Side reactions that produce PP and PS chain scission occur for high catalyst contents. The corresponding molecular weight decrement was also considered for the interpretation of the mechanical behavior. For 1.0% AlCl3, elongation at break and toughness dropped due to this effect, even when the interfacial adhesion was very good. The homopolymer chain scission also affected the tensile strength. This property remained invariable notwithstanding the particle size decay. Then, it can be inferred that the corresponding reduction in molecular weight must be compensating the strength increment.