Elemental organic compounds. Part XXI. Polymerization of ethylene by dibenzenechromium

Abstract

It has recently been reported that the polymerization of biitadiene takes place on surfaces of various transition metal atoms such a s Ti, Cr, Fe, Co, and Ni, each of which is formed by the decomposition of corresponding organic derivatives.The polymerization mechanism can be explained by the concept of catalysis at the atomic level which, when created, increases the activity of the transition metal towards addition polymerization by means of stimulatec. interaction of r-electrons of the monomer with the metal d orbitals. The oligomerization of butadiene by a zero-valent nickel atom has also been interpreted by essentially the same mechanism.2We have, however, been able to dimerize a monoolefin, ethylene, selectively on atomic nickel which was produced by the decomposition of nickelocene. 3 We have now found another example of this type of ethylene polymerization catalyst. A solution of dibenzenechromium (0), 1.1 g. in nitrogen-saturated n-heptane (50 d.), was placed in a 300-ml. autoclave under a nitrogen atmosphere. Ethylene was charged into the autoclave to 630 psi. The pressure did not change on prolonged heating a t 200°C. for 16 hr. However, a t 250OC. and 2800 psi, a rapid decrease to 1100 psi was observed after 7 hr. The white solid polyethylene formed was dissolved in hot xylene and boiled with hydrochloric acid three times to remove the catalyst and was reprecipitated several times with xylenemethanol.According to infrared analysis, it seems to have contained only an internal trans double bond (965 cm.-I) as the unsaturated bond.This value corresponds to 88% ~rystalliinity.~The intrinsic viscosity measured in tetralin at 130°C. was 0.99, from which the molecular weight calculated by Tung's equation was 34,000.6On the basis of these findings, it appears that the polymerization reaction may proceed through atomic level catalysis, since the reaction commences a t a temperature close to the decomposition poin t 6 of dibenzenechromium.The trans double bond in the polymer implies that the double bond was fixed to the catalyst during the propagation reaction and the chain grew from both sides independently. Thus, the mechanism of eqs. (1) is postulated.The catalyst was found decomposed in the cooled autoclave.The purified polymer weighed 3.7 g.The density was 0.966 g./cc. by the density-gradient tube method. JOURNAL OF POLYMER SCIENCE: PART A-I vor,. 5 (1967) Soon after metallic chromium is formed from the thermal decomposition of dibenaenechromium, the surrounding ethylene molecules start interacting with the vacant d orbitals of the metal to form n-bonding (I). Since the active chromium atoms shows a strong ability to dehydrogenate orgauic groups forming chromium hydride,' the ethylene ligands form a biradical olefiiiic chain (111).Each radical can add additional ethylene molecules assisted by the central metal which can interact with the ethylene *-system (111, IV). Thus, the successive radical propagation continues catalytically until hydrogen radicals from the decomposition of chromium hydride finally...

Review of Recent Literature on Ziegler-Type Catalysts

“…before mixing them with TiCl,. Tsutsui and Kayano (1967) found active at 250°C, and at 2800 psi the dibenzenechromium compound and high density (0.966 gram per cc) polyethylene was formed. The formation of partially crystalline polypropylene and highly linear polyethylene by means of ball-milled TiCL was described by Werber, Benning, and coworkers (Werber et al, 1968).…”