Some aspects of the mechanism of the stereospecific polymerization of α-olefins
INTRODUCTION a-Olefins can be polymerized stereospecifically to give crystalline isotactic polymers and, in the case of propylene, under certain specific conditions, also crystalline syndiotactic polymers. The most active and stereospecific catalyst systems in the polymerization to isotactic polymers consists of a halide of some transition metal (e.g. violet TiCJ3 or other metals of the second period) and a halide of an organometallic compound of a light metal (e.g. A1R3, A1R2C1)"2. Other systems which are active in this polymerization are prepared from only one transition metal compound [e.g. TiC13CH3 (refs. 3, 4) or Ti ± TiC13 (ref. 5)]. The last named systems are less active and are also often less stereospecific. All the systems quoted so far are heterogeneous in hydrocarbon diluents. The most interesting catalyst systems for the polymerization of propylene to syndiotactic polymer also consist of a transition metal compound (e.g. VC14, VAc3) and an organometallic compound of a light metal (e.g. A1R2C1)6. These systems are homogeneous—or anyhow very dispersed— in hydrocarbon solvents, and active and very stereospecific in the polymerization under consideration at sufficiently low temperatures, e.g. <—50°C. These different catalyst systems are generally called Ziegler— Natta catalysts. Since the discovery of these processes by Natta7, which dates back to 1954, several hypotheses have been put forward as to the nature of the catalytic complexes that are active in the stereospecific polymerization and the causes determining stereospecificity8; however, none of these hypotheses has been supported so far by completely demonstrative experimental data. The catalyst systems, which have been investigated or considered in more detail by various workers are those quoted first; these are bimetallic and active in the polymerization to isotactic polymers. The discussions on these systems not only concern the reaction mechanism, but also the nature of the components of the catalytic complexes and of the metal—carbon bond the polymeric chain grows on. For instance, it has not been demonstrated so far whether for the bimetallic systems considered [e.g. the systems TiC13—A1(C2H5)2ClJ, the catalytic complexes contain or do not contain both metals, and whether the polymeric chain grows on a Ti—C or Al—C bond. In this communication, it is my desire to particularly discuss these topics, essentially on the basis of the experimental data (most of which are recent)