Journal of Research of the National Bureau of Standards Vol. 48, No.4, April 1952 Research Paper 2313

Heats of Copolymerization of Butadiene and Styrene from Measurements of Heats of Combustion

Raymond A. Nelson/ Ralph S. Jessup/ and Donald E. Roberts

The h eats of combustion of two samples of polybutadiene and four cop olvm ers of buta­ cli ene and styrene prepared at 50° C (122° F ), and one sample of polybutadiene and four copolym ers polym eri zed at 5° C (41 ° F) were m easured in a bomb calorimeter. The stn ene contents in th e copolymers varied from approximately 8 to 55 weigh t percent. . A value for the heat of polymerization (or copolym eri zation) was calculated for each sample by combining th e exper im ental value for t he h eat of combustion wi th other known thermoche.mical data. The m ean value obtained for th e heat of polymerization (- tlI-J p 0) of polybutad lene IS 17.4 kcal per monomer unit, and the values obtained for t he heat of co­ polymerization varied from 17.1 to 17.7 kcal p er m onomer uni t. No app er ciable difference was found betweep ti? e heats of copolymerization of t he 5° C and 50° C pol.vmers. The h eat of copolymeriza t IOn can be r e pr ese n~ e d as a function of styr ene content of the polym er b.v a n equatIOn of a form den ved theor etICall y by Alfrey ancl Lewis, although th e co ncli t ions under whICh th e polymers were formed were such t hat some of the a s~ u m pt i o n s underl Yinp: the de ri vation of the equation wer e not satisfi ed. .

1. Introduction T A TlL" l. Analyses oj polymer sa m/)/es

M easurements have been made, by means of a Phenyl. Polymer Sly- Car- H ydro- Ox),- 8ul- A sh beta- bomb calorimeter, of the heats of combustion of rene bon gon gcn fur naphthyl- three samples of polybutadiene and eight copolymers a lll ine of butadiene and styrene, ranging in styrene content from approximately 8 to 55 percent by weight (5 to 50° C (122° F ) POI)'llWrS 40 mole %). Two of the samples of polybutadi ene I lrt cQ I 'let c ~ 'lrt 00 II A ______'1I'1 ~ 'ld ~ 'wI c:o 't "t ~ and four of the copolymers 'werc prepared by poly­ 0 88. 48(j 11. 166 O. 14 5 0. 109 O. 100 0.00 G L- 65i ____ 0 88.05(; 11. I i9.) · 145 '. 072 . 09i . 12 merizatiol1 at 50 ° C (122° F ), whereas the others GL- 658 ____ 8.58 88.00i 10.8982 . 16 . 136 . 104 . 1 were polymerized at 5° C (41 ° F ). The data have X-452 _____ 23 89 89. 42 10.36 .090 . 08i . 13 . Oi GL-OG2 __ __ '12.98 VO . 1OO 9. i 100 · 14 . 081 . 111 I been combined with other thermochemi cal data to 48-B ______55. i3 00.561 9. 2643 · 15 . 0·16 . 158 . 07 obtain values for heats of polymerization and co poly­ merization. 5° C (41 ° F) polymers

B ______0 88.4i4 11. 1000 O. 12 0. 083 0. 34 O. J4 2. Source and Purity of Material X-45'L ____ 8.58 88.869 10.872 .235 '. 056 . 18 . 13 X-4i8 ______22.61 89. 443 10. '1008 . 15 . 048 . 098 . Il G r.- 600 ____ 36.20 89.913 9.93'12 . 08 . 050 . 095 . 11 The materials used in this investigation were made GL-66L ___ 53.09 90.513 9.3585 . 11 . 045 .09i . 11 available by the R econstruction Finance Corp., Office of Rubber R eserve, and were purifLCd in the , Added after analysis. " rhe values reported here arc corrected val ues for sulfur content. For G L-657 Rubber Section of the Bureau. Some of the prop­ the experimelltall y determined ,-slue is 0. 150% S 81ld for X-454 0.087% S. See erties of the purified materials are given in table 1. text p. 2ib. The pol ymers were prepared by emulsion polymer­ ization. The " modifiers" used were commercial described above and after further evacuation to n- dodecyl mercaptan (DDM) for the 50° C polymers, about 10- 4 mm of mercury for several days at room and commercial tert-dodecyl mercaptan (S ulfole) for temperature. the 5° C polymers. The purified samples were analyzed quantita­ The purification procedure involved dissolving the tively, and the results arc reported in table 1. The sample in benzene and precipitating it by pouring styrene contents reported in this table were calcu­ the solution slowly into methyl alcohol while stirring. lated from the carbon-hydrogen ratios (corrected for This was done three times. Then the purified mercaptan) . sample was again dissolved in benzene, a small A part of the experimentally determined sulfur amount of phenyl-beta-naphthylamine was added content of sample X-454 was found to be present a as an antioxidant, the mixture was frozen, and the an inorganic impurity, rather than a mercaptan benzene was removed by sublimation in vacuum to attached to the polymer chain_ The sulfur con ten t give a product similar to foam rubber in appearance. r eported in table 1 for this sample is the excess of This procedure removed soap, fatty acid, and stabi­ the total observed ulfur over that found to be pres­ lizer, as well as som e of the low-molecular-weight ent in the inorganic impurity. The experimentally polY111.e1'. In the presen t work no significant dif­ determined sulfur content of sample GL-657, which ference was found in the heats of combustion of contained no styrene, wa high er than that calculated samples burned as received after the treatment from th e observed carbon-hydrogen ratio on the as- 275 -- sumption that the sample consisted entirely of do­ decrease in intrinsic energy accompanying the reac­ decyl mercaptan and polybutadiene. It was assumed tion that the difference was due to sulfur in inorganic im­ purity (as in the case of sample X-454) , and the b-?c sulfur content reported is that calculated from the CaH bOcSd(solid polym er) + (a + d + -:f- ) 0 2(g)~ carbon-hydrogen ratio. (1) 3 . Method and Apparatus aC02(g) + d S02(g) + %H20 (liq) ,

The apparatus and procedure used in the bomb using the Washburn procedure [7] modified so as to calorimetric measurements have been described pre­ apply to 28° C and to the energy con ten t of the gases viously [1 , 2, 3, 12).1 The bomb used has a capacity at zero pressure instead of 1 atm. of 381 ml. One milliliter of water was placed in th e The measuremen ts were made in terms of the abso­ bomb before each combustion experimen t. The lu te joulc as the unit of energy. Conversion to the polymer samples used were first compressed into conventional thermochemical caloric was made by pellets. means of the rela tion The calorimetric system was calibrated with NBS Standard Sample 39g of benzoic acid , using the valu e 1 cal= 4.1840 abs j . 26433.8 abs jig for the heat of combustion of this material under the conditions of the standard bomb 4. Results of the Calibration Experiments process [3, 4, 5]. In calculating the resul ts of the h eat of combustion All the measurem en ts of heat of combustion re­ measuremen ts, the amount of the combustion reac­ ported in this paper were made with the same calo­ tion in each experimen t was determined from the rimetric system . However, the work was carried out mass of carbon dioxide formed, using the value 44.010 over a period of about 2 years, and, because of nec­ g for the molecular weight of carbon dioxide [21 ]. essary repairs to the calorimetric system , it was re­ T ests were made for carbon monoxide in the gaseous calibrated several times during the course of the products of combustion after the carbon dioxide had work. T able 2 presents a summary of the valu es been removed, using a colorimetric method developed obtained for the energy equivalent of the calorimeter at the Bureau [6]. No carbon monoxide was found, and the numbers of th e polymer samples whose heats and no soot or other evidence of incomplete combus­ of combustion were measured, using each value fo r tion was found in the bomb in any experiment. the energy eq uivalent. Corrections were applied to the observed h eat of combustion in each experimen t for hea t of stirring, T A BLE 2. Enel'YY equivalent of calorimetej' heat transfer b etween calorimeter and surroundings, ignition energy (22 j), and energy produced by the Energy equivalen t Polymr l's inn:'stigatcd formation of aqueou s nitric (58 .8 kj /mole) and sul­ furic (3 05 .7 kj/mole) acids in the bomb. The last jre 13784.4 ± 0. i " II two corrections were made on the basis of the anal­ 13372.2± 0.8 48-n , X - 452. 01,- 658 ysis of the washings from the bomb after each com­ 1~970.2 ± 1. 8 0L-657 13964 .9± 1. 8 01,-662 bustion experiment. The air initially in the bomb 1395o.7 ± 1. 4 { 0L-660 o T~f\6 1 (e xperiment, 1 and 2) was not Hushed out with oxygen in these experimen ts. { 0 L--601 (ex periment, 3 and 4) The atmospheric nitrogen was left in the bomb be­ 13% 6.9± l.1 X - 454 13943.4 ± 0.5 X-4i8 cause the oxides of nitrogen are known to catalyze 13933.1± 1.0 B the formation of sulfuric acid from suHur dioxide,

oxygen, and water. a T he numbers following the ± signs are the stand a. rd d ev iation ~ of the ('orrQ­ The aqueous solution in the bomb at the end of sponding mean values of energy equivalent. each experiment was analyzed for the acids formed during the combustion by first determining the total 5. Experimental Results acidity of the bomb washings by titration wi th a standard solu tion of sodium hydroxide, using methyl The results of the heat of combustion experiments orange as the indicator, then removing the nitrate are reported in tables 3 and 4, where - 6. UB represents by evaporating the solution to dryness, and fin ally, the observed heat of combustion at 28° C under the determining the sulfur by precipitation as barium conditions specified by the volume (38 1 ml) of the sulfate and weighing. The percen tage of sulfur so bomb, the mass (1 g) of water placed in the bomb determined averaged 0.01 lower than the values re­ before each experiment, the temperature (28° C) to ported in table 1. The total correction for acids did which the combustion reaction is referred, and the no t amount to more than 0.2 percent of the total data given in columns 3 and 5; and where - 6. U ; heat produced in the combustion reaction. represen ts the decrease in intrinsic energy accom­ The observed h eat of combustion, - 6. UB , in each panying reaction (1) with all the reactan ts and prod­ experimen t was reduced to the value of - 6. U:, the ucts in their standard thermodynamic states at 28 0 C. (The data on sample A of 50° C polybutadiene are

1 Figures in brackets indicate tbe literature referenres at tbe end of tb is paper. for 30° C.) 276 TABLE 3. Results of bomb-calo!'irnetl'ic measurements TAB LE 4. Results of bomb-calorirnet!'ic measurements 50°0 polymers 50 C polymers

F. I1 r rgy of co m hu sti on I nitial 0 2 [n itial li z at 28 0 C . Mass of l\'lass of pressure Mass of Mass of sample CO, Experi ment sample CO, preS'i ure a t 28°C at 28°C -----1------d UB - d U~

Sample A a Sample B

g g aIm j ig CO, j ig CO, g g aim jfg CO, J/g CO, L ______L ______2 ______0. 909 14 2. 94555 31. 9 13878 '2 13872.8 0.90075 2. 91395 30.9 13876.6 J3871. 1 . 92983 3.01269 31. 9 13884.5 13879.2 2 ______3.04831 13875. 0 J3869.3 3 ______3 .. ______. 94306 31. 6 L ______. 90266 2. 92347 32.0 13883.4 13878.0 . 93426 3.02307 31. 8 13875.3 13869.6 .90516 2.93225 32.2 13882.0 13876.6 4 ______.91407 2.95543 31.2 13885.8 J3880.2 M ean ______13876.6 M ean ______13872.6 Standard d eviation of t he m ean______± 1. 7 Standard deviation of the m ean ______± 2.6

Sample G L- 657 Sample X - 454

l. ______0.93720 3. 04067 31. 8 13898.3 13892.6 1.. ______1 0. 90962 1 31. 6 13670. 5 13664.8 2 ______2 .. ______.91191 2.95790 1 3 ______.86631 2.81143 32. 1 13890.8 13885.2 2. 96572 32.6 13672. 7 13666.9 .89738 2.91277 32.3 13891. 8 13886.1 3 .. ______.89449 2. 90938 33. 4 1 13675. I 13669.2 4 ______4 .. ______.90816 5 ______.83177 2. 70001 32.9 13894. 7 13889. 0 2.95355 32. 1 13665. I 13659.3 ------2. 85568 32.8 13899.5 ]3893.8 6 ______M can ~ .89462 2.90321 3 1. 9 13892.8 13887. 2 ______. __ 13665. 1 Stancl al'd dC\7 iation of the mean ______± 2. 1 M ean .. ______]3889. 0 Standard d eviatioLl of t he mean .. ______± 1. 4 Sample X-478 Sample G L - 658 1. _ .... ______1 O. H1363 2.9914 6 1 32. 0 13508. 7 13504 .6 2 ______. 9·1549 3. 09588 31. 6 13502. 7 13498. 7 l. ______0. 92538 3. 014 53 3l. 8 13747.3 1374 1. 6 3 .. ______.89638 1 2. 93456 3 1. 9 1 13493. 7 J3494 . 6 2 ______2. 910 14 4 ______.. ______. 92034 31. 2 1350 1. 7 13495.9 3 ______. 89375 32.0 13753. 4 13747.7 3. 01664 4 ______. 90330 2. 9409 1 3 1. 2 J3747. 1 1374 1. 5 M ean ______. ___ .. ______.88723 2.88905 3 1. 3 J3747. 3 J374 1. 7 13498. '1 5 ______Standard deviation of th e mea n ______± 2. 2 6 ______. 85106 2. 77228 30.6 13747.2 13741. 7 .88153 2.87141 3 1. 0 13746.3 ]37'10. 7 M eall .. ______13742.5 Sample G L - 660 Standard deviation of the mean______± 1. 2

L ______0. 91793 3. 02034 31. 9 13287. 8 2 .. ______. 9356 1 3.07898 ~2. 1 1 ~2S9. 6 3 .. ______.8879 1 2. 92170 32. 0 13283. 4 4 .. ______.87460 2.87793 32. 0 13284.0 ; l. ______13498.7 ]3492.5 5 ._ .. ______31. 9 J3292. 2 2 ______1. 15567 3. 78352 3 1. 4 . 93032 3. 06082 0. 95626 3. 13200 30.7 1349 1. 9 13486.7 3 ______13499.7 Mean .. ______.. ______.. ______13231. 4 4 ______. 78384 2. 56602 3 1. 5 13494. 0 .80454 2. 63482 30.8 13498. 2 13492.6 Standard deviation of the mean _____ ,______± l. 7 5 ______]3494 .8 6 ______.89381 2.92658 :ll. 7 13500.6 7 ______. 88746 2.90639 30.7 13495.3 ]34R9.6 .85707 2.80670 3 1. 6 1350 1. I 13495.3 Sample G L- 66 1 M ean .. ______13492. 1 Stalldard dev iati on of th e mean.______± l. 2 1 .. ____ .. ______1 0. 93293 3.03957 32. 7 130 15. 4 13009. 1 2 ____ .... _____ .9204 1 :3. 04775 32.6 J30 11. 2 13004 . 9 :3 ______.. _____ 32.6 130 15. 4 13009. 1 4 ______.90656 3. 00149 Sam pIe G L -662 .91953 1 3. 0-1417 I 32.9 1 13023.4 130 17. 0 Mean ______.. ______.. ______13010.0 L ______0.9'1737 3. 126 10 3 1. 6 13183.4 1317i.3 Standa rd deviatioll of the mean ___ .. _.. ______±? 6 ~ ------. 94541 3. 12919 32.0 13180. 1 13173. 9 3 ______. 96905 3. 1982 1. 3 1. 6 13189.0 13182.8 4 ______. 854 75 2.82085 31. 7 13183.8 1317i. 9 5 ______.89713 2.95068 30.7 13182.3 13176.3 6 ______.89258 2.94579 31. 5 13188. 2 13182.2 The values obtained experimentally for the mass Mean ______13178. 4 of carbon dioxide formed during the combustion are Standard deviation of the m ean ______± 1.4 less than those calculated from the carbon content , of the samples based on the elemental analysis, the j- Sample 48- B maximum difference being about 0.2 percent. The apparatus for determining the carbon dioxide was 0.92042 3. 05117 33.9 12963.7 1295i. 1 tested several times during the co urse of the work, ,I 3~ ::::::::::::::______. 91594 3.03606 30.6 12963.9 12957. 8 . 94733 3. 13950 30.9 12965.5 12959.4 4 .. ______using NBS Standard Sample 39g of benzoic acid, 5 ______. 93335 3.09382 3 1. 4 12965. 4 12959.2 . 96567 3. 201 12 30.9 12970.3 12964.2 and was found to give results that check the stoichio­ 6------. 943 15 3. 12581 30. 8 12971.1 J 2965.0 metric values within 0.01 percent on the average. M ean ______12960.4 The difference between the observed and stiochio­ l St andard d eviation of the m eall ______± 1.4 metric values for the mass of carbon dioxide formed

.a T he data on this sample arc for 30° C. during the combustion of a polymer is probably due to moisture absorbed by the sample. The results reported in tables 3 and 4 include the heat of combustion of the phenyl-beta-naphthyla­ mine that was added to the samples as an anti­ oxident. These results were corrected for the heat

D93374- 5:2--2 277 - of combustion, (- /::" U ~ , 25 ° C = 8305. 7 kj /mole, of the sample. Then the heat of formation of th~s [9]) of this material on the a~su!nption that there was molecule can be obtained by combining the expen­ no reaction between the antlOxldent and the polymer l~ental value for its heat of combustion, according or air. Table 5 presents these corrected values of to eq 1 with values for the heats of fo.rm~tion of _ /::,. U O and also the corresponding valu es for - /::,.H , gaseous carbon dioxide, gaseous sulfur dlOxl~le , and (25 ° 0) and t::,.H ~ (2'5° C) used in the calculations liquid water. The heat, t::,.Ho, of the reactlOn of hea t of polymerization. C12H 25SCH2CH= CHCI-I3(liq ) + (m - 1)CH2= TABLE 5. Corrected values oj heat oj combustion, and the c01Tes ponding values oj heat oj JOT1na/ion per gTa1n oj cOTbon CHCH= CH z(liq ) + n CH z= CHC6H 5 (liq )~ dioxide JOTmed in com b11stion C I2H z5S(C4H 6)m(CsHs)nH (solicl polymer) (2)

_~U~ _ £lH ~ can then be calculated from th e heats of formation of Sample I Buta'diene l Styrene I capiaM er·n I I LIN" (28° C) I (25° C) WO) the solid polymer and of the reactants. The heat of 50° C poly mers copolymerization is taken as ( fj,Er: = Ml~ / [(m + n ) ] . Accurate values for the heats of formatIOn of 1,3- M ole % lvIole % Mole% jig CO 2 jig CO, jfg C O, butadiene [13, 17] and styrene [18] are available, A ...... •... 99. 815 0 0. 184 13877. 9 ± 3.3' 13901. 0 73.3 and are given in table 6. No data on the heat of GT~65 7 •••. 99.878 0 . 122 13891. 5 ±3.6 13914.6 92. 0 GL-65L .. 95. 123 4.656 .241 13744 .4 ± 3.2 13766.9 78.8 formation of ClzH z5SCHzCH= CHCH3 have been X - 452 . ••.. 85.840 13.994 . 166 13493. 2 ±3. 1 135 14. 6 87.5 GL-662 .. _. 71. 742 28.085 . 173 13 179. 7 ± 3.6 13200.0 86.5 found in the literature. Fortunately, only an ap­ 48-B .•..••. 60. 402 39. 492 . 107 12961.2 ±3. 1 12980.3 79.3 proximate valu e is required for th e present purpose, and such an approximate value was cal cul at~ d from 5° C polymers other data given in table 6, together WIth the indicated estimated valu es for the heats of the B ...... ••.• 99.859 0 0. 141 13875. 4 ± 4. 0 13898. 6 74.5 X-454 ...•.• 95. 258 4.644 .099 13667. 5 ± 3. 8 13690.0 13.5 following reactions at 25 ° C: X -478 ..••.• 86. 74 6 13. 162 . 091 13498. 4 ± 3. 7 13519.8 77. 8 GL-660 ••.• 77. 114 22. 784 . 103 13283. 0 ±3. 5 13302.5 84.6 GL-66L ... 62. 916 36. 982 . 102 13011. 3 ± 4. 0 13030. 4 87. 1 nC16H 34 (liq) + S ( rhombic ) ~

• T he number fo ll owing each ± sign was obtained by a ppropriate l y combinin~ nC 16H 33SH(liq) ; 6.[-]0 = 5.6 kcal (3) the standard deviations of the mean res ults of calibration and combustion experi­ ments with an estimate (0. 005 %) of the standard deviation of the value used fo r the heat of combustiou of benzoic acid, and an allowance (0.02%) for systematic nC16H 33SH (liq) ~ errors due to other factors, especially impurities in the samples. nClzH z5SC4H9(li q) ; ~H o = 2.4kcal (4)

6. Calculation of Heat of Polymerization nC IzH z5SC 41-19(li q) ~ In the case of a polymer such as polystyrene, which nClzH zsSCH zCH = CHCH 3( liq) + consists entirely of styrene units joined together, the heat of polymerization is easil y calculated as the H z(g) ;M-Io= 28.1 kcal. (5) difference between heats of combustion per monomer unit of monomer and polymer. A similar calculation The value of t::,.Ho for reaction (3) was taken as could also be made of the heat of copolymerization of equal to the difference between the heats of formation two monomers, provided the copolymer contained of n-pentane and n -pentanethlO~ ; the value for reaction (4) was taken as the chfference between practically nothing except the two kinds ?f mO ~l o mer the units. The polymers and copolymers lllvestlgateci heats of formation of liqllld ethanethlOl and liquid dim eth~Tl s ulphi.d~ ; the value for reaction (5) in the present work, however, were prepared by llSlllg was taken as the dIfference between the heat of docl ecyl mercaptan as a modifier, so that the calcula­ formation of liquid n-butane and the mean of the tion of heat of polymerization could not be made by heats of formation of cis- and trans-2-butene. The the simple method indicated above. values fo r th e heats of formation of the two butenes The calculations made here are based on the were obtained from the corresponding values for th e chain-transfer theory of pol~ · merization , from which ideal gases [13], together with valu es of heats. of it can be assumed that each polymer molecule. con­ vaporization at 25 °. C to th ~ :'eal gas~s at saturatlOn tains one dodecyl mercaptan molecule ch em l ~ally pressure [22], that IS, the dIfference III h eat content combined with it [10 , 11] . In other words, It IS between the real and Ideal gases was neglected. assumed that the formula for a polymer molecule can The vllJue for the heat of formation of liquid be written C1zH z5S (C4H 6)m(C8H g)nH , where m and C12H zsSCH zCH= CHCH 3 obtained in the manner n represent the number of butadiene and styrene z indicated is - 72.5 kcal/mole. As suggested pre­ 11llits respectively, in the molecule. The values of viously, and as is evident from th e method of cal­ m and n can be calculated from the elemental analysis culation, this value can only be regarded as ' Although this assumption is plausible, and is su pported by ex perimental approximate. data {lO] , it is made here merely as a matter of conv e~[l1 e n ce , III order to sImplIfy Values of heats of polymerization and copoly­ t he calculation of heats of polymerization . Substantlo.ll y the same result would be obtained if it were assumed that some of the polymer molecules had mer· merization calculated as described above are given captain units.at both ends and that the others had h ydJ'Ogen atoms at both en d ~ . The calculations were actuall y made on the assumptIOn that a butadIene lllllt in table 7 and are shown plotted a.gainst styrene was attached to the m el captan unit in each molecul e. The results would not be content in' fiO'ure 1 where the valu e - t::,.H;= 16 .68 changed a.ppreciably by assuming that a styrene uni t was attacHed to the mer­ b , . I 1 captan. kcal per monomer unit for styrene [12] IS a so s lown. 278 TAB LE 6. l'allles oJ heats of fOl'mati on at 25° C used in 21 calculating heats of polymeTization • Compound MIl Reference

kcol/wole n-Pentane (Uq ) ______- 41.36 1141 n-PenLanethiol (Ii q ) ______- 35.8 llo] Ethanethiol (Ii q ) ______- j() 9 Dimethylstllfid e (Iiq ) . _ _ __ - J3.6 POlllG n- Butane (Iiq ) ______- 35.51 113. 14] !i-Butene (Iiq ______(1- 7.43 II a, 221 1.3-Butadiene (Iiq ______+20.88 [la.17] St)-rcne (I iq) ______+24 S3 n- l-l c,adecanc (Iiq) ______IIS1 o 18 - J08. 59 114 U :L • ( G i\Ican of "niue's fo r cis a nd tranfl isomers. ° • --=:::::----. ,...... ~ r2=o 17 ° • ----:~ ° rl r2- 1 TARLE 7. Heats of polymeTization -- r------..,

Sa mple Styrene con- I _tJ. lf:, 16 I tont (25° C ) 0 01 02 U 0.4 C!) Cb C. C.t! e.9 o MOLE FRACTION STYRENE .>00 C' poJ)'mors FIG U RE L Heats oJ copolymerization_ kelll/m.ono­ 0 , 50° C polymers; . ,5° C polymers; 6. polystyrene. ~ I rol e % " unit .1 ______o 17.6 (1L- 65i o IG. 9 G L-6fiS ______4.65 J7. 4 parL in some reaction iuhe bomb. Since the X--4 .12 ______14. 02 17. I (I L- C,62 ______28. J3 li.2 llaLure oJ Lhe sulfur compound is no t known, no 48- B ______39. 53 17.7 correcLion for Lhe cf}'('ct of sncil reacLions co uld P olysl)-rcne ______100 b 16. 7 be made. Calculated valu es of the correction on Lhc basis of various assumptions as to whaL sulfur compounds ma.v have been prescnL and whaL re­ n ______o 17.6 actions may have takrn pla ce indicaLe that the X - 454 ______4.65 20.4 X - 4iS ______n 17 17. 6 error introduced by neglecting the thermal effe ct (1 L- 6GO ______22.84 17. 3 (1 L- 661 ______37. 02 17. 2 of the reaction, and by m aking co rrections for nitric and sulfuric acids as described previously, probably • Mole percent styrene plus mole pcrcen t butadiene= 100_ does not exceed 0.02 01' 0.03 percent of Lhe heat of ' Roberts, W alton, and Jessup [12]. This sample contained no sulful', and t he heat of polymerization was calculated as the d ift'erenee between the heatof combustion. This is fa r Loo small to account for com bustion per monomer·unil, of pol ym er ancI monomer. the abuormal res ult obtained with sample X - 454. It, will be seen from figure 1 thaL the values of 7. Discussion - 6.H~ for samples other than X -454 do not incli­ caLe allY appreciablr diiIerencc between Lh r 5° C It is apparcnt from fi gure 1 that the valu c 20.4 and the 50 0 C polymers. It will be seen also that kcal PCl' monomer unit for - 6.Hol, for the 5° C the points for polymers other than X - 454 lie reason­ pol~T m e r X - 454 is cntirely out of line with the valu es ably neal' to the curves shown. The maximum devin.­ for the other polymers, which range from 16.9 to tion from either curve corresponds to abouL 0.06 17 _7 kcal pN monomer 1l1li t. No satisfactory ex­ percent in heat of combustion of Lhe polym cr . This planation of this high valu e has been found, bu L is probably as good agreement as co uld be expected . the reasonably good consistency of the data on thc considering the nature of Lh e polym eric mat e ri ll.h~ other polymers suggests that the high valu c of and the approximations madc in calculating- t.hfl - 6.[J0 p 1'01' sample X - 454 may be related to some heats of polymerization. abllo rm alit~ · in the composition of this sample_ The curves shown in fi gure 1 were calculateCl Somc abnormalit.\~ in this sample is also indicated from an equation of a form derived theoretically by lw a co rrC'lation of the refractive indices of the Alfrey and Lewis [19], which expresses heat of co­ various poly'm ers with styrene content [20] . It has polymerization as a function of copolym er compo­ been found that the stYl' ene contents of the other sition. A value for one of the t wo parametcrs in 5° C polymers, as determined from analyses for this equation was chosen empirieally to fit Lho ca rbon, h.Hlrogen, and sulfur, can be rep l'rsented experimental data. According to Alfrey and L ewis b~- an empirical function of refractive index within [19] the other paramet.er, clcsigna ted as rlr2, was less a few thousandths of 1 percent of styrene. However, than 01' equal to unity in all copolymer pairs studied this relation gives a value for st.vrene con tent for by them . The Lwo curves shown in figure 1 may sample X - 454 that is lower than the value calculated therefore represent Lhc limits between which the from the analysis of the sample by approximately best theoretical curve would lie. It is seen from 0.5 perce nt. figure 1 that the data are at least not inconsistent As sta t ret previou sly, a par t of the sulfur in samplc with the theoretical curvcs_ It may he noted also X - 454 was prese nt as i norga ni c impurity, and it that the deviation of the curves from a straight may be that the inorganic sulfur compound took line connecting tll e values for 0 and 100 percent of 279 styrene is in the direction predicted by Alfrey and [6] M. Sh epherd, Anal. Chern. 19,77 (1947). Lewis. It should be pointed out, however, that the [7] E. W. Washburn, BS J. Research 10,525, (1933) RP546. [8] E. J. Hart and A. W. Meyer, J . Am . Chern . Soc. 71, polymers investigated here were prepared under 1980 (1949). conditions such that conversion ranged from 60 to [9] D. E. Roberts and R. S. J essup, J. Research N BS 40, 75 percent, whereas Alfrey and Lewis [19] state that 281 (1948) RP1873. their analysis applies only for low-conversion poly­ [10] H . R. Snyder, J. M. Stewart" R. E. Allen, and R. J . D earborn., J. Am . Chem Soc. 68, 1422 (1946) . merization, without appreciable drift in monomer [11] F. T. Wall, F . W. Banes, and G. D. Sands, J. Am . ratio. Ch em. Soc. 68, 1429 (1946). It seems unlikely from the information available [12] D. E. Roberts, W. W. vVal ton, and R. S. Jessup, J. on the polymers investigated and from the results Research NBS 38, 627 (1947) RP1801. [13] E. J. Prosen, F. W. Maron, and F . D. Rossini, J . R e­ of infrared studies on similar polymers by Hart and search, NBS 46, 106 (1951) RP2181. Meyer [8] that any appreciable differences in heat [14] E. J. Prosen and F. D. Rossini, J . Research NBS 34, of polymerization would arise from differences in the 263 (1945) RP1642. relative amounts of 1,2 and cis- and trans-1,4 struc­ [15] W. N. Hubbard and Guy Waddington, U. S. Bureau of Mines Experiment Station, Bartlesville, Okla., Un­ tures in various polymer samples. published data. [16] Selected values of ch emical thermodynamic properties, N BS Circular 500 (1952) . The authors are indebted to Max Tryon of the [17] E. J. Prosen and F. D. Rossini, J. R esearch NBS 34, 59 Rubber Section of the Bureau for purifying the (1945) RP1628. [18] American P etroleum Institute Research Project 44 at various polymer samples; to Rolf A. Paulson of the the National Bureau of Standards. Selected values of Analytical Chemistry Section for the chemical properties of hydrocarbons. Table 13p, Styrenes (Feb. analyses reported in table 1; to W. N. Hubbard and 28, 1949). Guy Waddington, U. S. Bureau of Mines Experiment [19] T . Alfrey , J1'. and Charles Lewis, J . P olymer Sci. IV, 221 (1949). Station, Bartlesville, Okla., for permission to use [20] Max Tryon, Rubber Section, National Bureau of Stand­ their preliminary value for the heat of formation of ards, unpublished data. See al so Specifications for n-pentanethiol. Government Synthet ic Rubber, Revi sed ed., issued b.v Reconstruction Finance Corp., Office of Rubber Re­ serve, 811 Vermont Ave. , vVashington 25, D. C. , 8 . References Section C 6, 1- 1- 51. [21] E. Wich ers, J . Am. Chem. Soc. 72, 1431 (1950) . [1] H . C. Dickinson, Bul. BS 11, 189 (1914) S230. [22] Selected values of properties of hydrocarbons. NBS [2] R. S. Jessup and C. B. Green, J. Research NBS 13, 469 Circular 461 , Table 8m, p. 139 (19471 (1934) RP721. [3] R. S. Jessup, J . Reaearch l\'BS 29, 247 (1942) RP1499. [4] R. S. Jessup, J. Research NBS 36,421 (1946) RP1711. [5] F . B. Silsbee, ~BS Circul ar 475 (1949) . WASHINGTON, July 26,1951.

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