POLYMER LETTERS VOL. 4, PP. 105-110 (1966)

COMMENT ON THE REPORT BY BALAS, DE LA MARE, AND SCHISSLER ( 1) REGARDING CIS- 1,4-POLYMERIZATION OF 1,3-BUTADIENE WITH ALKYL-FREE COBALT CATALYST

The claim of Balas and co-workers that, in contrast to their findings, my co-workers and I found it necessary to use thiophene in the polymer- ization of 1,3-butadiene with a cobaltous chloride-aluminum chloride catalyst to obtain polymers with a high cis unsaturation is inconsistent with our reports (2,3).

The substance of our findings regarding basic catalyst requirements is given in the title of our first report (2), which mentions only a cobalt- ous chloride-aluminum chloride complex as the catalyst, in agreement with findings that although the presence of thiophene offers important practical advantages it is not essential. W e referred to thiophene a s a useful modifier which aids cis-directed polymerization (ref. 2, p. 3233). The claim of Balas and coworkers (1) that in the absence of thiophene they obtained polybutadiene with a high cis content, whereas w e ob- tained only low-molecular-weight polybutadiene with low unsaturation, is consistent with some, but not all , of our findings. W e obtained such polymers only by polymerization in benzene in the absence of thiophene, using catalytic oils prepared from aluminum chloride, cobaltous chlo- ride, and aromatic solvent in the absence of aluminum powder (ref. 2, p. 3238). However, when aluminum powder was used in the preparation of these catalytic oils, a high conversion to high molecular weight poly- butadiene, with at least 94% of its theoretical unsaturation in the cis configuration, w a s obtained in the absence of thiophene and aluminum powder in the polymerization mixture (ref. 2, pp. 3239, 3242). Further- more, with the non-oily benzene solution of catalyst complex studied by Balas and co-workers, we obtained, in 7 hr. in the absence of thiophene, polybutadienes with about 50% of the unsaturation in the cis configura- tion (ref. 2, Fig. l), a finding which is not terribly inconsistent with those reported by Balas and co-workers, particularly in view of the dif- ferent reaction conditions involved in our respective studies. The co- balt content was much higher (100 x), the reaction t ime much longer (28 x), and the conversion much higher (5 x) in our study of this particu- lar system in benzene (ref. 2, Fig. 1). The cumulative effects of these factors could explain the higher cis content of the polymers reported by Balas and coworkers for this system. Also, we did not contend that “even in the presence of thiophene, polymerization failed in the ab- sence of aromatic solvent” as claimed by Balas and co-workers (1). We reported that effective stereospecific catalysis required a solution of catalyst complex in an aromatic solvent (ref. 2, pp. 3233, 3234). This

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statement should be viewed within the context of our report which, un- like the report of Balas and co-workers (l), describes procedures lead- ing to high yields of stereoregular polymer.

Our views (2) regarding the merits of using thiophene were based on examination of results obtained with polymerization procedures de- scribed in previous reports (2-4). The reaction conditions described by Balas in his previous reports (4) and cited in our report (2) are not the s a m e a s those described in his recent report (I), particularly with regard to limitations on conversion and exceedingly stringent impurity- exclusion precautions needed to obtain polymers with a high cis content in the absence of thiophene. These conditions appear to be more criti- cal than those required to bring about cis-directed polymerization with the thiophene-free system w e described (2,3). With thiophene present, however, impurity exclusion requirements are even less critical, and a gel-free polymerization to high conversion is facilitated under all report- ed conditions, particularly i f considerations regarding order of addition of polymerization-mixture components are taken into account, and precip- itation of a catalytically inactive adduct of thiophene with the complex of aluminum chloride and cobaltous chloride is avoided by adding the cobaltous-ion component las t (ref. 2, p. 3237, and the footnote on p. 3239). In view of the difficulties of stereoregular polymerization under conditions described by Balas and co-workers, thiophene modifica- tion still offers practical advantages similar to those of catalyst sys- t e m s based on alkylaluminum and cobalt compounds.

We reported (2) that thiophene does not function merely a s an acid scavenger like aluminum powder, a s proposed by Balas and co-workers (1). Their view is not supported by the evidence, and is inconsistent with findings that conversion is reduced initially and then enhanced on addition of increasing amounts of thiophene (ref. 2, Fig. l), and also with the combined implications of our findings that thiophene promotes cis-directed polymerization only i f its addition precedes contact between monomer and the complex of aluminum chloride with cobaltous chloride (ref. 2, p. 3244), and that the sulfur content of stereoregular cis-1,Cpoly- butadiene prepared in the presence of a large excess of thiophene corre- sponds to one sulfur atom per polyme; chain (ref. 2, p. 3242).* The thio-

*We deleted from our report (2) information on a second sulfur analysis of Soxhlet-extracted polymer reprecipitated after the reported analysis (which confirmed that the sulfur content of the polymer corresponds to one sulfur atom per polymer chain), since a referee believed that this in- formation was unnecessary because most readers would assume we dou- ble-checked our result in this manner. Due to loss of a high-molecular- weight fraction which was oxidatively crosslinked on reprecipitation, the reprecipitated sample had a somewhat lower molecular weight and a cor- respondingly higher sulfur content, indicating that thiophene was not at- tached randomly along polymer chains.

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phene content of a polymerization mixture had no significant effect on the molecular weight of stereoregular polymer (ref. 2, p. 3329) and, a t high concentration, increased rather than decreased the yield of polymer (ref. 2, Fig. l ) , indicating that thiophene does not terminate stereoregu- lar chain growth. Inconclusive findings regarding protonation ( 1) of polymer are not necessarily applicable to thiophene incorporation, since different types of reactions are involved, i.e., protonation involves addi- tion of a cation to polymer, whereas polymer alkylation of thiophene in- volves addition of a cation to thiophene. The function of thiophene, therefore, should not be confused with that of aluminum powder, which was used as a proton scavenger in preparing catalytic oils (2), is not consumed (ref. 2, p. 3252), does not need to be transferred to a polymer- ization mixture, does not need to be present during a thiophene-free polymerization (ref. 2, p. 3239) to obtain polybutadiene with a high cis content, and, hence, cannot be involved in the mechanism of stereoregu- lar cis-l,4-polymerization.

In view of these findings, we proposed that thiophene modification of the catalyst system leads to incorporation of thiophene in the polymer chain during initiation of polymerization (ref. 2, pp. 3245, 3247). How- ever, in light of the findings that did not involve thiophene, we c a m e to the same conclusion a s Balas and co-workers that cis-directed polymer- ization can occur in the absence of thiophene and aluminum powder, a s indicated by our view of the mechanism in general. Our statement on this w a s that “Basically, this mechanism suggests that for cis propaga- tion to occur, an A1 of the complex Co(AlCI,), should have attached to it two groups, a t least one of which should be a polarized diene mole- cule, while the other can be another group possessing a nucleophilic site adjacent to A1 which permits this group to be transferred to the electrophilic end of the polarized diene via a cyclic transition state,” and, a s indicated in related discussions, we suggested that butadiene (ref. 2, left column in Fig. 2) or aromatic solvent (ref. 2, p. 3251) might be the second group which serves to provide, in a less favorable manner, the same function a s thiophene when the latter is not present.

The mechanism we proposed, in principle, need not be inconsistent with findings that catalyst systems other than the cobaltous chloride- aluminum chloride complex also yield polybutadienes with similar micro- structures, and it need not matter whether the catalyst is alkyl-free, thio- phene-free, aluminum-free [e.g., cobaltous hexafhorosilicate (5) and n-- allylcobalt halides (6) ] , or transition metal-free [e.g., aluminum chloride complexes of beryllium and other nontransition-metal ions (i’)]. The ba- sic feature of this mechanism is that the catalyst provides a dual-bind- ing single electrophilic site at which a cyclic cis-directing insertion of monomer into the polymer chain can occur ( 2 ) . It is conceivable that silicon provides such a site in the cobalt hexafluorosilicate catalyst system, which might account for the bonding stability and activity of

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this catalyst in an aqueous system ( 5 ) , and that in the n-allylcobalt halide catalyst system (6 ) , cobalt itself provides this site.

Far more experimental findings can be explained by this mechanism for the cis-directed polymerization of 1,3-dienes than by one requiring n-coordination to a transition metal (l), e.g., with regard to the cis-con- figuration of monomer added to growing polymer chain in the presence and absence of thiophene (2), the effects of thiophene on yield and cis content of polymer (2), the ineffectiveness of thiophene in aiding cis- directed polymerization when added after polymerization is initiated (2), incorporation of one sulfur atom per polymer chain when thiophene is used (2), the cis-directing catalytic activity of aluminum chloride com- plexes of beryllium and other nontransition-metal ions (3, and the lack of stereospecific activity of catalysts containing spin-free cobaltous ion in polymerizations of olefins (2). In connection with the latter, it is interesting to note that, unlike polymerizations with catalysts contain- ing spin-free cobaltous ion, stereoregular olefin and trans-1,4-diene poly- merizations generally involve catalysts containing transition-metal ions, such as those of titanium, vanadium, or cobalt, which have vacant d- orbitals available for n-bonding. Since, furthermore, such ions can be used for either stereoregular trans-l,4 or cis-1,4-polymerization of 1,3- dienes by changing ligands [e.g., by replacing C1- by I- in Ti-A1 based catalysts or by changing solvents associated with n-allylcobalt bromide catalyst (8)], it is suggested that stereoregular cis- 1,4-polymer- ization in these cases arises from a-bonding of monomer to a catalyst component when n-coordination is made less favorable by steric or other ligand effects.

The structure of the aluminum chloride-cobalt chloride complex in benzene proposed by Balas and coworkers (1) is essentially the s a m e a s the structure we proposed (2,3), since one usually assumes interac- tions with polarizable solvents to occur at unoccupied positions of a solute. The basic structure of the complex dissolved in benzene or tol- uene a t room temperature involves cobaltous ion coordinated in a square- planar configuration to two aluminum tetrachloride tetrahedrons, since the unusually large chemical shift (316 ppm) of the NMR signal of A12’ complexed to cobalt is in reasonable quantitative agreement with that expected to arise from the presence of anisotropy in the magnetic mo- ment of cobaltous ion coordinated in a square-planar configuration (ref. 3, Appendix). A 245 ppm chemical shift of the A12’ signal of an alumi- num bromide catalyst complex with cobaltous bromide calculated on the basis of square-planar coordination of cobalt is in particularly good agreement with the 235 ppm shift observed experimentally (ref. 3, Ap- pendix). These findings indicate that solvent molecules involved in charge-transfer interactions, a s observed by Balas and co-workers ( l), are not close enough to complexed cobaltous ion to convey octahedral- coordination characteristics, which would involve a reduction in the

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magnetic anisotropy of Cot’ complexed to AlZ7 and, consequently, a reduction in the NMR shift of the latter, e.g., AH/H= (gL2 - g112) and in the octahedral form of Co(A1C1,)2, g 1 2 gll 2 4 (ref. 3, Table I and Appendix). In view of possible questions regarding the applicability of low-temperature EPR studies of complexed cobalt, it should be empha- sized that it is the NMR shift of A127 complexed to cobalt which pro- vides information about the coordination of cobalt in the active complex a t room temperature. The EPR findings (3), are consistent with the NMR results in the sense that they indicate that no organic polymeriza- tion-mixture component becomes attached to the cobalt of the catalyst, since freezing a solution of complex could be expected to stabilize organocobalt bonds formed in reactions at room temperature, and the EPR spectra of the isolated solid catalyst complex and complex dis- solved in benzene or toluene in the presence and absence of butadiene and thiophene are all very similar, whereas significant spectral changes could be expected to occur on attachment of strong field ligands, such a s olefins and aromatic compounds.

It is questionable whether charge-transfer interactions have a signifi- cant bearing on the mechanism of cis-directed polymerization of dienes. W e observed the EPR signal of a charge-transfer interaction product, the radical-cation of butadiene, during reactions of monomer with stereo- specific cobaltous chloride-aluminum chloride catalyst and with non- stereospecific antimony pentachloride catalyst systems (ref. 3, p. 3271; ref. 2, p. 3252).

The findings of Balas and co-workers (1) extend the range of condi- tions yielding stereoregular catalysis in the absence of thiophene and compliment rather than contradict our findings (2,3).

References

(1) Balas, J . G., H. E. De L a Mare, and D. 0. Schissler, J . Polymer Sci., 4, 2243 (1965).

(2) Scott, H., R. E. Frost, R. F. Belt, and D. E. O’Reilly, J . Polymer Sci., A2, 3233 (1964).

(3) O’Reilly, D. E., C. P. Poole, Jr., R. F. Belt, and H. Scott, J. Polymer Sci., A2, 3257 (1964).

(4) Shell Oil Co.: Belg. Pat. 579,689 (June 15, 1959) and Austral. Patent 49,765 (June 12, 1959).

( 5 ) Canale, A. J., W. A. Hewett, T. M. Shryne, and E. A. Youngman, Chem. Ind. (London), Q62, 1054.

(6) Wilke, G. paper presented a t the 148th Meeting of the American Chemical Society, Div. of Petroleum Chemistry, August 1964, Chicago, 111.

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(7) Goodrich-Gulf Chemicals, Inc.: Belg. Pat. 637,273 (Dec. 31, 196 3).

(8) L. Porri, G. Natta, and M. C. Gallazi, Chim. Ind. (Milan), 6 428 (1964).

Harvey Scott

The Franklin Institute Research Laboratories Philadelphia, Pennsylvania

Received August 27, 1965 Revised November 9, 1965