Overcoming the low reactivity of biobased, secondary diols in polyester synthesis

Shifting away from fossil- to biobased feedstocks is an important step towards a more sustainable materials sector. Isosorbide is a rigid, glucose-derived secondary diol, which has been shown to impart favourable material properties, but its low reactivity has hampered its use in polyester synthesis. Here we report a simple, yet innovative, synthesis strategy to overcome the inherently low reactivity of secondary diols in polyester synthesis. It enables the synthesis of fully biobased polyesters from secondary diols, such as poly(isosorbide succinate), with very high molecular weights (Mn up to 42.8 kg/mol). The addition of an aryl alcohol to diol and diacid monomers was found to lead to the in-situ formation of reactive aryl esters during esterification, which facilitated chain growth during polycondensation to obtain high molecular weight polyesters. This synthesis method is broadly applicable for aliphatic polyesters based on isosorbide and isomannide and could be an important step towards the more general commercial adaption of fully biobased, rigid polyesters.


Evolution of reactants during esterification between isosorbide, succinic acid and p-cresol
Supplementary Figure 1. 1 H NMR spectra recorded during esterification between isosorbide, succinic acid and pcresol at t = 0 to t = 5 h. Highlighted are the peaks between 2.8 and 3.0 ppm, which indicate the presence of succinic anhydride, bis(p-cresyl)succinate and mono-p-cresyl succinate esters.

Relevant end groups after 5 h esterification between isosorbide, succinic acid and p-cresol
Supplementary Figure 2. 1 H NMR spectrum of the reaction mixture after 5 h esterification. The relevant peaks for the calculation of the alcohol to cresyl succinate end group ratio are highlighted. Isosorbide assignments were done on the basis of integral ratios, considering the lower reactivity of endo-OH. For a complete assignment of the isosorbide related end group signals, see Noordover et al. 1

Calculation of alcohol to ester end group ratio after esterification
To estimate the conversion of esterification reactions, the alcohol to ester end group ratios in reactions with isosorbide were calculated as follows.
For an assignment of the relevant end groups, see Supplementary Figure 2. Successful polyester syntheses typically yielded an alcohol to ester end group ratio after esterification between 0.85 and 0.95. The p-cresyl succinate peak at 7.22 ppm was used for calculations of the end group ratios despite a slight overestimation due to the presence of bis(p-cresyl) succinate (see Supplementary Figure 1). This is due to the easy identification of the p-cresyl esters around 7.2 ppm, independent of the diacid moiety used (see Supplementary Similarly, calculations were carried out for isomannide-based polyesters. The equivalence of isomannide's two endo-OH groups resulted in only one signal for the respective monoesters.
For an overview of alcohol to ester end group ratios after esterification for all synthesized polyesters, see Supplementary Table 2.

Calculation of unreacted isosorbide
The amount of total unreacted isosorbide was calculated as follows: Poly(isosorbide-1,4-cyclohexanedicarboxylate) has been characterized by 2D NMR spectroscopy by Yoon et al. 3 . 2D NMR spectra recorded in DCM-d2 were comparable to those reported by Yoon et al. except for the absence of signals corresponding to 1,4-sorbitan (not detected in our samples).

Collection of volatiles during esterification of poly(isosorbide succinate) in 2 L autoclave
Supplementary Figure 52. Mass of volatiles collected during esterification between isosorbide, succinic acid and p-cresol in the 2 L autoclave. Volatiles were collected at the indicated times from both the short path receiving flask and the long path receiving flask (see Supplementary Figure 51). Typically a biphasic fraction was collected, consisting of H2O and p-cresol. At later stages of esterification, only p-cresol was collected. The reaction temperature was increased from 220 °C to 240 °C after 1 h. The plateau value for m(H2O) corresponds to 87.7% of theoretically expected H2O.

Ring-opening hydration of isosorbide to 1,4-sorbitan
The ring-opening hydration of isosorbide to 1,4-sorbitan was only observed in experiments conducted in the 2 L autoclave. It can be explained by a lower surface of evaporation, which can cause an increased reflux of water during esterification. Indeed it was found that 1,4sorbitan only forms during esterification. It might be avoided with a different reactor design, for example with a heated lid that decreases water reflux.  3 . The other signals expected to appear can not be identified due to their small signal intensity, which is likely due to their secondary alcohol functionality. This, combined with the low overall amount of 1,4-sorbitan in the polymer chain (1.5 mol%), leads to low overall concentrations of protons of each species (connected or unconnected) in the polymer chain. The assigned peak 1 has a higher reactivity as a primary alcohol, which likely facilitates its complete esterification and thus enables quantification.

Calculation of mol% (respective total isosorbide units) of 1,4-sorbitan during esterification in 2 L autoclave
Due to the partial ring-opening hydration of isosorbide during esterification, small amounts of 1,4-sorbitan are formed. The 1 H chemical shift of 1,4-sorbitan in the oligomer chains is at 4.22 ppm, which overlaps with peaks of endo-OH monoesters of isosorbide and unreacted isosorbide (see Supplementary Figure 2 for assignments of relevant end groups after esterification).

Synthesis conditions for isosorbide-and isomannide-based polyesters
Supplementary Esterification was typically conducted until a steady state equilibrium was reached, which indicated full conversion of carboxylic acid end groups. Unless noted otherwise, reactions were conducted with a 1:1:1.5 molar ratio of diol:diacid:p-cresol, on a 180 mmol (of the respective diol) scale and a BuSnOOH catalyst. a Respective diacid. b Refers to the reaction time at full vacuum (<1 mbar). c Reaction was conducted with 60 mmol of diol. d A 2 mol% (respective isosorbide) excess of adipic acid was added due to decomposition of adipic acid to cyclopentanone during esterification. e Amount of trans 1,4-cyclohexanedicarboxylic acid in product: 58.2%. Before reaction: 22.8%. Isomerization during reaction was also described by Yoon et al. 3 . f Lower thermal stability of diglycolic acid required lower initial esterification T and lower polycondensation T. g A 0.7 mol% (respective isosorbide) excess of thiodiglycolic acid was used due to decomposition of the monomer. The final product was brittle. h Ti(OBu)4 was used as a catalyst, BuSnOOH did not catalyse the polycondensation reaction sufficiently. The catalyst was added in two portions (before and after esterification) to compensate for the hydrolytic sensitivity of Ti-alkoxides. i A 0.7 mol% (respective isomannide) excess of adipic acid was added due to decomposition of adipic acid to cyclopentanone during esterification. j Amount of trans 1,4-cyclohexanedicarboxylic acid in product: 60.6%. Before reaction: 22.8%.

Alcohol to ester end group ratios and mol% of unreacted diol monomers of all polyester compositions after esterification
Supplementary