Controlling Polymer Composition in Organocatalyzed Photoredox

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Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Vinylcyclopropanes Dian-Feng Chen, Bret Boyle, Blaine McCarthy, Chern-Hooi Lim, and Garret M. Miyake J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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Journal of the American Chemical Society

Controlling Polymer Composition in Organocatalyzed Photoredox Radical RingOpening Polymerization of Vinylcyclopropanes Dian-Feng Chen, Bret M. Boyle, Blaine G. McCarthy, Chern-Hooi Lim, and Garret M. Miyake* Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United States

ABSTRACT Although radical polymerizations are among the most prevalent methodologies for the synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and selectivity of radical addition challenges the ability to impart control over the polymerization propagation and produce polymers with defined microstructure. Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening polymerization to produce polymers possessing linear (l) or cyclic (c) repeat units providing opportunity to control polymer structure and modify the polymer properties. Herein, we report the first organocatalyzed photoredox radical ring-opening polymerization of a variety of functionalized VCP monomers, where high monomer conversions and spatial and temporal control were achieved to produce poly(VCPs) with predictable molecular weight and low dispersity. Through manipulating polymerization concentration and temperature, tunable l or c content was realized, allowing further investigation of thermal and viscoelastic materials properties associated with these two distinct compositions. Unexpectedly, the photoredox catalysis enables a postpolymerization modification that converts l content into the c content. Combined experimental and computational studies suggested an intramolecular radical cyclization pathway, where cyclopentane and cyclohexane repeat units are likely formed.

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INTRODUCTION The merger of photoredox catalysis 1 and controlled polymerizations 2 have drawn increasing attention as to provide spatial and temporal control over the chain growth and produce polymers with predictable composition, molecular weight (MW), and low dispersity (Ð) under mild conditions.3 A variety of methodologies which implement light to affect diverse reaction mechanisms have been discovered, including photoinduced atom transfer radical polymerization (ATRP),4 photoinduced electron/energy transfer reversible addition-fragmentation chain transfer polymerization,

5

photocontrolled cationic

polymerization,6 metal-free ring-opening metathesis polymerization,7 and others.8 Our interest in photoredox-controlled polymerization originated with the motivation to develop an organocatalyzed ATRP (O-ATRP). Our group and others have employed highly reducing organic photoredox catalysts (PCs), such as perylene,3e phenothiazines,3f N,Ndiaryl dihydrophenazines3i and N-aryl phenoxazines,3j,9 for O-ATRP. To further advance this nascent polymerization methodology we are motivated to discover new reactivity of these organic photosystems that are otherwise inaccessible. Vinylcyclopropanes (VCPs) are an intriguing class of monomers in that they can be polymerized through various pathways to yield drastically different polymeric structures, exhibiting low volume shrinkage or even volume expansion—a unique property of modeling and restorative materials. 10 In a radical pathway, the radical ring-opening polymerization (rROP) proceeds to yield a polymer possessing a combination of unsaturated linear (l) and saturated cyclic (c) repeat units.11 These c repeat units have been proposed to be cyclobutanes generated through a 4-endo-trig radical cyclization (Figure 1A).12 To evaluate l/c selectivity, a linear factor SL is defined here as the total percentage of l repeat units. Most rROP approaches have utilized traditional free radical polymerization techniques, which achieve low selectivity between l or c content and produce polymers with ill-defined MW characteristics (Figure 1B).

12

To address this

limitation, Cu-ATRP was attempted for the rROP (ATrROP) of 1,1-diethyoxycarbonyl-2vinylcyclopropane (EtVCP) to produce well-defined polymers possessing a high l content (SL = 98%).13 However, presumable bidentate coordination from EtVCP or poly(EtVCP) to the Cu(I) catalyst inhibits polymerization to reach high monomer conversions ( 90%) under mild conditions. By manipulating polymerization concentration and temperature, unprecedented regulation on the linear and cyclic compositions of obtained polymers was achieved, which allowed investigations of composition-associated thermal and viscoelastic properties. Significantly, we have discovered a novel polymer-chain modification that converts l composition into c composition. The combination of a model reaction and DFT computations disclose several tandem radical cyclization pathways to produce cyclopentane and cyclohexane repeat units in the modified polymer. This postpolymerization modification also suggests a novel mechanism, besides previously proposed backbiting cyclization, to form the c composition during polymerization. Combined, these findings provide valuable insights to guide the future design and fabrication of poly(vinylcyclopropane)-based materials. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Dian-Feng Chen: 0000-0003-1821-9783 Chern-Hooi Lim: 0000-0003-1823-6305 16 ACS Paragon Plus Environment

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Garret M. Miyake: 0000-0003-2451-7090 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Colorado State University, the Sloan Research Foundation, and the National Institute of General Medical Sciences of the National Institutes of Health under Award Number R35GM119702. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. We acknowledge the use of computational resources provided by the XSEDE Comet supercomputer (NSF ACI-1053575). C.-H.L. is grateful for an NIH F32 Postdoctoral Fellowship (F32GM122392). B.G.M. is grateful for support from an NSF GRFP. D.-F. C. thanks Bonnie L. Buss for technical assistance and helpful discussions. REFERENCES 1

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the preparation of functional polyethylenes via thiol‐ene chemistry. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1957–1960. 20 Baldwin, J. E. Rules for ring closure. J. Chem. Soc., Chem. Commun., 1976, 734-736.


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Table of Content image for Controlling Polymer Composition in Organocatalyzed Photoredox Radical RingOpening Polymerization of Vinylcyclopropanes Dian-Feng Chen, Bret M. Boyle, Blaine G. McCarthy, Chern-Hooi, Lim, and Garret M. Miyake* Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United States

RO2C

CO2R

PC Visible Light

Visible Light

PC

and/or

n n = 1, 2, 3 Mild Conditions; > 90% Conversions; Controlled MW; Ð = 1.03-1.24; Tunable Composition


Controlling Polymer Composition in Organocatalyzed Photoredox

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