Abstract
The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.
Highlights
The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility
Most characteristics arise from the beneficial structure−property relationship of heteroatomcontaining polymer backbones, which are accessible by CROP in great variety
CROP of 2-oxazolines has been in the focus of interest for the structural versatility and polymer characteristics of poly(2-alkyl/aryl-2-oxazoline)s (PAOx), as well as the accessibility of copolymers by its living polymerization character.[1−4] Several reviews have exclusively focused on the potentials of this polymer class in the biomedical field, with the most prominent aim being to replace polyethylene glycol (PEG) in medicinal use for nonfouling and nonadhesive surfaces, biomedical analysis, protein and drug conjugation, and polymer-based drug delivery.[5−12] Most applications in structural materials are, yet, limited to functional poly(2oxazoline) precursors that do not yield polymer networks via a cationic polymerization mechanism.[2,5,13]
Summary
Insignificant photopolymerization could be observed for the aromatic PhOx below 140 °C, whereas OctOx showed apparent photoreactivity at temperatures above 100 °C. Based on earlier reports of thermally cross-linked bis(2-oxazoline)-terminated building blocks, promising (thermo)mechanical properties were expected from materials after cationic photopolymerization of difunctional 2-oxazolines.[32] Formulations of BisOx in mixtures with the monofunctional monomers OctOx and PhOx were prepared to yield sufficiently resistant photopolymers for (thermo)mechanical investigations by DMTA. 9,10-dibutoxyanthracene was used to enhance the resolution of the selective photopolymerization in the irradiated area by its excellent spectral compatibility with the 375 nm laser (λmax = 381 nm) This electron-transfer photosensitizer greatly supports the reactivity and the control over curing depth in the layer-bylayer cationic photopolymerization.[40] As the PAG S−B caused inevitable diffusional overpolymerization, the Hot Lithography of BisOx was conducted using a mixture of triarylsulfonium hexafluoroantimonate salts Experimental procedures of synthesis and characterization, analytic devices and methods, structural analysis, material testing and sample preparation, 3Dprinting by hot lithography (PDF)
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