Values of ΔG for the polymerization of lactams as a function of the number of atoms in the ring

604

A . K . BONETSKAYA and ~. M. SKU~ATOV CONCLUSIONS

(1) I t h a s b e e n e s t a b l i s h e d t h a t t h e p o l y m e r i z a t i o n of o c t a m e t h y l c y c l o t e t r a siloxane u n d e r t h e action o f stannic chloride t a k e s place a t t e m p e r a t u r e s u p t o 270°C a t a n a p p r e c i a b l e r a t e o n l y in t h e presence of water. (2) I n t h e absence o f water, t h e chemical r e a c t i o n b e t w e e n m e t h y l s i l o x a n e s a n d SnC14 t a k e s place a t a considerable r a t e o n l y a t t e m p e r a t u r e s o f 270°C a n d a b o v e , m e t h y l c h l o r o s i l o x a u e s , m e t h y l t i n t r i c h l o r i d e and" also m e t h y l c h l o r i d e being f o r m e d p r e d o m i n a n t l y . (3) T h e w a y in w h i c h t h e yield o f t h e final p r o d u c t s d e p e n d s on t h e t i m e o f h e a t i n g a n d t h e c o m p o s i t i o n o f t h e initial m i x t u r e h a s b e e n o b t a i n e d , a n d m a k e s it possible t o select t h e o p t i m u m conditions for s y n t h e s i z i n g t h e m e t h y l tintrichloride. (4) A possible m e c h a n i s m has b e e n discussed for t h e r e a c t i o n b e t w e e n m e t h y l siloxanes a n d stannic chloride. Translated by G. F. MODLEN REFERENCES

1. K. A. ANDRIANOV and S. Ye. YAKUSHKINA, Vysokomol. soyed. 2: 1508, 1960 (Not translated in Polymer Sci. U.S.S.R.) 2. K. A. ANDRIANOV and V. V. SEVERNYI, Izv. Akad. Nauk SSSR, Otd. khim. nauk, 82, 1963 3. K. A. ANDRIANOV and A. I. PETRASHKO, Dokl. Akad. Nauk SSSR 131: 561, 1960 4. V. V. YASTREBOV and A. I. CHERNYSHEV, Zh. obshch, khimii 37: 2140, 1967 5. M. P. BROWN and D. E. WEBSTER, J . Phys. Chem. 64: 698, 1960 6. K. MOEDRITZER and J. R. Van WAZER, J. Amer. Chem. Soc. 86: 802, 1964 7. H. SCHMIDBAUR and W. FINDEISS, Angew. Chem. 76: 753, 1964 8. W. NOLL, Chemic und Technologie der Silikone, Weinheim, 139, 1960 9. B. N. DOLGOV, N. F. ORLOV and M. G. VORONKOV, Izv. Akad. Nauk SSSR, Otd. khim. nauk, 1408, 1959

VALUES OF A G FOR THE POLYMERIZATION OF LACTAMS AS A FUNCTION OF THE NUMBER OF ATOMS IN THE RING* A. K . BO~ETSKAYA a n d S. 1~I. SKURATOV (dec.) M. V. Lomonosov State University, Moscow

(Received 23 February 1968) THERE are many examples of a sharp change in the reactivity of heterocyclic compounds when the number of methylene groups in the ring is changed. For example, it has been known * Vysokomol. soyod. A l l : No. 3, 532-537, 1969.

Values of AG for polymerization of lactams

605

for a long time t h a t whereas seven- and eight-membered lactams polymerize in the temperature range 200-240°C when small quantities of water are added, five- and six-membered lactams completely fail to polymerize under these conditions. At the present time one cannot consider the following question as solved: to what extent is this difference a consequence of thermodynamic causes, and to what extent does it arise from the kinetic conditions created during the course of these reactions? The present article is devoted only to the thermodynamic aspect of this question. The first a t t e m p t to give a thermodynamic description of the reversible reaction ring polymer was made by Strepikheyev in 1950 [1]; unfortunately, this work was not widely published. The author undertook a sound and rigorous approach to the thermodynamic discussion of the problem of the polymerization of ring compounds. Using the very incomplete and partly contradictory experimental material existing at that time, Strepikheyev gave a quantitative discussion of certain examples of the comparative thermodynamic stability of rings and the linear polymers corresponding to them, and drew conclusions about the reasons for the ability of some rings to polymerize, and about the inability of other rings to do so. A similar discussior~ on the same problem was carried out independently in 1955 by Dainton e t a l . [2]. They assessed values of zig for the polymerization of cyclanos with the number of atoms in the ring varying from 3 to 8 and for methyl- and 1,1-dimethyl-substit ut ed cyclanes with the number of atoms in the ring varying from 3 to 6. The calculation referred to a hypothetical reaction occurring at 25°C and involving the transformation of i mole of the monomer (in the liquid state) into the corresponding unit of the linear polymer (also in the liquid state). In calculating the values of AG of polymerization, use was made of values of 3 H of formation existing in the literature or assessed by the authors, together with values of absolute entropies for the corresponding monomers and polymers. The values of A H of polymerization wore found from the difference between the heats of formation of the polymer unit (this quantity was calculated from the contributions of the corresponding gToups to the heat of formation) and the experimentally determined heats of formation of the cyclanes. The values of zlS of polymerization were found from the difference between the absolute entropies for the polymers ~per unit) and for the corresponding eyclanes. The autors used experimentally determined values of absolute entropies [3] for cyclanes up to cyclohoxane, and the values for cyclohoptano and eyclooetane, for which there were no experimental data, were assessed by extrapolation.* On the basis of the data obtained for AG of polymerization, the authors [2] were led to the conclusion t h a t all the compounds in the series o f u n su b st i t u t o d cyclazles from cyclopropane to cyelooctano, apart from cyclohoxano, are thermodynamically unstable with respect to the corresponding polymer (values of z i g of polymerization are negative). Fo r mono- and 1,1-dimothyl-substituted eyclanos the values of ~ A G are positive for 5- and 6-membered cyclanos, and negative for 3- and 4-membered cyclanos. Small [5] a t t e m p t e d to extend the calculations made for eyclanos [2] to a number of unsaturated heterocyclic compounds, in the first instant to those in which the hetoro-atom did not differ too much from the carbon atom in size and bond angle (for example, cyclic * The values of absolute entropies determined experimentally in 1956 [4] for cycloheptano and cyelooctane differ considerably from those found by extrapolation [2]. This difference gives rise to a change in the calculated values of AG of polymerization: for cycloheptano this is ~ 1 kcal/mole, and for cyclooctano, ~ 3 keal/mole. However, even this largo difference does not materially affect the qualitative conclusions which were made by Dainton e t a l . [2].

606

A . K . BONETSKAYAand S. M. SXU~ATOV

esters or imines). The author showed that the difference between the values of AS of polymerization for cyclanes and for the cyclic esters or imines corresponding to them (in the number of atoms in the ring) was small. A larger difference was found in the value of ztH of polymerization. However, in the author's opinion the general trend of the dependence of .4G of polymerization on the number of atoms in the ring remained similar to that established for cyclanes [2] in the case of these heterocyclic compounds. Values of AG of polymerization under standard conditions have been calculated for lactams with the number of atoms in the ring varying from 5 to 8 [6]. The values of AH of polymerization were calculated by the authors from the difference between the experimental data for the heat of combustion of the cyclic lactams [7, 8] and the data for the corresponding polymers (per monomer unit) [6]. In view of the impossibility of obtaining experimental data about the heats of combustions of polypyrrolidone and polypiperidone, the author determined values of these heats of combustion by extrapolation of the data for polycaproamide and polyenanthoamide. Values of AS of polymerization were found as the difference between the experimentally measured [6] absolute entropies of the polymers (per monomer unit) and those of the corresponding lactams. The absolute entropies for polypyrrolidone and polypiperidone were also calculated by extrapolation of the corresponding data for polycaproamido and polyenanthoamide. The authors showed that the values of AG of polymerization under standard conditions was very small: it was negative for e-caprolactam and ~-enantholactam and positive for a-pyrrolidono and a-piperidone. There are no other papers in the literature in which the different polymerization capacities of heterocyclic compounds of a particular series are discussed from the thermodynamic point of view. In particular, in the case of lactams with more than eight atoms in the ring, not only are there no values of zig of polymerization for these compounds, but values of ztH have been determined only for 9- and 13-membercd lactams [9, 1OJ.

The p r e s e n t p a p e r s u m m a r i z e s the d a t a a b o u t t h e values o f A H , AS and AG for t h e p o l y m e r i z a t i o n of l a c t a m s w i t h f r o m 5 to 13 a t o m s in t h e ring. Values of A H of polymerization for lactams. The e n t h a l p y of the polymeriz a t i o n process m a y be f o u n d b y various m e t h o d s . I n certain cases, it m a y be successfully m e a s u r e d b y the direct calorimetric m e a s u r e m e n t of p o l y m e r i z a t i o n process [11-14]. I t is generally n o t difficult t o assess the a c c u r a c y of the results t h e r e b y o b t a i n e d or to define clearly t h e therm o d y n a m i c conditions to w h i c h the m e a s u r e d value of A H refers. U n f o r t u n a t e l y p o l y m e r i z a t i o n t a k e s place, as a rule, u n d e r conditions w h i c h m a k e calorimetric m e a s u r e m e n t s difficult, a n d the a c c u r a c y of the results o b t a i n e d is generally low. The second m e t h o d of m e a s u r i n g A H of p o l y m e r i z a t i o n is also e x p e r i m e n t a l b u t indirect; it is t o calculate A H from a c o m p a r i s o n of the heats of c o m b u s t i o n o f t h e m o n o m e r a n d p o l y m e r (the l a t t e r being calculated per m o n o m e r unit). T h e a c c u r a c y of m e a s u r e m e n t in these cases is also low, firstly because t h e small q u a n t i t y A H of p o l y m e r i z a t i o n is calculated as t h e difference between two large quantities e x p e r i m e n t a l l y determined, a n d secondly, because it is difficult to characterize with t h e r m o d y n a m i c rigour the p o l y m e r specimen being b u r n t . Finally, the t h i r d m e t h o d of d e t e r m i n i n g A H of p o l y m e r i z a t i o n is as follows: since the reaction o f t r a n s f o r m i n g the ring into a linear p o l y m e r is n o t a c c o m p a -

Values of AG for polymerization of lactams

607

nied b y the formation of new t)Tes of bond or a change in their total number, we m a y consider that the polymerization of lactams will be principally determined b y the "enthalpy" of cyclization* of the corresponding lactam. In this way, the enthalpy of polymerization m a y be calculated as the difference between the experimentally determined heat of combustion of the lactam and the heats of combustion calculated additive!y from the contributions of the appropriate groups. This method of calculation also cannot lead to accurate results, firstly, because this approach to the assessment of AH of polymerization has an undoubtedly approximate nature in this case, and secondly because the results of the calculation will be different depending on what contributions for the appropriate groups are used in calculating the enthalpy of cyclization. Table 1 shows the values of AH for the polymerization of lactams having from 5 to 13 atoms in the ring, obtained b y the various methods. Values of AH of polymerization for lactams measured experimentally at 230°C b y means of a double calorimeter with a compensating method of measurement are shown in column A. t TABLE 1. E N T H A L P Y OF P O L Y M E R I Z A T I O N OF LACTAMS

I Number Laetam

atoms

inOrfing ~-Pyrrolidone ~-Piperidono e-Caprolaetam ~-Enantholactam 1/-Capryllactam Pelargonlactam Caprinlactam Undocanlactam Laurinlactam

A, AH, Refer- B, AH, kcal/ /mole

ence

5

6 7" 8 9 10 11 12 13

--3"3 --5'2 --7"8

[12] [13] [9]

kcal/ /mole 1.3 --1.1 --3.0 --5.7

Reference

[6, 7]

[6, 7] [6, 7]

[6, 7]

C, AH, kcal/ /mole --1'1 --2'2 --3"8 --5"3

Refer - D, AH, kcal/ once /mole

[7] [7] [7] [7]

0.1 --1.7 --3.3 --5-4

--5-6 --2'8 0'5

~0

[9]

Column B gives values of AH for the polymerization of lactams determined b y the second method, that is, b y comparing the experimentally measured heats of combustion of the lactams with the heats of combustion of the corresponding polymers. * We u n d e r s t a n d by the term " e n t h a l p y of eyclization" the difference between the enthalpy of the cyclic compound and the enthalpy of the compound with an open chain having the same n u m b e r of bonds of a n y given type. t The value of AH of polymerization for the 13-membered lactam has boon provisionally taken b y us as zero. I t was established practically [9] in studying the polymerization of" a 13-momborod lactam that the enthalpy of the reaction was very much less t h a n for the remaining laetams studied, and the sensitivity of the calorimeter used by us did not. make it possible to determine the enthalpy accurately.

608

BO~-ETSKAYAand S. M. SKURATOV

A.K.

Values of ZIH of polymerization calculated from the enthalpy of cyclization of the lactams are shown in column C. The enthalpy of cyclization for lactams with from 5 to 8 atoms in the ring refers to 75°C and to the liquid state of the lactams. (The values of the contributions used in the calculation were 155.85 keal/mole for the CH2-group, and 79.4 kcM/mole for the CONH-group). In the case of lactams with 10, 11 and 12 atoms in the ring, the enthalpy of cyclization refers to the solid state of the lactams, because data about the heat content of the lactams in the temperature range 20-75°C are lacking. (The values of the contributions used in the calculation were 156.3 kcal/mole for the CH2-group and 77.2 kcal/mole for the CONH-group). One would expect t h a t the values of AH of polymerization determined by the three methods mentioned should turn out to be different depending on the temperature and phase state of the monomers and pol)nners to which t h e y refer. However, as m a y be seen from the data shown in Table 1, the values of ZIH of polymerization for the lactams obtained by the different methods agree closely with each other. I t is therefore entirely permissible to adopt the average values of these quantities in calculating the values of zig of polymerization. These are shown in column D.

A H, kcGl/mo/e I 0

.

.

.

.

.

/ /

.

.

--

----

/

-i

/

2O

/

/

/ /

-5

I

I

I

I

I

5

7

9

ii

13

Fro.

1

n

5

7

/

I

T

I

9

II

13

n

Fro. 2

Fxo. 1. Enthalpy of polymerization of lactams as a function of the ntrrnber (n) of atoms in the ring. Fie. 2. Values of A S under standard conditions for the polymerization of lactams as a function of the number (n) of atoms in the ring.

Figure 1 shows values depending on the number of polymerization rises on to zero on going from the

of AH of polymerization for 5-13-membered laetams of atoms in the ring. As m a y be seen from Fig. 1, AH going from the 5- to the 9-membered lactam, and falls 9- to the 13-membered lactam.

Yaloes of AG for polymerization of lactams

609

Estimation of AS of polymerization of lactams (under standard conditions). Values of t h e s t a n d a r d a b s o l u t e e n t r o p i e s for 5-, 6-, 7- a n d 8 - m e m b e r e d l a c t a m s a n d p o l y c a p r o a m i d e a n d p o l y e n a n t h o a m i d e h a v e b e e n d e t e r m i n e d [6] on the basis of m e a s u r e m e n t s of t r u e h e a t capacities a t low t e m p e r a t u r e s . T h e values of the a b s o l u t e e n t r o p i e s for p o l y p y r r o l i d o n e a n d p o l y p i p e r i d o n e were calculated in this p a p e r , as m e n t i o n e d above, b y e x t r a p o l a t i o n . On t h e basis of these d a t a , t h e a u t h o r s calculated values of AS for h y p o t h e tical p o l y m e r i z a t i o n of these l a c t a m s u n d e r s t a n d a r d conditions. T h e s e values are s h o w n in T a b l e 2. T h e r e are no d a t a in t h e l i t e r a t u r e for t h e e n t r o p i e s of l a c t a m s w i t h f r o m 9 to 13 a t o m s in t h e ring, nor to t h e corresponding p o l y m e r s . H o w e v e r , on the basis of t h e f a c t t h a t t h e values of AS o b t a i n e d in reference [6] for the p o l y m e r i z a t i o n of 5- to 8 - m e m b e r e d l a c t a m s l a y a p p r o x i m a t e l y on a s t r a i g h t line, * we considered it possible to assess values of AS of p o l y m e r i z a t i o n for l a c t a m s w i t h f r o m 9to 1 3 - m e m b e r e d rings b y linear e x t r a p o l a t i o n of t h e values o b t a i n e d in reference [6]. T h e values of AS f o u n d in this w a y are s h o w n g r a p h i c a l l y in Fig. 2 a n d are g i v e n in T a b l e 2. TABLE

2 . CHAIXIGE

in AG ) ' O R

NUMBER

Number of atoms in lactam ring 5 6 7 8 9

l0 11 12 13

T H E P O L Y M E R I Z A T I O : N O F L A C T A M S W I T H C H A N G E I1% TH]b3

OF A T O M S I N

THE

AH,

AS,

kcal/molo 0.1 --1.7 --3.3 --5.4 --7.8 --5"6 --2.8 0-5 0

RING. FROM

5 TO

13

TAS,

AG,

entropy units

kcal/mole (T:298.16°K)

kcal,/mole

--7.3 --6"6 1"1 4"0 10 15 20 25 30

-2.2 -1.8 0.3 1.2 3-0 4.5 6.0 7.5 9.0

2.3 0.1 -3.6 -6.6 -10.8 -10.1 --8.8 --7.0 --9.0

I I

O b v i o u s l y these values are v e r y a p p r o x i m a t e , b u t t h e p r o b a b l e error in t h e i r a s s e s s m e n t is of little consequence in the discussion g i v e n below.

Assessment of values of AG of polymerization for lactams under standard conditions. T a b l e 2 gives t h e values of AH a n d AS discussed a b o v e for the p o l y m e r i z a t i o n of l a c t a m s with f r o m 5 to 13 a t o m s in the ring, a n d values of AG calculated for the corresponding reactions f r o m t h e m . * I t is interosting to noto that in tho case of cyclanes with from 3 to 8 atoms in tho ring (apart from cyclohexano) the valuos of AS of polymerization also dopond linearly on the num~ bor of atoms in the ring.

610

A. K. B O ~ T S ~ Y A and S. M. SK~ATOV

As may be seen from Table 2, the values of AG of polymerization change from positive (for the 5-membered lactam) and close to zero (for the 6-membered lactam) to negative values which reach a maximum for 9- and 10-membered lactams, and then somewhat decrease. The value of AG of polymerization is chiefly determined by the value of A H in the case of 7- and 8-membered lactams. Also, in the case of large rings the entropy term makes a considerable contribution .to the value of AG of polymerization, the value of A H of polymerization being negligibly small for 12-and 13-membered lactams; polymerization of these lactams takes place principally because of the change in entropy. ,~#,kcagmo/e 2-

!

0

. . . . . .

-2

-10

-20 I

I

f

I

1

I

3

5

7

3

11

I3

11

FIG. 3. Values of AG for polymerization under standard conditions as a function of the number (n) of atoms in the ring: / - - f o r lactams; 2--for lactones [2].

Values of AG of polymerization as a function of the number of atoms in the ring are compared in Fig. 3 for lactams and cyclanes (the latter are taken from Dainton's work [2]*). As may be seen from Fig. 3, the trend of the change in AG of polymerization is much the same for lactams and for cyclanes. AG is aproximately zero for 5- and 6-membered rings, and becomes negative for large rings.

.CONCLUSIONS (1) The existing data about the values of A H and AS for the polymerization ~f lactams with from 5 to 13 atoms in the ring have been summarized. (2) On the basis of these data, values of AG of polymerization for these lactams ;have been calculated and discussed. It has been showa that the enthalpy term * We have verified the values of AG Of polymerization for cyclohoptane and cyclooetano b y t a k i n g into account the experimentally determined values of the absolute entropies for ~hose cyclanes [4].

Values of AG for polymerization of laetams

611

m a k e s t h e p r i n c i p a l c o n t r i b u t i o n t o t h e v a l u e of AG of p o l y m e r i z a t i o n in t h e case of 7- a n d 8 - m e m b e r e d l a c t a m s , w h e r e a s for 9:, 10- a n d l l - m e m b e r e d lact a m s t h e c o n t r i b u t i o n s of A H a n d T A S are a p p r o x i m a t e l y t h e s a m e in m a g n i t u d e ; t h e v a l u e of A H of p o l y m e r i z a t i o n is n e g l i g i b l y s m a l l in t h e case of 12- a n d 13-membered lactams, and the polymerization takes place principally because of t h e c h a n g e in e n t r o p y . (3) A c o m p a r i s o n h a s b e e n m a d e b e t w e e n t h e v a l u e s of AG of p o l y m e r i z a t i o n for l a c t a m s a n d c y e l a n e s .

Translated by G. F. MODLEI~ REFERENCES 1. A. A. STREPIKHEYEV, Dissertation, 1950 2. F. S. DAINTON, T. R. E. DEVLIN and P. A. SMALL, Trans. F a r a d a y Soc. 51: 1710, 1955 3. Petroleum Inst. Res. Project 44, Carnegie Press, Pitt., Penn., 1955 4. H. L. FINKE, D. W. SCOTT, M. E. GROSS and G. WADDINGTON, J. Amer. Chem. Soe. 78: 5469, 1956 5. P. A. SMALL, Trans. F a r a d a y Soe. 51: 1717, 1955 6. V. P. KOLESOV, I. Ye. PAUKOV and S. M. SKURATOY, Zh. fiz. khimii 36: 770, 1962 7. A. A. STREPIKHEYEV, S. M. SKURATOV, O. N. KACHINSKAYA, R. S. MUROMOVA, Ye. P. BRYKINA and S. M. SHTEKHER, Dokl. Akad. Nauk SSSR 102: 105, 1955 8. S. M. SKURATOV, A. A. STREPIKHEYEV and Ye. P. BRYKINA, Ueh. zap. MGU 164: 73, 1953 9. A. K. BONETSKAYA, S. M. SKURATOV, N. A. LIfKINA, A. A. STREL'TSOVA, K. Ye. KUZNETSOVA and M. L. LAZAREVA, Vysokomol. soyed. B1O: 75, 1968 (~ot translated in Polymer Sci. U.S.S.R.) 10. K. DACHS and E. SCHWARZ, Angew. Chem. 15: 540, 1962 11. L. K. Y. TONG and W. A. KENYON, J. Amer. Chem. Soc. 67: 1278, 1945 12. S. M. SKURATOV, A. A. STREPIKHEYEV, V. V. VOYEVODSKI and Ye. N. KANARSKAYA, Dokl. Akad. Nauk SSSR 86: 1155, 1952 13. S. M. SKURATOV, V. V. VOYEVODSKII, A. A. STREPIKHEYEV, Ye. N. KANARSKAYA, R. S. MUROMOVA and N. V. FOK, Dokl. Akad. Nauk SSSR 95: 591, 1954 14. S. M. SKURATOV, M. P. KOZINA and V. D. KALMYKOVA, Bull. Chem. Thermodynamics, No. 2, 30, 1959