U. S. Department of Commerce Research Paper RP20,]3 National Burea u of Standards Volume 44, March 1950

Part of the Journal of Research of the National Burea u of Standards

Heats of Polymerization. A Summary of Published Values and Their Relation to Structure

By Donald E. Roberts

This paper contains a table showing values of heats of polymerization assembled from a survey of the li terature. There are 42 substituted vin yl compounds arranged as follows: vinyl alkyls, vinyl aryls, other vinyls, vi nyl acids and esters, dienes and copolymers. Values reported by different authors are given for each compound, with corresponding states of monomers and ploymers and a notation on the methods used in obtaining t he values. A second table gives structural formulas of monomers. Some values of heats of polymerization to hypothetical polymers having no steri c hindrance are calculated from published values of heats of form ation of hydrocarbons, mak­ ing certain ass4mptions regarding branch groups. The m ethod of calculation is expla in ed. H eat of polymerization is a rbi trarily assigned to four energy effects: (1 ) t he reaction I I I I C= C ---> - C-C-; (2) the eff ect of side groups on bond energies wh en t he re is no in ter- I I I I . action between the groups; (3) thc eff cct of steri c hindrance between ide groups; and (4) thc "end eff ect" arisin g from the nearncss of t he double bond to the end of t he monomer molecule. Values of hcats of polymerization a re compared, and their relation to structure is ex­ amined, with particular emphasis on the effect of steric hindrance. Etyhlene has the highest and alpha-methylstyrene the lowest heat of polymeri zation ; isobute nc and the methacry­ lates also are low. D isubstitution on the same vinyl carbon is a frequent cause of steri c interference, with consequent recluctionin heat of polymerization . Large b ra nched substit­ uents may causc steri c interference. The substitution of chlorine on the aromatic ring of styrene has little effect on heat of polymerization. Steric interference may prcvent poly­ merization above the dimer. The heat of copolymerization of butadiene an d styrene lics between the values for t he separate monomers. H eat of copolymerization of other monomcr pairs may be higher or lower than the value for t he separate components. H eat of poly­ merization depends somewhat on t he ratio of 1,2- a nd 1,4-addition, and on the amount of crystallinity of the polymers.

1. Introduction German sources. Since then a large n umber of experimental observations have been made, and Values of the heats of polymerization of various many interesting comparisons are now possible. materials are recorded in many journal articles, This paper has been written to collect the scattered but there is no recent systematic compilation and data and list them in a convenient form for refer­ comparison of these values. Flory [1] 1 calculated ence so that useful values of heats of polymeriza­ a number of values in 1937 before there were many tion can readily be obtained without extensive experimental observations · available, and Roth searching. A second purpose is to permit com­ and Rist-Schumacher [2] published in 1942 a sum­ parison of the data for different compounds, so mary of values almost entirely obtained from that systematic trends and anomalies can be recognized. The discussion that follows calls I Figures in brackets indicate the literature references at the end of this paper. attention to some of the relations between the

Heats of Polymerization 221 871926- 50--2 heats of polymerization and the structures of the substituent groups as follows: vinyl alkyls, vinyl monomers and polymers. The reader may wish aryls, other vinyls, and vinyl acids and esters; to make further comparisons of the types illus­ these are followed by dienes and copolymers. trated. Within each class, the compounds are arranged according to increasing size and molecular weight II. Description and Use of Tables of the monomer. The valu es reported by the The table of values of heats of polymerization several authors are arranged chronologically for (table 1) is arranged for convenience according to each compound.

TABLE 1. Heats oj polymerization

Experi­ mental States of monomer (E) or Compound - t:.T-Ip a and polymer Calcu· M ethod and comment lated (C ) b

viN YL ALKYLS

Ethylene b keal/mole oj monomer Polymer is 05 t070% cr ystalline [36, 37]. Flory [1] (1937 ) ...... 23.1...... gas ... gas ...... C Heats of form ation. Jessup [1 5] (1948) ...... '12.4· ...... do ..• ...... r Do. Do...... d 22.3 ...... do...... Linlit value for 00 chain, no sterie hindrance C Do...... 24.7 ...... Gas... liquid ...... between side groups. AHv=2.4, AHm= Do...... 25.9 ..... ," '" ...... Gas... s olid ...... 11.2 . Jessup (Roberts) [1 5] (1948i, Prosen & Rossini [S] (1946) ...... 2.5.4 ...... d o ...... E Hea ts of combustion.

Propylene Flory [I] (l93il ...... 22.S ...... Gas ...g as...... C Heats of fo rmation. Roberts .(1949) ...... 20.5 ...... do...... C Do. Fontana & Kidder [42] (1948) ...... 16.5 (-74.5°C) ...... Liquicl ->solid...... E Equilibrium constants.

I·Butene Flory [1] (1937) ...... 23.4 ...... •...... Gas ...ga s...... C Heats of rormation . Roberts' (1949) ...... 20.9 ...... do...... C Do.

Isobutene Flory [1] (1937) ...... "". '...... 23 .0 ...... do ...... '.. C Do. Roberts' (1949) ...... 19.2 ...... do...... C Do. Tbomas, et al [43] (1940) ...... 10 (-7S0 C) ...... Liquid ...s olid g...... E Kazanskii & Rozengart [44] (1942) ...... 10.2' (1 30° to 295' C) ..... Gas->gas g •••• ••••••••••• E Equilibrium constants; dimerized with ca talyst, prodnct is mixed isomers E vans & Polanyi [31 (194;1) ...... 12.8...... Solution in hexane ...... E Calorimetric. E vans & T yrl'all [4] (1947) ...... 9.9· ...... Gas->gas...... C H ea ts of formation ; dimer is 2,4,4·tri· methyl·l ·pentene. Do ...... 19.2 (20.3) ...... do...... C H eats of formation, head·tail, no steric hindrance between side groups. Do ...... 12.6 ...... _. .. Liqnid->liquid...... E H eats of formation and combustion, with steric hindrance.

cis·2·Butene F lory [l] (l937) ...... 23.2 ...... Gas->gas...... C Heats of formation. Roberts' (1949) ...... 19.4 ...... do ...... ,...... C Do.

trans·2·Butene Flory [1] (1937)...... 22.3 ...... ".' ..... do ...... C Do. Roberts' (1949) ...... IS.4 ...... •...... do ...... C Do.

2·Methyl·l·butene Flory [1] (1937) ...... 23.6 ...... •.••...... do ...... C Do. Roberts' (1949)...... IS.7 ...... do ...... C Do.

cis·2·P entene Flory [I] (1937) ...... 23 .1...... do .....· ...... C D o. Roberts' (1 949)...... 19.2 ...... do ...... C D o.

trans·2·Pcntcnc Flory [1] (1937) ...... 23.1...... do ...... C Do Roberts' (1949) ...... 18. 1...... do ...... C D o. See footnotes a t end of table.

222 Journal of Research ------~

TABLE 1. B eato of polymerization- Continued

Expcri ~ mental States of monomer (E) 01' Compound -ilHp · and polymer Calcu· Method and comment lated (C) b I

VINYL ALKYLS-Continned

2,3·Dimetbyl·l·butene kcal/mole of monomer Flory [I] (1937) ...... 23 .1...... Gas->gas ...... ------C neats of formation. Roberts' (1949) ...... _...... lS.4 __ ...... do ...... C Do.

l·Heptene Flory ll] (1937) ...... ___ ...... 2.1.6 __ ... _...... do ...... C Do. Roberts' (1949) ...... _...... 20.5 ...... _...... do ...... C Do.

VINYL ARYLS

Styrene F lory [I] (1937) ...... __ ...... __ ..... 20 ...... Gas~ gas ...... c II eats of formation. Roberts' (1949) ...... 19 ...... do ...... C Three kcal less than for ethylene [1]. Lusehinsky [45] (l93S) ...... 13.S .... __ ...... __ .. Liquid ~so 1id ...... E D eats of combustion, fractionated polymer. Roth & Ri st·Sc huma e h~r [2J (1942) ..... 20.2' ...... Liquid ~ liquid ...... E Ileats of co mbustion . P erhaps tho value is per mole of dimer. Votinov, et al [46] (1942) ... __ ...... 21.9 __ ...... LiQuid ~ so 1id ...... E Ileats of combustioll . Goldfinger, et al [47J (1943) ...... 15.0 (70° to 140° C) .•.... Liquid ~ so luti o n (35 to E Adiabatic calorimeter. S5%) in monomer. Ferguson , et al [4SJ (194 5) ...... 17.2 ...... __ .. Liquid ~ so luli on (12%) E Do. in monomer. Tong & Kenyo n [49J (1947) ...... 16.3 to IS.0 (76.8° C) ..... Liquid ~ solid __ ...... E I so thermal calorimeter. Do ...... __ .. ____ . __ . __ ...... 16.1 (76 .So C) ..... __ ...... do ...... E Extrapolated to zero catalyst . Roberts, et al [50J (1947 ) ...... __ ... 16.7 ...... __ ...... __ ... do ...... E D eats of co mbustion. Do ...... __ ...... 17.5...... Liquid ~ so lution (7%) E Heat of sol li t ion (-il J1S=O. 6) . in monomer.

Indene Roth & Rist·Schumaehcr [2J (1942) ..... 16 .9' .. """ __ """'" Liquid ~ so lid ...... E H eats of combustion. P erhaps the value is per mole of dimeI' . alpha Methylstyrene Roberts & Jessup [I i] (i94S) ...... S.S to 10.1...... do ...... E Heats of combustion; fractionated polymer, average D . P . 15 to 7.5, respectively. para·Ethylstyrene Tong & Kenyon [49] (1947) ...... 16.4 to 16.9 (76.So 0) ...... do...... E I so thermal calorimeter. D o ...... __ ...... __ . 16.3 (76.8° 0) ...... do ...... __ .. . E Extrapolated to zero catalyst.

ortho·ChlorostYl'cne Tong & Kenyon [49] (1947) .. __ ...... 16.5 to 17.6 (76.So 0 ) ..... __ ... do ...... E Isothermal calorimeter. '00. __ ... __ ...... 16.4 (76 .So 0 ) ...... •. __ ...... do ...... E Extrapolated to ze ro catalyst.

para·Chlorostyrene Tong & Kenyon [49J (1947) ...... 16.2 to 17.5 (76.8° 0 ) .. _...... do ...... E Isothermal calorimeter. D o.... __ ...... __ ._ ...... 16.0 (76.So 0 ) ...... do ...... E Extrapolated to zero catalyst.

2,5·Diehlorostyrene Tong & Kenyo n [49] (19ti) ...... 16.8 to IS.0 (76.So 0) ...... do .. __ ...... E Isothermal ealori meter. Do .. __ ...... _...... 16.5 (76.So C) ...... do ... " ...... E Extrapolated to zero catalyst. Anethole Stau

OTHER VINYLS

Ethylene oxide b Polym.r may be crystalline . Staudinger & Sch J ~ pfer [51] (1939) __ .... 22.6 ..... ____ ...... Liqll i d ~ so l id .. __ ..... __ Heats of combust ion. Acrylonitrile h Polymer may be cr ystalline. Tong & Kenyon [41 ] (1947) ...... 17.3 (i6.8° 0) ...... do ...... E Isothermal calorimeter; extrapolated to zero catalyst . Vinyl acetate Tong & Kenyoll [41] (1947) •...... 21.3 (76.So 0) ...•..••..•...•.. do ...... E Isothermal calorimeter; not dependent on catalyst concentration. See footnotes at end of table.

Heats of Polymerization 223 TABLE 1. Heats of polymerization - Continued

Experi­ mental States of monomer (E l or Compound -t;lIp' and polymer Calcu­ Method and comment lated (C) b

OTHER VINYLS-Continued

Vinylidene chloride b kcallmole oj monomer Polymer is largely crystalline [39, 40] . Tong & Kenyon [41] (19+7) ______14.4 (76.8° C) ______Liqllid~ so lid _ E [sothermal calorimeter; extrapolated to zero catalyst. Vinyl butyl ether b Shostakovskii & Bogdanov 152] ( 1942) __ 14.4 (4 0° to 60° C) ______Liquid ~li quid g ______E Polymerized with alco holic FeCIs.

VINYL ACIDS AND ESTERS

Acrylic acid Staudinger & SchHipfer [51] (1939) __ _ 15.0______Liquid ~ solid g ______E Heats of combustion. Evans & Tyn'all [4] (l9n) ______18.5 ______Solution 5 ml in 100 ml E Calori metric. H,O. Mcthacry lie acid Evans & 1'yrrall [4] (1947) ______15.8 ______Solution 10 ml in 100 lUI E Do. H ,O. Methyl acrylate Evans & Tynall [4] ( 1947) 20.2 ______Solution 50 ml in 50 ml F. Do. EtOH. Tong & K en yon [41] (1947) ______18.7 (76.8° C) ______Liqnid ~ solid ______E lsothermal calorimeter; not corrected for unrcacted monomer; 0.01% benzoyl perox ide. Methyl methacrylate Tw ai [53] (1946) ______16.3 ______do g ______E Heats of com bustion. Tong & Kenyon [54, 55] ( 1945-46) ______13.0 (76.8° to 110° C) ______do ______E Isothermal calorimeter. Tong & Kenyon [54] (1945) ______13.1 (76.8° C) ______Solution (5 1%) in CCI, . E Do. Do______13.6 (76.8° C) ______Solution 26 to 32% H 20, E I sothermal calorimeter; po lymer insoluble 26 to 32% MeO H. in MeOH- H 20 . Kunst & Magat [56] (1947) ______10 to 13 ______F. Evans & 'i'yrall [4] (194 7) ______12.9 ______Emulsion 25 ml in 100 E Calorimetric. mIH,O. Do______20 ______Gas ~ gas ______C F. stimated fr om iso butene, head-tail, no sterie hindrance between Side_groups. Ethyl methacrylate Twai [53] (1946) ______14. 1. ______Liquid-+solid • ______E Heats of com bust ion . normal-Butyl methacrylate Tong & Kenyon [55] (1946) ______13 .5 (i6.8° C) ______do ______E Isothermal calorimeter. Phenyl methacrylate Tong & Kenyon [55] (1946) ______12.3 (76.8° C) ______do ______E Do. Cyclohex yl methacrylate Tong & l{enyo n [55] (lD46) ______12.2 (i6.8° C) ______do ______E Do. Benzyl methacrylate Tong & Kenyon [55] (1946) 13.4 (i6.8° C' ) ______do ______E Do.

DIENES

1,3-Butadiene Lebede\', et a l [31] (935)_-- ___ ------18.1 to 38.4. ______Gas ~ s o l id ------l E {H eats of combustion; several polymers, Do ______12.2 to 32.5 ______Liqnid-+solid ______formed in different ways. Flory [I] (1937) ______19.9 ______Gas~ gas______C H eats oC formation, 1,2-poiymerization. Roberts' ( 1949) ______17.4 ______do ______C H eats of formation, 1,2-poly merization. Flory [I ] (1937) ______20A ______do ______C- lIeats of format ion, 1,4-polymerization. Roberts. (1949) ______18.7 ______do ______C Heats of form ation, 1,4-polym erjzaJ.ion. Roberts & Jessup 15i] (1948); Prosen & Ross ini [5] (1945) ______23 (prcliminary) ______Gas ~so Jid ______E Heats of com bustion. Do ______18 (preliminary) ______Liquid ~ sofid ______E D o. I ,~-Cyc l o rentacliene BallI' & Frater [5S] (941 ) ___ ,------8.7' (149° to 185° C) _____ Gas ~ gas g ______E Equilibrium co nstant, dissociation at COll­ stant volume. Cyclic dimeI'. 1,4 -Pentaclien c Flory [I] ( 19:li) ______24.1. ______do ______C Heats of formation , 1,2-polymerization. Roberts' ( 1940' ______20.7 ______do ______C H eats of [ormation, 1,2-polymerization. See footnotes at eud of table.

224 Journal of Research TABLE 1. H eats of polymerization- Continued

E xpcri­ mrnLal States of monOlncr (Elor Compound -/H-Jp · aDd poly mer Calcu­ Method and commen t later\ ( 0 ) b

DIEN E S-Continned

Isoprene kca l/mole of monomer Jessup [59J (1938) ______24.1. ______GaS ~ SOJid ______} E [Heats of combustion; polymer is purified Do ______17.9 ______Liqnid ~solid ______j nat ural rubber. Staudinger & SchUipfer [51J ( 1939) ______21. 6 ______do g______E Heats of combustion. Evans & T yrrall [4 J (1947) ______16. 1 (1 6.9) ______GM~gM ______C Heats of formation, no sterie hindrance between side groups. Roberts ! (1949) ______16.9 (17.7) ______do______C Heats of formation, no steric hindrance between side gro ups.

COPOLYMERS

Butadiene-styrene Jessup [60J (1944 ) ______17 to 18 ______Liquid, l iquid ~ so lid ___ _ IlI ea ts of co m bustion, 23% (w t) styrene. Roberts & Jessup l50,61J ( 1947, 1944); Prosen & Ross ini [5J (1945) ______17.9 ______d o ______E lIea ts of co mbustion, 25.5% (wt) styrene. Vinyl acetate-maleic anh ydride Tong & Kenyon [29J ( 1948) ______20.2 (76.So C) ______Solu tion of m o norn e rs~ E Iscihermal calorimeier; J: 1 copoly mer in­ so lid . so lu ble in t he system. Isopropenyl acetate- maleic anhydride Tong & Kenyo n [29J ( 194S) ______17.S (76. ° 0) ______. ____ do ______E Lso thermal calorimeter; 1: 1 co polymer in so lu ble in system. Vinyl ace tate-diethyl maleate Tong & Kenyo n [29J (l94 S) ______20.0 (76.So 0) ______do ______Iso ther mal calorimeter; 1:1 copolymer. Vinyl ace tate-dieth ),1 fumarate 'rang & Kenyo n [291 (l9-IS) ______I S.6 (76.So 0) ______do ______E Iso tllrrmal calorimeter; 1: 1 co polym er_

• At or ncar 25° C unless otherwise indicated. b Tbe calculated values (except those for ethylene and dimeri zation if iso bu tene) arc not the actual heats of poly meri zation which wo uld be obtained experimentall y, but apply only to hypothetical ploymers that arc free fro m steric interfe rence between substituent groups attached to the polymer chain. o *deuotcs heat of dimcl' ization. d Recalculation of Flory's values by ihe auihor was dooe in the same mann er and gives this sam€' valu e. • Recalculated by the author following F lory [1] (sec Section Ill) . f Reference [62] gives a value fo r tho heat of formation of iso pre ne, supposed to be the " bes t value" to date. It was obtained by appl yin g certain corrections to the data of Jess up [59j (which was used unchanged hy Evans and 'l'yrrall), ..ld combining_this.resul t wi th calculations by P rosen and R ossini. Using this value for iso prene, calculations like those of E vans and Tyrrall give thc values shown for -t!.ilp . • Probable states. h Sec comments in last column.

A table of structural formulas (table 2) is in­ stituent groups to th e side. The polymer chain cluded so that the reader can readily compare the would be formed by joining th ese' vinyl or diolcfin structural features of the polymers. The arrange­ groups_ Since the diagrams in table 2 represcnt ment of this table is the sam e as that of table l. only one view of the moleculc, th cy do not show The monomers are shown with the vinyl or the zig-zag nature of the polymcr chain, which IS diolefin group as th e principal portion with sub- shown here in idealized form:

Polyeth.vJ en e

Po]ybutadiene (cis- l ,4-polymcri zat.ion )

Heats of Polymerization 225 TABLE 2. Formnlas and molecular weights

R R R R Ethylene __ __ . __. __ c = c 28. 052 c = C R R H para·C h lorostyrcnc l ~8. 593 R R " Propylene ___ . ___ __ C= C 42. 078 R C> CI CR , " R R R R 2,5·DichI0l·ostyrene. c = c 173, 042 I-Eutene ____ . __ .. _ C= C 56. 104 H I CI R C,R , "CR , C> R / C I l sobutene __ ___ . . __ C= C 56. 104 R II,C CH , R " Anethole ______.. _ C= C 148.196 R R "R cis-2·Eutene _____ . _ C= C 56.104 / R 3C CR , " b OCll R ,C R R , II, trans·2·E utene __. _ C= C 56. 104 Ethylene ox ide ____ C--C 'H. 0.12 "R "c n, " 0 / CR , R R R / Acrylonitrile . .. . _. C= C 53.062 2·Methyl·l -butene. C= C 70.130 R R " c C,H , Iii " N R R cis·2·Pentel1e . . __ __ C= C 70. 130 R R / V illyl acetate .. ____ C= C 86. 088 R 3C C,H, H " 0 R 3C "/ R O= C tran s-2·Pentene. ___ C= C 70. 130 "R CR, C,H, " " Cl C II, R / R / Vinylidene chlo· C= C 96. 950 2, 3· Dimet hyl-I- C= C 84. 156 ride ______R butene ______R " CI " C II / II R II, C CII, C= C " Vinyl butyl ether ._ R 100.156 R II " 0 1-Heptene ______C= C 9R, 182 / H C,II, C SH ll " H IT R R C= C C= C Acrylic acid ______R 72. 062 Styrene ______R 104. 144 C= O "/ C>" R O CR, R R R / C= C C= C l ndene __ __ _. ______I 11 6. 154 M ethacrylic acid __ R 86.088 C= O R~--C>" RO"/ CR, IT R R I C= C C= C M ethyl acrylate ___ R 86. 088 alpha· Methylsty· R 118. 170 C= O rene. ______. __ "/ C>" II, CO CH 3 II R R / C= C C= C R Methyl methacry· R 100. 114 para-Ethylstyrene. 132. 196 late ___ . ______C= O " "/ C> H, C O C , I L CH, H II R / C= C C= C ortho . Chloros ty· IT CI 138. 593 Ethyl methacry- R 114.140 re ne. . ______late ______.. _ C= O " "/ C> H, C, O

226 Journal of Research TABLE 2. Fo rrmdas and molecular weight s ~ Co ntinu ed

CR3 11 11 R / 1,4-Pcntadieno (1 ,2- C= C 68.11 4 C= C polymerization). R Dormai-B ut yl R H2. 192 ri n methacrylate ____ '--C= O n / / C= C II,C , O H IJ CR 3 n 3C R / I n Phenyl methaery- C= C 162.180 Isoprene ______C- C 68. 114 late. n ,? '--C= O RC CD R "'" u 0 0 / R II C= C CR3 / n / M ale ic a.nh y dride ~ O= C '--C= O 98. 056 Cyclohexyl meth· C= C 168. 228 acrylate. R ~ '-- C= O 0 / / HIlC , O C EI , n / Cn 3 C= C H / Isopropenyl acc- n Benzyl methacry- C= C 176. 206 tatc ______'-- 0 100. 114 late. n / '-- C= O 0 C / '-- Cil3 Oeil,o II Jl n J[ C= C 1,3 -B utadie ne ______C- C 54. 0SH Dicthylma lcatc __ . / 172. 176 ,? O= C '--C= O H C c n / H "'" U ][,C,'--O O C,II, H lJ,C ,O C ,? '-- C= O 1,3-C y clope n t a- n c \ H c /I,-- ~Ai\ n / diene ______I H CH [ II IIHC n l II cn 66.098 Di r thyl fum arate .. C= C 172. 176 11 C 1 / n / n C,--l/E cf O= C C "'" IJ n II '-- 0 ,n, climor

Some values of h eat of polymerizaLion LhaL can auLho1' , and no con ections have been applied. be found in the literature have not been included 'The value have been rounded to the nearest 0.1 in the table for one of three reasons: (1) the kcal in some cases, sincc unce rtainLi es in general compounds arc not closely related to those listed may amount to the order of 0.5 kca1. Values of here (e. g., tricyano compounds) and involve a heat capacities, and of heats of fusion, vaporiza­ different type of reaction; condensation reactions tion, and solution are known only for a few also arc excluded; (2) the values r eported appear monomers and polymers, so i t is not often possible to be in serious error, e. g. , because of oxidation to convert values of heats of polymerization to of the material ; (3) values h ave been superseded correspond to temperatures and sLaLes other Lhan by later values by the same authors. Early values those in which the measurements were made. It reported by other authors have been included for can sometimes be assumed [3, 4] that the heats completeness, although some of these values may of fusion, vaporization and solution for the polymer be regarded as obsolete. H eats of polymerization are not very different from those for the monomer can be calculated not only from heats of combus­ because of the chemical similarity with the tion and formation, but also from activation monomer. energies and equilibrium constants. There are In making comparisons of h eats of polymeriza­ many data in the literature other than those li sted tion, values for equivalent sLates should be used, in this paper, from which heats of polymerization otherwise Lh e included heaLs of vaporization, co uld be calculated if desired, although in some fusion, and solution may result in misleading cases the uncertainty of such values will be large. values , especially if Lhese h eats arc unusually large, 'rhe states of monomer and polymer and the as in styrene [5]. For polymers that are non­ corresponding values of heats of polymerizalion crystalline or have a second-order transition with given in table 1 arc those reported by the several no heat of fusion, as in polystyrene [61, the solid Heats of Polymerization 227 state is equivalent to the liquid state for purposes The above relations are due partly to the of comparison. Strictly, comparisons of h eats of assumption of certain values for branch correc­ polymerization should be made only when the tions and apply to polymers having no steric monomers and polymers are all referred to the interference between side groups. The calcula­ same state. In converting values of heats of tions were performed as fo llows: polymerization to other states, heats of vaporiza­ The expression for heat of formation per mole tion, fusion, and solution should b e used which of a long normal saturated hydrocarbon, CnH2n+2 are for the same temperature as that which applies (n> 5,gas) [7] is: to the h eat of polymerization. The possibility of dependence of heat of solution upon concentra­ llHj029S= - 10.408 - 4.926n kcaljmole, tion should also b e considered.

= - 4.926n as n approaches 0). III. Recalculation of Flory's Values If the chain is sufficiently long, any part of it will The author of this paper has recalculated the have a h eat of formation proportional to the values given by F lory [1] for heats of ploymeriza­ length of that part. The heat of formation of a tion to hypothetical h ead-to-tail polymers having structural unit of the hypothetical polymer is no steric hindrance between branch groups then llHp(unit) = - 4.926n+ llb kcal/unit where attached to the polymer chain. The m ethod of llb is the cOl'l'ection for the bond eff ect of branches calculation was essentially the same as Flory's on the unit. The heat of polymerization is except that a later expression for heats of forma­ obtained by taking the difference in heats of tion of hydrocarbons [7] was used in calculating formation in the gas state of the polymer unit heats of formation of polym ers, and the heats of and the monomer, llHjO(m) [8,9]: llHpo= - 4.9267/ formation of the monomers were ob tained directly + llb-I:1HjO(m) kcal/mole of monomer. The un­ from r8], or for 1,4-pentadiene from [9], instead of saturated group at the end of the polymer chain calculating them. The estimated corrections for is neglected. Since the above calculations apply branching in the polymer were obtained by to saturated polymers, a correction must b e applied comparing the heats of formation of the octanes for diolefins in which an unsaturated group remains. [7] (see also [4]). These were chosen from the The same values of h eats of hydrogenation m ay available data as being more likely than shorter b e used as in Flory's calculations: + 29.95 kcal molecules to give the best approximations. for - H C = CH2 and + 27.80 kcal fo r - HC= The recalculated values are 1 to 5 kcal lower CH-. These values were obtained as the mean than those given by Flory, varying between 18 of those given in [11 ,12] for mono- and 1,2- di­ and 21 lecal/mole of monomer; there are also substituted ethylenes and red uced to 298° C by greater differences among the recalculated values. th e correction - 0.25 lecal. The calculated valu es for the diolefins are not In calculating corrections for branching, the appreciably lower than those for the monolefins. h eat of formation [7] of n-octane was compared Hypothetical polymers without steric hindrance with that of 4- m ethylhep tan e for th e correction having double-branch structures have lower heats where there is a single branch per polymer unit, of polymerization (18.1 to 19.4 lecal/mole of 3,3- dimethylhexane where there are two branch es monomer) than do those with single-branch on one of the carbons of the polymer unit, and structures (20.5 to 20.9); polymers with trans 3,4- dimethylh exane where there are adjacent structure have h eats of polymerization about 1 single branches in th e polymer unit. The follow­ kcal less than those with cis structure. The ing assumptions wer e made in api}lying the branch calculated value for 1,2-polymerization of 1,3 - corrections for hypoth etical polymers: butadiene (17.4) is much lower than that for 1. L ength of a normal alkyl substituent makes 1,4-pentadiene (20.7). The hypothetical value for little d iff erence in bond effect. (This is supported polywerization of styrene (19 kcal), obtained by by data of Kistiakowsky and coworkers [12] on use of the calculations of Kharasch [10] as was heats of hydrogenation.) done by Flory, falls about 1. 5 kcal lower than 2. Branches in the octanes mentioned are far that for th e single-branch structures. enough removed from the end carbon atoms to

228 Journal of Research avoid end effects, so that a suitable value for the The end effect will be different for each monomer branch effect is obtained by comparing these and can generally be neglected in polymers. The octanes. value of this effect is given by the deviations from 3. The application of branch correction to th e linearity with number of carbon atoms of the heats hypothetical polymer uni t is appropriate wh en of formation of the lower members of the homol­ those corrections were obtained from correspond­ ogous series and is known at present for only a ingly branched octanes. This n eglects the small few monomers, as follows [14]: bond effect of having substituents on nearby C atoms of the polymer chain, which are not present Ethylene + 2.76 kcal, in the octanes . Although certain objections can be raised to the usc of these branching corrections, Propylene 0.7, it is felt that they afford a better approximation than those available at the time of Flory's cal­ I-Butene O. 39, culations, and they were obtained from the best data available at the present time. I-Heptene O. 00.

IV. Comparison of Values and Steric For the others, it is estimated to be of the order of 0.5 le cal or less. This effect will make a small Effects difference in the values obtained in comparison of Thc heat of polymerization of vinyl compounds heats of polymerization, depending on the differ­ can be arbitrarily assigned to four energy eff ects, ence in the end eff ects of the monomers to be com­ which are not entirely separable, the second and pared. The true value of effect (1) above is 19.59 third being negative with respect to the first and k cal, obtained by deducting the end effec t in fourth. These energy eff ects are: (1) the reaction ethylen e from the value 22.35 le cal [15] . Substituent groups of larger sizes may cause 6=6 -----> -6-6- . (2) the effect of side I I I I ' greater amounts of steric interference, resulting in groups on bond energies when such side groups lower experimental heats of polymerization. are so spaced that there is no interaction between Length of the n-alkyl substituents makes little them; (3) the effect of steric hindrance between diff erence, although branch ed substituents m ay side groups attached to the polymer chain; (4) cause steric hindrance [1 2]. The substitution of the "end effect" arising from the nearness of the chlorine on the aromaLic ring of styrene has little dou ble bond to the end of the monomer molecule eff ect on th e heat of polymerization, ince the sub­ [1 3]. stituen t is well removed from the polymerizing The first and fourth effects are represented by bonds and is not likely to interfere with oLher the polymerization of ethylene, whose polymer has branch groups. In ethylen e whose polymer has no side groups and therefore no bond effect nor no steric hindrance between side groups, the steric hindrance from such groups. The' second experimental and calculated valu es of h eat of eff ect arises from the fact that the bonds to the polymerization agree within experimental error substituents and the chain bonds in the monomer (the calculated , values for ethylene were derived have different effects on each other from those they from experimental data on hydrocarbons), whereas exert in the polymer. The value of the third effect isobutene, alpha-methylstyrene, and the methac­ can be obtained by the difference between the cal­ rylates have values of heat of polymerization culated values of heats of polymerization, which about two-thirds of what would be expected if include effects (1), (2), and (4), for the hypo­ there were no steric hindrance between side thetical polymers, and the experimental values, groups. Ethylene has the highest and alpha­ which include all effects. Both calculated and meLhylstyrene the lowest h eat of polymerization experimental values are available in only a few among the compounds listed. cases; isobutene may be used as an example. The A frequ ent cause of steric interfer ence between difference between 19.2 calculated and 12.8 ob­ side groups is the presence of two substituents on served is - 6.4 lecal/mole of monomer, attributable the same vinyl carbon atom, especially if one or to steric hindrance b etween side groups. more of the substituents is a methyl group. This

Heats of Polymerization 229 can be seen by comparing the experimental values of the breaking of one C- O bond and the forming of heat of polymerization for styrene (16.7 k:cal) of another C- O bond in the chain: and alpha-methylstyrene (8.8 to 10.1 kcal). If H2 H2 . the latter values are increased by an allowance of c- c 2.2 kcal for the bond effect of the methyl group, / "- 0 - they become 1l.0 to 12.3 kcal, r espectively. (The value 2.2 kcal is obtained by taking the difference This simple C- O bond rearrangement in itself can­ bet\yeen the calculated values for I-butene and not account for the heat of polymerization. The 2- methyl- 1- butene in table l.) Effect (4) above chain is terminated by - OH groups: HO- H 2C cancels in this comparison. A substituent phenyl (H 2C- 0 - CH2) "CH2- OH, but if the average group may be able to rotate so that its plane is molecular weight is large, the contribution to the perpendicular to the direction of the polymer chain heat of polymerization from the addition of H­ thus making the steric interference less than might OH would then be very small. The explanation be expected [3] . Another example of the effect of for the heat of polymerization is to be found in disubstitution can be seen by comparing methyl the following consideration. The three-member acrylate (18.7 kcal) and methyl methacrylate ring of ethylene oxide has been regarded as a (13.0+ 2.2 = 15.2 kcal). strained stru cture [2 1] but Walsh [22 , 23J (see also Steric hindrance becomes so great in some mole­ [24, 25 , 26, 27] ) believes it advantageous to trans­ cules that polymerization becomes impossible, late much of the old idea of strain in ethylene, although in some cases a dimer may be formed cyclopropane, ethylene oxide, and other similar [3 , 16] . There is generally no steric interference molecules into terms of hybridization. H e has between side groups in dimers, but it becomes suggested that ethylene oxide contains overlapping noticeable in the trimers and higher polymers in orbitals and carbon atoms that are more nearly those polymers that show that property [4J. For trigonal (ethylenic) than tetrahedral. WIlen this actual polymers, in the absence of steric hindrance molecule is converted to a straight-chain polymer between side groups, heat of polymerization per structure containing ordinary tetrahedral C atoms, molo of monomer should increase as the polymer energy will be evolved, as with ethylene. chain becomes longer, because of the factor The eff ect of the esterification of the acrylic (I - l in), which takes account of the facL that acids is confusing. In the case of acrylic acid. there is one less bond reacting than there are esterification increases the heat of polymerization, monomer units, as shown in Jessup's calcula­ whereas the opposite eff ect occurs with meth­ tions for ethylene [15]. (The expression for hea t acrylic acid. ()f polymerization of ethylene [13 , 14 , 15] may be The heat of copolymerization of the butadiene­ written !1Hpo=-19.59 (1- 1(n) - o, wh ere 0 is styr ene mixture lies between the values for the the end effect in ethylene, 2.76 kcal.) If steric separate monomers. Copolymers having dif­ hindrance occurs in the polymer, its effect should ferent monomer ratios from those given will of increase rapidly at first as more units are added course have different heats of polymerization. to the chain, and after the first three units the The heats of copolymerization of vinyl ace taLe heat of polymerization pCI' monomer unit should with maleic anhydride, diethyl maleate, a.nd either increase more slowly or ' decrease, and the diethyl fumarate a.re lower than the heat of poly­ steric energy per unit added should approach a merization of vinyl acetate alone. In some cases, constant value as the chain lengthens. The data however, the eff ect of sterLc hindrance between for alpha-methylstyrene [17] show some of these side groups in copolymers may be diminished by eff ects; here the heat of polymerization per mole 1 : 1 alternation of the units. Tong· and Kenyon ()f monomer decreases as the molecular weight [28] have observed that with methyl acrylate and 11lcreases. methyl methacrylate the heat of copolymerization Ethylene oxide appears to be a somewhat differ­ is higher than the sum of the heats of polymeriza­ ent monomer from the others listed. The reaction, tion of the components alone. These authors [29] which differs from vinyl polymerization in being a point out that with the diethyl fumarate and stepwise addition process rather than a free radical maleate copolymers with vinyl acetate, the dif­ mechanism [18, 19, 20] involves the net difference ference in their heats of copolymerization is less

230 Journal of Research than half of the hea t of isomerization of maleic to be adap ted by subtracting 0.7 kcal for heat of fumaric es ters, suggesting th at differen t stereo­ fusion of the polypropylene ( ame as for the isomers are produced. A discussion of heat of monomer), and adding a fevI Len ths of a kilo­ copolymerization appears in r eferenee [3 0). calorie for raising the temperature from - 75° to Polymers having different structure, although 25° C. The value th en becomes about 16 kcal/ derived from the same monomer ( ee butadiene mole for liquid-liquid a,t 25 ° C. The second esti­ (31)) may have different heats of polymerization. mate, obtained from ethylene and vinylidene In a polymer or copolymer containing a diene, chloride, should b e somewhat above the midpoint there is the possibility that there will be present of the difference b etween 19.6 and 14 .0, since !? ub­ a mixture of 1,2- and 1,4-a ddition (or 3,4- in stitution of a second chlorine causes a greater isoprene), and the value of heat of polymerization steric eff ect than that of the first. The final will vary according to the ra tio [32, 33, 34, 35) . es timate for vinyl chloride might then be placed The 1,2-addition may lead to cross-linking. The at about 17 keal/mole (liquid-liquid or gas-gas, 1,4-polymers may be partly cis and partly trans 25° C). structure. References Ethylene [36, 37, 38) and vinylid ene chloride [39, 40) both form polymers that are rather highly [1] P . J . F lory, J . Am. Che rn . Soc. 59, 241 (1937). crystalline. The polymers of ethylene oxide and [2] W. A. R oth a nd E . Ri t-Schumacher, K auLschuk 18, 137 (1942). acrylonitrile also are probably crystalline. The [3] A. G. Evans and 1\ 1. Pola nyi, N at Ul" e 152, 738 (1943). apparent heat of polymerization to solid polymer [4] A. G. Evans a nd E. Tyrra ll , J . ])oly me" Sci. 2, 387 must then include a small quan tity representing (1947) . the heat of crystallization, depending on the per­ [5] E. J . Prosen a nd F. D . R ossin i, J. Resea rch NBS 34, centage of crystallinity. If we usc tb e. value for 59 (J 945) RP] 628. heat of fu sion of polyvinyliden e chloride (0. 3 ](cal) [6] R . F. Boyer a nd R . S. Spencer, J . App li ed Phys. 15, 398 (1944). given by R einhardt [39 ), tben the heat of poly­ [7] E. J. P rosen a nd F. D . 110ssini, J . R esearch KBS 34, merization in the liquid states would be reduced 263 ( 1945) RP1642. from 14.4 [41) to 14.1 kcal/mole of monomer at [ ] E. J . Prosen a nd F. D . Rossini, J . R e earch NBS 36, 76.8° C. 269 (J 946) RP1702. [9] Selected Va lue of Properties of H y lrocarbons. Table Up (Par t 1). National B ureau of SLa nd­ a rds, ·Washington 25, D . C. (F ebruar y 28, ] 949). The au thor expresses his appreciation for help­ [10] M. . Kha rasch, BS J . l1esearch 2, 359 (1929) R Nl. ful commen ts offered by R aymond F . Boyer, D . [ll] G. B . Kistiakowsky, J . R. Ruhofl' , I-I. A. Smi th, a nd R. St ull, L. K. J. Tong and W . O. K enyon, R alph W . E. Vaughan, J . Am. Chem . Soc. 57, 76 (1935). S. J essup , E d"Iard J . Prosen, and Leo A. Wall. [12] G. B. Kistiak ow ky, J . R. R uhofl' , H . A. Smi th, a nd W . E. Vaughan, J . Am. Chem. Soc. 58, 137 (1936). It has been brought to our attenLion that there [1 3] E. J . Prosen, private com muni cation. is no reported valu e for the heat of polymerization [1 4] E. J . Prosen, ' V. H . J o hnson, a nd F. D . R os in i, of vinyl chloride. An estimate of the value can J . R esearch NBS 37, 51 (1946) RPI728. be made by considering its structure and com­ [15] R. S. J essup, J . C hem. Phys. 16, 661 ( 1948). p aring with other values in the table. Not [16] V. V. K orshak a nd K . K . Samplanskaya, D oklady Akad. Nauk SSSR 59, 497 (1948) . (Chem. Ab­ enough data are available for an accurate pre­ stracts 42, 6203 (1948». diction, but at least a rough idea can be obtained. [17] D. E . R ober ts and R . S. J essup, 19'18, Lo be pub- Assuming that the end eff ects are negligible in all li shed. cases, and that the h eats of vaporiza tion arc the [18] H . Moureu and M. D ode, Bu!. oc. chi m. [5 ] 4, 637 (1 937) . same for the monomers as for their polymers, [19] P . J. F lory, J . Am. Chem. Soc. 62, 156J (1940). vinyl chloride should be between eLhylene (19.6 [20] S. J)erry a nd H . Hi bbert, J . A m. ('hem. Soc. 62, 2599 kcal/mole, no end eff ect, gas-gas) and v inyliclcne (1940) . chloride (at 25°, about 14.0 k cal/mole, liquicl­ [2 1] 1[. de V. Robles, Hecueil des Travaux C hi miq ues des liquid), and somewhere near propylene si nee Lhe Pays-Bas 59, J84 (1940) . [2 2] A. D . Wa lsh, Nature 159, 165 a nd 71 2 (1947). bond and steric effects of Cl arc similar to Lhose for [23] A. D. Walsh, Trans. F ara day Soc. 45, 179 (1949) . Cl Ia. T o obtain an estimate from Lhe daLa of [24] T. M . Sugden, Nature 160, 367 (1947) . Fontan a and Kidder for propylene, Lheir value may [25] P . Karrer, H elv. Chim. Acta. 30, 1780 (~ 9 4 7) .

Heats of Polymerization 231 [26] C. L. Arcus, Chern. and Ind. 1947, 646. (45) W. von Luschinsky, Z. physik. Chen1. :\.182, 384 [27] C. A. Coulson and W. E. Moffitt, Phil. Mag. 40, I (1938) . (1949) . [46] A. Vot inov, P . Kobeko, and F. Marel, J . Phys. Chem. [28] L. K. J . Tong and W. O. Kenyon, unpublished, (USSR) 16, 106 (1942) . Chem. Abstracts 37, 5304 private comunication. (1943) . [29] L. K . J . Tong and W. O. Kenyon, J. Am. Chern. Soc. [47] G. Goldfinger, D . Josefowitz, and H. Mark, J . Am. 71, 1925 (1949). Chern. Soc. 65, 1432 (1943) . [30] T . Alfrey, Jr., and C. Lewis, J . Polymer Sci. 4, :221 [48] W. J . F erguson, I. C. Schoonover, and F. G. Brick­ (1949) . wedde, National Bureau of Standards 1945, un­ [31] S. V. Lebedev, G. G. KoblyanskiI, M. A. Khokhlov­ published. kin, N . I. Kulbina, and M. M. Gol'dman, Trudy [49] L. K. J. Tong and W. O. K enyon, J . Am. Chem. Gosudarst. Opyt. Zavoda Sintet. Kauchuka Litera Soc. 69, 1402 (1947) . B. IV. Synthetic Rubber 1935, 46. (Chem. [50) D. E. Roberts, W. W. Walton, and R. S. Jessup, Abstracts 31, 6958 (1937)). J. Research NBS, 38, 627 (1947) RP1801. Also {32] A. Saffer and B. L. Johnson, Ind. Eng. Chem. 40, J. Polymer Sci. 2, 420 (1947). 538 (1 948). [51] H. Staudinger and P. Schlapfer, unpublished data, [33] H . L. Dinsmore and D. C. Smith, Anal. Chern. 20, quoted in chapter by G. V. Schulz in Chemie und 11 (1948). Technologie der Kunststoffe, edited by R. R ou­ [34] A. W. Meyer, Ind. E ng. Chern. 41, 1570 (1949). See wink, Vol. 1, p. 60, Table 5, Leipzig, (1942); or also' E. J Hart and A. W. Meyer, J . Am. Chern . (one vol. ) p. 79, Table 6, Leipzig (1939). Soc. 71, 1980 (1949) . [52] M. F. Shostakovskil a nd I. F . Bogdanov, J . Applied [35] I. M. Kolthoff, T. S. Lee, and M . A. Mairs, J . Polymer Ch ern. (USSR) 15, 249 (1942). (Chen1. Abstracts Sci. 2,220 (1947) . 37. 2486 (1943) ). [36] E. Hunter and W . G. Oakes, Trans. Faraday Soc. [53] S. Iwai, J . Soc. Chem. Ind. (Japan) 49, 185 (1946). 41, 49 (1945). (Ch e rn . Abstracts 42, 6161 (1948)). [37] H. C. Raine, R. B. Richards, and H. Ryder, Trans. [54] L. K. J. Tong and W. O. Keynon, J. Am. Chem. Soc. Faraday Soc. 41, 56 (1945). 67,1278 (1945). [38] G. S. Parks and J. R. Mosley, J . Chem. Phys. 17, [55] L. K. J . T ong and W. O. Kenyon, J . Am. Chem. Soc. 691 (1949). 68. 1355 (1946). [39] R. C. Reinhardt, Ind. Eng. Chem. 35, 422 (1943). [56 E. K unst and M. Magat, Com pt. rend. 225, 499 [40] W. C. Goggin and R. D . Lowry, Ind. Eng. Chern . . (1947). 34, 327 (1942). [57] D. E. Roberts and R. S. Jessup, 1948, to be published. [4 1] L. K. J. T ong and W. O. Kenyon, J . Am. Chem. Soc. [58] E. Baur and S. Frater, R elv. Chim. Acta. 24, 768 69,2245 (1947) . (1941) . [42] c. M. Fontana and G. A. Kidder, J. Am. Chem. Soc. [59] R . S. Jessup, J . Research, N BS 20, 589 (1938) 70,3745 (1948). RP1093. [60] R. S. Jessup, Preliminary Report, 1944, unpublished. [43] R. M . Thomas, W. J. Sparks, P. K. Frolich, M. Otto, [61] D . E. Robert s, R. S. Jessup, 1944, unpublished. and M. Mueller- Cunradi, J. Am . Chem. Soc. 62, [62] J . E. Kilpatrick, C. W. Beckett, E. J . Prosen, K. S. 276 (1940) . Pitzer, and F. D. Rossini, J . Research N BS 42, {44] B. A. Kazanskii and M . I. Rozengart, J. Gen. Chem. 225 (1949) RP1964. (USSR) 12, 246 (1942). (Chern . Abstracts 37, 3045) (Hl43)). ,\VASHINGTON, October 6, 1949.

232 Journal of Research