More than half of the 170 million metric tons of polymers produced each year are polyolefins. Current technology uses highly active cationic catalysts, which suffer from an inability to tolerate heteroatoms such as oxygen, nitrogen, and sulfur. These systems require scrupulously clean starting materials and activating cocatalysts. A family of catalysts has been developed whose members are tolerant of both heteroatoms and less pure starting materials. These heteroatom-tolerant neutral late transition metal complexes are in fact highly active systems that produce high-molecular-weight polyethylene, polymerize functionalized olefins, and require no cocatalyst.
A metal‐free approach to the living ring‐opening polymerization (ROP, shown schematically) of lactide has been developed using strongly basic amines such as 4‐(dimethylamino)pyridine as transesterification catalysts. These organic catalysts must be used in combination with a nucleophile such as an alcohol, which is the actual initiating species.
A novel metal-free, organocatalytic approach to living polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening polymerization (ROP) of cyclic ester monomers. The catalysts is used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional omega-chain end that propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1-2 h. Catalysis with respect to both initiating alcohol and monomer was observed. Control of the alpha and omega end-groups was demonstrated with a pyrene-labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the N-heterocyclic carbenes is believed to ensue through a monomer-activated mechanism.
A metal-free, organocatalytic approach to living polymerization using N-heterocyclic carbenes as nucleophilic catalysts generated and used in situ in a single-pot process is detailed. The N-heterocyclic carbene catalyst platform is extremely versatile, as the nature of the substituents has a pronounced effect of catalyst stability and activity toward different substrates. The generation of imidazolium- and thiazaolium-based carbenes was accomplished from the reaction of the corresponding salts with the appropriate bases. This allowed the rapid screening of libraries of catalysts that provided a basic understanding of catalyst structure (sterics, electronics, etc.) with the polymerization rate, control, substrate, and range of molecular weights. The imidazole-based catalysts were significantly more active toward ROP than the thiazolium-based analogues. No appreciable differences between imidazol-2-ylidene and imidazolin-2-ylidene catalysts were observed. Less sterically demanding carbenes were found to be more active toward ring-opening polymerization (ROP) than their sterically encumbered analogues for lactone polymerization. These data prompted the investigation of ionic liquid as a precatalyst reservoir in a phase-transfer polymerization with an immiscible THF solution of monomer and initiator. In situ activation of the ionic liquid generates carbene that migrates to the organic phase effecting living ROP. Precatalyst (ionic liquid) regeneration terminates polymerization. This simple reaction/recycle protocol readily allows repetitive ROPs from the ionic liquid using commercially available materials.
Neutral Ni(II) salicylaldimine catalysts (pendant ligand ϭ NCMe or PPh 3 ) were used to copolymerize ethylene with monomers containing esters, alcohols, anhydrides, and amides and yielded linear functionalized polyethylene in a single step. ␣-Olefins and polycyclic olefin comonomers carrying functionality were directly incorporated into the polyethylene backbone by the catalysts without any cocatalyst, catalyst initiator, or other disturber compounds. The degree of comonomer incorporation was related to the monomer structure: tricyclononenes Ͼ norbornenes Ͼ ␣-olefins. A wide range of comonomer incorporation, up to 30 mol %, was achieved while a linear polyethylene structure was maintained under mild conditions (40°C, 100 psi ethylene). Results from the characterization of the copolymers by solution and solid-state NMR techniques, thermal analysis, and molecular weight demonstrated that the materials contained a relatively pure microstructure for a functionalized polyethylene that was prepared in one step with no catalyst additive.
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