Ring-Opening Polymerization of L-Lactide Catalyzed by Potassium-Based Complexes: Mechanistic Studies

Two non-toxic potassium compounds, 1 and 2, with a commercial oximate ligand have been prepared and fully spectroscopically characterized. Their activity as catalysts for the ring-opening polymerization (ROP) process of LLA has been studied, showing that they are extremely active and able to polymerize the monomer in a few minutes. For derivative 2, the presence of a crown ether in the potassium coordination sphere affects the nuclearity of the compound and consequently its solubility, with both aspects having an influence in the polymerization process. Detailed studies of the polymerization mechanism have been performed, and an unusual anionic mechanism was observed in absence of a co-initiator. Indeed, the monomer deprotonation generates a lactide enolate, which initiates the polymerization propagation. On the contrary, when a 1:1 ratio of cat:BnOH is used, a mixture of mechanisms is observed, the anionic mechanism and the activated monomer one, while from a cat:BnOH ratio of 1:2 and over, only the activated monomer mechanism is observed

Aluminum complexes of mono-pyrrolidine ligands for the con-trolled ring opening polymerization of lactide

In this paper we report the full characterization (solution-state NMR spectroscopy and solid-state structures) of a series of Al­(III) half-salan complexes and their exploitation for the ring-opening polymerization of <i>rac</i>-lactide. Depending on the ligand employed and stoichiometry of the complexation, structures of the form Al­(<b>X</b>)₂Me or Al­(<b>X</b>)­Me₂ were isolated. Interestingly Al­(<b>2</b>)₂Me and Al­(<b>2</b>)­Me₂ produce PLA with a strong isotactic bias (<i>P</i>_m up to 0.80), whereas all other complexes produced atactic PLA. This is in contrast to recent studies on similar salan ligand systems. PLAs with predictable molecular weights and narrow distributions were achieved. The results are discussed in terms of steric and electronic properties of the ligands

Tuning the thiolen: Al(III) and Fe(III) thiolen complexes for the isoselective ROP of rac-lactide

A series of five iron and aluminum complexes bearing {ONSO} imine thiobis(phenolate) ligands have been prepared and applied to the ring-opening polymerization (ROP) of rac-lactide. Fe(1)Cl produced polylactide with a very strong isotactic bias (Pm = 0.79–0.89) and well-defined melting temperatures (Tm = 154–181 °C). The polymers have been characterized by a combination of 1H{1H} NMR, 13C{1H} NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction. Fe(1)Cl has also been shown to activate CO2, at atmospheric pressure and concentrations, to form a carbonato-bridged dimer. Fe(2–5)Cl and Al(1–5)Me were also active for lactide ROP demonstrating good-molecular-weight control (Đ = 1.04–1.12) and moderate isotactic preference (Pm = 0.56–0.72), with polymerization outcome correlating with ligand substituents

Design and synthesis of new C1 and C2-symmetric ansa-metallocene catalysts for isotactic-polypropylene formation

Several ansa C1-symmetric cyclopentadienyl-fluorenyl metallocenes based on zirconium have been prepared with different substituents at position 3 on the cyclopentadienyl ring. Isotactic polypropylene production from these systems depends highly on the size of these substituents. Therefore, large groups such as 1-methyl-4-tbutylcyclohexyl (metallocene 6), 1-methyl-cyclohexyl (metallocene 7), 1,3,3,5- tetramethylcyclohexyl (metallocene 8), and 2,3,4-trimethyl-3-pentyl (metallocene 9) have been investigated. In combination with methylaluminoxane (MAO), they showed good activity and produced high molecular weight of isotactic polypropylene. In terms of the tacticity of the polymers, metallocene 6 made the best isotactic polypropylene with ~88% mmmm pentad content. Also, it has been found that if the size of this substituent is large as in 2,3,4-trimethyl-3-pentyl (metallocene 9), then it will block the polymerization active site which will deactivate the metallocene. New synthetic pathways for the synthesis of cyclopentadienyl-fluorenyl metallocenes based on titanium have been achieved. Anchoring these types of ligands onto titanium by following the conventional method of using TiCl4 in the metallation step has failed for the production of Me2C(3-(diphenylmethyl)-C5H3)(C13H8)TiCl2 (metallocene 12), Ph2C(C5H4)(C13H8)TiCl2 (metallocene 14), and Ph2C(C5H4)(C13H8)TiMe2 (metallocene 15); this is possibly due to the high reactivity of TiCl4. Therefore, TiCl4·2THF has been prepared and used in that step to produce these new titanocenes with good yields. A new ansa-C2-symmetric substituted bis-indenyl metallocene for isotactic polypropylene production has been successfully prepared. It is known that ansa-C2- symmetric metallocenes are good catalysts for isotactic polypropylene production, but in general, their synthesis suffers from the production of the meso Cs-stereoisomer of these catalysts, which generally produces only atactic polypropylene. Therefore, the meso stereoisomers must be removed and this is considered a loss of the material that increases the cost of the catalysts. Addition of bulky substituents on the indenyl groups as in Me2Si(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)2ZrCl2 (metallocene 5) has prevented the meso stereoisomer production. 5/MAO produces isotactic polypropylene with up to ~80% mmmm pentad content

TARGETED DESIGN OF CO-CONTINUOUS NANOSTRUCTURES IN COPOLYMERS

Microphase separated copolymers with nano-scale morphologies are critically important in designing next generation materials. Cocontinuous nanoscale structures, in which domains of multiple different phases each simultaneously percolate in three dimensions, provide opportunities to synergistically combine properties of the constituent polymers in a wide variety of contexts. While cocontinuous nanostructures are fabricated through equilibrium self-assembly of block or graft copolymers and kinetically trapped phase separation of polymer blends or crosslinked copolymer networks, their formation is highly sensitive to changes in chemical details, synthesis and/or processing conditions, bringing practical challenges to generalization to multiple systems. In this dissertation, we focus on transforming the design of cocontinuous morphologies from complicated protocols to general and robust principles by pre-designing telechelic polymers with well-defined end functionality, molecular weight and polydispersity. Relying on end-linking of telechelic polystyrene (PS) and poly (D,L - Lactide) (PLA) with a multi-functional crosslinker, randomly end-linked copolymer networks (RECNs) are synthesized and thoroughly characterized. Particularly, for the first time we are able to map the phase diagram of symmetric (Mn, A ≈ Mn, B) RECNs, highlighting the critical microphase separation transition (6 \u3c (χN)critical \u3c 12), above which disordered cocontinuous nanostructures span over 30 vol% and morphologies with dispersed domains reside on either side. The critical impacts of chemical parameters (strand length Mn, strand asymmetry, strand dispersity Đ, junction functionality) are further evaluated to influence the microphase separated structures. While maintaining cocontinuity, uniaxial stretching of PS/PLA RECNs above the glass transition temperatures introduces controlled orientation through a two-step process (domain stretching and domain rotation), which is found to provide substantial improvements in yield strength, toughness, and stiffness for bulk materials at room temperature. Nanoporous materials with interconnected porous structures are then fabricated by selective removal of the easily degradable PLA domains. Lastly, linear and branched multi-block copolymers (MBCs) with various block length are fabricated using step polymerization of telechelic PS and PLA. In addition to their ability to form cocontinuous morphologies within microphase separated and non-crosslinked MBCs, their solubility is dramatically improved in comparison to crosslinked copolymers