Full Quantification of the Light-Mediated Gilch Polymerization

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Full Quantification of the Light-Mediated Gilch Polymerization Ann-Kathrin Schönbein,† Jonas Kind,‡ Christina M. Thiele,*,‡ and Jasper J. Michels*,† †

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Clemens-Schöpf-Institut für Organische Chemie und Biochemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 16, 64287 Darmstadt, Germany

‡

S Supporting Information *

ABSTRACT: Alkoxy-substituted poly(p-phenylenevinylenes) (PPVs) continue to be major workhorse materials in optoelectronics, ranging from thin film electronics to bioimaging. An attractive synthetic route toward PPVs is the Gilch polymerization. Nevertheless, obtaining control over molecular weight and chain constitution is challenging due to the free-radical nature of this reaction. In this work we quantitatively show with in situ UV-irradiation NMR spectroscopy how control over the Gilch polymerization can be enhanced by irradiation with UV-light. The potential of this method has been demonstrated but never interpreted within a quantitative framework, resulting in a lack of mechanistic and kinetic insight. We account for this not only by in situ analyzing and modeling the photochemical Gilch polymerization but also by characterizing the photolysis of the starting material. The latter shows that the solvent THF likely acts as radical transfer agent in the Gilch pathway and similar precursor-based biradical routes. We perform two photopolymerization runs: (i) under continuous UV-irradiation and (ii) by applying a short UV pulse while monitoring the chemical response of the mixture. Since existing models of the Gilch polymerization are inadequate for describing the recorded time−concentration profiles, we develop a new model that couples thermal and photoinduced polymerization. Numerical curve fitting quantifies the rate constants associated with both pathways. We demonstrate that (i) a photoactivated p-quinodimethane species Q* reacts in the (re)initiation and propagation steps, (ii) photopropagation is significantly faster than photoinitiation, and (iii) the photochemical reactions are considerably faster than their thermal analogues, which allows for decoupling the thermal and photochemical pathways at temperatures low enough to suppress the former.

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halides,28−30 anthracene,31 or oxygen,32 however with limited success. Modulation of the reaction by means of an external stimulus seems a very interesting alternative. A recent communication demonstrates the potential of initiating the Gilch polymerization using UV-light irradiation.33 Thermal (dark) polymerization is suppressed by performing the experiment at −100 °C. As shown in Scheme 1 for poly(2,5bis((2-ethylhexyl)oxy)-1,4-phenylenevinylene) (BEH-PPV), the reaction is believed to occur via photoexcitation of the intermediate p-quinodimethane Q, formed by prior deprotonation of premonomer PM. The resulting activated species Q* then reacts, together with ground state Q, to form prepolymer Pre. Base-induced halide elimination gives the final conjugated PPV. Kuch et al. claim extremely high chain constitutional homogeneity and suggest polymerization to only occur upon switching on UV-irradiation, which would indeed provide the possibility to synthesize PPVs in a targeted way. However, to establish a firm basis for optimizing and utilizing the irradiationmediated synthesis of PPVs, an exploratory and qualitative

INTRODUCTION Alkoxy-substituted poly(p-phenylenevinylenes) (PPVs) have been used in optoelectronic applications, such as organic lightemitting diodes (OLEDs),1−3 field-effect transistors (OFETs),4 and photovoltaics (OPV).5−7 More recently, they have proved great potential as bioimaging agents8,9 since they combine high fluorescence quantum yields with low toxicity and biocompatibility. Despite the fact that their charge carrier mobilities are somewhat suboptimal, PPVs remain work horse conducting polymers in fundamental studies concerning charge carrier transport and trapping in disordered media10−13 as well as degradation phenomena.14−16 It is understood that such studies fully rely on reproducible polymer quality and minimal batchto-batch variation. Unfortunately, these prerequisites remain challenging to meet. For the synthesis of PPVs, step-growth polymerizations17−20 and especially precursor routes21−27 are methods of choice. Of the latter the Gilch route23 stands out owing to easy synthetic access of monomers, one pot polymerization and halide elimination, and high molecular weight products. Novel applications of PPVs require tailored properties, which can only be obtained by controlling the Gilch polymerization to desirable extent. It has been attempted to control chain length and polydispersity chemically with the use of benzyl © XXXX American Chemical Society

Received: March 21, 2018 Revised: May 28, 2018

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DOI: 10.1021/acs.macromol.8b00607 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Conventional (Thermal) and Photochemical Reaction Cascade of the Gilch Polymerization toward PPVsa

a

Key reactant in the photochemical pathway is the photoexcited state Q* of intermediate p-quinodimethane Q. Q* reacts to form biradical dimer D, and likely both pathways lead to prepolymer Pre and product PPV.

Scheme 2. Proposed Photofragmentation Reaction for the Premonomer PM, Used in the Gilch Route PPV Synthesis, Initiated by Homolytic C−Br Bond Cleavagea

a The radical intermediate may (a) react in radical processes such as hydrogen abstraction or (b) undergo electron transfer to yield benzyl cation 3, which may react with nucleophiles or eliminate to yield the p-quinodimethane Q.

discussion such as given by Kuch et al.33 is not sufficient.a Instead, elucidating and understanding the photo-Gilch reaction mechanistically requires quantitative in situ measurement of concentration transients. To date, neither in situ experimental evidence nor a quantitative description of the photomediated Gilch polymerization, covering the full reaction cascade and the involved photochemical reactions, exists. In this work we account for this hiatus by (i) providing in situ UV-irradiation experiments quantitatively monitoring the development of the actual concentration profiles of precursor, intermediates, and products during both the thermal (dark) and photochemical Gilch polymerization and (ii) providing indepth kinetic and mechanistic insight through quantifying the photochemical rate constants. In analogy to our recent work on the thermal Gilch polymerization,34 we employ in situ 1H NMR spectroscopy to record concentration−time profiles, however this time in combination with in situ UV-irradiation.35−42 We study the photolysis of the premonomer PM in detail to assess under what conditions this process is likely to interfere with the (photochemical) Gilch polymerization and proceed with performing two in situ photopolymerization runs: (i) under continuous UV-irradiation (“continuous experiment”) and (ii)

by applying a short irradiation interval and monitoring the chemical response of the mixture (“pulsed experiment”). In order to fit the recorded concentration profiles and quantify photochemical rate constants, we develop a new kinetic model for the Gilch polymerization, for the first time providing a unified description of the coupled occurrence of the thermal and photomediated pathways. In our previous study, we examined in detail the kinetics of the thermal Gilch polymerization.34 The significance and the challenges associated with the current work stand out considerably due to the significantly increased level of complexity, both experimentally and computationally. It provides for an encompassing description of the Gilch reaction, since it not only considers both the thermal and photochemical pathways but also addresses the coupling between the two as a function of the reaction temperature. Nevertheless, we use our previous study as a firm basis in the fitting procedures and quantification of rate constants associated with the new UVlight-mediated in situ polymerization runs described in the present work. B

DOI: 10.1021/acs.macromol.8b00607 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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RESULTS AND DISCUSSION Premonomer Photolysis. Since benzyl radical fragments43,44 resulting from UV-photolysis of PM might interfere with the Gilch reaction, we study the extent of this process prior to discussing the polymerization runs themselves. Lightinduced homolytic cleavage of the C−Br bond of PM may yield two different reactive intermediates (Scheme 2): (i) radical intermediate 1 and (ii) cationic intermediate 3. Evidence for both intermediates is found by performing the photolysis in alcoholic solution, which yields radical recombination products, such as a dimer, as well as benzylic ethers.43,44 During the Gilch reaction, 1 may be incorporated as radical chain end, whereas 3 may convert into Q upon proton abstraction by base. In principle, the latter hypothetically poses the possibility of initiating the prepolymerization in the absence of a base. To establish the occurrence and importance of these photolysis pathways, a solution of the premonomer PM in THF-d8 is irradiated at 300 K (room temperature) with UVlight and studied with in situ UV-irradiation NMR (365 nm LED). Only upon prolonged irradiation, i.e., typically in excess of 2 h, new 1H signals slowly become visible, indicative of the occurrence of photolysis (see Figure 1). We identified the

Figure 2. 1H NMR spectrum (700 MHz proton resonance frequency) of the reaction mixture, in THF-d8, after irradiation with UV-light (320−400 nm) for approximately 16 h at 300 K. Resonances of the −CH3 and −CH2D groups of the major decomposition product 2 (2.18 and 2.16 ppm) and two additional decomposition products 5 (2.14 and 2.12 ppm) and 6 (2.23 and 2.25 ppm) are displayed with corresponding labels.

be to test whether the presumed species 3 is more acidic than the premonomer itself, which in the case of a weak base would leave the possibility for an alternative and exclusive photopolymerization pathway proceeding via premonomer photolysis. However, because the initial photolysis step is so slow (even at room temperature), we consider it to be fully unimportant during (photo)polymerization. For this reason we consider further investigation of the reactivity of possible photolysis intermediates outside the scope of this study. In addition to the appearance of new species, an 1H resonance is found at 2.16 ppm (Figure 2), appearing as a pseudo triplet with a line splitting of approximately 1.9 Hz, in agreement with the typical magnitude for 2JHD coupling, i.e., between a hydrogen and a deuterium over two bonds. This resonance is strong evidence for the formation of a deuterated isotopomer of major decomposition product 2 (2-d1, Figure 2) via deuterium transfer from THF-d8. The observation of isotopomer 2-d1 has far-reaching implications for understanding the termination mechanism in all precursor-based routes toward PPVs, as it strongly suggests the solvent to indeed act as a chain transfer agent. Thus far, this has been assumed,34,45 but evidence was so far lacking. The concentration of the protonated isotopomer exceeds the concentration of the deuterated isotopomer. Hence, the presence of at least one other hydrogen source is mandatory. The observation that the ratio 2/2-d1 decreases with water content (see Figures S7 and S8) indicates that water might act as additional hydrogen source. However, since we do not know whether homolytic hydrogen transfer from water to photolysis intermediate 1 is indeed energetically favorable, this pathway remains a hypothesis. Besides, changes in the aliphatic region of the 1H NMR spectrum during irradiation might hint toward hydrogen abstraction from aliphatic side chains (see Figure S6). Although the above shows that at room temperature photofragmentation of the premonomer PM does take place upon prolonged UV-irradiation, the associated time scales significantly exceed that of the standard (dark) base-induced dehydrohalogenation of PM into the p-quinodimethane Q (first reaction step in Scheme 1). As we will see, at the very low temperatures applied to the polymerization runs described

Figure 1. 1H NMR spectra in THF-d8 (700 MHz 1H resonance frequency) of the premonomer prior to (bottom) and after irradiation with UV-light (365 nm) for 2 h (middle) and 8 h (top) at 300 K. New signals are marked with dashed boxes.

newly formed species using additional 2H NMR, COSY, HSQC, and HMBC NMR experiments as well as APPI mass spectrometry (see Supporting Information, Figures S7 and S10−S15). The main decomposition product is 1-(bromomethyl)-2,5-bis((2-ethylhexyl)oxy)-4-methylbenzene 2 (Figure 2), likely generated via hydrogen abstraction by the benzylic radical 1. The other products are 1,4-bis((2-ethylhexyl)oxy)2,5-dimethylbenzene 5 and benzaldehydes 6−8 (data for 7 and 8 are not shown in the main text; chemical shift assignments for all compounds mentioned are given in the Supporting Information, Figures S1 and S2). Formation of Q or prepolymer Pre is not observed, which immediately disproves the hypothesis suggested above of baseless polymerization. If benzyl cation 3 indeed forms, it converts into the mentioned aldehydes via nucleophilic addition of residual water and subsequent oxidation instead of converting into Q. One could argue that if the photolysis would have been performed in the presence of a base, conversion of intermediate 3 into Q, possibly followed by prepolymerization, might have been more likely. In that case, an interesting experiment would C

DOI: 10.1021/acs.macromol.8b00607 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules below, i.e., typically smaller than −70 °C, photolysis is suppressed to the extent that the photo-Gilch polymerization proceeds without interference of any radical fragments or species derived from premonomer photolysis. In Situ Irradiation NMR of the Photo-Gilch Polymerization. Next, we present the light-mediated Gilch polymerization of the BEH-PPV premonomer, studied by in situ UVirradiation 1H NMR spectroscopy. The first polymerization experiment is performed at −74 °C under continuous UVirradiation, whereas the second is carried out at −85 °C while exposing the mixture to a brief (15 min) irradiation “pulse”. Both experiments are carried out in the presence of the base KOtBu. Although at low temperature no signals appear that are consistent with photolysis, in the continuous experiment we irradiate the mixture only after full conversion of PM into Q. Assignment of the 1H NMR signals (Figure 3) of premonomer

(PM), p-quinodimethane (Q), prepolymer (Pre), and PPV is carried out in accordance with our previous work in ref 34. We note that the NMR instrument was preshimmed using a (static) solution of premonomer in THF-d8, as shimming on the reaction mixture would have been impossible due to its transient nature. This might have led to minor line broadening, expected to be of no influence on our results, quantifications, and conclusions. Figures 4 plots the concentrations of PM, Q, Pre, and PPV (symbols), recorded as a function of time for the continuous (a) and pulsed experiment (b).b The shaded regions indicate the time intervals during which the UV lamp is switched on, which for the continuous experiment occurs after full conversion of the premonomer into Q. The jump in slope of the curves for Q and Pre in both figures shows that, irrespective of temperature, UV-irradiation results in a steep increase in the rate of prepolymerization. The continuous experiment exhibits some thermal prepolymerization for times