Abstract
Sustainable and biobased surfactants are required for a wide range of everyday applications. Key drivers are cost, activity and efficiency of production. Polycondensation is an excellent route to build surfactant chains from bio-sourced monomers, but this typically requires high processing temperatures (≥200 °C) to remove the condensate and to lower viscosity of the polymer melt. In addition, high temperatures also increase the degree of branching and cause discolouration through the degradation of sensitive co-initiators and monomers. Here we report the synthesis of novel surface-active polymers from temperature sensitive renewable building blocks such as dicarboxylic acids, polyols (d-sorbitol) and fatty acids. We demonstrate that the products have the potential to be key components in renewable surfactant design, but only if the syntheses are optimised to ensure linear chains with hydrophilic character. The choice of catalyst is key to this control and we have assessed three different approaches. Additionally, we also demonstrate that use of supercritical carbon dioxide (scCO2) can dramatically improve conversion by reducing reaction viscosity, lowering reaction temperature, and driving condensate removal. We also evaluate the performance of the new biobased surfactants, focussing upon surface tension, and critical micelle concentration.
Highlights
Successful synthesis of linear polyol-polyesters can lead to development of new sustainable surfactants[1,2,3,4,5] and there is particular focus on the use of carbohydrates as a renewable resource for polymer design,[3,6,7] because they bring a rich variety of structures with abundant stereochemical diversity.[8]
Molecular weight determinations using acid value titrations (MAnV) indicate that the poly(sorbitol adipate) (PSA) synthesised using K2CO3 has a higher MAnV than the equivalent compound synthesised enzymatically; 1700 Da compared to 1050 Da respectively (Table 1, entries 2 and 3)
We have developed an efficient and clean synthesis of D-sorbitol and diacid based polyesters by combing the use of the inexpensive K2CO3 as selective catalyst in the green medium scCO2
Summary
Successful synthesis of linear polyol-polyesters can lead to development of new sustainable surfactants[1,2,3,4,5] and there is particular focus on the use of carbohydrates as a renewable resource for polymer design,[3,6,7] because they bring a rich variety of structures with abundant stereochemical diversity.[8]. The use of pTSA, Novozym 435 and K2CO3 for the synthesis of poly(sorbitol diacid) polyesters were compared in terms of the conversions reached, molecular weights achievable, OH : COOH ratios and the visual appearance of the polymers obtained.
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