External and Reversible CO2 Regulation of Ring-Opening

Article pubs.acs.org/Macromolecules

External and Reversible CO2 Regulation of Ring-Opening Polymerizations Based on a Primary Alcohol Propagating Species Olivier Coulembier,* Sébastien Moins, Richard Todd, and Philippe Dubois Center of Innovation and Research in Materials and Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials, University of Mons - UMONS, 23 Place du Parc, 7000 Mons, Belgium S Supporting Information *

ABSTRACT: We report the use of an organic catalyst system capable of switching between active and dormant propagating states during the ROP of cyclic monomers. While the ROP of both ε-caprolactone and trimethylene carbonate proceeds under nitrogen, the simple addition of CO2 results in a dormant “off” state. Cycling between atmospheres provides the ability to regulate the molecular weights of the resulting polymers without appreciable loss of catalytic activity for several “on/off” cycles.

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INTRODUCTION

which upon purging with nitrogen or argon reverts back to its nonionic form (Figure 1).22

The ability to switch on demand a controlled polymerization reversibly “on” and “of f ” represents a useful tool in modern polymer chemistry. In addition to both controlled polymerization processes and orthogonal polymer functionalizations, the external regulation of polymerizations may contribute in solving the grand challenges in material science.1 In an ideal case, the in situ switch between active and dormant growing polymer chains must be quick, quantitative, and fully reversible to an appropriate external stimulation. Temporally controlled polymerizations can be regulated by a variety of stimuli including exogenous reagents,2−8 mechanical force,9−11 applied voltage,12 or light.13−17 Among those available external stimuli, the chemical catalyst modulation obtained by either allosteric or simple (redox) chemical control represents an effective method for regulating ring-opening polymerizations (ROPs) in situ.1 These two strategies have made significant progress by employing chemical reagents to modulate the activity of metalbased catalysts/initiators while a powerful outcome should consist of the design of a fully reversible and controlled polymerization catalytic system exclusively based on organic molecules totally exempt of any metal trace. In an effort to promote the green character of polymer chemistry, organo-based catalysts for the ROP of various cyclic monomershave been developed and studied.18−20 As part of those catalysts, amidines and guanidines such as 1,5,7triazabicyclododecene (TBD) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) represent the spearhead of the modern green chemistry field.21 Jessop et al. demonstrated that exposing a 1:1 mixture of DBU and a primary alcohol (e.g., 1-hexanol) to gaseous CO2 at 1 atm and at room temperature (rt) generates an ionic liquid © 2014 American Chemical Society

Figure 1. Reversible fixation of CO2 on a 1:1 mixture of primary alcohol and DBU.

Reasoning that the primary alcohol involved in the CO2 fixation might be a polymer end-group, the design of a switchable polymerization process based on an active (“on”) to dormant (“off”) equilibrium purely dependent on organic molecules is then possible (Figure 2).

Figure 2. Reversible CO2/DBU/ROH reaction used in a switchable “on/off” controlled ROP process (with X = CH2 or O). Received: December 5, 2013 Revised: January 13, 2014 Published: January 16, 2014 486

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RESULTS AND DISCUSSION Because the reversibility of the CO2 fixation has been demonstrated to be efficient from primary alcohol under very mild conditions (1 atm of CO2 or N2; rt), the importance to conduct a ring-opening process from a primary alcohol propagating species has dictated the choice of the polymerizable monomer. To that end, ε-caprolactone (CL) and trimethylene carbonate (TMC) have been selected as pertinent candidates to demonstrate this critical “on/off” organo-based proof-of-principle. Polymerization of CL. While TDB is active toward the ROP of CL, DBU however requires a thiourea cocatalyst to promote the polymerization.23 In order to suppress any competitive intermolecular interactions and only focus on the possibility to halt in situ the polymerization via addition of CO2, we then decided to employ a mixture of TBD/DBU to promote the polymerization and the CO2 fixation, respectively. Polymerization of CL by TBD in the presence of benzyl alcohol (BzOH) and for a polymerization degree (DP) of 50 ([CL]0/[BzOH]0/[TBD]0: 50/1/0.25; [CL]0 ∼ 0.5 M) is generally accomplished within 5 h (conv ∼ 0.8) at rt23 No trace of polymerization was observed after 3 h when CL (0.42 M in either CH2Cl2 or toluene) was reacted with an equimolar quantity of DBU ([TBD]0/[DBU]0 = 1) under similar conditions ([CL]0/[BzOH]0/[TBD]0/[DBU]0: 50/1/0.5/ 0.5), whereas only 10% monomer conversion was obtained after 30 h for an initial [CL]0/[BzOH]0/[TBD]0/[DBU]0 ratio of 50/1/10/10. The diminished reactivity of the TBD/DBU (1:1) activation mixture for CL polymerization compared to that observed with pristine TBD suggested that some process was attenuating the reactivity of the TBD catalyst toward polymerization. A reversible interaction between TBD (MeCNpKBH+ = 25.98) and DBU (MeCNpKBH+ = 24.33) may provide an explanation for the low reactivity of the pristine guanidine in the polymerization of CL when aTBD-to-DBU initial ratio of 1 is used. In the IR spectrum of the semiprotonated complex of TBD and DBU (proton source: BzOH; [TBD]0/[DBU]0/[BzOH]0 = 1), the two CN intense bands appearing at 1643 and 1610 cm−1 tend to demonstrate a homoconjugated hydrogen bond (N+− H···N) between the amidine and the guanidine superbases (Figure 3).24−26 Confirmation of such complexation is reinforced by the appearance of a broad band complex in the region 3700−2400 cm−1 (see Supporting Information, Figure SI1).

The presence of an exchangeable proton is crucial for the TBD/DBU complexation since no clear IR modification is observed without proton source (Figure SI2). The BzOH has then been intentionally used to copy as much as possible the polymerization condition but also to observe and attest the formation of the semiprotonated complex by 1H NMR analysis. Free of any other component, the methylene hydroxy protons (Ph−CH2−OH) of BzOH appear at 4.68 ppm in CDCl3. Interestingly, those protons are upfield-shifted by 0.32 ppm when BzOH is mixed with equimolar quantities of TBD and DBU in identical conditions of concentration and temperature (Figure SI3). To limit the complexation between TBD and DBU during the polymerization process and promote the polymer formation, the ROP of CL was then carried out with an initial TBD-to-DBU molar ratio of 10, with all other experimental parameters unchanged ([CL]0/[BzOH]0/[TBD]0/[DBU]0: 50/1/10/1). Under these conditions, TDB might not be considered anymore as a catalyst but more as an activator since its loading content is higher than the initiator one. It has to be noted however that such activator-to-initiator proportion is in the same range than the majority of other metal-free “catalysts” used so far.27 By utilizing this ratio, a controlled polymerization was achieved, resulting after 2 h in PCL with a DP of 14 (Mn = 1400 g/mol) and a dispersity value (ĐM = Mw/Mn) of 1.12 as determined by size exclusion chromatography (SEC). While TBD is inactive for CO2 fixation from a primary alcohol (see Supporting Information), evidence for the successful formation of the carbonate end-group generation from the TBD/DBU (10:1) mixture was in part provided by a FTIR-scale single-turnover experiment. A solution of CL, BzOH, TBD, and DBU in dry CH2Cl2 ([CL]0/[BzOH]0/ [TBD]0/[DBU]0: 1/1/10/1) was, after 5 min, bubbled with CO2 until the total evaporation of the organic solvent yielded a viscous oil. Quick analysis by FTIR revealed that the major vibrational carbonyl band present at 1710 cm−1 is accompanied by a very strong signal at 1645 cm−1 corroborating the carbonate formation (Figure 4a) as compared to the state-ofthe-art.22,25,28 After some thermal treatment (5−10 min at 40 °C), the FTIR spectrum of the product revealed a significant decrease of the −OCO2(−) vibrational signature suggesting reversibility of

Figure 4. FTIR spectra of a CL/BzOH/TBD/DBU/CO2 mixture as a function of time: (a) t0 (23 °C) [red], (b) t0 + 5 min (40 °C) [black], and (c) t0 + 10 min (40 °C) [blue].

Figure 3. FTIR spectrum of a TBD/DBU equimolar mixture in the region 1690−1590 cm−1 (proton source: BzOH, 1 equiv). 487

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Figure 5. 1H NMR analysis (CDCl3, rt, 500 MHz) performed on a [CL]0/[BzOH]0/[TBD]0/[DBU]0 (1/1.2/1/0.5) mixture after CO2 bubbling.

Scheme 1. General Scheme of CO2-Based Switchable CL Polymerization Allowed by a TBD/DBU (10:1) Organocatalytic Mixture

the CO2 fixation from a primary caproyl end-group under mild conditions (Figure 4b,c). 1 H NMR analysis allows for attesting the nature of the caproyl end-group when bubbling CO2 as well as the importance to strictly respect the equimolarity between the primary alcohol and the DBU to totally turn off the ROP process. To that end, CL was ring-opened in CH2Cl2 by BzOH using a slight excess ([BzOH]0/[CL]0 = 1.2) in the presence of 1 equiv of TBD. After 2 min, gaseous CO2 was bubbled thorough the solution in the presence of 0.5 equiv of DBU ([BzOH]0/[CL]0/[TBD]0/[DBU]0 = 1.2/1/1/0.5) until the total evaporation of the organic solvent. Figure 5 presents the 1 H NMR analysis focused on the 2.3−5.1 ppm region of the spectrum. As compared to reported spectroscopic data,29 both the hydroxyl and carbonate methylene end-groups (He and Hf) are visible at 3.6 and 4.05 ppm, respectively. Taking into account the inherent experimental error of the NMR technique, 60% of the caproyl moieties have been functionalized by carbon dioxide. Such value corroborates the expected value of 50% and demonstrates the importance of the stoichiometry between the initiating alcohol and the DBU organocatalyst. Both the caproyl

methylene protons (He) and methylene hydroxy protons of BzOH are not upfield-shifted since DBU is implied in the carbonate salt. The feasibility of an organo-based chemical switch to modulate in situ the nature of the propagating PCL endgroup, and then activate or deactivate the ROP process (Scheme 1), was demonstrated by repetitive stepping of CO2 additions and eliminations triggered by a TBD/DBU (10:1) mixture in toluene.30 Upon starting the polymerization under nitrogen ([CL]0/[BzOH]0/[TBD]0/[DBU]0: 50/1/10/1; [CL]0 = 0.42M; rt), 19% conversion was observed after 26 min, at which point gaseous CO2 was bubbled through the polymerization solution for 1 min. Instantly, a white precipitate appeared in the organic phase, halting then the monomer consumption over the following 28 min. This dormant state polymerization was then converted back to its active form by bubbling N2 through the milky solution at rt (or 40 °C) until regeneration of its initial “transparent” aspect (∼5−10 min at rt, 2−3 min at 40 °C). This chemical cycle was also repeated three times for various CO2 and N2 exposure periods (see Supporting Information for details). 488

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Figure 6 clearly demonstrates that the polymerization can be activated and deactivated on demand via the reversible fixation

Figure 8. Time dependence of ln([CL]0/[CL]t) ([CL]0 = 1.7 M; [CL]0/[BzOH]0/[TBD]0/[DBU]0: 200/1/10/1) with alternation of N2 (blue lines) and CO2 (red lines) saturated atmospheres plotted from data obtained at 21 °C (●; full lines) and 70 °C (○; dotted lines). A few ĐM are reported in parentheses.

Figure 6. Time dependence of ln([CL]0/[CL]t) ([CL]0 = 0.42 M; [CL]0/[BzOH]0/[TBD]0/[DBU]0: 50/1/10/1) with alternation of N2 and CO2 saturated atmospheres.

CL polymerization and the CO2 removal are performed at 21 °C. After only two CO2 cycles, it was impossible to restart properly the propagation process by displacing the carbonate end-group by N2 bubbling at rt. It is important to note here that the nitrogen refreshment allows however obtaining back the transparency of the medium. If the apparent kinetics clearly declines as a function of CO2 cycles to totally level off at 0 (Figure 9), both transparency and the nonincrease in dispersity conclude on a somehow side reaction.

of CO2. Throughout the cycling process, experimental molecular weights (M n ) correlate strongly with their predetermined theoretical values and increase linearly with recorded conversions while low ĐM are maintained during the whole “on” and “off” process (Figure 7).

Figure 7. Absolute number-average molecular weights (Mn) and associated dispersity values (ĐM) as determined by SEC with respect to monomer conversion. The dashed line represents the theoretical molecular weight evolution as determined by the following relationship: Mnth = DPth × 114 × conv + 108.

Figure 9. Apparent rate constants (kapp) calculated from data shown in Figures 6 and 8. Polymerization conditions: (●) [CL]0 = 0.42 M, DP = 50; (○) [CL]0 = 1.7 M, DP = 200.

In order to extrapolate this “on−off” process to higher PCL molar masses in a reasonable time scale, a polymerization degree (DP = [CL]0/[BzOH]0) of 200 was targeted for an initial monomer concentration of 1.7M. Both DP and [CL]0 were not totally chosen arbitrarily since such conditions infer an initial initiator concentration ([BzOH]0) of 8.5 × 10−3 M. Such initiator concentration is more or less comparable to the one used for the polymerization performed at 0.42 M when a [CL]0-to-[BzOH]0 of 50 is targeted ([BzOH]0 = 8.4 × 10−3 M). From a kinetic point of view, apparent constant rates (kapp) obtained from semilog plots will then be directly comparable since they equal the kp[BzOH]0 product. Figure 8 (full line evolution) presents the semilog plot recorded for the new experimental conditions when both the

Although effective, the polymerization performed at lower monomer concentration starts to plateau when comparing kapp’s in regard to the number of CO2 cycles (Figure 9). This trend seems then to be the sign of a recurrent but not yet explained phenomenon which is more pronounced for higher initial monomer concentrations. Interestingly, this decrease in apparent kinetics is not observed when both ROP and CO2 removal are performed at 70 °C (Figure 8, dashed line evolution). It is important to note that whatever the number (and duration) of CO2/N2 cycles, a lesser loss of activity is observed (cycle 0: kapp = 1.62 × 10−2 min−1; cycle 1: kapp = 1.51 × 10−2; and cycle 2: kapp = 1.01 × 10−2 min−1), regardless of the ĐM of the product polymer (1.8 at 96% conversion). The high conversion associated dispersity values is indeed marginally higher than the others which is 489

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probably due to some loss in polymerization control but in any case is correlated to the chemical switching. To prove that the initial monomer concentration is an important factor in controlling the “on/off” CL ROP process, a final experiment was performed using a [CL]0 of 2.92 M (DP = 200, 21 °C). After 73 min a conversion of 52.6% was reached (Mn = 12 000 g/mol, ĐM = 1.27, kapp = 1.02 × 10−2 min−1), with all attempts to restart the polymerization after the first CO2 addition unsuccessul (no matter the temperature utilized). Polymerization of TMC. As already detailed in the state-ofthe-art, the TMC organo-based ROP might be performed utilizing pristine DBU.30 To that end, TMC ROP was performed at 21 °C for a [TMC]0/[BzOH]0/[DBU]0 ratio of 50/1/1 and for an initial monomer concentration of 0.4 M. Because of its partial insolubility in toluene at rt, TMC has been polymerized in DCM despite such solvent does not allow observing visually the CO2 quenching step. The high volatility of DCM also represents a problem to keep the polymerization volume invariable during the carbon dioxide bubbling step. This drawback was taken at our advantage by graduating the polymerization flask and then comparing kapp for two different concentrations. Figure 10 demonstrates that the TMC polymerization can also be activated and deactivated on demand with reversible

Figure 11. Apparent rate constants (kapp) evolution as a function of the [TMC]0. Data available from Figure 10.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis and characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (O.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the European Commission and RégionWallonne FEDER program (Materia Nova) and OPTI2MAT program of excellence, by the Interuniversity Attraction Pole Programme (P7/05) initiated by the Belgian Science Policy Office, and by the FNRS-FRFC. O.C. is a Research Fellow of the F.R.S.-FNRS.

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REFERENCES

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Figure 10. Time dependence of ln([TMC]0/[TMC]t) ([TMC]0/ [BzOH]0/[DBU]0: 50/1/1) with alternation of N2 and CO2 saturated atmospheres. Dispersity values are reported in parentheses. Initial TMC concentrations recalculated due to partial DCM evaporation after the first CO2/N2 cycle (top of the figure).

fixation of CO2 while maintaining low ĐM values. As compared to the polymerization performed in toluene (with CL monomer), the increase in monomer concentration is not a problem in DCM since kapp increase linearly with the calculated [TMC]0 (Figure 11).

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CONCLUSION This work clearly highlights the unique possibility to switch on demand a controlled polymerization reaction reversibly “on” and “off” via the reversible fixation of gaseous CO2. This external organoregulation of polymerization is demonstrated for monomers polymerizing from a primary alcohol end-group such as CL and TMC. Interestingly, we demonstrated that pristine DBU is able to promote both ROP and quenching process but may also be judiciously used with TBD when needed. 490

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