Isomerization of Linear to Hyperbranched Polymers: Two Isomeric

Letter Cite This: ACS Macro Lett. 2018, 7, 1144−1148

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Isomerization of Linear to Hyperbranched Polymers: Two Isomeric Lactones Converge via Metastable Isostructural Polyesters to a Highly Branched Analogue Grant W. Fahnhorst, Daniel E. Stasiw, William B. Tolman,*,† and Thomas R. Hoye* Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, Minnesota 55455, United States

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ABSTRACT: We report here the Zn(II)-catalyzed convergence of two metastable and isostructural polyesters to an isomeric polymer having a hyperbranched architecture. Ring-opening transesterification polymerization (ROTEP) of 4-carbomethoxyvalerolactone (CMVL) under Brønsted catalysis is known to give the linear polyester PCMVL. We show here that this can be isomerized to the equilibrated (and highly branched) polyester EQ-PCMVL. Analysis of the fragments obtained from eliminative degradation of EQPCMVL were critical in the formulation of its structure. The isomerization of PCMVL to EQ-PCMVL is a direct consequence of the presence of the second ester functional group in the CMVL ester-lactone, a rarely studied class of monomer. Zn(II)-catalysis of the ROTEP of the isomeric β-lactone, 2-(2-carbomethoxyethyl)propiolactone (isoCMVL), as well as isomerization of the isostructural linear homopolymer derived from that isomeric monomer, led to the same EQ-PCMVL. These results suggest a new strategy for the introduction of branching into various polyesters.

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he extent and nature of branching architecture in both synthetic and natural polymers have significant ramifications on material properties.1 Methods and strategies for incorporating these features are valuable. Using reversible Diels−Alder reactions, Sumerlin and co-workers recently demonstrated such topological reorganization of preformed polymers.2 Another illustrative example is the synthesis of either linear or branched polyolefins from a common monomer, a feature that can be controlled by the choice of catalyst that either permits, or not, chain walking during the polymerization reaction.3 A second example is the synthesis of hyperbranched polyesters by ring-opening transesterification polymerization (ROTEP) of lactone monomers bearing a hydroxy group, which polymerize to give highly branched polyesters.4 This example involves a class of monomers known as inimers4 (or dual initiator/monomer compounds) that, per force, produce polymers with a highly branched architecture.5 In the last two of these examples, the linear or (hyper)branched nature of the polymer is dictated from the outset of the polymerization. By contrast, we are unaware of any instances of the isomerization of a preformed, linear polymer to an isomeric (hyper)branched polymer. We report such a process here. We recently described6 the polymerization of 4-carbomethoxyvalerolactone (CMVL, Figure 1a), readily available from malic acid (via coumalic acid7). Under the action of Brønsted acid catalysis [diphenyl phosphate (DPP)], CMVL gave poly(CMVL) (PCMVL) as a semicrystalline, linear homopolymer (Figure 1a) via ROTEP. Exchange at the methyl ester carboxyls, a process that would have introduced branching (or © XXXX American Chemical Society

Figure 1. (a) Brønsted acid catalyzed ROTEP of CMVL to (linear) PCMVL. (b) Use of a the Zn(II) catalyst 2 leads to a new, isomeric polyester.

a ring), was not detected. The assignment of a linear structure to PCMVL was supported by its highly efficient, reagentdependent degradations. Namely, when PCMVL was heated in the presence of a Sn(II) catalyst, reverse-ROTEP8 smoothly converted it back to CMVL. On the other hand, when treated Received: August 16, 2018 Accepted: September 5, 2018

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DOI: 10.1021/acsmacrolett.8b00621 ACS Macro Lett. 2018, 7, 1144−1148

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ACS Macro Letters with the organobase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), PCMVL gave the methacrylate derivative 1a by elimination at every backbone ester junction. We now report (Figure 1b) that the zinc complex 29 catalyzes the conversion of CMVL to a highly branched, isostructural (and equilibrated) polyester, EQ-PCMVL. We further show (i) that the linear PCMVL is a metastable intermediate enroute to EQ-PCMVL and (ii) that catalyst 2 is capable of isomerizing preformed linear PCMVL to EQPCMVL. The key enabler of the intricate and fascinating interconversions associated with the overall equilibration is the presence of a carboalkoxy substituent on every repeat unit in the polyesterto our knowledge, a unique structural feature. Significantly, these findings pave the way for use of related strategies for introducing branching in a considerable array of polyesters. The polymerization of neat CMVL by catalyst 2 (Figure 2a), initiated by benzyl alcohol (BnOH), was rapid and proceeded to essentially the same equilibrium monomer conversion (>98%) as the DPP-promoted6 polymerization. However, the resulting EQ-PCMVL had a different macroscopic morphology; the melting characteristic of PCMVL was no longer observed in the DSC of this now amorphous polyester. The proton NMR spectra revealed distinguishing features between EQ-PCMVL and PCMVL (Figure 2a, inset), most notably, a second methyl ester and broadening of the backbone resonances. We again6 used degradation experiments to give valuable information about the structure of the new EQ-PCMVL. Exposure to Sn(Oct)2 under the retro-ROTEP conditions used for complete PCMVL degradation (150 °C, 24 h, 0.1 Torr) gave, now, only a trace of CMVL, leaving the remainder of the macromolecular sample intact. On the other hand, incubation in DBU (3 equiv, ambient T, neat, 20 h) resulted in about 75% overall degradation, producing substantial amounts of each of the four methyleneglutarates 1a−1d as the only monomeric decomposition products (gray box in Figure 2a). This contrasts with the formation of 1a alone from analogous eliminative fragmentation of the homopolymer PCMVL.6 Through retrodegradative reasoning, we deduced that the structure of EQ-PCMVL contains the four unique subunits 3a−3d (Figure 2b). Each of these gives rise to one of the four corresponding elimination products 1a−1d. The 4-atom backbone repeat unit 3b (red square) is isomeric with the 6atom unit 3a (blue rectangle), the only backbone subunit present in PCMVL. The remaining two subunits suggest that EQ-PCMVL is highly branched (cf. 3d, triangle) and that each branch has at its terminus a dimethyl ester (cf. 3c, ball). A corollary is that the structure of EQ-PCMVL (Figure 2b) contains essentially an equivalent number of triangles (3d) and balls (3c). Moreover, the total number of backbone ester moieties and methyl esters in the ensemble of macromolecules is conserved during the multiple transesterification events through which they arise (see SI for a more detailed discussion). The absence of crystallinity in EQ-PCMVL is also consistent with this highly branched structure. We then investigated the polymerization of CMVL with catalyst 2 more carefully by in situ 1H NMR monitoring in CDCl3 (Figure 3a) rather than in the bulk. A typical plot of reaction progress showed the consumption of monomeric CMVL and growth of PCMVL at the same rate in the early phase of reaction, as measured by the resonances for their methyl esters at δ 3.75 (green) and 3.71 (blue), respectively.

Figure 2. (a) Polymerization/reorganization of CMVL and PCMVL with catalyst 2. (b) Structure of EQ-PCMVL deduced from the eliminative fragments 1a−1d. Green depicts a closure to a cyclic substructure (see MALDI discussion below).

The half-life for monomer loss was less than 1 h. However, even at early time points a third methyl ester resonance (at δ 3.67, red) had appeared and this continued to grow with time. Gradually, as the amount of EQ-PCMVL increased, the concentration of PCMVL diminished until, ultimately, the isomerization progressed to the point of having an ca. 1:1 ratio of the two methyl ester resonances at 3.71 and 3.67.10 This result is consistent with the presence of a similar number of each of the subunits 3a−3d10 in EQ-PCMVL (degree of branching = ∼0.511). The ratio of these resonances remained essentially invariant even after extended time, a hallmark of a process having reached equilibrium. In an independent experiment that further solidifies the interpretation that PCMVL is isomerizing to EQ-PCMVL, 1145

DOI: 10.1021/acsmacrolett.8b00621 ACS Macro Lett. 2018, 7, 1144−1148

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ACS Macro Letters

EQ-PCMVL, significantly greater than even that of the 4K starting sample of PCMVL, is a clear reflection of a reduced radius of gyration14 in the equilibrated polymer, reflecting the high degree of branching and the presence, perhaps, of cyclic structures. We turned to MALDI analysis to provide further insight about the nature of the end-groups present in EQ-PCMVL (see Figures S5−S7 and the associated discussion in the SI for more details). Specifically, MALDI data indicated that isomerized EQ-PCMVL formed from (a low MW) PCMVL contained a large portion of hyperbranched polymers containing a ring (see green in Figure 2b). This feature arises from intramolecular transesterification of either a side-chain or midchain ester by the terminal hydroxy group, processes that are again supported by interpretation of the MALDI data.15 The result is a lower than targeted molar mass,16 a feature also observed by light scattering SEC of higher MW samples of EQPCMVL (see SI). isoCMVL is the β-lactone constitutional isomer of CMVL (Figure 4) and shares the same carbon skeleton. We wondered

Figure 3. Time courses for (a) polymerization of CMVL and (b) isomerization of preformed homomeric PCMVL by zinc complex 2 to produce the equilibrated EQ-PCMVL.

preformed PCMVL homopolymer (from the DPP ROTEP) was exposed to the zinc catalyst 2 in bulk at 80 °C with no added initiator alcohol (Figure 2a). The 1H NMR spectrum recorded at the final resting-state was essentially indistinguishable from that of the sample of EQ-PCMVL produced starting from the CMVL monomer. Thus, the zinc complex 2 and the lone hydroxy group in PCMVL are capable of promoting the chain-scrambling, transesterification events.12 A related experiment, also using preformed PCMVL, provided additional perspective (Figure 3b). Equal masses of two samples of PCMVL of different molecular weight (Mn = 4 vs 85 kg·mol−1) were treated with catalyst 2 in CDCl3 at ambient temperature. This corresponds to a molar ratio of the two of about 21:1. Aliquots were analyzed in parallel by both 1 H NMR spectroscopy and SEC. The latter showed initial broadening of both polymers with eventual merging of their retention volumes. These changes, however, occurred considerably faster than the growth of the new methyl ester resonance. Ultimately, both the SEC and NMR data converged to those of the resting state. These complementary experiments indicate that transesterification at the backbone esters (chain transfer) is faster than that at the methyl ester side chains in PCMVL, consistent with the fact that the latter are more sterically hindered (α-branched13) than the former. The greatly increased SEC retention volume of the final sample of

Figure 4. (a) ROTEP of the β-lactone isoCMVL to (linear) P(isoCMVL) and EQ-PCMVL. (b) Relative rates of ROTEP of CMVL and isoCMVL and their convergence to EQ-PCMVL with 2.

whether the isomeric homopolymer composed only of fouratom backbone repeat units 3b [i.e., P(isoCMVL), Figure 4a] could be prepared from isoCMVL. Moreover, could the overall isomerization to EQ-PCMVL be accessed from isoCMVL by way of P(isoCMVL)? Treating isoCMVL (see SI for its synthesis from methyl acrylate) with 2 (and BnOH as initiator) gave rise, initially, to the methyl ester resonance at 3.67 ppm, consistent with formation of P(isoCMVL).17 Again, growth of resonances associated with 3a and 3c at 3.71 ppm, indicative of the onset of formation of EQ-PCMVL, was observed prior to full consumption of the β-lactone (see Figure S3 for a kinetic scan analogous to that shown in Figure 3a). Under these polymerization conditions, P(isoCMVL) is, like PCMVL, a metastable state. Indeed, treating preformed, linear P(isoCMVL) with catalyst 2 gave EQ-PCMVL (see SI). The following overall picture emerges (Figure 4b). (i) The rates of ROTEP of CMVL and isoCMVL catalyzed by 2 are quite similar to one another. (ii) The isomerizations of each of the respective homopolymers are again (and surprisingly), similar to one another. (iii) Each isomerization is somewhat 1146

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ization aspects. Chem. Rev. 2009, 109, 5924−5973. (b) Zheng, Y.; Li, S.; Weng, Z.; Gao, G. Hyperbranched polymers: Advances from synthesis to applications. Chem. Soc. Rev. 2015, 44, 4091−4130. (2) Sun, H.; Kabb, C. P.; Dai, Y.; Hill, M. R.; Ghiviriga, I.; Bapat, A. P.; Sumerlin, B. S. Macromolecular metamorphosis via stimulusinduced transformations of polymer architecture. Nat. Chem. 2017, 9, 817−823. (3) Dong, Z.; Ye, Z. Hyperbranched polyethylenes by chain walking polymerization: synthesis, properties, functionalization, and applications. Polym. Chem. 2012, 3, 286−301. (4) Trollsás, M.; Löwenhielm, P.; Lee, V. Y.; Möller, M.; Miller, R. D.; Hedrick, J. L. New approach to hyperbranched polyesters: Selfcondensing cyclic ester polymerization of bis(hydroxymethyl)substituted ε-caprolactone. Macromolecules 1999, 32, 9062−9066. (5) (a) Fréchet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.; Grubbs, R. B. Self-condensing vinyl polymerization: An approach to dendritic materials. Science 1995, 269, 1080−1083. (b) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M. Preparation of hyperbranched polyacrylates by atom transfer radical polymerization. 1. Acrylic AB* monomers in “living” radical polymerization. Macromolecules 1997, 30, 5192−5194. (6) Fahnhorst, G. W.; Hoye, T. R. A carbomethoxylated polyvalerolactone from malic acid: Synthesis and divergent chemical recycling. ACS Macro Lett. 2018, 7, 143−147. (7) (a) von Pechmann, H. On the cleavage of α-oxyacids. Liebigs Ann. Chem. 1891, 264, 261−309. (b) Wiley, R. H.; Smith, N. R. Coumalic acid. Org. Synth. 1951, 31, 23−24. (8) (a) Zhu, J.-B.; Watson, E. M.; Tang, J.; Chen, E. Y.-X. A synthetic polymer system with repeatable chemical recyclability. Science 2018, 360, 398−403. (b) Hong, M.; Chen, E. Y.-X. Chemically recyclable polymers: A circular economy approach to sustainability. Green Chem. 2017, 19, 3692−3706. (c) Hong, M.; Chen, E. Y.-X. Completely recyclable biopolymers with linear and cyclic topologies via ring-opening polymerization of γ-butyrolactone. Nat. Chem. 2016, 8, 42−49. (d) Schneiderman, D. K.; Vanderlaan, M. E.; Mannion, A. M.; Panthani, T. R.; Batiste, D. C.; Wang, J. Z.; Bates, F. S.; Macosko, C. W.; Hillmyer, M. A. Chemically recyclable biobased polyurethanes. ACS Macro Lett. 2016, 5, 515−518. (9) Williams, C. K.; Breyfogle, L. E.; Kyung Choi, S.; Nam, W.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. A highly active zinc catalyst for the controlled polymerization of lactide. J. Am. Chem. Soc. 2003, 125, 11350−11359. (10) The resonance at 3.71 ppm reflects the total content of both 3a and 3c, whereas that at 3.67 ppm reflects the total amount of the 3b and 3c subunits. The 1H NMR methyl ester resonances of the bismethyl ester i (prepared by methanolysis of CMVL followed by acetylation, see SI) are nearly identical to those in PCMVL and EQPCMVL.

slower than the ROTEP, but sets in prior to equilibrium consumption of each lactone monomer. (iv) Finally, both reactions eventually converge to essentially the same final resting state, EQ-PCMVL. In conclusion, we have demonstrated an unprecedented isomerization pathway by which a preformed linear polymer is converted to a highly branched architecture. The process here involved multiple transesterification events and is a direct ramification of the use of monomeric lactones containing a carboalkoxy substituent. The nature of the catalyst can influence whether the linear homopolymer from initial ROTEP predominates or whether the isomerization to a highly branched polymer ensues. Degradation studies of these isomeric polymers were essential for deducing the structures of each. The work here also demonstrates that two isomeric monomers can converge, through the intermediacy of isostructural, linear homopolymers, to the same, equilibrated, final resting state, an intricate example of a classic chemical principle. Finally, we note that other lactones bearing a pendant ester group should function in a very similar fashion and that the fundamental concepts exposed through this investigation should allow for the introduction of branching into other families of polyesters, a potential strategy for management of polyester properties.18

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00621. Experimental protocols, new compound and polymer characterization data, and copies of 1H and 13C NMR spectra (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Grant W. Fahnhorst: 0000-0002-2193-4246 Daniel E. Stasiw: 0000-0003-0526-7372 William B. Tolman: 0000-0002-2243-6409 Thomas R. Hoye: 0000-0001-9318-1477 Present Address †

Department of Chemistry, Washington University in St. Louis, One Brookings Drive, Campus Box 1134, St. Louis, MO 63130−4899, United States.

(11) The degree of branching is calculated as [# of branched (3c) + # of terminal (3d) subunits]/[# of branched + # of terminal + # of unbranched (i.e., “linear”) subunits (represented, in aggregate by # of 3a + # of 3b)]. We observe loop formation (cyclization) on some polymer chains; however, the contribution from this will be negligible for EQ-PCMVL having higher molecular weights. Hölter, D.; Burgath, A.; Frey, H. Degree of branching in hyperbranched polymers. Acta Polym. 1997, 48, 30−35. (12) See Figure S1 and associated discussion for a description of the various transesterification events that reversibly interconvert either of the subunits 3a or 3b with either 3c or 3d. (13) Rule of six: “In reactions involving addition to an unsaturated function containing a double bond, the greater the number of atoms in the six position the greater will be the steric effect.” (a) Newman, M. S. Some observations concerning steric factors. J. Am. Chem. Soc. 1950, 72, 4783−4786. (b) Newman, M. S. In Steric Effects in Organic Chemistry; Newman, M. S., Ed.; Wiley: New York, 1956; p 206.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work was provided by the Center for Sustainable Polymers at the University of Minnesota, an NSFsupported Center for Chemical Innovation (CHE-1413862). Some of the NMR data were obtained with an instrument whose purchase was enabled by the NIH Shared Instrumentation Grant Program (S10OD011952).

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REFERENCES

(1) (a) Viot, B. I.; Lederer, A. Hyperbranched and highly branched polymer architecturesSynthetic strategies and major character1147

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ACS Macro Letters (14) (a) Gaborieau, M.; Castignolles, P. Size-exclusion chromatography (SEC) of branched polymers and polysaccharides. Anal. Bioanal. Chem. 2011, 399, 1413−1423. (b) McKee, M. G.; Unal, S.; Wilkes, G. L.; Long, T. E. Branched polyesters: Recent advances in synthesis and performance. Prog. Polym. Sci. 2005, 30, 507−539. (15) Dušek, K.; Š omvársky, J.; Smrcková, M.; Simonsick, W. J., Jr.; Wilczek, L. Role of cyclization in the degree-of-polymerization distribution of hyperbranched polymers. Modeling and experiment. Polym. Bull. 1999, 42, 489−496. (16) Because of this alteration of the molecular weight, the conversion of PCMVL to EQ-PCMVL is not an isomerization in the strictest sense. Our use here of the term “isomerization” refers to the fact that the overall process conserves the structural elements within each repeat unit before and after reaction. That is, in each of the macromolecules, regardless of its linear, hyperbranched, or cyclic architecture, every repeat unit (i) has the same carbon skeleton and (ii) contains exactly two alkoxycarbonyl groups. Recall also that, in aggregate, there is an equal number of methyl and backbone esters in every molecule. (17) In two occasions, a sample of neat isoCMVL slowly polymerized over the course of about 3−4 months to the homopolymeric, linear P(isoCMVL), while stored in a nitrogen-filled glovebox. This linear P(isoCMVL) was used for characterization (SEC, TGA, DSC, NMR, degradation) and was subsequently isomerized to EQ-PCMVL (see SI). (18) Corneillie, S.; Smet, M. PLA architectures: The role of branching. Polym. Chem. 2015, 6, 850−867.

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DOI: 10.1021/acsmacrolett.8b00621 ACS Macro Lett. 2018, 7, 1144−1148