71 - STEREOSPECIFIC POLYMERIZATION OF CONJUGATED DIOLEFINS WITH CATALYSTS CONTAINING DIFFERENT TiCl3 MODIFICATIONS

Abstract

At least three different crystalline modifications, indicated by the authors α, β, γ respectively, are possible for titanium trichloride. The α modification is obtained by reduction of TiCl4 with H2 at high temperature; its colour is violet and it is characterized by a layer lattice. The β modification, which is brown and characterized by a fibre lattice can be obtained: (a) by treating AlEt3 at room temperature with TiCl4 (Al/Ti molar ratio 1) or AlEt2Cl with TiCl4 (Al/Ti≥ 1) in hydrocarbon solution; (b) by decomposition of TiCl3 mono alkyls in hydrocarbon solution; (c) by the action of dark charges at room temperature on a gaseous mixture of H2 and TiCl4, according to the process described by Bock and Moser in 1912 (Monatsh.33, 1407, 1912). By heating the β modification above 200°C, a violet modification with layer lattice was obtained, which, however, is not similar to α—TiCl3; this new modification has been indicated as γ-TiCl3. Catalysts prepared from aluminum trialkyls or aluminum dialkylhalides and TiCl3 show a different stereospecificity in the polymerization of conjugated diolefins, depending on the modification of TiCl3 used. The AlR3-α-TiCl3 or -γ-TiCl3 systems give, in the polymerization of butadiene, crystalline polymers, which are chiefly trans-1,4:. In the case of isoprene, these systems yield polymerizates, the crystalline fraction of which is practically identical, by I.R. and X-ray analyses, with natural guttapercha. The systems obtained from AlR2Cl and α-TiCl3 or γ-TiCl3 respectively show the same stereospecificity as those obtained from AlR3. On the contrary the catalytic systems obtained from aluminum alkyls and β-TiCl3 show a quite different behaviour. In the polymerization of butadiene the AlEt3-β-TiCl3 system (Al/Ti≥ 1) yields polymerization products consisting of a mixture of products amorphous at room temperature with chiefly cis-1,4 arrangement, and of crystalline products which are substantially trans-1,4: (<90 per cent). The AlEt2Cl-βTiCl3 system independently of the Al/Ti ratio, yields products which are predominantly 1,4 (<90 per cent), and which appear to be cis-1,4 and trans-1,4 copolymers. In the polymerization of isoprene, the AlEt3–β-TiCl3 and AlEt2Cl–β-TiCl3 systems yield polymers which are prevailingly cis-1,4 (<90 per cent), and which exhibit properties similar to those of natural rubber. No appreciable difference was observed between the systems prepared from β-TiCl3, obtained in the AlEt3-TiCl4 reaction and according to Bock and Moser respectively. The results obtained when using catalysts prepared from different modifications of TiCl3 are interpreted by admitting that, due to the different crystalline structure, different catalytic complexes are formed on the surface of the TiCl3 crystals, in the reaction with aluminum alkyls.