POLYMER LETTERS VOL. 3, PP. 279-282 (1965)

STEREOSPECIFIC POLYMERIZATION OF CONJUGATED DIENES BY NEW SOLUBLE CATALYSTS

Catalyst systems of alkylaluminum/transition metal chelates were re- ported previously by the authors to polymerize various epoxides to crys- talline polymers of high molecular weight (1,2).

Recently, we have found that analogous catalyst systems are very ac- tive and stereoselective in the polymerization of conjugated dienes, es- pecially of butadiene, when organoaluminum halides such a s diethylalu- minum chloride (AlEt 2C1) are used, instead of trialkylaluminum, together with metal chelates of cobalt. The importance of obtaining polybuta- diene with a high content of cis-structure is well known. According to this novel method, polybutadiene with exceptionally high cis-content is readily obtainable.

The chelate compounds are different from usual inorganic or organic m e t a l s a l t s in the nature of the chemical bond and often show unique physical or chemical properties. They are generally understood to arise from the coordinate bond between the metal and electron donor at- oms of the chelating agent. There are three kinds of electron donor at- oms: N, 0, and S; and these atoms form the coordinate bond by donating lone paired electrons to the metal to give the chelate compound. Conse- quently the chelate compounds are conveniently classified by the combi- nation of the electron donor atoms. Therefore, there are six kinds of chelates containing pairs of electron donor atoms such a s (O,O), (O,N), (N,N), (O,S), (N,S), and (S,S). According to our study, most of the che- la tes examined were very active for the polymerization of butadiene and its derivatives (isoprene, l,+pentadiene, etc.,) and showed surprising selectivity in stereoregularity when they were used with organoaluminum halides such a s AlEt2C1.

Recent patent literature reveals that catalysts prepared from metal acetylacetonates and alkylaluminum halides polymerize butadiene to a high cis-content polybutadiene (3,4).

Characteristic of a chelate is its ability to form a soluble complex with AlEt 2C1 in aromatic or halogenated hydrocarbon solvents. Some of the chelates dissolve in organic solvents without addition of AlEt 2C1. A second characteristic is that even a small amount of the chelate is active enough to produce large quantities of polybutadiene having an ex- ceptionally high content of cis-structure; generally one molecule of the chelate produces a t least one polymer molecule, whereas scores of co- balt atoms are needed to form one polymer chain with an insoluble cata- lyst such as AIEt2Cl/CoC12 (5). Thirdly, the reaction systems are com- pletely homogeneous throughout the reaction period and give rise to a

279

TABL

E I

I4

03 0

Polymerization of Butadiene

by AlEt 2C1/Co-Chelate Catalysts a

Exp.

No.

Co-Chelate

[All/[Co]

React. Yield,

mmole/l.

Intr.

p-Structure, moI.-%

C T

V

toluene

%

visco.

mol.

time,

ratio

min.

1 bis(Salicylaldehyde)Co(II)

0.57

100

150

92

3.35

99.3

0.4

0.3

2 bis(a-Oxyacetophenone)Co(II)

0.57

100

60

88

1.97

98.7

0.7

0.6

3 4 5 6 7 8 9 10

11

bis(Quinizarine)Co(II)

bis(o-Vaniline)Co(II)

bi s(

5-Oxy- 1 , 4-naphthoquinone)Co(II)

bi s( 8-Oxyquinoline)Co(II)

bis(Salicyla1dehyde imine)Co(II)

bis(Salicy1aldehyde)ethylenediimine-Co(II)

tris(a-Nitroso-/3-naphthol)Co(III)

bis(Mercaptobenzothiazole)Co(II)

bis(Metcaptobenzoxazole)Co(II)

0.57

0.57

0.86

0.57

0.70

1.00

1.00

0.43

0.57

120

100 70

100

100 70

70

100

100

60

80

50

85

90

54

60

100

40

86

60

74

50

86

90

100

60

96

2.75

3.23

2.77

3.50

2.72

1.70

1.67

2.74

3.38

99.4

0.4

0.2

99.4

0.3

0.3

99.5

0.3

0.2

99.3

0.4

0.3

99.3

0.4

0.3

99.0

0.5

0.5

98.9

0.7

0.4

9 99.4

0.3

0.3

99.3

0.4

0.3

12 bis(Mercaptobenzimidazole)Co(II)

0.43

100

60

100

4.06

99.5

0.3

0.2

3 w v,

‘Polymerization wa

s carried out in a ice bath.

bC = cis-1,4; T =

trans-l,4; V

= vinyl-1,2.

2

POLYMER LETTERS 28 1

soluble polymer containing no gel fractions. The general procedure of polymerization was a s follows: the desired

quantity of the chelate was weighed into a reaction tube and toluene was added. After cooling the reaction mixture in an ice bath, AlEt,Cl was added under a nitrogen atmosphere. Then butadiene was bubbled into the catalyst solution and the polymerization was continued for the required period of t ime. When the reaction was completed, the viscous reaction mixture was stirred vigorously into large quantities of methanol containing a small amount of phenyl-B-naphthylamine. The elastic solid polymer was coagulated and dried a t 5OoC. in vacuo. The microstructure of the resulting polymer was determined by IR spectroanalysis according to Silas’s method (6). Intrinsic viscosity was measured at 3OoC. in ben- zene solution.

chelate catalyst systems polymerize butadiene to a high molecular weight polymer having a high content of cis-structure.

ed by stereoregulated adsorption of the monomer and polarization of the unsaturated center a t an active solid surface of the catalyst, but it is difficult to apply this hypothesis to diene polymerization brought about by soluble catalyst systems. It is probable that the initial s tep in the reaction is complexation of the butadiene molecule at the catalyst cen- ter prior to addition to the polymer chain. There is indirect evidence to suggest that a complex is formed between the diene and the soluble com- plex catalyst of AlEt&l/chelate, since the color of the catalyst solu- tion is changed upon introduction of the diene (usually from a pale yel- lowish green to almost colorless or a slight yellowish red). It is sup- posed that orientation in the polymer is determined by the conformation of the diene in the complex formed between the monomer and the catalyst. Hallman and Pauson have pointed out that butadiene is added to the iron carbonyl complex in the cis-conformation (7). Thus the essential step in the stereospecific polymerization of the butadiene seems to be the or- ientation of the diene molecule in cis-conformation by complexation with Co or N i of the catalyst, where the bulky ligand of the chelate may play an effective role in controlling the coordinate direction of the monomer to the transition m e t a l in sterically hindered form. Thus the diene in the pre-determined conformation might then be transfered to the organo- aluminum part of the complex catalyst, and the polymer chain may grow further stereoregularly in a similar manner.

Direct evidence for those views are scanty now and further discussion is difficult with the few experimental results which have been obtained in this study. However, further investigations in these directions are in progress and the results will be published in the near future.

The results are summarized in Table I. It shows that the A1Et2C1/Co-

In the case of a-olefins, stereospecific polymerization is best explain-

The authors wish to express their grateful appreciation to Dr. W.

282 POLYMER LETTERS

Cooper of the Dunlop Research Center for valuable suggestions, and to Mitsui Chemical Industry Co. Ltd., for their support of this work.

References

(1) Kambara, S., and A. Takahashi, Makromol. Chem., 6 3 89 (1963). (2) Takahashi, A., and S. Kambara, Makromol. Chem., 121 92 (1964). ( 3 ) Natta, G., L. Porri, and L. Fiore, Ital. Patent 597,770. (4) Natta, G., L. Porri, and L. Fiore, Brit. Patent 849,589. (5) Cooper, W., Rubber Plastics Age, $ 44 (1963). (6) Silas, R. S., J . Yates, and V. Thornton, Anal. Chem., & 529

(1959). (7) Hallam, B. F., and P. L. Pauson, J . Chem. SOC., 1358, 642. It

i s reviewed recently by M. A. Bennett i n Chem. Rev., a 611 (1962) and also by E. 0. Fisher and H. Werner in Angew. Chem., 75, 57 (1963).

A. Takahashi S. Kambara

Laboratory of Polymer Chemistry Tokyo Institute of Technology Tokyo, Japan

Received June 15, 1964.