Organic Pyropolymers STEPHEN D. BRUCK
Among the interesting materials issuing from the chemical laboratories of industry and the universities are polymers which can conduct electricity. In addition to their potential use in electronic devices, some of these materials
The possibility that such ma-
terials can be formed into unusual shapes opens a number of potential areas o f application that have not been known previously.
Though
chemists and chemical engineers seldom deal
with semiconducting materials directly, the advent o f a new materials technology will probably abruptly change this state of affairs.
The
author brings us up to ahte in one such area 18
falls between insulators and metallic conductors, irrtspective of the mechanism of the conduction process. As will be seen, attempts to apply situations prevailing in typical inorganic semiconductors to organic systems lead to difficulties. These difficulties are compounded especially in the case of pyropolymers-i.e., polymers which have been converted by pyrolysis to electrically conducting systemebecause of impurities and lack of proper structural characterizations.
SOME BASIC FUNDAMENTALS
may find increasing use as catalysts, and in other areas.
term “semiconducting” polymer has been loosely Tneapplied . to organic polymer systems whose resistivity
INDUSTRIAL A N D ENGINEERING CHEMISTRY
It seems appropriate first to dwell briefly on the mode of operation of inorganic semiconductors and to defme some of the parameters which will be frequently mentioned later in conjunction with organic pyropolymers. Semiconductors are characterized by two energy bands, one which is filled with electrons (valence band), and the other which is empty (conduction band). These two bands are separated by a forbidden energy gap. If an electron acquires sufficient energy (from heating or from the absorption of light), it can jump into the upper conduction band so as to conduct electricity. When this occurs, the removal of electrons from the valence bands leaves empty levels and creates the appearance of a flow of positively charged electrons (holes). In a perfect intrinsic semiconductor, the number of electrons in the conduction band is equal to the number of holes, and electrical conduction depends on the inherent atomic and crystal structure of the material. In extrinsic semiconductors, on the other hand, electrical conduction by electrons and holes results from impurity atoms present in the crystal lattice. Depending on
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whether the impurity atom (or atoms) donates or accepts an electron (or electrons) from the valence band, the semiconducting material will be n-type (because of the negative charge of the carriers) or p-type (because of the positive charge of the holes). The electrical conductivity (1/resistivity) of a semiconductor may be expressed by the following relationships : = (PL~ He) (1a)
+
= leln, =
where lei
(1
+
(1b)
c)
-E / 2 k T aoe
(14
electronic charge n = concentration of current carriers p h , @= mobilities of holes and electrons =
c = pe/ph = energy required to form charge k = Boltzmann constant T = absolute temperature e = base of natural logarithm
E
k
C - I --p(m)(&)+2
This relationship permits the calculation of the mobility ratio, c, since E may be obtained from a study of the temperature dependence of conductivity. If the carrier mobilities are only slightly dependent on temperature, a plot of log u us. 1 / T as per Equation I C should yield a straight line having a slope of - E / 2 k . Another important tool in the study of the mechanism of semiconduction is the Hall effect (6, 72). This is extremely difficult to observe in organic polymeric semiconductors because of the low mobilities of the current carriers in these systems. The Hall effect arises when a current-carrying conductor is placed in a transverse magnetic field, thus producing an electric field, E,, which is perpendicular to both the magnetic field and to the flow of current. When an electric field, iEL, is applied, the current carriers moving with a velocity of *zv are deflected by the magnetic field, kB,, where i and k are unit vectors in the directions of IC and Z , respectively. Consequently, the excess electrons create 20
eEH = f e v B , Since the velocity of the electrons is u
E, With current density J
= PELB, = nev =
(3) =
pEL, then (4)
nepEL:
E, = *(l/ne)BJ (5) The term =t(1/ne) in the above equation is called the Hall coefficient, R. Hall measurements thus can yield information on the concentration and sign of the charge carriers and, in conjunction with conductivity data, can give information on mobility values. ORGAN IC PY ROPOLYMERS
carriers
The value of c in Equation l b may be obtained from thermoelectric measurements, which yield the so-called thermoelectric power or Seebeck coefficient, Q . This represents the thermoelectric force per degree which arises when two different conductors are joined at both ends and the two junctions are kept at slightly different temperatures. By convention, the thermoelectric power is positive when the thermoelectric current flows from the specimen to the reference metal at the cold junction. Hence for a positive thermoelectric power, the Seebeck voltage will be positive at the cold junction. The Seebeck coefficient may be expressed as (77)
Q =
an electric field having a force eE, which, in a steady state, equals the force on the electrons due to the magnetic field; hence
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I n general, there are three approaches to the preparation of organic semiconductors : direct synthesis, modification of the polymer chains by methods such as chelating and complexing, and pyrolytic conversion of an appropriate polymer. The present paper deals with this third area and is limited to a brief review of graphite, followed by a discussion of “natural” pyropolymers, such as cokes, carbon blacks, and chars, and synthetic p) ropolymers, such as pyrolyzed divinylbenzene, polyvinylidene chloride, an aromatic polyimide, and polyacrylonitrile. These materials all appear to be intrinsic semiconductors. A discussion is also included on the catalytic activity of p) rolytic polyacrylonitrile which secms to be related to its semiconductiiig and electron spin resonance properties. Graphite and “Natural” Pyropolymers
Graphite has a layer structure which is composed of planes of hexagonal carbon rings fused into a covalently bonded large network (Figure 1). That the bonding between each layer is much weaker than the covalent bonding u ithin the layers serves to explain graphite’s good lubricating properties. The metallic-type electrical conductivit) of graphite originates in the high concentration of w-electrons associated with the 2 p , orbitals of the carbon atoms. The distance between the layers (in the direction of the c-axis) is 3.35 A . The stacking of the layer planes (or basal planes) in the crystal lattice is such that one half of the carbon atoms in a layer are positioned above and below the centers of the hexagonal carbon rings in adjacent lay-ers. The specific resistance parallel to the c-axis is several times greater than that perpendicular to it (25). In graphite, one full energy band is separated from an empt) band by a negligible energy gap. The electrical conductivity of pyrolytic graphite is due to both electrons and holes which are present in about equal numbers, The intrinsic carrier concentration (of a
I
1
I
I
1
specimen heat treated at 2500' C.) increases from 2.0 X lo'* per cc. at 4.2' K. to 6.5 X loB per cc. at 300" K., whereas in the same temperature range the hole mobility decreases from 4.0 X loa to 2.1 X lO'cm.'/volt-sec. with the mobility ratio, fiJfi,,, remaining constant at 1.1 (78). Impurities by virtue of their electron donating and accepting properties may form charge transfer complexes between expanded layer planes and affect the electrical conductivity of graphite (77). When cellulose and coal are pyrolyzed, internal condensation and dehydrogenation reactions occur. Up to approximately 600' C., the materials contain considerable quantities of oxygen and hydrogen, and exhibit strong ESR absorption. Further pyrolysis beyond 700' C. results in the disappearance of the ESR absorption, the loss of oxygen and hydrogen, and the appearance of electrical conduction. At approximately 1400' C. the conducting network structure is composed essentially of carbon but the presence of cross-links and lamellar imperfections prevents the graphitization p m ess. These carbonaceous polymers exhibit strong ESR absorption with a Line width of up to 6 gauss and spin concentration of approximately 10" spins/gram. The variation in intensity of the ESR absorption of two series of carbonaceous solids as a function of carbonizing temperature is illustrated in Figure 2. The maximum ESR absorption occurs between 550' and 600" C., then falls rapidly to zero and later reappears at about 1400' C. (27). According to Ingram (75) the ESR absorption origiites from unpaired spins as the result of the removal of edge groups by homolytic bond scission. These unpaired spins become stabiliied as relectrons in the aromatic network. The possibility that aromatic radical ions may be responsible for the ESR signal has also been suggested. The ESR absorption of graphites and carbons heated above 1500' C. has been attributed to charge carriers by Singer (27). As the temperature of pyrolysis is increased from 600" to 800' C., the resistivity of the system measured at room temperature falls rapidly to approximately 10"
2
=
0
z
I0 '
0.8
I@
0.7
101
0.6
10 '
0.5
lo)
04
lo?
0.3
i IO'
02
0
0.I
3l
-
*
E
= x
5
s
0
10-1 10-2
400
My)
em
IWO
lxo
1m
(UMNIZIHE IUII<UM 1'CI
Figure 3. The effect of heat treatment on the elcchicnl rcsisriOity and uctinafionm g y of a petrolcum coke (Refs. 74, 20)
~
AUTHOR S t q ' ~ hD . Bruck w a j ~ a n 1 ~ 4 ySenior St& Scientist
at Johs Hapkim Uniwsity, Applidd Pfiysics Laboratory, where some of the work discussed was conduc&d. This review
was adajkd from a tutorial paper presmkd at the 153rd National Meeting of the ACS, Miami. The author thanks Dr. R. E. Florin of the National Bureau of Standmdsfor his mitical review of this paper. Ths authoispresent address is B e l l c m , Inc., Washington, D. C. V O L 5 9 NO. 7 JULY 1 9 6 7
21
WROlVSU RLIltuNII 1"O
104fl
(OK-11
Rigurc 4. Resistivity of ( a ) prcoxidizcd polymnylbmunc pyrolyzates and (b) polyzinylidene chlaidc prOyvurtcs at abart 25O C. (Rcf. 29)
Figure 5. Pmamagnetic r ~ s o n a mabsorption in pnoxidized polydiuinyIbenmc as a function of higher p y d y f i c tmpnatwar (*. 29)
ohm-cm., but only small changes occur beyond 800" C. (74, 20), as seen from Figure 3. If the temperature of pyrolysis were to be raised beyond approximately 1600" C., the resistivity of the system would decrease further to about ohm-cm. In'the case of cokes, Hall coefficients fall from zero to approximately -0.25 in the pyrolysis range of 900' to 1200" C. The absolute thermoelectric power decreases with a changing sign (20). According to Akamatu (I), holes in large excess are produced by the cleavage of peripheral hydrogen atoms, which can trap electrons at the broken u-bonds to form new G states which show no ESR absorption. Hirabayashi (14) believes that as the temperature of the pyrolysis is raised beyond 700' C., extensive conjugation develops providing a large pool of r-electrons for electrical conduction. As the layer planes grow, inter-
granular boundary resistance and the activation energy of conduction fall, and peripheral u-bonds function as electron traps. A modification of this hypothesis by Weiss (28) suggests that the absence of ESR absorption in cham pyrolyzed between 700' and 1200" C. may involve quinone carbonyl groups serving as traps for electrons, some of which are donated by heterocyclic oxygen atoms. As the temperature of the pyrolysis rises up to 1200' C., the ratio of holes to electrons decreases which, together with the reduction in interboundary resistance, produces a metallic-type conductivity.
22
INDUSTRIAL AND ENGINEERING CHEMISTR'
Synlh.lic Pyropolymen
Polyvinylidene Chloride and Polydivinylbenzene. Among organic polymers, Winslow and colleagues found some yeam ago that the pyrolytic derivatives of poly-
vinylidene chloride and preoxidized polydivinylbenzene show strong ESR absorption (29). Pyrolysis of preoxidized polydivinylbenzene in vacuum at 400" C. increased the free radical concentration with time of pymlysis. The d.c. resistivities of preaxidized divinylbenzene pyrolyzed below 700" C. were characteristic of typical insulators. When pyrolyzed above 700" C., the room temperature resistivity values changed to the range of semiconductors (R < 10' ohm-cm.) as shown in Figure 4. The plot of the ESR absorption us. the reciprocal of absolute temperature shows a maximum at about 500" C., then falls drastically with increasing temperature (Figure 5). Similar observations were reported for polyvinylidene chloride. This polymer pyrolyzes without significant loss of carbon but with the loss of HC1 (Figure 6). It has been found that the rate of this reaction below 200" C. follows essentially first-order kinetics. According to Boyer (4) this indicates a chain reaction mechanism with the formation of unsaturated linkages but without appreciable cross-linking below 200" C. If the pyrolysis is carried out above 200" C., aromatic structures form via internal cyclization which show strong ESR absorption and have resistivities of less t b n 10' ohm-cm. A plot of ESR absorption us. the reciprocal of absolute temperature of pyrolysis shows a maximum at approximately 375" C., followed by a sharp drop in the intensity of the ESR signal, as illustrated in Figure 7. T o explain the above phenomenon, Winslow and his colleagues proposed that during pyrolysis there is a progressive increase in unsaturation with the development of aromatic structures which are arranged in a network by covalent bonds according to the hypothetical scheme on the opposite page. As the pyrolysis temperature is raised, more and more unpaired electrons form which eventually reach a concentration level beyond which their recombination to form covalent network structures exceeds their rate of formation. This causes a diminishing of the ESR absorption. The continuous formation of condensed ring structures adds to the overall resonance energy of the system and reduces the energy required for thermal excitation so that small changes in composition give rise to continuous paths of conjugation. As a result, the resistivity of the system is sharply reduced to the level of semiconductorsand electronic conduction occurs through mobile electrons in overlapping oribitals. Aromatic Polyimides. With the recent interest in thermally stable nonmetallic materials, a new class of polymers, aromatic polyimides, has attracted considerable attention. These polymers show no appreciable weight losses and structural changes up to approximately 500" C. during isothermal heating in vacuum (8, 9), and are quite stable up to 700" C. under non-
WIOLISIS IMPERkNII 1'0
Figurc 6. Weight loss croC~tm'sticsof polyvinylidmc chloride and preoxidiwd polydimnylbmunc pyolywd by iwcaring tempnature at thc conrtonf rote of 7000 c.per hr. (Ref. 29)
1Wfl I'K-II
Figure 7. Variation in paramagnetic resonance absorption with heatwuni fernperatwe for polyvinylidmc chloride residus (Ref.29)
V O L 5 9 NO. 7 JULY 1 9 6 7
23
I
I
I
I
I
1 90
nME 1M.l Figure 8. Relatiue EPR abrorpfion of poly[N,N'-(p,p'-o.ydiphcnylenr)pyromeliifimidc] as a function of pyrolysis time at 620' C. IYI1I"TE.D . I, E " I I U I * .
W L W l " 10*m11. ..*#I ( l U I
isothermal conditions (22). They are also highly resistant to ionizing radiation, withstanding a dose of 10,000 Mrad without damage (22). Bruck has recently reported (70)that when poly [N,N'-(p,f '-oxydiphenylene)pyromellitimide] is pyrolyzed in vacuum between 550' and 620' C., it develops strong ESR absorption with a line width of 4.5 to 7.5 gauss, and a spin concentration in the order of lo1#spins per gram of the pyrolyzate. The pyrolyzed material shows a room temperature conductivity of up to 20 (ohm-cm.)-I. Pyrolysis is accompanied by the production of carbon monoxide as the result of the cleavage of the carbonyl groups (8,9). No appreciable loss of weight occurs after about 1 hr. of pyrolysis. Apparently, an essentially chemical change is superseded by a predominantly physical change. The relative ESR absorption us. time curves (Figures 8 to 10) show distinct inflections during the early part of the pyrolyses, which become more pronounced as the temperature decreases and roughly correspond to the leveling off in weight loss. Figure 11, which is a plot of the maximum relative ESR absorption as a function of temperature of pyrolysis, shows a gradual increase in the microwave absorption up to 620° C., followed by a drastic drop. A similar phenomenon was observed by Winslow and his coworkers (29) with pyrolyzates of polydivinylbenzene and polyvinylidene chloride and by Uebersfeld (26),Bennett (3), and Ingram (76) with coals and carbonaceous solids. This drop in the ESR absorption coincides with a sharp drop in the room temperature resistivity leveling off at approximately 5 X ohm-cm. [u = 20 (ohm-cm.)-'1. Simultaneously, the density increases to 1.65 grams/cc. indicative of a polycondensation process. Figure 12 shows the 24
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
changes in the resistivity and density with increasing temperature of pyrolysis. A comparison between pyrolyzed polydivinylbenzene and pyrolyzed polyimide indicates that the polyimide develops electrical conduction at an appreciably lower temperature than polydivinylbenzene. This may be due to the greater number of aromatic rings, an increased tendency of the polyimide to form a polynuclear condensation product, and the consequent earlier development of optimum overlap of a-orbitals. When the rates of increase of relative ESR absorption are plotted against time of the pyrolysis, well defined maxima become evident (70). It is possible to demonstrate an apparent Arrhenius-type relationship between the logarithm of the maximum rates of ESR absorption and the reciprocal of the absolute temperature of pyrolysis. The slope of the straight line yields a nominal activation energy of -24 f 3 kcal./mole. This negative activation energy is unusual and no unequivocal explanation can be advanced at this time. However, this activation energy could represent the energy required to form a polyconjugated condensed network of aromatic and heterocyclic rings once primary scission of the appropriate bonds has occurred. It is not the activation energy of conduction. Elemental microanalyses of the pyrolyzates indicate progressive carbonization with increasing temperatures; however, even at 800' C., considerable amounts of nitrogen and oxygen are retained. The fact that very high electrical conduction was observed with samples containing considerable quantities of nitrogen and oxygen indicates that graphitization is not an essential requirement for electrical conduction in pyrolytic polymers. The conversion of the polyimide
nsr
ink]
Figwe 70. Rcloliw EPR absorption of poly [N,N'-@,p'-ovdiphmyl~)pyromcNitimidc]os ofuncrion of time of pyrolysir ut 575' C. m P R I I T s D BY ~ m 4 , Y I * P . O L W E I a0"Do"J.
.,.,.
ow>
to electroconductive products is accompanied by distinct changes in the x-ray diffraction pattern of the material (7). Prior to pyrolysis, the polymer is semicrystalline and largely unoriented. Pyrolysis in vacuum at 850' C. for 1 hr. yields a more diffuse pattern, characteristic of disordered carbon structures. The pyrolyzed polyimide shows a room temperature electrical conductivity of 0.1 to 20 (ohm-cm.)-' when heated in a vacuum between 600" to 850' C. for 1 br. The electrical conductivity did not change significantly even after the sample was left in contact with air for several weeks. Similarly, repeated thermal cycling between room temperature and 600" C. caused no appreciable diminishing of its conductivity at r w m temperature. Consequently, the
conductivity of the pyrolyzate seems to be electronic rather than ionic. Based on the above evidence, the pyrolytic conversion of the polyimide to semiconducting products may be illustrated by the hypothetical scheme below (9). Most of the weight loss takes place during the early part of the pyrolysis with the cleavage of the carbonyl groups and the formation of semi-isolated regions of condensed polynuclear systems (I). However, such semi-isolated regions are insufficient to bring about enough r-oribital overlap to cause electrical conduction. During the second stage of the pyrolysis (11) there is an increasing reorganization of the pyrolyzate, and the semi-isolated condensed ring systems gradually merge
V O L 5 9 NO. 7 JULY 1 9 6 7
25
Polyacrylonitrile
into a continuous network of fused aromatic rings in which an increasingly effective r-orbital overlap develops. At this stage electrical conduction sets in. Further growth in the network of fused polynuclear regions and increasing unsaturation (111) give rise to a still larger increase in the conductivity. Additional work is needed aimed at measuring the thermoelectric power and Hall coefficients. Polyacrylonitrile. Another system of considerable interest is polyacrylonitrile. When this 'plymer is pyrolyzed in the temperature range of 400" to 700" C., the resistivities of the pyrolyzed material approach those of semiconductors (73,24). The electrical conductivity of polyacrylonitrile may be increased by pyrolysis in an ammonia atmosphere. Evidence suggests that the thermal treatment promotes successive intramolecular cyclizations to yield annularly condensed pyridine rings. This is indicated by a decrease in the infrared Cz=N absorption band and the appearance of the CT-N band, as well as changes in the mechanical properties of the material. Structural 26
INDUSTRIAL AND ENGINEERING CHEMISTRY
Pyrolyzed polyacrylonitrile
Geiderikh and colleagues (73) reported activation energies of conduction of pyrolyzed polyacrylonitrile between 0.32 and 0.65 e.v. and thermoelectric powers of +72 to +lo8 pvolts/" C. with samples pyrolyzed between 400' to 500" C. ESR absorption measurements indicated free spin concentrations of 2.0 to 2.6 X spins/gram. Figure 13 shows the dependence of electroconductivity on absolute temperature, whereas Table I summarizes the electrical and ESR properties of pyrolyzed polyacrylonitrile (24). In another study Brennan, Brophy, and Schonhorn (5) reported activation energies from 0.06 to 0.33 e.v., depending on the temperature of the pyrolyses (400" to 900" C.). An apparently constant value of 0.2 e.v. was observed for the activation energy of conduction when the polymer was pyrolyzed between 800" and 900"
..
I u u t u r u R L i0O
Figure 14. Depmdeme of thnmoc[lllric coe$.unr on the tmpnnlurc of po6yanylonitrilc/lrnr treated at um'ous tnnperahlrcs (Ref.27)
Figure 75. Demtioation of pyrolyzd polyonylonitrile during deitydrogmtion of propanol-2 and restoration of its actimty by air oxidation (Ref. 19)
C. The Hall constant of the pyrolyzed polyacrylonitrile was approximated to be less than 0.1 cc./coulomb. 'The variation of the thermoelectric power from +20 to +ZOO #volts/' C. with conductivity suggests that the material is an intrinsic semiconductor. According to Voitenko (27),the thermoelectric power varies little with temperature, as seen in Figure 14. This seems to indicate that the conductivity of pyrolyzed polyacrylonitrile is due principally to an exponential increase in the mobility rather than to an increase in the relative concentration of the current carriers. The exponential dependence of electrical conductivity on temperature is thought to be due to the barrier of structural elements that can be surmounted by thermal activation. Averkin et al. studied the effects of hydrostatic pressure and tensile stress on the electrical conductivity of pyrolyzed polyacrylonitrile (2). They found that hydrostatic pressure of up to 8000 kg./cm." causes a reduction of resistance by approximately 30 to 45% in comparison to the resistance measured at atmospheric pressure. They attributed this phenomenon to a reduction in the intermolecular distance and a lowering of the potential barriers between the molecules. This process increases the mobility of the current carriers, thus reducing the electrical resistance. In contrast to the effect of hydrostatic pressure, the application of tensile stress increased the electrical resistance of the
samples as the result of increasing the intermolecular distance and potential barrier. Catalytic Activity of Pyrolyzed Polyacrylonitrile. Another interesting property of pyrolyzed polyacrylonitrile is its catalytic activity. It has been reported to catalyze the decomposition of hydrogen peroxide and the dehydration of formic acid (23). Manassen and Wallach (79) have shown that the pyrolyzed polymer is a strong hydrogen acceptor, being capable of dehydrogenating alcohols and olefins. For example, propanol-2 is converted to acetone in the vapor phase at 250" C., as seen in Figure 15. In contrast to the action of conventional dehydrogenation catalysts, no gaseous hydrogen is produced. Prolonged heating in nitrogen does not reactivate the catalyst, but a short heat treatment in air completely restores its activity. This suggests that the hydrogen is chemically bound to the surface of the catalyst. When cycloherene is passed over the pyrolyzed polyacrylonitrile at 350' C., almost 50% of the cyclohexene is converted to benzene without the production of cyclohexane and hydrogen. Normally, in the presence of palladium, cyclohexene disproportionates to cyclohexane and benzene. Table I1 compares the behavior of various other catalysts to that of pyrolyzed polyacrylonitrile (79). The absence of cyclohexane and the efficient production of benzene in the case of pyrolyzed polyacrylonitrile are evident. Pyrolyzed polyacrylonitrile can also cause &trans isomeriza-
TABLE 1.
1
2 3 4 5
ELECTRICAL AND PARAMAGNETIC PROPERTIES OF PYROLYZED POLYACRYLONITRILE(PAN)
PAN, redox plymnization PAN, redox polymuization PAN, ndox polymerization PAN, redox polymeriratiolr PAN (LiWICatalyn)
2.0 X 1OLD
400 400
in'",
500
in NH.
500
cucl,
5M)
...
(Cmrpilcdfim:
2.6 X 10" 2.0 x 1019 1.5 X 101'
...
lLfnrnu
2 x 2x 2x 1x
3
x
10-0 10101010-1
0.64 0.64 0.64
...
0.51
+75 +108 +72
0.32
+81
73) V O L 5 9 NO. 7 JULY 1 P 6 7
27
TABLE II. CATALYTIC EFFECT O F VARIOUS MATERIALS ON CYCLOHEXENE A T 350’ C. (19) Products, 70 CycloCatalyst hexane Benzene Others Pyrolyzed polyacrylonitrile .. 33 .. Animal charcoal 20 27 .. Coconut charcoal 63 35 .. Graphite 35 3 20
..
Boiling stones
..
. I
tions and double-bond shifts in olefins (19). For example butene-1 isomerizes to yield cis-butene-2 and trans-butene-2 between 300” and 450’ C . whereas butene-2 gives cis-butene-2 and butene-1 between 300” and 350” C. In the latter case the experimentally obtained ratios of cis-butene-2 to butene-1 are between 2.2 and 2.6. Optically active limonene undergoes both double-bond shift and dehydrogenation. The former mechanism yields terpinolene and a-terpinene, whereas the latter gives p-cymene and isopropenyltoluene. From a study of the model compound 5-ethyl-jmethyl-l,3-~yclohexadiene, Manassen and W-allach ( 79) have shown that the hydrogen transfer from the substrate to the catalyst surface occurs partly in the form of a hydrogen atom and partly in the form of a hydride ion. As has been mentioned above, the pyrolyzed polyacrylonitrile catalyzes the dehydrogenation of alcohols and olefins without the formation of gaseous hydrogen or saturated hydrocarbons, unlike other dehydrogenation catalysts. This behavior may be explained by considering that the pyrolyzed polyacrylonitrile has a condensed aromatic structure of annularly condensed pyridine rings as the result of the thermal treatment which promotes successive intramolecular cyclizations. Such a structure has been proposed previously in explaining its semiconducting behavior. During dehydrogenation reactions, the condensed aromatic structure, A, can transform into the hydroaromatic structure, B, which, upon treatment with oxygen, reverts to A :
hydrogen
H A
B
Conclusions
In conclusion, it seems that the semiconducting organic pyropolymers discussed in this review differ from inorganic semiconductors and graphite in having a large number of charge carriers of low mobilities. The limited data on thermoelectric power measurements suggest that the temperature dependence of organic pyropolymers is due primarily to an exponential in28
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
crease in the mobility of the current carriers with rising temperature rather than to an increase in the concentration of the current carriers. The existence of heteroatoms and edge atom substituents in the pyropolymers may influence the relative mobility of the current carriers rather than their relative concentration. These systems are characterized by their ability to form extensive charge delocalization within the rr-electron system because of internal cyclization and polycondensation reactions. This charge delocalization gives rise to the ability of these polymers to form quite stable charge transfer complexes with suitable electron donors and acceptors. Besides being electrical conductors, the reported catalytic activity of some of these organic pyropolymers may open up an important area for industrial exploitation. References
(1) Akamatu, H . , Mrozowski, S., Wobschall, D., “Proceedings of the Third Biennial Carbon Conference, Buffalo,” p. 135, Pergamon Press, New York, 1959. ( 2 ) Averkin, A. A., Alrapetyants, A. V., Plisavskii, Yu. V., Lutsenko, E. L., Serebryanikov, V. S., Dokl. Akad. ,Vauk SSSR 152, 1140 (1963). (3) Bennett, J. E., Ingram, D. J. E., Tapley, J. G., J. Chem. Phys. 23, 215 (1955). ( 4 ) Boyer, R . F., J . Phys. Colloid Chem. 51, 80 (1947). ( 5 ) Brennan, W. D., Brophy, J. J., Schonhorn, I
