Ring-Closing Depolymerization: A Powerful Tool for Synthesizing the

Article pubs.acs.org/Macromolecules

Ring-Closing Depolymerization: A Powerful Tool for Synthesizing the Allyloxy-Functionalized Six-Membered Aliphatic Carbonate Monomer 2‑Allyloxymethyl-2-ethyltrimethylene Carbonate Peter Olsén, Karin Odelius, and Ann-Christine Albertsson* Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Ring-closing depolymerization is demonstrated to be a powerful synthetic methodology for the formation of six-membered functional aliphatic carbonate monomers, providing a rapid, straightforward, inexpensive, and green route for obtaining six-membered functional aliphatic carbonate monomers at a scale greater than 100 g. The utility of this technique was observed via the synthesis of the allyloxyfunctionalized six-membered cyclic carbonate monomer 2allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC). The synthesis was performed in a one-pot bulk reaction, starting from trimethylolpropane allyl ether, diethyl carbonate, and NaH, resulting in a final AOMEC yield of 63%. The synthetic methodology is based upon the reversible nature of this class of polymers. The anionic environment produced by NaH was observed to be sufficient to mediate the monomer equilibrium concentration; thus, an additional catalyst is not required to induce depolymerization. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) was demonstrated to be a very active catalyst for the ring-opening polymerization (ROP) of AOMEC, resulting in a rapid (kpapp = 28.2 s−1) and controlled polymerization with a low dispersity (Đ = 1.2). The availability and activity of the functionality of poly(AOMEC)s were established through subsequent postpolymerization functionalization via the UV-initiated thiol−ene chemistry of poly(AOMEC) with 1-dodecanethiol and benzophenone as a radical initiator. The functionalization proceeded with high control and with a linear relation between the molecular weight and conversion of the unsaturation, revealing the high orthogonality of the reaction and the stability of the carbonate backbone. Hence, as a synthetic methodology, depolymerization provides a straightforward and simple approach for the synthesis of the highly versatile functional carbonate AOMEC. In addition, formation of the monomer does not require any solvents, reactive ring-closing reagents, or transition-metal-based depolymerization catalysts, thereby providing a “greener” route for obtaining functional carbonate monomers and polymers.

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INTRODUCTION

The synthesis route for preparing aliphatic functional sixmembered carbonates generally involves a reactive ring-closing reagent, a base, and a 1,3-diol that bears the desired functionality. Some of the commonly used ring-closing reagents include carbonyldiimidazole,5,6 bis(pentafluorophenyl) carbonate,7−9 triphosgene,10−12 enzymes,13,14 carbon monoxide,15 and ethyl chloroformate,16−20 among which ethyl chloroformate is the most frequently used. The selection of the ringclosing reagent and starting materials can offer great diversity in the accessible monomers in very few steps. Sanders et al. demonstrated that the combination of bis-MPA and 2 equiv of bis(pentafluorophenyl) carbonate produces an active ringclosed cyclic carbonate intermediate in only one step, which can subsequently be easily transformed into an array of different monomers.8 However, a major drawback of this synthetic methodology is that it requires an extensive use of solvents and

The synthetic toolbox for producing functional monomers, adapted for various applications in polymer science, is constantly expanding and refined. However, the “tools of the past” are sometimes forgotten, yet they can hold unexploited potential. The ring-opening polymerization of aliphatic six-membered carbonates is an area of research that has received considerable attention over the past decade, particularly regarding functional monomers.1−4 The “great boost” in interest for functional carbonate monomers can be attributed to many factors, but from the perspective of polymer synthesis, the most protruding factors are the great diversity in monomer preparation techniques and the retained ability to be polymerized via ring-opening, even with very demanding functional groups. This interest has resulted in an almost endless range of available functionalities for six-membered carbonates, and consequently, their application areas cover a large portion of the field of polymer science.1−4 © 2014 American Chemical Society

Received: June 13, 2014 Revised: August 7, 2014 Published: September 7, 2014 6189

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Monomer Synthesis. The employed synthesis procedure is influenced by the work of Carothers et al., Sarel et al., and Kricheldorf et al. in combination with work performed by our group.26,27,33,36−39 In a two-necked, 500 mL round-bottom flask equipped with a magnetic stirrer, 200 mL of diethyl carbonate (1.74 mol) was submerged into an ice bath. After cooling, 4.5 g of NaH (60% dispersion in mineral oil) (0.112 mol) was added, leading to the formation of a white slurry. The reaction vessel was placed under an inert atmosphere (N2(g)), followed by the addition of 200 g of trimethylolpropane allyl ether (1.12 mol). The addition was initially performed dropwise until the white slurry cleared, which was followed by a rapid addition. The reaction vessel was equipped with a fractionation column and a distillation apparatus, and it was placed in a thermostated oil bath at 120 °C. The reaction was maintained overnight (14 h), and the formed ethanol and some unreacted diethyl carbonate were collected in a 100 mL round-bottom flask; see the 1H NMR of the distillate in Spectrum S.1 of the Supporting Information. After 14 h, the reaction showed signs of a higher viscosity, and the 1H NMR spectrum contained peaks that are associated with the oligomeric structure of the carbonate, as shown in Spectrum S.2 of the Supporting Information. The reaction temperature was decreased to 80 °C, followed by the application of vacuum. The reaction temperature was increased in 10 °C increments every 15 min until the temperature reached 150 °C. At this stage, the reaction temperature was increased to 200 °C, followed by collection of the product, 2allyloxymethyl-2-ethyltrimethylene carbonate, with a yield of 77% (173 g). The product was further purified by vacuum distillation over CaH2, resulting in a yield of 82% (142 g); see Figure 2. The total yield obtained from the reaction with respect to trimethylolpropane allyl ether was 63%. Polymerization of 2-Allyloxymethyl-2-ethyltrimethylene Carbonate. All reaction vessels were dried in an oven at 150 °C for 48 h before use. In general, the desired amounts of the reactants were weighed into a two-neck, round-bottom flask (25 mL) under a nitrogen atmosphere in a glovebox (Mbraun MB 150-GI). Each flask was equipped with a magnetic stir bar and sealed with a two-way valve and a septum. All reactions were stirred at a constant temperature that was maintained (±2 °C) using an IKAMAG RCT basic safety control magnetic stirrer. Aliquots for 1H NMR and GPC measurements were withdrawn from the reaction vessels at regular time intervals using new disposable syringes while the vessels were flushed with nitrogen gas. TBD-Catalyzed Polymerization. The desired amounts of allyloxymethyl-2-ethyltrimethylene carbonate (3 g, 15 mmol) and the initiator benzyl alcohol (0.018 g, 0.15 mmol) were weighed inside of a glovebox into a dry 25 mL two-necked round-bottom flask, followed by the addition of 3 mL of dry THF. The reaction vessel was placed on a stirring plate, and a constant temperature of 25 °C was maintained. In a separate 5 mL round-bottom flask, the TBD (0.021 g, 0.15 mmol) was dissolved in 1.5 mL of dry THF. The TBD solution was injected into the reaction mixture, resulting in a total concentration of 2 M, under a flow of N2(g) marking t = 0. After t = 6 min, (0.042 g, 0.30 mml) of TBD dissolved in 1 mL of dry THF was added to the reaction mixture. Sn(Oct)2-Catalyzed Polymerization. The desired amounts of allyloxymethyl-2-ethyltrimethylene carbonate (5 g, 25 mmol), the initiator benzyl alcohol (0.018 g, 0.25 mmol), and Sn(Oct)2 (0.1 g, 0.25 mmol) were weighed inside of a glovebox into a dry 25 mL twonecked round-bottom flask. The reaction vessel was placed onto a stirring plate under a N2(g) atmosphere. The temperature was varied from 25 to 200 °C in a noncontinuous manner, i.e., from 100 °C−170 °C−60 °C−25 °C−130 °C−200 °C. The time required to reach the equilibrium point was highly dependent on the temperature. Therefore, aliquots were continuously withdrawn to monitor the change in monomer concentration. When the equilibrium point was reached, five aliquots were collected at different time points. All changes in polymer and monomer compositions were analyzed via 1H NMR. Thiol−Ene Click Chemistry on Poly(allyloxymethyl-2-ethyltrimethylene carbonate). The reaction was performed in a similar manner as that utilized for grafting on PLLA particles recently

often several steps, including protection/deprotection, prior to the ring-closing reaction. To circumvent the extensive use of solvents and reactants and to consequently reduce the environmental burden, other strategies have been developed. These strategies can roughly be denoted as the “ring-expansion route”. This synthesis pathway can start from either the epoxide or the cyclic equivalent of the 1,3-diol, an oxetane. The epoxide or oxetane can then either be ring expanded to form the cyclic monomer or copolymerized with carbon dioxide or carbon monoxide by the aid of different metal complexes.21−25 Although this approach is in many ways a green approach, it is limited to use in conjunction with carbon dioxide or carbon monoxide, and problems may arise during copolymerization with more reactive monomers, such as cyclic esters. Therefore, for specific applications, great importance is placed on preparing the monomer prior to polymerization. To develop a more straightforward approach for obtaining functional six-membered monomers, we focused on the beginning of ring-opening polymerization. In the 1930s, Carothers et al. highlighted the reversible nature of the sixmembered carbonate and utilized a very straightforward method to form its monomer: ring-closing depolymerization.26,27 Although ring-closing depolymerization has been demonstrated to be a good tool for forming numerous other heterocyclic monomers, in addition to the nonfunctional cyclic carbonates, it has not yet been utilized to obtain functional sixmembered carbonates. Over the years, our group has focused considerable research interest in the synthesis of functional degradable polymers, with a bias toward functional polyesters.28−32 In addition, we have explored the polymerization and ring-closing depolymerization of the nonfunctional aliphatic carbonate monomer trimethylene carbonate (TMC).33,34 Our previous work combined with work on how the ring size of lactones affects the ceiling temperature led to the following hypothesis: it should be possible to utilize the inherent reversible nature, i.e., the relatively low ceiling temperature of the functional six-membered carbonate polymer, without the aid of an additional ring-closing depolymerization catalyst to form the monomer.31,35 The objective was to observe this process and thereby demonstrate ring-closing depolymerization to be a powerful tool for the synthesis of functional six-membered aliphatic cyclic carbonates. The synthesis scheme focused on the functional sixmembered carbonate monomer 2-allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC). This synthetic methodology will ideally provide a more straightforward, less expensive, and greener approach for obtaining functional aliphatic sixmembered cyclic carbonate monomers.

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EXPERIMENTAL SECTION

Materials. Stannous octoate (Sn(Oct)2) (Sigma-Aldrich, Sweden) was dried over molecular sieves (3 Å) before use. All other chemicals were used as received and included benzyl alcohol (≥99%, SigmaAldrich, Sweden), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (98%, Sigma-Aldrich, Sweden), sodium hydride (NaH) (60% dispersion in mineral oil, Sigma-Aldrich, Sweden), diethyl carbonate (99%, SigmaAldrich, Sweden), trimethylolpropane allyl ether (98%, Sigma-Aldrich, Sweden), chloroform (HPLC grade, Fisher Scientific, Germany), tetrahydrofuran (THF) (anhydrous, containing 250 ppm BHT as an inhibitor, ≥99.9%, Sigma-Aldrich, Sweden), tetrahydrofuran (THF) (HPLC grade, Sigma-Aldrich, Sweden), benzophenone (BPO) (ReagentPlus 99%, Sigma-Aldrich, Sweden), 1-dodecanethiol (≥98%, Sigma-Aldrich, Sweden), and chloroform-d (99.8%, with silver foil, Cambridge Isotope Laboratories). 6190

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Scheme 1. Outline for the Synthesis of the Functional Six-Membered Aliphatic Carbonate Monomer via Ring-Closing Depolymerization

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described by us.40 The desired amounts of poly(allyloxymethyl-2ethyltrimethylene carbonate) (0.5 g, 2.5 mmol repeating unit), 1dodecanethiol (3 mL,12.5 mmol), and BPO (0.02 g, 0.125 mmol) were dissolved in 6.5 mL of THF in a 15 mL Pyrex glass tube equipped with a magnet and positioned in an ice bath. The tube was equipped with a rubber septum and was placed under an inert atmosphere. The reaction vessel was irradiated with a UV lamp (Ohsram Ultra Vitalux, 300 W) with a wavelength range of 280−320 nm and an output intensity of 38 mW/cm2. At regular time intervals, aliquots of the reaction were withdrawn and analyzed using 1H NMR and GPC. Instruments. Nuclear Magnetic Resonance (NMR). 1H NMR (400.13 MHz) and 13C NMR (100.62 MHz) spectra were recorded at 298 K using a Bruker Avance 400 spectrometer. For the measurements, either ∼10 mg (1H NMR) or ∼100 mg (13C NMR) of the polymer was dissolved in 0.8 mL of CDCl3 in a 5 mm diameter sample tube. The spectra were calibrated using the residual proton of the solvent signal: 7.26 ppm (1H NMR) and 77.0 ppm (13C NMR) for CHCl3. For the equations used to calculate the conversion, see eqs E1 and E2 in the Supporting Information. Gel Permeation Chromatography (GPC). GPC was used to determine the number-average molecular weights (Mn) and the dispersity (Đs) of the polymers after polymerization using a Verotech PL-GPC 50 Plus equipped with a PL-RI detector and two PolarGel-M Organic columns, 300 × 7.5 mm (Varian, Santa Clara, CA). Samples were injected with a PL-AS RT autosampler (Polymer Laboratories), and chloroform was used as the mobile phase at a flow rate of 1 mL/ min at 30 °C with toluene as an internal standard. The calibration was performed using polystyrene standards with a narrow molecular weight distribution ranging from 160 to 371 000 g/mol. Electrospray Ionization Mass Spectrometry (ESI-MS). A Finnigan LCQ ion trap mass spectrometer (Finnigan, San Jose, CA) was used to analyze the oligomeric condensation products and the monomer. The samples were dissolved in a water/methanol system (1:2 v/v) and continuously injected into the ESI source by the instrument syringe pump at a rate of 3 mL/min. The LCQ ESI source was operated at 5 kV, and the capillary heater was set to 180 °C with nitrogen as the nebulizing gas. The ions were isolated monoisotopically in the ion trap and collisionally activated. Helium, present as the damping gas in the mass analyzer, acted as the collision gas. The RF amplitude, which had a significant voltage range, was set to a value that caused the peak height of the parent ion to decrease by at least 50%. The positive ion mode was used for analysis.

RESULTS AND DISCUSSION

Ring-closing depolymerization has been demonstrated to be an excellent and green synthesis route for the preparation of carbonate monomers. The original example for this synthetic methodology dates back to the beginning of the 1930s in work of Carothers et al., in which the polymerization and ring formation of the glycol ester of carbonic acid were studied.26 In the same context, the synthesis of functional carbonate monomers via ring-closing depolymerization provides a simple yet very powerful route to obtain functional monomers. In contrast to the most commonly employed methods for synthesizing cyclic functional carbonate monomers, this method does not require any solvents, halogenic-ring closing reagents, or long synthetic routes.1−4 In other words, ringclosing depolymerization provides a “greener” route for obtaining functional monomers. The outline of this study will be divided into two different parts. Initially, focus will be placed on the oligomerization and depolymerization to yield the desired monomer, highlighting the underlying thermodynamic features. The second part will explore its polymerizability with the commonly used organic catalyst TBD, followed by subsequent postpolymerization functionalization via UV-initiated thiol−ene “click” chemistry. From Oligomers to the Functional Cyclic Monomer, AOMEC. The key compound in this synthetic route for the functional carbonate monomer is trimethylolpropane allyl ether, i.e., an alloxyl-functionalized 1,3-diol. We selected this starting compound for several reasons: (i) it is very inexpensive, (ii) it provides an orthogonal functionality, and (iii) its sixmembered cyclic carbonate monomer, 2-allyloxymethyl-2ethyltrimethylene carbonate (AOMEC), has previously been reported in the literature.41−43 Previous studies have reported that when homopolymerizing AOMEC via anionic ROP initiated by sec-Bu-Li, the formation of cyclic oligomers is clearly observed.42 Based on this result, the most convenient activator for the first condensation substep is a simple hydride donor. Sodium hydride (NaH) was selected for this step. Another positive consequence of using NaH is that it also serves as a “locking” agent for the ring-opened short oligomers that otherwise would 6191

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Figure 1. ESI-MS results from the oligomerization step (left) and the later ring-closing depolymerization step (right).

Figure 2. 1H NMR (left) and 13C NMR (right) spectra of AOMEC obtained by ring-closing depolymerization.

Figure 3. (left) Dependence of the monomer and polymer concentrations on temperature initiated with 1 mol % benzyl alcohol and 1 mol % Sn(Oct)2 as a catalyst in bulk. (right) Linear fit of the calculations of the thermodynamic features of the monomer.

the monomer concentration equilibrium, thereby eliminating the need for other depolymerization catalysts. The mass spectrum analysis of the product revealed that the monomer had been produced because most of the observed peaks could be associated with the monomer, as shown in Figure 1 (right). The higher mass peaks are due to the formation of a cluster of two monomers and are not an indication of the formation of a 12-membered carbonate ring. This was confirmed by the obtained 1H NMR results and was in perfect agreement with the results previously reported by Höcker et al., thereby confirming that the six-membered monomer had been produced, as shown in Figure 2 (left).42,43 In addition, the different conformations of 12-membered rings would lead to a difference in chemical shifts; see Figure 2 (right).36 Thermodynamics Underlying the Formation of the Monomer. The core concept underlying the monomer synthesis is based on the thermodynamic features of ringopening polymerization itself. At a certain polymerization temperature, there is an equilibrium concentration of polymer to monomer. However, note that the monomer concentration can either be favored by a decrease or an increase in temperature depending on the monomer and subsequent

have a similar boiling point as the cyclic monomer during distillation. Diethyl carbonate was selected as the cocondensation reactant simply due to the greater difference in boiling point between EtOH (bp = 78 °C) and EtOC(O)OEt (bp = 127 °C) in comparison to, e.g., MeOC(O)OMe (bp = 90 °C) and MeOH (bp = 65 °C). After the condensation reaction, a small change in viscosity was observed. This observation indicated that the reaction yielded a higher molecular weight species. To achieve controlled depolymerization, the repeating unit should correspond to the desired cyclic monomer. The mass spectrum analysis of the crude product revealed that the reaction settings resulted in oligomers with the desired structure, with the number of repeating units ranging from 1 to 7, as shown in Figure 1 (left). The monomer synthesis was performed in one pot, meaning that the depolymerization was performed directly after the condensation step in the same reaction vessel. Our hypothesis is based on what has previously been reported by Endo et al., that the anionic environment is sufficient to catalyze the depolymerization reaction.46 In other words, the anionic environment produced by NaH should be sufficient to mediate 6192

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Figure 4. Polymerization of AOMEC in THF (1 M) with benzyl alcohol (1 mol %) as an initiator and TBD as a catalyst (1 mol % for the first 6 min and 3 mol % for the remainder of the reaction). (left) Mn and Đ evolution with conversion. (right) Dependence of the monomer conversion on time.

Figure 5. Postpolymerization modification of poly(AOMEC) with 1-dodecanethiol (500 mol %) in THF (0.3 M) with BPO (5 mol %) as an initiator. (left) Mn and evolution of Đ with conversion of the unactivated alkene. (right) Conversion of the unactivated alkene with time.

the lower the Tc is.46 This concept has also found use in the recycling of carbonates and is therefore very important when investigating new monomers and polymers from a life cycle perspective.47,48 From Monomer to Functional Polymer. Organic catalysts have been found to be very active in the ring-opening polymerization of cyclic six-membered monomers,1,3,49 in particular TBD.50−53 The TBD-catalyzed polymerization of AOMEC proceeded with high control and with a linear relationship between molecular weight and conversion, as shown in Figure 4 (left) (for the 1H NMR spectrum of poly(AOMEC), see Spectrum S.3 in the Supporting Information). In addition, it was observed that the molecular weight evolution during polymerization is independent of the amount of TBD added to the system, as shown in Figure 4. Upon the further addition of TBD under the pending polymerization, the linearity of the molecular weight with conversion remains and Đ remains unaffected; the only observed effect is an increase in the polymerization rate. These results are consistent with the proposed mechanistic feature of the TBD-catalyzed polymerization of L-lactide and εcaprolactone, which is believed to act primary via hydrogen bonding rather than acyl transfer.50,54 Although the rate constants of polymerization do not follow a linear relationship with the amount of TBD added to the system, kpapp(1 mol %) = 4.2 s−1 and kpapp(3 mol %) = 28.2 s−1, as shown in Figure 4 (right). This result is believed to be an artifact of potential impurities in the system that deactivates some of the catalyst upon the addition of TBD. However, this result clearly shows that the initial TBD ratio enables an easy modulation of the reaction time with retained control over the Mn and Đ. Additionally, the ability of AOMEC to ROP from the monofunctionalized macroinitiator m-PEG 2000 was evaluated,

polymer. The synthesis of the monomer is completely dependent on the thermodynamic features of the ring in terms of enthalpy and entropy.44,45 The cyclic six-membered carbonates have a very convenient temperature range in which this feature can be utilized for their monomer synthesis. Although an increase in temperature leads to an increase in monomer concentration, the increase in this case is not high enough to favor side reactions such as decarboxylation. The reversible nature of the polymerization of AOMEC is illustrated by its difference in monomer and polymer concentrations at different temperatures in Figure 3 (left). The polymerization was performed in bulk with benzyl alcohol (1 mol %) as an initiator and Sn(Oct)2 (1 mol %) as a catalyst. Sn(Oct)2 was selected because of its high stability in this temperature range. Although the system used in the monomer synthesis is anionic rather than based on a coordination− insertion mechanism, it can be assumed that other reactions can result from the more nucleophilic nature of the active species.42 However, these results highlight the underlying thermodynamic features of AOMEC that we are utilizing for its monomer formation and in addition that no side reactions occur. The ceiling temperature was observed to be in good agreement with the polymer and monomer concentration curve (Figure 3 (left)), and a Tc value of 190 °C was obtained from the ratio of ΔH0p/ΔS0p (Figure 3 (right)). This value corresponds to the temperature at which the concentration of polymer is equal to the concentration of monomer. It has been shown that the Tc is greatly affected by the size of the substituent at the position that, in our case, bears the ethyl group and the orthogonal functionality, as deduced from work by Endo et al., who polymerized a wide range of carbonates in which the substituent had varying degrees of “bulkiness”. Therein, it was concluded that the bulkier the substituent was, 6193

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Scheme 2. Synthetic Outline for UV-Initiated Thiol−Ene Click of 1-Dodecanethiol on Poly(AOMEC)

The reversible nature of the polymerization of AOMEC was observed by its difference in monomer and polymer concentrations at different temperatures and by the ceiling temperature, which was obtained from the ratio of ΔH0p/ΔS0p and determined to be 190 °C. This value was in good agreement with the polymer and monomer concentration curve. These results highlight the reversible nature of poly(AOMEC) (and hence, the cyclic monomer AOMEC), indicating great potential for its chemical recycling. In addition, AOMEC was found to be very active toward ROP with the organic catalyst TBD and offers high control over molecular weight at a high rate. This was determined from the ROP of AOMEC with two subsequent additions of TBD during the polymerization, resulting in a rate shift from kpapp(1 mol %) = 4.2 s−1 to kpapp(3 mol %) = 28.2 s−1 with retained control over Đ and linearity between the conversion and Mn. The subsequent poly(AOMEC) orthogonal functionality was explored via a postpolymerization procedure utilizing UVinitiated thiol−ene chemistry with 1-dodecanethiol and BPO as a catalyst. The functionalization proceeded with high control and with a linear relation between the degree of functionalization and an increase in the Mn of the polymer. Depolymerization provides a straightforward and inexpensive route for obtaining functional carbonate monomers, particularly for the synthesis of AOMEC. Additionally, it was observed that the AOMEC monomer is very active in ROP with the organic catalyst TBD and, in conjunction with UV-initiated thiol−ene chemistry, offers a high degree of control over the final properties while maintaining the structural integrity of the polymer. These results are proposed to provide the starting point for achieving functional green and degradable polymers for every level of application, thus enabling a future in which functionality is not limited to a select few industries.

consequently revealing its potential as an amorphous functional monomer for micellar systems. We have previously examined how the amorphicity and electroactivity of the micelle core dictate the final structure and responsiveness of the micelle.55,56 The initiator had a starting Mn of 4000 and Đ of 1.04, and after extension, it had a Mn of 9800 and a Đ of 1.44. This result indicates the possibility of utilizing the AOMEC monomer in multiblock systems as well as in micellar systems. Thiol−ene chemistry has proven to be a valuable asset in the polymer synthetic toolbox and provides high functional group tolerance along with a high abundance of commercially accessible reactants.57,58 The route chosen for the thiol addition most often depends on the activation degree of the unsaturation, i.e., if the unsaturation is activated, the Michael addition is most commonly used,59 whereas for a nonactivated unsaturation, the route is via free radical addition.20 However, note that for an activated unsaturation both modes of addition are possible. Considering a degradable polymeric chain, the most preferable route is via free radical addition because the carbonate structure is fairly resilient toward free radicals. The UV-initiated thiol−ene enables the degree of functionalization to be varied depending on the exposure time, as shown in Figure 5 and Scheme 2. The parent polymer used for the thiol−ene functionalization was polymerized with TBD (2.5 mol %) as a catalyst for 30 min at a M/I ratio of 100, yielding a polymer with a Mn = 11 500 and a Đ =1.5. The increase of Đ compared to the previous polymerization is believed to be a consequence of the prolonged reaction time that results in more transesterfication reactions. The molecular weight of the poly(AOMEC) increased linearly with the conversion of the unactivated alkene, indicating that the addition of 1dodecanethiol proceeds with high control, as shown in Figure 5 (left). In addition, the time dependence of the thiol−ene reaction enables the degree of functionalization of the parent polymer to be tailored for specific applications.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR of condensation products and poly(AOMEC). This material is available free of charge via the Internet at http:// pubs.acs.org.

CONCLUSIONS Depolymerization as a synthetic methodology was demonstrated to be a powerful tool for the synthesis of the allyloxyfunctionalized six-membered cyclic carbonate monomer 2allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC) at a scale greater than 100 g. The monomer synthesis was performed in bulk with trimethylolpropane allyl ether (1 equiv), diethyl carbonate (1.5 equiv), and NaH (0.05 equiv). The reaction procedure was as follows: first, the oligomers (1− 7 repeating units) were formed through the removal of EtOH at 120 °C. Second, direct depolymerization was achieved in the same reaction vessel at 200 °C under vacuum without the addition of additional reactants. The synthesis resulted in a final yield of 63% of the pure AOMEC monomer with respect to trimethylolpropane allyl ether.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +46-8-790 82 74; Fax +46-820 84 77 (A.-C.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the ERC Advance Grant PARADIGM (Grant agreement no. 246776) for their financial support of this work. Dr. Sofia Regnell Andersson is gratefully 6194

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acknowledged for her assistance with the ESI-MS measurements. Last, but not least, Dr. Helmut Keul is gratefully acknowledged for his valuable scientific input during the preparation of this manuscript.

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dx.doi.org/10.1021/ma5012304 | Macromolecules 2014, 47, 6189−6195