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

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Abstract

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<1.09). NHOs with an unsaturated backbone are essential for polymerization to occur, while substitution at the exocyclic carbon atom has an impact on the reaction pathway and ensures the suppression of side reactions.

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Optimization of an AMBER Force Field for the Artificial Nucleic Acid, LNA, and Benchmarking with NMR of L(CAAU)

David Condon, Ilyas Yildirim, Scott D. Kennedy … (2013)

Locked Nucleic Acids (LNAs) are RNA analogues with an O2′-C4′ methylene bridge which locks the sugar into a C3′-endo conformation. This enhances hybridization to DNA and RNA, making LNAs useful in microarrays and potential therapeutics. Here, the LNA, L(CAAU), provides a simplified benchmark for testing the ability of molecular dynamics (MD) to approximate nucleic acid properties. LNA χ torsions and partial charges were parametrized to create AMBER parm99_LNA. The revisions were tested by comparing MD predictions with AMBER parm99 and parm99_LNA against a 200 ms NOESY NMR spectrum of L(CAAU). NMR indicates an A-Form equilibrium ensemble. In 3000 ns simulations starting with an A-form structure, parm99_LNA and parm99 provide 66% and 35% agreement, respectively, with NMR NOE volumes and 3 J-couplings. In simulations of L(CAAU) starting with all χ torsions in a syn conformation, only parm99_LNA is able to repair the structure. This implies methods for parametrizing force fields for nucleic acid mimics can reasonably approximate key interactions and that parm99_LNA will improve reliability of MD studies for systems with LNA. A method for approximating χ population distribution on the basis of base to sugar NOEs is also introduced.

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Macromolecular Design via an Organocatalytic, Monomer-Specific and Temperature-Dependent “On/Off Switch”. High Precision Synthesis of Polyester/Polycarbonate Multiblock Copolymers

Peter Olsen, Karin Odelius, Helmut Keul … (2015)

The employment of a monomer-specific “on/off switch” was used to synthesize a nine-block copolymer with a predetermined molecular weight and narrow distribution (Đ = 1.26) in only 2.5 h. The monomers consisted of a six-membered cyclic carbonate (i.e., 2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC)) and ε-caprolactone (εCL), which were catalyzed by 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD). The dependence of polymerization rate with temperature was different for the two monomers. Under similar reaction conditions, the ratio of the apparent rate constant of AOMEC and εCL [k p app(AOMEC)/k p app(εCL)] changes from 400 at T = −40 °C to 50 at T = 30 °C and 10 at T = 100 °C. Therefore, by decreasing the copolymerization temperature from 30 °C to −40 °C, the conversion of εCL can be switched “off”, and by increasing the temperature to 30 °C, the conversion of εCL can be switched “on” again. The addition of AOMEC at T = −40 °C results in the formation of a pure carbonate block. The cyclic addition of AOMEC to a solution of εCL along with a simultaneous temperature change leads to the formation of multiblock copolymers. This result provides a new straightforward synthetic route to degradable multiblock copolymers, yielding new interesting materials with endless structural possibilities.

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N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.

Maréva Fèvre, Julien Pinaud, Yves Gnanou … (2013)

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

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Author and article information

Journal

Journal ID (nlm-ta): Angew Chem Int Ed Engl

Journal ID (iso-abbrev): Angew. Chem. Int. Ed. Engl

Journal ID (publisher-id): anie

Title: Angewandte Chemie (International Ed. in English)

Publisher: WILEY-VCH Verlag (Weinheim )

ISSN (Print): 1433-7851

ISSN (Electronic): 1521-3773

Publication date (Print): 10 August 2015

Publication date (Electronic): 01 July 2015

Volume: 54

Issue: 33

Pages: 9550-9554

Affiliations

Department of Chemistry, University of Warwick, Materials and Analytical Science Building CV4 7AL Coventry (UK)

Author notes

[*] Dr. S. Naumann, A. W. Thomas, Prof. A. P. Dove Department of Chemistry, University of Warwick, Materials and Analytical Science Building, CV4 7AL Coventry (UK) E-mail: s.naumann@ 123456warwick.ac.uk a.p.dove@ 123456warwick.ac.uk

[[**]]

S.N. gratefully acknowledges his DFG research stipend (NA 1206/1). The Royal Society is also acknowledged for the award of an Industry Fellowship to A.P.D. The University of Warwick and EPSRC are also acknolwedged for funding.

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201504175.

Article

DOI: 10.1002/anie.201504175

PMC ID: 4539597

PubMed ID: 26136456

SO-VID: f54e7587-6f58-434f-a33a-93416302c4b5

Copyright © © 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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

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Most cited references 65

Optimization of an AMBER Force Field for the Artificial Nucleic Acid, LNA, and Benchmarking with NMR of L(CAAU)

Macromolecular Design via an Organocatalytic, Monomer-Specific and Temperature-Dependent “On/Off Switch”. High Precision Synthesis of Polyester/Polycarbonate Multiblock Copolymers

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.

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