Early transition metal catalyzed synthesis of complex polymer architectures

While early transition metal (Ti, Zr) complexes are heavily exploited in α-olefin metallocene polymerizations, their radical polymer chemistry is virtually neglected. One of the objectives of this research was the exploration of the radical and coordinative ring opening chemistry of Ti in the synthesis of complex polymer architectures through development of novel, inexpensive and simple catalysts/initiators systems for mechanistically dissimilar living polymerizations. Cp2Ti(III)Cl generated by room temperature Zn reduction of Cp2Ti(IV)Cl2 was explored as a catalyst for living radical polymerization (LRP) of styrene and isoprene initiated from four different classes of initiators such as epoxides, aldehydes, peroxides and halides as well as for the living ring opening polymerization (ROP) of cyclic esters via the corresponding in situ generated Ti-alkoxides. Moreover, these two mechanistically dissimilar polymerizations can occur simultaneously, thereby providing convenient fast access to novel, topologically complex block, graft, miktoarm stars, and hyperbranched systems via simple one pot procedures. ^ Zirconium complexes were also demonstrated to be extremely efficient, low toxicity initiators for the ring opening of cyclic esters. Thus Zr-alkoxides generated by in-situ hydrozirconation of cyclic esters (e.g., caprolactone) and aldehydes catalyze the synthesis of ultra-high molecular weight poly(caprolactone) (PCL) by living ROP. The resulting polymers show unprecedented improvement in mechanical and rheological properties over commercially available PCL and will expand the applications of biocompatible polymers both as commodity plastics and in biomedical field due to their controlled biodegradability. ^ These advances provide powerful new synthetic tools for polymer science on the way of bridging radical, ring opening and coordination polymerizations. Their implementation will converge in the controlled synthesis of monodisperse polymer structures with complex new architectures and unusual topologies and compositions based on a wide, previously inaccessible selection of monomers and therefore leading to new material properties.