Progressive Photocuring Approach: A Macrocyclic Photoinitiator Enabling Exceptional Toughness in Poly(TMPTA) Thick Coatings

Abstract

Achieving thick coatings (>200 μm) remains a significant challenge in the field of photopolymerization, primarily due to light attenuation. Despite the use of photobleaching initiators, rapid polymerization frequently results in the formation of a crosslinking gradient and the development of internal stresses, which adversely affect the mechanical properties, particularly the toughness, of the photo-cured materials. In this study, a novel macrocyclic photobleaching initiator [2](BPC)[2](C12) was developed specifically for thick-coating photopolymerization applications. In contrast to the photobleaching mechanism associated with the commercial initiator TPO, [2](BPC)[2](C12) exhibits a deeper color, which reduces light penetration and significantly decelerates the photobleaching rate, thereby generating a distinctive light-gating effect. This effect modulates the polymerization kinetics by inhibiting premature surface gelation and mitigating both the crosslinking density gradient and internal stress. As a result, the curing process is optimized in terms of uniformity, while maintaining a high degree of monomer crosslinking. To further elucidate the underlying mechanisms, this study is based on the density functional theory (DFT) calculations, steady-state photolysis, in situ nuclear magnetic resonance (NMR) analysis, providing an in-depth investigation into the photobleaching behavior and progressive curing mechanism of [2](BPC)[2](C12) in the presence of an EDB system. The findings from compression tests demonstrate that this progressive curing strategy significantly enhances the mechanical toughness of thick coatings. Specifically, for cylindrical poly(TMPTA) specimens (1 cm in diameter × 1 cm in height), the strain energy density increased from 0.6415 MPa to 0.9207 MPa, reflecting a 43.5% improvement. Moreover, the maximum strain exhibited a substantial increase, from 33% to 77%. This research not only introduces a new molecular platform for photopolymerization but also provides critical mechanistic insights that will aid in the rational design of high-performance photocuring systems for thick coatings.