Letter pubs.acs.org/journal/ascecg
Multicomponent Coupling Approach to Cross-Conjugated Polymers from Vanillin-Based Monomers Laure V. Kayser, Elizabeth M. Hartigan, and Bruce A. Arndtsen* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Quebec H3A 0B8, Canada S Supporting Information *
ABSTRACT: We describe the use of vanillin-based monomers as a renewable feedstock for the synthesis of crossconjugated polymers. This transformation exploits a catechylsubstituted phosphonite mediated multicomponent polymerization to convert vanillin-derived diimines, commercial diacid chlorides, and simple alkynes or alkenes into conjugated pyrrole-based polymers. The flexibility of the multicomponent polymerization has allowed for the efficient formation of families of vanillin-derived fluorescent polymers with tunable properties. This includes coupling vanillin with furan-based acid chlorides as the first cross-conjugated polymer composed of both components of lignocellulosic biomass. KEYWORDS: Multicomponent polymerization, Vanillin, Lignin, Conjugated polymers
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INTRODUCTION The rapid depletion of petroleum resources has become of growing concern over the past decade, and has stimulated a movement toward the design of replacement materials based upon renewables.1−7 Common examples include poly lactic acid (PLA), a biodegradable polymer derived from corn starch extracts,8,9 cellulose fibers or nanocrystals,10 polymers from biorenewable terpene derivatives,11−14 and triglyceride-based monomers (fatty acids).15,16 More recently, lignin, a complex amorphous aromatic polymer extracted from wood, has begun to attract attention as a feedstock material.17−21 Lignin represents the world’s largest renewable source of aromatics, and over 40 million tons is produced annually as a byproduct of the pulp and paper industry. The most common current use of lignin is its combustion for cheap energy production. As an alternative, lignin can be depolymerized to afford small aromatic molecules such as vanillin, a well-known food flavouring agent. Although vanillin can also be obtained from petroleum feedstocks, the biorefinery company Borregaard in Norway is currently the second largest producer of vanillin.22−24 The large scale availability of lignin-derived aromatic monomers from a nonedible biomass source make them ideal candidates for the synthesis of renewable polymers.23,24 Several examples have been reported of the polymerization of vanillin-based monomers into polyesters,25 the electropolymerization of bis-vanillin,26 or thiol−ene polymerization of alkene substituted vanillin.27 In considering the aromaticity of lignin-degradation products, one potential area where these materials could be uniquely suited is as renewable feedstocks for the synthesis of conjugated polymers. π-Conjugated polymers have emerged over the past several decades as important materials for a range of electronic applications, including as transistors, solar cells, light-emitting devices, and others.28−32 Their synthesis typically involves a © XXXX American Chemical Society
metal-catalyzed coupling reaction of monomers synthesized from petroleum feedstocks via multistep procedures.33−36 These not only create significant chemical waste but also can make structural modifications to influence polymer properties difficult. Lignin-derived compounds such as vanillin could in principle serve as attractive, renewable monomers in such polymerizations (Scheme 1a). However, the use of these as substrates has also presented challenges.37,38 First, many of lignin degradation products contain oxygen functional groups that are not directly amenable to assembling conjugated polymers via traditional procedures (e.g., cross coupling chemistry) without resorting to more complex, waste intensive Scheme 1. Synthesis of Cross-Conjugated Polymers from Vanillin-Based Monomers
Received: September 23, 2016 Revised: October 18, 2016 Published: October 24, 2016 A
DOI: 10.1021/acssuschemeng.6b02302 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering Table 1. Multicomponent Polymerization Reaction Developmenta
entry
T (°C)
R′
yield (%)
M̅ n (kDa)b
M̅ w (kDa)
Đ
DP n
c
25 25 45 55 75 45 55 55
TMH TMH TMH TMH TMH Me Me Me
41 61 66 58 59 67 71 76
3.0 7.6 9.7 12.1 7.2 12.7 11.6 11.4 (22.3)f
5.7 16.8 17.5 23.0 16.3 24.1 24.4 25.6 (31.7)f
1.9 2.2 1.8 1.9 2.3 1.9 2.1 2.3
6 14 18 22 14 29 26 26 (51)
1 2 3d 4d 5 6 7 8e a
2a (0.1 mmol), terephthaloyl chloride (20 mg, 0.1 mmol), CatPPh (50 mg, 0.24 mmol), 0.6 mL CH2Cl2, 24 h; DBU (46 mg, 0.3 mmol), 15 min. 3a precipitated in CH3CN then DMAD (43 mg, 0.3 mmol). bBy GPC vs polystyrene standards. c5 mL CH2Cl2. d48 h. eDBU (34 mg, 0.22 mmol) followed by in situ DMAD (85 mg, 0.6 mmol) addition, r.t. 15 min. fAbsolute M̅ n determined by GPC-MALLS-RI.
vanillin and other available monomers (e.g., with acid chlorides, alkynes, and/or alkenes) can be generated in one-pot polymerizations. This includes the first cross-conjugated polymer composed of both components of lignocellulosic biomass.
monomer synthesis. Equally important, a central feature of conjugated polymer utility is their tunability, wherein structural changes are typically required to create finely tuned electronic materials for various applications. The latter presents a particular challenge with biomass-derived feedstocks, where the monomer (and resultant polymerization product) is often defined. In principle, an approach to address both of these features of accessibility and structural modification with biomass would be to exploit multicomponent coupling reactions. Multicomponent polymerizations have recently emerged as important platforms for the rapid synthesis of families of well-defined polymers from combinations of monomers, and often do so with high step-economy.39−46 In this regard, we have recently reported that the phosphonite-mediated multicomponent polymerization of aromatic diimines, diacid chlorides, and alkynes/alkenes can provide a nonpalladium based approach to synthesize pyrrole-containing conjugated polymers.45 A feature of this reaction is its wide substrate scope, and its reliance on monomers arising from oxygenated substrates (e.g., imines from aldehydes, acid chlorides from carboxylic acids). These suggest the potential use of this transformation to convert biomass-derived materials into cross-conjugated polymers. While this would require the use of a phosphonite reagent, it avoids the waste intensive steps required to convert vanillin into monomers for more classical cross coupling polymerizations (e.g., vanillin-based organotin reagents). Toward this end, we describe herein how this phosphonite-mediated multicomponent polymerization can open a novel method to construct vanillin-based polymers (Scheme 1b). Importantly, by exploiting the tunability offered by the multicomponent polymerization, a range of new hybrid polymers derived from bis-
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RESULTS AND DISCUSSION Our initial studies toward this polymerization involved the use of the vanillin-based diimine 2a in reaction with terephthaloyl chloride (a commodity chemical used in the synthesis of Kevlar), and catechylphenylphosphonite. Diimine 2a can be readily generated in near quantitative yield from the dimer of vanillin (itself a lignin degradation product) via alkylation and condensation with a primary amine. The mixing of these three components for 14 h followed by deprotonation with DBU leads to the in situ formation of the poly(1,3-dipole) 3a. The subsequent addition of the alkyne dimethylacetylene dicarboxylate (DMAD) results in rapid 1,3-dipolar cycloaddition to afford the pyrrole-containing cross-conjugated polymer 4a. Unfortunately, GPC analysis shows this material in low molecular weight (M̅ n = 3 kDa, Table 1, entry 1). We postulated the low molecular weight of 4a arises from the first step in the polymerization, where the sterically encumbered diimine 2a may not react rapidly with the phosphonite compared to small molecule reactions.45 After probing various conditions and additives, it was found that this could be addressed by performing this step at high concentration and with mild heating (entries 2−5), which allows the formation of 4a in good molecular weight (M̅ n = 12.1 kDa, entry 4, vide inf ra). The moderate yields observed in both of these polymerizations are likely the result of the partial solubility of the polymers in the washing solvents used for isolation B
DOI: 10.1021/acssuschemeng.6b02302 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering Scheme 2. Diversity of Vanillin-Based Poly(pyrroles)
regioregular polymers (>95% selectivity by 1H NMR analysis). Alternatively, electron deficient alkenes such as 2-chloroacrylonitrile and vinylphosphonium bromide can also be employed in this reaction. These undergo HCl or PPh3·HBr loss to allow the formation of 4h′ and 4i, respectively. The acid chloride unit can also be modulated. As examples, the use of a naphthalenederived diacid chloride provides a route to generate the naphthalene-containing polymers 4b′. Thiophene and 3,4ethylenedioxythiophene (EDOT) can also be incorporated into the polymer from the acid chloride fragments (4c, 4c′, and 4d′). Pyrrole-based conjugated polymers have been shown to be fluorescent materials of potential relevance in polymer-based LEDs.51 As shown in Table 2, these lignin-based polymers are also blue-emitting compounds, and the structural modifications
(methanol and acetonitrile). This can be partially offset by use of a less solubilizing group on the backbone of bis-vanillin (R′ = Me, polymer 4a′). This both accelerates the polymerization and leads to an improvement in the degree of polymerization (entries 7, 7). The polymerization can also be performed without the precipitation of the poly phospha-Münchnone 3a (entry 8). This latter reaction is made possible by the use of a stoichiometric amount of DBU to avoid its side reaction with DMAD. The purity of 4a′ (from entry 8) was assessed by 1H, 13C, and 31P NMR spectroscopy (see Supporting Information for details). Of note, 1H NMR spectra show signals for the symmetrical vanillin (e.g., δ 7.07 (2H), δ 7.01 (2H)), phenylene (δ 7.49 (4H)), and pyrrole (δ 3.87 (4H)) units with no visible defects, together with small signals corresponding to aldehyde (δ 9.92 ppm), carboxylic acid (δ 8.33 ppm), and amide (δ 6.16 ppm) end-groups. Interestingly, integration of the end group signals suggests a degree of polymerization (DP n) of 90 (or M̅ n = 40 kDa) that is much higher than that noted with GPC analysis using polystyrene standards. Absolute molecular weight analysis using GPC-MALLS-RI detection shows that the actual molecular weight of this polymer is 22.3 kDa (M̅ n), which corresponds to a degree of polymerization of 51. With the phosphonite-mediated generation of polymer 4a in hand, we next probed the generality of this transformation (Scheme 2). In this regard, the multicomponent nature of this polymerization makes it straightforward to systematically vary the monomers. For example, a range of electron deficient alkynes and alkenes can be incorporated into these vanillinderived polymers (4a−i). This includes DMAD, diketonesubstituted alkynes (to form the diketo-substituted 4e′), and terminal alkynes (to generate asymmetrically substituted polymers 4f′ and 4g′).47 As we have previously noted, phospha-Münchnones undergo regioselective cycloaddition with unsymmetrical alkynes,48−50 and in this case form
Table 2. Optical Properties of the Vanillin-Based Polymersa polymer
λabs (nm)
λem (nm)
Eg (eV)
Φem
4a 4a′ 4b 4c 4c′ 4d′ 4e′ 4f′ 4g′ 4h′ 4i 4j 4j′
285 292 276 290 294 296 262 298 309 323 335 291 296
414 409 412 463 465 460 351 427 436 431 426 457 449
3.42 3.50 3.29 3.08 3.31 3.19 3.21 3.32 3.15 3.24 3.13 3.29 3.24
0.30 0.43 0.36 0.25 0.38 0.08
