Photoswitchable Macromolecules: The Foundation of Photodynamic Block Copolymer Lithography

The microphase separation of block copolymers leads to diverse thin-film nanostructures, such as spheres, cylinders, lamella and gyroids. Such thin-film nanostructures – typically on the scale of 10-100 nm – are of profound importance for applications such as microelectronics, catalysis and lithography. However, these conventional block copolymer morphologies are typically static once formed. In the current literature, manipulation of these nanostructures is often achieved via physical and chemical methods. Generally, a change in the volume fraction and/or the relative polarity between blocks is the driving force for a transition in morphology.
Given the remote and precise control enabled by light as an external stimulus, reversible switching between morphologies by light of a specific color is thus highly appealing. Therefore, inclusion of light-sensitive molecules into block copolymers is essential. One attractive class of photo-responsive entities are photoswitches. A photoswitch interacts with light to change its conformation and reverses by another color of light or via a thermal pathway. An ideal photoswitch should exhibit a large geometrical and/or polarity change upon irradiation. ... mehrThe former feature (i.e., large difference in geometry) is maximized only when photoswitches are installed as main-chain repeating units. This is particularly critical for the manipulation of block copolymer nanostructures. However, there is a critical lack of research focused on main-chain photoswitchable block copolymers despite a large library of photoswitches.
Herein, we establish the underpinning synthetic strategies for the integration of photoswitches into polymers and block copolymers as main-chain groups and as side-chain groups where applicable. We explore head-to-tail Acyclic Diene METathesis (ADMET), hydroxyl-yne click polymerization and a sequence-defined approach combining hydroxyl-yne click, protection/deprotection, and thiol-Michael click chemistries as synthetic tools. We demonstrate that photoswitchable polymers and block copolymers can be obtained efficiently via the developed strategies, successfully incorporating three different photoswitches, namely -bisimine, hydrazone and spiropyran. We further show that solution photoisomerization of the embedded photoswitches is independent of the polymerization methods as well as dispersity of polymer chains. Furthermore, we found a general pathway to enhance the efficiency of solid-state photoisomerization (or photoswitching) – one of the key factors for light-induced morphological switching of block copolymer nanostructures, referred to herein as ‘photodynamic block copolymer lithography’. The current thesis thus introduces a chemical and synthetic foundation for the realization of advanced polymeric materials that have rarely been explored in the current literature. Finally, preliminary study on thin-film and bulk morphologies of selected photoswitchable block copolymers shows promising results, providing guidance for our future endeavors towards realizing our vision of photodynamic block copolymer lithography.