Ind. Eng. Chem. Prod. Res. Dev. 1984,23,28-32
28
REVIEW SECTION
The Development of Carborane-Siloxane Polymers Edward N. Peters General Electric Company, Plastics Technology Department, PittsfieM, Massachusetts 0 120 1
The discovery of carborane-siloxane polymers in the early 1960’s furnished a promising lead in the search for high-temperature performance properties. However, the pioneering
processes for carborane and carborane-siloxane synthesis involved difficult procedures and/or low efficiencies. This severely limited not only the large-scale preparation of carboranes and linear carborane-siloxane polymers but also effectively eliminated the preparation of carboranes in the research lab& ratory. Thus a need existed for processes which overcame these difficultiis and which would allow for the full exploitation of these materials. This review describes the development of a second generation of technology which makes carboranes and linear carborane-siloxanes readily available for the first time.
Monomer Preparation Carborane Synthesis. C2BloHlzexists as three interconvertible isomeric species of nearly regular icosahedral structure with adjacent carbon atoms (1,2 or ortho), once removed carbon atoms (1,7 or meta), and twice removed carbon atoms (1,12 or para) as shown in Figure 1. The meta and para isomers are useful in preparing polymers. The early process for preparing carboranes involved a multi-step process which began with the difficult and hazardous pyrolysis of diborane to produce decaborane, B1oH14 (Holzman, 1967). This step suffered from low efficiencies.
Introduction Since the discovery of polyhedral boranes in the early The preparation of the ortho or 1,2 isomer of carborane 1960’s, considerable efforts have been expended at various from decaborane was accomplished in two steps (Fien et laboratories on the preparation of polymers based on these al., 1963; Heying et al., 1963). First the decaborane was materials. Pertinent studies have concentrated on the treated with a donor molecule, L: (i.e. amine, sulfide, or utilization of carboranes as building blocks for high-temnitrile), to produce the bis-ligand material, B10H12L2, perature polymers because of their high energy content, followed by reaction with acetylene. high stability associated with their pseudo-aromatic character, and ease of derivatization. Initially a wide variety of monomers and polymers containing carboranes were investigated (Peters, 1981, 1983; Heying, 1970). The most promising were the carborane-siloxane polymers (Peters, 1979). Indeed, the development of these polymers was heralded as a breakthrough in the search of a highThe conversion of 1,2-C2Bl,,Hl2into the meta or 1,7-isomer temperature elastomer. Incorporation of the carborane was effected by the thermal isomerization in the gas phase moiety into siloxane polymers significantly enhanced the at about 470 “C for several hours or at 600 O C for about overall thermal stability of these polymers (Peters, 1979). 30 s (Grafstein and Dvorak, 1963; Papetti et al., 1964). The term “carborane” is commonly used in a generic Normally yields are greater than 90%. The para of 1,12sense to describe compounds composed of boron, hydrogen, isomer of C2BIoHl2is prepared in about 20% yield by and carbon whose molecular geometries are polyhedra or heating the l,7-isomer in the gas phase at 700 “C for a few polyhedral fragments. Several families of carboranes exist with the general formula: CBnHn+2CBnHn+4C~BnHn+2seconds (Papetti et al., 1964). Clearly these early routes to carborane severely limited not only the large-scale (Dunks and Hawthorne, 1973). Of the families the neutral, preparation of carborane but also effectively eliminated ( n = 5 and 10) species have closed, polyhedral C2BnHn+2 their preparation in the research laboratory. These difbeen used in the preparation of carborane-siloxane polyficulties were circumvented by recent synthetic developmers (Papetti et al., 1966). Polymers prepared from 1,7ments which were based on an efficient one-step synthesis dicarba-closo-dodecacarborane(l2)-1,7-diylnuclei conof BllH14 ion from NaBH, and BF3 etherate (Dunks and taining two siloxane moieties per carborane (I) were shown Ordonez, 1978; Dunks et al., 1981). The reaction is conto have the best balance of thermal stability and elastomveniently carried out in diglyme at about 105 “C. The eric properties and hence their development will be reoverall process is represented by the proposed stoichiomviewed here. etry of eq 4.
-
1 7 N d H 4 + 20 BF3*O[C2H,], 2BllH14- + 15BF4- + 200[C2H,],
+ 20H2
(4)
Yields of B11H14 are typically greater than 90%. The 0196-4321 /84/1223-0028$01.50/0 0 1984 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 1, 1984 29 CH3 CH3 I]2 butyl lithium I b CI-Si o-CBloHloCSi-CI o-C~BIOHI~ 21 [CH3I2SiCb I 1 CH3 CH3
ORTHO
310-360
@=BH
==CH
Figure 1. Structures of C2B1&II2isomers.
Table I. Molecular Weight of Polymers from Silane Comonomers ~~~
-
~
X in VI11 --N(CH,)*
silane bis amino
10-15 000
bis carbamate
8000
bis ureido
30-40 000
Mw
-0CNRz 0
I1
-NCNlCH&
1
-
R R R SiCB,,H,,CSi-O-Si-O
CSH5
-N"
I
R
K
I
'
R
I1
(9)
-
+F
-
-
R R R SiCBloHloC- F Si-0-Si-0 R R R V
reaction involved the initial formation of B3H8-ion followed by its conversion to BllH1c ion. BloHl, is conveniently prepared via the oxidation of BllH14- (eq 5 ) . BllHl4-
-
(5)
BIOHl,
By use of typical oxidizing reagents such as H20z,KMn04, and Na2Cr2O770430% yields are obtained. Hence by use of standard kettles and equipment carborane can now be prepared in good yield via eq 4, 5, 2, and 3. Carborane Disilanol. m-Carborane disilanol was originally prepared via the silylation of m-C,B,&,, followed by hydrolysis as in eq 6 and 7 (Papetti and Heying, 1964). CH3 CH3 I]2 butyl lithium m - C2B10H12 *CI-Si m-CBlaHloCSi-CI 21 [CH3]2SiCI2 I I CH3 CH3
(6)
II
II
7H3
H20 A
U
b
CH3
HO-Si m-CB1oHloCSi-OH ( 7 ) CH3
CH3
-
VI
R
V
H20+-SiCBloHloCH
- OH-
R
Vi
R R HO-Si-O-Si-0-
+ OH-
I
R
(01
A
R
> 250 000
bis ureido
OC
Recycling Carboranes. Compared to conventional polysiloxanes, carborane-siloxanes are expensive to produce because of the carborane moiety. There are economic incentives to be able to reclaim the carborane portion from scrap, off-grade, or otherwise used-up polymer. Hence a highly selective method for cleaving carboranyl carbonsilicon bonds and the subsequent recovery of the carborane was developed (Beard and Moffitt, 1978). This method involves the treatment of carborane-siloxanes in tetrahydrofuran solvent with an aqueous solution of KF and leads to cleavage of the C-Si bond without disrupting the carborane cage. This two-phase reaction is promoted by phase-transfer catalysts and presumably proceeds by the following reactions
PARA
META
IV
-
IV
(8)
1
R
It was demonstrated that greater than 90% of the carborane in off-grade monomer, polymer, and even vulcanized polymer can be converted back to m-carborane. Thus this most expensive building block can be efficiently recycled. Ureidosilanes. Of the several reactive silane comonomers used in the preparation of poly(carboranesiloxanes), the bis(ureid0)silanes were the preferred monomer (Peters et al., 1977). These reagents are powerful silylating reagents which do not form acid or basic byproducts during reaction. Although a large structural variety of bis(ureid0) silanes were prepared, the particular ureido group chosen to provide the most easily synthesized and purified crystalline solids was the N-phenyl-N',N'tetramethylene structure, VII, in which the group R could be varied at will. These are prepared by reaction of the corresponding bis(pyrro1idino)silanes with phenyl isocyanate (eq 10). R CN-SiL.3
+2
CsH5N=C=O-
R
(10) 111
lk I However, as previously mentioned, the vapor-phase hightemperature isomerization of o- to m-carborane was not suitable for large-scale work. Papetti and co-workers showed that the skeletal isomerization of o-carborane derivatives containing large substituted silyl groups at the adjacent carbon atoms takes place at much lower temperatures than the rearrangement of o-carborane itself followed by hydrolysis, eq 8,9, and 7 (Papetti et al., 1964).
VI1
Polymer Preparation Carborane-siloxanes were first prepared by the ferric chloride catalyzed polymerization of dichlorodimethylsilane, with bis(methoxydimethylsily1)-m-carborane,eq 11 (Schroeder et al., 1966).
30
Id.Eng. Chem. Prod. Res. Dev., Vol. 23, No. 1, 1984
Table 11. Tabulation of Various Carborane-Siloxanes
IX a
g
h
meta
CH,
i
meta meta
CH 3 CH 3
b C
d
e f
j
a
R,
CB*oH,oC meta meta (85) para ( 1 5 ) meta (70) para ( 3 0 ) meta ( 5 0 ) para (50) meta (15) para ( 8 5 ) para (100) meta
T,, "C
R3
R,
Tg,"C
CH 3 CH3
CH 3 CH 3
CH3 CH3
68,90 39, 63
CH3
CH 3
CH,
47
CH 3
CH,
CH3
-
CH,
CH3
CH 3
160-200
CH CH3
CH3 CH3 ( 7 6 ) C6H5 (24) CH3 (67) C6H5 (33) C,H* CH 3
CH3 CH3 (76) C6H5 (24) CH3 (67) C,H5 (33)
220 37-57
-33
-
-25 22
-
-37
-
-22 -12 -29 -12 -3
C6H5
CH3 (67) (33) CH3 (33) C6H5 (67)
-50
C6H5
k
meta
CH3
CH3
1 m n
meta meta meta meta
CH 3 CH, -CH,CH ,CF , -CH,CH,CF3
CH 3 CH3 -CH,CH,CF3 -CH,CH,CF,
0 a
C6H5
-CH,CH,CF CH3 -CH,CH,CF
Values in parentheses are mole percent. CH3 CI-SI-CI I
CHI CH3 --Si
CH3
CH3
CH3
CH3
pyrrolidino-"-phenyl ureido)silanea, VII, in chlorobenzene at 0 to -10 OC, eq 14 (Peters et al., 1975; Stewart et al., 1979).
FeCk - CH30-Si m CBloHloCSi-OCH3 n
(11)
CH3 CH3
m CBl~H,~CSi-O-Si-O-
I
-
CH i
CH3 R 7
Ill
2 CH3Cl
VII--
SI m CBjoH,oCSi-O-Si-O-
0
t 2 , N-C-N,
/H
I
CH3
CH? CH3
Molecular weights around 10OOO were obtained. However, serious fabrication difficulties which resulted in a network polymer rather than a linear polymer prevented exploitation of these polymers (Papetti et al., 1966). These difficulties were related to the mode of polymer synthesis, which involved a ferric chloride induced cross-linking process (Dietrich et al., 1974). To obviate the deleterious effects of ferric chloride, a process based on the condensation of bis(hydroxydimethylsily1)-m-carboranewith reactive silane comonomers, VIII, was investigated (eq 12, Peters et al., 1977). R 111
(12)
- x-si-x ---R
Vlll
The results obtained using various reactive silane comonomers appears in Table I. Linear, low molecular weight polymers were obtained with bis(NJV-dimethy1amino)dimethylsilane. The low molecular weight was ascribed to a dimethylamine-inducedcleavage of the carborane-silicon bond in carborane-disilanol (eq 13, Peters et al. 1977). 111
+
HN[CH&
-
Polymers were also been prepared directly from mcarborane by converting it to the dilithio salt followed by reaction with a 1,5-dichlorotrisiloxe, eq 15 and 16 (Peters and Stewart, 1979). HCBloHloCH+ 2n-butyllithium LiCBloHloCLi (15) XI1
-
XI1
t
R R R CI-Si-0-Si-0-Si-CI R R R
-
? R ? -CBloHtoCSi-O-Si-O-SiR R R
(16)
Polymer Characterization The use of bis-ureidosilanes has permitted mild reaction conditions which have increased the versatility of the polymerization reaction. For example, structural modification of the polymer backbone can be readily achieved (Peters et al., 1977). Modifications of the basic polymer structure IX are shown in Table 11. -SiCB,oH,oCSi-O-Si - 0i I I CH3
R3
CH3
IX[a-01
CH3
HO-SiCB,oHloCH CH3
(14)
~-
CH? R
CH3
(13)
This production of monofunctional monomer terminates the polymer chains and prevents the advancement of molecular weight. Similar problems were encountered with the biscarbamates. The bis ureidosilanes with neutral, unreactive urea leaving groups eliminated this cleavage reaction. Indeed, the condensation reaction of I11 with bis ureidosilanes provided high molecular weight, linear polymers. The polymerization process employed is facile and involves the slow addition of I11 to a solution of bis(N-
Moreover vinyl, X, and hydrido, XI, groups can be incorporated into the polymer backbone to facilitate vulcanization. CH
CH
CH CH
- S I m CB H C S I - 0 SI 0CH
CH
X
CH
CH -SI
CH
H
m CB -H CSI 0 Si-0-
CH
CH
CH
XI
Linear, high molecular weight m-carborane-dimethylsiloxane, IXa, has a crystalline melting point around 68 "C. This crystalline phase detracts from the elastomeric properties. Since consistent properties in the -40 to 300
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 1, 1984
Table 111. Mechanical Properties as a Function of Temperaturea tensile temp, Young's "C modulus, psi strength, psi 300 260 200 25 -20 -3 0 -4 0 Figure 2. Thermogravimetric analysis in air of carborane-siloxane and conventional silicone.
"C range are required in applications intended for carborane-siloxane elastomers, it is important to disrupt the crystallinity. The replacement of 33 mol 5% of the dimethylsiloxane moieties with methylphenylsiloxane or diphenylsiloxane moieties (IXb and IXj, respectively) results in an amorphous polymer. The outstanding thermal stability and resistance toward reversion (chain scission) at higher temperatures of carborane-siloxanes is revealed in the thermogravimetric analysis shown in Figure 2 (Peters et al., 1977). The carborane-siloxane polymer exhibits no significant weight loss at 400 "C. For comparison, the weight loss of a conventional polydimethylsiloxane rubber is precipitous at 400 "C.
Fabrication of Elastomers Vulcanization. Linear, high molecular weight carborane-siloxane stocks can be readily formulated with fillers and other additives by using a two-roll mill or Brabender (Peters, 1979, 1980). They can be vulcanized under standard silicone-peroxide conditions. Dicumyl peroxide and a,a'-bis( tert-buty1peroxy)diisopropylbenzeneare the preferred vulcanization agents. Vulcanization under slight pressure is followed by postcure at end use temperature (usually 250 to 300 "C). Reinforcing fillers, such as highsurface silicas, and oxidative stabilizers, such as ferric oxide, are necessary to improve the mechanical properties and to promote high temperature stability (Peters, 1980; Peters et al., 1977). Reinforcing Fillers. Substantial improvements in the physical properties of vulcanizates generally can be expected from siliceous fillers with small particle size (Boonstra et al., 1975; Bachmann et al., 1959). The specific effect of silicas on the mechanical properties of vulcanized carborane-siloxane polymers was determined to provide information on the performance profile achievable with filler reinforcements. Silica reinforcing agents can exhibit a dramatic effect on the heat aging characteristics of carborane-siloxane polymers. Indeed, the use of silica with surface silanols rapidly deteriorated mechanical properties at 315 "C. It is known that acidic protons of silanol groups can cleave phenylsilane moieties with the resulting formation of siloxane bonds (Schroeder, 1972). In phenyl modified carborane-siloxanes this process involving silica silanols and polymer phenylsiloxane moieties generates new cross-links at high temperatures (eq 17).
I
I
500 540 576 500 2 500 30 800 1 4 0 000
31
elongation at break, %
59 116 157 519 558 1510 2100
15 30 40 130 180 55 20
Stock IXj with 30 phr surface treated fumed silica; 2.5 phr ferric oxide, and vulcanized with 2.5 phr dicumyl peroxide.
Table IV. Mechanical Properties at 25 "C after Heat Aging in Air at 315 "C tensile elongation aging Young's time, modulus, strength, at break, h
psi
psi
%
0 50 100 300 600 1000
536 712 855 1240 2000 3300
527 434 581 47 0 500 457
220 165 80 50 35 15
This deleterious resin-filler interaction was minimized by the use of trimethylsilated silica and silicates as reinforcing agents and has led to the preparation of vulcanizates with good retention of properties at high temperatures.
Properties of Vulcanizates The temperatureproperty profile of carborane-siloxane vulcanizates appears in Table 111. The decrease in tensile strength and elongation with increasing temperature is characteristic of filled vulcanizates. As the temperature is decreased and approaches the Tgof the system (-37 "C), the modulus and tensile strength increase and the elongation decreases. Heat Aging. Carborane-siloxane materials can remain elastomeric after long-term exposure to high temperatures. A vulcanizate containing 30 phr trimethylsilated silica and 0.25 phr finely dispersed ferric oxide exhibited good retention of mechanical properties after heat aging in air at 315 "C for up to loo0 h (Peters et al., 1978). The data in Table IV show that there is a gradual decrease in elongation along with a gradual increase in modulus. The tensile strength remains essentially constant. Flammability. Carborane-siloxanes have excellent flammability characteristics. The oxygen index test (0.1.) measures the minimum amount of oxygen (expressed as percent by volume) necessary to sustain combustion of a material (Fenimore and Martin, 1966). A carborane-siloxane vulcanizate (30 phr filler) exhibited an 0.1. of 62 (Peters et al., 1975). For comparison, silicone rubber has an 0.1. of 30 to 33. This result indicates that carboranesiloxane polymers should be of interest in applications where flammability is of major concern. Solvent Resistance. Resistance to various solvents is a critical characteristic of high-performance elastomers. After immersion in various solvents for 7 days at ambient temperature, carborane-iloxane vulcanizates exhibit only minor changes in mechanical properties but have large swelling in hydrocarbon based solvents and fuels similar to that experienced with conventional silicone rubbers (Arnold et al., 1973). Further improvements in solvent resistance are required for certain end use situations. The role of fluorine and the carbon-fluorine bond in
32 Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 1, 1984
[ -T., t
’0
E 60
r~ 70
’
..-
TRIFLUORO PROPYLCARBORANE SILOXANE
FLUOROSILICONE
+
80 r 90
loo
’d
I
1w m
,
1
j
-1
300 400 500 Temperature [’C]
m
7w
Figure 3. Thermogravimetric analysis in air of carborane-fluorosiloxane and fluorosilicone.
Table V. Solvent Resistance % ’ swelling toluene ref fuel B
vulanizate carborane-siloxane trifluoropropyl modified carborane-siloxane fluorosilicone
162 23
113 20
-
21
achieving a high degree of solvent resistance is well-known in organic polymers (Pierce and Kim, 1971). In fluorinecontaining silicones, the position of the fluorine on the alkyl side chain influences the thermal and hydrolytic stability of the compound. The y position is preferred for maximum stability. For example, poly(y,y’,y”-trifluoropropyhnethyl)siloxane,XIII, has excellent solvent resistance and has been reported to be superior in many respects to the fluorocarbon elastomers but fall short of them at temperatures above 230 “C (Kalfayan et al., 1975). Unfortunately, fluorosilicones undergo depolymerization at high temperatures with resultant deterioration of physical properties. The incorporation of the m-carborane moiety into the fluorosilicone backbone, Le., 1x0, significantly enhances the overall thermal stability and retards depolymerization as shown by TGA (Figure 3) (Peters et al., 1977). Improved solvent resistance with low swelling for carborane-siloxane vulcanizates was also obtained. The results appear in Table V. CF
CF
CF
CF
CH
CH
CH.
CH
CH,
CH,
CH
CH --
SIO-
CH Xlil
__ SI m CB,pHInCS~-OSi 0-CH
CH
ix
CH
0
Conclusion Carborane-siloxanes are truly unique polymers which add measurebly to the arsenal of design engineers for advanced systems. They can be fabricated like conventional silicones; however, carborane-siloxanes have superior thermal stability and excellent flammability characteristics. The high-temperature capabilities of these polymers have found utility as liquid phases in gas chromatography (Yancey and Lynn, 1974). Carboranesiloxanes have been fabricated into Q-rings, gaskets, and wire coatings which
are capable of performing at temperatures exceeding 300 OC (Schroeter et al., 1966; Peters et al., 1977). The properties of carborane polymers such as transition temperatures, stability, and solvent resistance can be changed by modification of the pendent groups on the polymer backbone or by varying the number of siloxane groups per repeat unit. A strong technology base now exists for the exploitation of carborane-siloxane polymers. The visionary support of the Office of Naval Research has been largely responsible for the important advancements of this area.
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-
Received for review June 6 , 1983 Accepted September 19, 1983
