3. Monosite catalysts

Catalytic systems based on titanium or chromium are characterized by the presence of several active sites responsible for broadening molar mass distributions, and above all by the fact that the copolymers are mixtures of fractions of different compositions, separable by solvent. Fundamental research has taken many years to elucidate certain aspects of these systems. To do so, it has had to draw on knowledge accumulated in a wide variety of chemical fields: heterogeneous catalysis, surface science, organometallic chemistry and molecular modeling. Since the 1950s, a number of models have been proposed, but are still being debated or called into question (Cossee's model, for example).

Monosite catalysts, on the other hand, are well-defined organometallic compounds that can be characterized by the usual analytical techniques (NMR, IR, centesimal analysis, etc.). They...