For an easier understanding of the research axes that have been investigated throughout this work, a simplified overview of the different materials designed and synthesized is presented in Figure 53.
Figure 53 Overview of the synthetic strategies and products developed in the course of this work.
(yellow = polymeric materials, green = monomeric materials)
Over the course of this work, several axes of research have been explored in order to gain a deeper understanding of UV-curing acrylated HBPs and various UV-curable monomers and polymers from itaconic acid for applications such as additive manufacturing, printing inks or coatings.
Firstly, an optimization of already existing HBPs from non-renewable acrylic acid has been investigated. The novel synthetic pathway allowed for an efficient synthesis of HBPs with a higher degree of substitution of the OH groups than similar materials previously reported, without the use of activated reagents. This method was used to obtain six acrylated HBPs from four different cores, with one or two generations of branching unit (bMPA). Propionic acid was used to achieve high conversion and low viscosity. These materials were found to exhibit excellent mechanical (tensile strength up to 3 times higher than standard materials) and thermal properties. In addition, they exhibited a very high reactivity, with a rate of polymerization more than twice as high as the highest RoP for similar materials (0.14 s-1 against 0.06 s-1). An
122 extensive characterization was also conducted with an in-depth study of possible side reactions and their likeliness. This optimization of HBP structures using acrylic acid allowed a better understanding of these macromolecular architectures, to be used for the synthesis of itaconic acid based HBPs.
In a second step, the limits of the classical esterification/condensation of itaconic acid were explored. It was proven that an uncontrolled polyesterification led to highly crosslinked materials leading to a gelation of the product. A more controlled method was developed using a secondary alcohol instead of a diol, which lead to a viscous but not crosslinked material, TMP_bMPA_It_Cy. Higher selectivity was obtained by preparing the end groups to be reacted with the polyol core. This second synthetic pathway to TMP_bMPA_It_Cy yielded a lower viscosity material with almost identical physical and thermal properties. It was also observed during the synthesis of star-shaped polymers, that the use of standard esterification conditions with primary alcohol on substituted branched polyol cores lead to a transesterification due to the higher reactivity of the β-carboxylic acid of itaconic acid and therefore the destruction of the architecture obtained before.
Thus, novel building blocks from itaconic acid were necessary to develop UV-curing monomers and branched polymers from itaconic acid. To this end, eight monoesters of itaconic acid were synthesized using an efficient and inexpensive method. No excess of alcohol or tedious purification steps were required, which were the main drawbacks of known methods.
This highly selective synthesis was made possible by a reaction of a range of mono-alcohols with itaconic anhydride, the latter was obtained by a ring-closing reaction of itaconic acid.
Simple and more complex alcohols were used: methanol, ethanol, propanol, butanol, cyclohexanol, benzyl alcohol, phenoxyethanol and isoborneol, to synthesize the monoesters in high purity and good to excellent yields. These monoesters were reacted with epoxidized soybean oil to assess the impact of each alkyl group on the viscosity and reactivity of the UV-curable materials. This structure-properties relationship study showed that all monoesters are good candidates as building blocks for UV-curing application, especially It_Cy, proven to be the most reactive. Resulting IESOs showed a decreased viscosity compared with the standard AESO and an increased conversion in some cases.
Thanks to the insights obtained during the design and the synthesis of HBPs from acrylic acid and monoesters of itaconic acid, several ways to synthesize HBPs from itaconic acid were investigated. As shown above, the simple condensation, once optimized, lead to viscous
UV-123 curable materials with high reactivity. The synthesis through transesterification using DMI was proven to be unsuccessful due to the insufficient difference in reactivity between the two carboxyl groups that did not allow for a good control over a selective transesterification.
Another alkyl-terminated HBP_It was synthesized through a more complex methodology using undecenoic acid, to be epoxidized before the reaction with cyclohexyl itaconate. This HBP_It was found to be very flexible and might be of interest as additive to be mixed with highly rigid UV-curable polymers. An acid-terminated HBP_It was developed and despite the high viscosity, the good degradation in basic conditions showed that it might be an interesting material as a UV-curable support material. The last type of HBP_It designed was hydroxyalkyl-terminated HBPs, which seems to be the most promising approach. This simple method uses polyol cores with itaconic anhydride and the resulting acid-terminated HBP is then reacted with an epoxide. Molecules with four different cores, two epoxides and 0 or 1 generation of bMPA were synthesized, yielding thirteen different HBP_It. Materials using cyclohexene oxide were found to exhibit a too high viscosity to be used as UV-curable materials, but the ones using butylene oxide were suitable for such an application. The synthesis of the first generation of HBP_It could not be fully optimized. However, these successful syntheses yielded to promising materials that could be further improved.
Finally, the knowledge acquired during these researches allowed the development of novel reactive diluents from itaconic acid. As it is more challenging to obtain a high reactivity material when designing a small structure containing itaconic acid, no study on the synthesis of itaconic acid based RDs was previously reported. Three types of reactive diluents were designed: diester, dihydroxyester and hydroxyester ester itaconates by exploiting the difference in reactivity of the two carboxylic acids. These monomers showed good reactivity towards UV-curing and showed varying viscosities, increasing with the number of hydroxy groups. These OH groups also increased the reactivities during UV-curing. The four best candidates were mixed with a standard bio-based resin with various amount (10, 30 and 50%) to assess their impact as reactive diluents. They were found to be extremely beneficial when formulated with the standard polyester, lowering the viscosity up to 20-fold, improving the thermal (increase of the Tg up to 40 °C) and mechanical properties significantly (more than doubled modulus at 25
°C in some cases) and improving the final double bond conversion and gel content (up to 35%
more), while keeping a similar rate of polymerization.
To conclude, numerous UV-curable materials were designed and synthesized using either improved greener synthetic pathways or novel synthetic routes. These methodologies were
124 designed to be as much in accordance as possible with the principles of green chemistry, while trying to keep the synthesis of such materials economically optimal and potentially scalable.
The original goal of this project: the synthesis of highly branched polymers from itaconic acid, was achieved and surpassed as novel building blocks and more challenging compounds such as reactive diluents were also developed. More generally, this work gathers valuable knowledge on the impact of each building block on the final properties of HBPs, but also on the delicate reactivity of itaconic acid both during the synthesis and the UV-curing process.
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