Temperature triggered alternating copolymerization of epoxides and lactones via pre-sequenced spiroorthoester intermediates

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer. The reaction is catalyzed by the temperature triggered, bifunctional cationic indium complex (±)-[(NNiOtBu)In(CH2SiMe3)][B(C6F5)4] (1). 1 can catalyze the coupling of epoxide and lactone to form spiroorthoester at 60 °C and its double ring-opening polymerization at 110 °C to form poly(ether-alt-ester). The post-polymerization modification and degradation of the poly(ether-alt-ester) are further investigated.

H(NNiOtbu) 6 and In(CH2Si(CH3)3)3 7 were synthesized according to previously reported procedures. 1 and 2 were synthesized according to a previous report. 8 Synthesis of spiroorthoester (SOE1) with 1. In a glovebox, a 100 mL schlenk flask was charged with 7.5 mL and 22.5 mL solution of a 1,2-epoxy-7-octene and ε-caprolactone stock solution, respectively (1,2-epoxy-7-octene: 12.5 mmol; [1,2-epoxy-7-octene] = 1.67 M; ε-caprolactone: 12.5 mmol; [ε-caprolactone] = 0.556 M). 7.5 mL solution of a catalyst stock solution (1: 0.1875 mmol; [1] = 0.025 M) were added to the schlenk flask. 12.5 mL solution of toluene was used to make up to the total volume of 50 mL. The resulting mixture was placed in an oil bath preheated to 60 °C and stirred. A sample of the crude mixture (ca. 0.05 mL) was dissolved in C6D6 to be analyzed by 1 H NMR spectroscopy to determine conversion. The solvent was removed under vacuum after 24 h. The residue was dissolved in hexane and filtered through silica flash column to remove catalyst. The collected solution was dried over vacuum to obtain the desired spiro orthoester (SOE1) as colourless oil in 92% isolated yield. The product was dried under high vacuum for 4 h prior to analysis.
One-pot synthesis of poly(ether-alt-ester) with 1. In a glovebox, a schlenk flask was charged with 0.6 mL and 1.8 mL solution of a 1,2-epoxy-7-octene and ε-caprolactone stock solution, respectively (1,2-epoxy-7-octene: 1.0 mmol; [1,2-epoxy-7-octene] = 1.67 M; ε-caprolactone: 1.0 mmol; [ε-caprolactone] = 0.556 M). 0.6 mL solution of a catalyst stock solution (1: 0.015 mmol; [1] = 0.025 M) were added to the schlenk flask. 1.0 mL solution of toluene was used to make up to the total volume of 4 mL. The resulting mixture was placed in an oil bath preheated to 60 °C and stirred for 24 h. A sample of the crude mixture (ca. 0.05 mL) was dissolved in C6D6 to be analyzed by 1 H NMR spectroscopy to determine conversion of epoxide and e-caprolactone and confirm the formation of corresponding SOE. The solvent was removed under high vacuum. The residue was stirred in an oil bath preheated to 110 °C for 24 h. The resulting viscous liquid was cooled down to room temperature. Some of the crude mixture was dissolved in C6D6 to be analyzed by 13 C{ 1 H} NMR spectroscopy to determine conversion of monomer. Cold methanol (0 °C, 15 mL) was added to the remaining polymeric product. The methanol-insoluble product was separated and dried under high vacuum for 4 h prior to analysis.
Homopolymerization of spiroorthoester (SOE1) with 1. In a glovebox, a Schlenk flask was charged with a catalyst (0.00416 mmol) and monomer SOE1 (0.832 mmol) was added directly to the flask. The mixture was stirred in an oil bath preheated to corresponding temperature for corresponding time. The resulting viscous liquid was cooled down to room temperature. Some of the crude mixture was dissolved in C6D6 to be analyzed by 13 C{ 1 H} NMR spectroscopy to determine conversion of monomer. Cold methanol (0 °C, 15 mL) was added to the remaining polymeric product. The methanol-insoluble product was separated and dried under high vacuum for 4 h prior to analysis.

Synthesis of cross-linked poly(ether-alt-ester).
In a glovebox, a vial was charged with poly(ether-ester) (0.416 mmol), 2,2'-(ethylenedioxy)diethanethiol (0.208 mmol), and 2,2'azobis(2-methylpropionitrile) (0.0298 mmol) in minimal amount of toluene. The mixture was stirred in an oil bath preheated to 80 °C for 5 min. The resulting soft solid was cooled down to room temperature. The insoluble product was washed with cold methanol and dried under high vacuum for 4 h prior to analysis.

Degradation of poly(ether-alt-ester).
A vial was charged with poly(ether-ester) (0.416 mmol) and 10 mL of 0.1 M of tetrabutylammonium hydroxide in MeOH/DCM. The mixture was stirred at room temperature for 2 h. Subsequently, 1 M HCl in diethyl ether was added to neutralize the reaction. The resulting mixture was dried under high vacuum for 4 h prior to analysis.
Lewis acidity study. We used a modified Gutmann-Beckett method to determine relative acidity of catalysts for SOE polymerization. This method involves the addition of 0.8 eq triethylphosphine oxide (OPEt3) to form an adduct with the metal complexes. The free triethylphosphine oxide shift is determined by the addition of a sealed capillary tube containing a solution of triethylphosphine oxide in CDCl3 into the NMR tube. The downfield shift of the phosphorus peak of the adducts are used to determine the relative strength of the Lewis acidity of each species.