Enzymatic synthesis of novel aromatic‐aliphatic polyesters with increased hydroxyl group density

Abstract

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.

Impact of enzymatically synthesized aliphatic–aromatic polyesters with increased hydroxyl group content on coating properties

“…The molar ratio of 1,6-hexanediol and 2,6-bis (hydroxymethyl)-p-cresol in the chain is 0.38 to 0.62. 18 sidering acid numbers decreasing from a theoretical value of 375 mg KOH/g to a value of 73 mg KOH/g during the reaction, around 20% of the polymer chains are acid functional before linking with glycerol. Because the molar composition and the initial weight are known, the mean functionality without additional aromatic OH-groups of the polyester can be calculated and has a value of F OH-theo.…”

“…After post-polymerization with an excess of glycerol (10:1), 18 the acid number decreased to a value of 33 mg KOH/g. Since a strong glycerol excess has been used, it can be assumed that mainly a simple addition of glycerol takes place instead of a chain extension and thus, the average OH functionality of the enzymatically synthesized polymer is increased by about F OH = 1.1.…”

“…= 2.00-0.20). 10 By using acid number and 1 H-NMR data of the oligoester of Seithu ¨mmer et al, 18 the molar composition of the enzymatic polyester before chain lengthening can be determined (Table 4).…”

“…Since, according to Table 3, only 31% of the polyester consists of the cresol component, the aromatic OH equivalent weight of the polymer is 168.19 g/mol: 0.31 mol% = 542 g/mol aromatic hydroxyl groups. Using number average molecular weight M n = 2400 g/mol from 1 H-NMR end group analysis of oligoester, 18 F OH = 4.43 (M n /OH equivalent weight of the polymer) aromatic OH-functionalities are added to the chain enzymatically without further reaction. When the terminal F OH-theo.terminal = 1.80 OH groups are added, the calculated hydroxyl functionality of the oligoester before chain extension is F OH = 6.23.…”

“…In particular, the combination of aliphatic and aromatic monomers in one polyester leads to excellent physical and mechanical properties and a favorable cost structure. 16,17 In the present work, the polyester based on adipic acid, 1,6-hexanediol and 2,6-bis (hydroxymethyl) -pcresol, first introduced in the publication by Seithu ¨mmer et al, 18 has been applied in developed coating formulations with different hardener components, such as polyisocyanates, epoxy and melamine building blocks. Therefore, the OH functionality and OH number were calculated using different, independent methods.…”