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 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.

No abstract

The distinctive features of well‐defined, three‐dimensional macromolecules with topologies designed to enhance solubility and amplify end‐group functionality facilitated nanophase morphologies in mixtures with organosilicates and ultimately nanoporous organosilicate networks. Novel macromolecular architectures including dendritic and star‐shaped polymers and organic nanoparticles were prepared by a modular approach from several libraries of building blocks including various generations of dendritic initiators and dendrons, selectively placed to amplify functionality and/or arm number, coupled with living polymerization techniques. Mixtures of an organosilicate and the macromolecular template were deposited, cured, and the phase separation of the organic component, organized the vitrifying organosilicate into nanostructures. Removal of the sacrificial macromolecular template, also denoted as porogen, by thermolysis, yielded the desired nanoporous organosilicate, and the size scale of phase separation was strongly dependent on the chain topology. These materials were designed for use as interlayer, ultra‐low dielectric insulators for on‐chip applications with dielectric constant values as low as 1.5. The porogen design, chemistry and role of polymer architecture on hybrid and pore morphology will be emphasized.

In the last three decades, a significant effort has gone into the development of biodegradable polymers with the object of designing resorbable biomaterials and, more recently, for designing commodity thermoplastics from renewable resources. Aliphatic polyesters, particularly poly(lactide), combine biocompatibility and biodegradability with remarkable physical properties and possess the requisite thermal stability at the processing temperatures. Advances in organometallic chemistry in the design and synthesis of single-site metal catalysts for olefin, [1] ring opening metathesis, [2] and ring opening polymerization techniques [3] have enabled the preparation of well-defined functional polymeric materials with predictable molecular weights and narrow polydispersities. The ring-opening polymerization (ROP) of lactide has been accomplished with a variety of metal catalysts including aluminum, tin, zinc, and yttrium through a coordination ± insertion mechanism. [4] Currently, considerable research is directed towards the preparation of organometallic compounds with tailored ligands that produced poly(lactides) with controlled stereochemistry and microstructure. [3, 5] However, there are few reports on the ROPs of lactides which do not use organometallic promoters. [6] Alternative strategies using only Figure 2. MCD (top) and electronic absorption (2nd from the top) spectra in THF and experimental (2nd from the bottom) and theoretical (bottom) ESI-MS spectra of Ni 3´1 .[1] a)