Single-site metallorganic catalysts for the polymerization of olefins has allowed the synthesis of new polymeric materials having molecular structures that cannot be obtained with conventional Ziegler–Natta catalysts, with exceptional control over the chain architecture, including control of the concentration of stereo- and regio-defects and efficient uniform placement of comonomeric units along the chains. The cases of isotactic and syndiotactic polypropylenes are illustrated as examples of a fine tuning of the physical and mechanical properties of polymeric materials achieved through the controlled incorporation of stereo- and regio-defects, and the rational choice of the catalytic system. Novel isotactic polypropylenes with controlled properties, intermediate between those of stiff-plastic and elastomeric materials have been produced through the design of the catalysts. For syndiotactic polypropylene, a new class of thermoplastic elastomers with finely controlled mechanical properties have been produced by using metallorganic catalysts of different stereoselectivity. Depending on the concentration of stereodefects, elastomers showing entropic or unconventional enthalpic elasticity, can be produced.
