The crystallinity of the hydrogenation products of cis-1,4-polybutadiene

Crystallinity of hydrogenation products of cis-l,4-polybutadiene 9. H. M. WOODBURN and J. R. FISHER, J. Org. Chem. 22: 835, 1957 10. Ye. I. TINYAKOVA, Ye. K. KHRENNIKOVA and B. A. DOLGOPLOSK, Izv. Akad. Nauk SSSR, Otd. khim. nauk, p. 1152, 1956 11. Ye. I. TINYAKOVA, Ye. K. KHRENNIKOVA and B. A. DOLGOPLOSK, Zh. obsheh. khim. 26: 2476, 1956 12. Ye. I. TINYAKOVA, Ye. K. KHRENNIKOVA and B. A. DOLGOPLOSK, Zh. ohsheh. khim. 28: 1632, 1958 13. Ye. I. TINYAKOVA, Ye. K. KHRENNIKOVA and B. A. DOLGOPLOSK, Zh. ot)sheh. khim. 28: 3269, 1958 14. B. L. YERUSALIMSKII, B. A. DOLGOPLOSK and A. P. KAVUNENKO, Zh. obshch. khim. 27: 267, 1957

THE CRYSTALLINITY OF THE HYDROGENATION PRODUCTS OF cis-I,4-POLYBUTADIENE* B. I. TIKHOMIIIOV, A. I. YAKUBCHIK and I. A. KLOPOTOVA Leningrad State University

(Receired 16 Ja~uary 1961) THE hydrogenation o f a butadiene polymer was carried out a long time ago by Staudinger [1]. He assumed that the butadiene units were joined in the 1,4-position and that the hydrogenation product would be a high-molecular, linear paraffin, i.e. a hard, isoluble material. However, the hydrogenated rubber obtained by Staudinger was readily soluble, and in tbrm it was reminiscent of natural rubber. Later he explained this by assuming that the original hutadiene polymer was not linear but had a branched structure. W h a t degree of hydrogenation was reached in Staudinger's experiments is not known. Ab that time these were no reliable methods for the determination of unsaturation in hydro;~nated polybutadiene. We have shown that when sodium-polybutadicne, with a branched structure, is hydrogenated, hydrogenated rubbers are obtained which even when the unsaturation is only 1 3°~o are readily soluble at norma.1 temperatures and which were shown hy X-ray analysis to be completely amorphous [2]. The solubility of sodium-polybutadiene is assisted by the presence in the Inaeromoleeules ot' hranehes, the length of which is commensurate with the distance between the branch points [3] (long-chain branching) and by the presence of pendant vinyl groups due to the existence of 1,2-butadiene units in the polymer chain (short-chain branching). Naturally long chain branching hinders crystallization of the hydrogenation products. Furthermore. when crystalline, syndiotaetic 1,2-polybutadiene was hydrogenated under conditions that prevented isomerization [4] amorphous hydrogenated polybutadienes were obtained even when the degree of' hydrogenation was 95Oo . This is associated with the fact t h a t the sterie hindrance of the ethyl group is greater than t h a t of the vinyl group. Sodium polybutadiene contains 50-70°.~ of 1,2-units. It is understandable that the randomly distributed vinyl groups, which are converted to ethyl groups on hydrogenation wouM hinder the crystallization of the hydrogenated rubber considerably. * Vysokomol. soyed. 4: No. l, 25-29, 1962.

B. I. TIKHOMIROV et al. We have hydrogenated cis-l,4-polybutadiene, obtaine d b y means of a Ziegler-type catalyst. The polymer contained 6% of 1,2-and 5% of trans-l,4 units. No 1,2,3-propanetricarboxylic acid was found in the products from the ozonolysis of this rubber, indicating that there is no branching at the a-methylene group [5]. Thus the small proportion of vinyl groups in the polymer form the only branches. I n the fully hydrogenated polymer the ratio of methylene to ethyl groups should be comparable to the ratio of methylene to methyl group in high-pressure polyethylene. I t is known that as a result of its marked crystallinity polyethylene shows a characteristic X-ray diffraction pattern, a n d moreover it is capable of forming spherulites which can be observed under the polarizing microscope. I t was found on hydrogenation of unfractionated eis-l,4-polybutadiene that even the partially hydrogenated products were insoluble at room temperature, and this is due to the crystallinity of the hydrogenated rubber [6]. The problem with which this paper is concerned was the examination of the phenomenon of crystallization in the hydrogenation products of eis-l,4-polybutadienes with various degrees of unsaturation.

EXPERIMENTAL RESULTS AND DISCUSSION

Un s atu r ated films of hydrogenated cis-l,4-polybutadiene, obtained from 1% solutions in toluene by rapid evaporation of the solvent at temperatures of 100-120 °, were examined b y the X - r a y diffraction method. The thickness of the films was about 0-2 mm. The X - r a y patterns of the specimens were obtained b y means of a URS-50I instrument and the scattered radiation was recorded b y means of a Geiger counter. Cu K radiation separated b y a nickel filter was used. The X- r ay tube was operated at a voltage of 35 kV and a current strength o f 10 mA. X - r ay scattering curves were obtained with angles between 2 0 = 1 3 ° and 2 8----26° for specimens of cis-l,4-polybutadiene with degrees of unsaturation of 6.5, 10.0, 19.0, 28-5, 48.0, 54.1 and 70.5%. All of these have tile same shape as the scattering curves of samples of polyethylene (Fig. 1). The two sharp peaks, corresponding to reflection from crystalline regions have maxima at

E

lq

76

/8

20

2Z

ZV ZB

FIG. 1. X-ray scattering curves for specimens of hydrogenated cis-l,4polybutadiene: continuous curve--unsaturation 70.5O/o; dotted curve--unsaturation 10"0%.

angles of 2 0 = 2 1 . 4 ° ( ± 0. 2) ° and 2 0 = 2 3 . 6 ° (±0.2°), i.e. the same angles as the m a x i m a in the polyethylene curves [7]. Separation of an intensity m a x i m u m corresponding to amorphous regions and the first crystalline m a x i m u m occurs

Crystallinity of hydrogenation products of

cis-1,4-polybutadiene

9

only on the curves for specimens of low degrees of hydrogenation. The angles of the maxima of amorphous scattering found from these curves coincide with those of the original cis-l,4-polybutadiene, which is completely amorphous at room temperature, and also with t h a t for amorphous polyethylene. This m a y indicate the mierostrueturM identity of these materials. There are no maxima on the X-ray scattering curves of the various specinlens of hydrogenated polybutadiene that could be attributed to interplanar scattering of crystalline cis-l,4-polybutadiene. These facts indicate t h a t the structure of the hydrogenated polybutadienes with different degrees of unsaturation is close to t h a t of polyethylene. The marked erystallinity even at low degrees of hydrogenation (unsaturation 70(}/o) compels us to assume t h a t the unsaturated units enter into the composition of the crystalline regions of the hydrogenated polybutadienes. Since the degree of hydrogenation of the polymer depends on its molecular weight [19], when an unfractionated specimen is hydrogenated a mixture of macromolecules with different degrees of unsaturation will be obtained. Only the average value is determined experimentally. Hence the maeromolecules of hydrogenated polybutadiene consist of randomly distributed hydrogenated and unhydrogenated sections, and moreover they (lifter from one another in their degrees of unsaturation. However, in spite of this irregularity of structure the hydrogenated products show a tendency to crystallization even when the proportions of hydrogenated and unhydrogenated units in the polymer are comparable. The presence of hydrogenated units in the chains has no significant effect on the cross-sectional dimensions of the molecules or on their cohesive energy, but it reduces the flexibility of the chains increases inter-chain interaction and facilitates regular packing of the maeromolecules in a three-dimensional lattice. The bundles of maeromolccules [9] in the hydrogenated rubber evidently (lifter less from one another than do the individual nmcromoleeules, and this assists crystallization. Comparison of the X-ray diffraction curves for the hydrogenated specimens of different degrees of unsaturation shows t h a t the erystallinity increases with increasing degree of hydrogenation. Since the structure of hydrogenated cis-l,4-polybutadiene has much in common with t h a t of polyethylene a tentative; calculation was made of the degree of erystallinity of the hydrogenated polybutadienes of different degrees of unsaturation by comparison of the integrated intensities of scattering from the crystalline and amorphous regions by the method developed for polyethylene [10]. The dependence of the degree of erystallinity on the unsaturation of hydrogenated cis-l,4-polybutadiene is shown below: U n s a t u r a t i o n (%) Degree o f c r y s t a l l i n i t y (°o)

70.5

54.1

48.0

:]8.5

19.1) 10.0

20

35

35

40

45

50

6.5 60

10

B.I. TIKt[OMIROVet al.

I n view of the fact t h a t spherulite crystallization occurs in p o l y e t h y l e n e it was o f interest to discover w h e t h e r the h y d r o g e n a t i o n p r o d u c t s of c i s - l , 4 p o l y b u t a d i e n e are capable o f forming spherulities or w h e t h e r crystallization leads o n l y to p r i m a r y s t r u c t u r a l formations. T h e spherulite s t r u c t u r e was studied b y m e a n s of an MP-6 polarizing microscope. 0.1 ~o solutions of the h y d r o g e n a t e d rubbers were m a d e up. Drops o f the h o t solution were deposited on a cover-slip a n d the solvent was e v a p o r a t e d i n vacuo. T h e p o l y m e r film was covered b y a second cover-slip, m e l t e d on a h o t - p l a t e at 150 ° a n d was t h e n rapidly t r a n s f e r r e d to a t h e r m o s t a t at 80-100 ° a n d k e p t there for 15-20 minutes. After cooling, the p o l y m e r film held b e t w e e n

FIG. 2. Specimens of hydrogenated cis-l,4-polybutadiene with degrees of unsaturation of: a--48.0°/o; b--24.4(~); c--13.6°,~) and d--6.5°/~), between crossed Nieols.

Crystallinity of hydrogenation products of cis-l,4-polybutadiene

11

the two cover-slips was placed on the stage of the microscope and examined between crossed Nicols. It was found t h a t a degree of hydrogenation of not less than 50% is necessary for the appearance of spherulite structure ill the polymer. With lower degrees of hydrogenation spherulites are either not formed or they are too small for observation with the limited resolving power of the optical microscope. The imperfection of the spherulite structure of polymers with ansaturation of 30-40% (Fig. 2a) probably indicates t h a t spherulites are not formed ill hydrogenated polybutadienes with high unsaturation (more than 50%). The size of the spherulites depends on the temperature at which the melted fihn is held in the thermostat. W'ith increasing degree of hydrogenation the spherulite structure becomes more clearly defined (Figs. 2b, c and d). This is understandable because the extent to which the structure approaches t h a t of polyethylene increases. The spherulite structure is more complex than primary crystalline formations; consequently for the former to arise a higher degree of regularity of the macromolecules is necessary. Whereas primary crystalline formations are detected by X-ray analysis in hydrogenated polybutadiene with 70~{) unsaturation spherulites can be detected with certainty only in [polymers with an unsaturation of 25% or less. The melting point of hydrogenated polybutadienes with degrees of unsaturation from 25}~ to 6%, determined by the disappearance of birefringenee under the polarizing microscope, lies within the interval of .()0-lI0 ~. Specimens with lower unsaturation have a higher melting point. The formation of spherulites in hydrogenated polybutadienes with a considerable degree of unsaturation can be understood in the light of the "bundle" structure theory of Kargin, Kitaigorodskii and Slonimskii [12]. In fact crystallization in these cases should lead to the formation of " b a n d s " but the formation of ';bands" from molecules of irregular structure can be understood only if it is assumed t h a t the molecules are organized in a bundle that as a whole takes up a folded configuration with a definite periodicity. CONCLUSIONS

X-ray analysis shows t h a t the hydrogenation products of ci~.-l,4-polybutadienes with a degree of unsaturation of 7(I ° / o r less show some degree of crystallinity. These products are capable of forming spherulites. The degree of erystallinity, perfection of the spherulite structure and melting point of hydrogenated cis-l,4-polybutadienes increase with increasing degree of hydrogenation. These facts can be explained on the basis of the "bundle" theory of polymer structure proposed by Kargin, Kitaigorodskii and Slonimskii.

Tra~.~laled by E. O. PttlLLIP~ REFERENCES 1. H. S T A U D I N G E R , K a u t s c h u k

10: 194, 1934

2. A. I. YAKUBCHIK, B. I. TIKHOMIROV and L. N. MIKHAILOVA, Zh. prikl, khim. 34: 652, 196i

i2

A. I. SHATENSHTEINet al.

3. 4. 5. 6. 7. 8. 9.

I. Ya. PODDUBNYI, Khim. prom., No. 2, 4, 1958 G. NATTA, Makromol. Chem. 35: 94, 1960 C. S. MARVEL, et al., J. Org. Chem. 16: 838, 1951 A. I. YAKUBCHIK and B. I. TIKHOMIROV, Zh. obshch, khim. 30: 128, 1960 S. L. AGGARWAL and G. P. TILLEY, J. Polymer Sci. 18: 17, 1955 G. NATTA and P. CORRADINI, Suppl. Nuovo eimento 15: Ser. 10, 9, 1960 A. I. YAKUBCHIK, B. I. TIKHOMIROV and V. S. SULIMOV, Vestnik LGU, No. 22, 135, 1961 10. V. A. KARGIN and G. L. SLONIMSKII, Kratkie ocherki po fiziko-khimii polimerov. (Short Essays on the Physical Chemistry of Polymers.) p. 116, 1960 11. J. L. MATTHEWS, H. G. PEISER and R. B. RICHARDS, Acta crystallog. 2: 85, 1949 12. V. A. KARGIN, A. I. KITAIGORODSKI and G. L. SLONIMSKII, Kolloid. zh. 19: 131, 1957

THE PROTONIC AND APROTONIC MECHANISM OF POLYMERIZATION INITIATED BY POTASSIUM AMIDE IN LIQUID AMMONIA* A. I. SHATENSHTEIN, YE. A. YAKOVLEVA, YE. A. KOVRIZHNIK I ). N. MANOCHKINA and N. A. PRAVIKOVA L. Ya. Karpov

Physico-chemical

(Received 25 J a n u a r y

Institute

1961)

ANIONIC p o l y m e r i z a t i o n initiated b y a solution o f p o t a s s i u m amide in liquid a m m o n i a is one o f a n u m b e r of processes t h a t are a s s u m e d to proceed t h r o u g h a stage of c a r b a n i o n f o r m a t i o n . The c a r b a n i o n s m a y be f o r m e d b y the a d d i t i o n of a n N H : ion to a multiple b o n d or b y the transfer of a proton, i.e. b y a n aprotonic or a p r o t o n i c m e c h a n i s m . Sanderson a n d H a u s e r [1] a n d E v a n s , Higginson a n d W o o d i n g [2] showed t h a t the first o f these m e c h a n i s m s applies to the polymerization o f s t y r e n e b y p o t a s s i u m amide in liquid a m m o n i a . U n d e r these conditions a - m e t h y l s t y r e n e does n o t p o l y m e r i z e [1, 2]. I n c o n t r a s t to this s t a t e m e n t we h a v e p r e p a r e d a n d f r a c t i o n a t e d the p o l y m e r from ~-methylstyreue. The molecular weight, nitrogen c o n t e n t a n d b r o m i n e value f o u n d lead to the conclusion t h a t t h e p r o t o n i c m e c h a n i s m p r e d o m i n a t e s in the p o l y m e r i z a t i o n of a - m e t h y l s t y r e n e b y this initiating system. I n order to estimate the s t r e n g t h o f ~ - m e t h y l s t y r e n e as a h y d r o g e n acid we used the m e t h o d of isotope-exchange of h y d r o g e n [4] a n d applied it also to some diene h y d r o c a r b o n s which we also f o u n d to form low-molecular p o l y m e r s on p o l y m e r i z a t i o n with K N H 2 in liquid a m m o n i a . As far as we k n o w there is no i n f o r m a t i o n in the literature on the acid-base properties o f monomers. * Vysokomol. soyed. 4: No. 1, 42-51, 1962.