Switching from Controlled Ring-Opening Polymerization (cROP) to Controlled Ring-Closing Depolymerization (cRCDP) by Adjusting the Reaction Parameters That Determine the Ceiling Temperature.

Full control over the ceiling temperature (Tc) enables a selective transition between the monomeric and polymeric state. This is exemplified by the conversion of the monomer 2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC) to poly(AOMEC) and back to AOMEC within 10 h by controlling the reaction from conditions that favor ring-opening polymerization (Tc > T0) (where T0 is the reaction temperature) to conditions that favor ring-closing depolymerization (Tc < T0). The ring-closing depolymerization (RCDP) mirrors the polymerization behavior with a clear relation between the monomer concentration and the molecular weight of the polymer, indicating that RCDP occurs at the chain end. The Tc of the polymerization system is highly dependent on the nature of the solvent, for example, in toluene, the Tc of AOMEC is 234 °C and in acetonitrile Tc = 142 °C at the same initial monomer concentration of 2 M. The control over the monomer to polymer equilibrium sets new standards for the selective degradation of polymers, the controlled release of active components, monomer synthesis and material recycling. In particular, the knowledge of the monomer to polymer equilibrium of polymers in solution under selected environmental conditions is of paramount importance for in vivo applications, where the polymer chain is subjected to both high dilution and a high polarity medium in the presence of catalysts, that is, very different conditions from which the polymer was formed.

Introduction

The assumption of an increasing future demand for more refined polymeric materials with niche properties for niche applications cannot be easily refuted.[1,2] The trend is not only to construct polymers in a controlled manner but also to equip the polymer chain with functions. Functions predesigned into the monomeric structure or generated via a postpolymerization modification that targets a specific site or performs an exquisite task in vivo or in natura, resulting in a desired macroscopic change.[3,4] In other words, we aim to achieve, via synthesis, the specificity of biological systems.

One highlighted group of materials for a diverse set of applications, especially in the biomedical arena, are degradable aliphatic polymers with repeating units of either ester or carbonate groups.[5−7] These polymers, which are most often obtained by ring-opening polymerization of the respective cyclic monomers, allow, through different addition schemes or inherent reactivity behavior, the construction of refined macromolecular architectural features.[8−10] This enables control of many polymer properties, such as degradation,[11−13] mechanical performance,[11,14] and the placement of functional groups along the polymer chain.[15,16]

A particular class of monomers/polymers that has evoked strong interest and shown high potential for a diverse set of applications is the substituted cyclic six-membered carbonates and the corresponding polycarbonates.[17−20] Their potential lies in the abundance of a large variety of 1,3-propane diols that are selectively substituted at the 2-position with residues carrying functional handles, such as hydroxyl and carboxyl groups, that are subsequently easily derivatized.[21−27] Together with recent advances in organocatalyzed ROP, these monomers/polymers have opened the possibility to construct highly refined functional polycarbonate materials at high rates, at ambient temperatures and with few side reactions.[28−33]

Additionally, a factor immensely important but alluring the lime-light is the favorable thermodynamic equilibrium polymerization behavior of six-membered cyclic carbonates. Cyclic six-membered carbonates, even though heavily substituted, possess good equilibrium polymerization behavior, that is, favorable thermodynamics of polymerization,[34−36] compared to the cyclic ester analogs.[37−40] The thermodynamic description of polymerization was first discussed in the seminal paper by Dainton and Ivin in the late 1940s, where they coined the expression “the ceiling temperature (Tc) of polymerization”.[41] In addition to being indicative of the highest temperature at which conversion of the monomer can be achieved, the ceiling temperature (Tc) is a measure of the propensity of the monomer to undergo polymerization. Tc is independent of the catalytic system used but dependent on the monomer concentration, and there are indications in the literature that suggest very different polymerization behavior in different surrounding media, even at the same concentration,[38,42,43] especially for the cationic polymerization of THF.[44,45] A specific understanding of how the monomer–polymer equilibrium is altered as a function of the reaction medium may be regarded as a black box, but it is highly important for the behavior of the polymer chain in real applications.

Our hypothesis is that the monomer–polymer equilibrium is dictated by the current state of the system. Our aim is to tailor the thermodynamic equilibrium by changing the surrounding reaction conditions. This will enable a reversible transition between the monomeric and polymeric states in the system. If successful, this process would enable both predesigned control of polymerization as well as a means to fine-tune the final behavior of the polymeric system in different environments, aspects that are highly important not only in the biomedical arena but also in the recycling of polycarbonates and polyesters.[46,47]

To explore this, the same monomer, 2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC), is polymerized in very different surroundings by varying the solvent type, temperature, and monomer concentration. We have centered our investigation on the perspective of the thermodynamics of ring-opening polymerization. The equilibrium polymerization behavior of AOMEC is thermodynamically intermediate, meaning that the equilibrium should be easily disturbed by changing the surrounding environment, such as the temperature, solvent type, and concentration.[48]

Experimental Section

Materials

Hexanediol (Sigma-Aldrich, Sweden) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (Sigma-Aldrich, Sweden) were stored under an inert gas atmosphere prior to use. Chloroform (HPLC grade, Fisher Scientific, Germany), methanol (general purpose grade, Fisher Scientific, Germany), dichloromethane (anhydrous, ≥ 99.8%, Sigma-Aldrich), toluene (anhydrous, ≥ 99.8%, Sigma-Aldrich), acetonitrile (anhydrous, ≥ 99.8%, Sigma-Aldrich), dichloromethane (DCM) (Fisher Scientific, Germany), sodium hydride (NaH) (60% dispersion in mineral oil, Sigma-Aldrich, Sweden), diethyl carbonate (99%, Sigma-Aldrich, Sweden), trimethylolpropane allyl ether (98%, Sigma-Aldrich, Sweden), acetic acid (technical grade, Fisher Scientific, Germany), and acetic acid anhydride (ReagentPlus, ≥ 99%, Sigma-Aldrich, Sweden) were used as received.

Synthesis of 2-Allyloxymethyl-2-ethyl-trimethylene Carbonate (AOMEC)

The monomer was synthesized via ring-closing depolymerization according to a previously reported protocol,[36,49] along with a consecutive distillation step with the addition of acetic anhydride (0.1 eq. to AOMEC) and triethylamine (0.1 eq. to AOMEC) to ensure that the residual hydroxyl groups were capped.

Polymerization of AOMEC

General Preparation and Polymerization Setup

All reaction vessels were equipped with a magnetic stirrer and were dried in an oven at 150 °C for 48 h, followed by further drying with a heating gun and three consecutive vacuum/N2(g) cycles. All reactants were weighed under an inert gas atmosphere (N2) in a glovebox (Mbraun MB 150-GI). All reactions were stirred at a constant temperature that was maintained (±2 °C) using an IKAMAG RCT basic safety thermostat. At different time intervals, samples were taken and analyzed by 1H NMR spectroscopy and size exclusion chromatography (SEC). Samples (0.1 mL, 0.6 mmol) were withdrawn from the reaction vessel using disposable syringes followed by termination with a solution of acetic acid (0.05 mL, 0.88 mmol) in dichloromethane (DCM, 1 mL).

Bulk Polymerization of AOMEC

AOMEC (1 g, 5 mmol) was weighed inside a glovebox into a dry 25 mL two-necked round-bottom flask, followed by the addition of the catalyst DBU (0.038 mL, 0.25 mmol) and polymerization at the selected temperatures.

Solution Polymerization of AOMEC

First, a stock solution containing 0.25 mmol DBU per ml of the selected solvent (toluene or acetonitrile) was prepared. Then, AOMEC (1 g, 5 mmol) was weighed inside a glovebox into a dry 25 mL two-necked round-bottom flask. To the reaction mixture, 1 mL of the stock solution and a prespecified amount of solvent were added. For more details of the polymerization setup, see general preparations and polymerization setup and Supporting Information Tables S1, S2 and S3. Additionally, for information regarding the instruments see Supporting Information.

Results and Discussion

The ability of a monomer to polymerize is highly dependent under which conditions the synthesis is performed. Parameters such as temperature, concentration, and perhaps even surrounding solvent dictate the final outcome of the polymerization and shift the equilibrium concentration between monomer and polymer.[50,51] Understanding how the equilibrium is influenced by external factors is immensely important during polymer synthesis and monomer synthesis but also for the final application of the envisioned polymeric material. The relationship between the enthalpy and entropy change, Δp and Δp, in the system, the monomer concentration, and temperature is summarized by the Daiton–Ivin equation, eq (T = Tc), which states the ceiling temperature (Tc) of polymerization decreases with decreased initial monomer concentration.[52] However, the influence of the solvent polarity on the enthalpy and entropy changes in the system with dilution, which may play a crucial role for the equilibrium state of the polymer, is omitted.

Influence of the Initial Monomer Concentration on Its Equilibrium Concentration

In the first paper on anionic ring-opening polymerization of cyclic carbonates with a lithium alcoholate as the initiator, it was observed that in the thermodynamic regime of the polymerization, a ring–chain equilibrium is established, which is dependent on the monomer concentration, temperature, and nature of the solvent.[42] However, no quantitative, systematic investigation of the phenomenon was performed. Recent reports on the polymerization of substituted trimethylene carbonates in dichloromethane (DCM) catalyzed by organocatalysts, that is, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), resulted in the respective polymer under mild condition in the absence of side reactions; low to no degree of decarboxylation was observed.[7] Although, likewise to the first paper a clear ring–chain equilibrium is established. Specifically, polymerization of TMC catalyzed by DBU has been reported to reach 99% monomer conversion at equilibrium after 40 h in 2 M DCM at ambient temperature.[53] However, when the TMC monomer was substituted with a triethylene glycol chain at the 2-position and the polymerization was performed under more dilute conditions (0.8 M DCM) with the same DBU catalyst, the polymerization reached an equilibrium point of 70% conversion after 8 h at ambient temperature.[54]

Inspired by these results, the first experiments on ring-opening polymerization of AOMEC were performed in dichloromethane (DCM) with different initial monomer concentrations using hexamethylenediol as initiator and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalyst for 72 h (to reach equilibrium) at 30 °C (Scheme ).

Scheme 1

Polymerization of AOMEC ([M]) with Hexanediol ([I]) as the Initiator Catalyzed with DBU ([C]), [M]/[I]/[C] = [100]:[1]:[5] at Different Concentrations Ranging from 0.125 to 4 M in DCM

As observed from Figure , with increasing initial monomer concentration ([AOMEC]0), the monomer concentration at equilibrium ([AOMEC]eq) decreases: [AOMEC]0 = 0.5 M ⇒ [AOMEC]eq = 66%, [AOMEC]0 = 1 M ⇒ [AOMEC]eq = 38%, [AOMEC]0 = 2 M ⇒ [AOMEC]eq = 10%, that is, the final concentration of the repeating units in the polymer increases. Hence, the carbonate monomer AOMEC, exhibits a ceiling temperature of Tc = 30 °C in DCM at an initial monomer concentration of 0.125 M (Supporting Information Table S1 and Figure ).

Figure 1

Equilibrium monomer conversion as a function of the initial monomer concentration for the polymerization of AOMEC ([M]) with hexanediol ([I]) as initiator and DBU as catalyst ([C]). [M]/[I]/[C] = [100]:[1]:[5] in DCM at 30 °C and ambient pressure.

The initial monomer concentration plays a crucial role in the concentration of AOMEC at equilibrium. However, the contribution of the enthalpy (Δp) and entropy (Δp) of polymerization in conjunction with the reaction environment remains unknown, that is, when the system is diluted, is this purely an entropic effect or is the ring strain influenced by other factors, such as the polarity of the medium.

Polymerization Thermodynamics in Different Environments

The influence of the reaction medium on the thermodynamics of the polymerization was determined by analyzing the influence of the initial monomer concentration, temperature and solvents with different polarity on the change in the monomer–polymer equilibrium and consequently on the enthalpy (Δp) and entropy (Δp) of polymerization and the ceiling temperature (Tc). The requirements for the choice of solvents are (i) a reasonably high boiling point combined with a reasonably high vapor pressure to ensure easy removal of the solvent; (ii) a solvent that is unable to initiate polymerization or deactivate the catalyst, meaning that it is inert under the polymerization conditions; and (iii) solvents with very different polarities. The two selected solvents that fulfill these criteria are toluene (PhMe with a boiling point of 111 °C and a relative polarity to water of 0.10 and acetonitrile (MeCN) with a boiling point of 82 °C and a relative polarity to water of 0.46.

The polymerization of AOMEC was performed with 5 mol % DBU (relative to the initial monomer concentration) as catalyst at different temperatures accompanied by the evaluation of the kinetic behavior to ensure that equilibrium conversion was reached (Scheme and Supporting Information Tables S2–S4 and Figures S1–S16).

Scheme 2

Polymerization of AOMEC in PhMe and in MeCN with 5 mol % DBU (Relative to the Initial Monomer Concentration) as Catalyst Using Different Initial Monomer Concentrations and Polymerization Temperatures: Evaluation of the Equilibrium Conversion

The thermodynamic equilibrium polymerization of bulk AOMEC ([AOMEC]0 = 6 M) revealed similar thermodynamic parameters as found by Endo et al.[55] for 2,2-disubstituted trimethylene carbonates. According to Dainton’s eq , the values obtained for [AOMEC]0 = 6 M are ΔH = −3.6 kJ mol–1 and ΔS = −6.9 mol–1 K–1 J (with a corresponding ceiling temperature of Tc = 247 °C (Figure ). Compared to the more thoroughly studied lactones, the ceiling temperature of this monomer is between that of ε-caprolactone and δ-valerolactone.[48] This result likely originates from the higher ring strain in a six-membered cyclic carbonate monomer than in a lactone with the same ring size.[56,57]

Figure 2

Thermodynamic equilibrium polymerization of bulk AOMEC (6 M) with hexanediol as initiator and DBU as catalyst at different temperatures: R ln(m/m0) = ΔHp/T – ΔSp. The intercept is used to calculate the ΔSp, and the slope of the line gives ΔHp. The ratio ΔHp/ΔSp= Tc.

The bulk thermodynamic equilibrium polymerization occurs at an initial monomer concentration of 6 M, Figure and Table ). Furthermore, AOMEC was polymerized in toluene at 2, 1, 0.5, and 0.25 M at 30, 60, 75, and 90 °C for each concentration and in acetonitrile at 2, 1, 0.5, and 0.25 M at 30, 45, , and 75 °C for each concentration. Tables S2–S4 present the experimental details for these experiments, and Supporting Information Figures S1–S16 display the results.

Table 1

Thermodynamic Data for the Polymerization of AOMEC in PhMe and MeCN at Different Concentrations

entry	solvent	C [mol L-1]	temp [°C]	ΔHp [kJ mol-1]	ΔSp [J mol-1 K-1]	Tc [°C]
1	bulk	6 M	30 → 200	–3.6 ± 0.006	–6.9 ± 0.28	247
2	toluene	2 M	30 → 90	–11.1 ± 0.026	–21.9 ± 1.5	234
3	toluene	1 M	30 → 90	–22.0 ± 0.048	–46.6 ± 2.6	199
4	toluene	0.5 M	30 → 90	–38.8 ± 0.072	–89.2 ± 4.0	162
5	toluene	0.25 M	30 → 90	–43.2 ± 0.156	–105.2 ± 6.8	137
6	acetonitrile	2 M	30 → 75	–7.6 ± 0.072	–18.3 ± 1.0	142
7	acetonitrile	1 M	30 → 75	–10.1 ± 0.025	–26.8 ± 1.4	104
8	acetonitrile	0.5 M	30 → 75	–12.7 ± 0.048	–35.8 ± 2.3	82
9	acetonitrile	0.25 M	0 → 60	–13.7 ± 0.06	–39.1 ± 3.5	77

All the results are summarized in Table . The polymerization behavior differs substantially based on the solvent. With a 2 M solution of AOMEC in PhMe, a ceiling temperature of Tc = 234 °C is obtained, while in MeCN at the same concentration, Tc = 142 °C (Table ). Specifically, in a 2 M solution of toluene at 30 °C, the concentration of AOMEC at equilibrium ([AOMEC]eq) is 100–97.6% = 2.4%, whereas in acetonitrile under the same conditions [AOMEC]eq = 100–73.0% = 27% (Figures S1 and S9).

The ring strain (the enthalpy of polymerization Δp may serve as a measure of the ring strain) and the contribution of the polymerization entropy ΔSp are strongly affected by solvents of different polarity (Figure ): Δp in PhMe (1 M) = −22.0 kJ mol–1, whereas Δp in MeCN (1 M) = −10.1 kJ mol–1. A more than a 2-fold increase in ring strain is observed in the nonpolar solvent toluene. The ring strain increases with dilution toward a maximum value in both cases; a larger increase in Δp is observed when using PhMe as the solvent (Figure a). The dielectric constants for a number of cyclic carbonates have been determined,[58] and within this selection the chemical structure that most closely resembles AOMEC is propylene carbonate, which has a dielectric constant of 64 at 25 °C (for comparison, the dielectric constant of MeCN is 37.5 and that of PhMe is 2.4). All cyclic carbonates have high dielectric constants due to their highly polar carbonate group. It is our hypothesis that solvents with different dielectric constants (different polarity) induce changes in the conformation of the cyclic carbonate monomer, forcing the monomer into a more or less favored conformation, which in turn determines the ring strain and Δp. A larger deviation in polarity between the monomer and solvent, that is, a larger difference in the dielectric constants, leads to increased ring strain in the monomer and consequently to a higher absolute value for Δp and a lower concentration of AOMEC at equilibrium. The increase in the Δp with dilution is an inescapable consequence of the change in system thermodynamics; however, the magnitude is related to the nature of the solvent. At 0.25 M in PhMe, Δp = −105.2 J mol–1 K–1, and in MeCN, Δp = −39.1 J mol–1 K–1,which is more than 2.5 times higher for the less polar solvent toluene (Table and Figure b). Because of the more favorable interactions between the solvent and monomer, a more ordered solvent network is produced. For MeCN, which has a more similar dielectric constant to cyclic carbonates, a more ordered solvent network is formed, which prevents a stronger decrease in entropy with dilution, and this is in contrast to PhMe, where the entropic increase is more pronounced (Figure b). This is however a hypothetical explanation, although the scientific literature regarding protein–ligand binding[59] and halogen bonding in different solvents supports this explanation.[60]

Figure 3

Thermodynamic equilibrium polymerization of AOMEC in PhMe and MeCN as a function of different initial monomer concentrations. (a) and (b) show the change in ΔH and ΔS, respectively, (c) and (d) display ΔH/ΔS = Tc as a function of different degrees of solvation (different monomer concentrations): (c) in PhMe and (d) in MeCN. (For numerical values, see Tables S2–S4).

The solvent polarity and monomer concentration that influence both Δp and Δp, which respected magnitudes in conjunction to temperature, determine the conditions at which polymerization occurs (Δp = Δp – TΔp < 0) and at which depolymerization is favored (ΔGp = Δp – TΔp > 0). We determined the polymerizability of AOMEC at different temperatures and different concentrations in PhMe and MeCN, which revealed substantial differences between the two solvents (Figure c,d). Tc was equivalent for the two solvents at very different degrees of solvation, Tc (PhMe, 0.25 M) = 137 °C and Tc (MeCN, 2 M) = 142 °C (Table , Figure c,d). The Tc of polymerization is indicative of the temperature at which Δp = 0, that is, monomer conversion occurs only up to this temperature, and if a polymer is subjected to temperatures above these conditions, depolymerization occurs.

The on/off between the monomeric and polymeric state, hence the Tc regime, was recently explored for the homopolymerization of γ-butyrolactone, a monomer that was previously considered “not polymerizable”. Polymerization was achieved at −40 °C; increasing the temperature to well above Tc enabled depolymerization and recycling of the monomer.[61,62] The Tc for the polymerization of a specific monomer can be adjusted by copolymerization; to increase the ceiling temperature of a specific monomer, a comonomer with a higher Tc can be added.[63−66] Adjustment of the monomer–polymer equilibrium by changing the concentration was also achieved for a highly substituted ε-caprolactone monomer.[67]

Polymerization with in Situ Change in the Reaction Conditions

To prove the ability of the AOMEC–poly(AOMEC) system to respond to changes in the environment, we performed sequential changes to the reaction conditions: (i) AOMEC was first reacted in bulk ([AOMEC]0 = 6 M) at 30 °C. Under these conditions, polymerization occurred because Tc > T0. (ii) After reaching equilibrium conversion of AOMEC, poly(AOMEC) was dissolved in MeCN to produce 0.5 M concentration of repeating units (AOMECru), and the temperature was increased to 82 °C. Under these conditions (Tc < T0) ring-closing depolymerization occurred. The time conversion plots and the change in the number-average molecular weight versus time for the polymerization of AOMEC and the depolymerization of poly(AOMEC) are shown in Figure ; the corresponding numeric values are shown in Tables S5 and S6. Specifically, 30% conversion of AOMEC to poly(AOMEC) was achieved after 20 min, whereas the corresponding value of the depolymerization of 30% of poly(AOMEC) to AOMEC was achieved after 70 min, as shown in Tables S5 and S6 and Figure .

Figure 4

Bulk polymerization of AOMEC ([AOMEC]0 = 6 M catalyzed with 10 mol % DBU): conversion of AOMEC versus time and Mn(SEC) versus time (green area). Depolymerization of poly(AOMEC) ([AOMEC]ru= 0.5 M with 10 mol % DBU) in MeCN at T = 82 °C: conversion of AOMECru versus time and Mn(SEC) versus time (red area).

The ability of AOMEC to be in the polymeric state or the monomeric state is dependent on the surrounding environment (Figures and 5). The initial polymerization conditions of the [AOMEC]0 = 6 M and 30 °C system favors polymerization (Tc > T0), and the conversion and molecular weight follow the conventional trend. After 2 h, when the system environment is changed to one that disfavors polymerization (solution of MeCN, [AOMEC]0 = 0.5 M and 82 °C, Tc < T0) direct ring-closing depolymerizaton is observed (the concentration of AOMECru decreases, where subscript ru indicates the repeating units).

Figure 5

Bulk polymerization of AOMEC ([M]0 = 6 M catalyzed with 10 mol % DBU): Mn(SEC) and Mw/Mn versus conversion of AOMEC (green area). Depolymerization of poly(AOMEC) ([AOMECru]0= 0.5 M with 10 mol % DBU) in MeCN at T = 82 °C: Mn(SEC) and Mw/Mn versus conversion of AOMECru (red area).

The depolymerization follows an analogous trend to the polymerization (Figures and 5), indicating that depolymerization occurred via ring-closing depolymerization from the chain end. This is further supported by the similar values of the molecular weight at the same conversion of AOMECru (where subscript ru indicates the repeating units) and AOMEC. However, in terms of kinetic behavior, the polymerization and depolymerization behave very differently, as shown in Figures and 6. The polymerization follows the classical pseudo first order kinetics, kpapp = 3.3 × 10–4 s–1, whereas the depolymerization follows pseudo zero order kinetics, kdpapp = 0.64 × 10–4 s–1 (Figure ). Therefore, the depolymerization of poly(AOMEC) is initially independent of the concentration. However, as the system approaches equilibrium, that is, Δp = 0, polymerization–depolymerization equilibrium is established, and a change in the kinetics is observed (Figure ). This observation cannot be generalized to ring-closing depolymerization but reflects the features of this particular system. The selected reaction parameters for depolymerization, MeCN 0.5 M and 82 °C, are at the borderline at which ΔG = 0 for the system. If the selected reaction parameters shift to more strongly favor depolymerization (Tc ≪ T0, hence Δp ≪ 0), it is believed that the entire depolymerization process would follow pseudo zero order kinetics.

Figure 6

Bulk polymerization of AOMEC ([M]0 = 6 M catalyzed with 10 mol % DBU): first order, ln([M]0–[M]e)/([M]-[M]e)) = kpappt, where the slope of the line gives kpapp (green area). Depolymerization of PAOMEC ([AOMEC]ru) = 0.5 M with 10 mol % DBU in MeCN at T = 82 °C: zero order, [AOMEC]ru = kdpappt + [AOMEC], where the slope of the line gives kdpapp (red area).

Conclusions

The surrounding conditions of cyclic carbonates and the corresponding polymers dictate their monomer–polymer equilibrium. In DCM at 30 °C and ambient pressure with an initial monomer concentration of [AOMEC]0 = 0.125 M, no monomer conversion is observed, Δp = 0, and the system is at its ceiling temperature, whereas at 4 M monomer, 95% equilibrium conversion is observed. The monomer–polymer equilibrium is highly dependent on the nature of the reaction conditions and the polarity of the surrounding medium. For AOMEC at [AOMEC]0 = 1 M and 75 °C in PhMe, the equilibrium monomer concentration is (100–86.6) 13.4%; in MeCN and at the same conditions, the equilibrium monomer concentration is (100–23.1) 76.9%. Tailoring the thermodynamic equilibrium via the reaction conditions enables selective transition between the monomeric and polymeric states. Exemplified by changing the reaction from conditions that favor polymerization (bulk polymerization at 30 °C) to conditions that disfavor polymerization ([AOMEC]0 = 0.5 M in MeCN and T = 82 °C) reformation of the cyclic monomer AOMEC is observed within 10 h.

These results indicate that the polymeric state should not be viewed only from the vantage point of its respective synthesis but rather as a continuing equilibrium between monomer and polymer. This underlines the importance of considering how the polymer will behave during its application. Specifically, if we consider in vivo applications where the polymer is subjected to a highly polar reaction conditions at low concentrations actively brings to question the most preferable state of the polymer under these reaction conditions.