The Rodriguez—Van Looy Model

The observations have led Rogriguez to conclude that the more active sites were in fact crystalline defects, i.e. Ti atoms with 2 vacancies (TiCl3RDn), one of those being occupied most of the time by the / -bridged Al-derivative (Fig. 14) these deficient situations will obviously be formed preferentially on the crystal edges (lower Cl extraction energy). The insertion process itself remains the same as formerly, of course. Obviously, Cossee type centers may coexist, but will be much less active, since the A1 derivative will strongly compete with the monomer for the vacancy. 

This proposal has the merit of explaining a number of experimental facts  

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