Biobased step-growth polymers:chemistry, functionality and applicability (2008)

Inspired by the opportunity to obtain materials with interesting new properties and further stimulated by the increasing oil prices and the augmenting environmental concerns, renewed interest in biobased polymers has recently arisen. Extensive efforts are being invested in extracting useful starting materials from renewable resources and to use these molecules to synthesize novel polymers. The aim of this study was to investigate the potential of several biobased monomers as starting compounds to synthesize polycondensate resins suitable for coating or toner applications. An in-depth study of the chemistry, functionality and the structure-property relations of such polymers was performed. Step-growth polymers with specific characteristics with respect to molecular weight (distribution), end-group structure and thermal properties were targeted. This part of the project included a detailed study of the suitable reaction conditions to polymerize the biobased starting materials, which often have limited reactivity and thermal stability. Polyesters were prepared by reacting the 1,4:3,6-dianhydrohexitols (DAHs, i.e. isosorbide, isoidide and isomannide) with dicarboxylic acids such as succinic acid. The bicyclic structures of the DAHs introduce sufficient chain rigidity and, thus, already for the relatively low molar masses required for coating resins sufficiently high glass transition temperatures (Tg) were obtained. Series of linear and branched terpolyesters were synthesized, of which the average number of reactive end-groups per polymer chain could be adjusted by varying the amount of polyols present in the reaction mixture. It was shown that the exo-oriented hydroxy-groups present in isoidide and isosorbide are more reactive in melt polycondensation reactions, using non-activated dicarboxylic acids, than their endo-oriented counterparts present in both isomannide and isosorbide. In addition, we found that the anhydro ether rings of isomannide are susceptible to ring-opening at elevated temperatures, in contrast to the ether rings of isosorbide and isoidide, which appear to be stable under these conditions. When using isoidide or isomannide to synthesize polyesters, semi-crystalline polymers are obtained, while polymerization of isosorbide with dicarboxylic acids yields amorphous materials. To obtain carboxylic acid-functional polyesters, linear hydroxy-functional polyesters were reacted with citric acid in the melt. Model reactions were carried out to investigate the chemistry of this modification reaction and the resulting end-group structures. Interestingly, citric acid is transformed into a more reactive anhydride species close to its melting temperature of 153 ºC. Therefore, the modification of hydroxy-functional polymers with the thermally labile citric acid can be performed at relatively low temperatures. In addition, the modified products can be cured with conventional cross-linkers (vide infra) at moderate temperatures, which is probably partly due to anhydride formation at the polyester chains ends, accelerating the curing reaction. Aliphatic, biobased polycarbonates were prepared by polymerization of the DAHs, in combination with other diols and/or polyols, using several types of carbonyl sources such as triphosgene, diphenyl carbonate and bis(ethyl/phenyl carbonate) species derived from the DAHs. It proved to be difficult to control the end-group structures of the polycarbonates when using the highly reactive phosgene derivatives, whereas the interchange reactions of the biobased diols with diphenyl carbonate required high reaction temperatures to achieve sufficient conversion. Thermal degradation occurred through an unzipping mechanism and decarboxylation. To prevent these detrimental side reactions, the hydroxy-groups of the DAHs were first converted to carbonate linkages using chloroformates, followed by melt interchange reactions of the resulting molecules with primary diols and/or polyols. These polymerizations do not require too high reaction temperatures, thereby limiting degradation and resulting in the desired hydroxy-functional copolycarbonates with satisfactory Tgs and molecular weights. Another route to functionalized, aliphatic polycarbonates was investigated, involving alcoholysis of high molecular weight poly(cyclohexene carbonate) by polyol species such as trimethylolpropane and 1,3,5-cyclohexanetriol. The obtained polycarbonates have significantly enhanced functionalities as well as reduced molecular weights and Tgs, all suitable for coating applications. The various hydroxy-functional polymers were mixed with free or e-caprolactam-blocked polyisocyanate curing agents and applied as coatings by either solution casting or powder coating. Polymers with carboxylic acid end-groups were cured using epoxy-compounds or ß- hydroxyalkylamides. The resulting polyester and poly(ester/carbonate urethane) coatings were tested for chemical, mechanical and UV stability. In addition, the rheological properties of these materials were investigated. Networks obtained by curing branched polymers perform better than those prepared from linear polymers, which is mainly due to the enhanced crosslink density of the former systems. Solvent and impact resistant coatings were prepared from linear and branched, biobased polycondensates. In addition to the conventional curing agents used in this study, several novel, biobased, e-caprolactam-blocked diisocyanates proved efficient in cross-linking branched polyesters and polycarbonates, leading to fully biobased, chemically and mechanically stable, glossy coatings with very promising properties.