Synthesis, Structure, and Properties of Poly(isoprene)s of Different Constitutions and Configurations from Catalysts Based on Complexes of Nd, Co, and Fe

Poly(isoprene)s of different molecular structures have been synthesized with various catalysts based on complexes of Nd-, Co-, and Fe-bearing pyridylimine, phosphine, and bipyridine ligands. Poly(isoprene)s with essentially cis-1,4 structure, 3,4 syndiotactic structure, and regular alternating cis-1,4-alt-3,4 structure have been obtained. Moreover, polymers with an unusual prevalent alternating cis-1,4-alt-3,4 structure but containing cis-1,4 units sequences of different lengths (3 or 5 units) within the chain have also been obtained. The structure and mechanical properties of all of the synthesized poly(isoprene)s, some of them completely new, have been investigated. Poly(isoprene)s with a cis-1,4 structure and with a perfectly alternating cis-1,4-alt-3,4 structure or prevalent alternating cis-1,4-alt-3,4 structure are amorphous, exhibiting mechanical behavior with a viscous flow at relatively high deformation. The sample of poly(isoprene) with almost a regular 3,4 syndiotactic structure (about 79% of 3,4 units) crystallizes in the stable orthorhombic form of 3,4-syndiotactic poly(isoprene). This crystalline sample shows better mechanical properties of deformability and flexibility than the other amorphous poly(isoprene)s without a viscous flow with plastic deformation and breaking at relatively high strain around 300–400% and remarkable elastic properties. The plastic deformation and the elastic behavior are associated with a reversible transformation during tensile deformation of the crystalline orthorhombic form into a disordered mesomorphic form, which recrystallizes into the orthorhombic form upon relaxation during elastic recovery. The glass transition temperature of the different poly(isoprene)s decreases linearly with the increasing cis-1,4 content, from 16 °C of the poly(isoprene) with the prevalent 3,4 structure and 21% of cis-1,4 units to −64 °C of the poly(isoprene) with 98% of cis-1,4 units.