Radical photopolymerization (RPP) has grown into a multibillion-dollar technology for reduced energy consumption and waste with increased productivity. However, radical-mediated polymerization ceases almost immediately following discontinuation of irradiation because of rapid termination of reactive centers. This restricts the wider use of RPP in applications that involve light attenuation or irregular surfaces because uniform polymerization is not guaranteed for these challenging exposure conditions and the resultant undercuring leads to compromised material properties and harmful leachable monomers. Herein, we developed a unique radical dark-curing photoinitiator (DCPI) that continues its polymerization beyond the cessation of irradiation. The DCPI achieved a remarkable 25–60% additional conversion over a 1 h period, when light was shuttered at 20% conversion, compared to the 1–3% additional conversion achieved by a Norrish type II control photoinitiator. We elucidated the origin of the high photon efficiency using computational studies and experiments, which suggest that the DCPI may be the most-photon-efficient photoinitiator to date. We also demonstrated that the mechanical properties of dark-cured polymer are similar to and even exceed those of the corresponding polymer obtained by extended photocuring. In particular, the initial 0.1 MPa storage modulus continuously developed to 4.3 MPa without further irradiation, while also exhibiting ∼30% less shrinkage stress than the full exposure control. With its superior photo- and dark-polymerization efficiency, the DCPI enhances the performance of many existing RPP processes while extending RPP to heretofore unattainable applications. The DCPI accomplishes such a performance through an inherent automatic rectification of initially undercured regions and defies the RPP paradigm that light dosage directly correlates to conversion.

中文翻译：

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自主暗固化增强了高效自由基光聚合

自由基光聚合（RPP）已发展成为一项数十亿美元的技术，可减少能耗和浪费，并提高生产率。然而，由于反应中心的迅速终止，自由基介导的聚合反应几乎在辐射中断后立即停止。这限制了RPP在涉及光衰减或不规则表面的应用中的广泛使用，因为在这些挑战性的曝光条件下不能保证均匀聚合，并且所产生的欠固化会损害材料性能和有害的可浸出单体。本文中，我们开发了一种独特的自由基暗固化光引发剂（DCPI），该引发剂在停止照射后仍能继续聚合。DCPI在1小时内的转化率提高了25-60％，当以20％的转化率关闭光时，与Norrish II型对照光引发剂实现的1-3％的额外转化率相比。我们通过计算研究和实验阐明了高光子效率的起源，这表明DCPI可能是迄今为止最光子效率最高的光引发剂。我们还证明了暗固化聚合物的机械性能类似于甚至超过了通过扩展光固化获得的相应聚合物的机械性能。特别是，最初的0.1 MPa储能模量在没有进一步照射的情况下连续发展到4.3 MPa，同时比全曝光控制的收缩应力低约30％。凭借出色的光聚合和暗聚合效率，DCPI增强了许多现有RPP流程的性能，同时将RPP扩展到迄今为止无法实现的应用程序。DCPI通过对最初固化不足的区域进行固有的自动矫正来实现这种性能，并违背了RPP范式，即光剂量与转化直接相关。