Letter Cite This: ACS Macro Lett. 2018, 7, 201−207
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Light-Induced Step-Growth Polymerization of AB-Type PhotoMonomers at Ambient Temperature Silvana Hurrle,‡ Anja S. Goldmann,†,‡ Hartmut Gliemann,∥ Hatice Mutlu,*,†,§ and Christopher Barner-Kowollik*,†,‡,§ †
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, QLD 4000, Brisbane, Australia ‡ Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 18, 76128 Karlsruhe, Germany § Soft Matter Synthesis Laboratory, Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Institut für Funktionelle Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany S Supporting Information *
ABSTRACT: We introduce two AB-type monomers able to undergo a facile catalyst-free photoinduced polycycloaddition of photocaged dienes, enabling rapid Diels−Alder ligations under UV-irradiation (λmax = 350 nm) at ambient temperature, closely adhering to Carother’s equation established by a careful kinetic study (17800 g mol−1 < Mw < 24700 g mol−1). The resulting macromolecules were in-depth analyzed via size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Additionally, SEC hyphenated to high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) enabled the careful mapping of the end group structure of the generated polymers. Furthermore, we demonstrate that both monomer systems can be readily copolymerized. The study thus demonstrates that Diels−Alder ligation resting upon photocaged dienes is a powerful tool for accessing step-growth polymers.
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work of Tchir and Porter,27,28 describes a light-induced tautomerization process29 and has led to a wide ranging and potent toolbox for synthetic organic chemistry,30,31 surface functionalization,32−34 and ligation chemistry on synthetic macromolecules.35−38 The so-called photoenols (i.e., photocaged dienes, see Figure 1a) are hydroxy-o-quinodimethanes formed in situ by the tautomerization of o-methylphenyl ketones or aldehydes.35 The photocaged diene can subsequently undergo a DA cycloaddition with various dienophiles. Based on photoenol chemistry, we recently introduced a stepgrowth polymerization process exploiting AA- and BB-type monomers using α-methyl benzaldehydes as functional diene precursor.39 Thus, specifically designed bis-benzaldehyde (AA) and bis-fumarate (BB) monomers were polymerized under untypical off-stoichiometric conditions (ratio of functional groups A and B close to 1.5) to ensure a sufficiently high incorporation of BB monomer by suppressing the parallel self-
tep-growth polymerizations (SGP), that is, polycondensations and polyadditions, are highly important processes both in fundamental and industrial research even 80 years after Wallace H. Carothers’s early studies.1,2 The range of reactions enabling SPG is vast.3 In addition to the numerous reactions resting on small molecule organic chemistry that have been adapted for SGP (such as esterification, amidation, urethane formation and aromatic substitution),4−6 recent years have seen thiol−ene chemistry,7−10 azide−alkyne 1,3-dipolar cycloadditions,11−13 azide/alkyne click chemistry,14,15 multicomponent reactions,16 as well as metathesis17 and electron transfer18−20 processes take a prominent role in SGP, as they allow for the introduction of functional groups in the backbone or the side chains.21 The exceptionally diverse mechanistic nature of SGP chemistry enables the tailoring of distinct property profiles of the resulting polymers with highly specific features including degradability,8,21,22 solubility,11 fluorescence,23,24 and residual functional groups for postsynthetic modification.10,25 A remarkable reaction, the photoenolization of, for example, α-methyl benzaldehydes,26 an example of a photoinduced Diels−Alder (DA) reaction harking back to the pioneering © 2018 American Chemical Society
Received: December 24, 2017 Accepted: January 25, 2018 Published: January 30, 2018 201
DOI: 10.1021/acsmacrolett.7b01001 ACS Macro Lett. 2018, 7, 201−207
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Figure 1. (a) Reaction mechanism of the photoinduced Diels−Alder cycloaddition and the structures of the herein described AB-type monomers M1 and M2 employed for step-growth polymerization; (b) Resulting homopolymers HP1 and HP2.
Figure 2. (a) SEC traces resulting from the kinetic study of the step-growth polymerization of M1 yielding HP1; (b) Molecular weight evolution vs monomer conversion (black) and monomer conversion vs time (blue; in DCM) of M1 (hollow) and M2 (filled), respectively, and final precipitated polymers (stars) in comparison to the theoretical Carother’s plot (solid line); (c) SEC traces of crude HP1 (dotted line) and after first precipitation in methanol (red) compared to the oligomer species (blue); (d) 1H NMR spectra in CDCl3 of the monomers M1 and M2, and the corresponding polymers HP1 and HP2. The following magnetic resonances are highlighted. red: aldehyde group of photoenol; green: aromatic pattern of the xylene backbone of M1; blue: cyclohexyl pattern of the repeating unit; yellow: methyl group of α-methyl benzaldehyde; asterisk: residual resonances of the deuterated solvents.
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depth of the light (dictated by Beer−Lambert’s law) and thus slower reaction kinetics (Mw = 900 g mol−1).43 Therefore, a concentration of 0.1 M was employed for all further photopolymerizations. Eventually, M1 and M2 were photohomopolymerized in dichloromethane at λmax = 350 nm at ambient temperature for preset time intervals (i.e., 1, 2, 4, 6, and 12 h) with monomer feed of 0.3 to 0.4 mmol. The resulting SEC traces for M1 are depicted in Figure 2a. Oligomeric cyclization, which competes with the chain growth is evident at 27.7−28.8 min retention time for M1 and 29.2−30.9 min retention time for M2 (refer to Supporting Information, section C.2, Figure S12), respectively. In general, cyclic oligomers can be distinguished from their linear analogs as their hydrodynamic radii differ, but also as the amount of cyclic species increases throughout the polymerization, whereas the linear oligomers slowly vanish. Nevertheless, both polymerizations affording homopolymers HP1 and HP2 reach close to quantitative conversions (>99.5%). The Carother’s plot in Figure 2b (black) underpins the formation of cyclic oligomers as the molecular weight is slightly below the expected values of Carother’s. This observation implies that the reactive functional groups are either undergoing decomposition or participate in the cyclic oligomerization rather than homopolymerization. Indeed, Kricheldorf explored cyclic oligomerization competing with the polymerization in step-growth processes42 and described self-dilution as a common characteristic for conventional SGP. As aforementioned, monomers with aromatic segments are prone to oligomeric cyclization. On the other hand, aliphatic backbones incline to an all-trans conformation, which is unfavorable for cyclization reactions. In the current system, however, M2 shows evidently a larger deviation from the theoretical values and, thereby, a high tendency toward the formation of small unreactive oligomers. Regardless, the final homopolymers HP1 and HP2 were isolated from the linear and cyclic oligomers by precipitation in methanol. The isolated polymers as well as the soluble oligomeric fragments were further characterized via SEC (Figure 2c), which clearly evidences the success of this facile separation method as the precipitating product is almost quantitatively free of oligomeric species present in the nonprecipitating fraction. Thus, polymers with Mw of 17800 and 24700 g mol−1 for HP1 and HP2 were isolated, respectively (Figure S13). The yield of polymer isolated after 12 h irradiation for HP1 (30%) varies significantly to HP2 (10%), indicating the high tendency toward side reactions during the polymerization of M2. The gravimetric ratio of generated cyclic to linear polymer is 2.3 and 11.5 for the polymerization reactions of M1 and M2, respectively. In addition, in Figures 2d and S14, the comparative 1H NMR spectra of the monomers (M1, M2) with the corresponding polymers (HP1, HP2) are depicted, and specific resonances are highlighted. The consumption of the α-methyl benzaldehyde species can be followed by the resonances for the aldehyde unit (red, ca. 10.70 ppm) and the methyl group (yellow, 2.58 ppm). The DA product shows specific resonances (blue, 2.69−3.50 ppm) in proximity to the former α-methyl benzaldehyde methyl group (yellow) and by comparing these integrals, the conversion of the polymerization can be calculated (see eq S1 in section C.2 in the Supporting Information). Moreover, the polymers HP1 and HP2 can be distinguished by the resonances of the xylene backbone of M1 (green, 7.30−7.50 ppm). In order to gain detailed insight into the polymerization process, the determination of the exact mass of the polymer
dimerization reaction of the AA monomer. However, a subtle change in the monomer structure may selectively drive the polymerization toward step-growth polymers under stoichiometric conditions, thus circumventing the occurrence of the self-dimerization or polymerization of the bis-aldehyde. Typically, AB monomers are challenging in terms of synthesis and storage as the functional groups are readily available for premature oligomerization reactions.11 Our current photoreactive AB monomer design avoids these disadvantages. Thus, based on our recent efforts,39 we herein introduce two monomers M1 and M2 (Figure 1a), consisting of 2-substituted 6-methylbenzaldehyde and the ethyl ester of fumaric acid as the photoreactive entities. While, in principle the reactive moieties of the monomers can be tethered via large range of spacers unit, we employ either a xylene (i.e., aromatic) moiety or an aliphatic propyl group, respectively, for M1 and M2. In contrast to earlier work,39 our monomers are engineered in an AB configuration, thus, leading to a fixed stoichiometric ratio of functional groups (r = 1), a clear step forward in the realm of photoinduced step-growth polymerization. We adapt the universal Carother’s equation (eq 1) for the determination of the degree of polymerization (DPn).40 DPn =
1 1−p
(1)
The equation illustrates the challenge of the step-growth mechanism: the conversion p must reach high values to achieve a reasonably high degree of polymerization. AB-type monomers disallow the adjustment of the ratio of functional groups; thus, in our case, it is crucial to employ a reaction with high yield and selectivity. Our current work employing a single AB monomer to deliver step-growth polymers at ambient temperature (a.t.) without any additional catalysts breaks new ground for industrial applications where step-growth polymerization is required with the smallest number of starting materials at ambient temperature. Accordingly, M1 and M2 were synthesized and fully characterized (for the detailed description refer to section B in the Supporting Information). The polymerizations described in the current work were conducted in dichloromethane, since the only demand the solvent has to fulfill is to dissolve the components (i.e., monomers and the resulting macromolecules) and not absorb in the employed UV-light (in our case λ = 310−400 nm with λmax = 350 nm).39 Introduction of aromatic functionalities within the polymer backbone, rather than pendant to the chain, is more exceptional,41 however, according to Kricheldorf and coworkers,42 monomers possessing aromatic segments (e.g., M1) usually show a high tendency toward oligomeric cyclization, therefore the initial concentration-dependent conversion studies were performed for the homopolymerization of M2 according to earlier described procedures,23,24,39 that is, at concentrations of 0.05, 0.1, and 0.2 M. SEC analysis of the crude polymer mixtures at 12 h at several concentrations (refer to the Supporting Information, section C.1, Figure S11) reveals a mixture of low molar mass termination product (30 min retention time) and a polymer distribution (23−29 min retention time) with apparent weight-average molecular weight of close to Mw = 1000 g mol−1 for a concentration of 0.1 M. Low monomer concentrations (0.05 M) not only result in slower reaction kinetics (Mw = 520 g mol−1), but also in a high probability of cyclic oligomer formation.42 A high monomer concentration of 0.2 M presumably leads to a short penetration 203
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Figure 3. (a) SEC-ESI mass spectrum of HP1 integrated from 16 to 21 min elution time: n is the number of repeating units, z corresponds to the charge and equally to the number of sodium counterions; (b) SEC-ESI mass spectrum of HP2 integrated from 17 to 22 min elution time; (c) Assigned species found for HP2; (d) SEC-ESI mass spectrum of CP1 integrated from 15 to 22 min elution time. The structures of the copolymers are depicted in the Supporting Information, section E, Figure S20.
identified masses suggest that the α-methyl benzaldehyde end group is cleaved (species Bn), followed by the removal of the fumaric ethyl ester (Cn). The detailed mechanism of the rearrangement and the consecutive fragmentation of the αmethyl benzaldehyde end group are additionally depicted in Figure 4a. The polymerization products are shown in Figure 3c, where An describes the main product with intact end groups. When An molecules lose the α-methyl benzaldehyde group via the proposed mechanism (Figure 4a), the masses of species Bn are obtained. Cn species display a loss of the fumaric acid group as well as an ethyl moiety which may originate from either the pendent chain or the newly formed ethyl ester end group. The fact that we observe Cn patterns for oligomers with more than one repeating unit and still only detect one ethyl group subtraction, suggests that it is indeed the ethyl ester end group that is cleaved. The isotopic pattern of Cn (Figure S19) indicates that two species, Cna and Cnb, are formed with an m/ z difference of 2 Da (detailed values are depicted in Figure 4b). We submit that the loss of end groups by fragmentation critically influences molecular weight control and prevents the formation of high molecular weight homopolymers of M2.
chain (including the low molecular weight components) is critical. Therefore, SEC hyphenated with high resolution electrospray ionization mass spectrometry (ESI-MS) was employed for the characterization of the molecular structure of the photohomopolymers. We highlight the fact that the below noted retention times relate to the SEC-ESI-MS experiment and not the SEC stand-alone experiment. For crude HP1, the SEC-ESI-MS (Figure 3a), according to the mass spectrometric data integrated from 16 to 21 min elution time, indicates polymer chains with intact end groups, including as double charged species (refer to Supporting Information, section D, Table S1). No evidence for side reactions or degradation was detected. In fact, the side chain of the fumaric acid ester is stable under the photopolymerization conditions and could thus serve as a handle for further functionalization. The mass spectrum of HP2 (Figure 3b), collated for the elution interval from 17 to 22 min, only shows monocharged species. The four most prominent signals can be assigned to the monomer M2, and its corresponding dimers, trimers and tetramers. However, and in contrast to HP1, the spectrum also suggests the presence of specific degradation products. The 204
DOI: 10.1021/acsmacrolett.7b01001 ACS Macro Lett. 2018, 7, 201−207
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the detection limit. In Figure 3d, the homopolymer species An and αn and the copolymer species Anαm are assigned. The fragmentation species for HP2, that is, Bn, however, are not assigned for clarity, yet can be clearly identified (refer to Table S2). The structures are unambiguously assigned, and it is evident that species containing M2 also generate corresponding structures that do not carry the α-methyl benzaldehyde end group. Only oligomers αn that consist solely of M1 do not show any fragmentation of the α-methyl benzaldehyde or the fumaric acid ester. Finally, the influence of the two monomers on the thermal behavior of the photochemically generated homo- and copolymers was monitored via DSC. Notably, the analysis clearly indicates the formation of amorphous materials with glass transition temperatures (Tg) varying in the order of 92.2 (HP2) > 81.3 (HP1) = 81.3 (CP2) > 81.2 (CP1) > 75.4 °C (CP3) after the second heating cycle (Section E, Figure S23). For instance, the Tg of homopolymer HP2 (∼92.2 °C) is higher than its analogue (∼70 °C for PET). Thus, the Diels− Alder cycloaddition groups contribute to the increase of Tg, most probably by interchain hydrogen bonding interactions. Moreover, the comparison of the DSC thermograms of homopolymers HP1 and HP2 with the corresponding copolymer CP3 highlights the change in the properties (shift of Tg) resulting from the combination of monomers with distinct structures. In summary, we introduce a photoinduced step-growth polymerization protocol based on AB-type monomers enabling catalyst-free and ambient temperature polymer synthesis. The polymerization kinetics were followed by NMR and SEC. Importantly, an in-depth SEC-ESI-MS analysis was carried out to verify the structures of the isolated polymers. The design of the spacer units between the ene and photocaged diene is critical for the generation of polymers with intact chain termini. The design of the spacer unit allows for the introduction of various segments on the polymer backbone. In particular, the fumaric acid ester segment can be further modified with a broad range of functional groups. We submit that our strategy can be extended to synthesize macromolecular structures with more advanced topologies, including networks.
Figure 4. (a) Proposed mechanism for the cleavage of the α-methyl benzaldehyde end group. (b) Table listing the experimental and theoretical masses found for HP2 in Figure 3b.
The latter assumption is underpinned by synthesizing copolymers based on M1 and M2, and a subsequent detailed molecular assessment via SEC-ESI-MS. To distinguish between the distinct monomer units, the repeating unit of HP1 in the respective copolymers is denoted as αn, and the repeating unit for HP2 (A) as well as the secondary products (B) are assigned as above. The copolymerizations were carried out in 0.1 M DCM with monomer feed ratios of 0.25, 0.50, and 0.75 mol % M2 to M1 (refer to section C.2 in the Supporting Information). The resulting copolymers are termed CP1, CP2, and CP3, respectively, and the detailed SEC characterization can be found in the Supporting Information (Figures S15 and S16). The 1H NMR characterization of the precipitated copolymers (Figure S17) reveals that M2 is less present in the copolymer compositions, evident from the comparison of the integral values of the resonances associated with the aromatic segment of M1 (7.30−7.50 ppm) with the CH2− and CHgroups of the cyclohexyl ring. Thus, the percentage of M2 units in the copolymers CP1, CP2, and CP3 was calculated as 9 ± 1, 39 ± 7, and 60 ± 3%, respectively. The subsequent SEC-ESIMS analysis of CP1 is shown in Figure 3d, and the relevant spectra of the copolymers CP2 and CP3 are displayed in Figures S21 and S22. The difference in the ratio of the intensities of the charged species can clearly be distinguished. For CP1, the most evident species are associated with the homopolymer segments along the series of the copolymer ions, consequently easing the comparison of both species. In contrast, for CP2 and CP3, the intensities of the ion fragments associated with HP1 segments are significantly smaller. Thus, any potential fragmented homopolymers are probably below
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b01001.
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Monomer syntheses, polymerization procedures, and additional experimental data (PDF).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]; christopher.
[email protected]. ORCID
Anja S. Goldmann: 0000-0002-1597-2836 Hatice Mutlu: 0000-0002-4683-0515 Christopher Barner-Kowollik: 0000-0002-6745-0570 Notes
The authors declare no competing financial interest. 205
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ACKNOWLEDGMENTS C.B.-K., A.G., and H.G. acknowledge funding for this project from the Sonderforschungsbereich 1176 (Project C4) funded by the German Research Council (DFG) as well as the Karlsruhe Institute of Technology (KIT), the Helmholtz association via the BioInterfaces in Technology and Medicine (BIFTM), and the Science and Technology of Nanosytems (STN) programs. C.B.-K. additionally acknowledges continued support from the Queensland University of Technology (QUT) and the Australian Research Council (ARC) in the form of a Laureate Fellowship. S.H. thanks Rebekka Schneider and Prof. Meier (KIT) for DSC measurements.
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