Bismuth subsalicylate, a low-toxicity catalyst for the ring-opening polymerization (ROP) of l-lactide (l-LA) with aliphatic diol initiators: synthesis, characterization, and mechanism of initiation

Abstract

The ring-opening polymerization (ROP) of l-lactide (l-LA) was induced by the catalytic action of bismuth subsalicylate (BiSS) using linear aliphatic diols [HO(CH₂) _n OH, where n = 2, 3, 4, 5, 6, and 8] as initiators and chain transfer agents. The theoretical and experimental degree of polymerization (DP) in all samples of α,ω-hydroxy telechelic poly(l-lactide) (HOPLLAOH) had a good agreement in all samples, an effect attributed to the interaction of BiSS with HO(CH₂) _n OH inducing a transfer reaction. HOPLLAOH was synthesized and characterized by a range of analytical techniques, confirming the insertion of methylene groups from the initiator into the main chain of the polyester. The glass-transition temperature (T _g) of HOPLLAOH was found to be proportional to the number of methylene groups present in the diol. Various parameters regarding the ROP of l-LA were studied, such as temperature, time of reaction, amount of catalyst, and the nature of the diols. A kinetic study of the reaction allowed the determination of the rate constants (k) and activation energy (E _a). A mechanism of initiation is proposed based on a computational study using density functional theory (DFT), evidencing the role of the alkyl diol as an initiator, producing an alkoxide (Bi-OROH). This species then acts as a nucleophile, attacking the carbonyl group, inducing its insertion, and ultimately completing the ring-opening of l-LA.

Scheme 1. Structure of the bismuth subsalicylate (BiSS) in 2D (a) and 3D (b).

Scheme 2. Synthesis of α,ω-hydroxy telechelic poly(l-lactide) (HOPLLAOH) by ROP of l-LA catalyzed by BiSS in the presence of an alkyl diol initiator.

Fig. 1. ROP of l-LA catalyzed by BiSS showing (a) the effect of temperature (2 h, 1 mmol BiSS), (b) the effect of the amount of catalyst used (140 °C, 2 h), and (c) the effect of the reaction time (140 °C, 0.03 mmol).

Fig. 2. (a) Semilogarithmic plot of the ROP of l-LA catalyzed by BiSS concentration against the reaction time at different temperatures: k = 2.10 × 10⁻⁴ s⁻¹ at 100 °C; k = 6.52 × 10⁻⁴ s⁻¹ at 120 °C; k = 2.34 × 10⁻³ s⁻¹ at 140 °C. (b) Graph of ln k vs. 1/T for the ROP of l-LA.

Fig. 3. ¹H NMR spectrum at room temperature of HOPLLA₈OH with DP_NMR = 18.8 (500 MHz, CDCl₃); asterisks indicate the residual monomer (l-LA).

Fig. 4. FT-IR spectrum of HOPLLA₈OH.

Fig. 5. ¹³C NMR spectrum of methine (CH) carbon, tetrad analysis of tacticity of HOPLLA₅OH in CDCl₃, where i and s correspond to isotactic and syndiotactic, respectively.

Fig. 6. (a) DSC thermograms of the poly(l-lactide) macrodiols: HOPLLA₂OH, HOPLLA₅OH, HOPLLA₈OH, and (b) the effect of multiple methylene groups (CH₂)_n on the glass transition temperature (T_g).

Scheme 3. Transfer reaction between BiSS and ethylene glycol.

Fig. 7. Energy profile (gas phase, 140 °C) of the mechanism of initiation of l-lactide (l-LA) initiated with ethylene glycol [HO(CH₂)₂OH] using bismuth subsalicylate (BiSS) as a catalyst.