addition and H-abstraction reactions of the phenyl radical in terms of the unique sites and the appropriate model compounds. The reaction path degeneracy (RPD) factor accounts for symmetry of
N sites
N sites
k;dd(Ph')(oligomer) -
H .=
--rei
i=l
M sites
--
14)
M sites
k"a(PPh')(oligomer) J=1
\ - I
kybs
J=1
The resulting values of R,1 are listed in Table 4. The value of 45.8 for BP is remarkably close to the value of 40 reported by Perkins (1973) for reaction of phenyl radical with benzene at 450 "C. R,1 for BP is greater than that for IS0 by more than a factor of 4. This is consistent with the experimental data, and provides semiquantitative support for the qualitative observations. In short, the trend in Rrel combines with the earlier established bond weakening effect of phenyl addition to predict higher rates of SO2 evolution from BP than from ISO. That is, the ratio of the rates of phenyl radical addition to Habstraction is greater for the BP oligomer, which is manifested in a more rapid increase in the rate of SO2 evolution and an increased rate of molecular weight growth. Summary and Conclusions Sulfur dioxide (S02) was the major gas product from reaction of both BP and ISO. SO2 evolution from BP showed apparent autocatalytic behavior and was with a slightly higher rate than for ISO. M, increased as reaction time increased during the pyrolysis of both oligomers. There was little difference in M , between the two oligomers until about 60 min, at which time the rate of molecular weight increase was higher for BP than for ISO.
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A molecular level explanation of these observations derives from the only difference between the oligomers, namely, the presence of the isopropylidene link in ISO. The selectivity to phenyl addition over H-abstraction is higher for the biphenyl oligomer than for the isopropylidene oligomer. The easily abstractable hydrogens in the isopropylidene oligomer act as aryl radical scavengers. Phenyl addition to the oligomer backbone results in a net weakening of the aryl-SO2 bond. This leads to increased rates of SO2 evolution as reaction time increases. Collectively, these factors show that the more rapid increase in the rate of SO2 evolution for the BP oligomer can be attributed to the increased selectivity to aryl radical addition over H-abstraction by aryl radical. The increased selectivity to phenyl addition also explains the experimentally observed changes in the weight-average molecular weight of the oligomers. The increase in M, for BP was greater than that for IS0 because of the higher selectivity to aryl radical addition in BP. Literature Cited Crossland, B.; Knight, G. J.; Wright, W.W. A Comparative Study of the Thermal Stability and Mechanism of Degradation of Poly(arylene sulphones). Br. Polym. J. 1986,18 (3),156-160. Danilina, L. I.; Teleshov, E. N.; Pravednikov, A. N. The Thermal Degradation of Aromatic Polysulphones. Vysok. Soyed. 1974,3, 581-587. Davis,A. Thermal Stability of Polysulphone.Makromol. Chem. 1969, 128, 242-251. Evans, M. G.;Polanyi, M. Inertia and Driving Force of Chemical Reactions. Trans. Faraday SOC.1938,34,11-29. Hale, W. F.; Farnham, A. G.; Johnson, R. N.; Clendinning, R. A. Poly(ary1 Ethers) by Nucleophilic Aromatic Substitution. 11. Thermal Stability. J. Polym. Sci., Part A-1 1967,5,2399-2414. Hansch, C.; Leo, A.; Unger, S. H.; Kim, K. H.; Nikaitani, D.; Lien, E. J. "Aromatic" Substituent Constants for Structure-Activity Correlations. J. Med. Chem. 1973,16 (ll), 1207-1216. James, D. G.L.; Suart, R. D. Kinetic Study of the Cyclohexadienyl Radical. 11. Patterns of Combination and Disproportionation with the Methyl, Ethyl, Sec-propyl and Tert-butyl Radicals. Tram. Faraday SOC.1968,64 (lo),2735-2751. Janzen, E. G.;Evans, C. E. Rate Constantsfor the Addition of Phenyl Radicals to N-(tert-Butyl-a-phenylnitrone)(Spin trapping) and
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Received for review November 16, 1993 Revised manuscript received April 15, 1994 Accepted April 25, 1994. ~
Abstract published in Advance ACS Abstracts, September 1, 1994. @
