Background Polyesters with pendant hydroxyl groups are attractive materials which offer additional functionalization points in the polymer chain. In contrast to chemical polycondensation, lipase regioselectivity enables the synthesis of these materials as certain hydroxyl groups remain unaffected during the enzymatic process. Methods and Major Results In this study, a combination of synthesis development and reactor design was used for the enzymatic synthesis of an aliphatic‐aromatic polyester with two different classes of pendant hydroxyl groups. Using 2,6‐bishydroxy(methyl)‐p‐cresol as diol in lipase catalyzed polycondensation with adipic acid required the addition of hexane diol as third monomer for polycondensation to take place. Reaction conditions were explored in order to identify the preferred reaction conditions for the incorporation of the aromatic diol and the enhancement of the hydroxyl group density. Post‐polymerization with glycerol at low temperature integrated additional aliphatic hydroxyl groups, reduced the polydispersity and increased the end group functionality. Conclusion A new material with aromatic building blocks and boosted polymer chain reactivity was obtained, which is suggested to find application in various areas of material development from coatings to adhesives.

Nowadays, coatings need to fulfill a variety of requirements such as having excellent mechanical, chemical, and optical properties at low baking temperatures. On a large scale, polyisocyanates, amines or melamines are used as crosslinking agents in the coatings industry. In this work, a new self-crosslinking agent based on a hydroxy functional 6-membered carbonate with high ring tension and thus presumably lower baking temperature was synthesized and the behavior as self-crosslinking agent was compared to the crosslinking agent derived from the commercially available 5-membered glycerol carbonate. The hydroxy functional 6-membered carbonate monomer was synthesized enzymatically under mild reaction conditions from commercially available substances, linked to a hexamethylene diisocyanate trimer and self-polymerized afterward. NMR- and IR-spectroscopy and GC-MS analysis were found to be suitable techniques to characterize monomers and crosslinking agents. DSC measurements were performed to evaluate appropriate reaction parameters for the attachment reaction of the 6-membered cyclic carbonate to the polyisocyanate without ring opening. The progress of self-crosslinking has been followed by characteristic changes in IR spectra as well as time and temperature-dependent changes of storage and loss modulus while oscillating rheological crosslinking. Furthermore, glass transition temperatures of the resulting coating films are determined, and sol gel analysis was performed to estimate the degree of crosslinking. After application on steel, aluminum and glass plates application tests were performed. In addition to excellent mechanical and chemical properties, the coating film showed good adhesion to the surface and was colorless. Combining these properties with relatively low baking temperatures, 6-membered cyclic carbonate crosslinking agents could represent a new technology for the coatings industry.

Nowadays, coating systems have to fulfill a wide range of requirements. In addition to mechanical properties such as hardness and elasticity, resistance and weatherability, specifically corrosion or chemical resistance are also important. Increasing attention is also being paid to points such as the use of sustainable reactants or the energy optimization of synthesis processes.1 The use of enzymes in the synthetic processes offers two main advantages: firstly, reaction temperatures can be significantly reduced, for example in the production of polyesters, and as a result and a major advantage, certain functional groups can be selectively retained during the reaction.2,3 Thus, for example, aromatic hydroxyl groups can be obtained, while aliphatic groups are esterified.4,5 This allows the preparation of polyesters that do not only have terminal OH groups, but hydroxyl groups within the chain that can act as additional crosslinking points during network formation or as adhesion-promoting groups.6,7 In this work, the influence of such an aliphatic–aromatic polyester, produced enzymatically at low temperatures, on the coating properties is investigated when using different hardener components. Coating formulations were created, and the required OH functionality and the hydroxyl number of the enzymatic polyester have been calculated by using two different, independent methods. Besides the development of guide formulations, the unique mechanical properties of coatings based on the enzymatic polyester were studied. In addition to comparative analysis of network densities, the coatings were also investigated by IR spectroscopy in order to assess the network formation reaction spectroscopically. It can be shown that additional OH groups in the polyester chain increase the network density, but this is not at the expense of elasticity. Thus, enzymatically produced polyesters combine the advantages of low reaction temperatures during production with a unique property profile due to aliphatic and aromatic moieties as well as the partial preservation of OH groups within the chain.