Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA
Electrochemically switchable ring-opening polymerization of lactide and cyclohexene oxide Miao Qi, Qi Dong, Dunwei Wang, and Jeffery A Byers J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02171 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
Electrocheimcally switchable ring-opening polymerization of lactide and cyclohexene oxide Miao Qi‡, Qi Dong‡, Dunwei Wang, and Jeffery A. Byers* Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill, Massachusetts, 02467, United States Supporting Information Placeholder ABSTRACT: An electrochemical method was developed for the redox switchable polymerization of lactide and an epoxide. Using a lithium reversible sacrificial electrode and a high surface area carbon working electrode, efficient transformation between formally iron(II) and iron(III) oxidation states of a bis(imino)pyridine iron alkoxide complex was possible, which led to the ability to activate the complex for ring opening polymerization reactions. In addition to serving as a redox trigger, an electrochemical toggle switch was developed in which the chemoselectivity for lactide and epoxide polymerization was altered in situ. These findings led to the synthesis of poly(lactic acid-b-cyclohexene oxide) block copolymers in which the sequence of monomers incorporated is controlled by the electrical potential applied.
The intimate connection between primary structure and macroscopic properties of macromolecules has inspired generations of chemists to develop new techniques aimed towards controlling composition and 1 2 sequence in synthetic polymers. Switchable catalysis is an emerging field that has shown great promise as a 3 method to control polymer sequence. In this type of catalysis, the reactivity of a catalyst is altered in situ through application of an external stimulus. Inherent to switchable catalysts is the temporal control needed for programmable polymer sequences. Various external stimuli have been used in switchable polymeriza2c tion reactions, but particularly effective has been re4 dox-switchable catalysts. Since their original discovery 5a for lactide polymerization, significant effort has been dedicated to controlling ring-opening polymerization processes by varying the oxidation states of metal cata5 lysts. Recently, we have been investigating a switchable polymerization system based on iron alkoxide com5e-g plexes bearing bis(imino)pyridine ligands. When these catalysts are in the iron(II) oxidation state (e.g. 1, Scheme 1), they are active for lactide polymerization but become dormant upon one electron oxidation to
form a cationic, formally iron(III) complex (e.g. 2, 6 Scheme 1). Sequential addition of chemical redox reagents deactivated and reactivated the catalyst towards 5e lactide polymerization. We have subsequently shown that the catalyst exhibits complementary reactivity for epoxide polymerization, being active when in the iron(III) oxidation state and inactive when in the iron(II) oxidation state. The orthogonal reactivity of the iron complexes for lactide and epoxide polymerization was exploited for the synthesis of sequence5f controlled block copolymers and redox-triggered 5g cross-linked polymer networks. Herein, we report an electrochemical method to afford the redox equivalents needed for switching using 6 this catalyst (Scheme 1). Electrochemistry is uncommonly used to control polymerization reactions with notable exceptions being electrochemical atom7 transfer radical polymerization (eATRP) and very recently, electrochemically controllable cationic 8 polymerizations. Using electrochemistry to supply the redox equivalents required for switchable polymerization reactions has some notable advantages: it obviates the need to add stoichiometric amounts of redox reagents thereby making it easier to remove metal impurities from the polymer, it is more easily programmable, it can be more easily applied in reactions that require elevated pressures of gaseous monomers, and it provides access to a wider range of redox potentials. Despite several examples of redox-switchable ring5 opening polymerization reactions, to the best of our knowledge, there are no reports where electrochemistry has been used as the redox switch. The eswitchable polymerization reaction reported here is also the first example where electrochemistry has been used to alter the chemoselectivity of any catalyst.
ACS Paragon Plus Environment
Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Scheme 1. Switchable ring-opening polymerizations using electrochemistry to toggle catalyst reactivity for lactide and epoxide polymerization (Ar = 4methoxylphenyl, Ar’ = 2,6-dimethylphenyl). To characterize the electrochemical properties of our catalysts, cyclic voltammetry (CV) was conducted for iron(II) species 1 (Figure 1a). For this purpose, a glassy carbon electrode was used as the working electrode, a platinum wire was used as a counter electrode, and a lithium ribbon separated from the bulk electrolyte by a selective iron permeable 9 membrane served as the reference electrode. The CV featured a reversible redox event that corresponds to 10 to Fe(III)Fe(II) interconversion (Figure 1a). Notably, the CV was void of additional redox reactions within + the measurement window (2.3 V to 3.7 V vs. Li /Li), which provided an opportunity to reliably convert the catalyst between oxidation states without worrying about parasitic chemical reactions.
Figure 1. (a) Cyclic voltagram of 1, scan rate = 25 mV/s; (b) Electrochemical cell used for e-switchable polymerization. Next, the electrochemical cell design was modified for bulk electrolysis (Figure 1b). To provide precise control over the electrochemical potential, potentiostatic electrolysis was employed throughout. A primary cell design principle was the need to compensate for the charge imbalance incurred by interconverting the neutral complex 1 with the cationic complex 2. Since lithium salts were found to exhibit no influence on the polymerization of lactide or epoxides (See Table S1-S2), lithium metal was identified as a suitable reversible sacrificial counter electrode that 11 could be used to balance charge. To complement the
counter electrode, a high surface area carbon fiber was used as the working electrode for rapid conversion of 12 the catalyst. Separation of the lithium counter and carbon working electrodes was necessary to prevent undesired redox events from occurring between the counter electrode and polymerization catalyst. Therefore, the counter electrode was separated from the working electrolyte, in which polymerization took place, by an ultrafine grade (
