The Solid State Polymerization of Hexamethylcyclotrisiloxane

14 The Solid State Polymerization of Hexamethylcyclotrisiloxane A. S. CHAWLA and L. E. ST. PIERRE

Downloaded by CORNELL UNIV on November 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch014

McGill University, Montreal, Quebec, Canada

The solid state polymerization has been investigated 60°C.

of

hexamethylcyclotrisiloxane

over the temperature

The rates of polymerization

range —196°

to

have been related to the

presence of ion scavengers, H2O, NH3, in the monomer and to the size of the crystals. Using large crystals dried over sodium, G values of polymerization tained at reported

50°C.

of 11 Χ 103 were ob

This is five times larger than

values. The reaction is concluded

previously

to be surface

initiated and to be terminated at a crystal face or at a defect.

' " p h e ability of high energy radiation to initiate polymerization of •*~ monomers in the crystalline state was first reported more than a decade ago (14) and has been an active area of investigation for both radiation and polymer researchers. The field has seen outstanding prog ress during this period (2, 3, 7, 13) but despite these advances, many important questions still remain only partially answered. F e w , if any, systems have been investigated i n sufficient detail to yield adequate answers to questions concerning the nature and the site of initiation, the nature of the propagating species, the effect of impurities, and the roles of crystal structure and size (8,9). Because of this lack of understanding, reproducibility continues to be a major problem for investigators in the field and, indeed, it is not difficult to discover in the literature direct contradictions i n results reported from two different laboratories. In this chapter we report some results from an intensive investiga tion of the radiation-induced polymerization of a single monomer—hexa methylcyclotrisiloxane—which by virtue of its ease of polymerization and very high solid state vapor pressure affords an opportunity for comment on two of the points mentioned above.

229 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

230

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES


The first reported solid-state polymerization of this monomer was that of Lawton, Grubb, and Balwit (6) in 1956. Subsequent studies involving irradiation initiation have been reported by Burlant and Taylor ( I ) , and Trofimova et al. (15); recently Prut et al. (12) reported the results of a study of the solid polymerization i n which they used S n C l * as the initiator. This chapter deals mainly with two aspects of solid state polymerization—the effect of impurities and the effect of crystal size on the rates of polymerization. From the conclusions drawn, a reaction mechanism is proposed. Experimental

Detailed experimental procedures are reported elsewhere (4). However, brief descriptions of these techniques follow. Monomer Preparation. The hexamethylcyclotrisiloxane used in these experiments was supplied through the courtesy of W . L . Robb of the Silicone Products Department, General Electric Co. It was purified through repeated sublimation and recrystallization and exhibited, by V P C analysis, an impurity level of less than 1 part in 1000. (m.p., 6 4 . 5 ° C . ) Polymerization Technique. Owing to the difficulty in obtaining reproducible results, comparisons were made only between samples of the same "batch." A batch was prepared by having the monomer in each of 10 or 12 tubes all derived from the same master sample and all equilibrated with the same vapor before sealing off. The customary procedure used in sample preparation was as follows : a tube containing ^ 5 0 grams of prepurified and silica dried monomer was connected to a vacuum line and degassed through repeated meltings. Monomer was then distilled simultaneously to each of the many tubes connected to a manifold. When approximately 2 grams were in each tube, the tubes were sealed under vacuum. Small crystals were then formed in the tubes by melting the monomer and quick quenching it in liquid nitrogen. Drying of Monomer. The monomer was dried by contacting it with finely dispersed sodium. The sodium was melted and degassed by holding it under a vacuum (10" 5 mm. H g ) for 3 hours at 4 0 0 ° C . The tube containing the sodium was then closed off, and monomer was distilled into it from a previously attached sidearm, separated by a breakoff. The tube was then heated and shaken to disperse the sodium. After 4 hours contact at 7 0 ° C . between the monomer and the sodium, the system was reconnected to the vacuum line, degassed, and the dried monomer was distilled into the reaction tubes as previously described. Addition of Impurities to Monomer. Minute amounts of H 2 0 and N H 3 were, in some cases, added to the monomer under controlled conditions. This was done by connecting 0.5-cc. thin walled ampoules to the vacuum line and equilibrating them with known vapor pressures of impurities. The ampoules were sealed off and placed in a reaction tube. A t any time desired, the contaminant was mixed with the reactant by simply shaking the reaction tube, thus causing the ampoule to break.

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


14.

CHAWLA

A N D ST. PIERRE

Hexamethylcyclotrisiloxane

231

Growth of Large Crystals. Large crystals were grown in the reaction tubes by cooling the empty end of a tube containing very small crystals. Mass transfer was effected through sublimation. The temperature of crystal growth was 2 5 ° C . with a temperature gradient of 2 . 5 ° C . being maintained between the ends of the tube. Irradiation. The irradiation source was a cobalt-60 Gammacell 220 (Canadian Atomic Energy Limited) operating at a dose rate of 3.7 Χ 10 4 rads/hr. Temperature control during irradiation was effected by placing the tubes i n holes i n a thermostatted aluminum block. Isolation of Products. Following irradiation, the reaction tubes were weighed and opened. Unreacted monomer was then pumped off, and the tube was re-weighed. Polymer yields were calculated from the weight differences. Results

and

Discussion

Effect of Impurities. The initial polymerizations were done using monomer which had been dried and vacuum transferred. The drying, however, d i d not involve the sodium treatment. Such polymerizations yielded results of a highly irreproducible nature ( Figure 1 ) and caused us to commence the drying procedures discussed above. Using monomer which had been dried rigorously, we were able to increase yields and to accomplish reproducibility, as is shown by the solid line i n Figure 1. The yields, using the dried monomer, were considerably higher than those reported by previous workers i n the field, and it was assumed that the differences were caused by higher levels of impurities i n their mono mers. Lawton, Grubb, and Balwit (6) had irradiated their samples i n open dishes and thus undoubtedly had appreciable quantities of water present, and while Burland and Taylor ( J ) d i d their work i n vacuum, no special drying procedures were reported. Before expanding on the role of impurities in defining polymerization rates and yields, it must be acknowledged that there is a dose-rate effect which may be a contributing factor to the discrepancies mentioned. Re ports of such an effect have been made, but they are at variance as to the nature of the change i n kinetics associated with changes in dose rates (1, 6, 15). In the current work the dose rates have been kept constant. To test the effect of impurity levels, very small quantities of N H 3 and H 2 0 were added to monomer which had been dried vigorously. The results (Figure 2) show a marked effect down to exceedingly low con centrations, and both H 2 0 and N H 3 are observed to be strong inhibitors. That such strong inhibition should occur with these agents suggests the mechanism to be ionic and evokes comparison with the results of Williams and Okamura (16). Those investigators observed similar rate effects in H 2 0-contaminated styrene during radiation polymerization. They pro-


232

A N D CONDENSATION

POLYMERIZATION

PROCESSES


ADDITION

Figure

2.

Effect

of impurities

on polymer sil'oxane •

Ο

yields in

hexamethyhyclotri-

Η,Ο NH,


14.

CHAWLA

A N D ST. P I E R R E


233


pose an ionic polymerization with the cations playing a predominant role. Quenching with water is proposed to take place via hydrogen transfer, neutralizing the propagating species. The present observations could well be explained on the same basis since hexamethylcyclotrisiloxane polymerizes via both anionic and cationic mechanisms (5). ( N o evidence exists to date for free radical polymerizations.) The greater activity of H 2 0 as an inhibitor, as compared with N H 3 , is explained on the basis of a more rapid ion transfer. Since this would be a hydrogen ion rather than a hydride ion, it further suggests an anion as the primary polymerization species in the present polymerization. The authors feel, however, that further work is required before definite conclusions can be drawn regarding the polymerization mechanism or mechanisms. Effect of Crystal Size. The effect of crystal size on the polymerization rate i n radiation-induced polymerizations has been the subject of much discussion (8, 9). However, since small levels of impurities have been demonstrated to have such profound effects, it has been difficult to compare meaningful rate data obtained on large crystals in one reaction vessel with those for small crystals in another. Hexamethylcyclotetrasiloxane allows a unique opportunity for experimentation into this size effect since, owing to the high vapor pressure of the solid (10), it is possible to grow large crystals from small crystals in the same tube, after purification is complete. Thus, crystals of both sizes exist in exactly the same environment. Large crystals were grown, in the manner described previously, in the empty ends of tubes containing small crystals. The total samples were then irradiated, and the yields from each end of the reaction tube were determined. Figure 3 shows the increase in rate with crystal size. It demonstrates a marked improvement in rate and is explicable in any one of three ways. First, should the termination occur at a surface, the larger crystals would allow a longer chain propagation. Secondly, the large crystals could, by virtue of an extra sublimation, be purer and therefore less susceptible to impurity inhibition. Thirdly, the large crystals may contain fewer defects. Since termination can be expected to occur at defects as well as at crystal faces, a diminution in such sites would enhance the kinetic chain length. The x-ray data of Peyronel (11 ) have been interpreted to show a diminished defect concentration in the large crystals. The analysis is, however, not quantitative in terms of numbers of defects but only points to fewer in the large crystals. Selection between Factors 1 and 3 is thus not possible at this time. Notwithstanding, the work of Peyronel shows that Factors 1 and 3 are both associated with crystal size.


234

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES


25|

TIME OF IRRADIATION (HOURS) Figure 3.

Effect of crystal size on polymerization Ο •

yields

Large crystals Small crystals

The importance of Factor 2, the extra sublimation of the large crys tals, was quickly discounted by a simple experiment. Large and small crystals were prepared in the same tube in the usual manner b y growing the large crystals i n the empty end of a tube containing small crystals. The tube was then sealed off at the center, and the large crystals were melted quickly and quenched to yield small crystals. These had now been subjected to one more sublimation than those i n the opposite end of the tube. Irradiation was then performed as before, and the respective yields were determined. The monomers i n both ends gave the same yields. The enhancement observed i n Figure 1 is thus a crystal size effect and is not caused by different impurity levels. Table I.

^(Monomer)

Present Temperature,

30 50

°C.

L-G-B (4)

30 80

B-T (5)

800 2200

T-B-K (6)

750 1680

Work

5,050 11,510

The combination of careful purification and the use of large crystals results i n rates of polymerization and polymer yields far in excess of those otherwise obtainable (Table I ) .


14.

CHAWLA

Literature

A N D ST. PIERRE


235

Cited

(1) Burlant, W., Taylor, C.,J.Polymer Sci. 42, 547 (1959). (2) Chapiro, Α., "High Polymers," Vol. 15, pp. 236-246, Interscience, Lon don, 1962.


(3) Charlesby, Α., Rept. Progr. Phys. 28, 463 (1965).

(4) Chawla, A. S., St. Pierre, L. E., J. Polymer Sci., in press. (5) Kantor, S. W., Grubb, W. T., Osthoff, R. C., J. Am. Chem. Soc. 76, 5190 (1954). (6) Lawton, E.J.,Grubb, W. T., Balwit, J. S., J. PolymerSci.,19, 455 (1956). (7) Morawetz,H.,"Physics and Chemistry of the Organic Solid State," Vol. 1, Chap. 4, Interscience, New York, 1963. (8) Okamura, S., Hayashi, K., Kitanishi, Y., J. Polymer Sci. 58, 925 (1962). (9) Okamura, S., Obayashi, Ε. K., Takeda, M., Tomikawa, K., Higashimura, T., J. Polymer Sci. C4, 827 (1964). (10) Osthoff, R. C., Grubb, W. T., Burkhard, C. Α., J. Am. Chem. Soc. 75, 2227 (1953). (11) Peyronel,G.,Accad. Lincei 15, 402 (1953). (12) Prut, Ε. V., Trofimova, G. M., Yenikolopyan, N. S., Vysokomolekul. Soedin 6, 2103 (1964). (13) Restaino, A. J., Mesrobian, R. B., Morawetz,H.,Ballantine, D. S., Dienes, G. J., Metz, D.J.,J.Am. Chem. Soc. 78, 2939 (1956). (14) Schmitz, J. V., Lawton, F. J., Science 113, 718 (1951). (15) Trofimova, G. M., Barkalov, I. M., Kuz'mina, S. S., Gol'danskii, V. I., Yenikolopyan, N. S., Dokl. Akad. Nauk, SSSR, 161, 882 (1965).

RECEIVED

April 1, 1968.


The Solid State Polymerization of Hexamethylcyclotrisiloxane

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