Development of Neodymium-Based Catalysts for the Polymerization of Dienes and Cyclic Esters

High-quality elastomeric materials from the polymerization of dienes are in high demand in industry. However, polymerizing the dienes with greater than 98% cis-1,4 content, with controlled molecular weight remains a prominent challenge. Therefore, the development of new catalysts that polymerize dienes with improved cis-1,4 content, well-defined molecular weights, and high catalytic activity is a topic of substantial interest in industry and academia. Neodymium-based Ziegler-Natta type catalysts are promising to achieve such goals and are the focus of the majority of fundamental studies. A variety of useful neodymium-based catalysts have been developed with oxygen-containing electron-donating ligands such as carboxylates and phosphates. These catalysts are often either highly active or highly stereospecific (yielding high cis-1,4), but seldom both and very often result in high polydispersity index. Recently, nitrogen-containing electron-donor ligands, such as β-diketiminiate and acenaphthene-diimine which offer greater potential for tunable steric and electronic properties, have shown promising results for achieving improved cis1,4 content, with well-defined molecular weight and narrow molecular weight distribution. While neodymium-based catalysts have shown high catalytic activity and stereoregularity for polymerizing dienes, their catalytic activities for ring opening polymerization of cyclic esters has seldom been explored. In this dissertation, two neodymium-based catalysts have been developed and their catalytic activities for the polymerization of dienes and the ring opening polymerization of cyclic esters are investigated. In Chapter 1, the development of neodymium-based catalysts for the polymerization of dienes and ring opening polymerization of cyclic esters are discussed. The effect of polymerization conditions and the mechanistic studies of the catalytic systems are also discussed in this chapter. Chapter 2 discusses the development of a new type of halide-free neodymium-based catalytic system for diene polymerization. The catalytic system consists of the neodymium diethylphosphate catalyst [Nd(μ-DEP)3]x and the co-catalyst triisobutyl aluminum (TIBA) and was used to polymerize β-myrcene. This catalytic system shows promising results and produced polymer with high cis-1,4 content and relatively narrow molecular weight distribution. The kinetic studies demonstrated the pseudo-living characteristics. Chapter 3 focuses on developing neodymium-based catalysts containing bis-diimine ligands. The diimine ligands offer greater potential for tunable steric and electronic properties compared to carboxylate and phosphate ligands. In an attempt to synthesize NdCl3·(L)1 and NdCl3·(L)2 with L = bis(2-pyridinal)ethylenediimine ligand, using azeotropic distillation, we instead obtained [NdCl(L)(H2O)3]Cl2·2(H2O) and L[Nd3Cl6(L)3(O)2]Cl. The crystal structures, luminescent properties, and the magnetic properties of these two complexes will be discussed in this chapter. Chapter 4 discusses the development of neodymium-based catalysts bearing phosphate ligands NdCl3·3L; L = triethyl phosphate (TEP) or tris(2-ethylhexyl) phosphate (TEHP) for the ring opening polymerization (ROP) of ε-caprolactone. These Nd catalysts were tested for ROP of ε- caprolactone (ε-CL) in the presence of benzyl alcohol to generate polymers with relatively narrow polydispersity indices and tunable molecular weights. An important result from the kinetic studies revealed that the catalyst with the sterically bulkier ligand TEHP gave a higher rate of polymerization and lower amount of the transesterified product. The livingness of this catalytic system was demonstrated by kinetic studies and by successful synthesis of the block copolymer poly(ε-caprolactone)-block-poly(L-lactide). In situ NMR studies by monitoring the reaction of NdCl3·3TEP and benzyl alcohol at room temperature are also discussed in this chapter.