Lipase selectivity in functional polyester synthesis

Abstract: Enzyme selectivity means that the enzyme´s preferences towards competing substrates will be different. In this thesis, the enzyme selectivity has been studied for utilization in synthesis of functionalized macromonomers. The aim was to study how the inherent –or introduced – selectivity of lipases can be used to introduce thiol? or enefunctionalities into short polyesters. Thiol? and ene?functionalized renewable organic precursor molecules in combination with thiol?ene click chemistry opens up for a sustainable material production. Lipases do not normally affect ene?moieties and the preference towards thiols is low, enabling introduction of these functional groups for further modifications. In addition, lipases have been shown to be good catalysts in the formation of polyesters, both via ring?opening and polycondensation polymerization.In paper I Candida antarctica lipase B was used to end?functionalize poly(??caprolactone) with free thiols in a one?pot reaction. The advantage of using achemoselective lipase as catalyst was that no protection of the thiol was needed. The chemoselectivity displayed by Candida antarctica lipase B turned out to be 88 000 in favour of the alcohol (paper II). Rhizomucor miehei lipase showed less pronounced chemoselectivity. The largest contribution to the selectivities was derived from the more than two orders of magnitude higer KM towards the thiol compared to the alcohol. Thiols can be cross?linked with enes in radical reactions to form networks, enabling formation of materials.One promising renewable molecule containing an acrylate moiety is itaconic acid. In paper III the selectivity towards the two esters in dimethyl itaconate was investigated and the active site of Candida antarctica lipase B was redesigned to generate variants with increased and decreased selectivity. One variant showed 14?fold higher selectivity and could regioselectively add dimethyl itaconate onto a diol. This variant could be used in end?functionalizations of polymers, introducing acrylate?ester end?groups.The enzyme selectivity towards lactones and their corresponding polyesters is of importance when designing a ring?opening polymerization reaction. In paper IV Candida antarctica lipase B was found to prefer ??pentadecalactone and polyesters over ??caprolactone ten?fold, while Humicola insolens cutinase preferred ??caprolactone and its corresponding polyester four?fold over ??pentadecalactone and its polyester. From a selectivity point of view, Candida antarctica lipase B and Humicola insolens cutinase would be equally good in ring?opening polymerization of ??pentadecalactone, while in the case of ??caprolactone Humicola insolens cutinase would be the preferred choice.