A Systematic Study of Nonionic Di- and Multiborane Catalysts for the Oligomerization and Polymerization of Epoxides.

Borane catalysis has emerged as a powerful technology in epoxide polymerization. Still, the structure-activity correlations for these catalysts are not fully understood to date, especially regarding compounds with nonionic backbones. Thus, in this work, 13 different borane catalysts of this respective type are described and investigated for their epoxide oligomerization and polymerization performance, using propylene oxide (PO), 1-butylene oxide (BO) and allyl glycidyl ether (AGE) as monomers.

Mesoionic N-Heterocyclic Olefins as Initiators for the Lewis Pair Polymerization of Epoxides.

Mesoionic N-heterocyclic olefins (mNHOs) have recently emerged as a novel class of highly nucleophilic and super-basic σ-donor compounds. Making use of these properties in synthetic polymer chemistry, it is shown that a combination of a specific mNHO and a Mg-based Lewis acid (magnesium bis(hexamethyldisilazide), Mg(HMDS)) delivers poly(propylene oxide) in quantitative yields from the polymerization of the corresponding epoxide (0.1 mol% mNHO loading).

Synthesis of Poly(propylene oxide)-Poly(-dimethylacrylamide) Diblock Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Solution.

Sterically-stabilized diblock copolymer nanoparticles comprising poly(propylene oxide) (PPO) cores are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous solution. '-Dimethylacrylamide (DMAC) acts as a cosolvent for the weakly hydrophobic trithiocarbonate-capped PPO precursor. Reversible addition-fragmentation chain transfer (RAFT) polymerization of DMAC is initially conducted at 80% w/w solids with deoxygenated water.

Sterically demanding binaphthol-based chiral diboranes for metal-free and isotactic poly(propylene oxide).

Chiral diborane polymerization catalysts with 3,3'-disubstituted binaphthol-backbones are presented. These compounds deliver isotactic poly(propylene oxide) from racemic monomer with isotactoc diad () and triad () placements of up to 92% and >80%, respectively. The resulting polyether is well-defined, of high molar mass and semi-crystalline.

Nontoxic -Heterocyclic Olefin Catalyst Systems for Well-Defined Polymerization of Biocompatible Aliphatic Polycarbonates.

Herein, -heterocyclic olefins (NHOs) are utilized as catalysts for the ring-opening polymerization (ROP) of functional aliphatic carbonates. This emerging class of catalysts provides high reactivity and rapid conversion. Aiming for the polymerization of monomers with high side chain functionality, six-membered carbonates derived from 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) served as model compounds.

Chiral diboranes as catalysts for the stereoselective organopolymerization of epoxides.

It is demonstrated that stereoselective polymerization of epoxides, long a domain of metal-based compounds, can also be achieved the application of organocatalysts. A simple two-step synthesis starting from widely available 1,1'-bi-2-naphthol (BINOL) backbones yields diboranes which, in tandem with organobases, deliver isotactic-enriched () polyethers from the homopolymerization of racemic propylene oxide (PO) and other epoxides. Thereby, isotactic diad contents of up to 88% can be achieved, resulting in well-defined (1.

A simplified approach for the metal-free polymerization of propylene oxide.

Triethyl borane (EtB), in combination with phosphazene-type superbases, has recently emerged as a powerful co-catalyst for the anionic polymerization of epoxides. Here, it is demonstrated that the monomer-activating property of EtB can also compensate for the application of much gentler organobases. This not only results in simpler setups, but also significantly reduces nucleophilicity/basicity-derived side reactions.

Dual Catalytic Ring-Opening Polymerization of Ethylene Carbonate for the Preparation of Degradable PEG.

A dual catalytic setup, consisting of a range of different Lewis bases (including -heterocyclic olefins, phosphazenes and nitrogen bases) and simple Lewis acids (such as LiCl, MgF, BEt), was employed to prepare poly(ether carbonate)s from five-membered, cyclic ethylene carbonate (EC). Polymerizations were conducted under microwave irradiation at = 160-200 °C with low catalyst loading (0.4-0.

Synthesis, properties & applications of N-heterocyclic olefins in catalysis.

N-Heterocyclic olefins (NHOs), a recently (re-)discovered type of electron-rich, polar alkene, are comprehensively presented. Along with synthetic aspects and chemical properties, special emphasis is put on the multi-faceted impact NHOs already have had on catalysis. This is discussed along the lines of small molecule organocatalysis, organo- and metal-assisted polymerization and of the understanding and application of NHO-ligated organometallic complexes.

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High-Molar-Mass Polyethers.

A dual catalytic setup based on N-heterocyclic olefins (NHOs) and magnesium bis(hexamethyldisilazide) (Mg(HMDS) ) was used to prepare poly(propylene oxide) with a molar mass (M ) >500 000 g mol , in some cases even >10  g mol , as determined by GPC/light scattering. This is achieved by combining the rapid polymerization characteristics of a zwitterionic, Lewis pair type mechanism with the efficient epoxide activation by the Mg species. Transfer-to-monomer, traditionally frustrating attempts at synthesizing polyethers with a high degree of polymerization, is practically removed as a limiting factor by this approach.

Proton Affinities of N-Heterocyclic Olefins and Their Implications for Organocatalyst Design.

The proton affinity (PA) of a range of structurally different N-heterocycles with an exocyclic double bond (= N-heterocyclic olefins, NHOs) has been determined using DFT calculations on the BLYP/def2-TZVPP level. It was found that NHOs belong to the upper end of the superbasicity scale, covering PA values from 262 to 296 kcal/mol. Different types of NHOs are compared with each other and with frequently employed organocatalysts.

A global checklist of the Bombycoidea (Insecta: Lepidoptera).

Background: Bombycoidea is an ecologically diverse and speciose superfamily of Lepidoptera. The superfamily includes many model organisms, but the taxonomy and classification of the superfamily has remained largely in disarray. Here we present a global checklist of Bombycoidea.

The Lewis Pair Polymerization of Lactones Using Metal Halides and N-Heterocyclic Olefins: Theoretical Insights.

Lewis pair polymerization employing -Heterocyclic olefins (NHOs) and simple metal halides as co-catalysts has emerged as a useful tool to polymerize diverse lactones. To elucidate some of the mechanistic aspects that remain unclear to date and to better understand the impact of the metal species, computational methods have been applied. Several key aspects have been considered: (1) the formation of NHO-metal halide adducts has been evaluated for eight different NHOs and three different Lewis acids, (2) the coordination of four lactones to MgCl₂ was studied and (3) the deprotonation of an initiator (butanol) was investigated in the presence and absence of metal halide for one specific Lewis pair.

Controlled preparation of amphiphilic triblock-copolyether in a metal- and solvent-free approach for tailored structure-directing agents.

Under mild conditions, PPO-PEO-PPO ("reverse Pluronics") and PBO-PEO-PBO copolyether were generated by way of N-heterocyclic olefin-based organocatalysis. Reverse Pluronics with molar masses > 20 000 g mol could be synthesized with excellent control (Đ ≤ 1.03) and were converted into (ordered) mesoporous carbons via organic self-assembly to showcase the need for tailor-made copolymer as structure-directing agent.

Application of imidazolinium salts and N-heterocyclic olefins for the synthesis of anionic and neutral tungsten imido alkylidene complexes.

The synthesis, single crystal X-ray structure and activity in olefin metathesis of novel anionic tungsten imido alkylidene complexes [1,3-bis-(2,4,6-trimethylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5-Me2Pyr)2Cl](-), [1,3-bis-(2,4,6-trimethylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5Me2Pyr)2(OC6F5)](-), and [1,3-bis-(2,6-diisopropylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5-Me2Pyr)Cl2](-) are reported. Additionally, the first example of a bis(N-heterocyclic olefinium) alkylidene tungstate, W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2-methylene-1,3,4,5-tetramethyl-imidazoline)2(OTf)2, is described, including preparation, crystal structure and catalytic activity.

Highly Polarized Alkenes as Organocatalysts for the Polymerization of Lactones and Trimethylene Carbonate.

In this work, the activity of N-heterocyclic olefins (NHOs), a newly emerging class of organopolymerization catalyst, is investigated to affect the metal-free polymerization of lactones and trimethylene carbonate (TMC). A decisive structure-activity relationship is revealed. While catalysts of the simplest type bearing an exocyclic ═CH moiety polymerize l-lactide (l-LA) and δ-valerolactone (δ-VL) in a non-living and non-quantitative manner, the introduction of methyl substituents on the exocyclic carbon radically changes this behavior.

Convenient preparation of high molecular weight poly(dimethylsiloxane) using thermally latent NHC-catalysis: a structure-activity correlation.

The polymerization of octamethylcyclotetrasiloxane (D4) is investigated using several five-, six- and seven-membered N-heterocyclic carbenes (NHCs). The catalysts are delivered in situ from thermally susceptible CO2 adducts. It is demonstrated that the polymerization can be triggered from a latent state by mild heating, using the highly nucleophilic 1,3,4,5-tetramethylimidazol-2-ylidene as organocatalyst.

Dual Catalysis for Selective Ring-Opening Polymerization of Lactones: Evolution toward Simplicity.

Much work has been directed to the design of complex single-site catalysts for ring-opening polymerization (ROP) to enhance both activity and selectivity. More simply, however, cooperative effects between Lewis acids and organocatalytic nucleophiles/Lewis bases provide a powerful alternative. In this study we demonstrate that the combination of N-heterocyclic carbenes, 1,8-diazabicycloundec-7-ene (DBU) and 4-dimethylaminopyridine (DMAP) with simple Lewis acids enables the ROP of the macrolactone pentadecalactone in a rapid and efficient manner.

N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide).

The metal-free polymerization of propylene oxide (PO) using a special class of alkene—N-heterocyclic olefins (NHOs)—as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal-free system for the polymerization of PO reported to date. The resulting polyether displays predictable end groups, molar mass, and a low dispersity (Đ(M)<1.

Latent and delayed action polymerization systems.

Various approaches to latent polymerization processes are described. In order to highlight recent advances in this field, the discussion is subdivided into chapters dedicated to diverse classes of polymers, namely polyurethanes, polyamides, polyesters, polyacrylates, epoxy resins, and metathesis-derived polymers. The described latent initiating systems encompass metal-containing as well as purely organic compounds that are activated by external triggers such as light, heat, or mechanical force.

Polymerization of ε-Caprolactam by Latent Precatalysts Based on Protected N-Heterocyclic Carbenes.

The cyclic amide ε-caprolactam (ε-CLA) is efficiently polymerized by thermally latent C-2-protected N-heterocyclic carbenes (NHCs) to give the corresponding polyamide (PA 6). Carbon dioxide- and metal salt-protected NHCs were used as precatalysts, out of which the free carbenes were generated thermally. This way, a premixing of the corresponding initiator with ε-CLA was possible to yield a storable and directly polymerizable mixture.

Morphology and molecules reveal unexpected cryptic diversity in the enigmatic genus Sinobirma Bryk, 1944 (Lepidoptera: Saturniidae).

The wild silkmoth genus Sinobirma Bryk, 1944 is a poorly known monotypic taxon from the eastern end of the Himalaya Range. It was convincingly proposed to be closely related to some members of an exclusively Afro-tropical group of Saturniidae, but its biogeographical and evolutionary history remains enigmatic. After examining recently collected material from Tibet, northern India, and northeastern Myanmar, we realized that this unique species, S.

Molecular dynamics studies on the aggregation of Y-shaped fluoroalkanes.

Molecular dynamics (MD) calculations have been performed on the aggregation of clusters with up to 128 Y-shaped perfluoroalkylated molecules of the type C10F20[C7H15]2 (Y-A/128) and C10H20[C7F15]2 (Y-B/128) as well as mixed clusters (Y-A/64+Y-B/64) using the AMBER 5 program. The effect of the segregation tendency of the chemically different parts and the influence of the steric repulsion due to the wedge shape of the molecules on the structure formation have been studied. The results have been analyzed by snapshots, radial atom pair distribution functions, orientational correlation functions as well as diffusion coefficients and are compared with the corresponding findings on clusters of alkanes and perfluoroalkanes.