U. S. Department of Commerce Research Paper RP1801 National Bureau of Sta ndards Volume 38, June 1947

Part of the Journal of Research of the National Bureau of Standards

Heats of Combustion and Solution of Liquid Styrene and Solid Polystyrene, and the Heat of Polymerization of Styrenel By Donald E. Roberts, William W . Walton, and Ralph S. Jessup

Bomb-calorimetric m easurements have yielded for the heats of combustion (-t:"Hc0) at 25° C of liquid styrcne and solid polystyrene to form gaseous carbon dioxide and liquid watcr the values 4394.88 ± 0.67 into kj/mole (1050.58 ± 0.14 kcal/mole) , and 4325.09 ± 0.42 int. kj/CsH s-unit (1033.89 ± 0.10 kcal/CsHs-unit), resp ectively, and for the heat of polymerization of liquid styrene to solid polystyrene at 25° C the value 69.79 ± 0.66 into kj/mole (16.68 ± 0.16 kcal/mole) . The results obtained on two samples of polystyrcne of different molecular weight were in agreement within the p'recision of the measurements. M easurements of the heat of olution of solid polystyrenc in liquid monomeric styrene gave the value 3.59 ± 0.21 l,nt. kj (0.86 ± 0.05 kcal) evolved per CsH s-unit of polystyrene at 25 ° C. Addition of this to the value for the heat of polymerization of liquid styrene to solid polystyrene gives the value 73.38 ± 0.69 into kj (17.54 ± 0.16 kcal) per mole of styrenc for the heat of polymerization of liquid styrene at 25° C, when the final product is a solution of polystyrene in styrene containing 6.9 percent by weight of polystyrene.

I. Introduction Although the indirect determination has the disadvantage that the heat of polymerization is The heat of polymerization of styrem· can be obtained as the small difference between two large obtained directly by measuring the heat evolved quantities, the precision of bomb-calorimetric when the polymerization reaction is allowed to measurements is such that the accuracy of the take place in a calorimeter, or it can be obtained indirect determination is comparable with that indirectly as the difference between the heats of of direct determinations which have been reported combustion of liquid monomer and solid polymer. [1 , 2].2 The indirect method has the advantage In the direct measurement, the polymerization is that the effe~t of the molecular weight of the not complete, as a rule, so that the heat evolved product on the heat of polymerization can be is that corresponding to a polymerization reaction investigated by using polymer fractions of widely in which the final product is a solution of polymer different molecular weights. in monomer. The indirect determination, on the The purpose of this paper is to present results other hand, yields the heat of polymerization of of bomb-calorimetric measurements on styrene the liquid monomer to soli d polymer. In order and on two samples of polystyrene having different to compare values of heat of polymerization ob­ average molecular weights, and a value of heat of tained by the two methods it is necessary to have polymerization derived therefrom. Data obtained a value for the h eat of solution of polymer in in measurements of the integral heat of solution monomer. of polystyrene in liquid styrene, which can be 1 Presented before tbe High Polymer Forum at the BOth meetin g of the American Chemical Societ y. Chicago , Sept. 11 , ]946. This paper also 2 Figures in brackets indicate the literature references at t he end of this will appear in the J . Polymer Sci. 2, No.3 ( l947). paper.

Heat of Polymerization of Styrene 627 used to compare the results of the measurements monomer or solid polymer at 25 0 C and a pressure reported here with values obtained by direct of 1 atmosphere. measurements of heat of polymerization made at The heat evolved at 25 0 C in the polymerization this Bureau ll] are also given. In themselves, reaction per mole of monomer - I::.. UpO (=- I::..HpO) calorimetric determinations of partial molar heats was then calculated by means of the relation of solution are of importance in connection with - I::..Up o= (- I::..UcO)m-(- I::..UcO)p, where the sub­ studies of thermodynamic properties of polymer scripts m and p refer to liquid monomer and solid solutions, and as the dependence of heat of solu­ polymer, respec ti vely . tion upon concentration is not yet known, further The samples of polystyrene, which were fine work is contemplated. powders, were compressed into pellets and then weighed in the platinum crucible in which they ·II. Methods and Apparatus were to be burned. The samples of liquid styrene were enclosed in thin-walled glass bulbs In this work, measurements of heats of combus­ [5 , 61 which were flattened on opposite sides and tion of liquid monomer and solid polymer were filled completely with the liquid. The method made with the same bomb calorimetric system, described in the references cited for filling such and the difference in the results was taken as the bulbs consisted in immersing the end of the stem heat of polymerization of liquid monomer to solid in the liquid, and then alternately heating the polymer. In calculating the results of the mea3- bulb to expel air, and allowing it to cool and draw urements of heat of combustion, the amount of in liquid. As it was thought undesirable to heat the combustion reaction in each experiment was the styrene, a different method of filling the bulbs derived from the mass of carbon dioxide formed, was used. This method consisted in immersing using the value 44.010 for the molecular weight of the end of the stem in the liquid, and th en alter­ carbon dioxide. nately applying pressure gently with th e fingers The calorimetric system was calibrated with to the flat sides of the bulb to expel air, and Standard Sample 39f of ben7.0ic acid, using the releasing the pressure. The weight of glass in value reported previously [3] for the heat of com­ each bulb after removing the stems was about bustion of this sample under the conditions of the , 0.05 g. bomb process. The apparatus and procedure followed in the The observed heat of combustion in each experi­ bomb-calorimetric measurements have been de­ ment was corrected for heat of stirring and heat scribed previously [7 , 8, 9]. The bomb used is transfer between calorimeter and surroundings, made of illium and is similar in design to that for energy used to ignite the charge, and for energy described by Prosen and Rossini [10]. The bomb produced by formation of nitric acid in the bomb. had a capacity of 380 ml. One milliliter of water In the experiments with polystyrene a small was placed in the bomb before each combustion amount (0.14 to 0.3 6 mg) of unburned carbon experiment. The carbon dioxide formed in com­ was left in th e crucible. Correction for this was bustion was absorbed in Ascarite and weighed, applied both to the observed mass of carbon following the procedure described by Rossini [11]. dioxide formed and to the observed heat of The absorption train used was similar to that combustion using 33 into kj /g as the heat of described in reference [10], but no provision was combustion of carbon. made for oxidizing products of incomplete com­ The observed value of heat of combustion III bustion and absorbing the resulting carbon dioxide. each experiment was reduced, in accordance with However, frequent tests for carbon monoxide in the procedure described by Washburn [4], to the the gaseous products of combustion were made, value of - I::..Uc0 (25 0 C), the decrease in intrinsic using an NBS colorimetric metbod [12], but no energy accompanying the combustion reaction carbon monoxide was ever found. The heat of solution of polystyrene in liquid styrene was determined by observing the temper­ with each of the reactants and products in its ature rise of the calorimeter produced by the thermodynamic standard state. The standard solution of about 1 g of polystyrene in 15.0 ml state for CsHs in this equation is that of the liquid of styrene.

628 Journal of Research T he calorimetric apparatus used in the measure­ III. Materials ments of the heat of solution was the same as that T he samples of styrene and polystyrene used in usell in the combustion rxperiments, excC'pt thaL th e bomb-calori m etric measurements were ob­ th e calorimeter cover was omitted, and a straigh t tained from tIl e Dow Chem ical Co. The sLy­ glass tube abou t 300 mrn in lrngth by 11.5 mm rene was a srleetecl sam ple, a nd was puri fied in si~l e diameter was uRed to con tain the samplc of a t this B ureau 3 by passinO' it tlll'oll cr h s i l i~a o'l,l hqUld styrene. Th is tube was immersed to a o b b , by three fractio nal freezings, ancl by el i Lilh tion dep th of 225 mm in th o watcr of the calorimeter in vac~ ulll . Freez ing ellrves indicated a p urity and extended abou t 40 mm above the jacket. of th e final product of 99 .95 mole percen t. Mass The finely powdrred sample of polystyrene was spec trographic analysis 4 indicated a C0mpo iti on contained in a cylillder 10 mm in diam ter and 110 of 99.9 1 mole per cent styrene and 0.09 mole per­ mm long made of 200 -mesh eopper gauze, whicJ l cen t ethylbenzene. The observed bomb-calOl'i­ was placed in the glass tube above the surface metric data on styrene were correct.ed for the of the liquid styrene, and was suspenrl rd by a thin· presence of this amount of ethylbenzene by usin O' walled copper-nickel tube. The Lemperatures of • b th.e heat of combustion data reported by Prosen, th e ealorimetcr and jacket were bo th brought Gilmont, and Hossini [14]. near to 300 C, with th e tempera Lure of the jacket After purifiy ing the sample it was stored in an slightly higher th an that of the calorim eter so evacua ted sealed glass container at the t f'mper­ that there was a small positive rate of chan o. ~ of atm e of dry i('e un t il time for making the CalOl"i­ calorimeter temperature due to thermal l eal~age . metric measuremen ts . It was then warmed to After a ,vait of several minutes for the attaimuC'nt. room temp E' rature, a sample was transferred to a of a steady positive rate, a series of observations glass bottle, and 10 ppm of tertiary butyl catechol o~ ealorimeter temperature and corresponding was addE' d to this sample to inhibit polymeriza­ tmle was made over a period of 10 minutes. The tion . Till' calorimetric measurements on the copper-gauze cylinder containing the salnple of monomer were completed within 1 month after it polystyrene was then lowered into the styrene and was warmed to room temperature. moved up and down by hand once a second to stir The two samples of polystyrene obtained for the the liquid. The temperature of the calorimeter bomb-calorimetric measuremen ts, samples I and rose rapidly at first, but the rate of rise decreased II, ~ ere commercial unfractionatecl polymers, with time and attained a small constant value contalllmg about 3 per cen t of methanol-soluble after about 10 minutes, indica ting that solution material (partly low-molecular-weigh t polymer of the polystyrene was complete. Stirring of Lh e and unpolymerized styrene) . The monomer ll sed solution was continued for 10 more minutes. and was 99.7 percent styr ene, and most of tbe impurity readings of time and temperature were continued was ethylbenzene. Each sample was prepared fo r 10 minutes after the stirring was stopped. by thermal polymerization without catalysts at a The rate of change of calorimeter temperature de­ range of temperatures, the average for th e two creased sligh tly wh en the stirring was stopped. samples being of the order of 100 0 C and above , This was attribut(,d to a combination of heat of and may have had a wide range of molec ular stirring of the viscous solution, and conduction of weights present. Howe\'er, precipi tation tests 5 heat to Lhe calorimeter from the hands during th e showed that as methanol was slowly add ed to a stirring. Correction to th e observed temperature toluene solution of the polymer, a large fraction rise during Lh e "reaction period" was made for of the material was precipita ted over a narrow these eff ects, and for heat transfer between calOl"i­ range of added precipitant, indicating that a large meter and jacl ('t. portion of each polymer probably was of nearly The energy u nit used in the original calculation of th e resul ts is th e in ternational joule. COllVer­ uniform molecular weigh t. sion to the conven t.ional thermochemical calorie 3 '['he purification and test ing Cor purity by observation ofCreez in cr behavior were done by mombers of t he staU of the low·temperature l abo r . t~ r y at the was made by the relation [1 3] 1 calorie = 4.1833 Bureau. into j . • By A. K . Brewer. , By 1. C. Schoonover oC t ho Bureau.

Heat of Polymerization of Styrene 629 742-178- 47- 4 The sample of polystyrene used i.n the measure­ process. One sample (not used for combustion ments of heat of solution, ssmple III, was obtained measurements) that was kept for about 1 month from Monsanto Chemical Co. It was a commer­ under the above conditions of drying increased in cial material designs ted as type A, and was pre­ weight by about 1 percent. Within the limits of pared in the form of a fine powder by solution in precision of the measurements, the heats of com­ toluene and precipitation with methanol in the bustion of the purified and unpurified samples per manner described below for samples I and II. gram of carbon dioxide formed wer e the same. The styrene used in heat of solution measure The heat of combustion of the purified material ments was a teclmical sample obtained from Dow per gram of sample burned was found to be about Chemical of Canada, Ltd. Freezing curves indi­ 0.06 percent less than that of the unpurified ma­ cated that it was 99.8 mole percent styrene, the terial. These facts suggest that the removal of most likely impurities being ethyl benzene and monomer and low molecular-weigh t polymer in the polystyrene. purification process did not affect the heat of com­ A few preliminary bomb-calorimetric measure­ bustion per gram of carbon dioxide formed by a ments were made on samples I and II of poly­ measurable amount, and that the increase in styrene without purification, except for removal weigh t of the material during drying was due to of visible particles of foreign material. The re­ the absorption of gas, which behaved as an inert mainder of each of these samples was purified at impurity. this Bureau, by the method of 1. C. Schoonover, The three samples of polystyrene were charac­ which produces a fine powder rather than a gel. terized, after purification, by viscosity deter­ According to this procedure, 20g of the material minations. Measurements were made with an was dissolved in 2 ~~ liters of chemically pure filtered Ostwald (Fenske type 50) viscometer at a tem­ and distilled toluene by allowing the mixture to perature of 30 .50 0 ± 0.05° C. The viscometer was stand for 24 hI', with intermittent shaking. The calibrated with water, and it was found that the solution was filtered through a fine sintered glass kinetic-energy correction to the viscosity was funnel. Two hundred milliliters of the filtered negligible over the range of viscosities measured solution was slowly added to 3 liters of chemically on the polystyrene samples. The viscosities of pure methanol in a 4-liter Erlenmeyer Hask, with the polystyrene samples were measured in toluene continuous stirring. Stirring was continued for solutions at concentrations of 1, 0.5, 0.25, and in ~ hr after all of the toluene solution had been added some cases, 0.125 g/100 ml of solution. The and the precipitate was then allowed to settle results were plotted by the method of Huggins overnight. [15], and the following values obtained for the The supernatant liquid was decanted, the pre­ intrinsic viscosity [7/] and the constant k' in the cipitate collected upon a large sintered-glass Huggins 6 equation: funnel, and washed eight times with methanol. When the entire amount of polystyrene solution l~l had been treated in this manner, the precipitates I .' were then dried for 18 hr in a thin layer at 70° C Sample L ______1.650 0.206 under a pressure of about 75 mm of mercury. Sample IL ___ __ 0.791 .232 The dried material was ground, mixed, and dried Sample III ___ __ 1.255 .217 for an additional 72 hI' under the same conditions I with intermittent stirring to expose fresh surfaces. A test made by moistening the dried material with These data indicate that samples I and II, used methanol and drying again indicated that the in the bomb-calorimetric measurements, differed material probably did not retain methanol under considerably in average molecular weight (sample these conditions of drying. I having about twice the molecular weight of It is believed that the above treatment removed sample II) and that the degree of polymerization most of any monomer and low-molecular-weight ' ~ ,p/c= [~l + k ' hh. p, where c is the concentration in grams per 100 ml of polymer that may have been present. It was solution, ~. p is specific viscosity, and k' is a constant that determines the found, however, that the polymer increased in dependence of the viscosity on concentration and depends on the t ype of polymer, on the sol ve nt and on the temperature, hut is supposecl to depend weight by about 0.1 percent during the drying only slightly, or not at all, on t he molecular weight of the polymer.

630 JoUInal of Research of sample III, used in the measurements of heat T A BLE 2.-Results of bomb-calorimetric measurements of solution was intermediate between those of 0 '1, polystyrene and styrene amples I and II. It may be noted that there I ileat o[ co mbustion at 30° C are not very wide differences among the values '''lass of IIniti al 0 2 I sample pressure, Mass o[ C02 I of k'. 30° C - ClUB - ClU,O IV. Results I P OLYSTYRE, E I (A)a The results of the calibration experiments are y atm 0 Int. j ig CO, Int. jIg C O, given in table 1. Because of sligh t changes in 1. 04199 35. 9 3.51789 12273.0 12265.5 the calorimetric ystem resulting from repairs to 0.83779 35. 8 2.82821 12276.5 12269. 4 .89085 35.7 3.00708 12275.3 12268. 1 I the bomb, and the sub titution of a new resistance . 871 26 35.7 2.94205 12273.2 12266. 0 , thermometer for the one used initially, several . 84294 :l5.7 2. 84516 12273. 6 12266.5 series of calibration exper im ents were made during . 70651 35.8 2.36437 12276.5 12269. 6 the course of the work. These are designated by I 12267. 5 the letters A, B , 0 , in table 1. ~:~:; ;; ' ~I ~Ie-~ i~t-i ~;; ~i -~~;l ~ - ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~~ ~ ~ ~ ~ ~ ~~ ~~ I ± 1. 3

T A B L E I.- Results oj calibration oj calorimeter with ben zoic POLYST Y RENE II (B) & acid I 0.54714 35.8 1. 84806 12279. 1 12272. 4 12274 .3 Observed energy equivalent at 28.5° C fo r sc ries- . 99097 35.8 3. 34687 12267.0 - --- . 97718 35.8 3.29996 12273. 4 12266.1 .98993 35.8 3. 341 74 12273.5 12266.2 B A I C . 96938 36.0 3. 27295 12275.3 12268. 0

Int. j /o C i nt. j /o C Int. j/o C M ean ______13828. 0 13825.9 13826. 9 ------. ------12267. 9 Standard deviation of m ean b ______13823.6 13827. 1 13821. 9 ±1.6 Weighted mean for polystyrene ______13828.6 13829.9 13822. 6 12267.6 mean b ______13826. 0 13828.2 13827. 4 Standard deviation of weighted ± 1.2 13827. 3 13823. 3 13835. 0 13823. 9 13825.3 13832.5 S'f Y R E E(C)& 13825. 2 13829.5 13829.0 138 18.8 13822. 4 13826. 4 13825.7 13822.6 13822. 9 1.17387 32. 5 3.96699 12474.1 0]2466. 6 13827.0 13822. 8 13823.8 0. 77992 32. 6 2.63602 124 78. 0 12-(71.1 13824.8 13818. 2 13825. 5 .79241 32. 3 2.67778 12478. 7 12471. 8 13817. 7 . 7371 4 32.8 2.49141 12473. 7 12466. 8 MOBIL 13825. 4±0. 8 a 13823. 2 M ean_ 13826. 7± 1. 2 a . 86098 32.8 2.9101 5 12469.4' 12462. 3 13828. 7 . 7286 1 32.7 2. 46261 12471.6 12464.8 13824. 0 . 78500 32. 8 2. 65316 12469.7 12462. 7 13824. 3

Mean _ 13824. 6± 0. 9 a 12466. 6 ~:~~ ~~d d-~~ i-;t-i~;; ~-f- ;';~~;; ~:~ ~:: ~: ~ ~ ~ ~ ~:::: ~ ~ ~~::~ ~ ~ I ± 1.9

• See footnote 7, page 632. • See preeed ing column [or an ex planation of tbe signifi cance of the letters A, B,3n d C. The results of measurements of heat of combus­ b Each value of standard dev iation of tbe mean was caleulated from the data of the experiments with the styrene or polystyrene sample and the corre­ tion are given in table 2, where - /:1 UB is the spon ding ex periments with benzoic acid, in the m anner described in foot­ observed heat of combustion in the bomb under note 7, page 632 . t he conditions specified in columns 2 and 3 of the • The values of - Cl U . ° for styrene were derived [rom the corresponding values of -ClUB by applying the Washburn correction and a correction of table, an d - /:1 U cO represen ts the decrease in 34 ppm for tbe etbylbenzene in t he sample. intrinsic energy accompanying the combustion reaction wh en the reactants (liquid styrene or The values obtained for the mas of carbon diox­ solid polystyrene and gaseous oxygen) and prod­ ide formed in combustion, which is a measure of ucts (gaseotls carbon diox ide and liquid water) are the amount of reaction, are lower than the corre­ all in their thermodynami c standard st ates. The sponding values calculated stoichiometrically from letters A , B, 0 , following the material names in the masses of samples burned, assuming that the table 2 in di cate the eries of calibration expen ­ composition of the samples is represented by ments listed in table 1, upon which the results (C8Hs)x. The average differences between ob­ are based. served and ca.lculated masses of carbon dixoide

Heat of Polymerization of Styrene 631 are, for styrenr, 0.029 percent, and for the samples 0.66 int.kj ,7 or 16.68 ± 0.16 kcal/mole of monomer. of polystyrene I and II, respectively, 0.146 and The data obtained in measurements of th e in­ 0.115 percent. Frequent measurements of the t egral heat of solution of polystyrene ill styrene are carbon dioxide fo rmed in comhustion of Standard given in table 4. The values given represent thp Sample 39f of hen zoic acid y ielded results in agree­ heat evolved when polystyrene is dissolved in . ment with calculated values within less than 0.01 styrene to fo rm a solution containing all average

percent on the average. The observed deficiency of 6.9 percent by weight of polystyrene. Thi'3 I of carbon dioxide in the case of styrene can proba­ average concentration of polymer in the resulting bly be atkibuted to air or moisture dissolved in solution was abou t the same as the eoncentratiol1 the liquid. The deficiency in the case of the of polymer in the final solution obtained in the samples of polystyrene is probably due to the direct measuremen ts of heat of polymerization [1] presence of nonhydrocarbon material III the made at the Bureau. samples as an impurity. I t will be seen from th e data of table 2 that the TABLE 3.- Yalues of heat of combvslion values of - ~ UC ° for the two samples of polystyrene arc practically identical, even though the indicated Polystyrene (solid) Styrene (liQu id)

amounts of impurity were different. This sug­ 4319.18 ± 0.42 in to kj /C',H,- 4389. 24±0.67 int. kJ/mole." unit.· gests that a part of the nonhydrocarbon content -~ U ,O(30° C) 1032.48 ± 0.1O kcal/ C,ll,- 1049.23 ± 0. 14 kca l/mole. of the samples behaved simply as an inert impurity, wl it. 4394 .28 ± 0.67 intokj /mo!e. but as the combined uncertainties of the calOl'i­ ±

dioxide formed is independent of molecular weight, 7 Except where otherwise noted, the Dumb€!' followi ng each ± sign is a at least over the range of the material studied. measure of the precision of the res ult which is defined as follows: I n I he case The weighted mean of the values for the two of a value of hcat of combustion, the measure of preCision is taken as [ Ii] sample" is therefore taken as the value derived from. the present m easuremen ts for the heat. of In this ex pression, s. is the standard deviation of the meau of the results of combustion of polystyrene. the series of experiments with benzoic acid to determine E , the energy equh-· alent of the calorimetric system; 8Q is the standard devia tion of the mean of In table 3 are given values of heat of combustion the results of the seri es of experiments to determine Q, the beat of combus­ t ion of the sample; (B B/ B) is an allowan ce of 5XIO- 5 for the standard devia· 8 g per mole (or per C H -unit) for styrene and poly­ tion of the valu e used for the beat of comhust ion of benzo ic acid; and (S R/ R) styrene, calculated from the mean values given is an allowance of 5X 10- 5 fo r the standard deviation associ ated with the deter­ mination of the amount of the combustion reaction from the mass of carbon in table 2. In reducing the values at30° to 25°C, dioxide formed. The corres ponding measure of precision of the value of heat use was made of unpublished data obtained at the of polymeriza tion is defined as Bureau on the heat capacities of polystyrene and styrene [16]. The values of heat capacities In this expression Sm' and sp were calculated from the data of the heat of were 128.48 into j/C8H 8-unit degree and 182.20 combustion measurements on monomer and polymer, respecLi vely, by into j/mole degree at 27 .5° C fOt" polystyrene and means of expressions of the form styrene, respectively. The value derived from the data given in table 3 H ere it is assumed tbat systematic errors in the value used [or the heat of for the heat of polymerization of liquid styrene to combustion of benzoic acid and in tbe method of dcterm ining the amount of the co mbustion reaction wiII cancel in taking the difference between the I solid polystyrene at 25° C, that is , the difference heats o[ combustion of polymer and monomer . between the values for heats of combustion of I'Standard deviation of the mean" as used above is defin ed as (I /.Jn)x.JT.J'/n, where d is the difference between a sing le observation monomer and polymer is - ~Hp ( 25° C)=69 .79 ±. and the mean, and n is the number of obscrvati on5.

632 Journal of Research T A RLE -J.- R esults of measurements of the heat 0/ solution of l6 .68 ± 0.16 lecal/mole for hent of polymerization polystyrene in styrene of liquid monomer to olid polymer to the value 3.59 ± 0.21 into kj /CsHs-unit, 01' 0.86 ± 0.05 lecal /­ ObsCl' ved hrat of sol uLion (- 6 11 ,25° C) CBB s-unit given in tnble 4 for tbe heat of soluLion Mass of pol y­ styrene a of solid polystyrene in liquid styrene yields the I n t. kj/C,II,· kcal/C,H,-unit 1I,dt valuc 73.38 ± 0.6 9 int. kj 0 1' 17.54 ± 0.1 6 kcal/mole of monomer for the heat of polymeri zation of g 0.897 ______3. 99 0.95 liquid styrene at 25 °C when the final product is a 0.998. ______. _ 3. 58 .86 solution of polystyrene in li quid stYL'(,J1 e. It wo uld 0. 986 ______2. 91 . 69 L lOo ______3. b8 . 93 have been more satisfactory if the same sample of - ---- polystyrene could have been used in both the Mean 0. 997 3. 59 0. 56 combustion and the solution measurements. Bow­ Standard de- ever, it is though t that the branching, ring fOl'lna­ v iation of mcan ______±0. 21 ±0.05 tion, or other differences that have been ascribed [30] to diffeT'('nt conditiolls of polymerization a D issolved in 15.0 ffil of styrene. probably would not. have an effect greater than experimen tal error on the thermal values r eported These measurements of heat of solution are here. It will be noted that there is good agree­ believed to be the only ones reported for a solution ment with the heat of polymeri za tion as measured of polymer in its own monomer. The observed direc tly [l]. evolution of heat reported in this paper has been For convenien t reference, a recapitulation of the surprising to some persons with whom this worle essential numerical res ults obtained is as follows: has been discussed. It was expected that the heat of solution would be quite small considering the chemical similarity of the two materials. Flory [1 8J assumes that the ]] cat of com busLion : heat of mixing of polystyrene and toluene is M_onomcr ______. ______1050. 58 keal/mole . Pol ymer . ______. 1033.89 keal/C,B ,-uni L. negligible, but Schlliz [19] found an evolution of H eat of solution of pol ymer in mOllomer 0.86 kcal/Csll,-uniL. heat upon dissolving polystyrene in benzene. I t HeaL of polym erization to : Solid polymer ______• ______may be noted that heat is evolved wh en cellulose 16.6S kcaJ /mole. Sol ution _------=------.-~- I 17.54 keal/molc. derivatives are dissolved in various solvents [20, 21 , 22, 23, 24 , 25]; heat is absorbed in rubber­ solvent systems [26 , 27, 28] and the heat effect is V. Discussion zero with gutta-perella and toluene [29].8 The The mothod that has been used for calculating fact that heat is evolved suggests that the attrac­ heat of polymerization from bomb-calorimetric ti ve force between adjacent polymer molecules is data is based on the assumption that the polymcr­ less than that between polymer and monomer izatioll reaction is represen ted by molecules. Addition of the value 69.79 ± 0.66 into lej or

S References numbered [1 9, 21 , 22, 2:3,24,2.1 a nd 28] are to work in which This equa tion mny be 1110]'e appropriately ,vriL ten 'I the heat was measured calorimetrically. I n references [20, 26, 27, and 20] the heat e fTect was calculated from os motic· or vapor·pressure measurements. ill the fo],m

(2)

The double bond could conceivably be located in tion of the terminal linkages with the production I other parts of the chain, or as suggested by Risi of a molecule having nn indane ring at tbe end: a nd Gauvin [:31] it could be destroyed by cycliza-

(3)

Heat of Polymerization of Styrene 633

) According .to information supplied by the Dow reported by Luschinsky [38]. One of his sample Chemical Co., polystyrene samples I and II were of polystyrene (sample A) was prepared by prepared by straight thermal polymerization of polymerizing styrene for 6 hr. at 180 0 C, dissolving styrene, which was at least 99.7 percent pure. the product in toluene and distilling off the toluene The impurity was mostly ethylbenzene, with and any remaining monomer in vacuum, and approximately 100 ppm of some organic peroxide. then redissolving the product in tolu ene and I t therefore seems unlikely that the composition drying in high vacuum at 100° C. His other of the polymer molecules differed from that sample (sample B ) was obtained by fractionating indicated by the formula (CsH s)x by amounts a part of sample A by precipitation from solution sufficient to affect the heat of combustion of the in methyl ethyl ketone in such a manner that polymer by a significant amount. In particular, a sample having a narrow range of molecular I the presence of fragments of catalyst or solvent weights was obtained. molecules on the ends of the polymer chains Measurements of heat of combustion were made ' [32, 33 , 34, 35, 36, 37] seems extremely unlikely. by means of a bomb calorimeter. The method As noted previously, the amonnt of carbon used to prevent evaporation of styrene was dioxide formed in the combustion of the poly­ probably not completely effective. The amount styrene samples was less than that calculated of reaction in each case was determined from the from the mass of sample burned. The fact that mass of sample burned. I the amonnt of nonhydrocarbon impurity indi­ The values reported for heat of combustion of cated by the deficiency of carbon dioxide is a bout stYTene, polystyrene A, and polystyrene Bare the same as the increase in weight of the polymer 1045.4, 1034.8, and 1031.6 kcal/mole of monomer, samples during drying makes it appear likely that respectively. Certain small corrections would I the nonhydrocarbon impurity was largely gas have to be applied to these values to convert absorbed during drying. If this gas was oxygen them to the same basis as the values of heat of i that reacted chemically to become a part of the combustion reported in this paper, but as these polymer molecules, the heat of combustion of corrections would be substan tially the same for the polymers per gram of carbon dioxide formed styrene, and for the two samples of polystyrene, i would be lower than that of a polymer having the resulting values for heat of polymerization I the composition (CsH s)x, and the value derived will not be affected by omission of these correc­ from the heat of combustion data for the heat of tions. The values of heat of polymerization of i polymerization according to reaction 1 would be styrene to solid polystyrene derived from Lus­ correspondingly too high . Making reasonfLble chinsky's data are 10.6 and 13.8 kcal/mole of assumptions as to the molecular weights of the monomer for PoJystyrene samples A and B, polymers, it is estimated that 0.15 percent by respectively. weight of oxygen chemically combined with Pr'osen, Oilmont, and Rossini [14] have reported sample I of polystyrene would lower its heat the v!'lue 1050.40 ± 0.20kcal/mole for the heat of of combustion per gram of carbon dioxide by an combustion, - /::1-1co, of liquid styrene at 25 ° C. amount corresponding to an increase of about 3 This valu e is in agreement within 0.017 percent percent of the heat of polymerization. The with that derived from the results presented in i corresponding effect of 0.11 percent combined this paper. Further references to earlier work I oxygen in the case of polystyrrne sample II would will be found in reference [14J. be of the order of 2 percent of the heat of polymeri­ Schulz [19] has made measurements of the in­ zation. As noted previously, however, the experi­ tegral heat of solution of polystyrene in benzene men tal data on heats of combustion indicate and has reported an evolution of 415 to 445 that the gas absorbed by the polymer in drying cal/CgHs-unit, the concentration of the solution did not combine chemically with it, but behaved being 3.3 percent by weight. This evolution of simply as an inert impurity. heat is only about half of that obtained in the work reported in tIllS paper in which styrene was VI. Previous Work the solvent, and the concentration was 6.9 per­ Measurements of the heats of combustion of cent. styrene and two samples of polystyrene have been Votinov, Kobeko, and Marei [39] derived the

634 Journal of Research ------

value 23 kcal/mole for the heat of depolymeriza­ [13J E. F. Mueller and F. D. Rosl:> ini, Am. J . Phy tion of polystyrene from the difference in activa­ (1944); see also, P remier Rapport de la Commission P ermanente de Thermochimie, International Union tion energies of polymerization and depolymeriza­ of Chemistry, p. 3 (1934). tion. They also reported, without experimental [14] E. J . Prosen, R G il monL, and F . D. R os ini, J. details, the values 9,831 and 10,041 cal/g (1023.8 Research NBS 34, 59 and 65 (1945) RP1628 and and 1045.7 kcal/mole of monomer) for the heats of RP1629. combustion of polymer and monomer, respectively, [1.5] M. L. Huggins, Ind. E ng. Chem. 35, 980 (1943). [16] R D. Rands, J . L. Prather, R. B . Scott, W . J . Fer­ and 21.9 kcal/mole for the heat of polymerization guson, and F . G. Brickwedde (ullpublished). of styrene derived from the values of heat of [17J F. D. Rossini and W. E. Deming, J . Wash. Acad. Sci. com bustion. 29, 416 (1939). Stanley [40] reported the value 20 kcal/mole of [18] P . J . Flory, J . Chem. Phys. 10, 51 (1942). monomer for the heat of polymerization in styrene [19] G. V. Schulz, Z. physik. Chem. [A] 179, 321 (1937). [20] O. HaggeI' and A. J, A. van c1er \Vyk, H elv. Chim. solution. The source of this figure was not given. Acta 23, 484 (1940). Direct measurements of the heat evolved during [21] W. L. H . Moll and G. W . F uller, 1<:o ll oid- Z. 80, 320 polymerization of styrene were made by Gold­ (1 937). finger, Josefowitz, and Mark [2]. They reported [22] G. V. Schulz, Z. physik. Chem. [B] 52,253 (1942). t he value 15.0 kcal/mole of monomer for the heat [23] T. Nakashima a nd N. SaiLo, J . Soc. Chern. Ind. Japan 38, Su pp!. bi nding 232 (1. 935). 0 0 of polymerization, at 70 to 140 C, the fin al [24] S. M. Lipatov and Z. A. P reobrazhenskaya, 1\:01- product being a solution in which the monomer loid- Z. 68, 324 (1934). was 35 to 85 percent polymerized. (25) V. A. Kargin and S. P. Papkov, J . Phys. Chern. Direct measurements [1] of the heat of poly­ (U. S. S: R ) 7, 483 (1936). merization of styrene made at the low temperature [26] G. Gee and L. R G. Treloar, T rans. Faraday Soc. 38, 147 (1942). laboratory of the Bureau have produced the result [27] Ie H. Meyer, E. \1'ollY, a nd C. C . Boissonas, H elv. 17.8 kcal/mole of monomer, which is in good Chim. Acta 23, 430 (1940). agreement with the corresponding value 17.54 [28] L. Hock a nd H . Schmidt, Kautschllk 10, 33 (1934) . kcal/mole of monomer reported in this paper. [29] E. Wolff, H elv. Chim. Acta 23, 4.39 (1 940), [3 0] T . Alfrey, A. Bartovics, a nd H. Mark, J . Am. Chern. References Soc. 65, 2319 (1943). VII. [31] J. nisi and D. Gauvin, Can. J . R esea r'ch 14, B, 255 (1936). [IJ 1. C. Schoonover, W. J. Ferguson, and F. G. Brick­ [32] F, R. Mayo, J. Am. Chem. . Soc. 65,2324 (1943). wedde. Publication pending. [33] C. C. P ric e, R W. Kell , and E. Kre bs, J . Am. Chem. [2] G. Goldfinger, D . Josefowitz, and H . Mark, J . Am. Soc. 6t, 1103 (19'12). Chern. Soc. 65, 1432 (1943). [34] C. C. Price a nd B. K TaLe, J . Am. Chern Soc. 65, 51 7 13] R S. J essup, J . Research NBS 29, 247 (1942) RP1499. (1943). [41 E. W. 'Washburn, BS J. Research 10, 525 (1933) [35] P . D . Bartlett a nd S. G. Cohen, J . Am. Chem Soc. 65, RP546. 543 (1943). [5] T. W . Richards and F. Barry, J . Am. Chem. Soc. 37, [36] H , F. PEann, D. J. Sall ey, and H . Mark, J. Am. Chern. 933 (1915) . Soc. 66, 983 (1944) . [0] R S. J essup, J . Research NBS 18,115 (1937) RP966. [37J W. K ern and H . Kammerer, J . p rakt. Chern. 1.61, 81 [7] H . C. Dickinson, Bul. BS 11, 189 (1914) S230. (1942) . [8] E . F. Mueller, Bul. BS 13, 547 (1916) S288. [38] W. v. Luschinsky, Z. physik, Chem. [A] 182, 384 19] R S. J essup, and C. B. Gree n, J . n eseareh NBS 13, (1938) . 469 (1934) RP721. [39] A. Vo tinov, P. Kobeko, and F. M areI, J. Phys. (lO] E. J. R Prosen and F. D . Rossini , J . Research NBS Chem. (U. S. s. H,,) 16, 106 (1942). 27,289 (1941) RP1420. [4 0] II. ~I. Stanley, Chemi,try & Industry 16, 93 (1938). [11] , F. D. R ossini, BS J . Research 6, 37 (1931) RP260. [121 M . Shepherd, Anal. Chem. 19, 77 (1947). W AS HINGTON, January 21, 1947.

Heat of Poly merization of Styrene 635