Department of Chemistry & Biochemistry, University of Denver, Denver, Colorado 80210, United States.
J Am Chem Soc. 2023 Jul 5;145(26):14202-14207. doi: 10.1021/jacs.3c03811. Epub 2023 Jun 21.
Thermosetting materials generated by photopolymerization frequently suffer from significant shrinkage stress, are often brittle, and have a limited range of mechanical properties. Various classes of chain transfer agents (CTAs) have been investigated and developed to reduce the cross-linking density of photopolymers by terminating chains and initiating new chains . Although CTAs are successful in manipulating the mechanical properties of photopolymers, they are traditionally consumed during the polymerization and are therefore required in high loadings (up to 20 wt % of the total formulation). Moreover, traditional CTAs frequently contain sulfur, which is malodorous and can create unstable formulations. Presented here is a catalytic, sulfur-free CTA that can be added in ppm quantities to existing commercial monomer feedstocks to create photopolymers similar to those prepared using traditional CTAs, but at 10 000-fold lower loadings. These catalysts, which are based on macrocyclic cobaloximes, were found to tunably reduce the molecular weight of the chain proportional to catalyst loading. It was shown, using only commercial monomers, that this catalyst could reduce the glass-transition temperature (), rubbery modulus ('), and stiffness of a cross-linked photopolymer while utilizing identical processing conditions and keeping 99.99 wt % of the formulation the same.
热固性材料由光聚合产生时，经常会产生显著的收缩应力，往往很脆，机械性能也有限。已经研究并开发了各种类型的链转移剂 (CTA)，通过终止链和引发新链来降低光聚合物的交联密度。虽然 CTA 成功地控制了光聚合物的机械性能，但它们在聚合过程中会被消耗，因此需要高负荷（高达总配方的 20wt%）。此外，传统的 CTA 通常含有硫，其气味难闻且可能导致不稳定的配方。本文介绍了一种催化的、无硫的 CTA，它可以以 ppm 数量添加到现有的商业单体原料中，以制备类似于使用传统 CTA 制备的光聚合物，但用量低 10000 倍。这些基于大环钴配合物的催化剂被发现可以根据催化剂的负载量，按比例调节降低聚合物的分子量。仅使用商业单体，证明了该催化剂可以降低交联光聚合物的玻璃化转变温度（）、橡胶模量（'）和硬度，同时使用相同的加工条件，且配方的 99.99wt%保持不变。
