Download - Organometallic polymers. III. Organometallic modifications of diene polymers

JOURNAL OF POLYMER SCIENCE Polymer Chemistry Edition VOL. 15, 461-467 (1977)

Organometallic Polymers. 111. Organometallic Modifications of Diene Polymers*

ALON GAL,? MICHAEL CAIS, and DAVID H. KOHN, Department o f Chemistry, Technion-Israel Insti tute of Technology, Haifa, Israel

Synopsis

A process for the chemical modification of polybutadienes and natural rubber by various metallocene compounds is described. Soluble products of up to 43% ferrocene content were obtained. The effect of substrate, metallocene, and reaction conditions on the course and extent of substitution was investigated. The glass transition temperature Tg was found to increase considerably with the degree of substitution, e.g., cis-polybutadiene substituted with ferrocene (18 mole-%) has a Tg of 30"C, as compared with -91°C for the unsubstituted polymer.

INTRODUCTION

In the past twenty years, metallocenes have been included in a large number of condensation and addition polymers. The literature on these polymers has been r e ~ i e w e d . ~ , ~

The ultraviolet and visible absorption characteristics of ferrocene compounds, coupled with their radiation resistance? suggest the use of metallocene polymers as transparent coatings and sheets for the protection of vulnerable substrates against photodegradation. As far as we could ascertain, the high molecular weight polymetallocenes reported in the literature were amorphous, glassy materials, unsuitable for use in the form of films. This is not altogether surprising, considering that the bulky ferrocene group is known to impart high glass transition temperatures Tg to polymer^.^ A direct approach to the preparation of flexible metallocene-embodying polymers would be copolymerization of ap- propriate metallocene compounds with monomers that produce materials of low TK. Of the commercially important polymers, polyethylene and polybutadiene have particularly low Tg values. However, the copolymerization of ethylene or butadiene with metallocene addition monomers should run into difficulties due mainly to the low reactivity of the latter. Thus, the copolymerization of vinylferrocene with butadiene resulted in viscous liquids containing less than 5 mole % of ferrocene.6

We wish to report now the incorporation of substantial amounts of metallocenes into flexible and even elastomeric polymers. This was achieved by chemical modification of various organic polymers, taking advantage of the re-

* For Part I1 see Gal, Cais, and Kohn.' + Present address: Hydrophilics Ltd., P.O. Box 6221, Haifa, Israel.

461

0 1977 by John Wiley & Sons, Inc.

462 GAL, CAIS, AND KOHN

markable nucleophilic affinity of the iron-group metallocenes. In this paper we describe the synthesis of metallocene-modified polybutadienes and of ferrocene-modified natural rubber. The assessment of the products as film-formers and ultraviolet shields will be the subject of another paper.

EXPERIMENTAL

Materials

The metallocene compounds used were ferrocene (practical), from Arapahoe Chemical Comp., ruthenocene, n-butyl-, tert-amyl-, and phenylferrocene (red label) from Research Organic/Inorganic Chemical Corp. Aluminum chloride (anhydrous powder) was from Coleman & Bell and 1,2-dichloroethane (purum) from Fluka A.G. The polybutadienes used were received as gifts from the manufacturers listed in Table I.

Modification Process

A typical process may be described as follows. Ferrocene (109 g) was dissolved in 1,2-dichloroethane (DCE) (555 ml), under a nitrogen atmosphere in a 2-liter graduated resin kettle, placed in a water bath and fitted with stirrer, dropping funnel, thermometer, gas inlet-tube, and sampling tube. To this solution, a suspension of aluminum chloride (20.8 g) in DCE (100 ml) was added from the dropping funnel and the mixture stirred for 20 min at a constant temperature of 28°C. The clear, reddish-brown solution was then cooled to 22OC, and a solution of Taktene-1200 (14 g) and ferrocene (109 g) in DCE (600 ml) was charged to the kettle. After 50 min of stirring at 22OC, about 200 ml of the reaction mixture was transferred under nitrogen pressure, through polyethylene tubing, into a stirred precipitation flask (1-liter resin kettle) containing 400 ml metha- nolic solution of antioxidant (diphenylamine, 0.5%) and reducing agent (ascorbic acid, 0.5%). Subsequent batches were precipitated in a similar manner after 100,122,160,170, and 190 min. The methanol-washed precipitates were added to toluene (100 ml containing 1% diphenylamine) and the gel fraction separated by centrifugation from the solution. After reprecipitation, the products were either dried under vacuum or redissolved and stored in the refrigerator in presence of antioxidant.

TABLE I Polybutadiene Starting Materials

Composition (%) Molec-

Trade name Manufacturer 1,4-cis trans Vinyl weight 1,4- 1,2- ular

Taktene-1200 Polymer Corp. of Canada 9 5 >lo5

Lithene QH Lithium Corp. of America 35 30 3 5 2700 Hystl B-3000 Hystl Development Comp., N.Y. 91.7 3040

ORGANOMETALLIC POLYMERS. I11 463

100

80

60

40

20

0 4ooo 3500 3000 2500 2000 1800 1600 1400 1200 KX)O 800 (cm-')

Fig. 1. Infrared spectrum (film) of ferrocene substituted cis-1,4-polybutadiene (Taktene 1200).

Characterization

Iron analyses were conducted on a Perkin-Elmer atomic absorption spectro- photometer, Model 403. In order to enable the determination of iron in aqueous solutions, the polymers were decomposed by treating weighed samples first with diluted nitric acid (50%) (to avoid autoignition) and subsequently heating them with fuming nitric acid until completely dissolved.

Infrared spectra were measured with a Perkin-Elmer infrared spectropho- tometer, Model 257. Calibration with known ferrocene solutions in DCE was possible because of the linear dependence of the absorbance at 1100 cm-l- characteristic to unsubstituted cyclopentadienyl units7-on concentration. Infrared spectrometry in conjunction with iron determination made possible estimation of homoannularity in various metallocene-modified polymers. (Homoannularity was defined as the percent of metallocene moieties substituted on one ring only. 8 )

The number-average molecular weight M , was determined in a vapor-phase osmometer (Model 115, Hitachi Perkin-Elmer).

The glass transition temperatures were measured by the penetrometric methodg by use of an automatic softening-point apparatus.

RESULTS AND DISCUSSION

The metallocene-modified diene polymers are yellow materials, whose infrared spectra exhibit the characteristic features of both substrate and substituent. Figure 1 shows the typical spectrum of a film of a ferrocene-substituted cis- polybutadiene. The bands at about 730,1440,1640, and 2900 cm-l belong to cis -polybutadiene segments,1° while the presence of the ferrocene moiety is ev- idenced by the peaks a t 820,1000,1100, and 3080 ~ m - l . ~ The shoulder a t 1700 cm-' points to the existence of carbonyl groups that may originate from the air-drying of the polymer film.

Experiments performed with high molecular weight cis-1,4-polybutadiene showed dependence of the rate of ferrocene substitution on the concentration of the Friedel-Crafts catalyst (Fig. 2). A t a reaction extent corresponding to about 10 ferrocene-substituted meric units out of 100 (27 w t % ferrocene), an insoluble gel began to form. The latter tended to become the main product at ferrocene contents approaching 40 w t %.

As shown in Table 11, the degree of substitution of other butadiene polymers, such as Lithene QH and Hystl-B-3000, was similar to that of Taktene 1200 (runs


0 7

0.6

0.4

0.2

0. I I

0 50 100 150 200 333

TIME (mln)

Fig. 2. Effect of reaction time and catalyst concentration on rate of substitution of cis-l,4-polybutadiene (Taktene 1200) by ferrocene. Parameter, AlC13 concentration (mole/monomole).

4-19, 5-2,8-2, and 6-6). In contrast, natural rubber-essentially composed of cis-isoprene units-appeared to crosslink much faster, even in relatively diluted polymer solutions, when the molar ferrocene: olefin group ratio was as high as 29:l (run 8-3). Experiments 6-9 and 6-10 demonstrate that the presence of a small amount of added hydrochloric acid, acting supposedly as a cocatalyst,'l accelerates markedly the rate of substitution.

Alkyl-substituted metallocenes, such as n-butyl- and tert -amylferrocene (runs

TABLE I1 Reactions of Diene Polymers with Metallocenesa

Star tin g materials

Organometallic compound Polymeric

product Concn, AlCI,, mole mole Reac- Sol Metallo- per per tion frac- cene

Run Concn, mono- mono- time, tion, content,

Substrate

no. Type g/dl 5 P e mole mole min % 5%

4-19 5-2 8-2 8-3 6-6 6-9 6-10 7-1 1-2

7-3

1-5

Taktene 1200 Hystl B 3000 Natural rubber Natural rubber Lithene QH Lithene QH Lithene QH Hystl B 3000 Hystl B 3000

Hystl B 3000

Hystl B 3000

1.0 1.0 1 .o 0.45 1 .o 1 .0 1.0 4.5 4.5

4.5

4.5

Ferrocene 4.5 0.6 170 99 34.4 Ferrocene 4.5 0.6 170 100 32.0 Ferrocene 4.5 0.6 3 0 27.5b Ferrocene 13.0 0.6 4 24 9.0 Ferrocene 4.5 0.6 150 85 36.4 Ferrocene 1.0 0.13 80 100 5.3 Ferrocene 1.0 0.13C 80 100 21.2 Ferrocene 1.0 0.13C 150 100 23.6 n-Butyl- 1.0 0.13C 150 100 14.8

tert-Amyl- 1.0 0.13C 150 100 8.2

Ruthenocene 1.0 0.13C 150 - d 15.0

ferrocene

ferrocene

a Solvent, 1,2-dichloroethane; temperature, 22°C. b Determined in the gel. C In presence of added HCI cocatalyst.

Not determined.


7-2 and 7-3) exhibited a somewhat diminished reactivity, probably due to steric hindrance by the alkyl group. Comparison of the concentration of unsubstituted cyclopentadienyl groups-as calculated from the peak exhibited in the infrared spectrum at 1100 cm-1-with the ferrocene content of the polymers points to the fact that both the unsubstituted and the alkylated rings participated in the reaction, the homoannular-to-heteroannular substitution ratio being about 1:l. As expected in virtue of its weaker electrophilic ~harac te r ,~ ruthenocene reacted to a smaller extent than ferrocene under the same conditions (run 7-5 vs. run

A series of experiments performed with Lithene QH are tabulated in Table 111. The substitution pattern was similar to that of Taktene 1200. Both the softening temperature and the number-average molecular weight of the soluble polymeric products increased with increasing substitution. The increase of was fairly consistent with the ferrocene content of the polymer; hence, it seems that practically no chain degradation occurred during substitution. As shown in Table I, Lithene QH is composed of three kinds of recurring units and has a rather low molecular weight. The complexity of its various substitution products with ferrocene prevented a meaningful correlation between structure and glass transition temperatures. It was therefore preferable to examine the substitution dependence of the Tg with the ferrocene-modified Taktene 1200, which has practically only one kind of recurring unit and is also well above the so-called "critical value" of the degree of polymerization. One can assume that the ferrocene-substituted Taktene is a random copolymer of the form (I):

7-1).

I

For the sake of convenience this polymer can be considered as a terpolymer of cis-butadiene, ethylene and vinylferrocene (11):

TABLE I11 Substitution Reactions of Lithene QH with Ferrocenea

Soluble polymeric product

Molecular weight Reaction Sol Ferrocene Substi-

Run time, fraction, content, tution, Measured no. min 72 % mole ?6 Calculated (E)

- - 0 100 0 0 24 00 -7 5 6-2 10 100 8 2.5 2600 3060 -4 9 6-3 40 100 21 7.5 3000 28 20 -18 6-4 80 95 35.5 16 3700 34 00 48 6-5 120 84 38 18 3850 3600 99 6-7 180 62 41 20.5 4000 4250 128 6-8 240 49 43 22.5 4200 5350 142

a Solvent, 1,2-dichloroethane; polymer concentration, 1 g/dl solution; ferrocene: polymer ratio, 4.5 molelmonomole; AlC1,:polymer ratio, 0.6 mole/monomole; temperature, 22°C.


Fig. 3. Effect of substitution on the glass transition temperature of cis-1,4-polybutadiene (Taktene 1200): (-1 observed penetrometric softening points; (---) TR evaluated by means of eq. (1).

I1

whose T, may then be calculated by the aid of the empirical eq. ( l).l2

1/T, = c (Wi/TgJ (1)

In eq. (l), W; is the weight fraction of the meric species and Tg, the glass transition temperature of the corresponding homopolymers. In the above case, the penetrometric softening point of unsubstituted Taktene 1200 is -91OC; the Tg values of polyethylene and polyvinylferrocene were reported to be -125"C13 and about 280°C,5 respectively.

In Figure 3, eq. (1) is represented by the dotted line, while the solid curve shows the measured penetrometric softening points. The two curves are in fair agreement up to a ferrocene content of about 12 mole %, but at higher substitu- tions they become divergent. It is noteworthy that as the substitution of Taktene 1200 proceeded from 12 to 18 mole %, the amount of crosslinked product increased from less than 1% to about 90%. The increasing disparity between the calculated and the observed values may be explained in terms of intramolecular side reactions leading to rigid cyclic structures within the polymer chains which take place simultaneously with substitution by ferrocene. Cyclizations, along with intermolecular crosslinkings of diene polymers in solution in the presence of Friedel-Crafts catalysts, have already been reported.14J5

Results of organometallic modifications of halogen-containing polymers will be presented in a subsequent paper.

This paper is taken in part from a dissertation submitted by one of the authors (AG) to the De- partment of Chemistry, Technion-Israel Institute of Technology, Haifa, in July 1973, in partial fulfillment of the degree of DSc.

References

1. A. Gal, M. Cais, and D. H. Kohn, J . Appl. Polym. Sci., 15,1571 (1971). 2. E. W. Neuse and H. Rosenberg, Metallocene Polymers, Marcel Dekker, New York, 1970.


3. C. U. Pittman, Chem. Tech., 1,416 (1971). 4. R. E. Bozak, in Adoances in Photochemistry, Vol. 8, J. N. Pitts, Jr., G. S. Hammond, and W.

5. A. J . Tinker, J. A. Barrie, and M. H. George, J. Polym. Sci. Polym. Letters Ed., 13, 487

6. C. U. Pittman, J. Polym. Sci. B, 6, 19 (1968). 7. M. Rosenblum, Chemistry of the Iron Group Metallocenes, Interscience, New York, 1965,

8. E. W. Neuse, and D. S. Trifan, J. Amer. Chem. SOC., 84,1850 (1962). 9. 0. B. Edgar, J. Chem. SOC., 2638 (1952).

A. Noyes, Jr., Eds., Wiley-Interscience, New York, 1971, p. 227.

(1975).

pp. 38,67.

10. H. W. Thompson and P. Torkington, Trans. Faraday Soc., 41,246 (1945). 11. S. H. Patinkin and B. S. Friedman, in Friedel-Crafts and Related Reactions, Vol. 2, G. A.

12. T. G Fox,Bull. Amer. Phys. Soc., 1,123 (1956). 13. R. A. V. Raff, in Encyclopedia of Polymer Science and Technology, H. F. Mark, N. G. Gaylord,

14. M. A. Golub, in Chemical Reactions of Polymers, E. M. Fettes, Ed., Interscience, New York,

15. J. R. Shelton and L. H. Lee, Rubber Chem. Technol., 31,415 (1958).

Olah, Ed., Interscience, New York, 1964, pp. 2-4.

and N. B. Bikales, Eds., Interscience, New York, 1967, Vol. 6, p. 296.

1964, p. 110.

Received April 6, 1976

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