An overview of catalysts based upon tetra(neophyl)zirconium and bis(arene)titanium complexes supported on partially hydroxylated alumina for olefin polymerization is presented. The chemistry of the surface species has been partially elucidated. When these catalysts are used to polymerize propylene, the resulting stereoblock homopolymer is a thermoplastic elastomer. Physical and mechanical characterization of the polymer indicates that the properties may be attributed to an ether-soluble, largely amorphous, but high molecular weight fraction. The isotactic blocks in this largely atactic fraction cocrystallize with the crystalline, isotactic fraction to provide crosslinks in a rubbery network.