Architectural control of isosorbide-based polyethers via ring-opening

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Architectural control of isosorbide-based polyethers via ring-opening polymerization Derek J. Saxon, Mohammadreza Nasiri, Mukunda Mandal, Saurabh Maduskar, Paul J. Dauenhauer, Christopher J. Cramer, Anne M LaPointe, and Theresa M. Reineke J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00083 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Journal of the American Chemical Society

Architectural control of isosorbide-based polyethers via ring-opening polymerization Derek J. Saxon,† Mohammadreza Nasiri,† Mukunda Mandal,† Saurabh Maduskar,‡ Paul J. Dauenhauer,‡ Christopher J. Cramer,† Anne M. LaPointe,§ Theresa M. Reineke*† †

Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemical Engineering and Material Science, University of Minnesota, Minneapolis, Minnesota 55455, United States § Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States ‡

Supporting Information Placeholder ABSTRACT: Isosorbide is a rigid, sugar-derived building

block that has shown promise in high-performance materials, albeit with a lack of available controlled polymerization methods. To this end, we provide mechanistic insights into the cationic and quasizwitterionic ring-opening polymerization (ROP) of an annulated isosorbide derivative (1,4:2,5:3,6-trianhydro-Dmannitol, 5). Ring-opening selectivity of this tricyclic ether was achieved, and the polymerization is selectively directed towards different macromolecular architectures, allowing for formation of either linear or cyclic polymers. Notably, straightforward recycling of unreacted monomer can be accomplished via sublimation. This work provides the first platform for tailored polymer architectures from isosorbide via ROP.

Scheme 1. Methods for the synthesis of isosorbide-based polymers.a Previous work O

H O O O H

• energy intensive • limited control

HO

H O O H

RP

O

• oligomers only (DP ~ 5) • undesirable crosslinking

5

O O

O = bridgehead; O = terminal

This work

O

O

OR • non-degradable backbone

O

ROP

H

H

ROP

O

O

OH

O O

O O

Isosorbide is an inexpensive sugar derived from sorbitol and has been studied in polymers for nearly half a century. Its rigid structure can impart high glass transition temperatures into materials, which exhibit excellent properties including good optical clarity and strong resistance to UV, heat, impact, and abrasion.1 These properties make isosorbide an interesting, sustainable candidate for a wide variety of applications such as packaging, electronic displays, and biomedical applications (e.g. drug formulations and tissue engineering).1–3 Until recently, isosorbide has been incorporated into materials – including polyesters,4,5 polycarbonates,6 polyethers,7 polyamides,8 and polyurethanes9 – almost entirely via stepgrowth polymerization.1 Most notably, a copolycarbonate from isosorbide (Durabio™) is currently used for automobile parts and touchscreen electronics.10 However, step-growth processes are energy intensive and the high temperature conditions often lead to thermo-oxidation of

SGP R

O

H O O H

• mild, catalytic conditions » architectual control a

(SGP = step-growth polymerization, RP = radical polymerization, ROP = ring-opening polymerization)

the materials, which easily become discolored from degradation.1 Step-growth polymerization can also restrict the molar mass and architectural control that can be achieved, especially for isosorbide due to the low reactivity of its secondary alcohols.11–16 The ability to control polymerization of this feedstock would be transformative for building new and unique sustainable polymers. There are limited examples of isosorbide-based polymers being synthesized under mild conditions via radical polymerization. For example, our group has shown isosorbide-based (meth)acrylates have glass transition temperatures near that of polystyrene and

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are good sustainable candidates for the glassy component in thermoplastic elastomers.17–19 However, radical polymerization typically employs isosorbide as a pendant group from a vinyl or (meth)acrylic backbone,20–23 rendering the polymer backbone non-degradable. Alternatively, ring-opening polymerization (ROP) has been employed as a strategy to incorporate heteroatoms into the polymer backbone, only to produce short oligomers and crosslinked material.24 In attempts to synthesize copolymers, 20 catalysts and a wide range of conditions (Tables S1-S3). Due to the large ring strain present, we were interested in determining whether 5 could polymerize under a variety of conditions. An array of metal-based catalysts, organocatalysts, Lewis acids, and anionic initiators were examined. However, cationic ROP (cROP) was the only method that resulted in successful homopolymerization (Mn up to 10,600 g mol–1, DP~80). Previously, only short oligomers (DP~5) have been achieved. Studies to investigate the cROP mechanism of 5 in detail were then performed. To this end, poly(5) [P5] was synthesized by a methyl triflate (MeOTf)-initiated polymerization in CH2Cl2 at room temperature for 3 d (Scheme 3, top left), which afforded a polymer in suboptimal conversion (Table S7). It was discovered that P5 did not have a purely intact


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Scheme 3. Cationic ring-opening polymerization of 5 (R = H, Me, Et, or Bn). ring-opening selectivity

multifunctional ROP

Me

O

O

H O O H

O

O

H O

O

MeO OR

O H

P5

P5L

O MeOTf

OR

why, we probed the partial charges (QCM5) of various atoms in the propagating species using the CM5 model (Figure S3).34 In the presence of PO, the α-carbon has increased cationic character, presumably due to the stronger C–O bond than Sc–O bond,35 resulting in a more electrophilic chain end. Scheme 4. Proposed system to induce ring-opening selectivity of 5 ([Sc] = Sc(OTf)3). [Sc]

MeOTf

δ+

O O

O

epoxide activation

O 5 O

[Sc]

Sc(OTf)3

HO

O

δ+

H O O O H P5L

OR

or

O

O

1

2 O O

H O

O 1

O H

2

P5C

ring-opening selectivity | architecture control

isosorbide core by 1H NMR spectroscopy (Figure S5b), which is hypothesized to be a result of the multi-functional reactivity of the monomer. Because 5 is constructed from three strained five-membered ethers, multiple ring-opening events could be taking place during the polymerization. DFT calculations reinforced this hypothesis, where the activation barriers (∆G‡) calculated for the various ringopening combinations spanned a window of about 3.4 kcal mol–1 (Table S4). To improve selectivity, we sought to design a system that would produce a greater disparity between ring-opening events. Activation of an epoxide to form a highly-strained oxonium initiating species was envisioned to induce selectivity by increasing the relative free energy of activation between bridgehead and terminal ring-opening events (Scheme 4). Thus, the reactivity bias could be shifted toward propagation through the bridgehead ether as both kinetics and thermodynamics of ring opening would be favored (see additional discussion, SI). Specifically, we were interested in employing Sc(OTf)3 and propylene oxide (PO) for polymerization of 5 as this system has exhibited successful cROP of tetrahydrofuran.33 This approach is also beneficial from a sustainability aspect since Sc(OTf)3 is catalytic and recyclable.17,33 This refined catalytic system was then modeled via DFT. Using Sc(OTf)3 alone, the ring-opening energetics did not vary much from those observed with MeOTf (Table S5). Introduction of a PO initiator, however, lowered the activation barrier by 4.0 kcal mol–1 and concomitantly favored bridgehead ring opening. This indicates that the epoxide plays a key role in inducing selectivity during ring opening (see additional discussion, SI). To understand

HO

O

O

H O O H

OR

bridgehead propagation

HO

O

O

OR

terminal propagation

Catalytic polymerization of 5 was then explored with Sc(OTf)3 and PO (Scheme 3, bottom). 1H NMR spectroscopy of the isolated polymer corroborated our DFT calculations, showing an intact isosorbide core (Figure S5c). No significant increase in the conversion of 5 to P5 was observed compared to our initial studies with MeOTf, even at high concentration and longer polymerization time (Table S6). To probe the occurrence of irreversible termination, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed, which revealed the sodium adduct of P5 ions separated by 128 mass units (Figure 1a-b). Additionally, we discovered the absence of end groups in the major species (~70%) for polymerizations conducted in dichloromethane, indicative of cyclic polymer formation (P5C; Figure 1a). Polymerizations conducted in acetonitrile showed only linear chains (P5L) with an alkoxy or hydroxyl end group (Figure 1b). This observation can be rationalized by the quasi-zwitterionic nature of the active chains initiated by Sc(OTf)3/PO (Figure 1c). Due to the poor ability of solvents with a low dielectric constant (ε) to solvate ions (e.g. dichloromethane), the chain ends are likely in close proximity to neutralize the charge, increasing the probability of backbiting reactions that result in the formation of cyclic polymers.36 The high ε of acetonitrile enhances solvation of the ions and allows for more charge separation. DFT calculations support this hypothesis as the ring-opened PO, while coordinated to Sc(OTf)3, possesses


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Figure 1. MALDI-TOF MS of P5 synthesized with Sc(OTf)3/PO in (a) dichloromethane and (b) acetonitrile. (c) Rationalization of the formation of cyclic and linear polymers using Sc(OTf)3 in low- and high-ε solvents due to the quasi-zwitterionic nature of the growing chains.

significant anionic character (QO,CM5 = –0.617). Thus, backbiting may occur in low-ε solvents under quasizwitterionic conditions (see additional discussion, SI). An analogous monotonic polymerization was also conducted by activating PO with MeOTf (Scheme 3, top right). MADLI-TOF MS shows nearly exclusive formation of linear P5 (85%; Figure S6), further demonstrating the Sc(OTf)3-initiated quasi-zwitterionic propagating chains are responsible for the backbiting that ultimately forms macrocyclic architectures in low-ε solvents. Overall the conversion of 5 with MeOTf/PO was also higher (15%, rt) compared to Sc(OTf)3/PO under the same conditions (Table S7). We attribute this in part to the difference in counterion mobility: the triflate is free in the case of MeOTf and coordinated in the case of Sc(OTf)3. During ROP, significant charge separation is observed by DFT in both the oxonium propagating center as well as the partially cationic α-carbon. These charges can be partially compensated by the free triflate counterion in the MeOTf system, which drastically lowers the activation barrier (∆∆G‡ = 6.3 kcal mol–1; Figure S4). ROP of 5 can be conducted at low temperature with

Sc(OTf)3/PO to achieve higher conversion (14%, 0 ºC; Table S8).26,37 The conversions obtained across all systems are consistent with previous reports of 5 having a low ceiling temperature (4-16%, 4 ºC).24,25 Unreacted 5 can also be recovered in a pure state by sublimation of the crude polymerization mixture, allowing for straightforward recovery of the monomer (see SI). This operation could be implemented as a recycle stream in an industrial process. Catalytic ROP of 5 may enable tunable design and synthesis of statistical and block copolymers. Additionally, 5 has the potential to be treated as a multifunctional monomer as careful selection of the catalytic system and reaction conditions allow for a variety of macromolecular (e.g. linear, cyclic, and branched) and micro-structures (via bridgehead or terminal ring opening) to be targeted. In addition to providing a new avenue for isosorbide-based polymers, shown to have desirable physical and degradation properties,1,2,4–7 exploiting the reactivity behavior uncovered in this work could prove to be useful for producing advanced materials from heteroatom-rich, renewable feedstocks. In conclusion, we have presented the first platform for


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tailored polymer architectures from isosorbide via ROP. We have demonstrated cROP of 5 and enabled ringopening selectivity by first activating an epoxide. Quasizwitterionic conditions with Sc(OTf)3/PO permit control over macromolecular architecture to yield cyclic or linear architectures. Furthermore, unreacted monomer can be recycled by sublimation. Ongoing efforts are aimed at enhancing the kinetic favorability of polymerization, increasing monomer conversion, and developing various copolymer compositions and architectures. Understanding cROP of this derivative will allow for the study of isosorbide-based polyethers and more generally, be useful for controlling the selectivity and polymerization of other complex cyclic ethers. Furthermore, we believe expanding the scope of polymerization methods available for isosorbide-based materials will aid in their adaptation as sustainable alternatives for a range of commodity and specialty applications. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: X. General information, experimental procedures, 1H and 13C NMR spectroscopy, HRMS, MALDI-TOF MS, and computational details (PDF) Cartesian coordinates of stationary points (XYZ)

AUTHOR INFORMATION Corresponding Author

*Email: [email protected] ORCID

Derek J. Saxon: 0000-0003-3683-9513 Mohammadreza Nasiri: 0000-0002-5308-272X Mukunda Mandal: 0000-0002-5984-465X Paul J. Dauenhauer: 0000-0001-5810-1953 Christopher J. Cramer: 0000-0001-5048-1859 Anne M. LaPointe: 0000-0002-7830-0922 Theresa M. Reineke: 0000-0001-7020-3450 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation under the Center for Sustainable Polymers (CHE-1413862). Mass spectrometry analysis was performed at The University of Minnesota Department of Chemistry Mass Spectrometry Laboratory (MSL), supported by the Office of the Vice President of Research, College of Science and Engineering, and the Department of Chemistry at the University of Minnesota, as well as The National Science Foundation (NSF, Award CHE-1336940). The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported within this paper. This work also

made use of the NMR facility at Cornell University, which is supported, in part, by the National Science Foundation (CHE-1531632). The authors would like to acknowledge Dr. Leon Lillie for helpful discussions.

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Architectural control of isosorbide-based polyethers via ring-opening

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