Kinetic Studies on Guanidine-Superbase-Promoted Ring-Opening Polymerization of ε-Caprolactone

◊These authors contributes equally to this work.

Abstract

The kinetics of the ring-opening polymerization of ε-caprolactone with butane-1,4-diol as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalyst were examined. A highly efficient and controllable polymerization of ε-caprolactone occurred with an activation energy of 22.5 kJ·mol^–1, which is much lower than that observed with butan-1-ol as the initiator (39.5 kJ·mol^–1). An intramolecular hydrogen-bonding-assisted mechanism is proposed to explain this lowering of the activation energy.

The synthesis of polycaprolactone (PCL), a member of the aliphatic polyester family, has attracted considerable attention due to the biocompatibility of this polymer and its biodegradability in nature.[1] Recently, the use of organocatalysts for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) has opened a new avenue for the synthesis of metal-free PCL.[2] Among the many highly active organocatalysts, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (pK _a = 26 in acetonitrile),[3] is a weakly nucleophilic guanidine superbase that has been widely studied as a bifunctional organocatalyst.[4] In 1999, Corey and Grogan[5] reported the use of a chiral bicyclic guanidine base as a bifunctional organocatalyst for the enantioselective Strecker synthesis of chiral α-amino nitriles and α-amino acids. This reactivity inspired us to gain deeper insights into the catalytic behavior of TBD in the ring-opening polymerizations of cyclic esters.[4b] [6] [7]

A number of elegant results have been reported in which TBD was employed as an organocatalyst for the synthesis of polyesters.[8] Many researchers have also studied the effects of various reaction conditions (temperature, catalyst concentration, monomer-to-initiator ratio, etc.) on the polymerization rate and the molecular weight of the product.[9] However, the effects of structurally different initiators on the polymerization kinetics have rarely been studied.[10] Here, we report our recent kinetic investigations on the polymerization of ε-CL polymerization by using TBD as an organocatalyst and butane-1,4-diol as an initiator.

To investigate the kinetic effect of different initiators, we first used butane-1,4-diol as an initiator for the ROP of ε-CL (Scheme [1]). The effects of various reaction parameters, such as the concentration of the monomer and the initiator or the catalyst loading, were investigated in detail to deduce the rate equation and the activation energy. The results are presented in Table [1]. Changes in the monomer concentration were monitored by ¹H NMR spectroscopy at various time intervals [see, Supplementary Information (SI), Figure S1], and the reaction was quenched by the addition of benzoic acid. At a loading of 1% of TBD and a monomer-to-initiator ([M]/[I]) ratio of 50, a 60% conversion of ε-CL was achieved after five hours (Table [1], entry 1). On increasing the amount of TBD from 1% to 2% or to 5%, the conversion of monomer reached 95% over run times of 220 and 110 minutes, respectively (Table [1], entries 1–3). The molecular-weight distributions of the resulting polymer were found to be unimodal and narrow (SI; Figure S2). To examine the lifetimes of the active species, polymerization tests were performed at different time intervals for TBD/butane-1,4-diol/ε-CL ratios of 5:2:100 and 5:1:100 at 25 °C (SI; Tables S1 and S2).

As can be seen in Figure [1], the linear correlation between the monomer conversion and the number-average molecular weight (M_n ) of the resulting polymer was characteristic of living polymerization (SI; Table S1, entries 1–5). At the same time, a modest change in the molecular-weight distribution was also observed (PDI = 1.05–1.16). Meanwhile, polymerization investigations with various concentration of the initiator were carried out. High conversions of up to 95% and 84% were achieved at [M]₀/[I]₀ ratios of 50 and 1000, respectively (Table [1], entries 3 and 7). The resulting polymer showed narrow to broad molecular-weight distributions at different concentration of initiator (SI; Figure S3). When the [M]₀/[I]₀ ratio was higher than 200, the peak distribution in GPC was asymmetric, showing that transesterification occurred during the reaction.

To determine the rate constant (k), the ROP of ε-CL was performed at [CL]₀/[I]₀ ratios of 50 (Figure [2]) and 100 (Figure S4) for various concentrations of TBD (1, 2, or 5%) at 25 °C. The rate constant k was determined by plotting ln([CL] _t /[CL]₀) against the polymerization time, where [CL]₀ is the initial ε-CL monomer concentration and [CL] _t is the ε-CL monomer concentration at time t. Semilogarithmic plots of ln([CL]₀/[CL] _t ) versus the time of polymerization at [CL]₀/[I]₀ ratios of 50 and 100 are shown in Figure [2] and Figure S4 (SI), respectively.[11] In each case, the linear plots of ln([CL]₀/[CL] _t ) versus reaction time indicated that the reaction shows a first-order dependence on the monomer concentration.

In all the semilogarithmic plots the deduced value of R ² was equal to or greater than 0.98. The kinetics are consistent with the rate law shown in Equation 1.

The kinetic analysis at various catalyst loading revealed that the polymerization rate increased with increasing concentration of TBD, whereas the propagation rate constant k _p remained almost constant, showing that the rate constant of propagation is independent of the concentration of TBD (Table [2]).

In further investigations, ε-CL was polymerized at various reaction temperatures, and the effect of temperature on the kinetics was examined. The PDI of PCL increased from 1.20 to 1.54 on increasing the temperature from 25 to 100 °C (SI; Figure S5). This indicates the occurrence of a transesterification reaction during chain propagation.

As can be seen in Figure [3], ln k increases linearly with 1/T.

The relationship between the rate constant and the polymerization temperature was in good agreement with the Arrhenius equation (Eq. 2). In addition, the calculated activation energy (E _a) for ε-CL polymerization by TBD with butane-1,4-diol as initiator in toluene was determined to be 22.5 kJ·mol^–1.

Further experiments were performed to confirm the kinetic results. A detail polymerization study was performed at 45 °C (Figure [4]). The experimentally determined rate constant of 57.7 × 10^–3 min^–1 was similar to the value of 52.2 × 10^–3 min^–1 from the Arrhenius equation plot.

For the purpose of a comparative study, the kinetics of polymerization of ε-CL initiated by butan-1-ol were also investigated. The rate constant and the activation energy were determined by performing the polymerization tests at various reaction temperature (SI; Figure S6). According to the Arrhenius plot, the calculated activation energy was 39.5 kJ·mol^–1, which is significantly higher than the value of 22.5 kJ·mol^–1 observed for the butane-1,4-diol-promoted polymerization. On the basis of these results, we speculate that folding of butane-1,4-diol leads to the formation of intramolecular as well as intermolecular hydrogen bonds with TBD (Scheme [2]). This hydrogen bonding results in enhanced nucleophilicity of the initiator, increasing the rate of the polymerization and, therefore, lowering the activation energy for the butane-1,4-diol-promoted polymerization.

In summary, the kinetics were examined of the controllable ROP of ε-CL in the presence of butane-1,4-diol as initiator and TBD as the catalyst. The activation energy of the butane-1,4-diol-promoted polymerization was found to be 22.5 kJ·mol^–1, which is lower than that with butan-1-ol as initiator under otherwise identical conditions. This lower activation energy can be ascribed to folding of butane-1,4-diol leading to intramolecular as well as intermolecular hydrogen bonding with TBD, thereby enhancing the nucleophilicity of the initiator and decreasing the activation energy.

• References

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  Search in Google Scholar
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• Supplementary Material